Power electronics
Power electronics is the application of solid-state electronics to the control and conversion of electric power. It also refers to a subject of research in electronic and electrical engineering which deals with the design, control, computation and integration of nonlinear, time-varying energy-processing electronic systems with fast dynamics.
The first high power electronic devices were mercury-arc valves. In modern systems the conversion is performed with semiconductor switching devices such as diodes, thyristors and transistors, pioneered by R. D. Middlebrook and others beginning in the 1950s. In contrast to electronic systems concerned with transmission and processing of signals and data, in power electronics substantial amounts of electrical energy are processed. An AC/DC converter (rectifier) is the most typical power electronics device found in many consumer electronic devices, e.g. television sets, personal computers, battery chargers, etc. The power range is typically from tens of watts to several hundred watts. In industry a common application is the variable speed drive (VSD) that is used to control an induction motor. The power range of VSDs start from a few hundred watts and end at tens of megawatts.
The power conversion systems can be classified according to the type of the input and output power
- AC to DC (rectifier)
- DC to AC (inverter)
- DC to DC (DC-to-DC converter)
- AC to AC (AC-to-AC converter)
Contents
History
Power electronics started with the development of the mercury arc rectifier. Invented by Peter Cooper Hewitt in 1902, it was used to convert alternating current (AC) into direct current (DC). From the 1920s on, research continued on applying thyratrons and grid-controlled mercury arc valves to power transmission. Uno Lamm developed a mercury valve with grading electrodes making them suitable for high voltage direct current power transmission. In 1933 selenium rectifiers were invented.[1]
In 1947 the bipolar point-contact transistor was invented by Walter H. Brattain and John Bardeen under the direction of William Shockley at Bell Labs. In 1948 Shockley's invention of the bipolar junction transistor (BJT) improved the stability and performance of transistors, and reduced costs. By the 1950s, higher power semiconductor diodes became available and started replacing vacuum tubes. In 1956 the Silicon Controlled Rectifier (SCR) was introduced by General Electric, greatly increasing the range of power electronics applications.[2]
By the 1960s the improved switching speed of bipolar junction transistors had allowed for high frequency DC/DC converters. In 1976 power MOSFETs became commercially available. In 1982 the Insulated Gate Bipolar Transistor (IGBT) was introduced.
Devices
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This section includes a list of references, but its sources remain unclear because it has insufficient inline citations. (December 2013) |
The capabilities and economy of power electronics system are determined by the active devices that are available. Their characteristics and limitations are a key element in the design of power electronics systems. Formerly, the mercury arc valve, the high-vacuum and gas-filled diode thermionic rectifiers, and triggered devices such as the thyratron and ignitron were widely used in power electronics. As the ratings of solid-state devices improved in both voltage and current-handling capacity, vacuum devices have been nearly entirely replaced by solid-state devices.
Power electronic devices may be used as switches, or as amplifiers.[3] An ideal switch is either open or closed and so dissipates no power; it withstands an applied voltage and passes no current, or passes any amount of current with no voltage drop. Semiconductor devices used as switches can approximate this ideal property and so most power electronic applications rely on switching devices on and off, which makes systems very efficient as very little power is wasted in the switch. By contrast, in the case of the amplifier, the current through the device varies continuously according to a controlled input. The voltage and current at the device terminals follow a load line, and the power dissipation inside the device is large compared with the power delivered to the load.
Several attributes dictate how devices are used. Devices such as diodes conduct when a forward voltage is applied and have no external control of the start of conduction. Power devices such as silicon controlled rectifiers and thyristors (as well as the mercury valve and thyratron) allow control of the start of conduction, but rely on periodic reversal of current flow to turn them off. Devices such as gate turn-off thyristors, BJT and MOSFET transistors provide full switching control and can be turned on or off without regard to the current flow through them. Transistor devices also allow proportional amplification, but this is rarely used for systems rated more than a few hundred watts. The control input characteristics of a device also greatly affect design; sometimes the control input is at a very high voltage with respect to ground and must be driven by an isolated source.
As efficiency is at a premium in a power electronic converter, the losses that a power electronic device generates should be as low as possible.
Devices vary in switching speed. Some diodes and thyristors are suited for relatively slow speed and are useful for power frequency switching and control; certain thyristors are useful at a few kilohertz. Devices such as MOSFETS and BJTs can switch at tens of kilohertz up to a few megahertz in power applications, but with decreasing power levels. Vacuum tube devices dominate high power (hundreds of kilowatts) at very high frequency (hundreds or thousands of megahertz) applications. Faster switching devices minimize energy lost in the transitions from on to off and back, but may create problems with radiated electromagnetic interference. Gate drive (or equivalent) circuits must be designed to supply sufficient drive current to achieve the full switching speed possible with a device. A device without sufficient drive to switch rapidly may be destroyed by excess heating.
Practical devices have non-zero voltage drop and dissipate power when on, and take some time to pass through an active region until they reach the "on" or "off" state. These losses are a significant part of the total lost power in a converter.
Power handling and dissipation of devices is also a critical factor in design. Power electronic devices may have to dissipate tens or hundreds of watts of waste heat, even switching as efficiently as possible between conducting and non-conducting states. In the switching mode, the power controlled is much larger than the power dissipated in the switch. The forward voltage drop in the conducting state translates into heat that must be dissipated. High power semiconductors require specialized heat sinks or active cooling systems to manage their junction temperature; exotic semiconductors such as silicon carbide have an advantage over straight silicon in this respect, and germanium, once the main-stay of solid-state electronics is now little used due to its unfavorable high temperature properties.
Semiconductor devices exist with ratings up to a few kilovolts in a single device. Where very high voltage must be controlled, multiple devices must be used in series, with networks to equalize voltage across all devices. Again, switching speed is a critical factor since the slowest-switching device will have to withstand a disproportionate share of the overall voltage. Mercury valves were once available with ratings to 100 kV in a single unit, simplifying their application in HVDC systems.
The current rating of a semiconductor device is limited by the heat generated within the dies and the heat developed in the resistance of the interconnecting leads. Semiconductor devices must be designed so that current is evenly distributed within the device across its internal junctions (or channels); once a "hot spot" develops, breakdown effects can rapidly destroy the device. Certain SCRs are available with current ratings to 3000 amperes in a single unit.
Solid-state devices
| Device | Description | Ratings |
|---|---|---|
| Diode | Uni-polar, uncontrolled, switching device used in applications such as rectification and circuit directional current control. Reverse voltage blocking device, commonly modeled as a switch in series with a voltage source, usually 0.7 VDC. The model can be enhanced to include a junction resistance, in order to accurately predict the diode voltage drop across the diode with respect to current flow. | Up to 3000 amperes and 5000 volts in a single silicon device. High voltage requires multiple series silicon devices. |
| Silicon-controlled rectifier (SCR) | This semi-controlled device turns on when a gate pulse is present and the anode is positive compared to the cathode. When a gate pulse is present, the device operates like a standard diode. When the anode is negative compared to the cathode, the device turns off and blocks positive or negative voltages present. The gate voltage does not allow the device to turn off.[4] | Up to 3000 amperes, 5000 volts in a single silicon device. |
| Thyristor | The thyristor is a family of three-terminal devices that include SCRs, GTOs, and MCT. For most of the devices, a gate pulse turns the device on. The device turns off when the anode voltage falls below a value (relative to the cathode) determined by the device characteristics. When off, it is considered a reverse voltage blocking device.[4] | |
| Gate turn-off thyristor (GTO) | The gate turn-off thyristor, unlike an SCR, can be turned on and off with a gate pulse. One issue with the device is that turn off gate voltages are usually larger and require more current than turn on levels. This turn off voltage is a negative voltage from gate to source, usually it only needs to be present for a short time, but the magnitude s on the order of 1/3 of the anode current. A snubber circuit is required in order to provide a usable switching curve for this device. Without the snubber circuit, the GTO cannot be used for turning inductive loads off. These devices, because of developments in IGCT technology are not very popular in the power electronics realm. They are considered controlled, uni-polar and bi-polar voltage blocking.[5] | |
| Triac | The triac is a device that is essentially an integrated pair of phase-controlled thyristors connected in inverse-parallel on the same chip.[6] Like an SCR, when a voltage pulse is present on the gate terminal, the device turns on. The main difference between an SCR and a Triac is that both the positive and negative cycle can be turned on independently of each other, using a positive or negative gate pulse. Similar to an SCR, once the device is turned on, the device cannot be turned off. This device is considered bi-polar and reverse voltage blocking. | |
| Bipolar junction transistor (BJT) | The BJT cannot be used at high power; they are slower and have more resistive losses when compared to MOSFET type devices. To carry high current, BJTs must have relatively large base currents, thus these devices have high power losses when compared to MOSFET devices. BJTs along with MOSFETs, are also considered unipolar and do not block reverse voltage very well, unless installed in pairs with protection diodes. Generally, BJTs are not utilized in power electronics switching circuits because of the I2R losses associated with on resistance and base current requirements.[4] BJTs have lower current gains in high power packages, thus requiring them to be set up in Darlington configurations in order to handle the currents required by power electronic circuits. Because of these multiple transistor configurations, switching times are in the hundreds of nanoseconds to microseconds. Devices have voltage ratings which max out around 1500 V and fairly high current ratings. They can also be paralleled in order to increase power handling, but must be limited to around 5 devices for current sharing.[5] | |
| Power MOSFET | The main benefit of the power MOSFET is that the base current for
BJT is large compared to almost zero for MOSFET gate current. Since the
MOSFET is a depletion channel device, voltage, not current, is necessary
to create a conduction path from drain to source. The gate does not
contribute to either drain or source current. Turn on gate current is
essentially zero with the only power dissipated at the gate coming
during switching. Losses in MOSFETs are largely attributed to
on-resistance. The calculations show a direct correlation to drain
source on-resistance and the device blocking voltage rating, BVdss.
Switching times range from tens of nanoseconds to a few hundred microseconds, depending on the device. MOSFET drain source resistances increase as more current flows through the device. As frequencies increase the losses increase as well, making BJTs more attractive. Power MOSFETs can be paralleled in order to increase switching current and therefore overall switching power. Nominal voltages for MOSFET switching devices range from a few volts to a little over 1000 V, with currents up to about 100 A or so. Newer devices may have higher operational characteristics. MOSFET devices are not bi-directional, nor are they reverse voltage blocking.[5] || |
|
| Insulated-gate bipolar transistor (IGBT) | These devices have the best characteristics of MOSFETs and BJTs. Like MOSFET devices, the insulated gate bipolar transistor has a high gate impedance, thus low gate current requirements. Like BJTs, this device has low on state voltage drop, thus low power loss across the switch in operating mode. Similar to the GTO, the IGBT can be used to block both positive and negative voltages. Operating currents are fairly high, in excess of 1500 A and switching voltage up to 3000 V.[5] The IGBT has reduced input capacitance compared to MOSFET devices which improves the Miller feedback effect during high dv/dt turn on and turn off.[6] | |
| MOS-controlled thyristor (MCT) | The MOS-controlled thyristor is thyristor like and can be triggered on or off by a pulse to the MOSFET gate.[6] Since the input is MOS technology, there is very little current flow, allowing for very low power control signals. The device is constructed with two MOSFET inputs and a pair of BJT output stages. Input MOSFETs are configured to allow turn on control during positive and negative half cycles. The output BJTs are configured to allow for bidirectional control and low voltage reverse blocking. Some benefits to the MCT are fast switching frequencies, fairly high voltage and medium current ratings (around 100 A or so). | |
| Integrated gate-commutated thyristor (IGCT) | Similar to a GTO, but without the high current requirements to turn
on or off the load. The IGCT can be used for quick switching with little
gate current. The devices high input impedance largely because of the
MOSFET gate drivers. They have low resistance outputs that don't waste
power and very fast transient times that rival that of BJTs. ABB Group
company has published data sheets for these devices and provided
descriptions of the inner workings. The device consists of a gate, with
an optically isolated input, low on resistance BJT output transistors
which lead to a low voltage drop and low power loss across the device at
fairly high switching voltage and current levels.
An example of this new device from ABB shows how this device improves on GTO technology for switching high voltage and high current in power electronics applications. According to ABB, the IGCT devices are capable of switching in excess of 5000 VAC and 5000 A at very high frequencies, something not possible to do efficiently with GTO devices.[7] |
DC/AC converters (inverters)
DC to AC converters produce an AC output waveform from a DC source. Applications include adjustable speed drives (ASD), uninterruptable power supplies (UPS), active filters, Flexible AC transmission systems (FACTS), voltage compensators, and photovoltaic generators. Topologies for these converters can be separated into two distinct categories: voltage source inverters and current source inverters. Voltage source inverters (VSIs) are named so because the independently controlled output is a voltage waveform. Similarly, current source inverters (CSIs) are distinct in that the controlled AC output is a current waveform.
Being static power converters, the DC to AC power conversion is the result of power switching devices, which are commonly fully controllable semiconductor power switches. The output waveforms are therefore made up of discrete values, producing fast transitions rather than smooth ones. The ability to produce near sinusoidal waveforms around the fundamental frequency is dictated by the modulation technique controlling when, and for how long, the power valves are on and off. Common modulation techniques include the carrier-based technique, or pulse width modulation, space-vector technique, and the selective-harmonic technique.[8]
Voltage source inverters have practical uses in both single-phase and three-phase applications. Single-phase VSIs utilize half-bridge and full-bridge configurations, and are widely used for power supplies, single-phase UPSs, and elaborate high-power topologies when used in multicell configurations. Three-phase VSIs are used in applications that require sinusoidal voltage waveforms, such as ASDs, UPSs, and some types of FACTS devices such as the STATCOM. They are also used in applications where arbitrary voltages are required as in the case of active filters and voltage compensators.[8]
Current source inverters are used to produce an AC output current from a DC current supply. This type of inverter is practical for three-phase applications in which high-quality voltage waveforms are required.
A relatively new class of inverters, called multilevel inverters, has gained widespread interest. Normal operation of CSIs and VSIs can be classified as two-level inverters, due to the fact that power switches connect to either the positive or to the negative DC bus. If more than two voltage levels were available to the inverter output terminals, the AC output could better approximate a sine wave. It is for this reason that multilevel inverters, although more complex and costly, offer higher performance.[9]
Each inverter type differs in the DC links used, and in whether or not they require freewheeling diodes. Either can be made to operate in square-wave or pulse-width modulation (PWM) mode, depending on its intended usage. Square-wave mode offers simplicity, while PWM can be implemented several different ways and produces higher quality waveforms.[8]
Voltage Source Inverters (VSI) feed the output inverter section from an approximately constant-voltage source.[8]
The desired quality of the current output waveform determines which modulation technique needs to be selected for a given application. The output of a VSI is composed of discrete values. In order to obtain a smooth current waveform, the loads need to be inductive at the select harmonic frequencies. Without some sort of inductive filtering between the source and load, a capacitive load will cause the load to receive a choppy current waveform, with large and frequent current spikes.[8]
There are three main types of VSIs:
- Single-phase half-bridge inverter
- Single-phase full-bridge inverter
- Three-phase voltage source inverter
Single-phase half-bridge inverter
The single-phase voltage source half-bridge inverters, are meant for lower voltage applications and are commonly used in power supplies.[8] Figure 2 shows the circuit schematic of this inverter.
Low-order current harmonics get injected back to the source voltage by the operation of the inverter. This means that two large capacitors are needed for filtering purposes in this design.[8] As Figure 2 illustrates, only one switch can be on at time in each leg of the inverter. If both switches in a leg were on at the same time, the DC source will be shorted out.
Inverters can use several modulation techniques to control their switching schemes. The carrier-based PWM technique compares the AC output waveform, vc, to a carrier voltage signal, vΔ. When vc is greater than vΔ, S+ is on, and when vc is less than vΔ, S- is on. When the AC output is at frequency fc with its amplitude at vc, and the triangular carrier signal is at frequency fΔ with its amplitude at vΔ, the PWM becomes a special sinusoidal case of the carrier based PWM.[8] This case is dubbed sinusoidal pulse-width modulation (SPWM).For this, the modulation index, or amplitude-modulation ratio, is defined as ma = vc/v∆ .
The normalized carrier frequency, or frequency-modulation ratio, is calculated using the equation mf = f∆/fc .
If the over-modulation region, ma, exceeds one, a higher fundamental AC output voltage will be observed, but at the cost of saturation. For SPWM, the harmonics of the output waveform are at well-defined frequencies and amplitudes. This simplifies the design of the filtering components needed for the low-order current harmonic injection from the operation of the inverter. The maximum output amplitude in this mode of operation is half of the source voltage. If the maximum output amplitude, ma, exceeds 3.24, the output waveform of the inverter becomes a square wave.[8]
As was true for PWM, both switches in a leg for square wave modulation cannot be turned on at the same time, as this would cause a short across the voltage source. The switching scheme requires that both S+ and S- be on for a half cycle of the AC output period.[8] The fundamental AC output amplitude is equal to vo1 = vaN = 2vi/π .
Its harmonics have an amplitude of voh = vo1/h.
Therefore, the AC output voltage is not controlled by the inverter, but rather by the magnitude of the DC input voltage of the inverter.[8]
Using selective harmonic elimination (SHE) as a modulation technique allows the switching of the inverter to selectively eliminate intrinsic harmonics. The fundamental component of the AC output voltage can also be adjusted within a desirable range. Since the AC output voltage obtained from this modulation technique has odd half and odd quarter wave symmetry, even harmonics do not exist.[8] Any undesirable odd (N-1) intrinsic harmonics from the output waveform can be eliminated.
Single-phase full-bridge inverter
The full-bridge inverter is similar to the half bridge-inverter, but it has an additional leg to connect the neutral point to the load.[8] Figure 3 shows the circuit schematic of the single-phase voltage source full-bridge inverter.
To avoid shorting out the voltage source, S1+ and S1- cannot be on at the same time, and S2+ and S2- also cannot be on at the same time. Any modulating technique used for the full-bridge configuration should have either the top or the bottom switch of each leg on at any given time. Due to the extra leg, the maximum amplitude of the output waveform is Vi, and is twice as large as the maximum achievable output amplitude for the half-bridge configuration.[8]
States 1 and 2 from Table 2 are used to generate the AC output voltage with bipolar SPWM. The AC output voltage can take on only two values, either Vi or –Vi. To generate these same states using a half-bridge configuration, a carrier based technique can be used. S+ being on for the half-bridge corresponds to S1+ and S2- being on for the full-bridge. Similarly, S- being on for the half-bridge corresponds to S1- and S2+ being on for the full bridge. The output voltage for this modulation technique is more or less sinusoidal, with a fundamental component that has an amplitude in the linear region of ma less than or equal to one[8] vo1 =vab1= vi • ma.
Unlike the bipolar PWM technique, the unipolar approach uses states 1, 2, 3 and 4 from Table 2 to generate its AC output voltage. Therefore, the AC output voltage can take on the values Vi, 0 or –V [1]i. To generate these states, two sinusoidal modulating signals, Vc and –Vc, are needed, as seen in Figure 4.
Vc is used to generate VaN, while –Vc is used to generate VbN. The following relationship is called unipolar carrier-based SPWM vo1 =2 • vaN1= vi • ma.
The phase voltages VaN and VbN are identical, but 180 degrees out of phase with each other. The output voltage is equal to the difference of the two phase voltages, and do not contain any even harmonics. Therefore, if mf is taken, even the AC output voltage harmonics will appear at normalized odd frequencies, fh. These frequencies are centered on double the value of the normalized carrier frequency. This particular feature allows for smaller filtering components when trying to obtain a higher quality output waveform.[8]
As was the case for the half-bridge SHE, the AC output voltage contains no even harmonics due to its odd half and odd quarter wave symmetry.[8]
Three-phase voltage source inverter
Single-phase VSIs are used primarily for low power range applications, while three-phase VSIs cover both medium and high power range applications.[8] Figure 5 shows the circuit schematic for a three-phase VSI.
Switches in any of the three legs of the inverter cannot be switched off simultaneously due to this resulting in the voltages being dependent on the respective line current's polarity. States 7 and 8 produce zero AC line voltages, which result in AC line currents freewheeling through either the upper or the lower components. However, the line voltages for states 1 through 6 produce an AC line voltage consisting of the discrete values of Vi, 0 or –Vi.[8]
For three-phase SPWM, three modulating signals that are 120 degrees out of phase with one another are used in order to produce out of phase load voltages. In order to preserve the PWM features with a single carrier signal, the normalized carrier frequency, mf, needs to be a multiple of three. This keeps the magnitude of the phase voltages identical, but out of phase with each other by 120 degrees.[8] The maximum achievable phase voltage amplitude in the linear region, ma less than or equal to one, is vphase = vi / 2. The maximum achievable line voltage amplitude is Vab1 = vab • √3 / 2
The only way to control the load voltage is by changing the input DC voltage.
Current source inverters
Current source inverters convert DC current into an AC current waveform. In applications requiring sinusoidal AC waveforms, magnitude, frequency, and phase should all be controlled. CSIs have high changes in current overtime, so capacitors are commonly employed on the AC side, while inductors are commonly employed on the DC side.[8] Due to the absence of freewheeling diodes, the power circuit is reduced in size and weight, and tends to be more reliable than VSIs.[9] Although single-phase topologies are possible, three-phase CSIs are more practical.
In its most generalized form, a three-phase CSI employs the same conduction sequence as a six-pulse rectifier. At any time, only one common-cathode switch and one common-anode switch are on.[9]
As a result, line currents take discrete values of –ii, 0 and ii. States are chosen such that a desired waveform is outputted and only valid states are used. This selection is based on modulating techniques, which include carrier-based PWM, selective harmonic elimination, and space-vector techniques.[8]
Carrier-based techniques used for VSIs can also be implemented for CSIs, resulting in CSI line currents that behave in the same way as VSI line voltages. The digital circuit utilized for modulating signals contains a switching pulse generator, a shorting pulse generator, a shorting pulse distributor, and a switching and shorting pulse combiner. A gating signal is produced based on a carrier current and three modulating signals.[8]
A shorting pulse is added to this signal when no top switches and no bottom switches are gated, causing the RMS currents to be equal in all legs. The same methods are utilized for each phase, however, switching variables are 120 degrees out of phase relative to one another, and the current pulses are shifted by a half-cycle with respect to output currents. If a triangular carrier is used with sinusoidal modulating signals, the CSI is said to be utilizing synchronized-pulse-width-modulation (SPWM). If full over-modulation is used in conjunction with SPWM the inverter is said to be in square-wave operation.[8]
The second CSI modulation category, SHE is also similar to its VSI counterpart. Utilizing the gating signals developed for a VSI and a set of synchronizing sinusoidal current signals, results in symmetrically distributed shorting pulses and, therefore, symmetrical gating patterns. This allows any arbitrary number of harmonics to be eliminated.[8] It also allows control of the fundamental line current through the proper selection of primary switching angles. Optimal switching patterns must have quarter-wave and half-wave symmetry, as well as symmetry about 30 degrees and 150 degrees. Switching patterns are never allowed between 60 degrees and 120 degrees. The current ripple can be further reduced with the use of larger output capacitors, or by increasing the number of switching pulses.[9]
The third category, space-vector-based modulation, generates PWM load line currents that equal load line currents, on average. Valid switching states and time selections are made digitally based on space vector transformation. Modulating signals are represented as a complex vector using a transformation equation. For balanced three-phase sinusoidal signals, this vector becomes a fixed module, which rotates at a frequency, ω. These space vectors are then used to approximate the modulating signal. If the signal is between arbitrary vectors, the vectors are combined with the zero vectors I7, I8, or I9.[8] The following equations are used to ensure that the generated currents and the current vectors are on average equivalent.
Multilevel inverters
A relatively new class called multilevel inverters has gained widespread interest. Normal operation of CSIs and VSIs can be classified as two-level inverters because the power switches connect to either the positive or the negative DC bus.[9] If more than two voltage levels were available to the inverter output terminals, the AC output could better approximate a sine wave.[8] For this reason multilevel inverters, although more complex and costly, offer higher performance.[9] A three-level neutral-clamped inverter is shown in Figure 10.
Control methods for a three-level inverter only allow two switches of the four switches in each leg to simultaneously change conduction states. This allows smooth commutation and avoids shoot through by only selecting valid states.[9] It may also be noted that since the DC bus voltage is shared by at least two power valves, their voltage ratings can be less than a two-level counterpart.
Carrier-based and space-vector modulation techniques are used for multilevel topologies. The methods for these techniques follow those of classic inverters, but with added complexity. Space-vector modulation offers a greater number of fixed voltage vectors to be used in approximating the modulation signal, and therefore allows more effective space vector PWM strategies to be accomplished at the cost of more elaborate algorithms. Due to added complexity and number of semiconductor devices, multilevel inverters are currently more suitable for high-power high-voltage applications.[9] This technology reduces the harmonics hence improves overall efficiency of the scheme.
AC/AC converters
Converting AC power to AC power allows control of the voltage, frequency, and phase of the waveform applied to a load from a supplied AC system .[10] The two main categories that can be used to separate the types of converters are whether the frequency of the waveform is changed.[11] AC/AC converter that don't allow the user to modify the frequencies are known as AC Voltage Controllers, or AC Regulators. AC converters that allow the user to change the frequency are simply referred to as frequency converters for AC to AC conversion. Under frequency converters there are three different types of converters that are typically used: cycloconverter, matrix converter, DC link converter (aka AC/DC/AC converter).
AC voltage controller: The purpose of an AC Voltage Controller, or AC Regulator, is to vary the RMS voltage across the load while at a constant frequency.[10] Three control methods that are generally accepted are ON/OFF Control, Phase-Angle Control, and Pulse Width Modulation AC Chopper Control (PWM AC Chopper Control).[12] All three of these methods can be implemented not only in single-phase circuits, but three-phase circuits as well.
- ON/OFF Control: Typically used for heating loads or speed control of motors, this control method involves turning the switch on for n integral cycles and turning the switch off for m integral cycles. Because turning the switches on and off causes undesirable harmonics to be created, the switches are turned on and off during zero-voltage and zero-current conditions (zero-crossing), effectively reducing the distortion.[12]
- Phase-Angle Control: Various circuits exist to implement a phase-angle control on different waveforms, such as half-wave or full-wave voltage control. The power electronic components that are typically used are diodes, SCRs, and Triacs. With the use of these components, the user can delay the firing angle in a wave which will only cause part of the wave to be outputted.[10]
- PWM AC Chopper Control: The other two control methods often have poor harmonics, output current quality, and input power factor. In order to improve these values PWM can be used instead of the other methods. What PWM AC Chopper does is have switches that turn on and off several times within alternate half-cycles of input voltage.[12]
Matrix converters and cycloconverters: Cycloconverters are widely used in industry for ac to ac conversion, because they are able to be used in high-power applications. They are commutated direct frequency converters that are synchronised by a supply line. The cycloconverters output voltage waveforms have complex harmonics with the higher order harmonics being filtered by the machine inductance. Causing the machine current to have fewer harmonics, while the remaining harmonics causes losses and torque pulsations. Note that in a cycloconverter, unlike other converters, there are no inductors or capacitors, i.e. no storage devices. For this reason, the instantaneous input power and the output power are equal.[13]
- Single-Phase to Single-Phase Cycloconverters: Single-Phase to Single-Phase Cycloconverters started drawing more interest recently[when?] because of the decrease in both size and price of the power electronics switches. The single-phase high frequency ac voltage can be either sinusoidal or trapezoidal. These might be zero voltage intervals for control purpose or zero voltage commutation.
- Three-Phase to Single-Phase Cycloconverters: There are two kinds of three-phase to single-phase cycloconverters: 3φ to 1φ half wave cycloconverters and 3φ to 1φ bridge cycloconverters. Both positive and negative converters can generate voltage at either polarity, resulting in the positive converter only supplying positive current, and the negative converter only supplying negative current.
With recent device advances, newer forms of cycloconverters are being developed, such as matrix converters. The first change that is first noticed is that matrix converters utilize bi-directional, bipolar switches. A single phase to a single phase matrix converter consists of a matrix of 9 switches connecting the three input phases to the tree output phase. Any input phase and output phase can be connected together at any time without connecting any two switches from the same phase at the same time; otherwise this will cause a short circuit of the input phases. Matrix converters are lighter, more compact and versatile than other converter solutions. As a result, they are able to achieve higher levels of integration, higher temperature operation, broad output frequency and natural bi-directional power flow suitable to regenerate energy back to the utility.
The matrix converters are subdivided into two types: direct and indirect converters. A direct matrix converter with three-phase input and three-phase output, the switches in a matrix converter must be bi-directional, that is, they must be able to block voltages of either polarity and to conduct current in either direction. This switching strategy permits the highest possible output voltage and reduces the reactive line-side current. Therefore the power flow through the converter is reversible. Because of its commutation problem and complex control keep it from being broadly utilized in industry.
Unlike the direct matrix converters, the indirect matrix converters has the same functionality, but uses separate input and output sections that are connected through a dc link without storage elements. The design includes a four-quadrant current source rectifier and a voltage source inverter. The input section consists of bi-directional bipolar switches. The commutation strategy can be applied by changing the switching state of the input section while the output section is in a freewheeling mode. This commutation algorithm is significantly less complexity and higher reliability as compared to a conventional direct matrix converter.[14]
DC link converters: DC Link Converters, also referred to as AC/DC/AC converters, convert an AC input to an AC output with the use of a DC link in the middle. Meaning that the power in the converter is converted to DC from AC with the use of a rectifier, and then it is converted back to AC from DC with the use of an inverter. The end result is an output with a lower voltage and variable (higher or lower) frequency.[12] Due to their wide area of application, the AC/DC/AC converters are the most common contemporary solution. Other advantages to AC/DC/AC converters is that they are stable in overload and no-load conditions, as well as they can be disengaged from a load without damage.[15]
Hybrid matrix converter: Hybrid matrix converters are relatively new for AC/AC converters. These converters combine the AC/DC/AC design with the matrix converter design. Multiple types of hybrid converters have been developed in this new category, an example being a converter that uses uni-directional switches and two converter stages without the dc-link; without the capacitors or inductors needed for a dc-link, the weight and size of the converter is reduced. Two sub-categories exist from the hybrid converters, named hybrid direct matrix converter (HDMC) and hybrid indirect matrix converter (HIMC). HDMC convert the voltage and current in one stage, while the HIMC utilizes separate stages, like the AC/DC/AC converter, but without the use of an intermediate storage element.[16][17]
Applications: Below is a list of common applications that each converter is used in.
- AC Voltage Controller: Lighting Control; Domestic and Industrial Heating; Speed Control of Fan,Pump or Hoist Drives, Soft Starting of Induction Motors, Static AC Switches[10] (Temperature Control, Transformer Tap Changing, etc.)
- Cycloconverter: High-Power Low-Speed Reversible AC Motor Drives; Constant Frequency Power Supply with Variable Input Frequency; Controllable VAR Generators for Power Factor Correction; AC System Interties Linking Two Independent Power Systems.[10]
- Matrix Converter: Currently the application of matrix converters are limited due to non-availability of bilateral monolithic switches capable of operating at high frequency, complex control law implementation, commutation and other reasons. With these developments, matrix converters could replace cycloconverters in many areas.[10]
- DC Link: Can be used for individual or multiple load applications of machine building and construction.[15]
Simulations of power electronic systems
Power electronic circuits are simulated using computer simulation programs such as PLECS, PSIM and MATLAB/simulink. Circuits are simulated before they are produced to test how the circuits respond under certain conditions. Also, creating a simulation is both cheaper and faster than creating a prototype to use for testing.[18]
Applications
Applications of power electronics range in size from a switched mode power supply in an AC adapter, battery chargers, fluorescent lamp ballasts, through variable frequency drives and DC motor drives used to operate pumps, fans, and manufacturing machinery, up to gigawatt-scale high voltage direct current power transmission systems used to interconnect electrical grids. Power electronic systems are found in virtually every electronic device. For example:
- DC/DC converters are used in most mobile devices (mobile phones, PDA etc.) to maintain the voltage at a fixed value whatever the voltage level of the battery is. These converters are also used for electronic isolation and power factor correction. A power optimizer is a type of DC/DC converter developed to maximize the energy harvest from solar photovoltaic or wind turbine systems.
- AC/DC converters (rectifiers) are used every time an electronic device is connected to the mains (computer, television etc.). These may simply change AC to DC or can also change the voltage level as part of their operation.
- AC/AC converters are used to change either the voltage level or the frequency (international power adapters, light dimmer). In power distribution networks AC/AC converters may be used to exchange power between utility frequency 50 Hz and 60 Hz power grids.
- DC/AC converters (inverters) are used primarily in UPS or renewable energy systems or emergency lighting systems. Mains power charges the DC battery. If the mains fails, an inverter produces AC electricity at mains voltage from the DC battery. Solar inverter, both smaller string and larger central inverters, as well as solar micro-inverter are used in photovoltaics as a component of a PV system.
Motor drives are found in pumps, blowers, and mill drives for textile, paper, cement and other such facilities. Drives may be used for power conversion and for motion control.[19] For AC motors, applications include variable-frequency drives, motor soft starters and excitation systems.[20]
In hybrid electric vehicles (HEVs), power electronics are used in two formats: series hybrid and parallel hybrid. The difference between a series hybrid and a parallel hybrid is the relationship of the electric motor to the internal combustion engine (ICE). Devices used in electric vehicles consist mostly of dc/dc converters for battery charging and dc/ac converters to power the propulsion motor. Electric trains use power electronic devices to obtain power, as well as for vector control using pulse width modulation (PWM) rectifiers. The trains obtain their power from power lines. Another new usage for power electronics is in elevator systems. These systems may use thyristors, inverters, permanent magnet motors, or various hybrid systems that incorporate PWM systems and standard motors.[21]
Inverters
In general, inverters are utilized in applications requiring direct conversion of electrical energy from DC to AC or indirect conversion from AC to AC. DC to AC conversion is useful for many fields, including power conditioning, harmonic compensation, motor drives, and renewable energy grid-integration.
In power systems it is often desired to eliminate harmonic content found in line currents. VSIs can be used as active power filters to provide this compensation. Based on measured line currents and voltages, a control system determines reference current signals for each phase. This is fed back through an outer loop and subtracted from actual current signals to create current signals for an inner loop to the inverter. These signals then cause the inverter to generate output currents that compensate for the harmonic content. This configuration requires no real power consumption, as it is fully fed by the line; the DC link is simply a capacitor that is kept at a constant voltage by the control system.[8] In this configuration, output currents are in phase with line voltages to produce a unity power factor. Conversely, VAR compensation is possible in a similar configuration where output currents lead line voltages to improve the overall power factor.[9]
In facilities that require energy at all times, such as hospitals and airports, UPS systems are utilized. In a standby system, an inverter is brought online when the normally supplying grid is interrupted. Power is instantaneously drawn from onsite batteries and converted into usable AC voltage by the VSI, until grid power is restored, or until backup generators are brought online. In an online UPS system, a rectifier-DC-link-inverter is used to protect the load from transients and harmonic content. A battery in parallel with the DC-link is kept fully charged by the output in case the grid power is interrupted, while the output of the inverter is fed through a low pass filter to the load. High power quality and independence from disturbances is achieved.[8]
Various AC motor drives have been developed for speed, torque, and position control of AC motors. These drives can be categorized as low-performance or as high-performance, based on whether they are scalar-controlled or vector-controlled, respectively. In scalar-controlled drives, fundamental stator current, or voltage frequency and amplitude, are the only controllable quantities. Therefore, these drives are employed in applications where high quality control is not required, such as fans and compressors. On the other hand, vector-controlled drives allow for instantaneous current and voltage values to be controlled continuously. This high performance is necessary for applications such as elevators and electric cars.[8]
Inverters are also vital to many renewable energy applications. In photovoltaic purposes, the inverter, which is usually a PWM VSI, gets fed by the DC electrical energy output of a photovoltaic module or array. The inverter then converts this into an AC voltage to be interfaced with either a load or the utility grid. Inverters may also be employed in other renewable systems, such as wind turbines. In these applications, the turbine speed usually varies causing changes in voltage frequency and sometimes in the magnitude. In this case, the generated voltage can be rectified and then inverted to stabilize frequency and magnitude.[8]
Smart grid
A smart grid is a modernized electrical grid that uses information and communications technology to gather and act on information, such as information about the behaviors of suppliers and consumers, in an automated fashion to improve the efficiency, reliability, economics, and sustainability of the production and distribution of electricity.[22][23]
Electric power generated by wind turbines and hydroelectric turbines by using induction generators can cause variances in the frequency at which power is generated. Power electronic devices are utilized in these systems to convert the generated ac voltages into high-voltage direct current (HVDC). The HVDC power can be more easily converted into three phase power that is coherent with the power associated to the existing power grid. Through these devices, the power delivered by these systems is cleaner and has a higher associated power factor. Wind power systems optimum torque is obtained either through a gearbox or direct drive technologies that can reduce the size of the power electronics device.[24]
Electric power can be generated through photovoltaic cells by using power electronic devices. The produced power is usually then transformed by solar inverters. Inverters are divided into three different types: central, module-integrated and string. Central converters can be connected either in parallel or in series on the DC side of the system. For photovoltaic "farms", a single central converter is used for the entire system. Module-integrated converters are connected in series on either the DC or AC side. Normally several modules are used within a photovoltaic system, since the system requires these converters on both DC and AC terminals. A string converter is used in a system that utilizes photovoltaic cells that are facing different directions. It is used to convert the power generated to each string, or line, in which the photovoltaic cells are interacting.[24]
Grid voltage regulation
Power electronics can be used to help utilities adapt to the rapid increase in distributed residential/commercial solar power generation. Germany and parts of Hawaii, California and New Jersey require costly studies to be conducted before approving new solar installations. Relatively small-scale ground- or pole-mounted devices create the potential for a distributed control infrastructure to monitor and manage the flow of power. Traditional electromechanical systems, such as capacitor banks or voltage regulators at substations, can take minutes to adjust voltage and can be distant from the solar installations where the problems originate. If voltage on a neighborhood circuit goes too high, it can endanger utility crews and cause damage to both utility and customer equipment. Further, a grid fault causes photovoltaic generators to shut down immediately, spiking demand for grid power. Smart grid-based regulators are more controllable than far more numerous consumer devices.[25]
In another approach, a group of 16 western utilities called the Western Electric Industry Leaders called for mandatory use of "smart inverters". These devices convert DC to household AC and can also help with power quality. Such devices could eliminate the need for expensive utility equipment upgrades at a much lower total cost.[25]
الکترونیک قدرت
الکترونیک قدرت شاخهای از الکترونیک صنعتی است. الکترونیک قدرت مبحثی است متشکل از مهندسی الکترونیک و مهندسی قدرت که در آن عملکرد الکترونیک حالت جامد برای کنترل و تبدیل توان الکتریکی بررسی میگردد. به عبارت دیگر الکترونیک قدرت به بررسی استفاده از نیمه هادیها در قدرت میپردازد.
محتویات
کاربردها
تقریبا تمام منابع تغذیه جدید همچون شارژرها، اینورترها و یو پی اسها از ساختارهای الکترونیک قدرت استفاده میکنند. برای مثال شارژر موبایل و لپتاپ، منابع تغذیه کامپیوتر و مانیتور و سیستمهای قدرت ماشینهای لباسشویی بر این اساس طراحی و ساخته میشوند. در صنعت رایجترین استفاده از الکترونیک قدرت در محرکههای با سرعت متغیر موتور القایی است. در سطوح توان بالاتر، الکترونیک قدرت در کاربردهایی مانند انرژیهای نو و بهینه سازی سیستم قدرت مورد استفاده قرار میگیرد.[۱]
تقسیم بندی
- بسته به ورودی و خروجی مبدلهای الکترونیک قدرت، میتوان آنها را به چهار دسته تقسیم بندی کرد:
- مبدلهای ac به dc یا یکسوسازها
- مبدلهای dc به dc یا چاپرها
- مبدلهای ac به ac (سیکلوکانورترها - مبدلهای ماتریسی)
- تبدیل وضعیت در موتورهای چند متغییره مانند شفت سلف
This is a very useful document to find answers to your questions regarding citizenship and permanent residency:
http://cms.horus.be/files/99935/Medi...zenship_en.pdf
It is worth reading as a whole, but pay more attention to page 10 where it says:
"The Directive does not cover asylum seekers, those
enjoying temporary protection or a subsidiary form of
protection, students, except for doctoral students, and
diplomatic personnel."
So, my intuition is that for PhD students, it's counted!
You may compare different countries residency permit in this document as well.
Look ate page 18:
Third country nationals can apply for naturalisation in
the member states after varying periods of legal residence:
Austria: 5 years.
Belgium: 3 years.
Denmark: 7 years.
Finland: 18 years.
France: 5 years.
Germany: at the latest after 8 years.
Greece: 10 years.
Irland: 5 years.
Italy: 10 years.
Luxembourg: 10 years.
The Netherlands: 5 years.
Portugal: 10 years.
Spain: 10 years.
Sweden: 5 years.
UK: 5 years.
But these info might have slightly changed, since the date of the doc goes back to 2001, but many of these information are still valid
This was the answer to many of my questions about Belgium!
Good luck
Accomodation Switzerland
ین اسامی سایت های رایج برای منزل و سایر ملزومات زندگی در سوئیس است. البته معمولا در سوئیس باید اول یک خانه مبله بگیرید. برای یک مدت کوتاه مثلا 2 ماهه و بعد بیایید به شهر مورد نظر و حضورا دنبال خانه غیر مبله باشید. ( چون خانه غیر مبله را بدون اینکه برای دیدن آن بروید به شما نمی دهند.)البته توجه داشته باشید که خانه های غیر مبله هم، یخچال، گاز خوراک پزی(البته برقی است!!!) و گاهی Dish washer هم دارند. سایتی که با رنگ قرمز هست، www.anibis.ch کمک اصلی در خانه پیدا کردن من در Lausanne بود.
اگر در یافتن سایتی مشکل داشتید بگید تا من اصل فایل به همراه لینک را براتون میل کنم.
موفق باشید.
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immoscout24 :rent or buy: houses, apartments, rooms, parking garages - all Switzerland
immosearch :rent: houses, apartments - all Switzerland
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wohnung24 :rent or buy: houses, apartments, rooms, parking garages - all Switzerland
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immomarktschweiz :rent or buy: houses, apartments - all Switzerland
markplatz :rent & buy apartments, rooms - mostly Zurich
marktplatz uni ZH :Apartments classifieds from Zurich University board - Language: E, G
immozug :apartments, various - Zug
immobern :apartments, various - Bern
anzeigerbern :apartments, various - Bern
immobiel :apartments, houses - Biel / Bienne
wgzimmer :apartments, rooms, flatshare - all Switzerland
easywg :room and room mate finder - all Switzerland - Language: All
lavaux :apartments, rooms - Lavaux, Montreux, Vevey region
justimmo :Houses for sale -Romandie - Swiss French region - Language: F
montreuxtourism :apartments, rooms, B&B - Lavaux, Montreux, Vevey, Villeneuve region
anibis :Classified adds including rental contracts and sublets - Romandie - Swiss French region - Language: F
oklogements : apartments, houses, Lausanne
residencedumidi :short-term furnished apartments, Lausanne
apartments-swiss-star :temporary furnished and serviced apartments
cityappartements :temporary furnished and serviced apartments - Zurich
wohnen ETHZ :rooms, apartments, short and long term - Zurich
pabs :furnished and serviced apartments - Zurich
ums :rooms, apartments, short and long term - all Switzerland
comparis :homefinder and general product and price comparison service
lodge-finder :adverts for lets, sublets, rooms
interhome :Holiday rentals - all Switzerland - Language: E
زندگی در انگلستان
یه حس Appreciation اگه در ابتدا یا انتهای پستتون باشه همه رغبت بیشتری برای جواب دادن پیدا می کنن1. صد درصد. همه آزاد هستند هر طوری دلشون می خواد لباس بپوشند. جلوی حجاب گرفته نمیشه، جلوی برهنگی هم همینطور.
2. از لحاظ فرهنگی متفاوت، ولی عموما آدم های خوبی هستند. آدم تعصبی هم کم و بیش توشون پیدا میشه، مثل همه دنیا.
3. نسبتا خوب. هرچقدر در شهرهای بزرگتر باشید مهاجرها بیشتر هستن و برخوردشون با مهاجرها به نسبت بهتر.
4. برخورد مردم با کسی به شخصیت و رفتار اون شخص برمی گرده، کسی کاری با دین شخص دیگه نداره.. ممکنه با یک مسلمان بهترین برخورد ممکن بشه و با یکی خیر. از روی دین درباره کسی قضاوت نمی کنند.
5. تو جامعه زیاد هستن (به خصوص در لندن)، برخوردشون مثل برخورد با زنان بی حجابه. حجاب داشتن یا نداشتن انتخاب هر شخصیه و به شما احترام می گذارن.
6. برای خارجی ها (non-EU) خیلی خوب نیست و همیشه اولویت با اروپایی هاست، ولی شما خوب باشید و پیگیرش باشید پیدا شدنیه.
7. هیچ کشور دنیا نژادپرست تر از ایران نیست. انگلیس جزو نسبتا خوب هاست.
8. کمبریج و امپریال که جزو 10 تای اول دنیا هستن، آکسفورد و منچستر و ساوتمپتون و ... ، دانشگاه های خوب ECE زیاده.
9. زبانتون خوب باشه بله. برای برقراری ارتباط باید فرهنگشون رو یاد بگیرید (وگرنه براشون "عجیب" به نظر میاید)، فرهنگشون هم مستقل از دینشونه.
10. به معنی اینکه شما رو دعوت کنن خونشون و ناهار و شام و ... اصلا، باهاشون بیرون می رید دنگی حساب می کنید و خبری از مهمون کردن و اینها نیست. اگه منظورتون اینه که شما رو در جمع خودشون راه میدن، بله، اگه شما خودتون رو باهاشون وفق بدید.
11. خیر.
12. به جز جیب بری (و دزدیدن موبایل و ...) هیچ مشکل امنیتی توی این 5 سال زندگی در لندن ندیدم، نه یک دعوا، نه بمب !!!، نه زورگیری و چاقوکشی و نه چیز دیگه.
13. تکنولوژی از چه نظر؟ شما به هر تکنولوژی (تا زمانی که مایل باشید هزینه اش رو بدید) می تونید دسترسی داشته باشید. اگه منظورتون شبکه های مخابراتی و الکترونیک و دسترسی به امکانات و ... است که خیلی خوبه، فقط اینجا شهرها استیل و شکل کلاسیک دارند. خارج از لندن شکل و قیافه شهرها خیلی چشم انداز نیست.
14. در لندن عالی (یکی از بهترین Public Transport های دنیا) رو داره، ولی خارج از لندن کاملا معمولی. هزینه حمل و نقل خیلی نسبت به ایران گرونتره. ماشین داشتن پر هزینه است.
15. -
16. قطعا مشکلات سیاسی هست بین دو کشور.
17. در پست قبلی هم بهتون گفتم، مردم رو از دولت جدا کنید. بی شک استعمارگرترین دولت تاریخ بودن ولی مردمشون همه آدم هستن، اتفاقا اکثرا از سیاست و ... هم سر در نمیارن.
18. انگلیس: آب و هوای بهتر، زبان انگلیسی، شهر بهتر و جذاب تر (فقط لندن البته)، دانشگاه های بهتر و ... ، البته من آلمان رو هم دیدم، جای خیلی خیلی خوبیه اونجا هم.
19. -
20. فقط منچستر!
21. ادبشون خوبه و جزو مودب ترین ها در دنیا هستن، و همینطور انتظار ادب خوب رو از بقیه هم دارن که باید یاد بگیرید چطوری جوابشون رو بدید.
22. بله مشخصه!
23. شما آزادید هر دینی داشته باشید، و دینتون شما رو از هیچی محدود نمی کنه. اصلا شما می تونید دینتون رو مخفی برای خودتون نگه دارید و به هیچ کس نگید! در هر فرمی که پر می کنید اگه دینتون رو بپرسند اختیاری است. البته منظور این نیست که اونها (مثلا در کار یا دانشگاه) شرایط دینی شما رو درک می کنن. مثلا سر کار بهتون 10-15 دقیقه وقت ناهار میدن که شما چیزی که از خونه آوردید رو می خورید، مدیرتون ممکنه درک نکنه شما یک ساعت به وقت نماز و نهار نیاز دارید، این رو به حساب برخورد نامناسب با مسلمون ها نگذارید.
24. حقوق بشر اینجا رعایت میشه
25. نظر اکثریتی که من باهاشون صحبت می کردم این بود که زندگی خودشونه و این قضیه شخصیه، به کسی ربط نداره.
26. نسبتا جزو بی اعتقادها هستن در اروپا. حتی پاپ در سخنرانیش مردم رو دعوت کرده بود به مذهبیت بیشتر که خیلی خوششون نیومده بود.
27. نسبی است!
28. رجوع به 21 !
موفق باشید.
کدوم کشورو با توجه به صدوره ویزا و تمکن مالی انتخاب کنم
با سلامخوب شما یه سری کشور اون بالا ردیف کردی... البته هر جا بری باید پیه یاد گرفتن زبونشون رو هم به تنت بمالی ،می خوام بدونم یعنی الان شما فرانسه فولی می خوای بری فرانسه یا می خوای انگلیسی تحصیل کنی؟؟؟ ???
اگه فرانسوی بلدی چرا نمی ری بخش فرهنگی خود سفارت از اینایی که بورسیه می دن اقدام نمی کنی؟
1- ایتالیا :رو من خیلی روش فکر نکردم تو نمایشگاه edutex دو تا دانشگاه از ایتالیا اومده بودن ، این آقایون حرفه حسابشون این بود که سالی 1500 یورو شهریه دانشگاه هست دانشگاه ما بهتون خوابگاه میده ماهی 90 یورو که کارتو راه بندازه ، خودت بخوای معمولی بخوری ماهی 150 یورو !!!! ( یعنی از هند و مالزی ارزونتر) اونها این جوری می گفتن ..... گفتم کار هم میشه کرد گفت دانشگاهای ما تو جنوب ایتالیاست
اونجا میزان بیکاری بالاست فکر کار کردن رو از سرت بیرون کن!رفت و آمد هم با دو چرخه خیلی مرسومه تو ایتالیا تا اونجایی که من می دونم
ولی این دفعه مشکل اینه که تا ایتالیایی بلد نباشی ویزا بی ویزا ، باید 3 ترم بری مدرسه ایتالیا تو فرمانیه ترمی 210 هزار تومان بدی هر ترم 2.5 ماه که بهت ویزا بدن...
2-انگلیس :یکی از آشناهای ما رفت سالی 40 میلیون داره خرج می کنه...دیگه بقیه اش با خودت
3- سوید :خوبه...دانشگاه مجانی.... دانشجوهای هم تیپ خودت که راه و چاه رو یاد گرفتن زیاده.... الان ها کار سخت شده گیر آوردن.....مبلغ 36 میلیون تومن باید تو بانک داشته باشی برای مدت کوتاه مثلآ یک هفته! که بدونن دستت به دهنت می رسه
4- سوییس: حقیقتش زیاد اطلاع ندارم ولی یا باید آلمانی یا فرانسه بلد باشی :'(
5-آلمان: دانشگاه هاش اکثرآ ترمی دارن حدود 500 یورو شهریه می گیرن به غیر از دانشگاه های ایالت اسن که مجانیه و یه دانشگاه دیگه که فکر کنم همون هایدلبرگ باشه ( حالا ته شو برات در می آرم ) که ترمی 375 یورو می گیره .... حداقل باید ماهی 600 یورو بزاری کنار واسه همین خرج ها و مخارج کار میشه کرد ساعتی 10 تا 15 یورو هم می دن از همون مرکز DAAD تو خیابون یخچال بپرسی اطلاعات کامل بهت می دن ولی با این شرایط خیلی سخته نمی گم غیر ممکن... آدم های زیادی بودن و هستن که تونستن ولی سخته...... در عوض مدرکت مدرکه
6- دانمارک : باید سالی 11000 یورو در قطع لیسانس 15000 یورو در مقطع فوق لیسانس شهریه بدی :'( خرج هم ماهی 650 یورو حداقلشه اجازه کار همراه ویزات نمی دن ، ولی می تونی بگیری ... و لی چقدر میشه کار کرد تا هم خرج دانشکاه و هم زندگی رو در آورد؟
7- بلژیک : من دو سال پیش رفتم بهم برگه دادن که توش شرایط رو توضیح داده بود باید حداقل 10000 یورو به ازای هر سال اقامت داشته باشی تو بانک ! از اونجایی که متقاضی کمه شانس موفقیت بالاست......شرایط دیگشو نمی دونم
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خوب حالا اون جاهایی که اسم نبردی: ;D
1- فنلاند: دانشگاه مجانی سطح علمی دانشگاه ها در حد مطلوب ولی پذیرش گرفتن در مقطع فوق لیسانس یه خورده مشکله...در مقطع لیسانس باید یه کنکور بدی.....( ساده ست)
در مورد حساب بانکی باید 6000 یورو پول تو حساب داشته باشی..تمام! موقع مصاحبه هم ازیتت نمی کنن .... کار هم تا فنلاندی بلد نباشی اصلآ مگر در شایط خاص اونم تو شرکت های بزرگ که این دفعه فکر نکنم نیمه وقت بهت این طوری کار بدن.... یاد گیری زبونشون خیلی سخته !
2- ایسلند : کشور 300 هزار نفریه ایسلند دانشگاه هاش در کل 3 یا 4 تا بیشتر نیست! سالی 500 دلار شهریه می گیرن ! تقریبآ همون 10000 دلار باید تو بانک داشته باشی هنوز تحقیقاتمو کامل انجام ندادم ولی باس حداقل ایلتس 6.5 داشته باشی... کار هم آسون گیر نمی آد.......منتظر باشید اطلاعات بیشتر بهتون می دم در آینده...
3-نروژ: به نظر من که خوبه دانشگاه ها مجانی... کار فراوون ...حتی برخی از دانشجو ها از سوید آخر هفته رو با کشتی میرن نروژ و کار می کنن بر می گردن...!
باید 10000 یورو تو بانک داشته باشی اونم تو بانک نروژی. من خودم نروژ رو انتخاب می کنم
بازم کشور هایی مثل اتریش، اسپانیا ، پرتغال هست... ولی من تحقیق زیادی روشون نکردم شاید بعدآ بی کار شدم تحقیق کردم
موفق باشید
تجربه دو سال زندگی در اتریش
دروود بر همه دوستان عزیزمیدونم که خیلی از دستم شاکی هستید چون دیگه خیلی اینجا نمیام ولی باور کنید خیلی سرم شلوغ بوده تویه این مدت
الانم تعطیلات زمستانی بین ترم هست و کمی بیکارم
چند روز پیش یه بنده خدایی ازم خواست به عنوان کسی که مدتها مدیر بخش اتریش بودم ، تجربیات این 2 سال رو در قالب یک تاپیک بنویسم تا بقیه استفاده کنن
منم دیدم شاید مفید باشه و سعی میکنم مواردی رو اینجا ذکر کنم :
در وحله اول اینو بگم اگه بازم برگردم به 2-3 سال پیش و قرار باشه برم خارج از کشور یکی از 3 تا گزینه اولم در اروپا حتما اتریش هست
این سوال خیلی ها هست که راضی هستی ؟؟؟ جوابش خیلی سخته چون من با توجه به خصوصیات و تمایلات شخصی خودم باید جواب بدم ولی من راضیم
ولی خیلی های دیگه شاید اینو نگن
ببینید اینجا در وحله اول براتون قشنگ هست و جذاب ولی کم کم که پول روبه تموم شدن میره شرایط سخت میشه و اون موقع باید دید که کی ، چند مرده حلاجه ؟؟
کسی که از ایران براش پول میاد قطعا اوضاش بدک نیست و اگه اهل کار باشه میتونه با 2-3 روز کار در هفته کلی خوش بگذرونه
ولی ، ولی کسی که پولی براش نمیاد یا کم میاد باید خیلی کار بکنه تا هزینه هاش جبران بشه و بتونه بدونه استرس زندگی بکنه
بعد از یه مدت میبینی 1 سال گذشته و تویه این مدت همه روزهات تکراری بوده : کلاس -- کار-- خونه
دیگه حتی وقت بیرون رفتن با دوستای روزهای ابتدایی رو نداری و اصلا نمیبینیشون
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حالا اگه بتونی تویه این شرایط دووم بیاری و یه 2 سالی با این شرایط زندگی کنی ، ( سخته و خیلی هم سخته مخصوصا برای کسی که تا حالا کار نکرده و شرایط سختی تویه زندگیش نداشته ) بعدش اوضاع کم کم خوب میشه
چون زبانت تویه این مدت راه افتاده ، کم کم با چم و خم کار آشنا شدی ، شرایط مدیریت کار و درس برات آسون تر شده ، و دیگه میتونی کار مرتبط با رشته تحصیلیت رو پیدا کنی
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ولی این شرایطم تا یه مدتی برات خوبه ، حالا دیگه به این فکر میکنی که : خوب من درسم تموم شد چکار بکنم ؟؟؟
این قسمتش خیلی سخته و استرس زا
من خوشبختانه هنوز به اون مرحله نرسیدم ولی بچه هایی که رسیدن رو دیدم و خوب دیگه برای موندن باید دنبال کار باشی و اونم کاری که زیر سقف حقوقی اداره کار نباشه که بتونی به واسطه اون ویزای کار بگیری که این قسمتش خیلی سخته ، کار پیدا میشه ولی با سقف درآمدی خیلی خیلی به ندرت
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شرایط زندگی روزمره در اینجا خوبه و آرومه
از ایران با خودتون خوراکی نیارین و نهایتا پلوپز که اونم خیلی واجب نیست کلا بجز لباس چیزی نباید بیارین و البته لوازم الکترونیکی شخصی
اینجا همه چیز هست و الان دیگه تقریبا به قیمت همون ایران شده با این وضع یورو و الکی برای خودتون اضافه بار درست نکنید
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سعی کنید با یه حداقلی معلومات زبان آلمانی اینجا بیاین و فکر نکنین : خوب برم اونجا در محیط قرار بگیرم خودش راه میفته ، اینجا اگه صفر کیلومتر بیاین میرین واسه 2 سال زبان خوندن خیلی خیلی راحت
پس سعی کنید حتی تویه خونه هم که شده با دانسته های ابتدایی زبان آلمانی اینجا بیاین
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اینجا درس خوندن اولاش سخته ولی کم کم که زبان راه میفته اوضاع خوب میشه
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مردم اینجا آروم هستند و کاری به کارتون ندارن و با خارجی ها خوبن چون خیلی خارجی تویه کشورشون زیاده
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به هیچ عنوان به هیچ عنوان و به هیچ عنوان با موسسات اعزام دانشجو کار نکنید ، الان قوانین خیلی سخت شده ،خیلی از این موسسات بدون اینکه بهتون بگن ، با توجه به اینکه شرایط پذیرش گرفتن سخت شده ، براتون مدرک سازی جعلی میکنن و به دانشگاه به اسم شما تحویل میدن و اگه خدایی نکرده دانشگاه استعلام بگیره و شما به واسطه اون مدرک اینجا باشین ، براتون خیلی خیلی گرون تموم میشه
پس این کار رو نکنین و بدونین که خودتون میتونین این کار رو انجام بدین
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هزینه های زندگیتون در سال اول چیزی بین ماهیانه 650-750 یورو هست ( بدون ریخت و پاش ) ولی میشه کم کم از سالهای بعد کمش کرد و در کنارش هم کار کرد
کارهای که اینجا قالب بچه های دانشجو انجام میدن : کار تویه رستوران به عنوان ظرفشور-کمک آشپز-گارسون-نظافتچی و گاها پخش روزنامه هست ، البته کارهای دیگه ای هم هست ولی اینها کارهایی هست که اکثر بچه ها انجام میدن و باید در نظر بگیرین که شاید مجبور باشین این کارهارو انجام بدین
با کار نیمه وقت مثلا 2-3 روز در هفته میشه نصف هزینه هارو جبران کرد ولی اگه قرار باشه کل هزینه هارو با کار دربیارین یه ذره اوضاع سخت میشه و اون موقع میفهمین که چجوری میشه که یکی که هر روز 7-8 ساعت اینجا میومد دیگه نمیتونه بیاد و باید هر روز کار بکنین تا بتونین هزینه هاتون رو جبران کنید و قطعا در روند درس خوندتون هم تاثیر میزاره و خیلی خیلی باید مدیریت زمانیتون قوی باشه که بتونین وقتی هم برای درس خوندن در بیارین
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من سعی میکنم این تاپیک رو کاملتر بکنم و بازم میگم تویه مدت خیلی خیلی درگیر بودم و وقت نداشتم که بیام اینجا
با تشکز از همه شما
تجربیات زندگی در فرانسه
تجربیات زندگی در فرانسه
با سلام خدمت دوستان عزیز و علاقمند به ادامه تحصیل در فرانسه
اگرچه این متن طولانی هست اما پیشنهاد میکنم حتما بخونینش.
مدتها بود به این موضوع فکر می کردم که آیا باید این موضوع رو مطرح کنم یا
نه، حقیقتش مطمئن نبودم که مطرح کردن این موضوع کار درستی باشه از این جهت
که فکر می کردم شاید این تجربه و واقعیت در مورد فرانسه فراگیر نباشه و
عمومیت نداشته باشه. از این جهت برای اشتراک تجربیاتم صبر و تامل می کردم.
اما امروز دیگه یقین پیدا کردم که باید شما دوستان رو از تجربیات زندگی در
فرنسه آگاه کنم، چه شما دوستان این تجربیات رو بپسندین و به به و چه چه
کنین چه موضغ مخالف بگیرین و هزار دلیل که نه، بلکه آسمون و ریسمون ببافین
که من یکطرفه به قاضی رفتم. به هر حال من آنچه شرط بلاغ است با شما بی هیچ
تعصب و غرضی و با تلاش برای رعایت انصاف می گویم، خواه پند گیرید خواه
ملال.
بنده به عنوان کسی که در فرانسه زندگی کرده و در حال اتمام دوران ارشدش
هست، قویا توصیه می کنم دور فرانسه رو با قلم قرمز درشت خط قرمز بکشین و به
هیچ عنوان به اینجا فکر نکنین. البته طرف صحبتم با دانشجوهای فنی به خصوص
رشته هایی که حساس حساب میشه مثل برق کنترل و روباتیک و رشته هایی از این
دست هست. حالا دلایلش رو هم اعلام می کنم.
در درجه اول، به وضعیت ویزای فرانسه برای این رشته ها توجه کنین و نهایت بی
انصافی سفارت چه در دریافت وجه برای به اصطلاح تایید مدارک دانشگاهی، که
هیچ ضرورتی جز پر کردن جیب و درآمدزایی برای سفارت نداره و چه بلاتکیفی و
رد درخواست ویزا با وجود تمامی هزینه های مادی و معنوی. علیرغم تمام این
هزینه ها و تحمل فشارهای روحی و استرس و این مسایل، در بسیاری از موارد
درخواست ویزای دانشجویان این رشته ها رد میشه و باعث ضربات روحی به متقاضی
میشه. حالا بگذریم از اینکه با آینده طرف و عمر از دست رفته اون بازی شده.
الان لابد خیلی از این دوستان عاشق فرانسه دلیل میارن که خوب اگه درخواست
ویزا ها رد میشه پس تو خودت چطور گرفتی؟ در پاسخ باید عرض کنم که بنده
مدتها پیش از اینکه بیام فرانسه از کشور خارج شده بودم و از طریق سفارت
فرانسه در تهران برای ویزا اقدام نکرده بودم و هیچکدوم از این مبالغی که
سفارت در تهران تحت عنوان تایید مدارک و در واقع درآمدزایی برای خودش
میگیره رو پرداخت نکردم. با این وجود بیش از 4 ماه طول کشید تا بنده ویزام
رو بگیرم و 2 ماه پس از آغاز کلاسها تو دانشگاه باشم.
در درجه بعدی با فرض گرفتن ویزا، از قدیم گفتن فرض محال محال نیست، اگر
دانشجوی دوره ارشد باشین باید برای پایان نامه در ترم سوم اقدام کنین. طبق
قوانین فرانسه به تمامی دانشجوها در این بازه زمانی حقوق پرداخت میشه. به
عبارت دیگر دانشجو به طور پاره وقت یک قرارداد کاری امضا می کنه تا ضمن
انجام پایان نامش مبلغی رو هم به عنوان حقوق دریافت کنه که البته این مبلغ
بسته به طرف قرارداد متفاوته. در بسیاری از موارد در فرانسه، پروژه های
دانشگاهی و تحقیقاتی توسط دو مرکز CNRS و INRIA تامین مالی میشن و این یعنی
دانشجو باید با این مراکز قرارداد ببنده و به نوعی به استخدام موقت این
مراکز و یا هرجای دیگه که طرف قراردادش هست در بیاد. بنده با بقیه رشته ها
کاری ندارم چون تجربه ای ندارم اما اگه کسی که در این دو رشته که بنده
تجربه دارم تصور میکنه میتونه با یکی از این دو مرکز قرارداد ببنده باید
بگم سخت در اشتباهه. الان ممکنه یکی بیاد بنده رو متهم کنه به نداشتن رزومه
خوب، اما در پاسخ باید عرض کنم که بنده اگر نگم بهترین قطعا در یکی از
بهترین دانشگاه های فرانسه در این زمینه دارم درس میخونم و رزومه بسیار قوی
هم دارم. اینقدر رزومم قوی هست که بدون اینکه برای دکترا اقدام کنم چه از
همین فرانسه و چه خارج از اینجا چندتا از استادها به بنده پیشنهاد دادن که
برای دکترا با اونها کار کنم. وقتی هم که صحبت از استاد می کنم، منظورم این
استادای تازه دکترا گرفته نیست بلکه افرادی که در این رشته تو دنیا بسیار
بسیار شناخته شده و صاحب کرسی هستند (از بردن اسامی دانشگاه و اساتید
معذورم، اما شما مختارین که حرفهام رو باور کنین یا نه). حالا اگه کسی تصور
می کنه همچین رزومه ای و یا قوی تر از این داره که می تونه این موسسات رو
قانع کنه اونها رو برای انجام پروژه ارشدشون حمایت مالی کنن میتونه تیری در
تاریکی شلیک کنه، البته اگه ویزا بیگیره و پاش به فرانسه برسه، اما بعدها
شاکی نشه که چرا کسی در مورد شرایط فرانسه به من توضیح نداد. حالا الان عده
دیگه ای میگن که تو که خودت داری میگی اساتید بهت پیشنهاد دکترا دادن پس
دیگه اعتراضت و شکایتت چیه؟ در جواب باید بگم که برای مثال یکی از این
اساتید دانشگاه خودم پس از صحبت با من برای دکترا به بنده گفت که من (یعنی
استاده) یک موضوع تعریف می کنم که تو هم کار پروژه ارشدت رو باهاش انجام
بدی و هم برای دکترا اقدام کنی و به بنده گفت که مدارک رو میفرستم CNRS و
فرآیند بررسی موضوع تز و استخدام تو حدود یک ماه طول میکشه. بنده همونجا به
استادم گفتم که CNRS بنده رو به دلیل ملیتم با احتمال بسیار بسیار زیاد رد
میکنه که استادم در پاسخ گفت (عین جمله استادم) "البته اینطور نیست که
ایرانی ها رو بطور سیستماتیک رد کنن، اگه قرار بود ردت کنن همون اول
نمیگذاشتن بیای فرانسه. تو الان حدود 2 ساله اینجایی و مشکلی پیش نمیاد" و
بنده هم در پاسخ گفتم امیدوارم همینطوری باشه. سرتون رو درد نیارم، چند روز
بعد با همین استاد امتجان داشتم. بعد از امتحان به من گفت وقت داری بیای
اتاقم که من هم گفتم حتما. بلافاصله پس از ورود به اتاق به بنده گفت خبر
بدی برات دارم که من هم بلافاصله در جواب گفتم ردم کردن؟ که استادم گفت
آره. یعنی کل پرونده ای که باید بررسیش حدود 1 ماه طول میکشید در کمتر از
2-3 روز و تنها به دلیل ملیت بنده رد شد. خلاصه، الان نه تنها برای دکترا
در فرانسه هیچ شانسی ندارم که اصلا ممکنه با این رزومه حتی نتونم مدرک
ارشدم رو از اینجا بگیرم. فراموش نکنیم که برای پایان نامه ارشد باید
قرارداد ببندیم که برای ما ایرانیها شاید تو خواب ممکن باشه و بدون پایان
نامه شما نمیتونین مدرک بگیرین. امکان داره یک عده ای بیان بگن که خوب ما
میگردیم برای پروژه هایی که توسط مراکز دیگه ای غیر از CNRS یا INRIA حمایت
بشه که در جواب باید بگم اولا تعداد این پروژه ها در مقایسه بسیار کمتره و
ثانیا بنده این راه رو هم امتحان کردم. پس از رد شدن پروپزالی که یکی از
اساتید خودم به همراه نام و رزومه بنده به CNRS فرستاده بود، ایشون بنده رو
به یکی دیگه از دوستان خودش در یک دانشگاه دیگه معرفی کرد و قرار شد این
استاد برای بنده از طریق منابع مالی خودش و دانشگاه اقدام کنه تا در واقع
بنده با دانشگاه طرف قرارداد باشم و نه دولت یا CNRS/INRIA. اما امروز پس
از گذشت حدود یک ماه به بنده ایمیل زده که مسئول مورد نظر در دانشگاه
علیرغم تمام دفاعی که ایشون از پرونده بنده کرده صراحتا با درخواست مخالفت
کرده. حالا شما در نظر بگیرین که این استاد رئیس دانشکده هست و با تمام
نفوذش نتونسته کاری کنه و از من خواسته که به جای دیگه فکر کنم.....
سرتون رو درد نیارم. با وجود رزومه به این قوی ای و تحصیل در دانشگاهی که
از صنایع مختلف برای استخدام فارع التحصیلان این دانشگاه میان، خود صنایع
میان به جای اینکه دانشجو بره دنبال کار یا پروژه، هنوز نتونستم پروژه ارشد
گیر بیارم و این در حالیه که تمام هم دوره ای های بنده از اوایل آوریل
پروژشون رو شروع کردن در حالی که رزومه به مراتب ضعیفتری از بنده داشتن. با
گذشت بیش از 5 روز از موعد مقرر برای ارسال مدارک مربوط به پروژه به
دانشگاه(ددلاین 15 آوریل بوده) هنوز بنده دنبال پروژه و قرارداد هستم و به
همه جا ایمیل زدم که یا پاسخی دریافت نکردم یا برای حفظ آبرو در پاسخ به
بنده گفتن دانشجوی مورد نظرشون رو گرفتن در حالی که من یقین دارم اینطور
نیست و خلاف واقع میگن.
البته در کنار این موضوع، باید عرض کنم که با فرض گرفتن ویزا، همونطور که
عرض کردم فرض محال محال نیست، در طول دوران تحصیل از زندگی در فرانسه لذت
خواهید برد. فرانسه کشوری بسیار بسیار زیبا و محیط بسیار آرومی با مردمان،
در اکثر موارد، خوبی هست. اما سئوال اینجاست که آیا شما برای لذت بردن بی
نتیجه به فرانسه میاین یا برای گرفتن مدرک و درس خوندن؟ لذا به هم رشته ای
های خودم قویا توصیه می کنم به فکر فرانسه نباشن.
بنده این حرفها رو نزدم که الان یک عده بیان به دولت فحش و بد و بیراه بدن و
تمام تقصیرات رو بر سر سیاستهای غلط دولت و نظام بندازن. در اینکه نظام
اشتباه های فراوانی داره شکی نیست اما اگه انشاا... پاتون به خارج از ایران
برسه می بینین که همه دولتها اینطورین و مردم رو بازی میدن. حالا یک دولتی
زورش با توجه به اقتصاد و رسانه ای که در اختیارش هست بیشتره و یکی دیگه
هم کمتر. به هر حال، دوست ندارم در اینجا وارد مسایل سیاسی بشم و دلیل
بیارم. شما دوستان خودتون بهتر از بنده خیلی چیزا رو میدونین.
باری، من آنچه شرط بلاغ بود، با توجه به تجربه چند ساله زندگی در خارج از
کشور، با شما گفتم. خواه پند گیرید خواه ملال. همچنین امیدوارم دوستان دیگه
ای که در این گروه هستن بیان و واقعیتها رو نسبت به زندگی در فرانسه بر
حسب تجربشون و بی هیچ حب و بغضی بیان کنن و دوستان داخل ایران رو که با
هزار امید و آرزو به دنبال کعبه آمالشون در فرانسه و یا احتمالا کشورهای
دیگه هستن با واقعیات موجود آشنا کنن. صمیمانه امیدوارم ماهایی که اینطرف
آب هستیم در عین انصاف تمام خوبیها و بدیها رو بگیم و سعی نکنیم برای
اونهایی که داخل ایران هستن فقط از خوبیها بگیم و دوستانمون رو در سرآب
آرزو فرو ببریم. بگذاریم دوستانمون با دونستن همه خوبیها و بدیها مقصدشون
را انتخاب کنن و برای آینده خودشون برنامه ریزی کنن.
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