Doubly-Fed Asynchronous Generators

One of the most common variable speed turbines is the doubly-fed asynchronous generator (DFAG). One design is shown in Figure 3-2. This design employs a series voltage-source converter to feed the wound rotor of the machine. By operating the rotor circuit at a variable ac frequency one is able to control the mechanical speed of the machine. In this design the net power out of the machine is a combination of the power coming out of the machine’s stator and that from the rotor (through the converter) into the system. When the unit is operating at supersynchronous speeds real power is injected from the rotor, through the converter, into the system. When the unit is operating at subsynchronous speeds real power is absorbed from the system, through the converter, by the rotor. At synchronous speed, the voltage on the rotor is essentially dc and there is no significant net power interchange between the rotor and the system. Most designs tend to supply reactive power to the system through the machines stator by effectively changing the d-axis excitation on the rotor. A vector control strategy is used, where the rotor current is split into d-axis (flux producing) and q-axis (torque producing) components. Each component is then controlled separately. The d-axis component is controlled in order to

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regulate the machine power factor, while the q-axis component is controlled in order to keep the electrical torque of the machine constant. It is possible, however, to also provide reactive power through the converter with a four-quadrant voltage source converter design. By employing this feature in the line side converter the wind turbine generator line side converter can essentially act as a STATCOM and supply or absorb reactive power to or from the system even when the actual wind turbine generator is not running and disconnected from the system. Providing this feature, though, will typically mean additional cost.

The fact that rotor currents are tightly controlled (kilohertz) means that the controls have the ability to, within limits, hold electrical torque constant (as opposed to the relation between torque and angle in synchronous machines). Thus, rapid fluctuations in mechanical power can be temporarily “stored” as kinetic energy, thus improving power quality.

As in the case of conventional induction generation, older designs of DFAGs would disconnect from the system during a close in fault. In the case of earlier DFAG designs, one might say they were more sensitive to system fault and would disconnect from the system in a much shorter time frame than conventional induction generators (within milliseconds. if the system voltage dropped below about 70%). Unlike the case of conventional induction generation, however, the process leading to separation might not be readily apparent from dynamic simulation results. The concern in DFAG is the fact that large disturbances will lead to large initial fault currents, both in the stator and in the rotor as well. These high initial currents will, of course, flow through the rotor-side converter. Due to low voltages at machine terminals during a disturbance, the stator- side converter is limited in its ability to pass power to the grid. Consequently, the additional energy goes into charging the dc bus capacitor and thus the dc bus voltage rises rapidly, depending on the design of the converter controls. This may give rise to protection acting to short-circuit the capacitor (via a crowbar) in order to protect the converter power electronic components (see Figure 3-10) [6]. In the past, when the crowbar circuit fired, the unit would be disconnected from the system.

The newer generations of doubly-fed asynchronous generators are now supplying low-voltage ride-through, achieved through changing the control and protection philosophy of the voltage- source converter. One example is the use on an active crow-bar circuit, which is described in the following subsection. Rotor Side Converter Stator Side Converter Fault

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3.2.6.1 Doubly-fed Asynchronous Generator Low-Voltage Ride-Through Using Active Crowbar

Active crowbar means a crowbar with a semiconductor switch that can in addition to turning on of the crowbar also switch it off. Typically the active crowbar consists of a diode bridge and an IGB-transistor as a fully controlled semiconductor switch, Figure 3-11.

Fully controllable semiconductor switch (IGBT) Small resistor Rotor side converter DC bus Stator side converter Active crowbar

Figure 3-11: Doubly-fed generator with an active crowbar.

It is common to have a resistor with a small resistance value in series with the transistor in order to limit the inrush current in the rotor when the crowbar is conducting.

The ride through sequence for a symmetric three-phase voltage dip starts when either the rotor current or the dc bus voltage increases above the tripping limit. In order to protect the converters and the dc bus capacitor from excessive voltages and currents, the crowbar transistor will be turned on and all the transistors of the rotor side converter will be turned off. As a result the rotor current is diverted to flow via the crowbar.

When the crowbar conducts the generator behaves much like an ordinary asynchronous induction generator that has a small external rotor resistor (roughly equivalent to 2/3 of the crowbar resistor's resistance per phase). Thus, after about a 10 to15 ms initial burst of reactive power to the grid the generator starts to consume reactive power. The active power depends on the generator slip. If the generator speed is above the synchronous speed it will continue to produce some power to the grid. If the speed is lower than synchronous speed the generator starts to consume active power. However, due to the low stator voltage the active and reactive power of the generator are quite low although the stator currents are high.

Due to the low impedance of the crowbar circuit and low stator voltage the generator flux reduces rapidly. The currents in the stator and rotor similarly decrease. Typically after 60 to 100 ms the measured crowbar current indicates that the transient has decayed enough for the rotor converter to be able to control them again. The dc bus voltage has also decreased to normal levels because the stator side converter has fed the extra energy stored in the dc bus capacitor to the grid. In the next step the crowbar transistor is turned off. The rotor current is then turning back to the rotor side converter. The transistors of the rotor side converter are still blocked, but the rotor current finds its way through the diodes that are parallel to the transistors. If the voltage induced in the rotor is now lower than the dc bus voltage, the rotor current will rapidly decrease to zero.

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The stator current is similarly reduced to a low value. After the rotor current has been close to zero long enough to be sure that the transient is over, the rotor side converter can be started again. If, however, the rotor current does not decrease fast enough, the dc bus voltage may rise to the tripping level again. Then the crowbar is retriggered and a new turn-off attempt is made when the dc bus voltage has decreased back to normal again. Thus, in severe voltage dips the crowbar may conduct several times before the rotor current is finally cut out.

When the rotor side converter has been successfully started, the rotor side converter can control the generator to produce reactive and active power to the grid. Typically the rotor side converter will start 80 to 150 ms after the beginning of the dip and rated generator current is available 200 to 400 ms after the beginning of the dip. The increase of the generator current has to be slow enough especially when the voltage in the grid during the dip is close to zero, because then the magnitude and phase angle of the voltage on the generator terminals will be largely defined by the generator itself, a situation that may cause loss of synchronism with the grid.

When the fault causing the voltage dip is cleared, the grid voltage will increase. This change will induce another transient in the generator stator and rotor. However, the circuit breaker will break the fault current in each phase when the phase current is near zero. Thus, the grid voltage increase is not as fast as the decrease has been. In many cases the resulting transient is so low that the crowbar is not triggered at all. The generator's active and reactive power can then be controlled according to the grid code requirements without delay. If the transient triggers the crowbar, the generator will start typically 50 to 100 ms after the triggering instant.

One example of a ride-through measured in a factory test is shown in Figure 3-12.

Figure 3-12. Measured ride-through of a doubly-fed generator with an active crowbar.

For unsymmetrical (that is, single- and two-phase) faults the ride-through of a doubly-fed generator is more difficult. The reason is the negative sequence in the grid voltage that is caused by the unbalance of the phase voltages. The negative sequence rotates in the other direction than the rotor and thus has a high slip value around 2 pu. Due to this high slip even a rather small negative sequence component in the stator voltage can induce in the rotor a voltage that is higher than the dc bus voltage. Because there are diodes parallel to the transistors in the rotor converter, the rotor circuit will feed current to the dc bus even when the rotor side converter's transistors are blocked.

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In contrast to the decaying transient caused by a symmetrical three-phase fault, the negative sequence will continue to exist until the asymmetric fault is cleared. Thus if the unbalance is high enough the rotor current will continue to boost the dc bus voltage up and cause repeating sequence of turn-ons and -offs of the crowbar. The active crowbar will then be operating much like a chopper that controls the dc bus voltage.

Typically the rotor converter cannot be started during the dip if the negative sequence component in the stator voltage is greater than 30 to 50 % of the rated voltage.

When the fault is cleared, the rotor side converter can control the generator in a normal way if it has succeeded to start during the dip. If the crowbar is still on, the crowbar will be turned off first and then the rotor side converter is started in a normal way.

There are several other alternative designs for a doubly-fed converter ride-through. One alternative is to dimension the rotor side converter to be able to handle the inrush current. Because the stator side converter cannot handle the power fed to the dc bus from the rotor when the grid voltage is close to zero, a transistor controlled resistor ("braking chopper") is needed to dissipate the extra energy in the dc bus. Naturally the required higher current rating of the rotor side converter increases the cost of the equipment.

Another alternative has semiconductor switches in the stator that temporarily disconnect the stator from the grid when a high current transient is detected. The main drawback of this scheme is the addition to the complexity of the circuit and the extra losses in these switches that decrease the efficiency of the generator during normal operation.