Experimental and Simulation Comparison for Timer Action Crowbar of Doubly-Fed Induction Generator

WenJun Chen, David Atkinson

School of Electrical, Electronic and Computer Engineering

Newcastle University Newcastle upon Tyne, NE1 7RU UK

Hamza Chaal, Milutin Jovanovic Electrical, Electronic Engineering School

Northumbria University Newcastle upon Tyne

NE1 8ST UK

Abstract—The sensitivity of the DFIG to the grid

disturbances such as a voltage dip restricts network

stability and risks damage to generator converters during

the fault period due to the over-current and/or over-voltage.

It is now a requirement that wind turbine manufacturers

demonstrate what is commonly called ‘Fault Ride Though’

(FRT) capability in their turbine systems. The rotor

crowbar, as a cost-effective and reliable method of

protecting the power converters of the DFIG, was employed

as a part of the FRT scheme. A computer model created in

Matlab/Simulink was validated with the results of a 7.5kW

experimental system.

Keyword-DFIG, FRT, Rotor Crowbar

I. INTRODUCTION

Wind power is gradually becoming a more significant part of worldwide electrical generation. With the scale of wind farm increasing and the capacity of a single generator rising, the interaction between the wind generator and the grid is of growing importance.

In conventional technology, when a grid fault occurs, all the wind power generators need to be disconnected from the grid for equipments protection purposes. The grid voltage and frequency cannot be maintained at this time. The behavior of the generator during the fault period does not assist the stability of the network. Thus, the new transmission system grid codes require that wind turbine generators do not disconnect themselves from the power network during fault conditions. Instead they should supply active and reactive power into the network and assist with power system stabilization during the fault

and in the immediate post-fault period. That means wind turbines should have ‘Fault Ride Though’ (FRT) capability.

These rules were initially suggested by a German expert [1]. Now they have become a common requirement in the wind energy generation system. Many grid codes have these rules. For example, the National Grid Company who supplies the electricity to the UK requires all the wind farms or electrical stations should remain connection in 140ms when the fault occurs in over 200kV transmission system [2]. Another example is the Scottish Hydro Electrical Company which also has similar requirements: the wind turbine isolation was not allowed during a fault condition [3].

The Doubly-Fed Induction Generator (DFIG) significantly dominates world wind power generation due to its cost effectiveness. However, the sensitivity of the DFIG to grid disturbances and voltage dips restricts network stability and risks the damage to the grid and generator converters due to over-current or/and over-voltage during the fault period. Thus much research has been carried out for the DFIG protection scheme and controller strategy during the voltage dips. According to the literature, there are three main classes of scheme for the low voltage ride through: ① rotor crowbar protection; ② new topological structure; ③ reasonable excitation control algorithm. In this paper, only the rotor crowbar method is discussed. Different crowbar structures are introduced. The behavior of a model created in Matlab/Simulink was then compared with experimental results for validation purposes.

978-1-4244-6255-1/11/$26.00 ©2011 IEEE

II. .METHODOLOGY

In order to protect the power electronic components in the DFIG converter, a rotor crowbar is widely used during the rotor-circuit fault condition. The rotor crowbar short-circuits the rotor-windings, diverting the rotor currents away from the converter. In this manner the DFIG can ‘ride through’ the fault transients and resume

control during the remaining period of the fault and during the voltage recovery period.

There are several kinds of rotor crowbar structures. The simplest and earliest configuration is constituted by anti-parallel thyristors or diodes. Figure 1(a) and Figure 1(b) illustrates this kind of crowbar [4].

Figure.1 (a) Anti-Parallel Thyristor Crowbar, (b) Diodes-Bridge Crowbar (c) Mixed Crowbar, (d) Rectifier and Thyristor Crowbar, (e) Rectifier

and IGBT Crowbar, (f) Triac with Bypass Resistor Crowbar (g) IGBT with Bypass Resistor Crowbar,

Figure 1(a) reveals the anti-parallel thyristor crowbar which is constructed by two pairs of anti-parallel thyristors. In the circuit, large dc component exists in the rotor current. This results the unavailability of the thyristor turn-off characteristic. Besides, the absorber circuits for thyristors are quite hard to design.

The diodes-bridge crowbar includes one Diode Bridge to commutate and one thyristor to control as shown in Figure 1(b). When the dc side voltage peaks, the thyristor would turn on to short-circuit. Meanwhile, the rotor winding would be disconnected from the rotor side converter, but would remain connected to the crowbar circuit until the stator is totally disconnected from the grid. This configuration is superior to the scheme in Figure 1(a), because fewer thyristors are employed, and easier controller is possible. However, the current through the thyristor is continuous in the Figure 1(b) configuration, thus the thyristor cannot be turned off. This results in the crowbar short-circuit remaining across the rotor windings, until the rotor current falls to zero, which disobey the grid code rules. The new rules require the DFIG to resume normal operation when fault clearance

occurs. Hence, in order to remove the crowbar circuit after the fault, an IGBT and GTO device would be required.

The improved crowbar layouts are displayed in Figure 1(c), 1(d) and 1(e) [4]. The mixed crowbar is designed based on the diode bridge crowbar. As shown in Figure 1(c), each arm of mixed crowbar is made up of one thyristor and one diode. Figure 1(d) and Figure 1(e) have a similar configuration. Both of them add a switch and an absorber resistor in series based on diode bridge crowbar.

In addition, the crowbar with bypass resistor circuit structure was used widely as well. There are two main types demonstrated in Figures 1(f) and 1(g) [5]. When this kind of crowbar is employed, the bypass resistor would be coupled with rotor windings during voltage dips. It supplies a bypass circuit to the rotor current during the grid fault. Then the target, limiting over current and protecting the power electronic converter, is achieved.

In these layouts of rotor crowbars, the structure 1(d), 1(e) and 1(g) have more merits and are widely used. Each

silicon-controlled rectifier in configuration 1(f) must be rated to block maximum rotor voltage and to carry worst-case rotor over-currents. The thyristor switches can be fired by a current fed gate signal, but can only be turned off at a through-switch current zero. Alternatively, GTO thyristors offer turn-off capabilities, but demand a far higher turn-off gate current. The configuration 1(d) and 1(e) use only one switching device. These devices must be able to block roughly 140% of rotor phase-phase voltage. Each diode in the bridge must carry the same ratings as for the silicon-controlled rectifiers in configuration 1(f). The single resistor in Figures 1(d) and 1(e) must carry an average current 135% of the value carried by each resistor in case 1(f). Thermally however, grid faults cover a very short period and hence the single resistor is not rated much higher than three separate resistors. Additionally, given the relative cost of diodes, diode rectifier configurations offer a major cost saving, however, the configuration 1(e) with IGBT has a considerable increase in single device cost compared to 4 with the Thyristor and GTO. Moreover, turn-off of the switch device in configuration 1(d) must wait for a rectified current zero; this will only occur once the rotor circuit flux has completely decayed. Near-instantaneous turn-on and turn-off can be achieved by using an IGBT power switch, as shown in the rectifier-IGBT configuration in 1(e). However, the swift turn-off ability of an IGBT is necessary for certain fault-ride-through control methods. Thus configuration 1(e) is employed on the test rig used in this work.

III. RESULT & DISCUSSION

The experimental results were produced by a 7.5kW DFIG test facility. The mechanical input torque was provided by a DC machine controlled to simulate a wind turbine and its flexible-coupling drive-shaft. The DFIG was controlled to generate at unity power factor under healthy voltage conditions. Pre-fault conditions consisted of rated rotor speed, specifically 12% above synchronous (1680rpm), and 67% rated power generation (5kW). In the crowbar test, the timer action crowbar method was employed. The value of the rotor crowbar resistance is an important consideration. It should be maximized to improve its relative ability to hasten the decay of rotor flux and protect the power electronic devices from the

over-current, but should remain safely below the maximum limit to avoid too high a voltage at the converter terminal during the fault. Accordingly the rotor resistance value should be selected in accordance with

. In the experimental system, using the base

rotor current of 3.35A, a maximum crowbar resistance of 33Ω is appropriate. The crowbar resistance was chosen to reduce the rotor transient time constant to roughly 1/4 of the test machine’s original value. This equates to 23Ω of crowbar resistance. Hence, a 25Ω, 0.6kW power resistor was selected for this purpose.

The crowbar is activated when the magnitude of the rotor current exceeds a threshold value, 2.0 p.u., which was the stated maximum IGBT pulse current. Then the crowbar remains engaged for a fixed time, 120ms. While the crowbar is engaged, rotor-side PWM is disengaged (all IGBT switches ‘off’). The rotor current and power PI controllers are all reset to zero output. The line-side converter’s controllers remain unaffected.

After 120ms, the crowbar is released. If a rotor over-current persists, the crowbar is re-engaged for another additional 120ms.

In the experimental case, the VAr support control was employed to minimize the voltage dip experienced by the generator. When the 15% fault condition was applied to DFIG without VAr control, the main supply voltage dropped to 0.15p.u during the fault, while with the VAr support, the stator voltage showed a reduced drop to 0.26 p.u. After a 50ms settling period, 28% voltage was maintained throughout the fault period (Figure 2(a)). However, when the crowbar engaged, the VAr control failed to support the supply voltage. Hence, the stator voltage of the crowbar test was illustrated in Figure 2(b). However, in Matlab model, the voltage change under the crowbar operation cannot be modelled. Thus, the constant 0.28 p.u dip voltage under VAr support was always given as a constant input voltage in simulation. The simulation stator voltage looks similar to Figure 2(a).

Figure 3 directly compares the timer action crowbar test results for a 15% fault applied to a DFIG. The left hand side graphs illustrate the experimental results, while

the right hand side graphs describe the simulation results. In general, the two sides of every group look similar.

Figure.2 The supply voltage without crowbar under VAr support device (b) The supply voltage with crowbar under VAr support device

Figure.3 the timer action crowbar experimental test (left) and simulation (right) results for a 15% fault applied to a DFIG:

In experimental tests the current sensors were placed at the output of the rotor-converter’s series filter chokes, between the crowbar and the converter. It is clear from this data that the converter leg currents are forced to zero throughout the crowbar period. The close-up graphs of both rotor current magnitudes reveal the 2 p.u. limit in effect. During the whole fault period the rotor currents never exceeds the calculated maximum limit of 9.5A (peak). In the comparison of the simulation and experimental results, the experimental rotor current magnitude is a little over 2 p.u., when the timer action

crowbar connected with the rotor windings. The main reason is the simulation switch device of crowbar is ideal. It has no switch delay time. However, although a near-instantaneous turn-on and turn-off can be achieved by using an IGBT power switch, it still takes a short time period to active in practical device. This turn-on time in practical case leads to the rotor current magnitude being a little over 2 p.u.

There are some differences in the power graphs between experiment and simulation results. The major

difference is the simulation result has less damping which indicates the oscillations last longer compared to experimental outcome during the crowbar operation period. It can be seen that the simulation model takes longer time to settle to a constant value.

Careful observation of the simulated power graph reveals that the frequency of the oscillation in the shaded regions is approximately 50Hz which is as same as the supply voltage frequency. This is due to the machine operating as a Singly-Fed Induction Generator during the crowbar period.

In this test, when the crowbar is released, rotor-side PWM and rotor current PI control are immediately resumed. Power control is resumed after a specified delay of 40ms, to allow the current controllers to settle. In the meantime, the current controllers are fed constant reference values. These interim values are deliberately constant to minimize the settling time of the current controllers. This value was set to [-1.00Pref+0.64jVs] in the test. The d-component of interim values compares favorably with the actively controlled rotor current values recorded in each of the VAr-support tests. The reactive-current reference saturation limits had been raised to ±1.0 p.u. in response to the VAr support tests. The PQ control delay should not become onerous as the interim reference currents will in any case deliver a reasonable power factor and will not act to destabilize the controller. To improve stability it is important to initialize the PQ PI feedback controllers’ integral components with values set equal to the interim reference values. This helps to smooth the resumption of outer-loop feedback control. As PQ PI control is resumed, note that the rotor current reference value changes were rate-limited to no more than 1.50 p.u./s to ensure current feedback control

stability. After the second crowbar activation period, experimental results shows that control is regained very quickly; active and reactive power levels returned to unity power factor and 5kW generation takes place within tens of milliseconds.

IV. CONCLUSION

A model has been developed which is capable of simulating the behavior of the DFIG during 15% voltage dip fault events. The simulation model has been validated by experimental results from a 7.5kW DFIG test rig.

For the 15% fault lasting for 1.5s, the rotor converter devices were fully protected. The rotor converter currents were immediately diverted upon crowbar activation and at no point did they exceed the calculated 9.5A peak limit for the converter.

The test results indicate the rectifier and IGBT crowbar with timer action crowbar method, is an effective and reliable method of protecting the DFIG power converter from rotor over-currents.

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transmission network[M]．Danmish，2000 [2] National grid electricity transmission Co.．The Grid Code Issue

3 Revision l3．9th Jan． 20(I6． connection conditions 6．3．15

[3] Dallachy J L，Tait I．Guidance Note for the Connection of Wind Farms．Issue No． 2．2．2． SP Transmission and Distribution．Scottish Hydro-Electric．2002

[4] NIIR ANEN ． Voltage Ride Through of A Doubly Fed Generator Equipped With An Active Crow bar．EPE—PEM C Riga Latvia，2004

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