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Improvement of Transient Stability Margin in Power Systems with Integrated Wind Generation Using a STATCOM: An Experimental Verification M. Molinas, Member, IEEE, S. Vazquez, Member, IEEE T. Takaku, Member, IEEE, J.M. Carrasco, Member, IEEE , R. Shimada, Member, IEEE , T. Undeland, Member, IEEE
Abstract-- The impact of the wind generation on the power systems is no longer negligible if high penetration levels are going to be reached. Significant barriers to interconnection are being perceived already with the severe requirements of the new emerged grid codes. Depending on the generator technologies, different solutions are found to support behavior in case of voltage sags. Voltage Source Static Var Compensator such as the STATCOM can be used to regulate voltage as shunt compensator with directly connected asynchronous wind generators. This paper has analyzed the extent to which the transient stability margin can be increased by the use of a STATCOM, first by simulations and then with an experimental verification on a setup of 7.5 kW. Measurement results confirm that the STATCOM provides a clear stability margin increase and with adequate rating it becomes possible to ride through severe faults.
Index Terms—Transient stability margin, Voltage Source Static Var Compensator, Voltage Sag, Power Quality, Grid Code, Asynchronous Wind Generator, Vector Control.
I. INTRODUCTION
G
IVEN the onshore and offshore wind resources available, the potential for large scale generation is huge, but the extent to which it can be integrated into the power system without affecting the overall stable operation depends on the technology available to mitigate the negative impacts. There is a strong need for preparing a framework for large scale M. Molinas is with the Norwegian University of Science and Technology, O.S. Bragstad plass 2E, 7491 Trondheim, Norway (e-mail:
[email protected]). S. Vazquez is with the University of Seville, Camino de los Descubrimientos s/n Sevilla 41092, Spain (e-mail:
[email protected]). T. Takaku is with Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo, Japan 152-8550 (e-mail:
[email protected]). J. M. Carrasco is with the University of Seville, Camino de los Descubrimientos s/n Sevilla 41092, Spain (e-mail:
[email protected]). R. Shimada is with Tokyo Institute of Technology, O-okayama, Meguroku, Tokyo, Japan 152-8550 (e-mail:
[email protected]). T. Undeland is with the Norwegian University of Science and Technology, O.S. Bragstad plass 2E, 7491 Trondheim, Norway (e-mail:
[email protected]).
integration of wind power if the vast resources are to be used. In establishing this framework, there are several issues that need to be considered. Some of these are familiar and related to power quality; others like the ride through capability introduced with the new grid codes are new challenges [1,2]. The new grid codes for integration of wind power into the power system has been in progress in several countries and some countries have already adhered to regional codes (Spain with E-ON). The grid codes have identified many potential adverse impacts of large scale integration. The impacts are wide ranging, but given the novelty of the industry and the scenarios, they are not fully understood. One of the issues addressed by these new grid codes is maintaining stability in the presence of external faults and riding through the faults. This requires particular control strategies for fixed speed induction generator wind turbines as well as robust power electronics for converter connected farms. Wind plants using fixed speed generators directly connected to the network will more acutely suffer from the new demands, since they would usually disconnect from the power system when the voltage goes below 30% of rated value. The current trend toward advanced power electronics devices will enable high scale integration of this type of generators and will allow wind plants to deliver electricity that meets quality requirements including the new grid codes. The STATCOM has been reported to have the capability to regulate voltage, control power factor, and stabilize power flow [3,4]. In this paper, the STATCOM is analyzed from the point of view of its potential for transient margin increase as reported in [5] but in the present scenario of new grid codes. The purpose is to quantify the enlargement of the transient stability margin by simulations and the respective experimental validation. This margin is the length of the fault that the wind generation is capable of riding through without loosing its stable operating condition. The paper concentrates on the directly connected asynchronous wind generator technology and analyses the improvement of transient stability margin in the event of a three phase-to ground fault when a STATCOM is used as the ride through solution. Vector control technique as presented in [6] is the one implemented in the STATCOM.
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II. THE STATCOM AS A POWER CONDITIONING DEVICE The STATCOM is a power electronics device based on the voltage source converter principle. The technology typically in use is a two level voltage source converter with a DC energy storage device, a coupling transformer connected in shunt with the power system, and DSP based control circuits. The main advantage of the STATCOM over thyristor type of static var compensators is that the compensating current does not depend on the voltage level of the connecting point and thus the compensating current is not lowered as the voltage drops. However, in the light of the new grid codes for wind generation, the most relevant feature of the STATCOM will be its inherent capability to increase the transient stability margin and thus contribute with ride through handling. III. SIMULATION MODEL Simulations have been performed with Matlab/Simulink software for the directly connected fixed speed generator technology. The wind farm consists of a 100 MW fixed speed wind turbines. The schematic configuration of the simulated system is shown in Fig. 1. For the simulation studies it was assumed that the power system was subjected to a three phase fault along the transmission line. The control strategy implemented is based on the vector control principle. The block diagram is shown in Fig. 2. The reactive current injected is controlled so as to obtain 1 p.u. rated grid voltage. The duration of the short circuit was controlled to investigate the maximum possible length that would not harm the system stability. The maximum duration of fault was investigated for the system compensated with the STATCOM and the system with only fixed capacitors. The main simulation results are summarized in Table 1. The results show that the system without the STATCOM goes unstable with a fault duration of 20 ms. This is the maximum time the system can ride through the fault with only fixed capacitors.
TABLE I SIMULATION RESULTS FOR THE SYSTEM WITH STATCOM AND FIXED CAPACITORS
Simulation results
Only capacitors
STATCOM
Compensation characteristics
Capacitors Q: 13.4 Mvar
Capacitors Q: 13.4 Mvar
Capacitors Q: 3.4 Mvar
Max. Fault duration
20 ms
186 ms
159 ms
1.1 p.u
0.55 p.u. steady state. (0.55 p.u. transient)
Current rating of STATCOM
The wind farm has limited capability to ride through the fault under this condition. However, when the STATCOM is set in operation the wind farm is able to ride through the fault for as long as 186 ms. The compensating reactive current of the STATCOM is shown by the response in Fig. 3. In this case, during normal operating conditions the STATCOM does not contribute to voltage support. All the voltage support in this case is provided by the 13.4 MVAr of fixed capacitors. Another operating condition is analyzed for the STATCOM by simulations. Under this condition, the amount of fixed capacitors is reduced to 3 MVAr and the short circuit simulation is run again. The response of the STATCOM is shown in Fig. 4, and it is clearly seen the current contribution to voltage support during normal operating conditions. However, the maximum fault duration in this case is reduced to 160 ms. But this is still a major improvement compared to the system with only fixed capacitors. DC link
Voltage Source Converter
Lf va ,b
* vabc conv.
Wind turbine
PWM
Clark
Clark-inv.
G Cage Induction Generator
Transformer
vd*
Vdc -
* Vdc
+
PI
Fig. 1. Schematics of the system configuration used in simulations. In the laboratory measurements the wind turbine and gear are replaced by an induction generator driven by a DC motor.
iq*
-
id
+ +
PI
+
STATCOM
Park
vd vq*
+
+
id *
iα , β
Park Park-inv.
Electric Grid
Clark
vα ,β
vα* , β
Gear
ia,b
-
ωL
ωL
PI
Fig. 2. Block diagram of the control implemented in the STATCOM
iq
3
Considering that
circuit capacities of the generator and the grid respectively, the STATCOM current rating can be expressed in pu as: ∆V I q = rsc− gen + rsc− grid (2) (1 − ∆V ) This last equation shows that both, the wind farm short circuit capacity and the grid short circuit capacity will influence the STATCOM current rating. Depending on the voltage drop, the rating will be determined by the short circuit capacities at the PCC. From this equation it is also clear that for voltage regulation purposes, the location of the STATCOM will be optimum at the middle point between the two voltage sources. The case of an off-grid wind farm is also analyzed to find the required rating of STATCOM for voltage regulation in case of voltage fluctuations produced by variations of wind. Figure 6 represents the off-grid farm and the STATCOM is placed at the load bus. The required rating is given by:
(
Fig. 3. Reactive current provided by the STATCOM when fixed capacitors amount is 13.4 MVAr
Fig. 4. Reactive current provided by the STATCOM when fixed capacitors amount is 3 MVAr
IV. DETERMINATION OF REQUIRED STATCOM RATING FOR VOLTAGE REGULATION
A mathematical relation between the wind farm and grid short circuit ratios, the voltage drop, and the rated power of the STATCOM was derived for the circuit represented in Fig.5.
1 1 expressed in pu are the short and xg xl
Iq =
)
1 ⋅ ∆V1 xk
(3)
In this case the maximum rating will depend on the maximum wind variation at the specific farm site. In general, from equations 2 and 3, it is clear that the rating of a STATCOM for an off-grid farm is lower than in the case of a grid integrated wind farm.
xk Off-grid Wind farm
I wind
I load
Iq
STATCOM Fig. 5 – Schematic representation of a grid integrated wind farm with STATCOM at the PCC
In Fig. 5, Eig represents the voltage generated at the wind turbine, E the grid voltage, Iq is the reactive current injection of the STATCOM, xst the coupling inductance, xsc the short circuit inductance, xg the generator internal inductance and xl the grid inductance to the point of common coupling PCC. Solving the equations for this circuit the rating of the STATCOM for voltage regulation as a function of voltage drop and short circuit capacities of the generator and grid is expressed in pu as: Iq =
(xl + xg )∆V
xl xg (1 − ∆V )
(1)
Fig. 6 – Off-grid wind farm with STATCOM connected at the load connecting point.
It is common practice to use switched capacitor banks for variable reactive support in case of off-grid wind farms, due to the low cost of the solution. However, using a STATCOM will eliminate mechanical stresses due to bank capacitors being switched on and off, greatly improving system life time. Moreover, the STATCOM solution will provide with the capability of handling transient stability. V. EXPERIMENTAL VERIFICATION OF THE TRANSIENT STABILITY IMPROVEMENT WITH STATCOM The experimental setup is shown in Fig. 7. Ratings and characteristics of all system components are summarized in Table II. In all measurements, the pu system is based on the rated VA of the generator used to emulate the wind generator.
4 Short Circuit Device
Wind Turbine + Wind Generator
AC DC
DC motor
System Component Wind Generator
Induction Generator
Weak Grid 0.11 pu
GRID
PCC AC DC
DC motor Local Load
TABLE II RATINGS OF DEVICES USED IN EXPERIMENTS
0.03 pu
Description Wound rotor induction machine with short circuited rotor windings
Parameters Pn = 7.5kW
Vn = 230V I n = 27.5 A n = 1430r / min rS = 0.05 pu
Induction Motor
rR = 0.04 pu
STATCOM 0.21pu
Independent AC GRID
X σ = 0.15 pu X M = 1.8 pu
DC AC
Fig. 7 – Experimental setup of the wind generation compensated with STATCOM
The wind generator is emulated with a 7.5 kW wound rotor induction machine with short-circuited rotor terminals. The wind turbine is emulated by a constant torque generated with a controllable DC motor. A constant torque is used to model the power from the wind, because transient phenomena are much faster than wind variations. A local load, having induction motor characteristics is placed at the point of common coupling as shown in Fig. 7, in order to force the power system towards its stability limit. The STATCOM is an IGBTbased inverter bridge, whose maximum current is about twice the rated current of the wind generator. Vector control technique, the same used in the simulation study, is implemented through a DSP and a host computer. A sudden severe drop in voltage is caused at PCC by simultaneously triggering all thyristors in the short circuit device (SCD) in Fig. 7. The depth of the voltage drop can be modified by properly adjusting the inductance in series with the SCD. In the experiments shown in Fig.8, a voltage drop of about 80% is produced, and the fault is cleared after 300ms. Before the fault, the wind generator is delivering about 1.3pu of active power, of which 0.75pu are absorbed by the local load, and the rest is sent to the weak grid. Fig. 8.a shows the system response when the system is not supported with a STATCOM. During the fault, the wind generator accelerates, since it is no longer able to generate enough electromagnetic torque to balance the torque coming from the wind, which is obviously unaffected by the grid fault. When the fault is cleared, the generator speed is about 1.6pu and, without fast reactive support; the generator is not able to produce enough braking torque to bring the speed back to its pre-fault value. The voltage at the PCC does not recover its nominal value and remains very low (0.4pu) with the resulting grid current very high (>3.0pu). The situation would have developed into voltage collapse, provided that we were not limited by the wind turbine emulator, which is unable to keep rated torque at such a high rotational speed. However, even if voltage collapse did not take place, the pseudo-stable postfault operating condition is not a sustainable one, and will not be accepted by any grid code.
Wind Turbine
Separately excited DC machine
Local Load
Induction Machine coupled to separately excited DC machine IGBT-based, 3-ph inverter bridge PWM controlled Short circuited 3-ph Thyristor bridge
STATCOM
Short Circuit Device
Pn = 10.0kW n = 2000r / min Same as Wind generator and wind turbine VDC ,MAX = 600V
I MAX = 78 A I Peak = 2000 A
The excessive currents in the system under this condition will also cause protections to trip very soon. Fig. 8.b shows the system response to the same fault condition, starting from the same initial operating point, but with a STATCOM connected at the PCC. The maximum current the STATCOM can deliver is fixed to 0.5pu of wind generator rated current. During the fault, the system behaves exactly like the one with no compensation, except for a small contribution of the STATCOM to the short circuit current. The voltage at the PCC and the generator speed are not affected by the STATCOM to any significant degree, during the fault time. However, it is clear that even with the relatively small rating of 0.5pu, the STATCOM is able to stabilize the power system when the fault is cleared; bringing the PCC voltage, grid current and generator speed back to their prefault values. Complete voltage recovery process takes about 0.9s from the instant of the fault clearance. Fig.8.c and Fig.8.d show the same experiment, with an increased rating for the STATCOM (1.0pu and 1.8pu, respectively). As expected, things do not change significantly during the fault time, but the voltage recovery process is considerably shortened as the STATCOM rating is increased. In particular, the wind generator is able to generate much more torque at the fault clearance, due to the increased reactive support at its terminals. This behavior can also be interpreted as an increased stability margin with the STATCOM ratings. That means that a system with higher rating STATCOM will be able to withstand longer short circuits and /or it will be able to withstand voltage drops while delivering higher amount of power. Analytical evaluation of the increase in stability margin, as a function of STATCOM ratings will be provided in a next paper.
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VI. CONCLUSION It is found in the simulation results that under short circuit conditions the STATCOM can provide a major increase in the transient stability margin of power systems that integrate wind generation. The fault duration is increased from 20 ms with only fixed capacitors to 186 ms with the STATCOM. Results of the experiments performed in the lab set-up of 7.5 kW confirm the increased transient stability margin as a result of the use of a suitably rated STATCOM. In this paper we could not provide a quantitative transient margin, but the experimental results provide a clear qualitative verification of transient margin increase compared to the system without the STATCOM support. In terms of ride through capability, these experimental results show that with a reasonable rating of 0.5pu and a fault of 80% voltage drop for 300 ms, the STATCOM is an optimum candidate for providing ride through in wind farms equipped with asynchronous generators directly connected to the grid. In addition, considering that wind turbines generators will trip when they detect a 30% voltage drop, it can be said that the STATCOM provides with a clear capability of handling ride through for an 80% voltage drop. The extent of this handling capability will depend on the power system configuration and
The rating of the generator used in experiments was not the appropriate one to be representative of the hundreds kilowatts turbines or megawatts wind farms. This is due to the equivalent resistance of the wind generator emulator which is relatively large in pu and provided a very good damping during the short circuit trials. Several short circuit trials were performed and it was indeed difficult to get the system unstable because of this reason. A local load was necessary to use for achieving instability in the non-compensated system. For this reason, the next step in this research will be to perform the same experiment with a higher rated generator (50kW) that has internal resistance in pu very similar to the ones of real wind turbines.
Vmains [pu]
Vmains [pu]
0 -1 0
0.5
1
2 0 -2 0.5
1
1.6 1.4 1.2 1 0
0.5
1
0.5
1
1.5
0
0.5
1
1.5
0
0.5
1
1.5
0
0.5
1
1.5
0 -2
1.6 1.4 1.2 1
2 I statcom [pu]
I statcom [pu]
0 2
1.5
2 0
-2
-1
1.5
IG speed [pu]
0
0
1.5
Imains [pu]
Imains [pu]
VII. FUTURE WORK
1
1
IG speed [pu]
rating of the device itself. This aspect will be thoroughly analyzed in a next paper. In the electrical system analyzed in this paper, in spite of the additional cost of power electronics converters and control, with the new grid codes, the achievement of ride through capability is of relevance. Therefore, investment in a STATCOM will certainly be justified in the scenario presented in this paper.
0
0.5
1
1.5
Time [s]
Fig.8.a – 300ms, 80% voltage drop at PCC without STATCOM
0
-2
Time [s]
Fig.8.b – 300ms, 80% voltage drop at PCC with STATCOM rated 0.5pu
1
1
Vmains [pu]
Vmains [pu]
6
0 -1 1
0 -2 0.5
1
IG speed [pu]
1.4 1.2 1 0.5
1
I statcom [pu]
I statcom [pu]
0
0
0.5
1
1.5
Fig.8.c – 300ms, 80% voltage drop at PCC with STATCOM rated 1.0pu
A comparison of time responses, required rating, and quantitative transient margin, among fixed capacitors, SVC and STATCOM will also be performed in the next step. Cost comparison will also be provided.
VIII. REFERENCES
[2] [3] [4] [5] [6]
1
1.5
0
0.5
1
1.5
0
0.5
1
1.5
0
0.5
1
1.5
0
1.6 1.4 1.2 1
2
Time [s]
[1]
0.5
-2
1.5
2
-2
0 2
1.5
1.6
0
-1
1.5
2
0 IG speed [pu]
0.5
Imains [pu]
Imains [pu]
0
0
Eltra specifications, “Wind farms connected to the grid with voltages over 100 KV. Technical regulations for the properties and control of wind turbines,” Eltra, Doc. NO. 17619 v6, in Danish, 2004. Nordisk Regelsamling (Nordic Grid Code), Nordel, 2004 L. Gyugyi, , “Dynamic Compensation of AC Transmission Line by Solid-State Synchronous Voltage Sources,” IEEE Power Engineering Society, Summer Meeting 1993, pp. 434 1-8. M.Molinas, J. Marvik, T. Undeland, “Impact of Large Scale Integration of Wind Power into the Electricity Grid,” International Conference of Women Engineers and Scientists, Seoul, Korea., (2005). E. Larsen, N. Miller, S.Nilsson, S. Lindgren, “Benefits of GTO-based Compensation Systems For Electric Utility Application,” IEEE Power Engineering Society, Summer Meeting 1991, pp. 397 1-8. M. Molinas, B. Nass, W. Gullvik, T.Undeland, “Control of Wind Turbines with Induction Generators Interfaced to the Grid with Power Electronics Converters,” Proc. of the International Power Electronics Conference IPEC 05, Niigata, Japan
0
-2
Time [s]
Fig.8.d – 300ms, 80% voltage drop at PCC with STATCOM rated 1.8pu
