Improved grid interface of induction generators for renewable energy by use of STATCOM

Transcription

1 Improved grid interface of induction generators for renewable energy by use of STATCOM Marta Molinas *, Jon Are Suul ** and Tore Undeland * * Norwegian University of Science and Technology Department of Electric Power Engineering, Trondheim, Norway **Sintef Energy Research, Trondheim, Norway marta.molinas@elkraft.ntnu.no Abstract This paper presents an overview of the STATCOM as a solution for voltage quality problems related to the interconnection of fluctuating renewable energy sources to the power network. Stationary, dynamic and transient modes of operation with emphasis on low voltage ride-through (LVRT) operation are described and simulation examples are presented to illustrate the role of the STATCOM as the grid interface solution using the example of a wind farm. Experimental results verify the validity of the simulation results indicating that the STATCOM is a good candidate for grid interface of renewable energy sources with induction generators. Keywords- STATCOM; voltage source converter; renewable energy; grid interface; reactive current; low voltage ride through; grid code. I. INTRODUCTION The potential for power generation from renewable energy sources such as wind and wave is high and environmental concerns are pushing for a higher penetration of these into the power network. The European community has set the Community Strategy and Action Plan for Renewable Energy Sources to double the share of renewable energies in gross domestic energy consumption in the European Union by 2 (to 2%) []. If this target is going to be reached efforts should be directed to the power quality related problems when fluctuating power from renewable sources is tapped into the power network. Power system operators are reluctant to accept the fluctuating and largely undispatchable generating resource of renewables in their pool because of their concern about the quality of power. Voltage quality is one of the technical problems to be faced when high amounts of renewables are penetrating the power network. There are several solutions found in the literature for voltage control under different operating conditions. Shunt connected compensation cover traditional equipment such as fixed and switched capacitors or inductors, transformer on-load tap changers and synchronous condensers; and power electronic equipment such as Static Var Compensator (SVC) and the STATCOM. The first two are suitable for stationary voltage control, [2]. On-load tap changers are used for stationary control when a transformer is available and to reduce or boost voltage according to the system need, but have no ability for compensating reactive power. The Synchronous condenser is an unloaded synchronous machine, and the exchange of reactive power with the grid can be controlled by the internal excitation. Depending on the design of the machine and the excitation system, the synchronous condenser can be made to perform dynamic voltage compensation. Although a synchronous condenser will have slower control than the fastest FACTS devices, it will act as voltage source, and has one fundamental advantage in the capability of supplying very high transient current compared to the stationary rating. Recent developments have been made of synchronous condenser that may be applied for flicker mitigation and low voltage ride through support of wind farms [3]. The SVC and the STATCOM are power electronics devices; the first one is based on thyristor technology, and acts as variable inductive/capacitive impedance in the power system, while the second one is a voltage source converter based on switched semiconductors and with the ability to control its exchange of reactive current with the grid, depending on the control objectives and the design of the control system. Both can be used for stationary and dynamic voltage control, but the STATCOM is more capable of transient voltage control because of its ability to almost instantaneously control the semiconductor devices, and because the compensation current can be controlled independent of the system voltage in a wide range [4]. This paper describes the principle of operation of the STATCOM in its three modes of operation by simulation examples when the STATCOM is installed at the sending end of a wind farm with stall-regulated turbines and constant speed induction generators directly connected to the grid. The schematic configuration of the wind farm example used in this paper is illustrated in Fig.. In this way the STATCOM is presented as a valid interface for renewable energy sources with the ability of improving voltage quality in stationary, dynamic and transient modes with emphasis in the ride through capability [5], [6]. An experimental verification has been performed on a 5 kw laboratory set-up, to validate the simulation results /7/$2. 27 IEEE. 25

2 Wind turbine Gear G C STATCOM Electric Grid Fig.. Schematic configuration of the system under study. There are several techniques to control the STATCOM [7], [8], [9]; the vector control technique is selected in this paper for its fast dynamics and decoupled control ability. The control strategy implemented in this paper in the three modes of operation is based on voltage oriented vector control. By use of this technique the control of the DC link voltage and reactive current is decoupled like in the control of torque and flux in the field oriented control of motor drives []. The block diagram of the vector control technique used for simulations and experiments is shown in Fig. 2. II. PRINCIPLE OF STATCOM OPERATION The STATCOM is a power electronics device based on the voltage source converter principle. The technology typically in use for distributed generation is a two level voltage source converter with only a small DC capacitor as voltage source, a coupling transformer connected in shunt with the power system, and digital control circuits [4]. The main advantage of the STATCOM over the thyristor based SVC 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. With its switching semiconductors and digital control techniques it is among the fastest technologies available for control, capable of instantaneous and continuous control. This ability is especially suitable for dynamic conditions in the power network. The principle of operation of the STATCOM is based on the injection or absorption of reactive current in the point of connection to the power network. Lack or excess of reactive current can be one of the sources for voltage drop or rise. By injecting/absorbing reactive current in the point of connection to the network a voltage drop/rise can be compensated for. V dc V dc, ref v dref, v d i dref, i qref, conv v abc, ref v αβ,,ref v dref, v qref, L f ωl v ab, i ab, v α, β vd i α, β i d ωl Fig. 2. Block diagram of vector control for the STATCOM i q III. SIMULATION STUDY A simulation example with a simplified model of a voltage source converter operated as a STATCOM is implemented and run in the PSCAD/EMTDC simulation tool []. An aggregated model is used for the wind farm with a two masses representation of the wind turbine drive train to represent the influence of the shaft dynamics [2]. The simplified model of the STATCOM neglects switching harmonics. The simulation model is based on the configuration of the Smøla wind farm in Norway with a capacity of 4 MW in its first stage [3], [4]. The rating of the STATCOM is 4 MVAr. Parameters of the main components of the system are given in Table. Without any additional compensation the system operates close to the voltage stability limit, and therefore an extra 8 MVAr capacitor bank at the point of common coupling (PCC) is used for simulations without STATCOM. In the following sub-sections, the stationary, dynamic and transient modes of the STATCOM operation are described with simulation examples, and the response of the system with and without the STATCOM are compared. TABLE I MAIN PARAMETERS OF SIMULATED SYSTEM Wind turbine rated power P n = 2. MW Rated apparent power S n = 2.2 MVA Rated voltage V LL,n = 69 V Stator resistance r s =. pu. Stator leakage inductance x s =.79 pu. Rotor resistance r r =.8 pu. Rotor leakage inductance x r =.74 pu. Magnetizing reactance x m = pu Number of coherent machines n =2 Terminal capacitor compensation Q C/n = 5 kvar Connection transformer rating S T =. P n Transformer series inductance x l =.6 pu STATCOM rating 4 MVAr STATCOM connection inductance x c =. pu STATCOM conduction losses r c =. pu. 26

3 v grid v pcc v pcc with STATCOM v grid v pcc v pcc with STATCOM..4 Voltage [pu].9 Voltage [pu].2 STATCOM Reactive power [MVAr] Fig. 3. Voltage at PCC during swell/sag sequence with constant capacitor compensation and with compensation by STATCOM A. Stationary modes of operation: voltage swell and sag A drop or swell of voltage can be compensated by an injection/absorption of controlled reactive current at the PCC. This may restore the voltage to a level closer to normal operating conditions and within the allowed operating range of the wind farm. For a stationary condition Fig. 3 shows the simulation results when a swell and sag occurs in the grid at maximum power production from the wind farm. The red dashed line indicates the voltage with the controlled injection/absorption or reactive current under a % voltage rise and drop respectively. Compared to the blue dashed line which is the voltage at the same point without the control of the STATCOM, there is a clear improvement of voltage condition after the control of the STATCOM. The black line is shown to indicate the voltage swell and sag that occur on the grid side that is the cause of voltage swell and sag at the PCC. In the same figure we see also the controlled reactive power absorbed and injected by the STATCOM. During the voltage swell the STATCOM absorbs reactive power and during the sag it injects reactive power as can be seen in the figure. The almost instantaneous control response of the STATCOM can also be observed in this figure. B. Dynamic operation: variable wind speed and flicker mitigation Short term fluctuations in power output from the wind turbines will influence the voltage along the grid. If such dynamic voltage variations are to be mitigated, a source of reactive power with higher controllability than mechanically switched capacitors is required, to compensate for the quick changes in reactive power need of the system. Operation of the wind farm at variable wind speed, and with a stationary oscillation in applied torque to the generator, are presented as two examples for illustrating the fast and continuous control capability of the STATCOM Q STATCOM [MVAr] Fig. 4. Voltage at PCC with variable wind speed. Constant capacitor compensation compared to compensation by STATCOM, and corresponding output of reactive power from the STATCOM A simulation with variable wind speed imposed to the wind farm model has been performed, and Fig. 4 shows the voltage at the PCC with and without control of the STATCOM. A quite drastic wind variation is implemented to better illustrate the high band-width control capabilities of the STATCOM. The red dashed line shows the advantage of the fast controllable reactive power over the fixed capacitor bank compensation of the blue dashed line. Without the STATCOM, the voltage is fluctuating around a value above the nominal value, because of the extra capacitors dimensioned for maximum power production. In the same figure we can see the corresponding variable reactive power generated and consumed (absorbed) by the STATCOM to reduce the voltage fluctuations. In addition to the power fluctuations caused by variable wind speed, wind turbines with induction generators directly connected to the grid will produce power fluctuations caused by turbulence and uneven wind shear on the blades. Depending on the design of the wind turbine, its operational conditions and the grid parameters, the voltage fluctuations caused by power pulsations may exceed restrictions on flicker emission [5], [6]. Periodic pulsations caused by blade tower passage can cause peak to peak power oscillations in the range of 2 %. If the pulsations are synchronized for all wind turbines in a wind farm, the total effect can give a significant contribution to voltage oscillations in the grid. Such synchronization is very unlikely to occur for most wind farms [7]. However, if the voltage is controlled synchronization of torque oscillations could be prevented even if such conditions are given. Then the power fluctuations and flicker emission of different wind turbines can be assumed uncorrelated, and the total influence of a wind farm will be approximately proportional to the square root of the number of turbines [6]. 27

4 .2 v grid v pcc v pcc with STATCOM v grid v pcc v pcc with STATCOM Voltage [pu] Fig. 5. Voltage at PCC with and without STATCOM when a stationary torque oscillation is imposed on the generator shaft For illustrational purposes, a simulation with a 5 Hz torque ripple with amplitude of.5 pu imposed on the generator shaft of the aggregated wind farm model is performed. This results in a power fluctuation of about.3 MW at the same frequency. The voltage at the PCC is shown in Fig. 5 for the two cases of only constant capacitor compensation and compensation by STATCOM. It is seen from the figure that the STATCOM is performing both stationary and dynamic voltage control, by increasing the average voltage and reducing the voltage fluctuation. The amplitude of the remaining voltage fluctuations is in this case dependent on the gain of the voltage loop of Fig. 2. If an integral effect, and possibly a slow droop, is included in the voltage control strategy, the voltage could be controlled to a smooth stationary level, almost eliminating the voltage fluctuations caused by the pulsations of input power. It should also be noted that like for the simulation with swell/sag, a voltage control strategy of the STATCOM will compensate also disturbances in the grid that are not caused by the wind farm. C. Transient operation: transient stability and low voltage ride through Power system operators in Europe and other parts of the world are developing or modifying interconnection rules and certification processes for renewable energy sources through a grid code [8], [9]. The new grid codes have introduced new demands when high penetration of renewable energy is expected. One of these demands is the low voltage ride through capability that requires that wind turbines to remain connected to the power network during and after severe network faults. Before the new grid codes for renewables, wind farms would usually disconnect from the power network when the voltage at its terminal drops more than -2 % below rated value [2]. There are various voltage profile requirements for ride through capability in different regions [2], [22]. Voltage [pu] Q STATCOM [MVAr] iq STATCOM [pu] Fig. 6. Ride through for 24 ms three-phase short circuit For illustration purposes, 9 % voltage sag with duration of 24 ms is implemented in the simulation example. As figure 6 shows, the PCC voltage is restored to its initial value with the control of the STATCOM. Without the STATCOM the PCC voltage does not recover the initial value. Ride through capability of the wind farm is achieved with a STATCOM rated 4 MVAr for this specific fault of 24 ms short circuit on the network. In Figure 6 we also observe the reactive power and the current injected by the STATCOM. The given system has a critical clearing time (CCT) [23], found by simulation trial and error, of about 5 ms with constant capacitor compensation. With a 4 MVAr STATCOM, the critical clearing time is about 247 ms. This difference demonstrate that the STATCOM has the capability to increase the transient stability margin of the system, and this larger CCT is the indicator or a better LVRT capability. A method for estimating the stability limit or the CCT from the system parameters and a specified STATCOM current rating is suggested in [24]. IV. EXPERIMENTAL STUDY Results from a laboratory setup with a 5 kw motorgenerator set emulating a wind turbine are presented in this section. Figure 7 shows the experimental set up, consisting of the wind turbine emulator, a weak grid connection, a short circuit device and the STATCOM. An inductance is placed between the induction generator and the PCC, to represent the series reactance of the transformer of Fig.. All the three modes of STATCOM operation can be investigated with this experimental setup, but only results from experiments with LVRT are presented here. 28

5 TABLE II MAIN PARAMETERS OF EXPERIMENTAL SETUP Parameters of induction generator Fig. 7. Layout of laboratory set-up with 5 kw induction generator The main parameters of the experimental setup are presented in Table 2. All per unit values are given on basis of the nominal voltage and nominal volt-ampere rating of the induction generator. The generator is a wound rotor induction machine, with the rotor windings short circuited, and the STATCOM is a two-level IGBT based voltage source converter connected to the PCC through a filter inductance. Rated voltage Rated power Rated apparent power Magnetizing reactance Stator leakage reactance Stator resistance Rotor leakage reactance Rotor resistance Grid side parameters Wind farm series reactance Weak Grid reactance Short circuit reactance Maximum STATCOM current rating STATCOM connection reactance V LL,n = 22 V rms P n = 5 kw S n = 2 kva x h =.85 pu x s =.3 pu r s =.52 pu x r =.3 pu r r =.6 pu x t =.4 pu x g ~.2 pu x sc =.7 pu I ST,max =. pu x c =.23 pu pu STATCOM Phase voltage-pu STATCOM No-control Phase voltage-no-control Grid voltage [pu] P-pu STATCOM Q-pu STATCOM P-No-control Q-No-control Grid power [pu] Speed-pu STATCOM Speed-No-control Generator speed [pu] Time [s] Fig. 8. Experimental results with fault duration of 3 ms and STATCOM current rating of pu compared to no reactive compensation 29

6 Fig. 8 shows the results from two experiments with fault duration of 3 ms; one experiment without voltage control at the PCC, and another experiment where a STATCOM with maximum current rating of pu is connected at the PCC. The first plot in the figure shows one of the phase voltages in per unit and the voltage amplitude, the second plot shows the flow of active and reactive power into the grid, and the third plot shows the speed of the generator. The fault sequence of the two experiments was by coincidence triggered 8 out of phase with each other referred to the grid voltage, and because of this difference, the voltage waveforms appear with half a period of phase shift in the figure. Before the fault, the generator is delivering about.5 pu power to the grid, which corresponds to about kw. The remaining voltage during the short circuit is in the range of.25 pu. From the curves of Fig. 8 it is obvious that the case of 3 ms short circuit without any compensation is unstable, and the generator is not able to return to its initial conditions. The speed continues to increase after the fault is cleared since the generator is not able to deliver the input power to the grid. At the same time, the voltage remains at a too low level, a high amount of reactive power is drawn from the grid. With a STATCOM present, the reactive support enables the generator to re-establish an electromagnetic torque that is larger than the applied mechanical torque, and the generator decelerates back towards its initial speed after the fault is cleared. During the deceleration, a higher power than before the fault is delivered to the grid, and when the speed approaches the initial value, the voltage recovers and the reactive power drawn from the grid is reduced. After some small oscillations in power and speed, the generator returns to its pre-fault condition. The same experiment with fault duration of 3 ms shows a stable behaviour also without any compensation. This means that the non-compensated system is just over the stability limit for a 3 ms fault, and that there is a significant stability margin for the system compensated by the STATCOM. Fig. 9 shows the results from an experiment with the STATCOM limited to.5 pu reactive current, and a fault duration of 35 ms. The general behaviour is the same as for the recovering case of Fig 8, but in this case the system is very close to the stability limit and the CCT with the specified fault. This can be seen by the very slow deceleration of the generator after the fault is cleared, which leads to a much longer recovery time than in the Grid voltage [pu] Statcom current [pu] Grid power [pu] Voltage amplitude Active current Active Power Phase Voltage Reactive current Reactive Power Generator speed [pu] Time [s] Fig. 8. Experimental results with fault duration of 35 ms and STATCOM current rating of.5 pu 22

7 previous experiment. In Fig. 8, also the active and reactive components of the STATCOM current are shown, and it can be seen how the reactive current is on its limit until the voltage is almost recovered. The active component of the STATCOM current is only covering the converter losses, and is therefore very small, although it must be increased when the voltage is low. These curves also show the fundamental advantage of the STATCOM over SVC or passive compensation regarding the ability to maintain the compensating current at low voltages. If the duration of fault is increased to 36 ms, the case with STATCOM limited to.5 pu current, becomes unstable. As an example, this means that with fault duration of 3 ms, where the non-compensated system is close to the stability limit, a system with.5 pu STATCOM will have a transient stability margin of about 5 ms for the same type of fault. Even though there are significant differences in system parameters and ratings, the response of the experimental setup shows the same general behaviour during recovery as the simulation model, for the short circuit faults that have been investigated. V. CONCLUSION The improvement of voltage quality at the point of common coupling of a wind farm by the use of a STATCOM is investigated with simulation and experimental examples. The ability of the STATCOM for voltage control in three different modes of operation is described and the improvement of dynamic and transient behaviour is demonstrated. The transient mode of operation is emphasized, due to the STATCOMs capability to inject constant current at even very low voltage levels. A clear increase of the CCT or the transient stability margin for specified faults, resulting in an improved capability of fault ride through, is achieved both for the simulations and the experimental results. During severe transients the STATCOM will have improved performance compared to mechanically switched or thyristor-based equipment. These are important advantages after the introduction of the new grid codes for renewable energy sources. If the STATCOM is to be selected as the ride through solution for renewable energy sources for a specific case, stationary and dynamic voltage regulation will be fulfilled by the same equipment with equally or improved performance compared to traditional solutions. The performance of the STATCOM will be further compared with the SVC and synchronous condenser under the three operating modes presented here. This will be the subject of further investigation. 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