Wind Power Integration in Isolated Grids enabled by Variable Speed Pumped Storage Hydropower Plant

Transcription

1 ICSET 008 Wind Power Integration in Isolated Grids enabled by Variable Speed Pumped Storage Hydropower Plant Jon Are Suul, Kjetil Uhlen, Member IEEE, Tore Undeland, Fellow IEEE Abstract This paper presents aspects of control and operation of a variable speed pumped storage power plant for integration of wind power in an isolated grid. A topology based on a synchronous machine and a full scale back-to-back voltage source converter is suggested for variable speed operation of the pump-turbine. With this topology, variable speed operation can be obtained in both pumping mode and generating mode, and reactive current can be controlled independently of the active power flow and the operation of the pump turbine. By utilizing the controllability of the variable speed system, power fluctuations from wind turbines can be compensated to limit the influence on the rest of the grid. At the same time the pumped storage can be controlled to take part in the frequency control of the system and also to control grid voltage or flow of reactive power. The operation of the proposed system is illustrated by simulations based on the situation on the Faroe Islands, where controllable energy storage could allow for higher penetration of renewable energy in the power system and by that reduced dependency on power generation based on fossil fuels. Keywords Pumped storage, wind power, isolated grid, power balancing, reactive power, energy storage, synchronous machine, voltage source converter. I. INTRODUCTION Integrating large shares of fluctuating renewable energy sources into electric power systems requires sufficient range of power control in production units and loads. In many cases, additional energy storage will be needed to ensure the power balance and maintain stable operation of the grid [1]. Islands and isolated power systems will be the first locations where it will be relevant and possible to achieve sustainable energy systems based entirely on environmental-friendly and renewable energy sources, and can therefore serve as good examples and test-cases. []. With increasing share of fluctuating renewable energy sources in an isolated power system, the use of pumped storage hydropower plants as energy storage can be among the most relevant alternatives with respect to feasibility and cost when suitable locations are available [3]. A few examples of such systems have been put into operation, and others are under development [4]-[9]. J. A. Suul is with the Department of Electric Power Engineering, Norwegian University of Science and Technology, 7491 Trondheim, Norway (Phone: ; fax: ; jon.are.suul@elkraft.ntnu.no). K. Uhlen is with SINTEF Energy Research, 7465 Trondheim, Norway ( Kjetil.Uhlen@sintef.no ). T. Undeland is with the Department of Electric Power Engineering, Norwegian University of Science and Technology, 7491 Trondheim, Norway ( Tore.Undeland@elkraft.ntnu.no). Operation of conventional pumped storage units with constant power in pumping mode will mainly help to improve the energy balance of an isolated power system. One way of introducing additional controllability so that the pumped storage can contribute to the steady state frequency control of the system will be to use several separate pumps, so that the load in pumping mode can be controlled in steps. The most flexible and effective solution will be variable speed operation, such that the input power in pumping mode can be controlled continuously within an allowable range of operation [3]. If a pumped storage hydropower plant with reversible pump-turbine is designed for variable speed operation, the controllability of the system can be utilized to balance power fluctuations from renewable energy sources both in pumping and generating mode [10]. The development of variable speed pumped storage power plants have until now been mainly focused on units for energy storage, load balancing and stability enhancement in large power systems. For such applications, the doublyfed asynchronous machine has been preferred, to limit the needed rating on the power electronic converter required for obtaining variable speed operation [11], [1]. Variable speed operation can be even more important in an isolated grid than in a larger power system, since introduction of fluctuating power production in a small system can represent more challenges to the grid operation. For lower power ratings and applications in isolated grids, full-scale converter topologies can be relevant for control of pumped storage units. The highest flexibility in control of active and reactive power can be obtained with a full-scale voltage source converter (VSC), but few applications of this topology for pumped storage units have been reported. The main objectives of this paper are therefore: 1. To present a configuration for variable speed operation of a pumped storage hydropower plant based on a synchronous machine with a full-scale back-to-back voltage source converter, and to discuss aspects of control and operation of this topology.. To investigate control strategies for the pumped storage unit that can enable higher wind power penetration in an isolated grid by compensating for active and reactive power fluctuations, and to illustrate the operation by simulations based on the power system of the Faroe Islands /08/$5.00 c 008 IEEE 46

2 II. PROPOSED CONFIGURATION FOR VARIABLE SPEED PUMPED STORAGE POWER PLANT Some of the early investigations into variable speed operation of reversible pump-turbines were based on synchronous machines with thyristor-controlled current source converters feeding the stator of the machine [13]. Only few plants with this topology have been constructed, and the majority of existing variable speed pumped storage units is based on doubly-fed asynchronous machines [1]. For applications in small isolated power systems, the ratings can be much lower compared to the large units with doublyfed machines. With the continuous development of forcecommutated semiconductors and high-power VSC drives, configurations with a back to back VSC for controlling a synchronous machine can therefore be relevant for variable speed operation of pumped storage units in isolated grids [10], [14], [15]. A schematic layout of this proposed configuration, including an overview of the suggested control system is shown in Fig. 1. The machine is considered to be a salient pole synchronous machine with static excitation system that can be operated both by the converter and with direct connection to the grid like a traditional power station, as indicated by the bypass switch in the figure. A. Operational characteristics of proposed configuration The configuration presented in Fig. 1 provides full flexibility in control of active and reactive power on the grid side, and the system can operate with variable speed both in pumping mode and in generating mode. The main motivation for introducing such a configuration will be to allow for controllable power in pumping mode, but the controllability and the speed of the response when delivering power to the grid can also be improved by variable speed operation. By controlling the reactive current component from the converter, also the grid voltage or the reactive power flow in the grid can be controlled by the presented configuration. This ability can be utilized even if the pumpturbine is not in operation, and the grid side converter will then be operating as a STATCOM. The configuration in Fig. 1 is also introducing extra redundancy to the basic operation of the system compared to a doubly fed asynchronous machine, since the power plant can be operated directly connected to the grid in generating mode. That also means that the power station can be used for a traditional black start of the system while the converter is out of operation. On the other hand, the presented configuration can make it possible to operate the pumped storage with variable speed even without any controlled production units based on synchronous generators operating in the system. In such a situation the converter for the pumped storage will have to control both the voltage and the grid frequency in stand-alone mode B. Description of control system The main structure of the control system for operating the suggested configuration in pumping mode is included in Fig. 1. The figure shows how the grid side converter, connected to the main transformer through a LC-filter, can be controlled by a traditional voltage oriented vector current control system in a synchronously rotating dq-reference frame. The estimate of the voltage phase angle used for the park transformation is obtained by a PLL that is also tracking the grid frequency and the voltage components in the rotating reference frame [16]. The d-axis of the rotating reference frame is aligned with the grid voltage vector, and the q-axis is leading the d-axis by 90. The current controllers can be PI-controllers in the rotating reference frame with feed-forward from measured grid voltage and decoupling terms depending on the filter inductance and the grid frequency [17], [18]. To avoid oscillations in the LCfilter, an active damping routine can be added to the function of the current controllers [19]. The output from the current controllers is divided by the DC-link voltage to decouple the current controllers from the dynamics of the DC-link. After transformation into phase coordinates and adding third harmonic injection, the reference voltages are used for PWM modulation of the switches of the converter. Grid C 1 LC-filter V g L 1 I g V I n 3 PWM V DC PWM 3 3 i f PLL g v dq, f g Grid side measurements i dq, v,,ref v dqref,, Current controllers i dref, i qref, f g P wind DC-link controller V/Q controller Power control p set f ref v g, dq VQ, grid p ref Torque control n s Flux control i m s i lref, s, ref i mref, i mref, p ref Power factor control im i f s i f, ref i dqref,, v,,ref v dqref,, Current controllers Current estimation i ml, & s Flux calculation i f n dq, i f i dq, n v, v dq, i, i dq, v f, ref Fig. 1. Suggested configuration for variable speed operation of pumped storage with control system for operation in pumping mode 463

3 Since the d-axis is aligned with the voltage vector, the input reference to the d-axis current controller is generated by an outer loop DC-link voltage controller that is maintaining the power balance of the system. The q-axis current reference can be generated by an outer loop controller for grid voltage or flow of reactive power. The grid frequency from the PLL is also used for the power control of the pump turbine. Different structures for controlling the power flow of the pumped storage system in pumping mode, and for generating the power reference to the drive system of the synchronous machine are discussed in [15]. The details of the drive system of the synchronous machine are not of main importance to the characteristics of the pumped storage system as seen from the grid if the response is fast and precise. In this paper a similar vector control structure as for the grid side converter is used, but since the machine has salient poles, a stator flux oriented mlreference frame is used for the control of torque and flux while a rotor oriented dq-reference frame is used for the current controllers [10], [0]. Basically the same control structure can be used for controlling the synchronous machine drive in generating mode, but an additional speed controller and the hydraulic control system of the turbine will have to be included in the model. III. COMPENSATING FOR WIND POWER FLUCTUATIONS IN AN ISOLATED GRID The controllability of a variable speed pumped storage unit can be utilized to compensate for the influence of power fluctuations in the output from renewable energy sources like wind power, so that a higher share of fluctuating power production can be allowed in the system. If the pumped storage unit is located close to a dominating source of power fluctuations like a wind farm, the variations in output power can be compensated directly based on measurements of the power flow. The pumped storage can be further utilized to take part in the primary frequency control of the system, and by that helping to improve the response of the system to other disturbances and sudden changes in production and load [10], [15]. With the presented topology the pumped storage system can also be used for controlling the grid voltage or the flow of reactive power in the system. A. Case study The power system on the Faroe Islands is taken as an example for a case study of an isolated system that can significantly benefit from a combination of wind power production and a variable speed pumped storage power plant [10], [15], [1]. The number of inhabitants on the Faroe Islands is around 48000, and the electric power system is currently dominated by diesel generator units supplying around 60% of the annual electricity consumption. The minimum load of the system is in the range of 14 MW, while the maximum load can reach 70 MW, and a pumped storage power plant in the power range around 10 MW can Diesel generators 6.6 kv 66 kv Z 1 66 kv kv 3.3 kv Hydro 60 MVA C d C d,1,1 5 MVA 5 MVA 5 MVA C d Load, 60 MVA Wind farm IM 690 V 10 MW C IM kv Load kv 1 km cable Z l3 C d,3 C d,3 PQ wind be considered relevant for practical implementation [10], [14]. A few wind turbine units have been introduced to the system, but if the dependency on fossil fuels for the electricity generation is going to be reduced by utilizing the available wind resources, there will be a need for energy storage and more controllability to stabilize the system [10], [1]. The most challenging situations for this system will be operation at minimum load when there is a high average power production from wind turbines, and this situation will be the focus for this investigation The operation of the proposed variable speed pumped storage system is illustrated by simulations based on a model in PSCAD/EMTDC of the main parts of the power system on the Faroe Islands, including the proposed converter topology and the corresponding control system of the pumped storage unit. The basic configuration of the isolated grid, including the voltage levels and the ratings of difference units is shown in Fig. while some more details are given in Table 1. The system is simulated for 80 seconds and the wind speed input used for simulation of the wind farm is based on use of the Kaimal power spectra developed for PSCAD simulation from [], [3]. The power output from the wind turbine model is not changing much with the operation of the pumped storage, and can be considered equal to the power series given in Fig. 3 for all investigated situations. After 40 seconds of simulation, the hydropower plant is tripped without reconnecting to the system. C d, Z PCC kv 3.3 kv PQ grid 3 km cable Pumped storage Fig.. Simplified grid model representing the main part of the power system on the Faroe Islands Wind farm Hydropower plant Diesel generators Pumped storage TABLE 1SYSTEM PARAMETERS - 10 MW aggregated model - Induction generators directly connected to the grid - Constant capacitors for reactive power compensation - Power set-point 0,8 pu = 4MW - Static droop; 5 pu =.5 MW/Hz - 18 MW aggregated model - Power set-point 0.7 pu = 1,6 MW - Static droop; 5 pu = 9 MW/Hz - Power control range 4-1 MW - Static droop; 5 pu = 5 MW/Hz 464

4 Wind power output [MW] Fig. 3. Power output from the wind farm Pump turbine speed [pu] Constant power Load following Droop with derivative term and load following B. Power control The possible influence on the rest of the power system by controlling the power input to the variable speed pumped storage power plant is illustrated by using three different control strategies based on [15]: 1. Constant power input to the pumped storage system as a reference case. Load following, where measured power fluctuations from the wind farm are directly compensated by the pumped storage power plant 3. Droop control based on grid frequency with a derivative term for higher transient frequency response and to damp oscillations in the system, combined with load following. The response of the diesel generators in the system to the wind power series from Fig. 3 is shown in Fig. 4. It can be seen that with constant power to the pumped storage, the diesel generators have to cover most of the power fluctuations from the wind farm, resulting in a wide power operating range. The diesel generators will in this case also have to cover all the loss of production when the small hydropower plant in the system is tripped. Controlling the pumped storage power plant to balance the power fluctuations from the wind turbine, it can be seen that the diesel generators are relieved from covering most of the power fluctuation, but that they still have to cover all the loss of production when the hydropower plant is tripped Fig. 5. Speed of pump-turbine with different power control strategies Adding a frequency droop to the power control of the pumped storage system, the diesel generators are relieved also from some of the steady state frequency control, and the derivative term added to the frequency control is damping the remaining power oscillations in the system. The response in speed of the pump-turbine of the pumped storage is shown in Fig. 5. This figure shows how the short term power fluctuations from the wind farm are filtered by the large inertia of the generator and the pump-turbine, so that mainly the slower power variations are reflected in the speed of the system. The grid frequency shown in Fig. 6 show how the rest of the system is relieved from the influence of the power output from the wind turbines when the fluctuations are compensated by the pumped storage system. It can also be seen how the frequency response of the power system is improved when the pumped storage is used for frequency control. The results in Fig. 5 and Fig. 6 indicate how the control of the pumped storage can limit the necessary operating range of the diesel generators, and by that also limit the fluctuations in grid frequency. This can allow for having less diesel generator capacity on line, and since the remaining units in operation can be operated at a 18 Constant power Load following Droop with derivative term and load following Constant power Load following Droop with derivative term and load following Diesel power [MW] Fig. 4. Power output from diesel generators with different power control strategies for the pumped storage power plant Grid Frequency [Hz] Fig. 6. Grid frequency with different power control strategies for the pumped storage power plant 465

5 higher average load, the efficiency of the diesel generators can be increased, and this will lead to further reduction in the fuel consumption of the electricity supply. C. Voltage and reactive power control With the back-to-back voltage source converter, the reactive current on the grid side can be controlled independently of the active power flow and used for controlling reactive power flow in the grid or for taking part in the voltage control. In contrast to frequency, that can be considered a global variable in steady state, the grid voltage is a local variable, and the design of the control loops for voltage or reactive power will therefore be dependent on the configuration of the local grid. The control objective can also be different depending on what kind of grid the system is located in and what kind of challenges that are more critical. The presented topology has the flexibility to easily implement different control structures for voltage or reactive power operating within the limit of total converter current. In the investigated model, the grid is mainly consisting of high voltage cables, and is therefore quite strong with respect to voltage. Still voltage flicker can be a problem with wind power integration, and mitigation of voltage fluctuations can therefore be important. The influence of different voltage control strategies on the grid voltage, and the flow of reactive power, is illustrated with the simulations shown in Fig. 7 and Fig. 8. The simulations are carried out with the 3 rd power control strategy from section III B. and the results with four different control strategies are shown in the figures: 1. Zero reactive current in the converter. Grid voltage controlled to 1.0 pu by PI-controller. 3. Reactive power flow to the grid from the point of common coupling (PCC) in Fig is controlled to 0 by PI-controller. 4. Grid voltage control with PI-controller and droop from filtered reactive current on the voltage reference. With reactive current from the converter controlled to zero, it can be seen that the voltage is fluctuating with the power variations from the wind turbines, and that these fluctuations in reactive power have to be provided by the other generators in the grid. If the grid voltage is controlled to 1.0 pu, the converter for the pumped storage is supplying the reactive power consumed by the wind farm, and also delivering reactive power to the grid to boost the voltage. The presented figures also show how the reactive power exchange with the grid can be controlled directly, and that the converter in this way can easily compensate all the reactive power fluctuations caused by the wind farm. The voltage in the grid is also stabilized at a level close to the rated value, depending on the characteristics and the response of the rest of the system. Since pure integral effect in outer loop controllers can lead to unintended interaction between different equipment, steady state droop characteristics should be allowed for the Grid voltage [pu] Zero reactive current Control of grid voltage Control of reactive power to the grid Grid voltage control with slow droop 0.95 Fig. 7. Grid voltage at PCC with different strategies for control of voltage or reactive power voltage or reactive power controller. In Fig. 7 and Fig. 8, one case is shown where reactive current from the converter is filtered and used to generate the droop. It is seen from the figures how this makes it possible to mitigate the short term fluctuations in voltage and reactive power flow, while the system has a droop characteristic for the longer term voltage variations. The voltage source converter is providing full flexibility with respect to control of voltage or reactive power, and the functionality and response of the system can easily be designed according to the most relevant control objectives for a specific implementation. IV. CONCLUSION A topology for variable speed operation of a pumped storage hydropower plant based on a full-scale back-to-back voltage source converter driving a synchronous machine is suggested for balancing of wind power fluctuations in an isolated grid. With the proposed topology, the pumped storage unit can also be operated with the machine directly connected to the grid, running at constant speed like a conventional power plant, such that the operation of the system will be less dependent on the converter reliability. Reactive power to the grid [MVAr] Zero reactive current Control of grid voltage Control of reactive power to the grid Grid voltage control with slow droop -4 Fig. 8. Reactive power flow from PCC to the grid 466

6 The back-to-back converter provides full flexibility in control of active and reactive power, and the system can be operated with variable speed in both pumping and generating mode, while grid voltage or reactive power can be controlled independently from the operation of the pumpturbine. The operation and control of the suggested topology is illustrated by simulating a grid model based on the power system of the Faroe Islands. The presented results show how the variable speed pumped storage can be controlled to limit the influence of wind power fluctuations and at the same time contribute to increased frequency response of the power system. The simulation results therefore indicate how the variable speed pumped storage can allow for a more wind power to be introduced to an isolated grid without undermining the frequency control and the instantaneous power balance of the system. It is also shown how the suggested topology can be utilized to control the grid voltage or the flow of reactive power in the system. Controlling the grid voltage, or compensating for the fluctuating reactive power consumption of a wind farm, can allow for a better distribution of reactive power flow in the system and by that reducing the power losses. Voltage control by the grid side converter can also mitigate possible power quality problems related to voltage flicker and improve the voltage stability of the system. The controllability introduced by the suggested configuration for a pumped storage power plant can in this way be used both to compensate for the consequences of fluctuating power production from wind turbines and to improve the general operation of an isolated power system. This can make it possible to allow for higher wind power penetration and significantly reduce the dependency on fossil fuels for electricity production in isolated power systems. 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