Dynamic Reactive Power Control for Wind Power Plants Ernst Camm, Charles Edwards, Ken Mattern, Stephen Williams S&C Electric Company, 6601 N. Ridge Blvd, Chicago IL 60626 USA ecamm@sandc.com, cedwards@sandc.om, kmattern@sandc.com, swilliams@sandc.com Abstract Hybrid reactive power compensation systems consisting of inverter-based dynamic compensators and mechanically-switched shunt capacitor and reactor banks provide an economical means of meeting typical grid code requirements for power factor and voltage control. Furthermore, the inverter-based dynamic component of the hybrid compensation system has short-time capabilities on the order of 264 percent of its steady-state rating for up to 3 seconds. These capabilities could reduce the overall reactive power system requirements in wind power plants where all wind turbine generators (WTGs) respond fast enough to allow dynamic power factor capabilities of the WTGs to be utilized to meet the grid code requirements for power factor control. The same short-time capabilities can also assist WTGs in meeting fault ride-through requirements. This paper provides an overview of the capabilities of the hybrid reactive power compensation system, typical configurations and ratings, and application examples. Index Terms grid code, hybrid reactive compensation system, power factor, wind power plant, wind turbine generators. I. INTRODUCTION Modern wind turbine generators (WTGs) have capabilities to dynamically control the power factor or voltage at the terminals of the generators. Grid codes typically require wind power plants to be able to vary their power factor to meet system operating conditions. In Spain the Royal Decrees 436/2004 [1] and 661/2007 [2] determines bonus payments for reactive power between 0.90 inductive and unity, based on an hourly assessment. Payments for the power factor at the point of connection (POC) can be positive or negative, depending on the actual power factor and the time of day as defined in Royal Decree 436/2004 [3] and summarized in Table I. The payment is defined as a percentage of the reference or average tariff and varies from -4% for a power factor of less than 0.95 inductive at peak and flat rate periods to +8% for a power factor of less than 0.95 inductive in valley periods or less than 0.95 capacitive in peak rate periods. In many cases the power factor at the wind power plant s POC is controlled using the dynamic power factor capabilities of the WTGs and a central wind plant management system. In cases where WTGs do not have the capability, switched power factor correction capacitors in the WTGs or medium-voltage mechanically switched capacitor banks in the substation are used to maintain the desired power factor at the POC. TABLE I. BONUS/PENALTY BY DEMAND PERIOD FOR SPANISH WIND POWER PLANTS. 1 Inductive Capacitive Power Factor Bonus/Penalty by demand period [% of reference tariff] Peak (Punta) Flat (Llano) Valley (Valle) <0.95-4 -4 8 0.95 to <0.96-3 0 6 0.96 to <0.97-2 0 4 0.97 to <0.98-1 0 2 0.98 to <1.0 0 2 0 1.0 0 4 0 0.98 to <1.0 0 2 0 0.97 to <0.98 2 0-1 0.96 to <0.97 4 0-2 0.95 to <0.96 6 0-3 <0.95 8-4 -4 II. INVERTER-BASED DYNAMIC COMPENSATORS Voltage source inverter technologies using pulse width modulation techniques to synthesize a voltage either greater than or less than the bus where the inverters are connected are now widely used in wind power plants for power factor or voltage control. See Figure 1. Commercial inverterbased dynamic compensators are available in modules of ±1.25 MVAR at 480 V and having short-term capabilities of 3.3 MVAR per module (i.e. 264% of the continuous rating) for up to 3 seconds. These units have the capability to swing from full inductive to full capacitive output, or vice versa, in about 2 milliseconds based upon the response time of the current control loop. Two modules can be connected to a single 2.5 MVA, 0.48/33 kv step-up transformer for connection to a 33 kv (or other medium voltage) collector substation. See Figure 2. Figure 3 shows two inverter modules for connection to a single step-up transformer. 1 The peak periods last from 11 to 21h in the winter, and from 12 to 22h in the summer. Valley periods last from 0 to 11h and 21 to 24h in the winter, and 0 to 12h and 22 to 24h in the summer. Other times fall in the flat period.
Larger dynamic compensators are comprised of multiple ±2.5 MVAR units with step-up transformers. Sometimes a single ±1.25 MVAR module is connected via its own stepup transformer if the total MVAR rating requires an odd number of inverters. Figure 1. Single-line diagram of ±1.25 MVAR, 480 V voltage source inverter module. Figure 2. Single-line diagram of two ±1.25 MVAR inverter modules and step-up transformer. III. HYBRID REACTIVE POWER COMPENSATORS Hybrid reactive power compensators consist of an inverter-based dynamic compensator and one or more medium-voltage mechanically-switched shunt capacitor banks and reactors, which are normally connecting to the collector bus of the wind power plant. The dynamic compensator can control up to six switched shunt devices (SSDs) and can be configured to control either voltage, reactive power, or power factor at the collector bus or at the POC. The dynamic compensator is typically sized such that the largest capacitor or reactor bank does not exceed about 70 to 75% of the rated total dynamic range (i.e. full inductive to full capacitive MVAR) of the dynamic compensator. The dynamic compensator will typically be configured to switch on a capacitor when it reaches a specified capacitive MVAR output level. Similarly, it will switch off a capacitor or switch on a reactor when it reaches a specified inductive MVAR output level. Figures 4 and 5 show the configuration of a typical hybrid reactive power compensation system and the actual physical arrangement of the same system. In this case the system consists of a ±6.25 MVAR dynamic compensator, two 8 MVAR, 33 kv shunt capacitor banks, and one 8 MVAR shunt reactor. This system provides reactive power in the range of 0.95 leading (inductive) to 0.95 lagging (capacitive) power factor at the 33 kv POC for a 48 MW wind power plant. Figure 3. Two ±1.25 MVAR inverter modules. Figure 4. Typical single-line diagram of a hybrid reactive power compensator.
Figure 5. Actual installation of the hybrid reactive power compensator shown in Figure 4. Collector and utility switchgear are located at the back. The inverters associated with the dynamic reactive power compensator are located in the container on the right. IV. POWER FACTOR CONTROL USING HYBRID REACTIVE POWER COMPENSATORS Hybrid reactive power compensators can be used to dynamically control the power factor at the POC with response times dictated by intentional delays associated with the switching of SSDs in the compensator. Through local collector bus voltage and current sensing and slow feedback of voltage and current at the POC through SCADA, the compensator can dynamically control the power factor at a remote POC using a line drop compensation algorithm. Figure 6 shows actual measurements associated with a hybrid reactive power compensator for a 90 MW wind power plant where the POC is at 345 kv, 37 km away from the collector substation. In this case the compensator is controlling the power factor at the POC at unity (i.e., zero net MVAR at the POC). MW, MVAR 100 80 60 40 20 0 WPP MW INVERTER MVAR WPP NET MVAR -20 0 1 2 3 4 5 6 7 8 9 10 11 Hours Figure 6. Actual hybrid reactive power compensator power factor control performance for a 90 MW wind power plant. Vertical changes in inverter MVAR output indicate where switching of capacitor banks occur. A hybrid reactive power compensator offers several advantages relative to additional switched power factor correction capacitors applied at the WTG low voltage terminals: The power factor of the wind power plant can be dynamically controlled without changing the power factor setpoints of individual WTGs, increasing the overall speed of response. The power factor at the WTG terminals can be independently controlled at or near unity power factor, thereby minimizing reactive current flow through the WTG step-up transformers and the collector system. This effectively reduces active and reactive power losses in the wind power plant. A wind farm management system is not required to control reactive power in the wind power plant. The available reactive power from the compensator can be utilized at all power output levels of the wind power plant, compensating for collector cable charging (capacitive) reactive power at zero and low power output levels or providing reactive power in accordance with a defined daily schedule based on power factor requirements such as those in [3]. Compared to using only mechanically-switched capacitor banks in the collector substation, the hybrid reactive power compensator offers the following advantages: The power factor at the POC can be controlled more precisely over the entire output range of the wind power plant. The number of switching events of capacitor banks is reduced due to the dynamic reactive power range of the inverters, thereby increasing the life of the capacitor bank switching devices. In addition, the number of occurrences of off delays due to capacitor banks discharging is reduced. The dynamic compensator control algorithm distributes the number of switching operations of any single capacitor bank based on historical number of switching operations, resulting in more even wear on capacitor switching devices and prolonged life in multiple capacitor bank applications. The severity of the capacitor-switching transients associated with the energization of the capacitor banks can be reduced slightly by the inverter control action. The same advantages listed in the last two items of the list above apply as well.
V. HARMONIC RESONANCE AND HARMONIC CURRENT INJECTION BY WTGS As in any situation where shunt capacitor banks are applied, the application review of the hybrid reactive power compensator must include a review of potential harmonic resonance conditions. The review will typically include detailed harmonic resonance analysis, including impedance frequency scans for various operating conditions of the wind power plant and harmonic distortion analysis based on representative ambient harmonic levels (i.e. harmonics present in the system without the wind power plant) or harmonic currents injected by WTGs. In cases where WTGs with power factor correction capacitors are involved, careful attention must also be paid to any potential resonance conditions caused by the WTG capacitors. In cases where resonance conditions with high local impedances at characteristic harmonic frequencies (i.e. 5 th, 7 th, 11 th, 13 th, etc. harmonics) are identified, capacitor banks in the hybrid reactive compensation system can be converted to harmonic filter banks. In wind power plants using WTGs with power factor correction capacitors where resonance conditions are caused by the WTG capacitors, a damped C-type filter is commonly used to lower the local impedance of the wind power plant over a wide range of frequencies. See Figures 7 and 8. In wind power plants utilizing WTGs with doubly-fed induction generators (DFIG) or full-converter WTGs with appreciable levels of harmonic current injection, it is sometimes necessary to apply a high-pass filter to prevent some of the harmonic currents from flowing into the system and causing high levels of harmonic voltage and current distortion. See Figure 9. If a hybrid compensator is applied in this situation, one or more of the capacitor banks can be converted to high-pass filters to address this issue. Z 100 10 1 0.1 0.01 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Harmonic number (n) Low Output High Output Damped Figure 8. Wind power plant impedance frequency scan with and without a damped C-type harmonic filter. Figure 9. Single-line diagram of a typical high-pass harmonic filter. VI. SUMMARY Hybrid reactive power compensation systems consisting of inverter-based dynamic compensators and mechanicallyswitched shunt capacitor and reactor banks provide an economical means of meeting typical grid code requirements for power factor and voltage control. The application and associated wind power plant and power system parameters must be carefully reviewed to optimize the design of the hybrid reactive power compensator and to identify any potential resonance conditions that may be caused by WTG power factor correction capacitors or collector substation capacitors applied in the hybrid compensator. REFERENCES Figure 7. Single-line diagram of a typical damped C-type harmonic filter. [1] Secretary of State for Energy, Industrial Development and Small Businesses, Spanish Ministry of Economy, Real Decreto 2818/1998 sobre producción de energía eléctrica por instalaciones abastecidas por recursos o fuentes de energía renovables, residuos ycogeneración, December 23 1998, http://www.mineco.es/energia.
[2] Ministry of Industry, Tourism and Commerce, BOE 126, Royal Decree 661/2007, por el que se regula la actividad de produccion de energia electrica en regimen especial, 25 May 2007. [3] Ministry of Economy, Real Decreto 436/2004, de 12 de marzo, por el que se establece la metodología para la actualización y sistematización del régimen jurídico y económico de la actividad de producción de energía eléctrica en régimen especial