In the Kingdom of Thailand, power-quality regulations applicable to small power producers and very small power producers were first issued in 2008. The regulations specify requirements for steady-state voltage, power factor, frequency, voltage fluctuations, harmonics and direct current.
The Provincial Electricity Authority is responsible for carrying out the on-site power-quality testing of generator installations for all new small power producers and very small power producers prior to the plant entering commercial operation. In late 2012, the first large-scale wind farm in Thailand — the 90-MW FKW project — came on-line followed in early 2013 by the neighboring 90-MW KR2 wind farm. The impact on power quality attributable to these wind farms — the largest in Southeast Asia — required a number of mitigation measures to comply with the power-quality regulations of the Kingdom of Thailand.
Pre-Grid Connection Monitoring
Power-quality monitoring results showed voltage fluctuation was not an issue as the wind turbine generators (WTGs) were decoupled from the grid by a fully rated converter. Also, the WTGs were designed to maintain voltage, power factor, frequency, voltage fluctuation and direct-current injection within acceptable levels. However, the harmonic current emissions and harmonic voltage distortion sometimes failed to comply with the regulation limits.
The total harmonic distortion in the voltage and the fifth harmonic current emission both exceeded the allowable limits under some operating conditions. The total harmonic voltage distortion exceeded the allowable limit during low wind speeds when the power output of the wind farm was between 0% and 30% of the installed capacity. The fifth harmonic voltage was the most significant in terms of exceeding the acceptable limit. The fifth harmonic current exceeded the limit when the wind farm power output was between 0% and 70% of the installed capacity.
The monitoring test results also revealed the fifth harmonic impedance changed dynamically depending on the wind speed and output power of the wind farm as the number of generators connected varied in accordance with the wind speed fluctuations across the wind farm site. Harmonics generated by the voltage source converter-based WTGs did not remain constant but varied according to the converter control and switching scheme.
The Large-Scale Wind Farms
Each of the wind farms comprise 45 Siemens SWT-2.3-101 wind turbines. The 690-V WTG voltage is stepped up to 33 kV, and the transformer is connected to a 33-kV underground cable collector system. This system is connected to the Provincial Electricity Authority’s 115-kV overhead transmission line by two parallel 115/33-kV, 60-MVA power transformers at the wind farm substation. Each wind turbine has a rating of 2.3 MW and an aerodynamic rotor diameter of 101 m (331 ft). An asynchronous generator is decoupled from the grid by a fully rated frequency converter.
A wind turbine with an induction generator directly connected to the grid is not expected to create any significant harmonic distortions during normal operation. However, wind turbines with power electronic converters do produce harmonic current emissions, so the possibility of harmonic voltage distortion must be considered. The harmonic current emission of such wind turbine systems is normally included in the manufacturer’s power-quality data information. The anticipated harmonic voltages can be calculated from the harmonic current emissions of the wind turbine, but this requires knowledge of the grid impedances at different frequencies.
The harmonic signature of a WTG cannot be predicted by mathematical equations such as the Fourier analysis. As a result, it is necessary to investigate the harmonic profiles obtained from field measurements such that some commonalities can be determined for various turbine types and operating under variable conditions. Harmonics have the potential to excite an internal or external resonance point or even destabilize the system operation.
On-Site Monitoring Tests
To study the impact of the wind farms on power quality at all voltage levels, power quality meters were installed at four locations, namely at the point of common coupling (PCC) at 115 kV, the 33-kV collector system, and the input and output terminals of the 33/0.69-kV wind turbine transformer. The measurement recorder confirmed the active power at the PCC was proportional to the number of WTGs, while the reactive power from the WTGs was not proportional to the number of turbines. This varies as the reactive power at the PCC is controlled with a closed-loop controller, and the reactive power output of the WTG is varied to achieve the set point target at the PCC.
Three modes are available to control reactive power at the PCC: reactive power control mode, voltage control mode and power factor mode. The simplest strategy for the wind farm is to operate in the reactive power control mode with a 0-MVar set point to maintain unity power factor. In this mode, the wind farm will not export or consume reactive power when the turbines are operating. However, when this control strategy was adopted, there were some steady-state overvoltage problems at high active power output levels because the output reactive power of the wind farm was controlled to 0 MVar and used reactive power measured at the 115-kV side of the wind farm transformers as the feedback signal.
With this control strategy, if the wind speed is high enough for the wind turbines to go on-line, the converter imports reactive power, compensating for the capacitance of the underground cables in the collector system, to try to control the reactive power to 0 MVar at the PCC. If the wind is low, there may be only a few wind turbines on-line and the wind farm may export reactive power (<-4 MVar). If there is no wind, and the wind turbines are off-line, the quiescent reactive output from the wind farm as a result of the underground cables is around -4.0 MVar and the voltage is not actively controlled by the wind farm. In this situation, the voltage at the PCC may exceed the grid code limit of 1.05 p.u. (120.75 kV).
Prior to Jan. 18, 2013, the wind farm always supplied reactive power to the utility, but following a change in the control mode from constant reactive power control to voltage control with a target voltage of 1.03 p.u. (118.4 kV), the wind farm supplied and absorbed reactive power from the utility. The results recorded indicated about +6.8 MVar was absorbed during maximum active power output generation and -4.1 MVar was supplied during low active power output generation.
Operationally, when the voltage fell below the target, the reactive export increased to support the voltage. When the voltage rose above the target, the reactive power import increased to reduce the voltage. With the wind farm operating in the voltage control mode, the steady-state voltage remained below the allowable maximum of 1.05 p.u.
The variable-speed wind turbines with fully rated frequency converters are capable of controlling the output of active and reactive power. It is possible to control the output reactive power appropriately with the variation of the output real power, so voltage changes from the real power flow may be compensated by the reactive power flow, minimizing the flicker emission.
The results recorded from Jan. 1, 2013, to Jan. 1, 2014, confirmed CP95 of the short-term flicker severity (CP95 of Pst=0.22) and long-term flicker severity (CP95 of Plt=0.46) — as per standard EN 50160 Voltage Characteristics in Public Distribution Systems, issued by the European Committee for Electrotechnical Standardization — complied with the limits in the regulations.
In the regulations, limits are specified for the total harmonic distortion in voltage and individual harmonic current emissions. However, on-site monitoring at the 115-kV PCC confirmed the harmonic distortion (THDv = 2.24%) and the fifth harmonic current emission (4.51 A) failed to comply with the regulations.
The results recorded from Jan. 1 through Jan. 31, 2013, showed the total harmonic distortion in voltage exceeded the limit at low wind speeds when the wind farm power output was between 0% and 30% of the installed capacity. The fifth harmonic current exceeded the limits for about 80% of the period when the wind farm output power was between 0% and 70% of installed capacity. These characteristics are attributable to the fifth harmonic impedance that changes dynamically depending on the wind speed and power output of the wind farm.
Harmonics generated by voltage source converter-based WTGs do not remain constant but will vary according to the converter control and switching scheme. To mitigate the harmonics issues, a fifth harmonic filter was installed downstream of one of the feeder circuit breakers supplying the 33-kV bus bar. These passive harmonic filters, which were retrofitted to existing substations, have mitigated the harmonics emissions successfully from the wind farms, allowing them to operate in compliance with the Kingdom of Thailand’s power-quality regulations.
Government policy in Thailand is for renewable energy and alternative energy sources to account for 25% of the installed generation within the next 10 years. According to the country’s 2010 Power Development Plan Revision 3, the total installed generation capacity by the end of 2030 will be around 20,500 MW, including 3,800 MW of wind energy and 2,000 MW of solar energy, some 29% of the total generating capacity.
With the increasing penetration of renewable energy, the impact of power quality from the renewable generators will become increasingly important. Experience from the first two wind farm projects has demonstrated, with the appropriate design, negative impacts on power quality can be mitigated successfully.
Chakphed Madtharad ([email protected]) graduated with a Ph.D. degree in electrical engineering from Chiang Mai University, Thailand, in collaboration with the University of Canterbury, New Zealand. He currently works in the smart grid planning division of the Provincial Electricity Authority, where his responsibilities include harmonics and power quality, power electronics, power system smart grids and microgrids.
Jeremy Warman ([email protected]) was awarded a ME degree in electrical engineering from the University of Canterbury, New Zealand. He currently works for LR Senergy in Melbourne, Australia. Warman’s interests include harmonics and power quality associated with wind farms and renewable energy integration.