t is well known that to keep a consistent frequency in the power network (normally 50 or 60 Hz), the active power supply should be identical to the load demand, at any given time. In case power imbalance happens in the utility grid, the frequency deviates from its nominal value. Therefore, in a power system, traditional generating units are normally equipped with a governor control, which works as a primary load frequency control during the imbalance situation.
This control should be activated directly to prevent the frequency from large deviation.69 Over the recent days, the distribution generators such as PVPPs have been replacing these conventional generators slowly.70 Recent studies confirm that with a high penetration level, large frequency deviation and instability problems are expected.44, 71, 72 Thus, some of the modern GCs require the next generation of PV inverters to provide the capability of frequency control with a view to ensure network stability.
The frequency control can be realized by decreasing the amount of active power output injected to the grid known as APC capability. Figure 9 shows the typical APC (power-frequency) curve of PVPPs, which explains that in case the system frequency increased, active power reduction is required.17 Although the capability of APC requirement is very important for frequency stability because of the high penetration of PVPPs, this issue is not defined in some GCs as the related research is at an early stage. In addition, in some countries, the APC is left to the transmission or distribution system operators, such as South Africa.73, 74 The APC had been defined in the German GC, which stipulated that when the network frequency exceeds 50.
2 until 51.5 Hz, PVPPs have to reduce the generated active power (P) with a gradient of 40% per Hertz of the available PV power, as illustrated in Figure 10. In case the frequency is under 50.05 Hz, the available active power must rise again without delay.18, 75 The US-PREPA regulations require PVPPs to regulate the frequency deviation via APC to the drop characteristic of 5%, which is applicable for traditional generators.26 The new Malaysian standards states that the PVPPs must remain in continuous operation mode as long as the frequency range is between 47 and 50.5 Hz. Nevertheless, during overfrequency events, if the system frequency exceeds 50.
5 Hz, the PVPP is expected to reduce its active power with a gradient of 40% per Hertz.32 In the case of China, the country does not mandate any APC and just requires the PVPP to withstand the overfrequency event (50.2-50.5 Hz) within 2 minutes.
Otherwise, the trip is mandatory. As a result, the most restricting requirements are imposed by the German GC. In the near future, it is anticipated that APC could be a requirement for other nations as more renewable energy is being added increasingly.