Summary and Future Trend
As can be seen from the above sections, every grid code reflects the technical features of the individual transmission network. Every network has its own unique technical features to deal with and these special features influence the grid code. The static part of grid code has shown a number of significant aspects including reactive power requirements at PCC as well as continuous voltage and frequency operating range. It has been observed from Table 2 that in several countries such as Germany, Denmark, and Ireland reactive power is ordered based on the amount of real power generation. In other countries, power factor/reactive power requirements have not been related to real power generation. Continuous voltage operation range has been found to be dependent on voltage level of PCC in Germany, India and UK. In other countries, permitted range of voltage deviation has been found mostly to be ± 10 %. RPPs are required to run continuously between 47 and 52 Hz in Australia and this is the widest frequency range of continuous operation among all the countries discussed. The analysis of the dynamic regulations has shown that (except for UK) a WPP under no circumstances is permitted to trip during the first 150 ms of voltage drop due to a nearby fault. Along with WPP other technologies have also been considered in LVRT requirements of UK and Australia. However, HVRT requirements have been defined in only a few countries including Germany, Spain, Denmark, and Australia.
A thorough analysis of the most recent drafts of international grid codes reveals that more rigorous requirements for RPPs are anticipated to be in place in the upcoming years. In particular, WPPs will be enforced to provide the following support to the grid–inject reactive power during faults, imitate synchronizing power feature, and damp power oscillations. TSOs in Germany, Spain, UK, and Australia have already included reactive power injection features in their updated codes. Recent development with variable-speed wind turbines boost flexible reactive power control capability, which is decoupled to their output active power. Hence, TSOs can willingly rely on those WPPs to further maintain the network voltage stability [47, 48].
Because of the back-to-back converter connection to the grid, variable speed wind turbines as well as solar PV plant operation is fundamentally decoupled from the rest of the grid . In a system with a large share of converter based plants, the lack of synchronizing power results in a reduction of total inertia of the system. To address this problem, some TSOs strongly propose WPPs to have inertia response similar to that of conventional synchronous generators. REE in Spain is an example TSO in this regard. Followed by Denmark, Spain had the highest penetration of wind electricity consumption (15.9 %) among EU countries in 2011 . Spanish regulations on the inertia response set by REE are as follows :
(a) The gains of proportional-integral (PI) controller must be set in a way that the active power output can vary by 5 % within 50 ms. It can be mentioned that an extra PI controller has to be fitted in the WPP control scheme, which acts on the input and changes the output power consequently suppressing the fre- quency variations ;
(b) In order to support WPPs, energy storage devices must be used. These devices must inject or absorb at least 10 % of the output active power for 2 s;
(c) Dead-band of frequency variations is equal to ± 10 mHz; and
(d) Inertia response must be disabled for voltage sags below 0.85 p.u.
Ireland and New Zealand are also expected to include similar requirements in their grid codes in the near future.