Discussion on presented frequency support strategies

De-loading WTs to provide a primary reserve for the grid will be considered in some countries with high wind penetration in the grid. De-loading by additional pitch angle has minimum adverse effect in comparison with the over-speeding method. In FSPC strategy presented in section 5.3.1.2, the pitch angle deviation is adjusted according to the operating

5 Frequency support by VSC-HVDC connected offshore WTs 83

point and once a frequency deviation is detected in the grid the pitch angle will be reduced to provide a frequency support to the grid. The time constant of the provided washout filter controls the duration of the frequency support by the WT. Accordingly, the active power (pitch angle) is ramped back to the original value.

If a fast inertial response support is required from the WTs which may be difficult to achieve by pitch actuators, the kinetic energy in the WT rotor can be extracted for fast but temporary frequency support. KEC I and KEC II methods were presented in sections 5.3.2.1 and 5.3.2.2, both strategies employ a lead-lag compensator that must be tuned properly. The main disadvantage of KEC I is that it requires adjusting the speed controller in the WT to avoid any adverse effects during the recovery (reacceleration) phase after the rotor speed drops. This problem is more present during under nominal speed operation for high wind power share in the grid. KEC II does not require any adjustments on the speed controller of the WT and shows a very good results for frequency support during low wind (under nominal) speed operation. For the case of high wind speed operation it was shown that the pitch controller will adversely interact with the control action of KEC II, this problem was solved by simply blocking the pitch control during the frequency support period. Additionally a slight change in the dynamic overload region of the WT power-speed characteristics can enhance significantly the performance of KEC II during high wind speed operation.

A combined strategy of FSPC and KEC provide further enhancement of the frequency response. Based solely on the results in Figure 5-22 and Figure 5-23 the combination of KEC I and FSPC offers the best option for frequency support, nevertheless, the fact that KEC II does not require any adjustments on the WT speed controller and provide more uniform behavior for different wind speeds might make the choice of the combined KEC II and FSPC strategy more attractive in practice.

Control parameter tuning in KEC methods is usually based on the assumption that the shape of the primary response of the grid frequency does not change significantly during different load flow scenarios and wind fluctuations. Such assumption is invalid in reality which makes the application of these methods more challenging in real grid with fluctuating wind speeds and variable wind power share. Additionally, the operating point of one single WT does not necessary correlate with the total wind power share in the grid, in other words a couple of WTs in a WF can be operating with lower wind speeds while the rest are operating with high wind speeds. If part of the WTs in the WF is operating with low wind speeds, their frequency support behavior could conflict with the other WTs operating with clearly higher wind speeds the WF. Therefore, the selectivity should be available to decide which WTs should participate

5 Frequency support by VSC-HVDC connected offshore WTs 84

in the frequency support in order to optimize the overall contribution. Wake effect and wind variation during the primary frequency response phase are additional factors that should be considered for more accurate analysis. It can be concluded that due to the above mentioned uncertainties of the KEC strategies performance in real applications, such control methods do not offer the grid operator a 100% reliable solution for supporting the grid frequency.

6 Overfrequency limiting by VSC-HVDC connected offshore WTs 85

6 Overfrequency limiting by VSC-HVDC connected

offshore WTs

Overfrequency caused by a sudden loss of load or energy excess in a grid is a scenario that is not properly addressed in literature. Most of the studies on primary frequency response in electrical grids focus on the common case of loss of generation which causes a certain frequency drop. However in certain grid configurations, the case of overfrequency due to loss of load or surplus energy generation is a possible scenario that should be carefully investigated. Figure 6-1 shows the newly planned AC corridors to transfer the energy generated from the large offshore WFs in the northern sea of Germany to the load centers in the southern part. Any disturbance event that requires a disconnection in the transmission lines would mean that the northern part of the grid will be isolated from the southern part. As a result, a frequency rise will occur in the northern part due to the excess generation from the offshore WFs. 0 20 40 60 80 50 F re q u en cy ( H z) Time (s) max

f

Offshore WFs planned and operating with subsea HVDC cables 0 20 40 60 80 50 F re q u en cy ( H z) Time (s) 20 40 60 50 F re q u en cy ( H z) Time (s)

Figure 6-1: Planned HVDC transmission corridors in Germany to transfer the generated offshore wind power in the north to the main loads in south [52]

This chapter aims to answer the following questions:

 How can the offshore WF limit the frequency rise for this possible scenario?

 Are the offshore WF capable of reducing its output power fast enough to avoid high frequency overshoots?

6 Overfrequency limiting by VSC-HVDC connected offshore WTs 86  Can the KEC strategies discussed in section 5.3.2 be applied for overfrequency limiting?

 Can the VSC-HVDC connection offer any alternatives or enhancements for the Overfrequency limiting control?