Results

The most important simulation parameters are listed in Table 3.3. A power system of150 GW is considered with a loss of 3 GW of production. This is a good representation of the reference incident in the European power sys-tem [10]. Wind power penetrations of 15 % and 50 % are considered, where conventional steam-based generators and CCGTs are replaced by wind tur-bines to obtain these penetrations. The primary frequency response is still provided by the conventional generators, the wind turbines only provide inertial response. The frequency nadir fmin and the initial rate of change of frequencydf /dtinit averaged over the first 2 s are calculated for a range of Kin and Kdroop. For each case, 9234 gain combinations (57 values for Kdroop and 162 values for Kin) are tested. The droop constant Kdroop

Param. Value Unit Param. Value Unit

Pt,nom 3.0 MW Pload 150 GW

v 10.0 m/s Preserve 6 GW

fnom 50 Hz Pdisturb 3.0 GW

Th 25 s Tl 0.64 s

Ωmin 0.85 rad/s Ωmax 2 rad/s

Case 1 Case 2

Pnuc 25 % Pnuc 25 %

Psteam 0 % Psteam 60 %

Pccgt 60 % Pccgt 0 %

Pwind 15 % Pwind 15 %

Case 3 Case 4

Pnuc 25 % Pnuc 25 %

Psteam 0 % Psteam 25 %

Pccgt 25 % Pccgt 0 %

Pwind 50 % Pwind 50 %

No wind turbines

Pnuc 25 % Pccgt 0− 75 %

Pwind 0 % Psteam 75− 0 %

Table 3.3: Simulation parameters.

is varied between 0 and 2.7 MW/Hz, whereas Kin varies between 0 and 7.8 MWs/Hz. The frequency nadir and ROCOF values are compared with the values obtained for the power system without wind turbines. It is indeed more interesting to compare this frequency response to the power system without wind turbines than to the power system with wind turbines with-out synthetic inertia as the frequency response is always worse for the latter due to the lower system inertia. This can be seen in the simulations for all different cases. Consequently, the system without any wind turbines is used as reference situation. The desired result is a high frequency nadir and a low ROCOF. The objective is to obtain a frequency response with the synthetic inertia strategy which is as good as or even better than the frequency response for the system without wind turbines.

3.6.4.3 Case 1: Low wind power penetration, 60 % CCGT The first case has a wind power penetration of 15 % and consists of 60 % CCGTs and 0 % steam-based power plants. This means that all primary control is provided by fast reacting CCGTs. The frequency nadir and

RO

Figure 3.13: Case 1: Frequency nadir and ROCOF for a wind power penetration of 15 % and 60 % CCGT.

COF are shown in Fig. 3.13. The thick black lines represent the values of fmin and df /dtinit for the equivalent power system without wind turbines, whereas the contour plots represent the values offmin anddf /dtinit for the system with wind turbines equipped with the synthetic inertia strategy for different values ofKin andKdroop.

Considering only the frequency nadir, the optimal parameters (i.e., re-sulting in the highest frequency nadir) areKin= 0 MWs/Hz and Kdroop= 0.45 MW/Hz. Furthermore, all the parameter combinations which have a frequency nadir above the value forfmin without wind turbines result in a better frequency response than the case without wind turbines (area inside the thick black curve, indicated by the plus sign in Fig. 3.13).

As expected, the ROCOF decreases for increasing values of Kin and Kdroop, which is the desired result. A lower ROCOF gives the generators participating in the primary control more time to adapt their power output.

High values forKin andKdroop would, however, result in a low frequency

0 5 10 15 20 25 30 35 40 45 50

Figure 3.14: Case 1: Frequency response for different scenarios and wind turbine behavior for SI. Optimal parameters for WT with synthetic inertia: Kin= 0 MWs/Hz, Kdroop= 0.45 MW/Hz.

nadir, which is unacceptable. All parameter combinations which have a ROCOF below the value for df /dtinit without wind turbines are indicated by the plus sign in Fig. 3.13. Consequently, it is clear from Fig. 3.13 that only parameters near the origin (Kin≈ 0, K^droop≈ 0) worsen the ROCOF.

Consequently, the parametersKinandKdroop that result in the highest frequency nadir also result in a slightly better ROCOF and are therefore chosen as the optimal parameters for this system composition. As is shown in the following cases, the parameter combination that results in the high-est frequency nadir, always results in an enhanced ROCOF, also for other system compositions. Therefore, when there is referred to the ‘optimal’ pa-rameters in this section, this corresponds to the papa-rameters that result in the highest frequency nadir. In § 3.6.4.7, the parameter ranges which result in both a higher frequency nadir and lower ROCOF are presented.

In Fig. 3.14, the frequency response is shown for three different sce-narios. The frequency response is slightly worsened when the wind power penetration increases, but no synthetic inertia is used in the wind tur-bines. Wind turbines with synthetic inertia slightly improve the frequency response, though the differences are small. The frequency nadir fmin is slightly higher and the ROCOF is almost the same. This is due to the high amount of gas turbines in the system, which quickly change their power output in case of a disturbance, leaving only little room for improvement.

When looking at the turbine-side behavior in Fig. 3.14, it can be seen that the rotational speed Ω only deviates little from the pre-disturbance value and the additional power output is quite modest.

Kin[^MWs_Hz ]

Figure 3.15: Case 2: Frequency nadir and ROCOF for a wind power penetration of 15 % and 60 % steam.

3.6.4.4 Case 2: Low wind power penetration, 60 % steam