POD and TSE using classical cascade control of E-STATCOM

As described in Section 5.3.3, the outputs of the POD controller are active and reactive cur- rent references to be injected into the grid by the E-STATCOM. The outer control loops that have been investigated earlier in this chapter together with the current controller described in Section 5.3.1 are included in the control of the E-STATCOM. To verify the POD controller performance, the setup in Fig. 7.31, which represents a single-machine infinite-bus system with the E-STATCOM, is used. The single-line diagram of the setup is shown in Fig. 7.32, where the possible connection buses of the E-STATCOM are marked as 1 and 2. For the tests, the gain KQis chosen to get a damping ratio of 10% at bus 2 when reactive power is used for POD. For

a fair comparison, the gain KP is then adjusted to get a similar order of maximum active and

reactive power injection for the tests. Once the values for the gains are selected, they are kept constant for all the experiments.

First, the power oscillation damping controller is tested for the two connection points of the E- STATCOM in Fig. 7.32. For this test, knowledge of the oscillatory frequency in the transmitted active power has been considered. To start the power oscillation, a three-phase fault is applied at bus 1 and the fault is cleared after 250 ms. The performance of the E-STATCOM for POD using the control strategy described in Section 6.3 is shown in Figs. 7.33 and 7.34. Observe that to facilitate the comparison, the presented measured signals have been filtered to remove noise and high-frequency harmonic components.

Fig. 7.32 Single line diagram of the laboratory setup for POD.

Fig. 7.33 Measured generator active power output following a three-phase fault with E-STATCOM con- nected at bus 1 (top) and bus 2 (bottom). POD byPinjonly (black solid),Qinjonly (gray solid),

bothPinj,Qinj(black dashed) and no POD (gray dashed).

As described in the small-signal analysis in Section 6.3.3, the injected active power decreases with the distance from the generator, where its impact to provide damping decreases (see Fig. 7.34). A better damping with active power injection is obtained when the E-STATCOM is closer to the generator, in this case at bus 1 (Fig. 7.33). With respect to reactive power injec- tion, the damping provided by the compensator increases when moving closer to the electrical midpoint. As the transmitted power Pg is used to control the reactive power injected by the

compensator, the same amount of injected reactive power is used at the two locations and a bet- ter damping action is achieved close to the electrical midpoint of the line, in this case at bus 2. With a proper choice of the control signals for injection of active and reactive power, effective power oscillation damping is provided by the E-STATCOM at both connection points of the compensator as shown in Fig. 7.33 (black dashed curves).

Fig. 7.34 Injected active and reactive power with E-STATCOM connected at bus 1 (black) and bus 2 (gray). Active power injection (top) and reactive power injection (bottom); bothPinj andQinj

used for POD.

To test the dynamic performance of the investigated POD controller in case of system parameter changes, a second set of experiments has been carried out assuming an oscillation frequency of 0.9 Hz, where the actual measured oscillation frequency is 0.42 Hz. This means that an error of 100% in the estimated oscillation frequency is here considered. Figure 7.35 compares the performance of the POD controller with and without the oscillation frequency adaptation in the RLS estimator. In both cases, the E-STATCOM is connected at bus 2 and injection of active and reactive power is used for POD. By using the frequency adaption as described in Section 4.3.2, the phase of the oscillatory component in the input signal is correctly estimated, thus providing an effective damping. This is advantageous when compared to the classical approaches, where the correct phase-shift is provided in the estimation only at a particular oscillation frequency. As shown in Fig. 7.35 (gray solid plots), if the POD controller is not adapted to changes in the system, its performance is significantly reduced. This is also shown in Fig. 7.36, where the total energy exchange (Wtotal) between the E-STATCOM and the grid until the oscillations are

completely damped for the two cases is displayed. The case without frequency adaptation is characterized by a longer settling time of the oscillation and therfore a larger amount of total energy exchange. This results in an uneconomical use of the energy storage.

Fig. 7.35 Top: measured generator output power with frequency adaptation (black solid), without fre- quency adaptation (gray solid) and with no POD (gray dashed); Middle: Injected active power with frequency adaptation (black) and without frequency adaptation (gray); Bottom: Injected reactive power with frequency adaptation (black) and without frequency adaptation (gray).

In the experimental tests performed so far, the applied three-phase fault results in a sustained oscillation without any risk of transient instability. In case of larger disturbances, a TSE con- troller is needed to keep the generator from losing synchronism. The tendency of the generator to lose synchronism increases with the fault-clearing time and the amount of transmitted active power. It has been found that the critical fault clearing time of the uncompensated system in Fig. 7.32 is about to 550 ms. Thus, a larger fault clearing time of 600 ms is considered for a three-phase fault applied at bus 1 to test the effectiveness of the TSE controller described in Section 6.6. This fault causes the uncompensated system to lose synchronism, as indicated by a continuous increase of the estimated generator angle deviation in Fig. 7.37. The estimated gen- erator angle (∆˜δg) and speed (∆˜ωg) deviations following the disturbance are estimated using

two PLLs, placed at the terminals of the generator and the infinite bus. By monitoring∆˜ωg as

described in Section 6.6, the performance of the TSE controller is validated here.

Fig. 7.37 Top: generator output power; Middle: estimate of generator angle deviation; Bottom: estimate of generator speed deviation; the E-STATCOM is in idle mode and the three-phase fault is applied for 550 ms (gray curves) and 600 ms (black curves).

For the 600 ms three-phase fault applied at bus 1 and the E-STATCOM connected at bus 2, Figure 7.38 shows the performance of the TSE and POD controllers. First, the TSE function without the POD controller is tested. As indicated in the figure in plots (a) and (b), the TSE controller guarantees the transient stability of the system. To damp the stable oscillation follow- ing the TSE action, the POD controller is also activated and the test is repeated. The results in plots (c) and (d) show that the sustained oscillations following the TSE operation are damped by

the action of the POD controller. It can be seen in the results that the reactive power controller is limited even during the POD operation due to the high power swing of the generator power following the fault. As described in the previous chapter, the TSE function starts following the disturbance and will be applied during the first swing of the generator angle. By the end of the first swing, the control action switches to the POD operation. In this example case, it has been found that the critical fault clearing time of the system is increased at least by 50 ms when using a maximum active or reactive current injection of 0.16 pu, corresponding to 50% of the active current rating of the energy storage in this setup. When both active and reactive power injection is used, the fault clearing time can be increased in total by at least 100 ms.

Fig. 7.38 TSE and POD performance with E-STATCOM connected at bus 2; (a) Generator angle devi- ation when active and reactive power are used for TSE only; (b) injected active power (black) and reactive power (gray) for TSE only; (c) Generator angle deviation during TSE and POD operation using only active power (black solid), only reactive power (gray) and both (black dashed); (d) injected active power (black) and reactive power (gray) for TSE and POD.