IMPACTOS EN LA ETAPA DE OPERACIÓN Y MANTENIMIENTO 1. IMPACTOS POSITIVOS

As described in Section 6.2, the level of stability enhancement provided by the converter de- pends on how much the active power output from the generators is changed by the injected currents. This in turn depends on the location of the converter a, the power system configu- ration as well as the active and reactive power rating of the compensator. On the other hand, the TSE function from the available maximum current ratings for the system in Fig. 6.2 is de- scribed in (6.42) and (6.43). Considering the same power system configuration, evaluation of the TSE function during the first swing of the machines will be made in this section. This can be achieved by investigating how much the stability margin or power transfer capability of a transmission system increase for a given disturbance using active and reactive power injection. For this analysis, the system in Fig. 6.2 with the second generator modeled as an infinite bus (Hg2= ∞) is considered for simplicity. The increase in power transfer capability of the system

without reducing its stability margin will then be investigated while using the TSE function at various locations in the power system. Assuming that the connection point of the converter is given by the parametera as before, the output power of Generator 1 with a constant active or reactive current injection during TSE is derived as

Pg1,P≈

Vg1Vg2sin(δg1−δg2)

X1+X2 ± 

V2

g1(1−a)2+Vg1Vg2cos(δg1−δg2)a(1−a) √

[(1−a)Vg1]2+[aVg2]2+2a(1−a)Vg1Vg2cos(δg1−δg2)  Id max Pg1,Q ≈ Vg1Vg2_Xsin(δ_1+Xg1₂−δg2) ±  Vg1Vg2sin(δg1−δg2)a(1−a) √

[(1−a)Vg1]2+[aVg2]2+2a(1−a)Vg1Vg2cos(δg1−δg2) 

Iq max

(6.45)

Consider that the system has a total series reactance of 1.5 pu with Vg1 = Vg2 = 1 pu and the

connection point of the converter is a = 0.3. Using the expressions in (6.45), an example of the power-angle curve for the system during TSE operation (with the current rating chosen as Id

max= Imaxq = 0.1 pu) is shown in Fig. 6.18.

If the generator is initially operating with an output power of 0.5 pu, the uncompensated system will have a maximum deceleration area of0.159 pu as calculated from the power-angle curve. It is possible to observe that an increase in the transmitted power will reduce the available deceleration area and hence the stability margin of the system. As it can be seen from the same figure, the compensated system has more deceleration area for the same initial transmitted power. This means that the active power transfer capability of the system can be increased

Fig. 6.18 power-angle curve without TSE (black dashed), TSE with constant active current injection (black solid), TSE with constant reactive current injection (gray solid) and initial transmitted power (gray dashed).

without reducing its stability margin by using a TSE controller. For the specific case in Fig. 6.18, it is found that the initial transmitted power can be increased to0.561 pu and 0.526 pu without reducing the stability margin of the system when active and reactive power is used for TSE, respectively.

Figure 6.19 summarizes the current rating of the converter needed to increase the power transfer in Fig. 6.18 to 0.6 pu at various locations, without reducing the available deceleration area of the uncompensated system with initial transmitted power of 0.5 pu. As it can be seen from the results, the required active power rating to increase the power transfer capability reduces closer to the generator location. At the locationa = 0.1, for example, the active current rating is 10% of the reactive current requirement to increase the power transfer by the same amount. On the other hand, the reactive power rating reduces when moving closer to the middle of the transmission line. By combining the use of both active and reactive power, it is possible to increase the power transfer capability of the system at various locations of the system.

Although the results in Fig. 6.19 give an idea of how much can be gained by using a TSE operation and the choice of active or reactive power depending on location of the compensator, it should be noted that the results are specific to the case studied and a similar evaluation should be made for other power system configurations in question.

Fig. 6.19 Current rating requirement to increase power transfer capability at various locations; Left: active power for TSE and Right: reactive power for TSE.

6.7

Conclusions

A POD and TSE control algorithms have been derived in this chapter. For this, the signal esti- mation technique described in Chapter 4 has been used to design an adaptive POD controller. It has been shown that with proper selection of the local measurement signals together with the proposed estimation method, the use of active and reactive power for stability enhance- ment function can be maximized. Moreover, the impact of various loads and the use of multiple compensators on power system stability has been investigated. The use of the proposed control method has been shown to provide independent damping for multiple oscillation modes without any negative interaction among the compensators. The control methods derived in this section will be verified using various power system configurations in the next chapter.

Verification of E-STATCOM control for

power system stability enhancement

7.1

Introduction

A detailed description of the outer control loops for stability enhancement has been derived in the previous chapter. In this chapter, the dynamic performance of the stability enhancement controllers will be verified through simulations and experimental tests.

7.2

Simulation verification

In this section, the performance of the POD and TSE controllers will be evaluated for various power system configurations.