Study System-1 Description

Figure 3.1 depicts the Study System-1 comprising a 110 kV transmission system. The transformer ‘TR1’ steps down the voltage to 20 kV and connects the grid through a cable to a combination of balanced and unbalanced load (respectively ‘Load 1’ and ‘Load 2’). A STATCOM is utilized at PCC with a coupling transformer ‘TR2’ to compensate the load reactive power and negative sequence current. STATCOM consists of six electrical switches S1-6 (IGBTs or GTOs) and a dc-link capacitor ‘C’. PCC voltage is denoted by ‘VS’. System data is given in Appendix A.

AC Rc Ls Lc 20 MVA 110/20 kV 110 kV 2 M V A 26 /0 .57 kV STATCOM C ` A Is L O A D 1 L O A D 2 Rs _Vs Transmission Line Cable S1 S2 S3 S4 S5 S6 TR1 TR2

Figure 3.1 Schematic diagram of Study System-1

Figure 3.2(a) presents the simulation results with the proposed study system whereas Figure 3.2(b) depicts the results published in [10]. Figure 3.2(a) presents (i) the three phase source currents IS , (ii) load current ILoad , (iii) STATCOM current ISTATCOM and (iv) dc-link voltage Vdc . The same sequence of waveform is followed in Figure 3.2(b).

1. Mode-1 floating state (inoperative) (t = 1.85 to 1.9 sec) 2. Mode-2 Power factor correction (t = 1.9 to 2.0 sec) 3. Mode-3 Load compensation (t = 2.0 to 2.1 sec)

(i)

(ii)

(iii)

(iv)

(a) (i) Source current, (ii) Load current, (iii) STATCOM current and (iv) DC-link voltage

(b) Source side, load side and STATCOM three phase current and DC-link voltage [10]

Figure 3.2 (a) Simulation and (b) published results for compensation of unbalanced load for Study System-1

In floating state, STATCOM does not exchange any reactive power with grid whereas in Mode-2 it corrects the load power factor by providing reactive power. In Mode-3, it compensates the load reactive power and negative sequence load current.

Before t = 1.9 sec, STATCOM operates in floating state. At t = 1.9 sec, STATCOM starts power factor correction and current starts to flow through STATCOM as shown in Figure 3.2(a) (iii). Because of the unbalanced load, load current and consequently source current also become asymmetric [10]. At t = 2.0 sec, negative sequence controller of STATCOM starts compensating the negative sequence load current. As a result, source side current

and (ii). STATCOM now supplies unbalanced current for load compensation and hence, ISTATCOM becomes asymmetric.

Because of negative sequence current flow through STATCOM, Vdc has 2nd harmonic component in voltage as seen in Figure 3.2(a) (iv) [10]. To lessen the ripples a large size capacitor is suggested in [15]. The simulated results are similar to the published results in Figure 3.2(b), and thus validate the design of controller.

Load power factor for the same study system is presented in Figure 3.3. Before t = 1.9 sec, the power factor is 0.99 which increases to unity after STATCOM transitions to mode-2 for power factor correction. Nonetheless, 2nd harmonic components in power factor are still observed. After t = 2.0 sec, STATCOM starts compensating negative sequence currents and hence, power factor becomes unity. No 2ndharmonic components are observed during this period.

Figure 3.3 Power factor during load compensation for study system-1

Figure 3.4 presents the pu values of STATCOM current in dq reference frame for the same study. Figure 3.4(a) depicts the simulation results with proposed study system and Figure 3.4(b) depicts the results published in [10]. Figure 3.4(a) presents (i) the positive sequence q axis component of STATCOM current 𝐼_𝑞𝑝 . Figures 3.4(a) (ii) and (iii) present the negative sequence d and q axis parts of STATCOM current 𝐼_𝑑𝑛 and 𝐼_𝑞𝑛 respectively. The same sequence is followed in Figure 3.4(b).

As discussed in Chapter 2, 𝐼𝑞𝑝 represents reactive power (positive sequence) flow through the STATCOM. The STATCOM is in mode-1 before t = 1.9 sec, therefore, 𝐼_𝑞𝑝 is zero. As STATCOM enters into mode-2, 𝐼_𝑞𝑝 reaches its reference value (Load reactive power) in less than a cycle. During this time, the negative sequence controller is inactive. Therefore

𝐼_𝑑𝑛 and 𝐼_𝑞𝑛 have zero steady state values. The transients at time t = 1.9 sec are attributed to incomplete decoupling of positive and negative sequence currents [10]. At time t = 2.0 sec, the STATCOM transitions to mode-3, thus compensates load negative sequence currents (𝐼𝑑𝑝𝑟𝑒𝑓 and 𝐼𝑞𝑝𝑟𝑒𝑓) in less than 2 cycles. The transients at this time are also caused by incomplete decoupling of positive and negative sequence currents.

(i)

(ii)

(iii)

(a) (i) Positive sequence q axis STATCOM current, (ii) and (iii) negative sequence d and q axis STATCOM current

(b) q axis part for positive sequence and d and q axis parts negative sequence STATCOM current Figure 3.4 Positive and negative sequence currents in dq reference frame in (a) simulation

and (b) published [10]results for load compensation for Study System-1

Figure 3.4(b) illustrates response of controller proposed in [10]. The change of sign of 𝐼_𝑞𝑝 in Figure 3.4(a) and Figure 3.4(b) is because of different convention of power flow used in this thesis. The magnitude difference in negative sequence current is due to Y-∆ transformer. As in this simulation, voltage and currents are measured at high voltage side (Y) of transformer and in [10] the measurements are done at low voltage side (∆) of transformer.

On comparison of Figures 3.4(a) and (b), at time t = 1.9 sec, 𝐼_𝑞𝑝 reaches its reference value in less than a cycle in Figure 3.4(a) (i) while it takes more than 2 cycles for the same in Figure 3.4(b). Also, in Figures 3.4(a) (ii and iii) at time t = 2.0 sec, 𝐼_𝑑𝑛 and 𝐼_𝑞𝑛 track their reference values in 2 cycles, whereas a settling time of 4 cycles is illustrated in Figure 3.4(b).

Therefore, a faster response of STATCOM is exhibited by proposed controller. This is attributed to the use of an all pass filter, explained in Section 2.3.1 with that instant separation of positive and negative sequence components is possible.