Coupling inductor and transformer reactance impact on design

In this chapter, the impact of coupling inductor’s and transformer’s reactance has been investigated. One of the main functions of the coupling inductor is to restrict the amount of harmonic current drawn from the grid due to presences of harmonics in VSC’s output voltage. Moreover, VSC’s net impedance seen from the grid side is the summation of coupling inductor and transformer’s reactance. Higher the value of this impedance is, better the harmonic attenuation would be. But increasing the reactance value of the cou-pling inductor comes with two possible consequences. First, large inductor value makes the system dynamic slower. Second, as per the equation 3.7 mentioned in chapter 3.3.1, to produce a fixed amount of capacitive reactive current, more VSC voltage is needed with higher reactance value.

To produce higher VSC voltage, more SMs are needed which ultimately affects its de-sign. Hence, to show the effect of how increasing or decreasing the reactance value affects the needed number of SMs, two cases for each coupling inductor and transformer reactance have been simulated. Thereafter, the results of these cases have been com-pared with the pre-defined system values.

Starting with coupling reactance, its system value of 13 mH was chosen in all of the simulations till now. But now, to check its effect on needed SMs, cases with 16 mH and 11 mH have been simulated. First simulation, with 16 mH of coupling reactance and assuming 20% of current capacity was available to produce combine 3rd, 4th and 5th

order positive sequence AHF currents, with their worst-case phase angles, was carried out. Thereafter, the same simulation was repeated with 11 mH of coupling reactance value. In last, VSC current and voltage waveform results with these two reactance values were compared with the results of 13 mH case, shown in Table 7.

Figure 27. Simulated (a) DC link, VSC valve and MV busbar voltages in MCOP region and com-bine 3rd, 4th and 5th order AHF currents with 0.20 p.u of overall magnitude, worst peak phase angle (60°, 20° and 0° respectively), 16mH of coupling reactance and 44 submodules, (b) VSC phase current in delta winding.

As shown in Figure 27(a), the needed number of SMs to produce desired AHF current with 16 mH of coupling reactance sought to be 44 and the peak value of VSC reference voltage is at 83.64 kV. However, comparing these values with Table 7 where the same AHF current was produced with 13 mH of coupling reactance, then the needed number of SMs was 42 and the peak value of VSC reference voltage was 79.71 kV. Hence, it can be clearly noticed that 16 mH of coupling reactance value causes the increment in peak value of VSC reference voltage, in comparison to the case with 13 mH, and there-fore result into a higher number of needed SMs as well.

Continuing further, simulation with 11 mH of coupling reactance and assuming 20% of current capacity was available to produce combine 3rd, 4th and 5th order positive se-quence AHF currents, with their worst-case phase angles, was carried out. Figure 28(a) shows that the needed number of SMs to produce desired AHF current with 11 mH of coupling reactance now sought to be 41 and the peak value of VSC reference voltage is at 77.59 kV. Comparing these results with 13 mH coupling reactance case, it can be concluded that since the peak value of VSC reference voltage has decreased from 79.71 kV to 77.59 kV, therefore the needed number of SMs to produce the same AHF current has also fallen from 42 to 41.

(a) (b)

Figure 28. Simulated (a) DC link, VSC valve and MV busbar voltages in MCOP region and com-bine 3rd, 4th and 5th order AHF currents with 0.20 p.u of overall magnitude, worst peak phase angle (60°, 20° and 0° respectively), 11mH of coupling reactance and 41 submodules, (b) VSC phase current in delta winding.

Transformer reactance is the other crucial passive element of VSC which affects the number of needed SMs to produce the desired voltage. Its system value of 0.10 p.u was chosen in all of the simulations until now. But now to check its effect, cases with 0.15 p.u and 0.05 p.u of transformer reactance with 20% available capacity (post RPC prioritisa-tion) for AHF current generation was carried out. Here, combined 3rd, 4th and 5th order of harmonic filtering current, positive sequence in nature and having worst-case phase angle reference, has been used. VSC current and voltage waveform results with these two reactance values were compared with the results of 0.10 p.u transformer reactance case of the same AHF current shown in Table 7.

Figure 29(a) depicts that the needed number of SMs to produce desired AHF current with 0.15 p.u of transformer reactance sought to be 45 and peak value of VSC reference voltage is at 86.31 kV. Comparing these values with Table 7 wherein the same AHF current was produced with 0.10 p.u of transformer reactance, then the needed number of SMs was 42 and the peak value of VSC reference voltage was 79.71 kV.

Figure 29. Simulated (a) DC link, VSC valve and MV busbar voltages in MCOP region and com-bine 3rd, 4th and 5th order AHF currents with 0.20 p.u of overall magnitude, worst peak phase angle (60°, 20° and 0° respectively), 0.15 p.u of transformer reactance and 45 submodules, (b) VSC phase current in delta winding.

(a) (b)

(a) (b)

In last, the value of transformer reactance was decreased to 0.05 p.u and simulation to produce the same AHF current (as did last case) was carried out. Here, now the needed number of SMs to produce desired AHF current sought to be 38 and the peak value of VSC reference voltage is at 73.03 kV, shown in Figure 30(a). Comparing these values with Table 7 where the same AHF current was produced with 0.10 of transformer reac-tance, the values of these parameters were 42 and 79.71 kV respectively.

Therefore, it can be concluded that increasing the transformer reactance value from 0.10 to 0.15 p.u result in increment in peak value of VSC voltage reference from 79.71 to 86.31 kV and consequently increment into the needed number of SMs from 42 to 45 as well. Similarly, decreasing the same reactance value from 0.10 to 0.05 p.u result into decrement in peak value of VSC voltage reference from 79.71 to 73.03 kV and conse-quently decrement in the needed number of SMs from 42 to 38 as well.

Figure 30. Simulated (a) DC link, VSC valve and MV busbar voltages in MCOP region and com-bine 3rd, 4th and 5th order AHF currents with 0.20 p.u of overall magnitude, worst peak phase angle (60°, 20° and 0° respectively), 0.05 p.u of transformer reactance and 38 submodules, (b) VSC phase current in delta winding.