Performance Of The Damping Controller At Different Values Of X q

SUPPLEMENTARY DAMPING CONTROLLER FOR AN SSSC-COMPENSATED TRANSMISSION LINE

7.4 Performance Of The Damping Controller At Different Values Of X q

generator's speed deviation; the resulting modulation in the inverter's dead angleyis also shown in Fig. 7.6 (c).

Although there is a clear stabilizing influence from the damping controller evident in Fig. 7.6, the torsional oscillations still persist for several seconds after the disturbance even with the damping controller activated. Fig. 7.7 therefore considers whether further improvement in the damping of the torsional oscillations can be obtained by increasing the supplementary controller's gainK_{SSR '}

Fig. 7.7 now shows the SSR characteristics for XqO = 0.3707pu but compares the fixed SSSe compensation (KSSR = 0) to modulated compensation with KSSR = 0.15. The results show that there is in fact further reduction in the amplitude of the Mode 1 (15 Hz) shaft torsional oscillations at the increased value of gain (c.f. Fig. 7.6); however, this comes at the expense of larger excursions in the compensating reactance and generator electrical torque during, and after the application of the fault in the transmission line. Also, there appears to be a less-damped higher frequency mode present in the speed deviation signal in Fig. 7.7 that is not as evident in Fig. 7.6.

Thus, while in principle it is possible to add significant damping to the torsional oscillations using the SSSC's supplementary controller, careful consideration would be required to apply appropriate limits to the magnitude ofLlX_qcommanded at the output of such controls.

It was shown in section 7.2 that for lower values of compensation, such as X^q* =0.178 pu, the damping of the torsional modes in the study system was not of concern. Thus, in the following studies, the values of compensation chosen for further study are Xq*

=

^{0.245 pu, Xq*= 0.315 pu}

and X

q*

=

0.385 pu; these values span the range of compensating reactances for which torsional oscillations were shown to be a concern in Fig. 7.2 and Fig. 7.3 earlier.

As in the previous simulation studies, the response of the system at each of these values ofX^q*to a three-phase fault of duration 0.075 seconds was considered. The responses of four system variables from the PSCADIEMTDC simulation, namely, generator speed, the electrical torque, the LPA-LPB turbine shaft torque and the GEN-EXC shaft torque with no damping controller, and with damping control and gainK_SSR=0.1 are shown in each study.

This section now compares the SSR characteristics of the SSSC form of compensation with and without supplementary damping controls, and for different values of compensating reactance, when the supplementary controller has been designed to damp out Mode 4 oscillations at a specific value ofX_{q *}

=

0.3707 pu. (Note: when examining the variables in the following plots, the oscillations may appear to exhibit higher amplitudes than in previous plots; however, this is simply because the scales on the plots have been adjusted from those used previously in the chapter.)

Fig. 7.8 shows the SSR characteristics of the First Benchmark Model when the SSSC is providing compensation of X_q*

=

0.245 pu. Fig. 7.8 also compares the situation when there are no supplementary controls (K_SSR = 0) to that when the supplementary controls are activated with a gain ofK_SSR

=

^0.1. It is clear that the system does not exhibit negative damping even when the supplementary damping controls are disabled. However, with the supplementary damping controls enabled, there are noticeable (but not pronounced) improvements in the damping of all four system variables.

In Fig. 7.9, when the SSSC is providing a compensating reactance ofX_q*

=

0.315 pu, there is a gradual increase in the amplitudes of the oscillations in all the variables when there is no supplementary damping control; this is particularly evident in the generator speed and LPA-LPB torque, as shown in Fig. 7.9 (a) and (c). Once again, with the supplementary damping controls activated, the damping of all the system variables is noticeably improved, to the extent that the system is no longer negatively damped at this value ofX_qwith supplementary damping control.

When the SSSC is providing a compensating reactance ofXq*

=

0.385 pu, with no supplementary controls, it is obvious that the system is extremely negatively damped from the post-fault response

(a) Generator Speed

~ ~

a. -2 -4

(b) Electrical Torque

- Without Damping Control - With Damping Control

._ 1.~ (c) LPA-LPB Torque

~

_-

^0: ---'--_~

_{0 . 5}

^{_ _}

_·

(d) GEN-EXC Torque

~ ^0:

-0.1o 0.5 1 1.5 2 2.5

Time(5)

Fig. 7.8:Performance of the SSSC compensated IEEE Benchmark Model transmission system at X_qo= 0.245 pu with no supplementary damping control and with damping control and gain

KSSR

=

0.1.

shown in Fig. 7.10. However, when the supplementary control is activated at this value ofXq·,

all system variables are once again positively damped and the stability is restored. It should be noted that in Fig. 7.10 (and in some of the other results shown in this chapter) the amplitudes of the generator electrical torques during, and immediately after the fault are significant when the SSSC's supplementary damping controls are activated. However, it should also be noted that in this time period the controller is responding to large excursions in the system variables which it should not, ideally, be trying to influence. Ideally, a small signal damping controller such as this should have its output limited in some manner to prevent such impact on the system during large-signal transients, whilst still allowing it to positively influence the small signal behaviour of the system once the large-signal response is over.

The above investigation has presented three sets of SSR studies of the three-level SSSC with different commanded capacitive compensation reactance values. The investigations compared

(a) Generator Speed

!:

0.. -2

-4 (b) Electrical Torque

- Without Damping Control - With Damping Control

-1L - - l - ---L- -h=~~==:====::!....______l

(c) LPA-LPB Torque

~ ^_i

(d) GEN-EXC Torque

~ ^0:

-0.20 0.5 1 1.5 2 2.5

Time (s)

Fig. 7.9: Performance of theSSSCcompensated IEEE Benchmark Model transmission system at X_qO

=

0.315 pu with no supplementary damping control and with damping control and gain

KSSR

=

0.1.

the response of the system when no supplementary damping control is provided, to that when the supplementary damping controller designed in section 7.3 is activated. It is observed that the supplementary controller designed for a single value of compensating reactance ofXq*

=

0,3707 pu still provides positive damping to the system when the value ofX_q*is changed from the design value. Although this fixed-design supplementary controller did not provide significant damping to the system for lower values ofX_q*, it did not negatively affect the stability of the system at those values ofXq*. Furthermore, the supplementary damping controller was shown to be able to stabilize negatively-damped torsional oscillations in the First Benchmark Model for values ofX_q*

other than that for which it was designed. Thus it can be concluded that the SSSC, with a single, fixed design of supplementary damping controller, could be used over a range of compensation values in the First Benchmark Model. However, the results have also shown that further refinement of the supplementary damping controller is required to limit its output under larger signal conditions, particularly at large values ofX_{q *,}

(a) Generator Speed

20,---.---,r---.---.----~

t o

a.. _- Without Damping Control -20 - With Damping Control

(b) Electrical Torque 3

§^t

~~i\illI~;~Aft!lti-Mk~~~~ ^4vWllVif

ⁿ

a.. 0 u

-1

(c) LPA-LPB Torque

~.::~-.---~-~,.---~-

.c 0.5

(d)GEN-EXCTorqu

~ 0 ,

<11

a..

-0.5o 0.5 1 1.5 2 2.5

Time(5)

Fig. 7.10: Performance of the SSSC compensated IEEE Benchmark Model transmission system at X_qo

=

0.385 pu with no supplementary damping control and with damping control and gain

KSSR

=

^0.1.