3.2 Operating units in parallel
3.2.2 Controller modifications for droop control
The inclusion of droop functionality in the control loop requires a simple modification to the AC voltage controller. The reactive current demand is fed back from the controller output, scaled by the droop gain, and sub-tracted from the voltage reference at the input. The revised controller is shown in Figure 3.14. The additional feedback cancels the integral action of the original controller at low frequencies, causing the controller gain to be reduced. At higher frequencies the controller gain is unaffected by the droop feedback and the ability of the droop controller to respond to tran-sients is therefore not compromised. The feedback gain is set according to the desired droop constant and the rating of the STATCOM. For the STAT-COM described in the previous section, with a 400 A rating and a droop constant of 5 %, the droop constant is 0.0425 VA−1. It should be noted that since positive reactive current causes a drop in system voltage and since an increase in reactive current demand now results in a proportional increase in the STATCOM AC voltage demand, the time taken for the controller to reach a new steady-state operating point is reduced. This is shown for a sin-gle droop controlled STATCOM in Figure 3.15. The response of the STAT-COM without droop control is shown for comparison. It can be seen that the droop-controlled STATCOM has a time-constant which is approximately
half the time-constant of the original controller, or 0.025 S.
Figure 3.14:The voltage controller with droop feedback.
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
0.93 0.94 0.95 0.96 0.97 0.98 0.99 1 1.01
Time (s)
Voltage (p.u.)
Figure 3.15:Response of the droop controller (green) compared with the original controller (blue).
In order to demonstrate that the droop controller behaves as expected, sim-ulations were performed with a second STATCOM with the same current rating as the first present. The STATCOMs were connected in parallel. Both STATCOMs had correctly tuned controllers and the droop constant for both was set to 0.0425 VA−1. A small additional impedance had to be placed between the STATCOMs for the simulation to run correctly; when this was not done, the system voltage failed to converge on any realistic value. The additional impedance was provided through a 10 µH inductance in series
with each STATCOM. Results showing the system voltage are shown in Fig-ure 3.16 and the reactive currents provided by each of the STATCOMs are shown in Figure 3.17.
Figure 3.16:PCC voltage of the system when two droop controlled STAT-COMs with equal ratings are controlling the voltage.
The new steady-state voltage is slightly above 0.99 p.u. This is only mar-ginally lower than the original value of 1 p.u., although the new voltage will vary with load. The two current curves cannot be distinguished from each other, which shows that the two STATCOMs share the load equally be-tween them and the control response for both STATCOMs is the same. The reactive current provided by each STATCOM is approximately 72 A, which gives 144 A total. For comparison, a single droop controlled STATCOM with the same ratings as the two used in these simulations regulates the voltage to approximately 0.985 p.u.and requires almost 120 A to do so. In the case of a single STATCOM without droop control, the current required to regulate the voltage to 1 p.u.is approximately 170 A.
The simulation was repeated with two STATCOMs having different droop
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Figure 3.17:Reactive currents supplied by the two STATCOMs.
constants and maximum current ratings. In this case one STATCOM, re-ferred to as STATCOM 1 for clarity, has a droop constant of 0.0567 VA−1 and a current rating of 267 A. The other, referred to as STATCOM 2 for clar-ity, has a droop constant of 0.0283 VA−1and a current rating of 533 A. The system voltage is shown in Figure 3.18 and STATCOM reactive currents are shown in Figure 3.19.
As was the case when two identical STATCOMs were used, the voltage is controlled to approximately 0.99 p.u. once it has returned to steady-state.
Once ins steady state, STATCOM 1 supplies approximately 48 A while STAT-COM 2 supplies approximately 93 A. In both cases, when the respective STATCOM current rating are taken as the base current, the per unit cur-rent is close to 0.18 p.u., although it can be seen that STATCOM 1 provides a slightly larger share than STATCOM 2. This result, although not ideal, is not entirely unexpected as imperfect load sharing between droop controlled sources has been reported by a number of previous authors [104–106].
In this section, modifications to the AC voltage controller have been made
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0.93
0.94 0.95 0.96 0.97 0.98 0.99 1 1.01
Time (s)
Voltage (p.u.)
Figure 3.18:PCC voltage of the system when two droop controlled STAT-COMs with different ratings and droop constants are control-ling the voltage.
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
−0.25
−0.2
−0.15
−0.1
−0.05 0 0.05
Time (s)
Current (p.u.)
Figure 3.19:Reactive currents supplied by STATCOM 1 (blue) and STAT-COM 2 (green).
in order to allow multiple STATCOMs to operate in parallel. Simulations have been used to demonstrate the behaviour of the revised controller and the operation of multiple parallel STATCOMs. Study of the droop con-trolled STATCOM does not form a major part of this work, but has been included for completeness and to demonstrate that the proposed controller with droop modifications is suitable for deployment on systems where mul-tiple devices may be installed in parallel or where future expandability is desirable.
3.3 Summary
This chapter has introduced the STATCOM control scheme used throughout the rest of this work. The control scheme works by continually estimating the reactive current required to reach the desired system voltage and adjust-ing the output of the STATCOM accordadjust-ingly. Simulation has been used to demonstrate that the controller functions as expected. The behaviour of the controller when the system X/R ratio is small, as is typical on distribution systems, has been considered. Unexpectedly, it was found that the sup-ply resistance does not have a significant impact on the controller dynamics when the supply X/R ratio is greater than one.
Droop control modifications required to operate STATCOMs in parallel have been described and it has been demonstrated through basic simulation that, with the appropriate modifications to the control structure, parallel opera-tion of multiple STATCOMs is possible, with reactive power shared between them.
Knowledge of the system impedance is required for the controller described to function correctly. In this chapter it has been assumed that the impedance of the system is known and fixed controllers have been used based on this assumption. The impedance of the system may not already be known or it may change from time to time. In the next chapter the applicability of
im-pedance estimation to tuning the STATCOM controllers will be discussed.
Impedance estimation
The impedance estimation method used in this work needed to be one which could be implemented using a power electronic converter, therefore allow-ing it to be an added function of the STATCOM control. In addition, it had to be possible to estimate the impedance when desired, without having to wait for a significant grid event. Ideally the method would allow the im-pedance over a range of frequencies to be calculated. Finally, although the method had to be invasive (in order to meet the previous requirement) the disturbance created by the method should be kept to a minimum. In order to meet these requirements, an impulse injection method was chosen.
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