Power control behaviour and transient of the a.c voltage-sensor-less control

The simulation results for the initial grid voltage estimation show that the fixed-band hysteresis current control and the proposed grid voltage estimation system are able to control the current thus the power without ac-side voltage sensors. The advantage of a fast transient response of the hysteresis current control is well presented. However, it also shows a high harmonic content which is approaching the limit of the grid code regulation. Considering it is only going to operate for a few fundamental cycles, this side effect can be accepted but should be improved by using linear current control as introduced in the previous sections.

After acquiring accurate grid voltage via the initialization procedure, the ac-side voltage- sensor-less control under PWM switching scheme can be started safely. The control algorithm

will be tested under balanced grid conditions. But before verifying the final system design, a few situations related to failed initial synchronisation is illustrated to provide comparison.

The initial feed-forward Vdand Vqare set different to the actual grid side to simulate the error

of the initial synchronisation. Firstly an error of Vdis made to be 50% of the nominal value

while Vq is set to be same as the grid side in order to simulate the case under an mismatch

between the amplitudes of the feed-forward and the actual grid voltage. The simulation results are shown in Figures 7.15 and 7.16. Then the case Vd is set to be 86.6% of the actual value

while Vq 57.7% of the value of Vd is tested to simulate a correct estimation of the voltage

amplitude but 30 degrees of phase error. The power output during the transient stage is also provided in the results shown in Figure 7.17. Finally the case of an accurate initial synchronisation is provided in Figure 7.18. The parameters of the system under study is listed in the table below, Table 7.2.

Parameters Values

Grid voltage 230V (RMS)

Grid frequency 50Hz

Coupling inductance 10mH

Series resistance 0.1

Target modulation depth 0.8

D.c. link voltage 800V

Power reference 3000W

Switching Frequency 10kHz

Figure 7.15 Waveform of the grid voltage

(a) (b)

Figure 7.16 Initial current (a) and power (b) transient response of the a.c. voltage-sensor-less current control with 50% of grid voltage amplitude feed-forward error

(a) (b)

Figure 7.17 Initial current (a) and power (b) transient response of the a.c. voltage-sensor-less current control with 30 degree of grid voltage phase angle feed-forward error

(a) (b)

Figure 7.18 Initial current (a) and power (b) transient response of the a.c. voltage-sensor-less current control with accurate feed-forward grid voltage

It can be verified that the system could reach steady state without very accurate initial synchronisation to the grid. However, the simulation results shown in Figure 7.16 and Figure 7.17 have a significant current overshoot during the first few cycles. The power modules are at a risk of being damaged by this high current. As a consequence, the power output has yielded large transient error during the start up. The power during the transient stage undergoes some reverse power flow which is not desirable at the first point. The d.c. bus voltage will be affected and at the risk of being over charged. Then the over-shoot of the power above the power reference may be too high that it could cause the d.c. bus voltage to drop below the minimum

required value if the output power is not regulated by the d.c. bus voltage control, then the SPWM will be sent into the range of non-linear modulation which means the modulation depth is above 1.0. Low order harmonics in this case will be significant. And the accuracy of the grid voltage estimation will be affected, bringing uncertainty to the stability of the controller. The comparison provided above suggests that initial synchronisation to the grid is important for avoiding these situations thus very necessary.

In practice, the grid voltage could deviate from its nominal value while the power flowing into the d.c. bus could vary. Therefore, the simulation study should take these situations into consideration. The a.c. side voltage-sensor-less control is further tested for its power tracking ability and voltage tracking ability.

A balanced voltage dip of the a.c. side is introduced to test the algorithm. In reality a voltage dip below 70% will allow the connected generation to quit in 1.25 seconds according to Figure 1.6. Therefore it is important to know if the controller could reach steady state within the required remaining connection time range otherwise it could be meaningless to control the power as the error will not be cleared effectively. In the simulation study a 50% voltage dip is provided so that the availability of the control scheme under smaller voltage dip can be verified.

In the following simulation results, a balanced grid voltage dip of 50% of its nominal value is applied at t=0.1 second after the control is active. In Figure 7.19, the voltage waveform is provided. The response of the basic a.c. voltage-sensor-less control system is shown in Figure 7.20. It is clear that the controller is well capable of tracking such a variation of the grid voltage as shown in Figure 7.21. In this case, the current reference generated based on the estimated voltage therefore is able to represent the desired power output. The current response is well damped and swift enough to track the desired power output. The current overshoot visible in Figure 7.20 is caused by the current control error due to the feed-forward voltage discussed in

Section 5.2.5, a similar case to the current overshoot of a current system equipped with voltage sensors and a slow PLL under the same grid conditions. The size of error is determined by the parameter design of the voltage estimation stage together with dynamic decoupling stage. As the voltage estimation stage naturally has a wider bandwidth than the current control stage while the dynamic decoupling is aiming at restricting the high frequency components in the estimated voltage signals, the grid voltage tracking ability thus the power tracking ability is mainly restricted by the dynamic decoupling. However it is small enough to be neglected.

Figure 7.19 Waveform of the grid voltage

(a) (b)

Figure 7.21 Estimated grid voltage Vdand Vq

The system response with slower voltage estimations is shown in Figure 7.22 to provide some comparison. It is obvious that the system response slows down with the decreasing PI coefficients of the dynamic decoupling stage.

(a) (b)

Figure 7.22 Current (a) and power (b) response with a slow grid voltage estimation system

Another situation could bring challenge to the performance of the system is the phase jump of the grid side voltage. In the simulation study, a forward phase jump of 30 degree is applied to the grid voltage at 0.1 second after the system been activated as shown in Figure 7.23. To

isolate the response of the proposed a.c. sensor-less current control from the response of its outer loops, e.g. the d.c. bus voltage control and PLL, the system is simplified without activating these controllers. Therefore this system became a linear system as described in Section 5.4.

Figure 7.23 Waveform of the grid voltage with 30 degree phase jump

As the current control in this simulation study is not voltage oriented without PLL where the angle of the reference frame of the control system is not locked to the grid angle. To maintain unity power factor, the current should respond to the change of the phase angle of the grid voltage, and its angle should be adjusted accordingly by changing the reference of Iq. The

reference for Id should also be adjusted to maintain the total active power output to be the

(a) (b)

Figure 7.24 Current (a) and power (b) response under 30 degree grid voltage phase jump

The simulation results have now verified the validity of the proposed voltage-sensor-less current control under grid voltage phase jump. The current injected is adjusted according to the estimated grid voltage which is accurately tracking the actual grid voltage. The current response to the voltage phase jump is within a desired range of current values without visible current overshoot. This ensures the safety of the power modules. With accurate estimation of the grid voltage, the power is able to be recover to its reference value. In Figure 7.25, the Vd

and Vqare referred to the angle of the angular reference of the system reference frame before

the phase jump. It can be verified that the voltage estimation is swift which ensures the smooth current response and the fast power output recovery. The current is in phase with its phase voltage at steady state which is shown in Figure 7.26. These simulation results suggest that without physical inertia, the VSI system is more capable of dealing with phase jump of the mains voltage compared to the actual generators.

Figure 7.25 Estimated grid dq components comparing to the actual values

Figure 7.26 Phase A current and the phase voltage before and after phase jump

To test the power tracking ability, a step change of the power reference is applied. The system starts with a reference power output of 3kW. At 0.1 second, the output power reference rises

to 5kW. The current response and the real power at the PCC is recorded and plotted as shown in Figure 7.27.

Two points should be verified by this simulation study. Firstly, is the stability of the system guaranteed if the power reference which determines the working point of the system step- changes? Secondly, is the system able to track the changed power reference in a reasonable amount of time? Thirdly, will error be introduced to the grid voltage estimation if the current injected which is the input to the grid voltage estimation system changes while the grid voltage remain the same?

(a) (b)

Figure 7.27 Current (a) and power (b) response to a reference power step change

The results show that although the grid voltage is estimated based on the output voltage of the VSI and the current response, the wider bandwidth of the grid voltage estimation allows the grid voltage to be tracked accurately without being interfered by the change of the working point of the system. The system performance therefore is subjective to the performance of the current control stage. A settling time of 30ms is approximately achieving which is a good match to the design objective.