Design of Reactive Power and Voltage Controllers for Converter-interfaced ac Microgrids

This paper aims at presenting design of two controllers for the study of a microgrid testbed. The response of the microgrid testbed to different short circuits would be investigated under these two control regimes, namely, reactive power and voltage controls. This paper therefore presents design of active power, reactive power and voltage regulators for a converter-interfaced ac microgrid. The design was performed using Simulink Control Design ® in the Department of Electrical and Computer Engineering, Curtin University, Sarawak, Malaysia between May 2015 and December 2015. The microgrid response under different control regimes in a project which is still ongoing, this paper presents an attempt to design two control regimes for the ac microgrid testbed.

response under different control regimes in a project which is still ongoing, this paper presents an attempt to design two control regimes for the ac microgrid testbed.

INTRODUCTION
The current power system is experiencing increased proliferation of distributed generation (DG) using distributed energy resources in order to increase its reliability and resilience. This trend leads to transformation of the distribution system from passive to active network. Active distribution network based on converter interfacing presents new challenges to the distribution system. These challenges include bidirectional power flow, incapacitating existing protection systems and requirement for advanced control schemes [1][2][3][4].
The protection challenge is associated with the limited contribution of the microsources to short circuits as a result of the effect of power electronic converter. When a microgrid is based on converter-interfaced microsource (s), such as the doubly-fed induction generator (DFIG) shown in Fig. 1, its contribution to short circuits (SCs) is limited by the converter capacity if it runs in islanded mode of operation. However, if the microgrid is utility-connected, its contribution to both microgrid and utility faults is limited by both the converter capacity and contribution from the utility's sources. Therefore, the microgrid's contribution to microgrid SCs in islanded mode is not same as its contribution to same fault when in grid-connected mode. This differential contribution to SCs challenges existing protective relays [5][6][7][8][9].
The control challenge is partly because the microgrid lacks the required stored energy (inertia) to quickly recover to a new steady state during and after disturbance such as a short circuit [10][11][12][13][14][15]. When in grid-connected mode, the microgrid experiences severe oscillation during and after utility disturbance. Also, the control regime determines the contribution of the microgrid to short circuits and its stability during and after disturbances. Implementation of control is, therefore, useful for full-scale deployment of microgrids [16][17][18][19][20].
This paper presents an attempt to implement two basic control strategies for a microgrid based on two 5.5kW converter-interfaced DFIGs. The control strategies are active-reactive power (PQ) control and active power-voltage (PV) control, using a testbed developed in SYMPOWERSystems ® . The controllers are designed in order to meet the requirements provided in Table 1.

Active power regulator
The open-loop transfer function of the generator is provided in (2.3).
Its step response is shown in Fig. 2.3. It fails to meet the required specifications set in Table 1. A proportional-integral (PI) compensator is therefore designed as shown in Fig. 2

Reactive Power Management System
The open-loop transfer function of the reactive var source is provided in (2.5).
Its step response is shown in Fig. 2.6. . The system meets the required system specifications provided in Table 1.

Grid ac voltage regulator
The open-loop transfer function of the voltage source is provided in (2.7). Where, Its step response is shown in Fig. 2.9.   Fig. 2.11 shows the closed-loop response of the designed grid voltage regulator. It is a stable loop with infinite gain margin and phase margin of 140 o at 13.5 rads -1 . The system meets the required system specifications provided in Table 1.

DC bus voltage regulator
The open-loop transfer function representing the dc voltage of the rotor-side converter is provided in (2.9). Its open-loop unit step response is provided in Fig. 2.12. A PI compensator whose transfer function is presented in (2.10) is designed such that the converter closed loop meets the requirements in Table 1. Fig. 2

Grid-side converter current regulator
The unit step response of the grid-side converter is shown in Fig. 2.14. Its open-loop transfer function is provided in (2.11).  . The closedloop system meets the required system specifications provided in Table 1.

Rotor-side converter current regulator
The unit step response of the rotor-side converter is shown in Fig. 2.16. Its open-loop transfer function is provided in (2.13).

.17. Closed-loop response of rotor-side current regulator in frequency domain
A PI compensator whose transfer function is presented in (2.14) is designed such that the controller-converter closed loop meets the requirements in Table 1  As shown in Table 2, the closed-loop systems meet the required peak value provided in Table 1

Implementation of Control Strategies
The converter is used for power control. The composite ac-ac converter consists of two converters, namely, ac-dc (rotor side) and dc-ac (grid side) converters.

Power and pitch control systems
The pitch angle is maintained constant at zero degree until the speed reaches point D of the tracking characteristic. After point D, the pitch angle is proportional to the speed deviation from point D speed. The control system is shown in Fig. 2.18.
The power is controlled so as to follow a predefined characteristic, called power-speed characteristic, as shown in Fig. 2.19.

Rotor-side converter control system
The rotor-side converter is used to control the wind turbine output power and the voltage (or reactive power) measured at the grid terminals, as shown in Fig. 2.20.

Pitch Angle Gain
Pitch Angle Speed_D Max. pitch angle