Hilmy Awad

Tuning software phase-locked loop for series-connected converters

Accurate phase information is crucial for most of the modern power-electronics apparatus such as the static series compensator (SSC). A software phase-locked loop (SPLL) has been proposed in literature to obtain phase and frequency information of the grid voltage. Either a lead/lag filter or a proportional-plus-integral (PI) controller is employed to control the performance of the SPLL. In this paper, a criterion to tune the SPLL is discussed and the gains of the PI-controller are determined to obtain the desired performance. The proposed criterion is based on the fact that a phase shift of the grid voltage is sensed as a frequency deviation by the load. If the deviation of the grid frequency is kept within the range /spl plusmn/1 Hz, most of the loads function properly. Hence and by the SSC, the response of the phase angle of the load voltage is designed to follow the grid voltage angle while satisfying the frequency range at all times. Consequently, the gains of the PI-regulator of the SPLL are dependent on the maximum phase shift of the grid voltage. Unbalanced grid voltages are separated into positive- and negative-sequence components and the SPLL is locked to the positive sequence. The response of the SPLL has been evaluated by using PSCAD/EMTDC simulation package.

Double vector control for series connected voltage source converters

The voltage source converter, connected in series with the grid, as a static series compensator (SSC) is suited to protect sensitive loads against incoming supply disturbances such as voltage dips. Dynamic performance of the SSC is very important since it is essential to start compensation for voltage sags without a sensible delay. This paper proposes a double-vector control algorithm to be implemented to the SSC to improve its dynamic performance. The proposed algorithm incorporates both current and voltage controllers with an inner current control loop and outer voltage control loop. The validity of the proposed algorithm has been demonstrated through a PSCAD/EMTDC simulation, when the grid is subjected to a symmetrical three-phase voltage dip. Simulation results have proved that the proposed algorithm is able to improve the transient response of the SSC, compared to traditional control concepts, such as concepts related to phasors of the sequence components and RMS calculations.

Operation of static series compensator under distorted utility conditions

The static series compensator (SSC) has shown a significant capability to mitigate voltage dips, which are the most severe problem to sensitive loads. Also, it has been declared that the function of the SSC can be extended to work as a series active filter. This work proposes a moving-average filter to detect the fundamental component of the measured voltages and currents (needed to control the SSC) while using a double vector control algorithm to improve the transient performance of the SSC. This is made in order to accurately control the fundamental voltage component at the load terminals in the case of distorted grid voltage. Furthermore, a selective harmonic compensation strategy is applied to filter out the grid harmonics. The operation of the SSC under distorted utility conditions and voltage dips is discussed. The validity of the proposed controller is verified by experiments, carried out on a 10-kV SSC laboratory setup. Experimental results have shown the ability of the SSC to mitigate voltage dips and harmonics. It is also shown that the proposed controller has improved the transient performance of the SSC even under distorted utility conditions.

Voltage Compensation in Weak Grids Using Distributed Generation with Voltage Source Converter as a Front End

In this paper, a three-phase voltage source converter (VSC) is implemented as a front end of a distributed generation (DG) unit. A voltage regulation feature has been added to the VSC controller in order to maintain the voltage at the load connection bus equal to the nominal voltage even in case of balanced or unbalanced voltage dips. In addition, the voltage regulation limits are calculated depending on the rating of the DG

Mitigation of unbalanced voltage dips using static series compensator

The static series compensator (SSC) is suited to protect sensitive loads against voltage dips. Because most of the power system faults are single- or double-phase, the control algorithms of the SSC should be adapted for unbalanced dips. This paper proposes two control strategies to improve the dynamic performance of the SSC. The first strategy uses a fast technique for separating positive and negative sequence components of the supply voltage, which are then controlled separately. Thus, two controllers are implemented for the two sequences, each based on vector control. The second strategy is based on using only a positive sequence controller and increasing the switching frequency. Consequently, the negative sequence due to the unbalanced dip is transformed into variations in the positive sequence. As the switching frequency increases, the ability of the controller to follow those variations improves. The validity of the proposed strategies is demonstrated through PSCAD/EMTDC simulation, when the grid is subjected to unbalanced three-phase voltage dips.

Static series compensator for voltage dips mitigation

Power quality problems encompass a wide range of disturbances, which cause costly loss of production to critical processes. Voltage dips are the major source of power quality-related problems. The static series compensator (SSC), commercially known as dynamic voltage restorer (DVR), is best suited to protect sensitive loads against such incoming supply disturbances. This paper presents an overview of configuration, design criteria, and rated power estimation of the SSC. The used control techniques are discussed aiming to compromise among them based on their suitability with the SSC requirements.

Transient performance improvement of static series compensator by double vector control

This paper presents the principle and verification of the double vector control (DVC) algorithm, which improves the transient performance of the static series compensator (SSC). Both current and voltage controllers are incorporated by the DVC, with an inner current control loop and outer voltage control loop. The loop gains are determined according to the system stability analysis, carried out on the closed loop system. Also a review of the reported control techniques that have been implemented to control the injected voltage by the SSC is presented. A 10 kV SSC experimental setup is exploited to carry out experiments with different load types: static linear; dynamic linear and nonlinear loads. Experiments have proved that the proposed algorithm is able to improve the transient response of the SSC compared to like using phasors of the sequence components, RMS calculations and even vector controlled SSC without an inner current loop.

Mitigation of voltage swells by static series compensator

Swells and overvoltages can cause overheating, tripping or even destruction of industrial equipment such as motor drives and control relays. This paper investigates the possibility of employing the static series compensator (SSC) to mitigate voltage swells/overvoltages. In the case of voltage swells, active power may be drawn from the grid into the energy-storage capacitor (ESC) of the SSC, depending on the load current and the SSC impedance. This active power may overcharge the ESC. Two possibilities to overcome this situation are explored in this paper: 1) if the DC-voltage of the ESC is lower than a predetermined voltage level, the active power is employed to charge the ESC to this voltage level; 2) otherwise, the overvoltage protection of the SSC must operate. This paper also applies an overvoltage protection scheme based on a combination of a DC resistor with a chopper and the valves of the SSC. The design of the DC resistor is discussed. A 10 kV SSC experimental setup is exploited to carry out experiments in the case of balanced and unbalanced voltage swells at the grid side.

ENERGY FLOW CONTROL BETWEEN STATIC SERIES COMPENSATOR AND DISTRIBUTION SYSTEMS

By proper control algorithms, the energy flow between the static series compensator (SSC) and the distribution system can be fully controlled. This paper proposes and describes two control techniques to exchange electric energy between the energy storage capacitor (ESC) of the SSC and the utility. The first is based on using a shunt diode rectifier, which is placed either on the load side or the grid side (both configurations are studied). The other exploits the voltage source converter of the SSC to charge its the ESC. A design guide for the dc-capacitor is given. The proposed techniques are validated by PSCAD/EMTDC simulation.

Testing static series compensator against capacitor-bank energizing

The static series compensator (SSC) has shown to be an effective power electronics apparatus for mitigation of voltage dips. However, the power system operation is not only affected by voltage dips but also other power system events may cause disturbances. In this paper, the behavior of the SSC is investigated in case of upstream capacitor-bank energizing. Upstream capacitor-bank energizing leads to a transient at the terminals of the SSC followed by a sustained overvoltage. This over-voltage is exploited to charge the energy-storage capacitor of the SSC to the maximum allowable dc voltage. Simulations with PSCAD/EMTDC show that the capacitor-energizing transient does negatively affect the performance of the SSC.