Power Quality (PQ) is an important measure of an electrical power system. The term PQ means to maintain purely sinusoidal current wave form in phase with a purely sinusoidal voltage wave form. The power generated at the generating station is purely sinusoidal in nature. The deteriorating quality of electric power is mainly because of current and voltage harmonics due to wide spread application of static power electronics converters, zero and negative sequence components originated by the use of single phase and unbalanced loads, reactive power, voltage sag, voltage swell,flicker, voltage interruption etc. To improve the power quality traditional compensation methods such as passive filters, synchronous capacitors, phase advancers, etc. were employed. However traditional controllers include many disadvantages such as fixed compensation, bulkiness, electromagnetic interference, possible resonance etc.. These disadvantages urged power system and power electronic engineers to develop adjustable and dynamic solutions using custom power devices.
Custom power devices are power conditioning equipments using static power electronic converters to improve the power quality of distribution system customers. These include APF, dynamic voltage restorer (DVR) and Unified Power Quality etc.
Series voltage controller
[Dynamic Voltage Restorer, (DVR)]
The series voltage controller is connected in series with the protected load. Usually the connection is made via a transformer, but configurations with direct connection via power electronics also exist. The resulting voltage at the load bus bar equals the sum of the grid voltage and the injected voltage from the DVR. The converter generates the reactive power needed while the active power is taken from the energy storage. The energy storage can be different depending on the needs of compensating.
DVR can compensate voltage at both transmission and distribution sides. Usually a DVR is installed on a critical load feeder. During the normal operating condition (without sag condition) DVR operates in a low loss standby mode During this condition the DVR is said to be in steady state. When a disturbance occurs (abnormal condition) and supply voltage deviates from nominal value, DVR supplies voltage for compensation of sag and is said to be in transient state.
Structure of DVR
The DVR is composed by the following main parts:
• voltage source converter (VSC)
• Voltage injection transformer
• Passive filter
• Energy storage
The DVR basically consists of a power circuit and a control circuit. Control circuit is used to derive the parameters (magnitude, frequency, phase shift, etc…) of the control signal that has to be injected by the DVR. Based on the control signal, the injected voltage is generated by the switches in the power circuit .
Ø Energy storage device; It is used to supply the real power requirement for the compensation during voltage sag. Flywheels, Lead acid batteries, Superconducting magnetic energy storage (SMES) and Super-Capacitors can be used as energy storage devices. For DC drives such as SMES, batteries and capacitors, ac to dc conversion devices (solid state inverters) are needed to deliver power, whereas for others, ac to ac conversion is required. The maximum compensation ability of the DVR for particular voltage sag is dependent on the amount of the active power supplied by the energy storage devices. Lead acid batteries are popular among the others owing to its high response during charging and discharging. But the discharge rate is dependent on the chemical reaction rate of the battery so that the available energy inside the battery is determined by its discharge rate.
Ø Voltage-source converter : It is a power electronic device, which can generate a sinusoidal voltage with any required magnitude, frequency and phase angle. Voltage source converters are widely used in adjustable-speed drives, but can also be used to mitigate voltage dips. The VSC is used to either completely replace the voltage or to inject the ‘missing voltage’. The ‘missing voltage’ is the difference between the nominal voltage and the actual. The converter is normally based on some kind of energy storage, which will supply the converter with a DC voltage. The solid-state electronics in the converter is then switched to get the desired output voltage. Normally the VSC is not only used for voltage dip mitigation, but also for other power quality issues, e.g. flicker and harmonics.
Ø Low pass passive filters: They are used to convert the PWM inverted pulse waveform into a sinusoidal waveform. This is achieved by removing the unnecessary higher order harmonic components generated from the DC to AC conversion in the VSI, which will distort the compensated output voltage. These filters can be placed either in the high voltage side or in the low voltage side of the injection transformers. When the filters are in the inverter side higher order harmonics are prevented from passing through the voltage transformer. And it will reduce the stress on the injection transformer. But there can be a phase shift and voltage drop in the inverted output. This can be reduced by placing the filter in the load side. But in this case since the higher order harmonic currents do penetrate to the secondary side of the transformer, a higher rating of the transformer is necessary. However the leakage reactance of the transformer can be used as a part of the filter, which will be helpful in tuning the filter.
Ø By-pass switch : The static switch is a versatile device for switching a new element into the circuit when the voltage support is needed. It has a dynamic response time of about one cycle. To correct quickly for voltage spikes, sags or interruptions, the static switch can used to switch one or more of devices such as capacitor, filter, alternate power line, energy storage systems etc .
Since the DVR is a series connected device, any fault current that occurs due to a fault in the downstream will flow through the inverter circuit. The power electronic components in the inverter circuit are normally rated to the load current as they are expensive to be over rated. Therefore to protect the inverter from high currents, a by-pass switch (crowbar circuit) is incorporated to by-pass the inverter. Basically the crowbar circuit senses the current flowing in the distribution circuit and if it is beyond the inverter current rating the circuit bypasses the DVR
circuit components (DC Source, inverter and the filter) thus eliminating high currents flowing through the inverter side. When the supply current is in normal condition the crowbar circuit will become inactive.
Ø Injection Transformer :The basic function of the injection transformer is to increase the voltage supplied by the filtered VSI output to the desired level while isolating the DVR circuit from the distribution network. The transformer winding ratio is pre-determined according to the voltage required in the secondary side of the transformer (generally this is kept equal to the supply voltage to allow the DVR to compensate for full voltage sag) .A higher transformer winding ratio will increase the primary side current, which will adversely affect the performance of the power electronic devices connected in the VSI. The rating of the injection transformer is an important factor when deciding the DVR performance, since it limits the maximum compensation ability of the DVR .
Operating states of DVR
ü During a voltage sag/swell on the line
The DVR injects the difference between the pre-sag and the sag voltage, by supplying the real power requirement from the energy storage device together with the reactive power. The maximum injection capability of the DVR is limited by the ratings of the DC energy storage and the voltage injection transformer ratio. In the case of three single-phase DVRs the magnitude of the injected voltage can be controlled individually. The injected voltages are made synchronized (i.e. same frequency and the phase angle) with the network voltages .
ü During the normal operation
Since the network is working under normal condition the DVR is not injecting any voltages to the system. In that case, if the energy storage device is fully charged then the DVR operates in the standby mode or otherwise it operates in the self charging mode. The energy storage device can be charged either from the power supply itself or from a different source .
ü During a short circuit or fault in the downstream of the distribution line
Here the by-pass switch is activated to provide an alternate path for the fault currents. Hence the inverter is protected from the flow of high fault current through it, which can damage the
sensitive power electronic components.