Power quality has a significant influence on high-technology equipments related to communication, advanced control, automation, precise manufacturing technique and on-line service. For example, voltage sag can have a bad influence on the products of semiconductor fabrication with considerable financial losses. Power quality problems include transients, sags, interruptions and other distortions to the sinusoidal waveform. One of the most important power quality issues is voltage sag that is a sudden short duration reduction in voltage magnitude between 10 and 90% compared to nominal voltage. Voltage sag is deemed as a momentary decrease in the rms voltage, with duration ranging from half a cycle up to one minute. Deep voltage sags, even of relatively short duration, can have significant costs because of the proliferation of voltage-sensitive computer-based and variable speed drive loads. The fraction of load that is sensitive to low voltage is expected to grow rapidly in the coming decades. Studies have shown that transmission faults, while relatively rare, can cause widespread sags that may constitute a major source of process interruptions for very long distances from the faulted point. Distribution faults are considerably more common but the resulting sags are more limited in geographic extent. The majority of voltage sags are within 40%of the nominal voltage. Therefore, by designing drives and other critical loads capable of riding through sags with magnitude of up to 40%, interruption of processes can be reduced significantly. The DVR can correct sags resulting from faults in either the transmission or the distribution system.
Basic Principle of DVR
To quantify voltage sag in radial distribution system, the voltage divider model, shown in Fig. 1, can be used on the assumption that the fault current is much larger than the load current during faults. The point of common coupling (PCC) is the point from which both the fault and the load are fed. Voltage sag is mostly unbalanced and accompanied by phase angle jump.
From Fig, the voltage at the PCC and phase angle jump can be obtained byThe DVR is able to compensate the voltage sag especially at sensitive loads by injecting an appropriate voltage through an injection transformer. Figure 2 shows a block diagram of the DVR power circuit. When examining the DVR it can be divided into four component blocks: 1) Energy storage device,2) DC to DC power controller,3) A three-phase voltage converter,4) Three single-phase series injection transformers.
The design of the DVR allows real and reactive power to be either supplied or absorbed when operating. If a small fault occurs on the protected system, then the DVR can correct it using only reactive power generated internally. For correction of larger faults, the DVR may be required to develop real power. To enable the development of real power an energy storage device must be used; currently the DVR design uses a capacitor bank. Once the fault has been corrected and the supply is operating under normal conditions, the DVR replenishes the energy expended from the healthy system. The rating (in terms of energy storage capabilities) of the capacitor bank is dependent upon system factors such as the rating of the load that protects and the duration and depth of anticipated sags. When correcting large sag (using real power), the power electronics are fed from the capacitor bank via a DC-DC voltage conversion circuit.
The core element in DVR design is the three-phase voltage converter. This inverter utilizes solid-state power electronics (insulated gate bipolar transistors, IGBTs) to convert DC to AC and back again during operation. The DVR connects in series with the distribution line through an injection transformer, actually three single-phase transformers. The primary side (connected into the line) must be sized to carry the full line current.The primary voltage rating is the maximum voltage the DVR can inject into the line for a given application.The DVR rating (per phase), is the maximum injection voltage times the primary current.The bridge outputs on the secondary are filtered before being applied to the injection transformer. The bridges are independently controllable to allow each phase to be compensated separately. The output voltage wave shapes are generated by pulse-width modulated switching. When voltage sag reaches a value below the limit for correction using zero energy, the energy storage system within the DVR has to be used to aid voltage correction.
The ideal restoration is to make load voltages unchanged. When DVR restores large voltage disturbances, active power or energy should be injected from DVR to distribution system. If the capability of energy storage of DVR were infinite, DVR could maintain load voltage unchanged ideally during any kind of faults. However, the stored energy in DVR is limited practically by the limit of DC link capacity of DVR. Namely, DVR cannot restore the load voltage constantly when the voltage across the DC link has gone down and stored energy has run out eventually during deep voltage sag with long duration. Therefore, it is necessary to minimize energy injection from DVR.
There are several methods how to inject DVR mitigating voltage to distribution system: pre-sag compensation, in-phase compensation, and phase advance
Conventional DVR Voltage Injection Methods
The possibility of compensating voltage sag can be limited by a number of factors including finite DVR power rating, different load conditions, and different types of voltage sag. Some loads are very sensitive to phase angle jump and others are tolerant to phase angle jump. Therefore, the control strategy depends on the type of load character-istics . There are three distinguishing methods to inject DVR compensating voltage, that is, pre-sag compensation method, in-phase compensation method, and phase advance method.
Pre-sag compensation methods is to track supply voltage continuously and compensate load voltage during fault to pre-fault condition. Fig. 3 shows the single-phase vector diagram of the pre-sag compensation. In this method, the load voltage can be restored ideally, but injected active power cannot be controlled and is determined by external conditions such as the type of faults and load condition.In in-phase compensation shown in Fig. 4, the injected DVR voltage is in phase with measured supply voltage regardless of the load current and the pre-fault voltage. The advantage of this method is that magnitude of injected DVR voltage is minimized for constant load voltage magnitude.
Pre-sag compensation and in-phase compensation must inject active power to loads almost all the time. However, the amount of possible injection active power is confined to the stored energy in DC link, which is one of the most expensive components in DVR. Due to the limit of energy storage capacity of DC link, the DVR restoration time and performance are confined in these methods. For the sake of controlling injection energy, phase advance method was proposed.
The injection active power is made zero by means of having the injection voltage phasor perpendicular to the load current phasor. This method can reduce the consumption of energy stored in DC link by injecting reactive power instead of active power. Reducing energy consumption means that ride-through ability is increased when the energy storage capacity is fixed. On the other hand, the injection voltage magnitude of phase advance method is larger than those of pre-sag or in-phase method and the voltage phase shift can cause voltage waveform discontinuity, inaccurate zero crossing, and load power swing. Therefore, phase advance method should be adjusted to the load that is tolerant to phase angle jump, or transition period should be taken while phase angle is moved from pre-fault angle to advance angle
A Three Phase DVR & its Control
A sample three phase DVR capable of maintaining the load voltage balanced and of constant amplitude against flicker, harmonics, sags, swells and unbalance in supply and unbalance in load is discussed in the remaining part of this lecture.
The three phase inverter is made by three single phase inverters connected to star connected primary of interface transformer. The secondaries are connected in series in the lines.The three phase inverter rating is 10kVA and the transformer has a turns ratio of 1:5.This means that the inverter can inject upto 20% of rated voltage in series with the supply.Inverter modulator will saturate after that and clip the injected voltage at around 65V peak (assuming 320V peak phase voltage). The maximum load in the supply line is assumed to be around 50kVA. The inverter uses sinusoidal PWM (unipolar switching) at 20kHz switching frequency.
The load voltage is stepped down using PTs and a PLL is locked onto R phase.The pure sinewave phase synchronised to R phase goes into a Positive Sequence Constructor circuit (all pass filter based) which generates unit amplitude positive sequence waves.These templates are multiplied by the desired amplitude (320V) to form the desired load voltage.The actual load voltage from the sensing circuit is subtracted from this to form the reference signals into Inverter Modulator.The inverter injects the required voltage.The control strategy is feed-forward and hence is fast, but suffers from the disadvantage of not having any feedback.The Dc Side is assumed to be a power source like a battery or a AC-DC converter running from same bus.Correction strategy is inphase and hence active power flow is involved.The saturation block in this subsystem is set at ± 65 Volts to reflect the overmodulation limit of inverter. The simulation results are not included here due to space limitations. However the following comments are in order.Three simulation files (Simulink files) are included in the accompanying download file this DVR. They are "dvr_simple.mdl", "dvr_filt.mdl" and "dvr_full.mdl". The first one models the inverter as an ideal voltage controlled voltage source and can be used only to illustrate the concepts involved. The second one models the inverter as an ideal voltage controlled voltage source but includes the filter at the output of the inverter.The third one includes the PWM switching also , but does not take care of the inverter losses.
The first model will give very optimistic results under dynamic conditions – for example it will show that the output voltage is not even aware of a sudden phase change at input. This will not be true in practice. The various unmodelled delays along with the inverter filter response time will really pass the sudden changes in the input voltages at least partially to the output side.
The first model will yield a performance, which simply does not depend on the load current, since this model has no impedance anywhere. But in practice the output voltage will get affected by load harmonics due to two reasons – the inverter output filter will call for harmonic drops when harmonic load currents flow through it and in the absence of feedback control the system does not correct anything to the right of inverter. Secondly the finite bandwidth of inverter (due to a finite switching frequency) will make it fail in generating high frequency content produced at the source bus by high frequency component of load currents flowing in source impedance (which is taken as zero in the first simulink model).
When the amount of sag, swell or flicker or harmonic content is excessive the inverter will saturate and clip its output. This will lead to distortion in output voltage. But simulation runs reveal that this distortion remains under 10% even for sags which take source voltage to 100V peak. All three models include this clipping effect.
The control of DVR is not a very complex problem and in fact field experience justifies feedforward control. However providing a suitable DC Side energy source to handle long periods of sag or swell or flicker throughout the day (like arc furnace) will be a problem. If it is a Battery it requires a charger. Some researchers have proposed drawing charging power from the line using the same inverter during periods which sag or swell is little and can be handled by 90-degree voltage injection. But that makes the control pretty complex. If it is a AC-DC Diode Rectifier the DVR can handle only sags and not swells since during swells the inverter will absorb power (in the ‘inphase injection strategy’ considered here) and dump it on the DC Side.So, then it has to be a Bilaterlal Converter based AC-DC Converter and then we get very close to what they call a ‘Unified Power Quality Conditioner’ – then it is no more a DVR alone, but can easily become a UPQC.
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