19-01-2011, 05:13 PM
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Abstract
Interest in Power Quality has been explicitly seen in Electrical Power Engineering since past decade, even though Utilities all over the world have for decades worked on the improvement of voltage quality, what is now known as power quality. There are numerous type of power quality issues and power problems each of which might have varying and diverse causes. Typical power problems include voltage sag, harmonics, notching,
Transients like surge, swell, etc. Control of power quality problems involves cooperation between network operator (utility), customer and equipment manufacturer. A Dynamic Voltage Restorer (DVR) is a distribution voltage DC-to-AC solid-state switching converter that injects three single phase AC output voltages in series with the distribution feeder and in synchronism with the voltages of the distribution system. A DVR is interface equipment between utility and customer connected in series between the supply and load to mitigate the three major power quality problems, namely, the voltage sags, swells, and interruptions. The equations are formulated for calculating the voltages and power injected (inverter rating) from each of the three DVR phases (3-phase DVR). The results are presented, considering an example system.
Introduction
Back Ground:
Power quality may be defined as any power problems manifested in voltage, current or frequency deviations that results in failure or mis -operation of customers equipment. Both electric utilities and end users of electrical power are becoming increasingly concerned about the quality of electric power. Power quality is an umbrella concept for multitude of individual types of power system disturbances. The issues that fall under this umbrella are not necessarily new. What is new is that engineers are now attempting to deal with these issues with a systems approach rather than as individual problems. One important and noticeable change seen is that the quality of electricity supplied is now subject to legislation which considers it to be no different from other goods and services.
Just a few years ago, momentary power outages, sags, swells, surges had relatively little effect on most industrial processes. Today, manufacturing systems, sensitive telemetry, and precision electronic equipment can be disturbed, halted, or even damaged by voltage sag of two or three electrical cycles . Production losses can soar. Power Quality problems evidence themselves in a variety of ways such as: Computer shut down, malfunction of errors, PLC (Programmable Logic Controller) malfunction or errors, variable speed drives tripping out, racing or blinking digital clocks, etc. These can give problems ranging from inconvenience to loss of manufacturing capability with substantial loss in income Understanding Power Quality and the range of associated problems becomes very important to mitigate the problems. The ideal power supply to a low voltage customer is 240 / 415 V at 50 Hz with a sinusoidal wave shape. The electricity supplier through his local network cannot keep the supply exactly at the ideal due to a range of disturbances outside its control and attempts to maintain its voltage within specified ranges. Power Quality problems arise when these ranges are exceeded and this can occur in three ways
a) Frequency events: change of the supply frequency outside of the normal range
b) Voltage events: change of the voltage amplitude outside its normal range (may occur for very short periods or be sustained)
c) Waveform events: distortion of the voltage waveform outside the normal range.
Actual voltage varies from the normal range because of disturbances on the supply system, within customer’s plant and / or within nearby plants. These disturbances can
i. Damage sensitive data processing, control and instrumentation equipment
ii. Interrupt supply
iii. Trip out variable speed drives
iv. Cause data processing, control and instrumentation equipment to malfunction
v. Cause capacitors, transformers and induction motors to overheat
vi. Cause annoying light flicker
While the electricity supplier (local network operator) has the responsibility of keeping the power supply voltage within specified limits, Customers have two basic responsibilities:
1) To ensure that their equipment is able to tolerate the normal range of supply disturbances (in accordance with the regulations)
2) To ensure that their equipment does not cause disturbances which will propagate into Supply system at an excessive level (this is very important aspect of power quality)Frequency, Voltage and Waveform events are the basic types of power quality disturbances. The frequency of a power system is established by the rotational speed of the power station generators and it is very rare that this frequency is significantly varied and it is not further discussed in this paper. Waveform events result in distortion of the normal sinusoidal wave shapes of the mains voltage. Harmonics, Inter-harmonics, notching, transients, noise disturbance etc. and these are mostly caused by the consumers of electricity due to their equipment, particularly power electronic related equipment and induction motors. These are also not further discussed in this paper. The third type of power quality disturbance, viz, Voltage events are considered in this paper. The role and certain deign aspects of Dynamic Voltage Restorer (DVR) to mitigate the power quality problems related with the voltage events are presented here. The voltage is normally held in the range of ± 6%
Voltage variations can be divided into several categories:
i. Long term variations lasting more than 1 minute,
ii. Short term variations of duration less than 1 minute, called sags (voltages between 10% and 90% of nominal) or swells (voltage greater than 110% of nominal). These are shown in iii. Voltage unbalance where the voltage on each phase conductor is different.
iv. Continuous or random fluctuations that are observed as light flicker
v. Interruptions where supply is lost completely
vi. Neutral-ground voltage rises that are usually
Associated with poor grounding/earthing practices.
Power quality problems can originate at the supply system, or the customer’s plant or even a neighboring installation which could propagate via the supply. Some effects of power quality disturbances for the voltage events are shown in the following Table
Disturbance Effect
Over voltage Overstress insulation
Under voltage Excessive motor current
Unbalance Motor heating
Neutral-ground voltage Digital device malfunction
Interruption Complete shut down
Sag Variable speed drive & computer trip-out
Swell Overstress insulation
Fluctuations Light flicker
Voltage Sag / Swell
Voltage sag/swell that occurs more frequently than any other power quality phenomenon is known as the most important power quality problems in the power distribution systems. And Voltage sag is defined as a sudden reduction of supply voltage down 90% to 10% of nominal.
According to the standard, a typical duration of sag is from l0 ms to 1 minute.
On the other hand, voltage swell is defined as a sudden increasing of supply voltage up 1l0% to 180% in rms voltage at the network fundamental frequency with duration from 10 ms to 1 minute
Voltage sag/swell often are caused by faults such as single line-to-ground fault, double line-to ground fault on the power distribution system or due to starting of large induction motors or energizing a large capacitor bank. Voltage sag/swell can interrupt or lead to malfunction of any electric equipment which is sensitive to voltage variations
Mitigation of Power Quality Problems:
Power quality problems can be mitigated with the following practices:
Proper earthing and its verification, Uninterruptible Power supplies (UPS), Local or embedded generation (such as diesel generators, microturbines, fuel cells, stirling engines, etc), Transfer switches, Static breakers, Active filters, Static VAR compensators (SVC), Passive filters, Energy storage systems, Ferro-resonant transformers, DVRs etc. The interface between the system and the equipment is the most common place to mitigate sags and interruptions. A DVR is one of such utility-customer interface equipment designed to mitigate the power quality problems associated with voltage sags, swells and interruptions, The next section gives an introduction to DVR and explains its functioning and suitable locations. It also presents a basic block diagram of DVR and explains the three major components of a DVR. Most of the mitigation techniques are based on the injection of active power, thus compensating the loss of active power supplied by the system. All modern techniques are based on power electronic devices, with voltage source converters being the main building block. Section 3 presents the equations for calculating the injection voltages and rating of a DVR to mitigate voltage sags for a 3-phase DVR. An example system is also given in the same section along with the results obtained.
Dynamic Voltage Restorer
A DVR, Dynamic Voltage Restorer is a distribution voltage DC-to-AC solid-state switching converter that injects three single phase AC output voltages in series with the distribution feeder, and in synchronism with the voltages of the distribution system. By injecting voltages
of controllable amplitude, phase angle, and frequency (harmonic) into the distribution feeder in instantaneous real time via a series-injection transformer, the DVR can restore the quality of voltage at its load side terminals when the quality of the source side terminal voltage is
significantly out of specification for sensitive load equipment. It is designed to mitigate voltage sags and swells on lines feeding sensitive equipment. A viable alternative to uninterruptible power systems (UPS) and other utilization solutions to the voltage sag problem, the DVR is specially designed for large loads of the order of 2 MVA to 10 MVA served at distribution voltage. A DVR typically requires less than one-third the nominal power rating of the UPS [6]. DVR can also be used to mitigate troublesome harmonic voltages on the distribution system.
DVR comprises of three main parts:
1. Inverter
2. DC energy storage
3. Control system
Basic Principle
The basic idea of a DVR is to inject the missing voltage cycles into the system through series injection transformer whenever voltage sags are present in the system supply voltage. As a consequence, sag is unseen by the loads. During normal operation, the capacitor receives energy from the main supply source. When voltage dip or sags are detected, the capacitor d elivers dc supply to the inverter. The inverter ensures that only the missing voltage is injected to the transformer. A relatively small capacitor is present on dc side of the PWM solid state inverter and the voltage over this capacitor is kept constant, by exchanging energy with the energy storage reservoir. The required output voltage is obtained by using pulse-width modulation switching pattern. As the controller will have to supply active as well as reactive power, some kind of energy storage is needed. In the DVRs that are commercially available now large capacitors are used as a source of energy. Other potential sources are being considered are: battery banks, superconducting coils, and flywheel.
DVR Components
With reference to Fig. the main components of DVR are
1) Energy Storage Unit: The required energy for compensation of load voltage during sag can be taken either from an external energy storage unit (batteries) or from the supply line feeder
Through a rectifier and a capacitor.
2) Inverter Circuit: Since the vast majority of voltage sags seen on utility systems are unbalanced, mostly due to single-phase events, the VSC will often be required to operate with unbalanced switching functions for the three phases, and must therefore treat each phase
Independently. Mitigation of Voltage Sags in a Refinery with Induction Motors Using Dynamic Voltage Restorer (DVR) 121 Moreover, a sag on one phase may result in a swell on another phase, so the VSC must be capable of handling both sags and swells simultaneously. The variable output voltage of the inverter is achieved using PWM scheme.
3) Filter Unit: The nonlinear characteristics of semiconductor devices cause distorted waveforms associated with high frequency harmonics at the inverter output. To overcome this problem and provide high quality energy supply, a harmonic filtering unit is used. This can cause voltage drop and phase shift in the fundamental component of the inverter output, and has to be accounted for in the compensation voltage.
4) Series Injection Transformers: Three single-phase injection transformers are used to inject the missing voltage to the system at the load bus. To integrate the injection transformer correctly into the DVR, the MVA rating, the primary winding voltage and current ratings, the turn-ratio and the short-circuit impedance values of transformers are required. The existence of the transformers allow for the design of the DVR in a lower voltage level, depending upon the stepping up ratio. In such case, the limiting factor will be the ability of the inverter switches to withstand higher currents.
5) Controller and auxiliary circuits: By-Pass switches, breakers, measuring and protection relays are some auxiliaries to the DVR block, in addition to the controller of the DVR.
Conclusion:
The Dynamic Voltage Restorer (DVR) is a promising and effective device for power quality enhancement due to its quick response and high reliability. The conclusion is that the DVR is an effective apparatus to protect sensitive loads from short duration voltage dips. The DVR can be inserted both at the low voltage level and at medium voltage level. The series connection with the existing supply voltages makes it effective at locations where voltage dips are the primary problem. However, the series connection makes the protection equipment more complex as well as the continuous conduction losses and voltage drop. The role of a DVR in mitigating the power quality problems in terms of voltage sag, swell and interruptions is explained. The equations for calculating the voltages and power injected from each of the three DVR phases are given. The results obtained for a single phase dip to zero volts on the red phase are presented to design the PWM Inverter rating of the DVR.
