Power Quality Improvement by Custom Power Devices-A Review

ISSN 2321 3361 © 2017 IJESC

Research Article

Volume 7 Issue No. 1

Power Quality Improvement by Custom Power Devices- A Review Pradeep Ku mar1 , Dr. Ritula Thaku r2 , Dr.Lini Mathew3 M.E Student 1 , Assistant Professor2 , Associate Professor and Head 3 Depart ment of Electrical Engineering, National Institute of Technical Teachers’ Training and Research, Chandigarh, India Abstract: Power quality is one of major concerns in the present era due to the increase in modern sensitive and sophisticated loads connected to the Distribution System. Recently, the Power electronics controllers are gain ing concern to provide the quality of power for both power suppliers and consumers. Many power filtering technology i.e. passive filters, active power filters, hybrid filters have applied fro m t ime to time fo r g iving the solution of power quality problems to end users, but the users could not fully satisfied them. New technologies, using power electronics-based concepts, have been developed to provide protection for commercia l and industrial customers fro m power quality problems on the electrical distribution system. Such power electronics based devices are known as Custom Power devices. There are many custom power devices and they are divid ed in two groups: network-reconfiguring type and compensating type. The network reconfiguring devices are: solid-state current limiter (SSCL), solid-state breaker (SSB) and solid-state transfer switch (SSTS). The co mpensating devices are: Distribution Static Co mpensator (DSTATCOM), Dynamic Vo ltage Restorer (DVR), and Un ified Po wer Quality Co mpensator (UPQC). This paper present an overview of custom power devices used in the distribution systems for mitigating the power quality issues. Keywords: Custom power, Distribution Static Co mpensator (DSTATCOM), Dynamic Voltage Restorer (DVR), Un ified Po wer Quality Co mpensator (UPQC). I. INTRODUCTION In many ac d istribution systems, due to installation of sensitive and sophisticated sensitive nature of loads, users continue to demand higher power quality. There are numerous power quality polluting loads are present in a power system. New technologies, using power electronics -based concepts, have been developed to provide protection for co mmercial and industrial consumers fro m power quality problems on the electrical distribution system. Such power electronics based devices are known as Custom Power devices. The custom power devices are installed in a distribution networks for mitigating the various power quality problems. The custom power enhances the reliability and quality of power that is delivered to customers. Custom power provides an integrated solution to these problems which are faced by the utilities and power distributors. The custom power devices are basically of two types – network reconfiguring type and compensating type. The network reconfiguring devices are: solid -state current limiter (SSCL), solid-state breaker (SSB) and solidstate transfer switch (SSTS). The co mpensating devices are: Distribution Static Co mpensator (DSTATCOM), Dynamic Vo ltage Restorer (DVR), and Un ified Po wer Quality Co mpensator (UPQC). II. CUS TOM POWER CONCEPT Custom Power is a concept based on the use of power electronic controllers in the distribution system to supply value-added, reliab le, high quality power to its customers. For many customers this is a preferred alternative to the customer improvising utility power by their own means, mostly in a band aid manner with numerous uninterruptible power supplies, as is done now. There are many utilit ies are moving in the direction of value-added Custom Power service to their end users (large customers). Custom Po wer means that the customer receives quality power with specified standards from a utility or a service provider. The equip ments installed by the

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end users in coordination with the utility, which includes an acceptable combination of the fo llo wing features:  

No (or rare) power interruptions. Magnitude and duration of voltage reductions within specified limits.  Magnitude and duration of overvoltages within specified limits.  Low harmonic vo ltage.  Low phase unbalance.  Acceptance of fluctuating, nonlinear and low power factor loads without significant effect on the terminal voltage. This can be done on the basis of an individual, large customer, industrial / Co mmercial Park or a supply for a high tech community on a wide area basis. III.

CUS TOM POWER DEVICES

Custom power is a strategy, which is intended principally to convene the requirement of industrial and commercial consumers.

Figure. 1. Cl assification of custom power devices .

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The concept of the custom power is tools of application of power electronics controller devices into power distribution system to supply a quality o f power, demanded by the sensitive users. The custom power devices used in the distribution networks are network configuring type and compensating type. The classification of custom power devices CPDs is shown Fig. 1. A.

NETWORK RECONFIGURING CUS TOM POWER DEVIC ES

TYPE

This network configuring device transfers the power fro m the preferred feeder to the alternate feeder. The power transfer takes place when the voltage sag/swell occurs in the preferred feeder. Solid state transfer switch consists of two opposite poled switch which is made of thyristors. It is used for th e protection of the sensitive loads. Let SW 1 and SW2 are the switches made fro m thyristors. Switch SW 1 is at p referred feeder side and switch SW 2 is at alternate feeder side as shown in Fig 4.

The network reconfiguring devices are usually called switchgear and they include current limit ing, circuit breaking and current transferring devices. These devices are series connected devices. However, most popular network reconfiguring device is SSTS. (i) Soli d state current li miter (SSCL): This network reconfiguring device limits a fau lt current by quickly inserting a series inductance in the fault path. The schematic diagram of the solid state current limiter (SSCL) is shown in Fig.2. The Solid state current limiter (SSCL) consists of two opposite poled switches in parallel with an inductor. This inductor is called current limit inductor. The current limiter is connected is series with a feeder such that it can restrict the current in case of a fault downstream. In the healthy state, the o pposite poled switches remains closed. These switches are opened when a fau lt is detected and then fault current is flows through an inductor.

Figure. 4 S olid state transfer s witch (SSTS) Let the voltage sag problem occurs in the preferred feeder then SSTS closes the switch SW 2 and current starts flowing through the alternate feeder to the load and the switch SW 1 disconnected. Uninterrupti ble power supply (UPS) Uninterruptible power supply (UPS) is the conventional response to circu mvent production interruption and outage costs. The single line diagram of ups is shown in the Fig 5.

Figure.2. S olid state current li miter (SSCL). (ii) Solid state breaker (SSB): Fig. 3 shows a schematic diagram of a solid state breaker SSB.

Figure. 3 S olid state breaker (SSB). The function of a solid state breaker SSB is to disconnect the faulty element fro m the system. It operates faster than a mechanical circuit breaker. This circu it breaker has almost the same topology as that of an SCL only the d ifference is that a limit ing inductor is connected in series with an opposite poled thyristors pair. The pair of thyristors is switched on simu ltaneously as the bidirectional switch GTO is switched off once a fault is detected. This will force the fault current to flow through the limiting inductor. (iii) Solid state transfer s witch (SSTS): Fig. 4 shows a schematic diagram of a So lid state transfer switch (SSTS).

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Figure.5. A single line diagram of Uninterrupti ble power suppl y (UPS). In UPS load has received the power fro m source via t wo stage operation: conversion (ac/dc) and inversion (dc/ac). During voltage dip or an interruption, the load voltage is made constant by energy, generated through battery. The performance of ups is depending on energy storage capacity of battery. For high power load financially, it is not suitable because of two conversions the maintenance cost of battery has become too high. B. COMPENSATING DEVICES

TYPE

CUS TOM

POWER

The co mpensating custom power devices compensate a load, correct its power factor, unbalance or improve the power quality of supplied voltage. The compensating type custom power devices are classified as: i) Dynamic Static Co mpensator D-STATCOM ii) Dynamic Voltage Restorer DVR iii) Unified Power Quality conditioner

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(i) Dynamic Static Compensator D-STATCOM The D-STATCOM is one of the most effective Custom power devices used in a distribution network. It is a three-phase and shunt connected power electronics based device which is connected near the load at the distribution systems. The schematic diagram of a D-STATCOM (Distribution Static Co mpensator), is shown in Figure-6. It consists of a two-level Vo ltage Source Converter (VSC), a dc energy storage device, a coupling transformer connected in shunt to the distribution network through a coupling transformer. The VSC converts the dc voltage across the storage device into a set of threephase ac output voltages. These voltages are in phase and coupled with the ac system through the reactance of the coupling transformer. Su itable adjustment of the phas e and magnitude of the D-STATCOM output voltages allows effective control of active and reactive power exchanges between the DSTATCOM and the ac system. Such configuration allows the device to absorb or generate controllable active and reactive power. The voltage-source converter (VSC) connected in shunt with the ac system provides a multifunctional topology which can be used for up to three quite distinct purposes: 1. Voltage regulat ion and compensation of reactive power 2. Correction of power factor. 3. Elimination of current harmonics. A voltage-source converter (VSC) is a power electronic device as shown in Fig 6. The function of a VSC is to generate a sinusoidal voltage with minimal harmon ic distortion fro m a DC voltage and connected to AC distribution line through coupling transformer. The DC side of the voltage-source converter (VSC) is connected to a DC capacitor. The AC terminals of the converter are connected to the Point of Co mmon Coupling (PCC) through an inductance as shown in Fig. 6, such inductance could be a filter inductance or the leakage inductance of the coupling transformer. A dc capacitor could be charged by a battery source, or could be precharged by the converter itself. The output voltage of the voltagesource converter VSC is co mpared with AC bus voltage. If the voltage of a converter is equal to the AC terminal voltage, the reactive power exchange is zero. When the AC terminal voltage magnitude is greater than that of voltage-source converter (VSC) voltage, the DSTATCOM is in the capacit ive mode of operation and vice versa. The amount of reactive power flow is proportional to the difference in the two voltages.

Figure.6. the Schematic diagram of a DSTATCOM (Distributi on Static Compensator)

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If the D-STATCOM has a DC source or energy storage device on its DC side, it can supply real power to the distribution networks. The active power flow is controlled by the angle between the ac terminal and voltage-source converter (VSC) voltages. When phase angle of the AC terminal voltage leads the voltage-source converter (VSC) phase angle, the DSTATCOM absorbs the real power fro m the distribution networks. If the phase angle of the AC terminal voltage lags the voltage-source converter (VSC) phase angle, the DSTATCOM supplies real power to distribution networks. The DSTATCOM operates in both current and voltage control modes. (ii) Dynamic Voltage Restorer DVR Dynamic Vo ltage Restorer (DVR) is a series compensating device, which can protect a sensitive load from the voltage distortions. DVR is one of the most efficient and effective modern custom power devices that used to mit igate the power quality problems in the distribution networks. Co mmercially, static series compensator is known as Dynamic Vo ltage Restorer (DVR). The DVR is installed in series with the line to inject voltage into the system in order to regulate the load side voltage. The location of a DVR installed into a distribution network is shown in Fig. 7.

Figure.7. Location a DVR i n a distri bution network. The main co mponents of the DVR are shown in Figure 8. 1. Voltage Source Converter (VSC) 2. Injection/coupling transformer 3. Energy storage device 4. Harmon ic filter 5. DC charg ing circu it 6. Control and Protection system 1. Voltage Source Converter (VSC): A VSC is a power electronic system consists of a storage device and switching devices, which can generate a sinusoidal voltage at any required frequency, magnitude, and phase angle. In the DVR application, the VSC is used to temporarily replace the supply voltage or to generate the part of the supply voltage which is missing. There are four main types of switching devices: Metal Oxide Semiconductor Field Effect Transistors (MOSFET), Gate Turn -Off thyristors (GTO), Insulated Gate Bipolar Transistors (IGBT), and Integrated Gate Co mmutated Thyristors (IGCT). Each type has its own benefits and drawbacks. The IGCT is a recent compact device with enhanced performance and reliability that allows building VSC with very large power ratings. Because of the highly sophisticated converter design with IGCTs, the DVR can compensate dips which are beyond the capability of the past DVRs using conventional devices. 2. Injecti on Transformer: The Injection / Booster transformer is a specially designed transformer that attempts to 4158

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limit the coupling of noise and transient energy from the primary side to the secondary side. The main tasks of in jection transformers are: i. It connects the DVR to the distribution network through the HV-windings and transforms and couples the injected compensating voltages generated by the -voltage source converters to the incoming supply voltage. ii. In addit ion, the Injection / Booster transformer serves the purpose of isolating the load fro m the system (VSC and control mechanis m).

schematic diagram o f a Unified Power Quality conditioner is shown in Fig.9. A UPQC, co mbines the operations of a Distribution Static Co mpensator (DSTATCOM) and Dynamic Vo ltage Regulator (DVR) together. This co mb ination allows a simu ltaneous compensation of the load currents and the supply voltages, so that compensated current drawn fro m the network and the compensated supply voltage delivered to the load are sinusoidal and balanced. It places in the distribution system to reduce the disturbances that impact on the performance load. UPQC is the only mult i functioning device wh ich can reduce several problems power quality problems.

3. Energ y Storage Unit: The purpose of storage devices is to supply the necessary energy to the VSC v ia a dc link for the generation of in jected voltages. During voltage sag, the DVR injects a voltage to restore the load supply voltages . The different kinds of energy storage devices are superconductive magnetic energy storage (SM ES), batteries, and capacitance. The energy storage can be different depending on the needs of compensating. The DVR often has limitations on the depth and duration of the voltage dip that it can compens ate. 4. Harmonic filter: The main function of a harmonic filter is to maintain the harmon ic voltage content generated by the voltage source converter (VSC) to the permissible level. It has a small rating appro ximately 2% of the load M VA connected to delta-connected tertiary winding of the in jection transformer. 5. DC Charging Circuit: The dc charging circuit has two main tasks. i. The first task is to charge the energy source after a sag compensation event. ii. The second task is to maintain dc link voltage at the nominal dc lin k voltage. 6. Control and protection: The control mechanism of the general configuration typically consists of hardware with programmab le logic. A ll protective functions of the DVR should be imp lemented in the software. Differential current protection of the transformer, or short circu it current on the customer load side are only two examp les of many protection functions possibility.

Figure.9. Schematic di agram of a Unified Power Quality condi tioner (UPQC). The applications of custom power devices are tabulated in Table I. Table I. Application of Custom Power Devices. Custom Power Devices 1. Static transfer switch (STS)

2.

3.

Applicati ons  Vo ltage sag and swell protection.  Transfer of power fro m different feeder (transfer load)

current /Static Breaker



Distribution Static Co mpensator (DSTATCOM )



Static limiter Circuit (SCL)



  

4.

Dynamic Vo ltage Restorer (DVR)

   

Figure.8. B asic configurati on of a DVR.

   5.

(iii) Unified Power Quality condi tioner (UPQC) The Unified Power Quality Conditioner (UPQC) is a custom power device that consists of two voltage source inverters (VSI) is connected to a dc energy storage capacitor. The International Journal of Engineering Science and Computing, January 2017

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Unified Power Quality Co mpensator ( UPQC)

 

Fault Current Limitation Break Faulted Circu it

Load current balancing Power factor improvement Flicker effect compensation Current Harmonic compensation Vo ltage regulation Flicker attenuation Vo ltage sag and swell protection Voltage balancing VA R co mpensation Harmonic suppression Current balancing Active and reactive power control Vo ltage balancing Vo ltage regulation

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IV.

CONCLUS ION

The power quality problems in distribution systems are not new but customer awareness of these problems increased recently. It is very difficult to maintain electric power quality at acceptable limits. The power electronics based devices known as CUSTOM POW ER (CP) devices can be used for providing high power quality, at reasonable cost. These Custom Power devices provide solutions to power quality at the med iu m voltage distribution network level. In many cases, depending on the frequency of events and the cost associated with lost production, the CUSTOM POW ER device can provide payback in less than two years. The Custom Power devices are installed into the distribution system to protect entire plant, feeder, and loads. The DSTATCOM, is connected in shunt and it can be provide good power quality in bo th transmission and distribution levels. While the DVR, which is a series-connected device, can reduce the vulnerability of a sensitive plant to voltage sags and swells. UPQC is the key of custom power devices, can co mpensate both voltage and current related problems at the same time.

[12] S.V.RaviKu marandS.SivaNagaraju,”S imu lation of DSTATCOM and DVR in Po wer Systems”, ARPN Journal nof EngineeringandAppliedSciences,vol.NO.3,June 2007. [13] P. Boonchiam, N. Mithulananthan, “Understanding of Dynamic Voltage Restorers through Matlab Simulat ion”, Thammasat Int. J. Sc. Tech., Vo l. 11, No. 3, July-Sept. 2006. [14] “Custom Power-State of the Art” Cigre W G14.31, 2000. 22. [15] N. G. Hingorani, "Introducing custom power, ” vol.32, pp.41-48, June1995 23. [16] H. P. Tiwari, S. K. Gupta, “Dynamic Voltage Restorer Based on Load Condition”, International Journal of Innovation, Management and Technology, Vol. 1(1), April 2010, pp. 75-81.

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