A Review of Voltage Control Techniques of Networks With Distributed Generations Using on-Load Tap Changer Transformers

UPEC2010 31st Aug - 3rd Sept 2010 A Review of Voltage Control Techniques of Networks with Distributed Generations using On-Load Tap Changer Transformers C. Gao University of Bath Abstract- Voltage is an important parameter for the control of electrical power systems. The Distribution Network Operators (DNO) have the responsibility to regulate the voltage supplied to consumer within statutory limits. Traditionally, the On-Load Tap Changer (OLTC) transformer equipped with automat
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  A Review of Voltage Control Techniques of  Networks with Distributed Generations usingOn-Load Tap Changer Transformers C. Gao University of  M. A. Redfern University of   Abstract- Voltage is an important parameter for the control of electrical power systems. The Distribution Network Operators(DNO) have the responsibility to regulate the voltage supplied toconsumer within statutory limits. Traditionally, the On-LoadTap Changer (OLTC) transformer equipped with automaticvoltage control (AVC) relays is the most popular and effectivevoltage control device.Connecting Distributed Generation (DG) to the network inherently affects the feeder voltage profiles and influences thevoltage control in distribution systems. In recent years, thenumber of DGs connected to the distribution networks iscontinuing to grow. Their impact on the network is thereforedemanding proper attention and is affecting the design of newautomatic voltage control schemes for OLTCs.The potential for using Smart Grids also promises to have amajor influence on schemes for voltage control in the powersystems.This paper presents an overview of existing OLTC controlschemes which are used to control the voltage in distributionnetworks. It includes a discussion of new techniques which havebeen introduced and those being proposed to improve voltagecontrol for networks containing various levels of DG. It alsoexamines the potential opportunities for OLTC voltage controlschemes offered by the use of Smart Grids.  Index Terms -- OLTC transformer, AVC relays, distributedgeneration (DG), voltage regulation, TAPP, Smart Grid,STATCOM. I.   I  NTRODUCTION   Nowadays, the electric power transmission and distributionsystems normally operate at multiple voltage levels. Voltageis one of the most important parameters for the control of electric power systems. The On-Load Tap Changer (OLTC)transformers are used between these multiple voltage levels toregulate and maintain the voltage which is supplied toconsumers within statutory limits. The OLTC voltageregulation is naturally operated by changing the number of turns in one winding of the transformer to physically alter theratios of the transformer. More or less windings can beswitched into the system by the OLTC transformer then thealteration of ratios therefore affects control of the transformer output voltage to keep the voltage within predicted limits [1].The On-Load Tap Changer mechanism is a transformer component controlled automatically by a relay to increase or decrease voltage by altering the tap position of transformer.When the secondary voltage detected is no longer within the permitted deadband, the relay issues a command to the tapchanger mechanism to alter its tap position in order to restorethe required voltage level. The On-Load Tap Changer transformer, coupled with its voltage control relay regulatesthe transformer output voltage to keep the voltage magnitudewithin limits. It is generally used in the distribution networksto transform from 33 kV down to 11 or 6.6 kV. Fig. 1. AVC relay scheme A block diagram illustrates the basic operation and thegeneral arrangement of the OLTC and a simple AutomaticVoltage Control (AVC) relay which is shown in Figure 1.The AVC relay [2] monitors the voltage at the secondary sideof the transformer. With the comparison between load voltageand target voltage, the AVC relay determines whether toadjust the tap position or not in order to maintain the requiredvoltage level. For short duration voltage excursions outside of the voltage deadband, a time delay is introduced into theAVC relay to prevent operation of the tap changer.Since the distribution networks become more complex andthe number of distributed generation (DG) is continuing togrow, conventional OLTC voltage control schemes are goingto be less effective. Whenever there is reverse power flowcaused by the integration of distributed generation, there arecomplications for the operation of the AVC. Meanwhile, the potential use of Smart Grids also indicates to have asignificant influence on schemes for voltage control in the power systems. To deal with the voltage control problems UPEC201031st Aug - 3rd Sept 2010  together with the increasing penetration of the DGs as well asthe use of Smart Grid, DNOs need more stable and effectiveOLTC voltage control schemes.II.   B ASICS OF VOLTAGE CONTROL SCHEMES  There are various control characteristics associated withOLTC such as Line Drop Compensation (LDC), time gradingfor accommodating operation in series of transformers, aswell as a variety of circulating current compensationtechniques for operation of parallel transformers. Fig. 2. AVC relay scheme with LDC LDC monitors the voltage at the secondary side of thetransformer and then using a measure of the secondarycurrent to simulate the voltage drop across the feeder thatexists between the transformer and the load [3]. This voltagedrop along the feeder impedance is used to boost the voltageregulated at the transformer terminal therefore ensuring thecorrect voltage level maintains at the load where it isrequired. The LDC provides voltage control at a nominal load point rather than at transformer terminals as shown in Figure2.II.1 Operation in seriesThere are multiple voltage levels used for generation,transmission and distribution in most power supply networks.In each area, On-Load Tap Changer transformer will be used between these different voltage levels as shown in Figure 3. Fig. 3. Different OLTCs operated in series Due to the uncoordinated control schemes between the up-stream and down-stream controller, the OLTC transformer can become unstable. The grading time (GT) is introduced asan additional delay to ensure the up-stream transformer hasfinished its operation before the down-stream transformer restores the voltage level [3]. The GT is set as the worst casevoltage correction time.II.1.1 Communications assisted voltage control schemes   A communications unit can be used to replace the need for GT delay. When the up-stream transformer starts theoperation, a blocking signal is issued to stop the operation of down-stream transformer. The blocking signal is removedwhen the up-stream transformer has done its correction [2].Hence, the time delay is reduced from worst case correctiontime to up-stream operation time.II.1.2 Enhanced Voltage Control AVC Relay SchemeBecause of the high cost of the communications assistedvoltage control schemes, the Enhanced Voltage Control AVCRelay Scheme provides an autonomous tap-changer controlwithout the communications unit. This scheme includes thefeatures of Source Drop Compensation (SDC) and Pre-emptive Tap Changer operation [4].SDC   determines the voltage at the regulation point by thesource current and the feeder impedance between the up-stream and down-stream transformer. The down-stream AVCrelay gains an insight into this voltage when a voltagedisturbance occurs. When the voltage at the regulation pointis within its deadband, the up-stream transformer operation isassumed to be complete and the down-stream AVC relay can proceed with the local voltage correction. However, if thevoltage of regulation point is without its deadband, the down-stream AVC relay will wait until the voltage correction of up-stream transformer completed.Pre-emptive Tap Changer Operation is that when a voltagedeviation occurs and the monitored voltage drop is changedwith the disturbance, it can be assumed that the cause of thevoltage drop is either in full or in part to be the load change.Therefore, it allows the local AVC relay to correct the voltagewithout any up-stream correction. The grading time delayscan be over-ridden and a pre-emptive tap change initiated.II.2 Operation in parallelWith the growing customer demand for the higher securityand reliability of supply, it is common practice for the DNOsto parallel transformers on one site or across the network inorder to meet the engineering recommendation as shown in  Figure 4.In this situation, the main aim of the AVC scheme is tomaintain the voltage within statutory limits, at the same time,to minimise circulating current between parallel transformers.There are some factors might affect the AVC schemes suchas power factor, DG integration or varying load. TheStandard voltage control schemes for paralleled transformersare as follows.    Fig. 4. OLTC in parallel II.2.1 Master-Follower A simple and extensively used AVC scheme is the master-follower scheme. One OLTC transformer is designed as the“Master” and all other OLTC transformers in parallel with itas “Followers”. In this scheme the master transformer monitors the required voltage and alters the tap position toregulate the voltage to desirable level. The other OLTCtransformers replicate the same actions to make all paralleledtransformers kept on the same tap position [5]. The master-follower scheme can be used with LDC and operates under varying power factor, reverse power flow and DG integration.The disadvantage of this scheme is that the circulatingcurrent will flow between transformers if the paralleledtransformers are not the same type. Additionally, the paralleltransformers must be on the same site. It is impossible to usethis scheme across a network due to the connection betweenthe AVC relays.II.2.2 True Circulating CurrentThe   True Circulating Current scheme is considered to useessentially identical transformers as the master-follower scheme. This scheme regulates the voltage as well as reducescirculating current between paralleled transformers. Thedifferent tap positions of the transformers will result in acirculating current. This current is calculated by theinterconnection between controllers to create a voltage bias.The biasing in opposite polarities is used to correct the OLTCto adjust the relay setting voltage therefore the circulatingcurrent is minimised [5]. It can also work with LDC andoperates under varying power factor, reverse power flow andDG integration. The backward is that it is difficult to paralleltransformers which are not in the same site.II.2.3 Negative Reactance CompoundingThe negative reactance compounding (NRC) method is oneof the common AVC schemes used in the distributionnetworks. The negative value of reactance with LDC settingsis used to make the tap positions of paralleled OLTCtransformers to be similar [5].The following formulas show the relationship betweenLDC settings and negative reactance compounding (NRC)setting:The Figure 5 illustrates the NRC principle. The tap positionof transformer T1 is much higher than of transformer T2 inthis situation. A circulating current occurs and flows betweenthe two paralleled transformers. The individual transformer current I T1 is shifted clockwise and current I T2 anti-clockwise by the circulating current. A voltage drop I T · Z  NRC is createdto modify the target voltage from V VT to V AVC and used byAVC relay to correct the tap position. The effective measuredvoltage V AVC1 is seen by AVC relay of T1 higher than thetarget voltage V VT and as a result the tap change is down. Asimilar action is done by AVC of T2 but the tap change is up.When the circulating current is eliminated and target voltageis achieved, the action stops with a similar tap position of  both the parallel transformers [5]. Fig. 5. NRC principle The paralleling transformers with the negative reactancecompounding scheme can operate with transformers atdifferent positions in the networks and it is unnecessary to beidentical anymore due to the independently action of eachtransformers. However, the NRC scheme is not accuratewithout unity power factor thus is susceptible to the varying power factor. The voltage error is increased due to the power factor deviation [5]. The performance of LDC is reduced dueto the negative value of X LDC setting and an increased value of R  LDC is necessary to keep the same boost.II.2.4 Transformer Automatic Paralleling PackageThe Transformer Automatic Paralleling Package (TAPP)scheme, based on the NRC scheme, reduces the circulatingcurrent between the paralleled transformers by dividing themeasured current into load transformer current andcirculating current. TAPP scheme uses techniques based onthe target power factor, to evaluate circulating current bycomparing the measured transformer load current (I TR  ) withthe target power factor (pf  targ ) as shown in Figure 6. Two )2()1(  LDC  LDC  NRC  LDC  LDC  LDC   jX  R Z  jX  R Z  −=+=  separate circuits, one for LDC and one for compounding, areintroduced into TAPP scheme to eliminate the LDCdegradation with NRC. However, the disadvantage of TAPPscheme is that the load power factor deviation will result inan error in the controlled voltage due to knowledge of theload current being considered as circulating current. Thespecified power factor is the necessary factor to make thevoltage control to be satisfactory. Fig. 6. Principle of TAPP scheme III.   I  NFLUENCE OF THE DG S INTEGRATION  Due to a variety of reasons, more and more distributedresources are connected to provide local power supplies tosolve the constraints of the transmission or distributionnetworks, reduce the transportation cost from power supplystations to consumers and to exploit renewable energy.Therefore, the capacity of Distributed Generation (DG)connected to distribution networks has grown and iscontinuing to grow. The presence of DG considerablyinfluences the voltage regulation of distribution network. Thissituation demands much more attention to improve voltagecontrol techniques to accommodate the increasing number of DGs in distribution networks as shown in Figure 7. Fig. 7. Diagram of a distribution network with DG The voltage drop across the feeder is compensated by thecompounding setting with traditional AVC relay schemes.However, the presence of DGs including wind turbinesconnected to distribution networks affects the AVC relay performance and results in voltage regulation problems due tothe interference. The DG integration changes the power flowand sometimes results in reverse power flow as well as avoltage increase occur at the point of connection. Thus thefeeder currents measured by traditional AVC relay are nolonger proportional to load currents. The measured voltage isshifted upwards or downwards depending on the power factor of transformer current and direction of power flow to the DGand load [6].Improved voltage control schemes are the topic of on-going research to accommodate the presence of the DGsconnected to distribution networks.IV.    N EW DEVELOPMENT SCHEMES  The Enhanced TAPP scheme operates efficiently under varying power factor and intermittent feature of the DGwithout the degradation of LDC performance [5]. Thedrawbacks of the above schemes associated with DGintegration are eliminated. Even the voltage profile of dynamic distribution networks can be improved by EnhancedTAPP scheme. The paralleled transformers of EnhancedTAPP scheme can be on the same site or at different locationsin the network due to this scheme combines the TrueCirculating Current scheme and TAPP scheme toaccommodate the two situations respectively. Its principle isillustrated in Figure 8. Fig. 8. Enhanced TAPP scheme in True Circulating Current mode In the True Circulating Current mode, the measuredtransformer current (I Tn ) and load current (I T1 + I T2 ) are used tocalculate the circulating current. The AVC relay has seen theincreased voltage with higher tap position and decreasedvoltage with lower tap position due to the voltage drop(I CIRC ·Z T ). Then the tap down action for the OLTC transformer on higher tap position and up action for OLTC transformer onlower tap position are operated until the V T is within thedeadband. It is compared between the group load current (I T1  + I T2 ) and a full load current to provide a proper LDC voltage boost as the follow formula: )3( 1  FLT nTn LDC  LDC   I  I  Z V  ∑ = ⋅=   where T transformers are feeding the local network.
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