Voltage Control in Distribution Networks using On-Load Tap Changer Transformers

  • Chao Gao

Student thesis: Doctoral ThesisPhD


Voltage is one of the most important parameters for electrical powernetworks. The Distribution Network Operators (DNOs) have theresponsibility to maintain the voltage supplied to consumers withinstatutory limits. On-Load Tap Changer (OLTC) transformer equippedwith Automatic Voltage Control (AVC) relay is the most widely used andeffective voltage control device.Due to a variety of advantages of adding Distributed Generation (DG),more and more distributed resources are connected to local distributionnetworks to solve constraints of networks, reduce the losses from powersupply station to consumers. When DG is connected, the direction ofpower flow can be reversed when the DG output power exceeds the localload. This means that the bidirectional power flow can either be frompower grid towards loads, or vice versa. The connection point of DG maysuffer overvoltage when the DG is producing a large amount of apparentpower. The intermittent nature of renewable energy resources which aremost frequently used in DG technology results in uncertainty ofdistribution network operation. Overall, conventional OLTC voltagecontrol methods need to be changed when DG is connected todistribution networks. The required voltage control needs to addresschallenges outlined above and new control method need to be formulatedto reduce the limitations of DG output restricted by current operationalpolicies by DNOs.The thesis presents an analysis of voltage control using OLTCtransformer with DG in distribution networks. The thesis reviewsconventional OLTC voltage control schemes and existing policies ofDNOs in the UK. An overview of DG technologies is also presented withtheir operation characteristics based on power output. The impact of DGon OLTC voltage control schemes in distribution networks is simulatedand discussed. The effects of different X/R ratio of overhead line andunderground cable are also considered. These impacts need to becritically assessed before any new method implementation.The thesis also introduces the new concepts of Smart Grid and SmartMeter in terms of the transition from passive to active distributionnetworks. The role of Smart Meter and an overview of communicationtechnologies that could be used for voltage control are investigated. Thethesis analyses the high latency of an example solution of which cost andavailability are considered to demonstrate the real-time voltage controlusing Smart Metering with existing communication infrastructurescannot be achieved cost-effectively.The thesis provides an advanced compensation-based OLTC voltagecontrol algorithm using Automatic Compensation Voltage Control(ACVC) technique to improve the voltage control performance with DGpenetration without communication. The proposed algorithm issimulated under varying load and DG conditions based on SimulinkMATLAB to show the robustness of the proposed method. A generic11kV network in the UK is modelled to evaluate the correct controlperformance of the advanced voltage control algorithm while increasingthe DG capacity.
Date of Award9 Oct 2013
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorMiles Redfern (Supervisor)


  • voltage control
  • DLTC
  • distributed generation
  • smart grid
  • compensation-based algorithm

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