• Ziye Guo

Student thesis: Phd


To achieve a net zero carbon network by 2050 in the UK, a significant increase in the integration of power electronic interfaced (PEI) renewable generation is required. The main problem of the increasing integration of PEI based renewable generation is the short circuit level (SCL) on the transmission network will vary significantly, because at certain times significant synchronous generators will be used, but at other times most of the power will be supplied via inverters. Under this condition, future transmission line protection must operate with varying SCL level. In addition, to the detection of solid faults, resistive faults should also be detected and the protection must remain stable when faults occur outside the protected zone or it is non-fault disturbance. Superimposed directional comparison (SDC) protection has a fast directional response time and is less reliant on high values of fault current. The SDC protection can be used as a main transmission line protection. This project focuses on the use of the SDC protection to protect a network based on double circuit transmission lines when the SCL of the network is significantly varying. The SDC protection works by comparing the polarity of the superimposed voltage and current signal. These superimposed signals are obtained by subtracting the signal observed one cycle before the fault or disturbance and comparing with the corresponding values after the change occurs. Accurate system frequency tracking is an essential part to ensure the time delay is exactly one cycle, hence ensuring the superimposed signals are accurately derived. Four methods are used to track frequency including superimposed elements extraction method (SSET) and all methods are tested and analyzed in MATLAB. Then, a reduced National Grid model with double circuit transmission lines is modelled in DIgSILENT. The SDC protection is developed and applied to protect a transmission line in the model. Several case studies with varying SCL are tested and analyzed. After that, a function based on superimposed technique in a new type relay is tested and analysed in physical laboratory. The results show the SSET is better for accurate frequency tracking because it can operate at a lower sampling rate than other frequency tracking methods and delivers similar results; noise and harmonics bring errors to all the methods, the SSET can track frequency without filters but with small errors; the use of 5th order low-pass filter can filter noise and harmonics and minimize the errors; the SDC protection can work in a network where the SCL is varying, it can cope with reasonable levels of fault resistance; it also remains stable during external faults; the time for this protection to make directional decisions is within 5 milliseconds. The SDC protection is highly dependent on the differences between the pre-fault signals and fault signals and it will not operate if the differences are very small. This is reasonable because other protections will not reliable operate when the fault current is very small. This project indicates the potential of using the SDC protection as a main protection for transmission lines.
Date of Award1 Aug 2023
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorHaiyu Li (Supervisor) & Jelena Ponocko (Supervisor)


  • Superimposed directional protection
  • Frequency tracking
  • Directional protection
  • Short circuit level

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