Abstract
This paper presents a non‐unit protection method for long transmission lines of multi‐terminal HVDC (MTDC) grids based on the magnitude and direction of modal voltage and current travelling waves. The method overcomes the difficulty resulting from high attenuation and distortion as reported in previous methods using fault‐generated transients for protection of long HVDC lines. The proposed method determines the faulted pole using multi‐resolution morphological gradient of the pole voltage. Then, the multiplication of multi‐resolution morphological gradient of the modal travelling waves is utilised as a criterion to discriminate between internal and external faults and non‐fault events. The criterion is a negative high value for internal faults, a negative low value for forward external faults and events, and a positive value for backward external faults and events. A four‐terminal HVDC grid with modular multilevel converters including long HVDC lines is modelled using the PSCAD/EMTDC software. The simulation study validates effectiveness of the method for detection and discrimination of high resistance faults on long lines, as well as stability against external DC and AC faults, and DC circuit breaker opening and reclosing transients. The proposed method uses a relatively low sampling frequency and local data, i.e. no requirement for a communication link.
Highlights
Multi-terminal HVDC (MTDC) grids with voltage source converter (VSC) are identified as a key solution for the integration of renewable energy resources, offshore wind farms, into existing power systems [1,2,3]
This paper presents a non-unit protection method for long transmission lines of multiterminal HVDC (MTDC) grids based on the magnitude and direction of modal voltage and current travelling waves
The robustness of the proposed protection method is analysed for the test MTDC grid depicted in Figure 5, for different cases of internal and external DC faults, and external AC faults
Summary
Multi-terminal HVDC (MTDC) grids with voltage source converter (VSC) are identified as a key solution for the integration of renewable energy resources, offshore wind farms, into existing power systems [1,2,3]. A protection method based on high frequency modal transient voltage energy by applying wavelet transform is presented and tested for relatively short cables and overhead lines in [27]. The method is tested for high resistance faults occurring on relatively long overhead lines Another improved travelling wave protection method using high frequency components of line voltages and currents obtained by wavelet transform is presented in [29]. A protection method using mathematical morphology gradient to detect fault-generated voltage travelling wave fronts is proposed by [30] This method is tested for both cable and overhead transmission lines; it is not suitable for long cables. The magnitude and the relative direction of the line mode voltage and current travelling wave fronts determined by multi-resolution morphological gradients are deployed to overcome the problem of non-unit protection of long transmission lines in MTDC grids.
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