This case study analyses the events of 05/10/2010 on the CLI connection with the ZESCO system where after energising a passive network, a resonance condition occurred causing the HVDC scheme to trip due to an AC harmonic overvoltage.
At 23:16, the Zambezi converter station went into passive network mode with the Southern ZESCO network after cascade tripping of generation and transmission equipment at the Victoria Falls Power Station. The 220kV Sesheke – Zambezi 1 line also tripped with this disturbance.
After the HVDC scheme supplied the passive local load at Zambezi substation with Katima Mulilo load for 10 minutes, the 220kV Sesheke Zambezi breaker was closed, energizing Sesheke substation including the 220kV Sesheke – Livingstone line with 15MVAR line reactor. This condition led to a resonance condition with high lower order harmonic content and subsequent overvoltage of 1.25 per unit causing the Zambezi converter station to trip on overvoltage. Supply to Zambezi was restored and the CLI was back to normal at 03:51 on 06/10/2010.
The aim of this case study is to determine the cause of the resonance condition and subsequent over-voltages that occurred. To do this, the frequency response of the AC network is done with DigSilent PF to determine which sensitive parallel and series resonant points exist. After this, an EMT study is performed to reveal the network energisation currents that would be characteristic of the line and reactor energisation. The harmonic content analysis of the inrush currents with sequence analysis is then done to establish the harmonic orders, amplitudes and phase rotations that would be characteristic of the line and reactor inrush condition. The theoretical analysis done in Chapters 2 and 3 on AC/DC harmonic interaction can then be utilised to ascertain how these harmonic orders would cross- modulate via the VSC terminal to give rise to new harmonic orders on the AC side. These newly cross-modulated harmonic orders can then be checked against the network frequency response for sensitivity. This would lead to possible reasons for the observed resonance condition.
In order to confirm the theoretical analysis and to consider possible solutions to the problem, a simulation in PSCAD or the RTDS can be done to replicate the case and to confirm the
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results. The detailed model that exists for the CLI in DigSilent PowerFactory model is sufficient for loadflows, frequency sweeps and RMS dynamic simulations. An EMT simulation model with valve firing is only available in PSCAD and RSCAD for the RTDS. Therefore simulations that require detailed VSC time domain analysis will be done in PSCAD or in the RTDS.
An important source of information is the event list registered at the NamPower National Control Centre. This information is used to correlate GPS time-stamped events originating from the substation automation system with the recorded TFRs to be able to help identify the effects seen on the TFR waveforms. (See the events list in Appendix B).
Another vital source of information are the TFRs from the Zambezi converter station that recorded the relevant voltages, currents and digital channels during the disturbances and can be used as input data to perform a harmonic sequence analysis of the waveforms during each stage of the abnormal condition. Because the record is stored in Comtrade format, a Matlab file capable of importing a Comtrade file and doing a harmonic sequence analysis was used as a tool for the analysis. This tool from the author of [10] is mathematically similar to the Excel harmonic sequence analyser that was created for the other examples in this thesis.
However, the Matlab code of this tool had to be modified by the author of this thesis in order to be able to continuously plot different harmonic sequence components over time. For this a loop was created to step a window of one period at a time and to register all the harmonic sequence components for that period. Outside the loop, plots of the recorded harmonic sequence components are executed. In this way, continuous harmonic interaction and saturation effects can be seen against time. (See Matlab program code in Appendix A).
From the transient fault records of this case study it can be seen the waveform shapes change from one 50Hz cycle to the next. To be able to monitor continuous harmonic sequence components of a transient condition that changes from cycle to cycle, it was decided to only look at one fundamental cycle at a time and letting the DFT or FFT assume an infinite periodicity of the waveform. Although the sampling rate of the TFR‟s are sufficient at 200 samples per cycle or 10kHz to monitor up to the 100th harmonic order in theory, any inter- harmonic distortion would be transformed to the two adjacent integer multiples of the fundamental component with some possible additional spectral leakage. Because of the fact that the inverse DFT or FFT would reproduce the original waveform, this possibility was accepted in order to be able to apply the common tools used that do not really cater for inter-
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harmonic distortion such as the DigSilent Power Factory‟s FFT function, the Matlab continuous harmonic sequence analyser and the Excel FFT harmonic sequence analyser.
The harmonic cross modulation theory would yield the same results using (2.65), (2.67) and (3.26) whether the additional frequency components are integer multiples of the fundamental component or not. Thus an inter-harmonic distortion treated as two separate major harmonic components that are integer multiples of the fundamental, each consisting of its PPS and NPS components, would yield the same harmonic interaction results in the time domain as when a single non-integer multiple of the fundamental component would be applied. Determining the sequence components of a three-phase unbalanced transient inter-harmonic component in order to determine which cross-modulation rules apply would be more difficult to accomplish. Because harmonic components are theoretical constructs that can be considered independent from each other and because the principle of super-position applies, the assumptions above are considered valid.