Ground fault protection for DC-filter high voltage capacitors based on virtual capacitance
Introduction
With remarkable advantages of long distance power transmission, large capability, flexible control, and low losses, high-voltage direct-current (HVDC) transmission technology has been applied wildly in the modern power system [1], [2], [3], [4], [5]. In a line-commutated converter (LCC) based HVDC system, the operation of converter results in generation of characteristic harmonics, which may cause negative effects on power quality, communication systems, and electrical apparatuses [6], [7], [8], [9]. Therefore, DC filters are installed at both ends of the HVDC transmission lines to avoid the harm of characteristic harmonics.
As a key component of DC filter, high-voltage capacitor (HVC) has to endure most of the DC-side voltage. And when a ground fault occurs on it, the resonance circuit and tuning frequency of DC filter will be changed, which not only can lead to harmonic pollution but can result in over-current or over-voltage, seriously threatening the safety and stability of the HVDC system [10], [11], [12], [13]. According to the statistical analysis, DC filter faults, including lead wire ground faults at the high voltage side, HVC ground faults and ground faults under HVC section, led to about 15% of the pole outages in HVDC transmission systems [14]. Differential protection is currently used as the main protection of DC-filter ground fault, but it is difficult to locate faulty component. Meanwhile, there are some problems exist with it, such as poor selectivity, insufficient sensitivity, and difficult threshold selection. To solve these problems, the authors of [12] have proposed a new type of HVC earth fault protection scheme, it is independent of the system operating conditions and of high reliability. The authors of [13] have introduced a protection scheme for based on characteristic harmonic impedance ratio, it can accurately identify ground faults of the high voltage capacitor. However, it is necessary to extract specific frequency signals for these protections. To improve the safety and reliability of HVDC system, therefore, it is of great significance to study a time-domain protection for DC-filter high voltage capacitors (DFHVCs).
Based on virtual capacitance, a novel ground-fault protection for DFHVCs is proposed in this paper. According to the equivalent circuit of DC filter in a LCC-HVDC system, virtual capacitance characteristics are analyzed under different ground faults of DFHVC. Then, these characteristics are used for constructing time-domain protection scheme to identify DFHVC ground faults from external faults. The performance of proposed protection is validated by simulation results under different fault conditions.
The remainder of this paper is organized as follows. In Section 2, DC filter and its differential protection are introduced. The proposed protection principle is described in Section 3. The proposed protection scheme is presented in Section 4. In Section 5, the simulation model is built and used to verify the performance of proposed protection, followed by the conclusions of the paper in Section 6.
Section snippets
DC filter
Due to the advantages of small footprint and maintenance friendly, double-tuned filter is most widely used in LCC-HVDC projects [15], [16], [17]. Therefore, this paper is focused on HP12/24 double-tuned filter.
Double-tuned filter, which is consisted of HVC C1, inductor L1, resistor R1, capacitor C2 and inductor L2, is shown in Fig. 1. The capacitance of each bridge arm of HVC C1 is C. CT1, CT2 and CT3 are the current transformers. f1 and f2 are the ground faults of the upper and lower bridge
The protection principles
During normal operation of HVC, voltages at both ends of the unbalanced branch of capacitor C1 shown in Fig. 1 are equal. Therefore, the theoretical value of unbalanced current measured by the current transformer CT2 is 0, which is not affected by the operation modes of HVDC system. However when a ground fault occurs on HVC, the structure of DC filter is changed, leading to unequal voltages at both ends of the unbalanced branch. And the unbalanced current can be a non-zero value affected by the
Ground-fault protection scheme based on virtual capacitance
According to the analysis in Section III, when a ground fault occurs on HVC, the virtual capacitance can be calculated by using the unbalanced current and the DC-filter voltage in time domain. And during normal operation, the virtual capacitance is 0. Therefore, the virtual capacitance characteristics can be used to construct a ground-fault protection scheme for DFHVC.
The protection criterion is formulated as follows:where N is the total number of sampling
Simulation results
A bipolar ± 500 kV HVDC system based on CIGRE HVDC benchmark model, is built for simulation under PSCAD/EMTDC environment [18], [19], [20]. The main parameters of this HVDC system are shown in Table 1. The simulation step and sampling frequency of signals in the proposed protection are set to 50 μs and 10 kHz, respectively. The ground faults occur at 1.0 s. According to (10), the setting coefficient kset is set as 0.01, and Cset = 0.0007 μF.
In order to verify the performance of the proposed
Conclusions
This paper proposes a novel ground-fault protection for DFHVCs based on virtual capacitance. According to the equivalent circuit of DC filter in a LCC-HVDC system, virtual capacitance characteristics are analyzed under different ground faults of DFHVC. Then, these characteristics are used for constructing protection scheme to identify DFHVC ground faults from external faults. A ± 500 kV HVDC system built in PSCAD/EMTDC is used to verify the performance of the proposed protection. Comprehensive
Acknowledgements
This work was supported in part by National Key Research and Development Program of China (2016YFB0900603) and Technology Projects of State Grid Corporation of China (52094017000W).
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