Fault Line Selection of Small Current Grounding System Research Based on Zero Sequence Circuit Current Phase

The probability of two-point same-phase ground fault (TPSPGF) is low in small current grounding system (SCGS), but the harm of TPSPGF can not be ignored. However, there is no effective fault line selection method for TPSFGF at present. A fault line selection scheme for TPSPGF is proposed in this paper. Firstly, the simplified equivalent circuit model of TPSPGF in SCGS is established, and the steady and transient fault characteristics of TPSPGF are analyzed. Secondly, two common line selection methods for single point ground fault are introduced, and their applicability for TPSPGF is analyzed. Finally, based on the fault component of phase current, the line selection procedure for TPSPGF is given. The correctness of the analysis results and the effectiveness of selection scheme is verified by simulation.

When single-phase to ground fault occurs in small current grounding system, the zero-sequence transient current contains a lot of complex fault information. In order to extract characteristic from the zero-sequence current accurately as the criterion of fault line selection, this paper puts forward a method of fault line selection based on VMD energy entropy and optimized K-means clustering. Firstly, the zero-sequence current of each line is decomposed by VMD (variational mode decomposition, VMD), and each sub mode of the original current is obtained. Secondly, the energy entropy of each sub mode is calculated as the characteristic quantity of each line state. Finally, the optimized K-means clustering algorithm is used to cluster the fault feature to realize fault line selection. In this paper, Simulink is used to simulate the single-phase to ground fault in resonant grounded system and the results show that the method that has high accuracy can provide reference for practical engineering application.

Based on the detailed analysis of single-phase grounding fault characteristics in the small current grounding system, an adaptive multi-criteria fusion method of fault-line selection based on transient signal is proposed in this paper. This method introduces fault membership and criterion confidence to achieve the fusion of multiple fault-line selection criteria. At the same time, each criterion adaptively determines whether to participate in the selection according to its confidence level. MATLAB/Simulink simulation results indicate that this fault-line selection method is not influenced by such factors as the transient characteristics of the fault, fault resistance, length of the faulty line, etc.

In order to improve the line selection accuracy of the fault line selection device for small current grounding systems, a line selection method based on information fusion is proposed, that is, several complementary single line selection methods based on transient and steady state characteristics are used for comprehensive line selection. Firstly, wavelet method, zero-sequence active component method, and fifth harmonic method are used according to the characteristic definition of transient and steady-state fault characteristics, and then the rough set theory algorithm model is used to fuse the faults with various fault characteristics to select the fault line. In Matlab, a distribution network model was built and a program of fusion algorithm was written for simulation analysis and verification of route selection methods. The simulation results show that the line selection method based on rough set theory and information fusion has high accuracy and good application value.

When a single-phase-to-ground fault occurs in the distribution network, there is a wealth of information in the transient process of the fault line, from which the fault line selection criterion can be extracted. In this paper, based on the analysis of the related principle of wavelet transform and the concept of entropy, the comprehensive wavelet entropy is defined, and the principle and process of fault line selection are put forward by analyzing the variation coefficients of a group of comprehensive wavelet entropy. Finally, the simulation software is used to verify the fault line selection method under different fault conditions to prove its applicability and feasibility.

Because of line parameters, running types, fault grounding impedances and other factors, the veracity of current fault line selection (FLS) method is not high enough and may contain some dead areas for FLS device of non-utility grounding system (NUGS). To overcome this shortage, a novel FLS method is proposed in this paper based on analyzing the fault characters of distribution network, combining merits of prevalent FLS methods, constructing these methods' subjection functions and then synthesizing them based on the fuzzy theory. Experiments show this method can achieve high sensitivity, adjustability and veracity. The dynamic simulation experiment shows in the neutral un-grounding system(NUS), the method which is made of the zero sequence inactive power (ZSIP) method, the base-wave virtual value (BV) method and the base-wave phase (BP) method of zero sequence current (ZSC) can achieve high accuracy; in the neutral resonant grounding system (NES), the method that combines zero sequence active power (ZSAP) method, 5-harmonic phase (HP) method and remaining current increment (RCI) method can even achieve 100% accuracy. Based on the PC104 industry control computer, a new FLS device of NUGS according to this new FLS shows very high accuracy in dynamic simulation experiments, and it should be widely used in China.

Transient analysis is carried out for the single-phase grounding fault in small current grounding system, the transient grounding current expression is derived, and the influence factors are analyzed. Introduces a method for non-stationary and non-linear signal analysis method-Hilbert Huang transform (HHT) to analyze the single phase grounding fault in small current grounding system, HHT can be better used to extract the abundant transient time frequency information from the non-stationary and nonlinear fault current signals. The empirical mode decomposition (EMD) process and the normalized Hilbert Huang transform (NHHT) algorithm are presented, NHHT is used to analyze and verify an example of the nonlinear and non-stationary amplitude modulation signals. Build a small current grounding system in the EMTP_ATP environment, by selecting the appropriate time window to extract the transient signals, NHHT is used to analyze the transient current signals, and the Hilbert amplitude spectrum and the Hilbert marginal spectrum of the zero sequence transient current signals are obtained.

This paper proposes a method of intelligent protection and location integration for single-phase grounding fault in small current grounding system, which can quickly transfer the grounding current at the grounding fault point and make the current at the grounding fault point tend to zero. The arc cannot maintain and extinguish, thereby protecting the personal safety of the staff. At the same time, the fault location is carried out according to the principle that the zero sequence electrical parameters on different lines and the zero sequence electrical parameters of each line after the active grounding of the fault phase bus before and after the single-phase grounding fault. Finally, through PSCAD/EMTDC and Matlab simulation, the feasibility and effectiveness of this method are verified from different ground resistance, initial phase angle, and fault distance in small current grounding system.

The main fault in distribution networks is single-phase-to-ground fault (PTGF). when PTGF happens in spark catcher grounded system, the fault will be eliminated by making spark catcher grounded directly to avoid the generation of arc over-voltage. But if a transformer internal grounding fault (TIGF)occurs in networks(such as turn-to-ground fault), spark catcher action makes both ends of the fault windings grounded which results to transformer internal winding short and further leads to the transformer burning. So how to discriminate line faults from transformer internal faults becomes a crucial issue waiting to be solved urgently in small current neutral grounding system. In this paper the zero sequence current (ZSC) distribution and negative sequence current (NSC) distribution are analyzed respectively in above two situations. The PTGF and TIGF can be discriminated accurately. A new spark catcher controlling method based on ZSC distribution and NSC distribution is proposed in this paper.

According to the obvious transient characteristics at the beginning of the fault in the non-effectively grounding distribution system, a new method for selecting the fault line, based on Hilbert--Huang transform (HHT), is proposed to extract the non-power frequency transient current from the zero-sequence current in distribution networks with a single-phase-to-earth fault. HHT is used to get the amplitude and phase angle of the zero-sequence current at the beginning of the fault. The non-power frequency currents, which have obvious characteristics, are calculated when power frequency currents are extracted from the zero-sequence currents. A neutral non-effectual grounding distribution network model is established with the Matlab/Simulink and this model is simulated in many cases. The simulation results suggest that the non-power frequency current of the fault line, extracted with HHT, have a contrary polarity as the currents of the non-fault lines and have a more amplitude than each of the other non-fault lines at the one tenth of the wave period time after the fault occurs. These characteristics are used as evidence for fault line selection. The result indicates this method is precise and reliable with the single-phase to ground fault in distribution network.

This paper introduces a distributed grounding line selection method based on GOOSE, which is also implemented in the protection device. In the small current grounding system, when single phase grounding fault happens, fault current is quite small due to the switching of arc-extinguish coil decreasing the capacitive current, which makes it difficult to recognize the fault line using steady-state determination method. By employing wavelets decomposition methodology, discriminating the high frequency component of transient current and recognizing the grounding line rapidly and accurately can be achieved. With exploiting GOOSE of station control layer to share information among several devices, wavelets method is utilized to judge grounding line locally. The algorithm is verified through MATLAB simulation and the microprocessor-based protection embedded in the algorithm is validated in RTDS test. The scheme presents great application prospects and is worth popularizing.

When the single phase ground fault occurs in resonant grounded system, there are significant differences in the components of fault phase current between fault line and non-fault line. This feature can be reflected by intrinsic mode entropy (IMEn) of fault current. IMEn is a new signal analysis over multiple oscillation levels. It corresponds to the sample entropy (SampEn) and the empirical mode decomposition (EMD). The results of fault simulations in different conditions indicate that the method is reliable.

Small current grounding system is mainly used in China’s distribution network. When single-phase grounding fault occurs in the distribution network, it is important to quickly and accurately select the fault line and locate the fault section to ensure the safe and stable operation of the system. In this paper, the fault location methods for the small current grounding system are described in detail, the advantages and disadvantages of various methods are compared, the main problems existing in the low-current grounding system are pointed out, and the future development direction is prospected.

Detection of high resistance earth faults in medium voltage networks is an important problem due to ineffectiveness of traditional earth fault protections. Such short circuits can be detected by the criterion of a reactive power of higher harmonics for zero sequence current and voltage. The main problem is determination of the power setting value in the protection, which depends on the asymmetry of phase-to-earth capacitances and higher harmonics in supply voltages, which are generated by non-linear loads. The intensive tests of the asymmetry of the zero sequence currents and voltages for harmonics and their reactive power have been carried out in 15 kV compensated network as a function of all relevant parameters, i.e.: maximum capacitance deviation of the network and protected line, percentage content of harmonics in supply voltages, capacitive current of the network and the line. It has been shown that third harmonics of the zero sequence voltage and current are the best suited for practical use, since the asymmetry reactive power of these components is the smallest among the considered harmonics and the protection sensitivity will be the highest.

The voltage unbalance between the upper and lower capacitors is an important problem in the three-level PWM converter system, and it is known that the voltage unbalance is directly related to the zero sequence. In this paper, we provide a zero sequence current path by connecting the neutral point of the source transformer to the midpoint of the DC link capacitors. Further, we control the unbalanced voltage with the zero sequence current. The zero sequence current is controlled by the zero sequence voltage made from sinusoidal PWM. The effectiveness of the proposed scheme is verified by computer simulations and experiment.