Ungrounded System Research Articles

Study on phase selection control characteristics of fast vacuum circuit breakers suitable for new power systems

Abstract With a high proportion of new energy access to the power system, the user side of the randomness and volatility of distributed photovoltaic large-capacity access to the 10 kV medium-voltage distribution grid operation results in more frequent fluctuations in the system voltage and more frequent switching of reactive compensation of the shunt capacitor bank on the 10 kV side of the conventional substation. Conventional reactive compensation switching circuit breakers adopt a spring mechanism whose mechanical stability is insufficient and the selection of the relevant fit capacitor bank accuracy is not enough. The 12 kV fast vacuum circuit breaker of small mechanical dispersion for selection related to the combination was proposed. First of all, based on the neutral grounded and ungrounded system of the capacitor bank, the strategy of capacitor bank phase selection closing was developed. For the action characteristics of the phase selection of the 12 kV fast vacuum circuit breaker, the variation laws of mechanical dispersion and the influence factors such as ambient temperature, capacitance voltage, capacitance degradation, mechanical wear, and conductive rod deformation were obtained by simulation and experiment. The results showed that the ambient temperature and capacitance voltage had a great influence on the switching time of the fast circuit breaker, while other factors had a small influence. Again, the pre-breakdown characteristics of the 12 kV fast vacuum circuit breaker were obtained by experiments. The results showed that the influence of contact opening distance and pre-breakdown voltage was a linear relationship, and the pre-breakdown slope was 30 kV/mm. According to the pre-breakdown closing characteristic curve, the pre-breakdown time of the fast circuit breaker was 0.45 ms at most. The feedforward compensation mechanism based on a neural network was proposed to compensate for the action characteristics of the fast circuit breaker and aimed to improve the phase selection accuracy. Finally, two rounds of capacitive current switching tests of 12 kV fast vacuum circuit breakers with C2 level were carried out in a third-party test station. The accuracy of phase closing is within the ±0.5 ms closing window, effectively preventing the occurrence of closing inrush current.

Resonance mechanisms of a single line-to-ground fault on ungrounded systems

Resonance mechanisms of a single line-to-ground fault on ungrounded systems

Faulted feeder identification and location for a single line-to-ground fault in ungrounded distribution system based on principal frequency component

Faulted feeder identification and location for a single line-to-ground fault in ungrounded distribution system based on principal frequency component

Early Warning Analysis of Grid Ferromagnetic Resonance Overvoltage Risk Based on Multi-source Data

In the impartial factor ungrounded system, ferromagnetic resonance overvoltage is a frequent fault that lasts for a lengthy time and is hazardous to the grid. In this paper, the mechanism of grid ferromagnetic resonance overvoltage is first explored in depth. The precept of impartial voltage shift and ferromagnetic resonance brought about through PT saturation is analyzed with the aid of graphical and mathematical analysis. Then, the characteristics of fault current information are extracted by wavelet transform, and indicators such as wavelet fault degree, wavelet singularity and wavelet energy measurement are obtained respectively. D-S evidence theory is used to fuse multi-source information of electrical volume and switching quantity, so as to obtain comprehensive fault results of power grid more accurately. Finally, based on the time series risk assessment, the distribution network time series risk index is calculated, the risk level and risk area of each period are determined, and the early warning results are issued. Finally, an example is given to verify the effectiveness of the proposed method.

Single-phase broken-line fault of a 10 kV ungrounded distribution Line

In recent years, a great many applications of overhead insulated lines in distribution networks have reduced the occurrence of single-phase ground faults, and at the same time made broken-line faults a problem that cannot be ignored. Timely fault detection and fault location are of great help to reduce power outages and improve the quality of power supply for users. Aiming at the ungrounded system, this article designs a single-phase broken-line model, considers the influence of abort situation and others, analyses the fault characteristics such as sequence current and neutral point offset voltage, and then advances a fault model according to negative sequence current and neutral point offset voltage fault detection method. Lastly, the validity of the suggested way is confirmed by MTALAB.

ERROR ESTIMATION OF THE DEVELOPED METHOD FOR UNGROUNDED AC SYSTEMS TO DETERMINE THE NETWORK INSULATION

The problem of efficient and safe mining has recently become more relevant due to the intensification of production processes, for example, during the operation of electrical installations at mining enterprises, the insulation of electrical networks and electrical equipment undergoes a change in the electrical insulating properties of materials, which reduces the electrical safety. The network insulation admittance determines the safety of electrical receivers and operational personnel in mining facilities, since the current of a single-phase earth fault is directly proportional to the value of the network insulation admittance, and by controlling its active and reactive components, this will allow for a violation of the integrity of the insulation, thereby taking preventive measures. The article depicts the analysis of the reliability of the devised method by estimating the error of insulation parameters in ungrounded AC systems which is done by indirect measurements. Graph-analytical studies of relative root-mean-square error reveal that the accuracy of the calculation in the devised method can be effectually managed by selecting the susceptance of the additional capacitor and accuracy class of measuring units.

"Approbation of the developed method for determining network insulation parameters on an operating excavator"

"One of the factors that determine the safety of electrical installations in ungrounded AC systems is the insulation resistance to earth. Early recognition and fault clearance, accompanied by a change in the active and reactive components of the insulation conductivity in ungrounded AC systems, will ensure work safety in electrical installations. Due to the fact the scale of cases of electrical injuries continues to remain significant, despite the tightening of the requirements of regulatory and technical documentation and the creation of newer effective methods and protection equipment against electric shock, therefore, increasing the level of electrical safety in the operation of mining machines and complexes is an urgent task. The article depicts the results of the approbation of the developed method for determining network insulation parameters on an operating excavator. The developed method is based on measuring phase voltages and angles of their vectors before and after connecting an additional capacitance to the phase of the electrical network and to the earth. According to the developed method, numerical data of the insulation condition in an electrical network with a voltage up to 1000 V and the single-phase earth fault current were calculated. During the approbation of the developed method, the lack of the existing protective residual current device was identified. Keywords: network insulation, admittance, susceptance, neutral shift, zero-sequence voltage. "

A New Model for Correcting and Improving the Sequence Networks Method of Single-Phase Earth Fault to Accommodate All Resistance Grounding Systems

The method of sequence networks is still well-known and widely utilized for analyzing and computing unsymmetrical faults in power system networks today, particularly in solid grounding systems where it was initially introduced for short circuit analysis. When it comes to single-phase earth fault that is a dominant majority type of fault in power distribution grids, different neutral grounding methods with varied impedances result in distinct fault characteristics, to continue utilizing sequence networks could cause inaccurate calculations by the reason of neutral voltage during earth fault does not remain as close to zero as it’s pinned in solid grounding system. As neutral resistance increases gradually, the impact of line-to-ground capacitance becomes more pronounced and can no longer be ignored. In this case, applying sequence networks could lead to calculated results that are far from the actual situations in grids. This paper discovers and proves an obscurely hidden defect of the sequence network, while proposing a new and novel modeling to derive comprehensive equations of fault calculations for all types of resistant grounding systems, including solid grounding, low resistance grounding, high resistance grounding, and ungrounded systems.

Analysis of A Few Continuous Ferromagnetic Resonance PT Faults in Power Plants and Improvement Measures of Harmonic-free Suppression Technology

Many vicious faults of burning PT and high-voltage fuse caused by ferroresonance occurred in power plants. Microcomputer harmonic elimination device and silicon carbide harmonic elimination device did not work; particularly silicon carbide harmonic elimination devices burnt together with PT. This paper, enumerating the excitation conditions of PT ferromagnetic resonance, analyzes the circuit characteristics of PT circuit ferromagnetic resonance in the power plant and the power frequency voltage characteristics of the sound phase when the single-phase grounding fault occurs in the neutral point ungrounded system. The defects of the existing harmonic elimination device are summarized, and the technical improvement measures are proposed to add the current-sensitive intelligent harmonic elimination device. Practice shows that this kind of harmonic elimination device can effectively avoid PT bursts and fuse-fusing faults caused by the resonance of the intermittent grounding of the system.

Cooperative Use of IMD and GPT in a 3-Phase Ungrounded Distribution System Linked to a Transformerless Photovoltaic Power Generation Facility

Distributed generations, using solar photovoltaic (PV) generation systems, are generally connected to ungrounded distribution systems to ensure operational continuity and avoid electro-chemical corrosions. The ungrounded power distribution system possesses an advantage of continuous operation regardless of primary fault occurrence due to a small fault current. Conversely, a subsequent secondary fault can induce a large fault current representing that of other electrical grounding types, resulting in inevitable power shutdowns. As preventative methods, both insulation status monitoring and primary failure detection have become of high importance. This paper presents a method enabling the cooperative use of IMD (Insulation Monitoring Device) and GPT (Ground Potential Transformer) in an ungrounded distribution system connected with a transformerless inverter. Moreover, factors leading to errors during IMD insulation monitoring, CLR (Current Limit Resistor) burnout of a GPT, and malfunctions of related protection devices are presented. Furthermore, a method for selecting the inductor and capacitor in consideration of the operating characteristics of IMD and GPT is discussed. The proposed cooperative operation method enables the accurate measurement of insulation resistance using IMD, while concurrently reducing the constitutively induced zero-sequence voltage in the CLR of a GPT to prevent CLR burnouts and malfunctions of connected protection devices. Hence, the method is anticipated to contribute to the stable operation of alternating current (AC) and Direct Current (DC) combined systems connected with transformerless inverters.

New Selective Directional Ground Fault Protection Method for Ungrounded Systems

Ground faults are the main cause of electrical failure in transmission and distribution systems. In ungrounded distribution networks, it is difficult to detect where a first ground fault has happened as only capacitive currents are available. Also, it is important to trip only the faulty line in order to ensure the system's reliability. Directional residual overcurrent relays are the standard solution for these systems, but their difficult commissioning and high costs make it not trivial. This paper introduces a new selective and directional protection method to determine the faulty line in ungrounded medium voltage networks with three or more feeders. The method evaluates the residual currents in the feeders coupled to the same busbars in combination with a reference current created by the sum of a combination of residual currents of all feeders. Both simulation and laboratory results validate this new method, which is easy to start up, as it does not need previous configurations for its implementation, and it presents an economical solution to this problem.

Single-Phase Grounding Fault Identification Based on Transition Conductance Tracking Method

The instability of single-phase grounding faults makes it difficult to track the fault development process accurately. In this paper, a transition conductance tracking method for single-phase grounding fault identification is established to perceive the health state of the distribution network. To solve the situation that the insulation parameters of the neutral ungrounded system are difficult to obtain, a phase-shift method is proposed to measure the insulation parameters accurately. Based on the measurement results of insulation parameters, a transition conductance tracking model is constructed to monitor the changes of transition conductance in real time. Since the transition conductance varies steadily, it can be used as an important parameter to evaluate the development of a single-phase grounding fault. The innovative tracking method is verified by the simulation model, and the error analysis shows that the tracking error of transition conductance is less than 9%. The simulation results demonstrate that the proposed method can track the development of grounding faults effectively and give a quick warning to the operator. The fault identification theory adopted in this paper is not limited by the neutral grounding mode and especially can be applied to situations where the neutral point parameters are not adjusted.

Measurement and automatic monitoring insulation resistance of AC/DC mixed unearthed networks

Where the danger of electric shock is high, for example underground mines or medical equipment directly connected to a patient, special ungrounded power systems (unearthed networks) may be used to minimize possible leakage current to the ground. In mixed unearthed networks (AC/DC-IT) comprising alternating and direct current circuits, AC part of system is connected with DC part through rectifyer valves. Commutation of the valves causes cyclic variation of configuration of the entire galvanically connected network. A distinct feature of AC/DC-IT systems is that voltages between all points of AC side and ground may have mean value different from zero. This characteristic parameter can be used as a signal to determine fault and position to ground or decrease in insulation resistance. However, it also affects the results of the network equivalent insulation resistance measurements when a DC source is used to check insulation. The paper presents a solution to eliminate this effect and proposes a device model to measure and monitor insulation resistance. The results of this study can be applied in the design and manufacture of automatic resistance measuring and monitoring devices for AC/DC mixed unearthed networks.

Modified Complete Ensemble Empirical Mode Decomposition based HIF detection approach for microgrid system

Detection of high impedance fault (HIF) in an active distribution system is a challenging task. It is learned from the fault characteristics that detection and discrimination of HIF during different critical conditions is impossible using the fault current magnitude. Under dependable situations such as energization of a transformer, nonlinear load and capacitor bank detection and discrimination of HIF is challenging. In a distribution system with an inverter based distributed generator (IBDG), the current contribution during islanding mode is very low and also for HIF condition. To mitigate these issues, an intelligent approach applying Modified Complete Ensemble Empirical Mode Decomposition with Adaptive Noise (MCEEMDAN) on residual current signal is developed. The second intrinsic mode function (IMF2) is extracted using MCEEMDAN and its Teager Kaiser Energy Operator (TKEO) is computed to detect and discriminate the HIF against other physical events. The novelty of MCEEMDAN approach lies with its noise free output and faster response as compared to other time–frequency approaches. The method is tested for several fault and non-fault cases including, presence of noise, harmonics, and unbalance loadings. The comparison with recently reported techniques for very HIF and ungrounded system proofs the efficacy of the method.

Mathematical description of the method for ungrounded AC systems to determine the network insulation

For the safety of humans and animals in the vicinity of single-phase earth faults in ungrounded AC systems, the development of methods for monitoring the condition of insulation and protection against electric shocks is an important issue. This paper presents a method for determining insulation parameters in ungrounded AC systems, which provides: (1) Satisfactory accuracy; (2) easy measurements of the voltage modules magnitude; (3) safety when working in electrical installations. The proposed method is based on measuring the zero-sequence voltage module and phase-to-earth voltage module, as well as the angle of phase-to-earth voltage vector and zero-sequence voltage vector before and after connection of additional conductivity between a phase and the earth. In addition, the obtained insulation parameters enable the zero-sequence current to be decomposed into an active and reactive component, thus ensuring accurate compensation of capacitive currents. By monitoring the state of the active conductivity value, the value of the single-phase earth-fault current can be determined at an early stage and the relay protection can thus be correctly set. Consequently, by knowing the insulation condition parameters, the single-phase earth-fault currents can be monitored and reduced more effectively, and thus the step voltage, which determines the degree of safety.

Application of 66kV Active Intervention Grounding Arc Suppression Device

The energy supply is mainly based on electric energy at present, and power supply reliability is an important indicator for evaluating power supply services. In view of the 66kV transmission voltage level unique to Northeast my country, the neutral point of the transmission system directly affects the stable operation of the power system. First, the composition of the substation in the system and the working principle of the arc suppression device are analyzed. Secondly, when a single-phase grounding fault occurs in a 66kV neutral point ungrounded system, an active intervention grounding arc suppression device is designed. Finally, the operating characteristics of the designed grounding arc suppression device are demonstrated based on the simulation results.

Provable Limitations of Acquiring Meaning from Ungrounded Form: What Will Future Language Models Understand?

AbstractLanguage models trained on billions of tokens have recently led to unprecedented results on many NLP tasks. This success raises the question of whether, in principle, a system can ever “understand” raw text without access to some form of grounding. We formally investigate the abilities of ungrounded systems to acquire meaning. Our analysis focuses on the role of “assertions”: textual contexts that provide indirect clues about the underlying semantics. We study whether assertions enable a system to emulate representations preserving semantic relations like equivalence. We find that assertions enable semantic emulation of languages that satisfy a strong notion of semantic transparency. However, for classes of languages where the same expression can take different values in different contexts, we show that emulation can become uncomputable. Finally, we discuss differences between our formal model and natural language, exploring how our results generalize to a modal setting and other semantic relations. Together, our results suggest that assertions in code or language do not provide sufficient signal to fully emulate semantic representations. We formalize ways in which ungrounded language models appear to be fundamentally limited in their ability to “understand”.

PSCAD/EMTDC를 이용한 ±35kV급 MVDC 하이브리드 배전망의 사고해석에 관한 연구

Based on ±35kV-scale MVDC hybrid distribution system to demonstrate MVDC distribution technology, this paper presents scenarios of contingency fault analysis and modeling for impact evaluation on MVDC hybrid distribution system in order to examine the operation characteristics of MVDC facilities and customers. And also, this paper proposes a modeling of the ±35kV-scale MVDC hybrid distribution system using PSCAD/EMTDC, which is composed of distribution substation, MVDC distribution feeder, converter station and PV system. From the simulation results of fault analysis in MVDC distribution system based on the modeling for impact evaluation on distribution system with the fault scenarios, it is confirmed that tremendous fault current can be momentarily occurred due to discharging of DC link capacitor in converter station regardless of fault type. Furthermore, it is found that the faults in MVDC feeder can cause the phenomenon of under-voltage at customers in other feeder of the main transformer. In addition, it is clear that the malfunction of recloser in other feeder can be prevented due to ungrounded system of MVDC system, and also the voltages of AC 22.9kV bus and MVDC feeder are maintained in a stable manner even if ground fault is occurred in AC 22.9kV feeder.

Single-Phase Ferroresonance in an Ungrounded System During System Energization

Ferroresonance often occurs in power systems during system switching, which creates overvoltage situations and poses great risks to the equipment safety. In this article, a single-phase ferroresonance incident during a system energization process is presented, which occurred at a single-phase station service transformer in an ungrounded system, causing a surge arrester's failure. This incident is investigated through the PSCAD simulation and the analytical analysis. To validate the PSCAD simulation model, the relay field record is compared with the PSCAD simulation results. Several case studies are conducted using PSCAD simulation to evaluate contributing factors to this ferroresonance incident. The root cause is found to be lack of ground in the system during energization. Field practices are recommended to avoid such incidents, which were proved to be effective by successfully energizing the system in the field.

The Signal Injection Method with High-Frequency for Distribution Network Capacitive Current Measurement

In order to solve the problem of limited frequency selection range and simplify calculations on the grounded capacitance measurement for the ungrounded distribution system, a high-frequency injecting method for the capacitive current measurement is proposed in the paper. The injecting frequency can be high enough to ignore capacitive reactance and leakage resistance comparing with leakage inductance. The leakage inductance value is obtained directly on the base of simplified calculation. Then the resonant frequency of the power system needs to be obtained by the traditional resonant method. Finally, the grounded capacitance can be calculated through the equivalent circuit and the simultaneous equations. The simulation and experiments verify the feasibility of the high-frequency method. The experiment results prove that this method not only solves the problem of frequency selection, but also has advantages of convenience and high precision (the relative error is less than 5%), which is also suitable for the larger grounded capacitance system.