Capacitor Voltage Transformer Research Articles

Analytical discrete Fourier transformer‐based phasor estimation method for reducing transient impact of capacitor voltage transformer

The output voltage of capacitor voltage transformer (CVT) is highly influential during initiation of contingencies due to its transient behaviour. This study proposes a novel method for fundamental component phasor estimation which reduces the transient impact of CVT. The proposed method (PM) first, based on hybrid full-cycle averaging window and also least squares (LS) technique, calculates the transient components of the CVT. In the next stage, by subtracting estimated transient behaviour from fault voltage signal, the fundamental phasor component of the obtained signal can be estimated utilising discrete Fourier transformer (DFT). Also, in order to prevent instability due to off-nominal frequency sampling rate, DFT formulation is improved. Several case studies have been carried out in order to show the robustness of the PM in the realistic simulation scenarios. Results show that the PM given in this study is able to estimate fundamental phasor voltage fault signal quickly and precisely with ignorable effect from transient impacts of CVT in almost always one cycle after fault occurrence.

A positive-sequence current based directional relaying approach for CCVT subsidence transient condition

In this work, a directional relaying algorithm is proposed for transmission line to prevent relay maloperation during coupling capacitor voltage transformer (CCVT) subsidence transient. The effect of CCVT subsidence transient during single-pole-tripping condition is highlighted in this paper. The proposed method which is based on phase angle difference of post fault and prefault positive sequence current can help the directional relay to take accurate decision during erroneous CCVT secondary response. The available CCVT model in PSCAD is not able to generate significant subsidence transient in the secondary side. So, a new CCVT model is developed in EMTDC/PSCAD domain for transient response analysis and to check the relay operation. Next, the performance of different voltage and current information based directional relaying techniques have been analyzed and compared with the proposed method. The results are evaluated for different system and fault conditions. Results demonstrate the accuracy of the proposed method.

Measurement Error Estimation for Capacitive Voltage Transformer by Insulation Parameters

Measurement errors of a capacitive voltage transformer (CVT) are relevant to its equivalent parameters for which its capacitive divider contributes the most. In daily operation, dielectric aging, moisture, dielectric breakdown, etc., it will exert mixing effects on a capacitive divider’s insulation characteristics, leading to fluctuation in equivalent parameters which result in the measurement error. This paper proposes an equivalent circuit model to represent a CVT which incorporates insulation characteristics of a capacitive divider. After software simulation and laboratory experiments, the relationship between measurement errors and insulation parameters is obtained. It indicates that variation of insulation parameters in a CVT will cause a reasonable measurement error. From field tests and calculation, equivalent capacitance mainly affects magnitude error, while dielectric loss mainly affects phase error. As capacitance changes 0.2%, magnitude error can reach −0.2%. As dielectric loss factor changes 0.2%, phase error can reach 5′. An increase of equivalent capacitance and dielectric loss factor in the high-voltage capacitor will cause a positive real power measurement error. An increase of equivalent capacitance and dielectric loss factor in the low-voltage capacitor will cause a negative real power measurement error.

Adaptive phasor estimation algorithm to enhance numerical distance protection

This study presents an adaptive phasor estimation approach for fast numerical distance protection. The proposed method is based on signal condition assessment and choosing the proper phasor estimation method. Effects of errors caused by decaying DC component and coupling capacitor voltage transformer transients, as the main problems for distance relay impedance measurement procedure, are alleviated using the proposed adaptive method. Meanwhile, a new short window DC removal method is introduced. Thanks to the fast DC value estimation method, a short window phasor estimation method is utilised for current phasor estimation. Using the proper phasor estimation method based on voltage and current signal conditions results in more accuracy, fast decision making procedure and consequently a more reliable protection scheme. To verify the performance of the proposed method, numerous simulation studies are carried out. The results assert that the proposed adaptive method not only accelerates the relay operation, but also improves the relay accuracy which enhances numerical distance protection from security and dependability viewpoints.

Evidence theory‐based identification of aging for capacitive voltage transformers

In this study, an aging identification approach is proposed for capacitive voltage transformers (CVTs). Aging is an unavoidable phenomenon which emerges and spreads in equipment structure and mainly affects the insulation parts. The aging may affect transformer output in its primary stages. If it grows, it may cause serious damages to the equipment, substation and the staff. Accordingly, it is necessary to diagnose the aging of CVTs in its primary steps in order to implement the required maintenance and repair. The proposed aging identification method is an efficient method which collects data from different sources in digital substations to diagnose aging in CVT structures. Moreover, the possible external disturbances are also taken into account. A modified method is proposed based on the distance between the evidences. In addition, the algorithm is capable to identify the CVTs providing bad data arising from their aging.

Controlled Switching of a 1200 MVA Transformer in Manitoba

This paper describes the application of a controlled switching device to energize a 500 kV/230 kV/46 kV, 1200 MVA autotransformer in Manitoba. The development of a detailed electromagnetic-transient (EMT) simulation model of the transformer formed the basis of the work described. The transformer model includes an accurate representation of hysteresis and remanance, which was validated using recorded waveforms and manufacturer data. This paper discusses a feasibility study conducted using this transformer model to evaluate the suitability of controlled switching to energize this transformer. A series of real-time hardware in the loop tests was conducted using a real-time simulator as part of precommissioning tests. The real-time simulation (RTS) tests and phase 1 of commissioning tests conducted in October 2014 showed that the controlled switching device would produce inaccurate residual flux estimates when it integrates the secondary voltage waveforms of a capacitive voltage transformer. Based on these RTS test results and EMT simulations, the installation of a wound potential transformer (PT) was recommended. This paper also discusses simulation studies conducted to manually program the controller until the PT was commissioned. During phase 2 of testing conducted in October 2015, a wound PT was commissioned and the controller was tested. The recorded waveforms showed good agreement with simulation results. The controller has accurately predicted residual flux and minimized inrush currents during transformer switching events.

Ferroresonance Characteristics on Capacitive Voltage Transformer Under Lightning Impulse Voltage

Nowadays, more papers explain about the occurrence of ferroresonance due to the switching operation. Indeed, the switching operation is more possible to occurr on system respect to other transient disturbances, such as lightning stroke. However, lightning stroke which is always characterized by a high impulse current is also able to initiate the ferroresonance on the system. In this paper, the effect of ferroresonance due to the lightning stroke on capacitive voltage transformer (CVT) is simulated and investigated. The ferroresonance causes an overvoltage as well as on overcurrent on CVT. ATP/EMTP is utilized to simulate and analyze the ferroresonace phenomena. In this study, Heidler’s lightning model is used to describe the lightning condition on the system. Lightning impulse parameters such as its amplitude and duration, including its front time (ts) and tail time (tr) are varied. The effect of these lightning parameters on peak voltage and peak current on the primary side of CVT is investigated. The simulation results show that the peak voltage increases to about 267.5% of its normal peak voltage while peak current increases up to about 266.1% of its normal current. These peak values increase linearly with the amplitude and they increase non-linearly with the duration of the lightning impulse. However, the front time almost does not affect the peak values of voltage and current. Therefore, it can be concluded that lightning impulse characteristics affect the occurrence of ferroresonance on CVT.

Ferroresonance conditions in wind parks

Wind parks are made up of a large number of saturable inductances (power transformers, inductive voltage transformers (IVTs)), as well as capacitors (cables, wind turbine harmonic filters, capacitor voltage transformers (CVTs), voltage grading capacitors in circuit-breakers). Therefore, they may present scenarios in which ferroresonance occurs. This paper presents the scenarios that can lead to ferroresonant circuits in doubly fed induction generator (DFIG) based wind parks. Ferroresonance conditions and their consequences are demonstrated by simulating the detailed model of the wind park in EMTP-RV. The wind park simulation model includes detailed models of wind turbines, medium voltage (MV) collector grid, high voltage/medium voltage (HV/MV) wind park substation, overvoltage, overcurrent and differential current protections, current transformers (CTs), IVTs and CVTs.

Implementation and Application of Practical Traveling-Wave-Based Directional Protection in UHV Transmission Lines

Traveling-wave-based protection is becoming attractive in ultra-high-voltage (UHV) transmission systems due to the requirement for high-speed protection. However, the poor transfer characteristics of coupling capacitor voltage transformers (CCVTs) for high-frequency voltage have strictly limited its application. A novel traveling-wave-based protection scheme considering the characteristics of CCVTs and current transformers is proposed in this paper. The dyadic wavelet transform is used to extract the polarities of voltage and current traveling waves for fault direction identification. A prototype is further developed for practical applications. Extensive laboratory tests are performed to assess the performance of the developed scheme. The prototype has been applied to the 750-kV substations in China and an external fault was recorded during the operation period, validating the effectiveness and reliability of the proposed protection principle.

Line voltage phasor reconstruction on capacitive voltage transformers using dynamic phasor approach

Capacitive voltage transformers (CVTs) are widely used in line voltage measurement of EHV/UHV power systems. When a fault occurs, CVT measurements may contain severe transients during the beginning 3–6 fault cycles, which have usually been ignored instead of processed in many previous algorithms. However, this may cause reach problems and postpone response time in digital relay operation. In the present paper, dynamic phasor approach was employed to reconstruct line voltage phasors. Using the proposed algorithm, line voltage phasors can be obtained rapidly after one-cycle plus 2 samples, which are still accurate even if transients are involved in the CVT measurements. The proposed algorithm is simulated and evaluated by the MATLAB/SIMULINK built system. Simulation and evaluation results illustrate the effectiveness and performance of the proposed algorithm in various fault conditions and CVT types.

Directional Relaying During Secondary Arc Using Negative-Sequence Superimposed Technique

The presence of secondary arc during single-pole tripping (SPT) poses a challenge for protection schemes. A conventional directional relaying algorithm based on the negative-sequence component is affected due to the presence of nonfundamental frequency components in voltage and current signals during secondary arc. In this paper, a negative-sequence-based superimposed technique is proposed to provide a solution to directional relaying during the SPT condition which is not affected by the presence of a secondary arc. The technique is independent of coupling capacitor voltage transformer location and not affected by high fault resistance.

English

Capacitive Voltage Transformers (CVTs) are widely used as source of voltage signals for monitoring, protection relays and control application at transmission and sub transmission voltage level. The main component of the CVT are the capacitive voltage divider, a tuning reactor, a step down voltage transformer, a Ferro-resonance suppression circuit and the burden made by the relay. CVT are used for protection of high voltage lines, substations and transmission of high frequency signals. These applications required low voltage signal, which is given by output of CVT. The CVT applications require fast and secure protection. The energy storage elements like inductors & capacitors cause CVTs to exhibit transient behaviour. Further the performance of protection devices is also affected by the transient of CVTs. In steady state, the secondary voltage of a capacitor voltage transformer mirrors the primary voltage. Because of the energy-storage elements used, the transient output, caused by sudden change in primary voltage, is no longer a replica of the primary. Transient over voltages, which exceed insulation ratings can stress electrical insulation, leading to abrupt failure or gradual breakdown of the dielectric of electrical component. This paper attempts to study the effects of changes in capacitor values on the transient response of the CVT. Furthermore, the various method which affects the transient response are discussed by simulating the mathematical model using MATLAB

Experimental study on the influence of the disconnecting switch operation on CVTs in UHV series compensation stations

In series compensation stations of the UHV AC demonstration project in China, capacitive voltage transformers (CVTs) failed several times during the operation of disconnecting switches (DSs). The reasons for this failure are analyzed in this paper. The stray inductor, the stray capacitor between the platform and ground and the capacitor of the CVT forms a resonant circuit in series compensation stations, causing repeated oscillating overvoltage and impulse current acting on the CVTs during DS operation. A full-scale experiment indicates that the oscillating frequency is approximately 200 Hz and the repeating frequency is approximately 0.5 MHz. The simulation experiment in the laboratory indicates that the partial welding mode and the partial high electric field are the main causes of damage to the CVT element insulation. A mechanism of the insulation damage is proposed, and some suggestions are given based on our results.

Mitigating Ferroresonance in Capacitive and Inductive Voltage Transformers with Ferroresonance Suppressing Methods

In order to restrain ferroresonance in coupling capacitor voltage transformers (CCVT), three kinds of ferroresonance suppression circuits (FSCs) have been presented. The resonant type, the fast-saturation reactor type and electronic type which are connected to secondary side of CCVTs, are proposed in this paper. Frequency domain analyses, for voltage ratio magnitude, are carried out for CCVTs to obtain preferable FSC. Furthermore, using surge suppressors such as metal oxide varistors (MOV) to improve the performance of the FSCs is investigated. Time domain simulations demonstrate that ferroresonance can be cleared in few cycles in presence of FSCs. In addition, to mitigate ferroresonanse in inductive transformers (VTs), we connect the damping resistor to the open-delta-arranged auxiliary winding.

Estimation of Electronic Suppression Circuit Resistance for Protective Relaying Applications

The transient response of instrument transformers affects the performance of high-speed relays, particularly speed and overreach in case of distance protection. Transient response of a capacitive voltage transformer depends on the type of ferro-resonance suppression circuit (FSC), i.e., active, passive, or electronic, provided in the secondary to damp the ferro-resonant oscillations. The objective of this article is not to estimate optimum resistance but to provide a systematic approach instead of the trial-and-error approach to estimate damping resistance for the electronic ferro-resonance suppression circuit used in the: capacitive voltage transformer for protective relaying application. The performance of the numerical distance relay for the: capacitive voltage transformer with estimated damping resistance in the electronic ferro- resonance suppression circuit is tested and validated in real time using a field programmable gate array.

A review of ferroresonance in capacitive voltage transformer

Ferroresonance incidences in electrical power system have been commonly regarded as unexplained phenomenon, which is not critical for utility engineers. As a result, research conducted in this area is limited and the awareness on ferroresonance is relatively low among utility engineers. However, as the electrical system evolves, its complexity increases in line with the increasing risk of ferroresonance. As a result, this paper provides a consolidated review on the research conducted on ferroresonance to highlight its importance. This paper covers the fundamental inductor–capacitor pair for ferroresonance initiation and the modes of ferroresonance, followed by ferroresonance in capacitive voltage transformer (CVT), constituting its impact, initiation, and suppression techniques. The core focus in this paper is ferroresonance in CVT due to switching events, on which the documented literature is very scarce. © 2014 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Effect of TCSC on Chaotic Ferroresonant Oscillations in Voltage Transformer

ABSTRACTFerroresonance is one of the complex phenomena that occur in power system. This can create thermal and insulation problems for power system equipment. It can also cause chaotic oscillations in the power system. One of the factors that influences on the occurrence of ferroresonance in the capacitor voltage transformers (CVTs) is the magnitude of transmission line capacitor. To study the ferroresonance nonlinear dynamics, in this paper, the chaos theory is used. By using this theory, changes in system parameters which cause chaotic ferroresonant oscillations can be better reviewed and analysed. Thyristor-controlled series capacitors (TCSCs) are used for series compensation in the transmission line. Behaviour of the system in ferroresonance occurrence, with TCSC, is shown in bifurcation and phase plane diagrams. By using these diagrams, behavioural changes of the system can be easily seen in this case. The results show that TCSCs lead increasing of ferroresonance in CVT.

English

Instrument transformers are the integral components of power system protection and metering. The objective of instrument transformers is to provide scaled down replicas of high voltages and currents of the power system. In case of high voltage transmission network (≥132KV), capacitive voltage transformers are widely used as an alternative to electromagnetic voltage transformers due to their techno economic advantages. They transform higher voltages to lower values which are easily measurable and can be directly used for metering, protection and control applications. During the system transient conditions, there is a deviation in the scaled down secondary voltage signal from the actual signal which comprises of wide range of frequencies, due to energy storing elements of capacitive voltage transformer. There are several practices in the industry to mitigate the deviation in the secondary voltage signal of capacitive voltage transformer. For satisfactory operation of capacitive voltage transformer, better frequency response is desirable. The simulation study to determine the effect of various factors influencing the frequency response of capacitive voltage transformer are presented in this paper.

Development of Combined Monitoring System of MOA Insulating Material and CT (Casing Tube) Insulating Material

Live testing of resistive current of metal oxide arrester (MOA) and on-line monitoring of capacitive current of capacitor voltage transformer or casing tube (CT) are two important parameters for stable and safe operation of power system. The present method is to separately monitor these two types of equipment. Through analyzing equipment structure, monitoring principle, integration mechanism and so on, this article realizes combined measurement of resistive current and capacitive current. Thus, the combined monitoring of metal oxide arrester (MOA), rheology and CT insulation status is achieved.

A novel fast distance relay for long transmission lines

The measuring accuracy and the measurement stability of conventional distance relay will be influenced by complex and remarkable harmonic components due to the large capacitance of the line when it is used for a long line. Correspondingly, the tripping speed will be delayed to some extent. To solve this problem, a fast distance relay for long transmission lines is presented, which is on the basis of the differential equation algorithm using π transmission line model and the theory of Equal Transfer Process of Transmission Lines (ETPTL). The shortcomings of π model differential equation algorithm due to the impact of high frequency components can be overcome by using a low-pass filter. The problem resulting from the difference between the transfer feature of the voltages used by the distance protection and that of the currents due to the transient characteristic of coupling capacitor voltage transformers (CCVT) can be solved by using virtual digital CCVT. Then, the new distance relay can trip quickly by re-structuring the voltage at the fault point and iterative calculations. A variety of ATP simulation tests show that the new relay has fast tripping speed and high reliability when applied to the long transmission lines.