Short-circuit Duty Research Articles

Closed Loop Control of Wind Power Based on Trans Quasi-Zsourceinverter

The paper presents a wind generator output power appropriately monitored using a closed loop controller engaging the buck-boost competency of Trans qZSI and is fed to grid. The converter getting all benefits of conventional ZSI and exchanges the traditional inverter in the arrangement. Moreover, the Trans qZSI has the exceptional benefits of minimum component count and lower component ratings. The feed forward controller cautiously controls the wind generator output power. It monitors the short circuit duty cycle of inverter using third harmonic injected control technique to create a boosted output. Simulation of the complete circuit is performed on MATLAB and results are discussed. It offers an exclusive voltage regulation by creating proper choice of shoot through duty ratio under variable wind speed. The proposed control system is suitable for transient and steady state operation retaining the preferred level of output.

Influence of tertiary stabilizing windings on zero-sequence performance of three-phase three-legged YNynd transformers. Part II: Tank overheating hazard and short-circuit duty

This paper analyzes the influence of stabilizing windings in three-phase three-legged YNynd transformers on tank overheating hazard and on the short-circuit performance of such transformers in the event of unbalanced conditions involving zero-sequence currents as asymmetrical faults. It complements a previous companion paper that presented experimental zero-sequence measurements and equivalent circuit calculations for three power transformers. Those previously calculated zero-sequence equivalent circuit parameters are used here to evaluate the comparative performance of YNynd power transformers when the delta stabilizing winding is closed and when it is open. Additionally, it analyzes the effects of other variables, such as relative position of the stabilizing winding, presence of magnetic shunts in the tank walls, and system grounding.

Influence of tertiary stabilizing windings on zero-sequence performance of three-phase three-legged YNynd transformers. Part I: Equivalent circuit models

The presence of a stabilizing winding (or tertiary stabilizing winding when is used to for auxiliary applications) in three-phase three-legged YNynd transformers remarkably affects the zero-sequence performance of both the transformer and the network. This paper presents a detailed analysis of the influence of the stabilizing winding on the zero-sequence behavior of three-phase three-legged YNynd transformers. Based on a complete set of onsite zero-sequence measurements taken in three power transformers, transformer zero-sequence performance is analyzed in relation to internal design features such as stabilizing winding position relative to high-voltage and low-voltage windings or the presence of magnetic shunts in the tank. Based on these measurements, this paper assesses the ability of various equivalent circuit models to reproduce zero-sequence performance accurately. A companion paper that complements this study evaluates the influence of stabilizing windings on tank overheating hazard and short-circuit duty in the event of asymmetrical faults.

Use of Duplex Reactor in an Oil Refinery

In order to reduce short-circuit level in a power augmentation generation project in an oil refinery, it installed a duplex reactor as an alternative solution when comparing to usual techniques. Duplex reactor concept was born in the beginning of XX century, and its application is similar to conventional series reactors: limiting system maximum short-circuit current to match rated equipment short-circuit duty, protecting personal and electrical installations. However, in some electrical networks, duplex reactors have best performances than conventional ones. Selection of duplex reactor instead of other classical options is discussed in this paper. Technical and economic characteristics about the duplex reactor are also presented along this paper.

Symmetrical, Total, and Peak-Oh My: Is Current All I Use to Select My Circuit Breaker?

American National Standards Institute (ANSI)/IEEE medium-voltage circuit breaker ratings have changed in recent years, causing a great deal of confusion over how to properly select a breaker for short circuit duty. This article reviews the basics of an asymmetrical short circuit current waveform. It illustrates the effect of the X/R ratio and how the relationship between the dc component and the circuit-breaker contact parting time determines the interrupting time of the short circuit current. Five examples are discussed, and the first set examines basic power distribution system applications that illustrate the potential issues of mixing older megavolt- ampere (MVA)-rated switchgear with new circuit breakers rated using the voltage range factor K =1. The second set examines the complexities that occur when a generator with significant X/R ratio is directly supplying distribution switchgear. The examination of these different conditions is intended to aid equipment users in specifying the correct circuit breaker by identifying the critical factors and conditions affecting the performance of the circuit breaker.

Power Plant Station Protection System against Voltage Fluctuation

Electrical Power System protection is required to protectboth the user and the system equipment itself fromany fault, hence electrical power system is not allowed to operate without any protection devices installed. Power System fault is defined as the undesirable condition that occurs in the power system. Some of these undesirable conditions are short circuit, current leakage, ground faultand over-under voltage. With the increasing loads, voltages and short-circuit duty in power plant, over voltage protection has become more important today. Here, the component that had been used is PIC 16F877a microcontroller to control the whole system and especially on the circuit breakers as well as the LCT display unit is used to display the voltage level and type of generator that used to serve the load. Sensors are used to measure both thevoltage and the load. The controlled digital signal from PIC microcontroller is converted by using the digital analog converter to control the whole circuit. Thus a device called protective relay is created to meet this requirement. The protective relay is mostlyoften coupled with circuit breaker in a way that it can isolate the abnormal condition in the system.

UNIVERSAL CONTROLLER FOR THE INTERCONNECTION OF DISTRIBUTED GENERATORS WITH UTILITY LINES AT CUSTOMER LEVEL VOLTAGES

This paper presents the design, construction and performance testing of a controller capable of protecting the utility from any negative side effect of the interconnection of distributed generation (DG) at customer level voltages. The controller is universal in the sense that it allows for the interconnection of all kinds of distributed generators: synchronous, induction and those that need a power electronic converter (solar, fuel cell, micro-turbine, etc.). The controller assures that the IEEE standards 1457 for the interconnection of DG and utility standards are complied. Additionally, the controller cuts the fault current in less than a cycle to prevent the increase to the shortcircuit duty of the already installed protection equipment. This is especially important in the case of a synchronous interconnection in locations where the short-circuit power of the substation breakers is close to its limits. A 25 kVA utility grade prototype controller has been built and tested in the lab.

On Power System Controlled Separation

This paper describes verification of five conjectures related to power system controlled separation. It attempts to verify that the location of uncontrolled separation (loss of synchronism or out-of-step operation) is independent of the location and severity (short-circuit duty or duration) of the initial faults, that the location depends on the prevailing network configuration and load level, and that it takes place one operation at a time (cascades). Verification of these conjectures would allow controlled separation during a disturbance in real-time, using the present communication and protection systems, and results in a minimal load and generation imbalance. Two actual power systems were used, a 50-bus system to establish the procedure for controlled separation and a 640-bus interconnection to apply the procedure. Data representing static and dynamic behavior of the two power systems were obtained from the operating utilities, and the many required simulations were conducted using EPRI Power System Analysis Package (including IPFLOW and ETMSP programs).

Application of the duplex Reactor-an unusual and forgotten technique to reduce short circuit duty

The duplex reactor is a center-tapped three-phase current-limiting reactor that can benefit power system operation when connected in certain configurations between two utility-fed distribution system buses and a local generator source. Its use was fairly common in the 1940s and 1950s. This paper discusses the physical and mutual inductance characteristics of the duplex reactor, its equivalent circuit model, and revisits its potential application to limit the three-phase short circuit duty on medium-voltage industrial systems. Its use can also result in beneficial system voltage support of local distribution buses during severe remote system faults. Results of simple system simulations using short circuit and load flow programs are explored on example systems.

Using short-circuit currents to perform a protective device coordination study

The article provides a brief background on the fundamentals of short-circuit analysis. Following that background is a discussion of how various types of phase overcurrent protective devices respond to the short-circuit current waveform. Based on this discussion, the short-circuit currents which are necessary to perform a protective device coordination study are specified. A comparison of the two types of short-circuit studies follows, based on the complex impedance network representation of the power system. The results of the comparison are then used to present recommendations for how the results of a computer-aided study to compute short-circuit current duties can be applied to determine the short-circuit currents necessary to select the settings of phase overcurrent protective devices. Finally, a sample industrial power system is provided to clarify the recommendations.

Limitations of fault-current limiters for expansion of electrical distribution systems

The application of fault current limiters (FCLs) has been recommended in the existing literature to retain underrated medium-voltage circuit breakers of lower short-circuit ratings in service, when the distribution systems are expanded and their short-circuit currents increase. However, the existing published literature does not describe the limitations of FCLs from system separation and loss of protective device coordination conditions, nor does it provide an analysis of short-circuit duties of medium-voltage circuit breakers used with FCLs. This paper shows how these evaluations can be carried out. Considerations of the high cost of replacements and the far greater loss of production, due to lack of coordination and availability of a proven alternate solution, should be included in an informed decision to use FCLs.

Investigations of voltage flicker in electric arc furnace power systems

Operation of rapidly varying loads such as AC and DC arc furnaces in large industrial power systems will cause voltage flicker on the utility system. System planning will help in determining the available short-circuit duty at the point-of-common-coupling to keep the voltage flicker within acceptable limits. Perceptible flicker limit curves are useful in determining the amount of flicker in a system. Short-circuit voltage depression calculation is one technique to estimate the amount of expected flicker in a system. On-site field tests with equipment that will accurately capture multiple frequencies will aid in measuring the existing voltage flicker. The authors discuss flicker estimation and field tests as well as harmonic filter tuning effects on voltage flicker. The ultimate determination whether unacceptable voltage flicker exists in a system will be complaints from customers served by the utility system actually experiencing objectionable or noticeable flicker.

Safety against Lightning for Linemen Working on de-Energized Power Lines

Where the risk of accidental power-frequency energization is concerned, the safety ground has a short-circuit discharge duty determined by the configuration and parameters of the power system. Safety is thus assured by selecting a safety ground having a short-circuit safety rating which matches the duty imposed by the power system. Up to now, a similar matching for lightning currents has not been possible. This paper addresses this problem by: 1. Determining the lightning current rating of typical safety grounds. 2. Calculating the lightning current discharge duty of safety grounds in terms of the insulation level of the line and the distance from the location of the thunderstorm. The analysis shows that special precautions are needed when working on power lines of voltage classes above 360 kV.

Relay Protection of Generator Station Service Transformers

Generator station service transformers, also called unit auxiliary transformers, are subjected to very rigorous short circuit duty. These faults fall generally into three areas: Faults within the transformers themselves, low-side faults between the transformer and the low-side bank breakers, and faults on the station service supply buses. The purpose of this paper is to show how station service transformers should be protected, and why any protective scheme can only be effective if these transformers are properly specified for UAT service.

Through-fault capability requirements for unit auxiliary transformers

The magnitude and duration of short circuit currents for faults on the low sides of unit auxiliary transformers (UATS) are calculated taking into account the generators' initial loadings, time constants, and saturation. The stresses imposed on UATS by these currents are compared with the short circuit withstand capabilities provided by present standards. In many cases it is found that the standard capabilities are not adequate for the short circuit duties encountered in UAT applications. A method is presented to specify the short circuit withstand capabilities when the standards do not apply.

Application of New Methods for Calculating Short-Circuit Duties and Power Circuit-Breaker Capabilities

The more salient points in the application of ac high-voltage circuit breakers according to new Standards C37.010 1964 and C37.5-1969 are outlined. The relationship between circuit-breaker rated interrupting and momentary capabilities and the corresponding calculated duties is defined. Areas of ambiguity, omissions, and controversy which exist in the standards are under review at the present time. The authors report some of these areas for information only.

Refinery Electrical Power System Reliability

Modern refinery process units require electrical power systems that do not have power interruptions. The backbone of a highly satisfactory 120-MVA system combines local generation with a power company captive dual substation (110 kV/12 kV) that serves dual feeders to area buses. It must be designed with the aid of computers to properly anticipate short-circuit duties, select and set protective relays, and provide adequate voltage regulation, especially during faults and switching. An adequate return on the investment in the system requires that the maintenance be organized, thorough, and technically competent. The system and its operating procedures must provide fault isolation in first zones of protection, and complete restoration of power to process units must be rapid and complete.

EHV Application of Autotransformers

The universal method of voltage transformation from EHV levels to lower voltage systems is through the use of the autotransformer. The characteristic of the autotransformer is profoundly influenced by the tertiary winding or the lack of it. This paper discusses the influence of the tertiary winding and its placement, fault magnitude and short circuit duty, autotransformer loading, overexcitation, protection of unloaded tertiaries, loading of paralleled autotransformers, and the use of single-phase units while presenting a circuit for studying unequal impedances in paralleled banks.

Dynamic Behavior of a 220-Kv Dead-End Suspension Bus during Short Circuit

A full-size 220-kv dead-end twin-bundled conductor suspension bus was subjected to short-circuit currents of various magnitudes and durations. Transducers and instruments available allowed the measurement and recording of many physical variables during these tests. Maximum short-circuit magnitude was 30,000 rms amperes. Short-circuit currents cause dynamic forces to be applied to the bus conductors, hardware, and towers. The major shock force is a result of the twin conductors being pulled together between spreaders by the short-circuit magnetic forces. The number and location of twin conductor spreaders have considerable influence on the peak magnitude of the dynamic forces. Electrical and mechanical damage to conductors as a result of a short circuit was negligible. This suspension bus design proved to be adequate for a shortcircuit duty of 10 million kva (kilovoltamperes) at 220 kv, and with minor modifications it may be adequate for 20 million kva at 220 kv.

A new welder busway distribution system

THE OHIO Stamping Plant of Chrysler. Corporation at Twinsburg, Ohio, covers an area of 1,500,000 sq ft (square feet), approximately 34 acres. Power is supplied by the Ohio Edison Company to supply the great number of motor loads and so forth for the heavy presses and other manufacturing equipment in the plant. The estimated short-circuit duty is 2,500,000 kva, based on long-range future plans. The estimated initial short-circuit duty is 600,000 kva 3 phase symmetrical.