Medium Voltage Power System Research Articles

Synchronized Flicker Measurement for Flicker Transfer Evaluation in Power Systems

Voltage fluctuations caused by rapidly changing loads, such as arc furnaces, can propagate to different parts of a power system. Although the flicker level at its origin can be high, levels that are measured at other sites are subject to attenuation, a process that is influenced by fault levels, transformer impedances, line impedances, and composition of the connected loads. This paper presents the methodology, measurement results, and data analysis in relation to synchronized flicker measurements carried out in a high-voltage (HV)/medium-voltage (MV) power system which contains an arc furnace supplied by a dedicated feeder connected to the HV busbar. The flicker transfer coefficients derived from measurement results clearly indicate that flicker transfer from the arc furnace site to the upstream HV busbar is governed by the fault levels at the two locations. However, the transfer of flicker from the upstream HV busbar to other downstream busbars is dependent on the downstream load composition. These flicker transfer coefficients are vital in the application of methodologies described in many reports and standards in relation to establishing planning levels at various voltage levels and in the allocation of flicker emission to customers.

Multifunction digital relay commissioning and maintenance testing

This paper discusses with the protective relay technology over the past 25 years has evolved from single-function electromechanical (EM) relays, to static (electronic) relays, and, finally, to digital multifunction relays. A significant number of these multifunction digital relays are being installed on medium-voltage electric power systems within industrial and commercial facilities. As was required in earlier EM and static relay technologies, digital relays also require commissioning and relay-setting verification. This article discusses the unique challenges the user faces in testing and commissioning digital multifunction relays. It also explores the impact on maintenance testing of self-diagnostics, the digital relay's internal capability to check itself for failures.

Solid-State Circuit Breakers and Current Limiters for Medium-Voltage Systems Having Distributed Power Systems

State-of-the-art mechanical circuit breakers in medium-voltage systems allow a safe handling of short-circuits if the short circuit power of the grid is limited. Using delayed turn-off times, the circuit breakers can be coordinated with lower level protection gear. Hence, a high availability of the grid can be guaranteed. However, during a short-circuit a significant voltage sag can be noticed locally in the medium-voltage grid. Sensitive loads such as computers will fail even if the voltage returns within a few seconds. A semiconductor circuit breaker, however, is able to switch fast enough to keep voltage disturbance within acceptable limits. The optimization and selection of power electronic switch topologies is critical. In this paper, different semiconductors are briefly compared considering the requirements of a solid-state switch integrated into a 20-kV medium-voltage grid. Based on these semiconductor characteristics, various switch topologies are developed, which are compared under technical and economical aspects. It is shown that solid-state circuit breakers offer significant advantages when compared to present solutions and can be used in today's medium-voltage power systems.

Protection, Metering, Monitoring, and Control of Medium-Voltage Power Systems

Traditional practices for industrial and commercial medium-voltage power systems to provide protection, metering, monitoring, control, power quality analysis, disturbance recording, and automation have changed. The new multifunction numerical protection relays using the processing power of the present generation of microprocessors have very high functionality. Many functions can be integrated into one device, allowing new power system problem-solving capabilities while permitting significant cost savings as compared with discrete protective relays, instruments, meters, recorders, and transducers of the past. This paper describes the vast capabilities and recent enhancements of these devices, which will be called intelligent electronic devices in this paper. Included are anecdotes obtained from an installation on a large industrial power system.

97/02860 A complete set of automatic compensation equipment of a capacitive charging current in the application of a medium-voltage power system

97/02860 A complete set of automatic compensation equipment of a capacitive charging current in the application of a medium-voltage power system

A complete set of automatic compensation equipment of a capacitive charging current in the application of a medium-voltage power system

The complete set of automatic compensation of a capacitive charging current is composed of a grounding-transformer-type arc-extinguishing coil, an automatic tuning controller and a fail-safe ground fault protective device. This paper introduces the overall scheme of the set, the design, the operation principle and the characteristics of its various parts.

The technique of finite-impulse-response filtering applied to digital protection and control of medium voltage power system

When developing a relaying algorithm, digital filtering is an important step. A method to design a FIR filter based upon two objectives is given. These objectives are to allow easy computation and to be as close as possible to a given frequency response. With the help of an example, an original method is also proposed to evaluate the complete sequence of a digital relaying algorithm, based on FIR filtering. It is shown how such an algorithm can be used, not only for protection but also for each function of an integrated protection and control system. >

Optimum Use of Oil-Filled Distribution Transformers to Design Resistance Ground Sources for Medium-Voltage Delta-Connected Power Sources

A procedure is developed to design a resistance ground source for medium-voltage power systems with delta-connected source transformers. The optimum connection is shown to be three distribution transformers in wye/broken delta configuration with a 600-V resistor in the secondary circuit. The assumptions are that the transformer banks are used solely as a source of ground fault current, that the power system has no loads at line-to-neutral voltage, and that the ratio of source transformer to distribution transformer three-phase kVA is greater than 25.

Vacuum Switchgear Application Study with Reference to Switching Surge Protection

The unique characteristics of, vacuum circuit breakers have raised the issue of whether special criteria should be developed relating to their application on medium-voltage power systems. A computer program is described which models vacuum switching surge phenomena and the measurements required to obtain the vacuum interrupter inputs to the program. The program has been validated by good correlation between computed and measured results relative to a single-phase laboratory circuit and a three-phase unloaded transformer circuit. Examples of the use of the computer program in switching transformer and motor circuits are presented.