Studies to Prevent Further Outages at Local Manufacturing Facility.

Background

An industrial manufacturer of engineered sealing solutions located in the Chicago suburbs turned to S&C’s Power Systems Services to perform short-circuit and coordination studies on their 4.16-kV medium-voltage power distribution system. The customer had recently experienced an outage to their entire manufacturing facility when the main circuit breaker operated as a result of miscoordination. During the initial project evaluation, the customer also indicated that the entire low-voltage distribution system had routinely been operating at 5% above nominal voltage.

As the first step to the analytical studies, Power Systems Services’ engineers performed a plant walkdown to collect data on the facilities’ electrical system. Each 4.16-kV substation transformer, as well as the serving 34.4-kV step-down utility transformer, was checked for its present operating tap to ascertain the cause for the higher-than-nominal voltage on the low-voltage distribution system.

Project Approach

The customer’s 4.16-kV primary system, which is shown on the one-line diagram linked here, consists of a main circuit breaker, a four-bay line-up of S&C Metal-Enclosed Switchgear featuring S&C Type SM-5S fuses with 600 Ampere disconnect switches, eleven step-down transformers (ranging in size from 300 kVA to 1500 kVA) with primary fusing, low-voltage switchboards, and several large chiller motors.

Using the Power Systems Analysis Framework (PSAF) for Windows® software application, S&C engineers calculated the available short-circuit currents at each bus in the system for three ANSI duty types: close/latching, contact parting, and low-voltage circuit breaker.

S&C engineers also examined the coordination of overcurrent protective devices from the main secondary low-voltage circuit breaker (or largest ampere rated feeder circuit breaker) in each of the unit substations to the primary-side fuses supplying the utility step-down transformer. The coordination studies were performed using CYMTCC for Windows®, a commercially distributed software program by S&C.

The study was performed in accordance with industry practices and procedures consistent with ANSI/IEEE Standard 242-2001.

Results

Results of the PSAF output reports indicated that all of the overcurrent protective devices that were studied were capable of interrupting the maximum available fault currents throughout the facility. The CYMTCC coordination plots indicated that the present relay settings were likely to result in a response from the main circuit breaker for almost any type of fault occurring on the primary system. S&C engineers also observed miscoordination between other protective devices on the system. Power Systems Services engineers provided recommendations on relay setting changes and fuse resizing.

The current transformers (CTs) that are presently installed in the main circuit breaker have a fixed ratio of 500:5. The utility transformer is capable of supplying 694 amperes of load current at 4.16 kV. Based on this data, the thermal capability of the current transformers may be exceeded if the transformer operates at full load.

S&C recommended checking the thermal capability of the current transformers to ensure continued viability of the equipment.

S&C Engineers also found that the utility step-down transformer was operating at a tap that would tend to boost the nominal output voltage on the secondary-side of the transformer to 5% above the nominal 4.16 kV. To prevent future overvoltage conditions, S&C engineers recommended changing the existing tap setting on the 4.16 kV bus as a means of voltage regulation.

Contact your local S&C sales office for more information on how Power Systems Services can improve reliability on your electric power system.