On the eastern end of Lake Erie, 21 urban distribution substations serve the power needs of the city of Buffalo, New York, U.S. The system, now owned and operated by Niagara Mohawk, a National Grid company, was built in the early 1920s. The substations, all of which are virtually identical in construction and layout, were built using standard equipment available at that time.

A typical Buffalo-style substation is composed of four bays. Each bay contains a self-cooled, 2500-kVA transformer, which is fed from a different underground subtransmission 2-kV cable. The transformers are operated in parallel and feed a common secondary bus. The secondary equipment consists of a 4.16-kV main bus, a transfer bus, and three normally closed bus tie-breakers. There are three feeder positions per bay for a total of 12 feeders per substation. The bus regulation is provided by a set of three single-phase induction voltage regulators for each transformer bank and is operated in parallel feeding the main 4.16-kV common bus.

Induction Voltage Regulator Issues

“Over the years, due to age and the electromechanical balance design, the controls have become increasingly difficult to adjust or maintain proper calibration, and spare parts are no longer available. A need was identified to find a solution in order to maintain proper substation voltage regulation,” stated Leonard Fiume, superintendent of power delivery.

The paralleling system on the induction voltage regulators used a two-wire scheme with current transformers (CTs) in conjunction with the breaker auxiliary contacts that make or break the paralleling circuit. The CTs are 80+ years old and are energized continuously. This exposure leads to heat and insulation breakdown, which, in combination with periodic failures of auxiliary switches (not closing or opening properly), eventually leads to CT failures. One initial idea was to replace all 12 regulators in each station with modern single-phase, step-type regulators with a five-wire paralleling scheme. Another consideration was to keep the induction regulators and replace the induction regulator controls with modern electronic controls. Preliminary costs indicated that it would be an expensive undertaking to replace the paralleling CTs and wiring needed for the new controls. The selected option was to incorporate a PLC-based control scheme that was developed by one of the department supervisors.

PLC Voltage Regulator Control

John Otabachian, supervisor of stations for the Frontier Region, developed a control scheme using the circulating megavar method to control the bus voltage and maintain parallel operation of the induction voltage regulators. The PLC regulator controls, which consist of Bitronic Meters with Modbus Plus (MB+) com ports located in each transformer bay, read bus voltage and bank VARs on all three phases. This data is transferred from each meter to the PLC via MB+ LAN.

Hard-wiring consists of proximity switches on the regulator dial indicator for maximum raise and lower travel indication, the regulator auto/manual switch, the raise and lower contactor on each regulator, and the bank breaker 52a switch. These points are all hard-wired back to the PLCs I/O modules. The bank breaker 52a contact is used to signal the PLC that the bank is out of service and is not to be used in the paralleling calculations.

Three Modicon compact PLCs, located in the front bay service/battery room, read all the data from each bank Bitronic meter. To improve response time, three PLCs were used to control all 12 regulators. Programming would have been extensive with only one PLC.

After performing calculations with the received data, the PLCs control all 12 regulators to adjust the bus voltage and keep the regulators running in parallel. If one or more induction voltage regulators is taken out of service, placed on manual, or alarmed and locked out, the remaining regulators will run in parallel and adjust the bus voltage. If any of the regulators fail to respond to the PLC commands, or conversely operate excessively, built-in alarming locks out the regulator from further operation.

“The voltage regulator systems were failing earlier than the transformers or circuit breakers. The PLC induction voltage regulator control provides us with life extension of the existing equipment. We can then rebuild the station in a more cost-effective manner. We have converted two substations to the PLC control, and they have been operating successfully for over a year. Potential customer voltage problems and power-quality concerns have been addressed, and we will continue the conversion program to PLC induction voltage regulator controls,” stated Doug Tulley, manager of energy services for the Frontier Region.

“This voltage control method could be used in various other regulator and load tap changers schemes and could be easily incorporated with substation integration systems, stated Otabachian.

William Lobko is the manager of substation operation and maintenance services, New York, for Niagara Mohawk, a National Grid company. Lobko has more than 25 years of experience in the electric-utility industry, mainly in construction, operation and maintenance activities of substation equipment. Currently, he is involved in the development of substation maintenance standards and procedures, work methods, work forecasts, testing requirements and other related support activities for the field operation organization to ensure the compliance and completion of corporate substation maintenance programs.
William.Lobko@us.ngrid.com