Innovative Transformer Load Tap Changer Control Using Ethernet-Based Communications

This paper describes the design of a load tap changer (LTC) control for up to four parallel transformers using a modified circulating current approach. The LTC control discussed in this paper was implemented using programmable logic, math functions, and IEC 61850 communications over fiber-optic Ethernet. One LTC control is associated with each transformer. The current angle from each LTC control is passed to all parallel LTC controls using IEC 61850 Generic Object-Oriented Substation Event (GOOSE) messaging. A decision is made in each control as to whether it should be the next control to tap up, tap down, or remain unchanged should the voltage magnitude go outside the upper and lower band settings for a predetermined time. This selection is then communicated to all LTC controls. The LTC control also keeps track of the tap position by monitoring raise and lower motor current in the LTC. The LTC control was field tested on three parallel 230/115 kV autotransformers on the Santee Cooper system. This paper describes the installation, supervisory control and data acquisition (SCADA) communications and control, and field experience gained with the IEC 61850 LTC control. This Ethernet-based system requires no wiring between transformers to derive circulating current because each LTC control is essentially standalone and all required information for controlling the LTCs is communicated over a network. This simplified installation and troubleshooting.