Using IEC 61850 Standard for Automatic OLTC Control of Parallel Transformers

This presentation describes a utility experience from a project where protection and control for the whole substation is done by using the IEC 61850 standard. Special attention is given to the control of parallel transformers with on-load tap-changer (OLTC), which has been achieved by using IEC 61850 analogue GOOSE messages between the regulators. In this substation, there are two 20MVA, 110/20kV, YNd5 transformers with OLTC. Each power transformer has a dedicated IED which provides integrated protection and control for the transformer. Thus, all power transformer protection functions like 87T, 87N, 50/51, 50N/51N, 49 as well as OLTC control function 90 are integrated into a single device. Transformer backup protection is allocated in the switchgear control IED for the transformer bay. Because the two transformers can operate in parallel, the OLTC control is based on the minimizing circulating current principle. The OLTC control functionality is implemented in accordance with the IEC 61850 standard. Thus, two logical nodes, namely YLTC and ATCC, are available in each of the two IEDs. The YLTC logical nodes represent the OLTC mechanism and integrate functionality like tap position reading, OLTC mechanism supervision and issuing of the manual and automatic raise and lower commands to the OLTC mechanism. The ATCC logical node represents the regulator itself and integrates functionality like voltage measurement and supervision, timing, line drop compensation, circulating current measurement and compensation etc. Because of the parallel control it is necessary to exchange information about measured current, measured voltage, etc. between the two IEDs. This information exchange is performed by using analog GOOSE messages in accordance with IEC 61850 standard between the two IEDs. The whole engineering flow follows the IEC 61850 standard and it has been performed by an external system integrator. Since September 2007 three substations with such control systems have been in full commercial operation.