RT22 | Real Time Performance Analysis of Transformer Differential Protection based on IEC 61850-9

IEC 61850 is a comprehensive standard for substation automation systems (SAS). The standard mandates integration of all protection, control, measurement, and monitoring functions of a power system network at both the process and station levels. After successful implementation of IEC 61850-8-1-based GOOSE at several power substations worldwide, major protective device vendors are currently developing interoperable products for IEC 61850-9-2-based time-critical sampled value (SV) messages over the digital process bus. As consequence several energy companies worldwide are developing projects for substations fully digital. Some of the benefits of the implementation of the IEC 61850-9-2 technology include: 1) a significant reduction in the overall cost of substation; 2) reduction in the cost of numerous copper conductors compared to the use of few optic fiber cable; 3) probability reduction in the of the occurrence of Current Transformer (CT) saturation and its consequences; 4) it is much safer to use than conventional methods where an open circuit could occur and also it eliminates high voltage copper lines from entering the relay room; 5) it permits wide availability of measurements to individual protection IEDs on the digital communication bus, and system reconfiguration can be done in less time. The performance of the process network in the digital substation is particularly important for relaying functions implementation, and the transmission quality of SV packets is becoming the key constraint on protections because they carry practical parameters of the power system. Unlike GOOSE, the same SV message is not repeated several times and, therefore, it is important to analyze the impact of SV loss, bit error, disorder, or delay on protection functions. The amount of SV packets is much larger than that of GOOSE packets, taking up major traffic in process bus. Moreover, SV packet is likely to be lost or delayed in network congestion, which may lead to the failure of protection function. Therefore, more attention should be paid to SV packet issues, especially for differential protection that needs synchronization of SV on both sides. Traditional protection IEDs are not equipped with any corrective measures for SV loss/delay and, therefore, lost, or delayed samples are neglected considering their values to be zero for the phasor estimation. Today, the solution adopted by IED vendors is blocking the protection functions for excessive packets loss/delay. However, it might deteriorate the system if backup protection fails to work during a fault. Therefore, this paper analyses the impact of smart algorithms for the treatment of the lost/delay SVs on differential protection IED. Different corrective measures will be tested, such as, zero-value replacement, linear interpolation, second-order Lagrange interpolation and cubic spline interpolation. Apparently, better calculation precision usually means more computing workloads and slower computing speed. Therefore, all algorithms will be implemented and tested in real time environment. In a first moment using virtual IED (software-in-the-loop simulation) and in a second moment using real IED (hardware-in-the-loop simulation). To examine it, a real substation from CPFL Energy, a Brazilian utility, is modeled in HYPERSIM software. Basically, it consists of two 69 kV transmission lines, one power transformer, and four 13.8 kV distribution feeders. The SV estimation algorithms are investigated as a part of the transformer differential IED for various SV loss/delay scenarios in a laboratory environment using IEC 61850 data integrity manipulation and fault insertion tool.