Fast Load Shedding Scheme for Enhancing Reliability and Stability of Expanded Liquified Gas Plant

A grid-isolated Liquefied Natural Gas (LNG) import terminal has been expanded to include liquefaction and export capabilities. The new electrical system contains two 65MW steam turbine generators and 30 load feeders with two intertie breakers connecting the generation systems of the import and export facilities. To prevent loss of power to the plant, during contingency, it is important to perform load-generation balance within 100ms. In addition, the system frequency should be maintained within the 95% of the nominal frequency to prevent system instability. An IEC 61850 based Fast Load Shedding (FLS) scheme is deployed in this new plant to operate for loss-of-power condition, either from the generators or from the existing intertie connections. Protection relays with communication capabilities to exchange IEC 61850 Generic Object-Oriented System Event (GOOSE) were used over redundant communication networks utilizing Parallel Redundancy Protocol (PRP) for high-speed communications within the plant. Two redundant FLS Controllers exchange GOOSE messages with feeder, motor, transformer and generator relays to capture breaker status and power flow information for reliable operation. In addition, compressor-loading interlocks over the same IEC 61850 PRP network are implemented using motor protection relays to ensure the availability of at least one fuel gas compressor at all times. This paper presents lessons learned from the design and deployment of the FLS scheme by utilizing the available protection relays and adding FLS Controllers. The design strategies, such as IEC 61850 GOOSE communications between relays and Controllers, redundant PRP networks, network quality of service, etc. are discussed. The challenges to integrate non-IEC 61850 devices into the scheme are also discussed. The FLS initiation logic (to arm and disarm the scheme), and adaptive load-generation balance design are presented with test results, with the objective of sharing experiences from the design and deployment of a FLS scheme that guarantees plant's availability and stability.