New Voltage Backfeeding Protection System Can Eliminate Expensive DER Connection Requirement
Objective: Inverter-based DER such as solar farms are frequently connected to utility distribution medium voltage (MV) lines. These lines can become disconnected from utility supplies, in which case the DER becomes the only source to the “islanded” MV circuits. When this happens, the connected DER can create sustained overvoltages on the associated transmission system. To prevent sustained overvoltages, protective relaying must be added to the utility system. These relay systems, known as 3V0 protection, are expensive and make the economics of DER connection less appealing. This paper will describe a low-cost alternative to 3V0 protection.
Methods: A typical distribution station is fed by a high voltage transmission line and a transformer. DER may be connected on MV lines emanating from the station. When a permanent single phase to ground fault occurs on the transmission line, circuit breakers at the transmission station will lock out. But the line will remain energized by the distribution DER. The energization voltage will have a steady state value given by the following equation: V island= V eps x SQRT (P gen/P load). Where P gen and P load are the aggregate generation and load prior to formation of the island, V eps is the electric power system voltage prior to formation of the island, and V island is the resultant voltage to which the island stabilizes. Note that if P gen/P load is greater than 1.0, sustained transmission overvoltages will be impressed on the unfaulted transmission phases. However, if this ratio can be maintained at a safe value, such as 0.77, the voltage will be reduced to safe levels. And at a ratio of 0.77 or below, the resulting under-voltage will be such that IEEE standards require the DER to trip within 2 seconds. Assurance that this will always occur reduces or eliminates the need for expensive 3V0 relaying. This paper describes a system that monitors P gen/P load in real time and controls P gen by sending power curtailment signals to the DER so the subject ratio is never exceeded. This system is based on an existing islanding protection platform that uses dual communication methods of powerline and secured cellular connections.
Results: The current islanding protection system has been employed for six years on over 60 circuits. Its performance has demonstrated an ability to support the required DER monitoring/control algorithm described above. In its current form, it can send control signals to DER using measurements obtained from utility SCADA. An enhanced version will provide direct monitoring eliminating dependency on utility SCADA.
Conclusion: The resulting system, proven 100% reliable, provides both islanding and backfeed protection. The benefits are cost reduction and improved performance that will make DER connections increasingly viable and enable large scale deployments.