Case Study: Using Synchrophasors and Off-the-Shelf Protection and Automation Equipment for Electrical Load-Shedding Services

The electrical systems of Alberta and British Columbia are connected via one 500 kV line and two 138 kV lines. When this interconnection is operated above certain levels and trips, the Alberta Interconnected Electric System (AIES) is at risk of firm load shedding. This requires the implementation of load-shedding schemes that monitor for contingencies and take mitigating actions to lessen impact during high import conditions. The primary purpose of AIES load-shedding schemes is to balance load and generation after the loss of resources, thereby preserving system stability and reliability.

The Alberta Electric System Operator (AESO) currently leverages an import load remedial action scheme (ILRAS), which uses interruptible loads that are automatically tripped after the loss of the interconnection during high import levels, and a load shed service (LSS). The LSS uses loads that are automatically tripped when the AIES frequency drops to 59.5 Hz or below, which can occur when the interconnection trips during high import levels.

This paper discusses the engineering design and implementation of an LSS system. The system installed at an Alberta plant was designed utilizing off-the-shelf, industry standard protection and control SCADA equipment to deliver sustainability and value through long-term, single-manufacturer support at an economically attractive cost.

The key provisions related to the LSS system are as follows:
•The committed load is disconnected from the AIES within 12 cycles of the frequency reaching 59.5 Hz, ± 0.02 Hz.
•The plant implemented electronic SCADA communications with AESO in order to respond to dispatch instructions and provide status information.
•The plant digitally records and provides event records to AESO following a trip. Event data (including the time stamp, measured frequency, and real power of the armed load) are captured and recorded for 60 seconds before and after a trip, at a sampling interval of 16 milliseconds.
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