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Challenges for existing adaptive approaches

The key challenges with existing AOCR schemes emerge with the upcoming DER interconnection requirements and transitioning/operating as microgrids. Addressing these will be formative for the development of more versatile AOCR schemes supporting grid/microgrid modes of operation.

A. Underreaching OCRs With Multiple DER Generation Levels

Considering DERs as constant current sources, existing AOCRs often downscale the DER fault current contribution for different levels of output w.r.t. rated power. However, DERs can feed maximum fault current even when producing low unfaulted output power because they behave like a terminal voltage controlled current source. depends on highly variable parameters like distance from the fault, fault impedance, and fault type. As a result, existing AOCRs may overcompensate DER output for lower output power and under-reach.

B. Delayed Trip Time for a Fault in the Primary Protection Zone

Microgrids can have significantly lower fault currents compared to grid-connected systems. This negatively affects the OCR sensitivity in primary/secondary zones. In the microgrid mode, traditional time-based coordination can cause a significant delay in a relay’s trip time for primary zone faults. Existing AOCR approaches do not discuss this problem.

C. Current Direction Reversal Due to the Mode Switching

Current direction reversal is possible for i) the grid-connected system given the DER output states, or ii) microgrids depending on the location of the grid forming DER. Existing AOCR approaches seldom discuss the directional aspects of protection. However, adapting directional supervision when available is important to ensure selectivity towards internal vs external faults. Without accounting for directional supervision, it is difficult to create a robust scheme for the coexistence of grid-connected and microgrid modes.

These observations show that the existing AOCRs lack applicability to ADNs with both grid and microgrid modes, are vulnerable to under-reaching and changing fault current, and can be too complex to implement in traditional distribution relays. The next section elaborates a new emerging vulnerability difficult to address with current practices to programming the relay.