Normally Open Point (NOP)

The primary function of an NOP is to break the mesh in a distribution network, ensuring the topology remains a tree structure without closed loops. This is essential because:

• Distribution protection schemes rely on unidirectional fault current flow
• Closed loops create circulating currents that complicate protection coordination
• Radiality simplifies voltage regulation and fault detection

An NOP physically separates two feeders that are otherwise connected through the tie switch, maintaining the radiality constraint required by utility operating standards.

During a fault or planned outage, the NOP can be closed to transfer de-energized customers to an adjacent healthy feeder. This process involves:

• Fault isolation upstream of the affected segment
• Closing the NOP to establish an alternative supply path
• Opening a sectionalizing switch to maintain radiality

The NOP enables service restoration without waiting for field crews, dramatically reducing SAIDI metrics. Modern distribution automation systems execute this sequence in under 60 seconds.

NOPs are positioned at feeder boundaries where two distinct circuits approach each other but remain electrically separated. Optimal placement considers:

• Load transfer capacity of the adjacent feeder during contingency
• Proximity to critical loads requiring high reliability
• Coordination with sectionalizing switches for flexible reconfiguration

Utilities typically place NOPs at the midpoint of feeder ties to maximize the number of customers that can be restored from either direction during an outage.

Modern NOPs are equipped with Intelligent Electronic Devices (IEDs) that enable remote operation and autonomous decision-making. Key capabilities include:

• IEC 61850 GOOSE messaging for peer-to-peer communication with adjacent reclosers
• Integration with Outage Management Systems (OMS) for automated fault response
• Voltage sensing on both sides to verify synchronization before closing

In a self-healing grid, NOPs participate in distributed automation schemes that detect faults, isolate affected sections, and restore power without human intervention.

A traditional NOP is a mechanical switch with binary open/closed states. A Soft Open Point (SOP) replaces this with power electronics:

• Traditional NOP: Simple, low-cost, provides only connectivity
• SOP: Back-to-back converters enabling active and reactive power flow control
• SOPs can balance load between feeders continuously, not just during contingencies

While NOPs remain the standard due to cost and simplicity, SOPs are emerging in networks with high distributed energy resource penetration where dynamic power flow control is essential.

When an NOP closes to restore power after a prolonged outage, operators must account for Cold Load Pickup (CLPU) — a temporary demand surge caused by:

• Simultaneous restart of thermostatically controlled loads (HVAC, refrigeration)
• Loss of load diversity that normally smooths demand profiles
• Inrush currents from motors and transformers

CLPU can exceed normal peak load by 2-5x and persist for minutes to hours. Reconfiguration algorithms must verify that the adjacent feeder has sufficient cold load pickup capacity before closing the NOP.