Soft Open Point (SOP)

Unlike mechanical switches that simply connect or isolate feeders, SOPs use back-to-back voltage source converters (VSCs) to decouple active and reactive power flows. Each converter independently regulates its output, enabling:

• Active power transfer between feeders to balance loading
• Reactive power injection or absorption at each terminal for voltage support
• Four-quadrant operation without affecting the other feeder's power factor This decoupling is impossible with conventional tie switches and forms the foundation of SOP flexibility.

SOPs provide continuous, fast-acting voltage control at the feeder interconnection point. By injecting or absorbing reactive power independently on each side, the device can:

• Mitigate voltage rise caused by high distributed generation penetration
• Compensate for voltage drop during heavy loading conditions
• Respond to fluctuations within milliseconds, far faster than mechanical tap changers This capability is critical for maintaining voltage within statutory limits on networks with high solar PV adoption.

By actively controlling the magnitude and direction of active power flow between feeders, SOPs enable dynamic loss reduction. The control system continuously solves an optimization problem to:

• Shift power from lightly loaded feeders to heavily loaded ones
• Reduce I²R losses in conductors by flattening the loading profile
• Adapt to changing load and generation patterns throughout the day Studies on UK distribution networks have demonstrated loss reductions of 10-20% compared to fixed normally open point configurations.

During fault conditions, SOPs enhance the self-healing capability of distribution networks. The power electronics can:

• Instantly block fault current contribution to prevent propagation
• Operate as a virtual circuit breaker by ceasing power transfer within microseconds
• Enable seamless load transfer to healthy feeders without the momentary interruption associated with mechanical switching Post-fault, the SOP supports cold load pickup by ramping power gradually, avoiding the inrush currents that challenge conventional restoration schemes.

SOPs directly increase the amount of renewable generation a feeder can accommodate without reinforcement. By providing a controlled path for reverse power flows, the device:

• Prevents thermal overload of the primary feeder by diverting excess generation to adjacent feeders
• Mitigates voltage violations at the point of common coupling through dynamic reactive power absorption
• Enables higher penetration of rooftop solar PV and small-scale wind without upgrading conductors or transformers This capability defers or eliminates costly network reinforcement projects.

Distribution networks frequently experience phase imbalances due to uneven single-phase loading. SOPs with four-leg inverter topologies or three-phase converters can:

• Transfer power selectively between phases to balance currents
• Reduce neutral current and associated losses in the neutral conductor
• Improve voltage symmetry at the feeder level This function extends transformer life by reducing thermal stress from negative-sequence currents and improves power quality for all connected customers.

Unlike a mechanical Normally Open Point (NOP) that is either fully open or fully closed, an SOP uses a back-to-back voltage source converter (VSC) topology to enable continuously variable active power transfer. This allows a lightly loaded feeder to share capacity with an overloaded neighbor without violating the radiality constraint—the converters decouple the feeders electrically while coupling them magnetically through the DC link. The control system can inject or absorb real power (P) independently on each AC terminal, effectively creating a controllable energy bridge that balances feeder loading in real time without reconfiguring the entire network topology.

Each VSC in an SOP can independently generate or absorb reactive power (Q) at its point of common coupling, functioning as a distributed STATCOM on each feeder. This four-quadrant operation provides dynamic voltage support without the need for discrete capacitor bank switching. Key benefits include:

• Voltage profile flattening along both connected feeders
• Mitigation of voltage rise caused by high distributed PV penetration
• Reduction of reactive power flows through upstream transformers, decreasing I²R losses
• Elimination of the hunting and transient inrush issues associated with mechanical switched capacitors

SOPs serve as actuators within a Model Predictive Control (MPC) framework that solves a mixed-integer nonlinear optimization problem to minimize total feeder losses. The objective function targets reduction of I²R losses across all line segments by continuously adjusting the SOP's P and Q setpoints. Unlike traditional Distribution Feeder Reconfiguration (DFR) which requires discrete switching events, SOP-based loss minimization is continuous and bumpless, avoiding the transient disturbances and equipment wear associated with mechanical switching. The optimization respects constraints including voltage limits, line thermal ratings, and the DC link power balance between the two VSCs.

During a Service Restoration (SR) event, an SOP provides capabilities far beyond a mechanical tie switch. When a fault occurs on one feeder, the SOP can:

• Instantly transfer load from the faulted feeder to the healthy adjacent feeder through the DC link
• Provide black-start capability to the isolated section by establishing voltage and frequency reference
• Maintain fault current limiting through the converters' current control loops, preventing fault propagation to the healthy feeder
• Enable seamless resynchronization when the fault is cleared, avoiding the phase-angle matching required for mechanical reclosing This reduces SAIDI by eliminating the minutes-long switching delays inherent in traditional restoration schemes.

High concentrations of Distributed Energy Resources (DERs)—particularly rooftop solar—create bidirectional power flows that can cause feeder congestion and voltage violations not anticipated in legacy radial designs. An SOP acts as a power flow valve, redirecting excess generation from a saturated feeder to a neighboring feeder with available hosting capacity. This defers or eliminates the need for traditional conductor upgrades and transformer replacements. The SOP's power electronics also provide harmonic filtering and negative sequence compensation, improving power quality at the interconnection point.

Traditional meshed distribution operation is avoided because it requires directional overcurrent protection and complex distance relaying coordination. An SOP achieves the reliability benefits of meshing—multiple supply paths—while maintaining radial protection simplicity. Because the converters' current control loops inherently limit fault current contribution to approximately 1.2–1.5 pu of rated current, the SOP does not appear as a significant fault current source. Protection engineers can retain existing overcurrent coordination settings on both feeders without recalculation, as the SOP's limited fault contribution does not alter fuse-saving or recloser sequences.