Patented Transient Limiting for Secondary Network Primary Feeders

A shunt-connected transient limiting device for secondary network primary feeders

Thunderbird Energy Systems is developing the Network Primary Feeder Self-Protected Transient Limiting Device, or NPF SPTL. One device connects in shunt to each protected primary feeder between the substation feeder breaker and the primary side of the network transformers. It is designed for low-voltage secondary network distribution systems that use network transformers and network protectors.

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How the device works

During normal feeder operation, the device remains electrically passive and dormant. Under the specified combination of an open feeder breaker, continued secondary-network backfeed, and a primary-feeder ground fault, the device inserts a calibrated resistance into the feeder’s zero-sequence network to limit transient overvoltage.

A neutral grounding switch may close only when the feeder breaker is confirmed open and the resistor voltage exceeds the specified threshold. If the switch operates to clear a network protector that failed to open, the device latches and requires manual reset.

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Principal device components

• Three single-phase core-coil assemblies, or an equivalent three-phase transformer assembly, housed within a sealed mineral-oil tank

• Grounded-wye high-voltage windings connected to the primary feeder, with broken-delta low-voltage windings in the principal embodiment

• A calibrated low-voltage resistor across the broken-delta point that inserts resistance into the feeder’s zero-sequence network

• Submerged MOV surge arresters that protect the device during high-energy fault conditions

• A self-protecting neutral grounding switch, control logic, temperature sensing, and a 120-volt power supply

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Where it Installs

• One NPF SPTL device is assigned to each protected network primary feeder. The device connects in shunt between the substation feeder breaker and the primary side of the network transformers on that feeder. It may be placed at a suitable point along the feeder and housed in an underground vault comparable to a network-transformer vault.

• The device does not carry normal feeder current and is not installed in series with the feeder.

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Specs

• Network type: Low-voltage AC secondary area networks and spot networks

• Primary voltage class: 15, 25, or 35 kV class, approximately 4.8 to 34.5 kV

• Connection: Shunt-connected

• Application density: One device per protected primary feeder

• Transformer requirement: Network transformers with delta-connected high-voltage windings

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Why it matters

• Designed to limit transient overvoltage affecting network transformers, network protectors, and open microprocessor-based protector relays

• Addresses specified single line-to-ground fault, double line-to-ground fault, backfeed, and feeder re-energization conditions

• Intended to reduce transient electrical stress within underground secondary network distribution systems

• Designed for development through independent modeling, laboratory testing, OEM product engineering, and utility evaluation