Single Pole Auto reclosing

Single-Pole Auto-Reclosing (SPAR) Assessment

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Single-Pole Auto-Reclosing (SPAR) is a protection strategy widely used on high-voltage (HV) and extra-high-voltage (EHV) transmission systems to improve network reliability and maintain system stability following temporary single-phase faults. Instead of disconnecting all three phases, only the faulted phase is opened, allowing power transfer to continue through the healthy phases. Once the fault has cleared and the secondary arc has extinguished, the circuit breaker automatically recloses the isolated phase and restores normal operation.

As most overhead transmission line faults are transient in nature, caused by lightning, vegetation contact, pollution flashovers, or temporary conductor interactions, SPAR significantly reduces outage durations and improves network availability compared to conventional three-pole tripping schemes.

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Importance of SPAR Assessment

Following the opening of the faulted phase, a secondary arc may continue to burn at the fault location due to electrostatic and electromagnetic coupling from the healthy phases. The success of a SPAR scheme depends on this secondary arc extinguishing before the circuit breaker recloses.

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If reclosing occurs too early:

• The fault may be re-established.

• The auto-reclosing operation may fail.

• Additional system disturbances and overvoltages may occur.

• Network stability may be adversely affected.

 

SPAR studies are therefore performed to determine the optimum breaker dead time and confirm successful arc extinction under a range of network operating conditions.

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Technical Assessment Methodology

Our SPAR assessments are performed using detailed Electromagnetic Transient (EMT) simulations in PSCAD, EMTP, or equivalent platforms. The studies accurately represent:

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• Frequency-dependent transmission line models

• Source impedances and network equivalents

• Circuit breaker operating characteristics

• Secondary arc models

• Shunt reactors and neutral reactors

• Transmission line transposition and geometry

• Transformer models

• Renewable generation and inverter-based resources

• Protection and auto-reclosing logic

 

The analysis evaluates both inductive and capacitive coupling effects between healthy and faulted phases to determine the resulting secondary arc current and recovery voltage characteristics.

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Technical Parameters Evaluated

A SPAR assessment typically investigates:

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Secondary Arc Current

The magnitude of secondary arc current sustained after fault clearing due to coupling from the healthy phases.

Factors influencing secondary arc current include:

• Transmission line length

• Phase spacing and conductor configuration

• Shunt reactor compensation

• Source strength

• Grounding arrangements

• Fault location

 

Arc Extinction Time

The time required for the secondary arc current to reach a level that allows the arc to self-extinguish.

Arc extinction time is one of the most critical outputs of a SPAR study and directly determines the required breaker dead time.

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Dead Time Requirement

The minimum period between circuit breaker opening and reclosing required to ensure successful fault clearance.

The dead time must be greater than the maximum expected arc extinction time, while remaining short enough to maintain system stability and synchronism.

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Recovery Voltage

The transient recovery voltage appearing across the fault path after current interruption.

The study assesses whether the recovery voltage could lead to arc reignition following extinction.

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Switching and Temporary Overvoltages

SPAR operations may produce:

• Reclosing overvoltages

• Trapped charge effects

• Temporary overvoltages (TOV)

• Resonance phenomena

These are evaluated to confirm acceptable equipment stresses and insulation performance.

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Sensitivity Studies

To identify the worst-case conditions, assessments are usually performed for:

• Multiple fault locations along the transmission line

• Different fault arc lengths

• Various system short-circuit levels

• Maximum and minimum generation dispatch conditions

• Different shunt reactor operating states

• Seasonal network configurations

• Outage scenarios

 

Typical Study Outputs

A comprehensive SPAR study includes:

• Secondary arc current waveforms

• Secondary arc voltage waveforms

• Arc extinction time results

• Recommended breaker dead times

• Reclosing success assessment

• Overvoltage analysis

• Sensitivity study results

• Protection and operational recommendations

 

Applications

SPAR assessments are commonly required for:

• New transmission line developments

• Grid connection studies

• Renewable energy integration projects

• Wind farm connections

• Solar farm connections

• HVDC converter station connections

• Network reinforcement projects

• Protection system upgrades

 

Benefits

Accurate SPAR studies provide confidence that auto-reclosing schemes will operate successfully under all credible operating conditions. The results help optimise protection settings, minimise fault-related outages, improve network reliability, and maximise transmission line availability while ensuring equipment stresses remain within acceptable limits.

As modern transmission networks experience increasing penetration of renewable generation and changing system dynamics, EMT-based SPAR assessments have become an essential component of transmission system design, operation, and grid compliance studies.