Insulation Coordination and Electromagnetic Transient Studies

Overview

APS delivers specialist electromagnetic transient (EMT) studies for transmission, distribution, renewable generation, industrial networks, GIS and AIS substations, cable-connected substations, offshore networks, HVDC systems and grid-connection projects. Studies are performed using industry-recognised EMT simulation platforms, principally EMTP and PSCAD/EMTDC, and are underpinned by assessment methods aligned with IEC, IEEE and CIGRE practice.

EMT studies become essential where conventional load-flow, short-circuit and RMS dynamic analysis are no longer sufficient. Fast switching events, nonlinear equipment behaviour, travelling-wave effects, surge-arrester operation, transformer saturation, cable resonance, GIS very fast transients and inverter-based resource control interactions often cannot be represented adequately in the phasor domain. APS resolves these phenomena in the time domain, quantifies the resulting electrical stresses, and relates them directly to equipment withstand capability, surge-arrester performance, protection behaviour and the applicable insulation coordination framework.

Study Types

APS provides EMT and insulation coordination studies across the full spectrum of transient overvoltage phenomena. The service portfolio covers all major study families required for equipment specification, substation design, grid-connection compliance, offshore system development and HVDC integration.

• Temporary Overvoltage Studies — earth faults, load rejection, islanding, transformer energisation, feeder tripping, open-phase conditions and renewable plant control response More info
• Switching Overvoltage Studies — line, cable, transformer, reactor, capacitor-bank and GIS/AIS switching, including statistical circuit-breaker pole-scatter assessment More info
• Lightning Overvoltage Studies — surge propagation, shielding performance, backflashover assessment, arrester placement, insulation withstand verification and protective-margin assessment More info
• Very Fast Transient Overvoltage Studies — VFTO assessment in GIS substations, disconnector operation, GIS busbar arrangements and cable/GIS interface conditions More info
• Transient Recovery Voltage Studies — circuit-breaker TRV duties for terminal faults, short-line faults, transformer-limited faults, reactor switching and out-of-phase switching More info
• Ferroresonance Studies — voltage transformers, power transformers, cable-fed transformers, open-phase conditions, CVTs and weak-source energisation scenarios More info
• Resonance and Frequency-Scan Studies — impedance and admittance scans, resonance curves, critical-frequency identification, long-cable resonance and offshore network resonance More info
• Resonance Mitigation and Protection Studies — damping requirements, switching restrictions, arrester duties, filter and reactor detuning, controlled switching and protection settings More info
• Transformer Energisation Studies — inrush current, sympathetic inrush, residual flux effects, controlled switching assessment and protection stability review More info
• Equipment Energisation Studies — capacitor-bank energisation, harmonic-filter energisation, shunt-reactor switching, cable energisation and transformer/cable interaction studies More info
• HVDC and Offshore DC-Side Studies — converter blocking, DC line and cable faults, control-interaction overvoltages, arrester duty under DC stress and offshore platform transient performance More info
• EMT-Based Protection Analysis — relay performance under DC offset, CT saturation, transformer inrush, ferroresonance, high-frequency transient content and non-standard waveform conditions More info

Technical Approach

Each study begins with a clear definition of credible operating scenarios, system configurations, switching conditions, fault locations and equipment operating states. The assessment then proceeds through EMT model development, equipment representation, sensitivity analysis and, where required, statistical switching assessment. Calculated stresses are interpreted against equipment withstand capability, resonance characteristics, surge-arrester residual voltage, arrester energy duty, protective levels and applicable insulation coordination requirements. Model fidelity is treated as a first-order engineering requirement. Depending on the phenomenon under investigation, studies may include:

• Frequency-dependent overhead line and cable models
• Detailed transformer models including saturation and winding capacitance effects
• Surge-arrester models selected according to the required voltage and energy assessment
• GIS and cable interface representations for very fast transient studies
• Circuit-breaker statistical switching and pole-scatter representation
• Frequency-scan and resonance-curve evaluation
• Inverter-based resource control representation where plant response influences the transient behaviour
• Validation against manufacturer data, available test records or field measurements where practicable

This approach ensures that the analysis is technically defensible, proportionate to the risk, and suitable for design, compliance or operational decision-making.

Insulation Coordination in Accordance with IEC 60071

APS applies the principles of IEC 60071 to assess representative overvoltages, coordination withstand voltages, required withstand voltages and selected standard insulation levels. The assessment is developed systematically from the simulated or calculated representative overvoltage through to the required insulation withstand level for each relevant equipment class and overvoltage category. Where required, the study can include:

• Determination of representative overvoltages
• Assessment of coordination withstand voltage
• Application of safety and correction factors
• Selection or verification of standard withstand voltage levels
• Evaluation of internal and external insulation requirements
• Review of equipment insulation margins
• Assessment of acceptable risk and statistical overvoltage performance

Surge-Arrester Application and Protective Margin Assessment

Surge-arrester application is reviewed against continuous operating voltage, rated voltage, temporary-overvoltage withstand capability, residual voltage, energy duty and protective margin. APS does not assess arrester suitability based on residual voltage alone. The assessment may include:

• Arrester continuous operating voltage and rated voltage selection
• Residual voltage and protective level verification
• Protective margin assessment against equipment withstand levels
• Arrester energy-duty calculation under switching, lightning or DC-side events
• Temporary-overvoltage withstand assessment
• Arrester placement optimisation
• Review of arrester coordination across transformers, cables, GIS, AIS and converter equipment

Governing Standards and Technical References

Assessment methods are aligned with relevant international standards and technical guidance, including:

• IEC 60071-1 / IEC 60071-2 / IEC 60071-4 — insulation coordination principles, application guidance and computational methods
• IEC 60099-4 / IEC 60099-5 — metal-oxide surge arresters and application guidance
• IEEE C62.82 series — insulation coordination application guidance
• IEEE C37.011 — application of transient recovery voltage requirements for high-voltage circuit breakers
• CIGRE TB 559 — guidelines for insulation coordination studies
• CIGRE technical guidance on wind farms, offshore networks, HVDC systems and electrical transients
• Applicable grid codes, including ENTSO-E NC HVDC and relevant national connection requirements

Deliverables

The outcome of each engagement is a practical engineering report, not simply a collection of simulated waveforms. Each report explains the physical mechanism driving the transient behaviour, quantifies the resulting risk to equipment, and presents clear mitigation measures where required. Typical deliverables include:

• Study methodology and modelling assumptions
• EMT model description and operating scenarios
• Time-domain waveforms and peak stress results
• Statistical switching results where applicable
• Resonance curves and frequency-scan interpretation
• Surge-arrester energy and protective-margin assessment
• Equipment withstand comparison
• Identification of non-compliant or high-risk conditions
• Recommended mitigation measures and operational restrictions
• Clear conclusions suitable for design, compliance, procurement or operational decision-making

APS combines EMT simulation expertise, insulation coordination knowledge and practical power-system engineering judgement to support clients in identifying, quantifying and mitigating transient overvoltage risks.