SERVICES

EMT & INSULATION
COORDINATION STUDIES

Independent expertise in insulation coordination, transient studies, and overvoltage assessment for high-voltage power systems.

Supporting utilities, engineering firms, manufacturers, and EPC contractors worldwide.

Understanding Transient Behavior Before It Becomes a Problem

Prof. Dr. Selma Grebović

Associate Professor & External Consultant Master’s and Ph.D. in Electrical Engineering, with specialization in energy stress of surge arresters. Ph.D. awarded by Graz University of Technology

ADVANCED CAPABILITIES

In Cooperation with Prof. Dr. Selma Grebović

TRANSIENT / EMT STUDIES

INSULATION COORDINATION STUDIES

Switching transient studies
Lightning overvoltage studies
Temporary overvoltage (TOV) studies
Transient recovery voltage (TRV) studies
Surge arrester rating and energy duty studies
Very fast transient (VFT) studies
Inrush and sympathetic inrush studies
Subsynchronous resonance (SSCI / SSTI) studies
Harmonic and resonance studies
HVDC and FACTS transient studies
Black-start and self-excitation studies
Geomagnetic disturbance (GIC) studies

Determination of representative overvoltages
Coordination and withstand voltage selection (BIL / SIL / LIWL / SIWL)
Surge arrester placement and coordination
Shielding and direct-stroke protection design
Backflashover analysis and tower footing impedance optimization
Air clearance and creepage studies
Cable-overhead line interface studies
Substation lightning and switching performance
Mitigation studies

SELECTED REFERENCES

Prof. Selma Grebović has collaborated with leading organizations and institutions worldwide.

Électricité de France (EDF)

French Electricity Producer

Réseau de Transport d'Électricité (RTE)

French Transmission System Operator

Hrvatski operator prijenosnog sustava (HOPS)

Croatian Transmission System Operator

Energoinvest, d.d.

Bosnian Multidisciplinary Engineering and Energy Company

Fakultet elektrotehnike i računarstva (FER)

University of Zagreb, Croatia - Faculty of Electrical Engineering and Computing

ENGINEERING CASE STUDIES

Case 1: 400 kV Insulation Coordination Under Lightning Surges

For a project of insulation coordination, a simulation model of a 400 kV transmission network built in an EMT tool. The study analysed overvoltage propagation caused by a direct lightning strike (~27 kA) and evaluated Metal-Oxide (MO) surge arrester placement for power transformer protection.

Read the case study

Case 2: 400 kV Shunt Reactor Switching Overvoltages: System Modelling & Risk at CB Terminals/Contacts

A 400 kV transmission substation equipped with shunt reactors on two double-circuit transmission lines was analysed to evaluate switching overvoltages when circuit breakers disconnect shunt reactors under normal operating conditions — for example during load variation or maintenance.

What was the problem?

When a circuit breaker interrupts the small inductive current of a shunt reactor, the current is chopped before its natural zero, creating a high-frequency oscillating overvoltage. Switching one of two parallel reactors while the other remains in service produces significantly higher overvoltages, reaching up to 3.33 p.u. at the circuit breaker terminals.

What was simulated?

Four scenarios were modelled in EMT simulation software for the transmission line with 60 MVAr: switching both reactors simultaneously vs. switching only one, each case run with and without Metal-Oxide (MO) surge arresters at the reactor terminals.

Read the case study

Case 3: 400 kV Shunt Reactor Switching: Reignition Modelling & Risk of Flashover at Reactor’s Windings

What was the problem?

This study focuses on a dangerous phenomenon during reactor switching: circuit breaker reignition. After current chopping, the reactor generates an oscillating recovery voltage across the breaker contacts. If the contact gap is still small, this voltage can re-ignite the arc — producing extremely steep transient overvoltages that threaten reactor winding insulation.
Part 2 then models the reignition event in detail and its impact on the interturn insulation at the first turns of the winding, where the front steepness is critical.

What was simulated?

Reignition overvoltages reach steepness up to ~2000 kV/μs for a 14 A chopping current — far above the tolerable limit of 1211 kV/μs (maximum permissible — tolerable — instantaneous rate of rise of voltage — dv/dt — for the reactor's internal insulation under repetitive switching duty). These steep transients concentrate on the first turns of the reactor winding, risking insulation puncture and complete winding breakdown.

Read the case study

Case 4: 400 kV Double-Circuit Transmission - Secondary Arc Extinction and Single-Phase AutoReclosure (SPAR) operation

What is this study about?

Single-phase-to-ground faults account for roughly 80% of all transient faults on high-voltage transmission lines. Instead of tripping all three phases — which disrupts power flow — Single-Phase Autoreclosure (SPAR) disconnects only the faulted phase, waits for the arc to extinguish naturally, then recloses. This study evaluated whether SPAR can be applied successfully on a 400 kV double-circuit interconnection, and what role shunt reactors play in enabling it.

What is the problem?

After the faulted phase is isolated by the circuit breaker, a secondary arc continues to burn — fed by electromagnetic and electrostatic coupling from the two healthy phases. This arc must extinguish on its own before reclosing; if it does not, the reclosure fails and the fault persists. The secondary arc current and the recovery voltage that appears after extinction are the two critical quantities. Both depend on line length, fault location, and shunt reactor compensation.

Read the case study

TOOLS & METHODOLOGY

We use industry-leading platforms to model and analyse complex transient phenomena with accuracy.

400 kV transmission network for insulation coordination studies

Power system network configuration for lightning surge and insulation coordination analysis

WHO WE SUPPORT

Utility transmission and substation infrastructure requiring overvoltage protection

Utilities

Support for transmission and substation assets requiring reliable overvoltage protection.

EPC Contractors

Technical support during project development, procurement, testing, and commissioning.

Engineering and consulting professionals performing power system studies

Engineering & Consulting Firms

Independent technical expertise for studies, specifications, and project reviews.

Manufacturers

Guidance on standards, testing requirements, product evaluation, and performance assessment.

Need Support with EMT & Insulation Coordination Studies?

Whether you’re evaluating protective margins, performing transient studies, or reviewing equipment protection strategies, Metarresters can help.

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EMT & INSULATION COORDINATION STUDIES

Independent expertise in insulation coordination, transient studies, and overvoltage assessment for high-voltage power systems.​

Supporting utilities, engineering firms, manufacturers, and EPC contractors worldwide.

Prof. Dr. Selma Grebović

In Cooperation with Prof. Dr. Selma Grebović

TRANSIENT / EMT STUDIES

INSULATION COORDINATION STUDIES

SELECTED REFERENCES

Électricité de France (EDF)

Réseau de Transport d'Électricité (RTE)

Hrvatski operator prijenosnog sustava (HOPS)

Energoinvest, d.d.

Fakultet elektrotehnike i računarstva (FER)

ENGINEERING CASE STUDIES

Case 1: 400 kV Insulation Coordination Under Lightning Surges

TOOLS & METHODOLOGY

WHO WE SUPPORT

Utilities

Support for transmission and substation assets requiring reliable overvoltage protection.

EPC Contractors

Engineering & Consulting Firms

Manufacturers

Guidance on standards, testing requirements, product evaluation, and performance assessment.

Need Support with EMT & Insulation Coordination Studies?

EMT & INSULATION
COORDINATION STUDIES

Independent expertise in insulation coordination, transient studies, and overvoltage assessment for high-voltage power systems.