INSULATION COORDINATION
INSULATION COORDINATION
Definition in IEC 60071-1
Selection of the dielectric strength of equipment in relation to the operating voltages and over
voltages which can appear on the system for which the equipment is intended, and taking into
account the service environment and the characteristics of the available preventing and protective
devices.
Dielectric strength of the equipment – Rated insulation level or standard insulation level
Definition in IEEE 1313.1
Insulation Co-ordination: The selection of insulation strength consistent with expected
overvoltages to obtain an acceptable risk of failure
• Insulation coordination study in a power system is a vital aspect of ensuring the reliable and
safe operation of the electrical network.
• It involves analyzing and determining the appropriate insulation levels for various equipment
and components within the power system to withstand the expected voltages and overvoltages
during normal and abnormal operating conditions.
OBJECTIVE OF INSULATION COORDINATION
➢ The objective of the insulation coordination study is to verify that the already selected equipment is
appropriate and it satisfies the performance criteria required for overvoltage switching surge and
overvoltage caused by lightning.
➢ The primary objectives of an insulation coordination study are:
• Overvoltage Protection
• Insulation Level Determination
• Clearances and Creepage Distances
• Surge Protection
• Grounding and Bonding
• Environmental Considerations
➢ Insulation coordination also helps us to determine the selection and verification of surge arrester at right
and to limit the various types of overvoltage.
➢ An effective insulation coordination study helps to prevent insulation failures, flashovers, and other power
system disturbances, thereby contributing to the overall reliability and safety of the electrical network.
STANDARDS FOR INSULATION COORDINATION
➢The following International Electrotechnical Commission (IEC) and Institute of Electrical and
Electronics Engineers (IEEE) are used commonly along with the Conseil International des
Grands Réseaux Electriques (CIGRE) recommendations.
➢ In India, the IEC standards (given below) was adopted by the Bureau of Indian Standards (IS)
on the recommendation of the High-Voltage Engineering Sectional Committee and approval of
the Electrotechnical Division Council.
• IS/IEC 60071-1:2019 Part 1: Definitions, principles and rules.
• IS/IEC 60071-2:2023 Part 2: Application guidelines.
• IS/IEC TR 60071-4:2004 Part 4: Computational guide to insulation co-ordination and modelling
of electrical networks.
• IEC 60071-11:2022 Part 11：Definitions, principles and rules for HVDC system
• IEC 60071-12:2022 Part 12: Application guidelines for LCC HVDC converter stations
• 82.1-2010 – IEEE Standard for Insulation Coordination–Definitions, Principles, and Rules
• 82.2-2022 – IEEE Guide for the Application of Insulation Coordination
Why Insulation Coordination studies are important?
• Reliability: Insulation failures can lead to interruptions in power supply, causing downtime and financial
losses for industries, businesses, and individuals. By designing systems with proper insulation, the
likelihood of faults and outages is reduced, contributing to improved system reliability and availability.
• Asset Protection: Electrical equipment, such as transformers, switchgear, and cables, is expensive to
replace or repair. Effective insulation coordination ensures that equipment is protected from overvoltages
and electrical stress, extending the lifespan of these assets and reducing maintenance costs.
• Compliance with Standards: Insulation coordination studies ensure that electrical systems meet the
relevant industry standards and guidelines set by organizations like the IEC and IEEE. Adhering to these
standards is often a legal requirement and ensures that the system is designed and operated safely and
reliably.
• Preventing Cascading Failures: In large, interconnected power systems, a failure in one part of the
network can potentially lead to cascading failures that affect a wider area. Proper insulation coordination
helps prevent these cascading failures by containing faults and minimizing their impact.
• Environmental Considerations: Electrical faults can lead to harmful emissions, fires, and other
environmental hazards. Effective insulation coordination reduces the likelihood of these incidents,
contributing to a safer and cleaner environment.
Why Insulation Coordination studies are important?
• Mitigating Lightning and Switching Surges: Lightning strikes and switching operations can cause
sudden voltage spikes that stress the insulation. Insulation coordination studies help design systems that
can handle these transient events without suffering breakdowns.
• Optimal Design: Insulation coordination studies assist in finding the right balance between insulation
levels, clearances, and other parameters. This results in efficient designs that provide adequate safety
margins without unnecessary over-engineering.
• Predictive Maintenance: Regular insulation testing and monitoring are integral to the maintenance of
power systems. Insulation coordination studies provide insights into the expected performance of
insulation materials over time, helping utilities plan for predictive maintenance activities.
• New Technologies and Developments: As technology advances, new materials and techniques are
introduced for insulation. Insulation coordination studies help incorporate these advancements into
system design, ensuring that the latest innovations are leveraged to enhance safety and reliability.
• Emergency Preparedness: In the event of a fault or breakdown, proper insulation coordination helps in
containing the effects and facilitating quicker restoration, minimizing downtime and operational
disruptions.
AIR INSULATED SWITCHGEAR
GAS INSULATED SWITCHGEAR
Types of overvoltages
1. Temporary overvoltages (TOV)
2. Slow-front overvoltages (SFO)
3. Fast front overvoltages (FFO)
4. Very fast-front overvoltages (VFFO)
Types of overvoltages
1. Temporary overvoltages (TOV): They are undamped or weakly damped oscillatory overvoltages
measured between phase to-earth or phase-to-phase for long duration. Temporary overvoltages occur
due to phase to earth faults, generator overspeed, load rejection, resonance and ferro-resonance due to
inductive and capacitive elements, or by a combination of all the above.
2. Slow-front overvoltages (SFO): They are unidirectional or oscillatory overvoltages, with a slow
front, highly damped for short-duration (microseconds to milli seconds). These overvoltages are
caused by switching overhead line, cables, indictive and capacitive equipments, during the initiation
and clearance of faults, or lightning strikes at the remote end of the overhead line / stations.
3. Fast front overvoltages (FFO): They are transient overvoltages whose fast front shape for short
duration (microseconds) is caused by switching operation at the nearest location or lightning strikes
or even during the initiation of the fault. The lightning effect can be (a). a direct stroke on a phase
conductor; (b) a direct stroke to earth wire or a tower top when lightning hits a shielded line; (c) an
induced voltage when the lightning strike occurs to earth in the vicinity of the line.
4. Very fast-front overvoltages (VFFO): They are the result of switching operations at the high
voltage GIS equipment or faults.
Schematic representation of
Insulation coordination of equipment
Events and most critical types of overvoltages
Process involved in
Insulation coordination study
The process of insulation coordination study typically
involves the following steps:
• Data Collection
• Modelling and Simulation
• Overvoltage Assessment
• Insulation Coordination Analysis
• Recommendations
• Documentation
Surge Arrester Selection & Location
Surge arresters should be placed for protection of station equipment, including
transformers, reactors, capacitor banks, circuit breakers, underground power cables, etc.
Step 1: Select Surge Arrester
1. MCOV ≥ maximum continuous line-to-ground voltage;
2. TOV capability ≥ system TOV;
3. Pressure relief/short-circuit current discharge capability ≥ system line-to-ground fault
rms current magnitude.
Step 2: Locate Surge Arrester
1. A surge arrester shall be located as close as possible to the equipment to be protected;
2. Otherwise, calculation is needed to determine maximum allowable separation
distance between the equipment to be protected and the arrester. If so, Step 4 can be
bypassed.
Surge Arrester Selection & Location
Step 3: Evaluate Insulation Co-ordination
1. Gather surge arrester protective levels, including lightning impulse protective level (LPL),
front-of-wave protective level (FOW) and switching impulse protective level (SPL);
2. Gather equipment insulation levels, including basic lightning impulse insulation level (BIL),
chopped wave withstand (CWW) and basic switching impulse insulation level (BSL);
3. 3. If effect of separation distance can be disregarded, the protective ratios for
lightning, PRL1 and PRL2, and switching impulses PRS are:
For acceptable coordination, PRL1 and PRL2 should be equal to
or greater than 1.2 and PRS should be equal to or greater than 1.15.
Step 4: Evaluate Alternates
If acceptable coordination cannot be achieved,
evaluate the following measures:
1. Increasing BIL and BSL;
2. Decreasing arrester separation distance;
3. Adding additional arresters;
4. Using arrester with lower protective characteristics.
