Insulation Coordination

Station Lightning Coordination: Summary, Rules & Conclusions

The final part of the series. This self-study draws the whole topic together — the open-circuit 2E_A and behind-arrester voltages, the transformer-capacitance parameter K₁, the timing of arrester operation, the consolidated transformer / external-apparatus / clearance BIL criteria, altitude and bushing treatment — and sets out the ten main conclusions and the final engineering lessons. The single message: station lightning insulation coordination is a layout-, waveshape- and reliability-dependent study, not a table lookup.

Reading time ≈ 40 min · Part Eight — the final part

Section 1

Bringing the Whole Series Together

This final part draws together the station lightning insulation coordination series — the incoming surge, arrester rating and location, transformer and behind-arrester voltages, transformer capacitance, BIL selection, insulation strength, GIS, the IEEE/IEC comparison and nonstandard waveshapes — into one set of key equations, practical rules and conclusions.

The central idea has not changed: insulation coordination is the comparison of stress (the surge voltage at equipment) with strength (the BIL, chopped-wave withstand, switching-impulse withstand or clearance strength).

A final consolidation page

This final part brings together the main engineering rules developed throughout the series. The purpose is not to introduce a new calculation method, but to summarise how the calculated station surge voltages are converted into equipment BILs, insulation margins and lightning clearances.

The full coordination chain

incoming surge selection;
arrester rating and location;
travelling-wave voltage calculation;
equipment voltage to ground;
insulation-strength comparison;
required BIL;
next available standard BIL;
clearance and altitude check.

\[ \text{Insulation coordination} = \text{stress} \le \text{strength} \]

What this final part summarises

the voltage calculations — open circuit, behind-arrester, and with capacitance;
the transformer, external-apparatus and clearance coordination rules;
altitude and bushing treatment;
the ten main conclusions of the chapter;
the final engineering lessons;
the one message that ties it all together.

Section 2

Voltage with \(C_T = 0\) — the Open-Circuit Case

These are summary rules

The equations on this page are summary rules from the simplified method — useful for preliminary checks, standard layouts and sanity checks. For high-voltage, non-standard or critical stations, the final coordination should be verified with EMTP® or an equivalent transient program.

Why an open circuit nearly doubles the voltage — and what \(K_2\) does

When transformer capacitance is ignored, the transformer terminal or open bus end behaves like an open circuit. A wave arriving at an open circuit reflects with the same polarity, so the incident and reflected waves add — which is why the voltage can approach twice the arrester surge voltage. The factor \(K_2\) corrects that ideal \(2E_A\) result for the actual arrester voltage–current characteristic and the number of connected lines, so it is not treated as an exact universal doubling rule.

The simplest case neglects the transformer capacitance (\(C_T = 0\)), so the transformer or bus end behaves like an open circuit. The wave reflects with the same polarity, and the open-end voltage can rise toward double the arrester voltage — reduced by a correction factor when the actual arrester characteristic is used:

\[ E_T = 2E_A \qquad\Rightarrow\qquad E_{T,\max} = 2K_2 E_A \]

\(E_T\): surge voltage at the transformer / open end
\(E_A\): surge voltage at the arrester
\(K_2\): correction factor for the actual arrester V–I behaviour (depends on the number of lines)

The recurring message

Even when the arrester limits voltage at its own terminal, the open end or transformer terminal can see a higher voltage because of travelling-wave reflection.

Section 3

Breaker and Junction Voltage with \(C_T = 0\)

Why steepness and distance matter here

The breaker and junction voltages increase with incoming-surge steepness and travel time: a steeper surge rises more during the time the reflections take to travel between the arrester and the equipment. This is why arrester lead length and equipment separation distance directly affect the voltage behind the arrester.

For equipment behind the arrester, the voltage grows with surge steepness, the arrester-to-equipment travel time and the arrester lead travel time:

\[ E_B = E_A + 2S(T_B + T_A) \qquad\qquad E_J = E_A + 2S\,T_A \]

\(E_B,\ E_J\): surge voltage at the breaker; at the arrester-bus junction
\(S\): incoming-surge steepness
\(T_B,\ T_A\): breaker-to-arrester and arrester-lead travel times

So breaker and junction voltages rise when surge steepness, separation distance or arrester lead length increase.

Section 4

Why Transformer Capacitance Must Be Included

What capacitance does, in practice

Transformer capacitance changes the travelling-wave behaviour at the transformer terminal. It can increase the transformer crest voltage and change its time to crest, and it can also modify the voltage behind the arrester by delaying arrester operation and changing the reflected wave.

A transformer is not only an open circuit — at surge frequencies it has a capacitance to ground \(C_T\) that affects both the transformer terminal voltage and the voltage behind the arrester. It can increase the transformer voltage while reducing some behind-arrester voltages, by changing the reflection and timing behaviour. Typical assumed values are \(2\) to \(4\ \text{nF}\) when better data are unavailable. So \(C_T\) must be considered.

Section 5

Transformer Voltage with Capacitance

Reading \(K_1\)

\(K_1\) is a compact way of expressing how severe the arrester-to-equipment separation is — it combines surge steepness, travel time and arrester voltage. A larger \(K_1\) generally means a higher transformer or equipment voltage.

With capacitance, the transformer voltage is read from fitted curves of a single dimensionless parameter:

\[ K_1 = \frac{S(T_T + T_A)}{E_A} \]

\(K_1\): dimensionless transformer-voltage parameter
\(T_T\): arrester-to-transformer travel time

\(K_1\) combines surge steepness, the arrester-to-transformer distance, the arrester lead length and the arrester protective voltage — the larger it is, the greater the transformer voltage. Transformer voltage is governed by surge steepness and electrical distance from the arrester, not only by the arrester protective level.

Section 6

Time to Crest of the Transformer Voltage

Why the time to crest decides the criterion

The time to crest of the transformer voltage decides whether the stress should be compared with chopped-wave strength or with full-wave BIL. A very fast spike may be coordinated with chopped-wave strength, but a slower or sustained voltage should be compared directly with BIL.

Transformer coordination depends on whether the surge behaves like a fast chopped-wave stress or a slower full-wave stress, decided by the transformer time to crest \(t_T\): if \(t_T \le 3.0\ \mu\text{s}\) the chopped-wave test is relevant, while if \(t_T > 3.0\ \mu\text{s}\) the full-wave BIL criterion is more appropriate. Transformer BIL selection is not based on crest voltage alone — the time to crest matters too.

Section 7

Breaker Voltage with Capacitance — a Timing Problem

The voltage behind the arrester depends on whether the arrester operates early or late relative to the returning reflected wave — the controlling comparison is:

\[ t_A \quad\text{versus}\quad t_f - 2(T_B + T_A) \]

\(t_A\): arrester operating time
\(t_f\): time to crest of the incoming surge

If \(t_A \le t_f - 2(T_B + T_A)\), the early-operation equation applies; otherwise the late-operation equation. Breaker voltage is a timing problem — it depends on when the arrester operates relative to the reflected waves.

Section 8

Transformer Coordination — the BIL Criteria

The three transformer BIL checks, in words

1 — Fast spike. If \(t_T \le 3\ \mu\text{s}\) and the transformer crest is sufficiently higher than the arrester discharge voltage, the waveform is a fast spike, and the chopped-wave strength of about \(1.10\,\text{BIL}\) may be used (with the 20% safety factor included). 2 — Slower / full-wave-like. If it crests more slowly, the calculated voltage is compared directly with BIL. 3 — Long tail. A long-tail surge may be severe not because of its first crest but because the voltage persists, so the switching-impulse strength of about \(0.83\,\text{BIL}\) can govern.

Transformer internal insulation is non-self-restoring, so a safety factor \(SF = 1.20\) is used. Coordination depends on the transformer crest, the arrester discharge voltage, the time to crest, the surge-tail duration, and the chopped-wave / full-wave / switching-impulse strengths. For a short-tail surge (\(t_c < 60\ \mu\text{s}\)), the chopped-wave level is used only when the surge is fast and spike-like; otherwise the full-wave criterion applies. For a long-tail surge (or shielding failure without flashover), the switching-impulse level governs. The highest BIL from the short-tail and long-tail checks is selected:

Table 1 — The coordination criteria, consolidated (select the highest BIL).

Insulation / Case	Required BIL
Transformer — fast spike (\(t_T \le 3\ \mu\text{s}\), \(E_t/E_d > 1.10\))	\(\dfrac{1.20\,E_t}{1.10}\)
Transformer — slow / full-wave (\(t_T > 3\ \mu\text{s}\) or \(E_t/E_d \le 1.10\))	\(1.20\,E_t\)
Transformer — long tail (\(t_c > 60\ \mu\text{s}\))	\(\dfrac{1.20\,E_d}{0.83}\)
External apparatus — spike	\(\dfrac{E_b}{1.15\,\delta}\)
External apparatus — full-wave	\(\dfrac{E_b}{\delta}\)
Air clearance	\(d = \dfrac{E_{\max}}{605}\) (then \(/\delta\))

Here \(0.83\,\text{BIL}\) represents the switching-impulse strength — a moderate-crest, long-tail surge can require a higher transformer BIL than a short spike.

Take the highest, then round up

Where more than one transformer criterion applies, perform all the relevant checks and let the highest required BIL govern — then round it up to the next available standard transformer BIL.

Section 9

Breaker and External-Apparatus Coordination

External apparatus, in words

Circuit breakers, disconnectors and bus supports are generally self-restoring. The comparison may use the short-duration strength allowance when the waveform is spike-like; if the waveform is not spike-like, the voltage should be compared directly with BIL.

Circuit breakers, disconnecting switches, bus supports and external insulation are mainly self-restoring, so no large safety margin is applied. The required BIL uses the \(1.15\) chopped-wave factor for a spike-like wave, but the full-wave value if the wave is not spike-like:

\[ \text{BIL} = \frac{E_b}{1.15\,\delta} \quad(\text{spike-like}) \qquad\qquad \text{BIL} = \frac{E_b}{\delta} \quad(\text{full-wave-like}) \]

\(E_b\): crest voltage to ground at the apparatus
\(\delta\): relative air density

External apparatus may tolerate spike-like waves better than full waves — but altitude must be considered.

Section 10

Altitude Correction

Altitude, in words

Altitude correction applies to external air insulation: lower air density reduces the effective withstand strength, so external BIL requirements and air clearances must be increased at higher altitude. It does not apply in the same way to transformer internal insulation.

External insulation strength falls with air density, so at altitude the required external BIL (and clearance) increases:

\[ \delta = e^{-0.121A} \qquad \text{BIL}_{\text{alt}} = \frac{\text{BIL}_{\text{sea level}}}{\delta} \qquad d_{\text{alt}} = \frac{d_{\text{sea level}}}{\delta} \]

\(A\): altitude in km

Altitude affects external insulation and air clearances — but not transformer internal insulation in the same way.

Section 11

Transformer Bushing Coordination

Check the bushing twice

A transformer bushing should be checked as two insulation systems. The internal insulation is coordinated with the transformer BIL; the external insulation is coordinated as external air insulation and must include altitude correction. The selected external bushing BIL should normally not be lower than the internal bushing BIL.

A bushing has internal insulation (treated like the transformer, so its BIL equals the transformer BIL) and external insulation (treated as self-restoring). The external BIL must be at least the internal:

\[ \text{BIL}_{\text{external bushing}} \ge \text{BIL}_{\text{internal bushing}} \]

At high altitude the external requirement may exceed the internal. Bushings must be checked twice — internally as transformer insulation and externally as air-exposed insulation.

Section 12

Air Clearances

Clearance is the final physical check

Air clearance is the final physical translation of the coordination result. Once the maximum station voltage is known, the required phase-ground and phase-phase distances must be checked so that the physical layout is consistent with the selected insulation level.

Phase-ground and phase-phase clearances follow from the maximum station voltage and a conservative air-gap gradient of \(605\ \text{kV/m}\), corrected for altitude:

\[ d = \frac{E_{\max}}{605} \qquad d_{\text{alt}} = \frac{d}{\delta} \qquad d_{\text{phase-ground}} \approx d_{\text{phase-phase}} \]

For the studied lightning conditions, the phase-ground clearance is generally adequate for phase-phase too. Clearances should be based on the highest relevant station surge voltage and corrected for altitude.

Section 13

Main Conclusions — Methods

EMTP® versus the simplified method

For high-voltage stations, EMTP® should be the preferred final verification, because it represents travelling waves, reflections, arrester characteristics, transformer capacitance and nonstandard waveshapes more realistically. The simplified method stays useful for lower-voltage stations, standardised layouts, early estimates and independent checks of simulation results — but it should not be used blindly for extensive, unusual or highly critical layouts.

Conclusions on the calculation methods

Detailed transient study is recommended for high-voltage stations — EMTP® / ATP model travelling waves, layout, bus lengths, \(C_T\), arrester characteristics, reflections and waveshape.
The simplified method suits lower-voltage stations — quick voltage estimates, preliminary BIL, separation limits, clearances and independent checks; best for small, standardised stations.
It also gives initial estimates for complex stations — likely voltages, critical locations, arrester locations, BIL classes and clearances; and a post-simulation sanity check.
The simplified method is conservative — it tends to give higher voltages than EMTP®; useful early, but excessive conservatism over-designs insulation.
Utility standards should rest on generic studies — realistic arrester models, \(C_T\), representative lines and layouts, and proper nonstandard-waveshape evaluation, not only historical margins.

Section 14

Main Conclusions — Configuration and GIS

Configuration and contingencies, in short

Multiple connected lines have two competing effects: they can reduce local surge voltage by providing extra paths for the travelling wave, but they also increase the number of possible incoming surges — so multiline stations do not always have lower transformer stress. Contingency cases must be weighed by both electrical configuration and probability of occurrence: a one-line-out case may be electrically more severe in some respects, but if it exists only a small fraction of the time, its design surge may be less severe.

GIS, and “ahead of” versus “behind” the arrester

GIS is usually easier to protect against ordinary lightning because of its compact geometry and short separation distances — but it has other concerns: very fast front transients, particle contamination, internal insulation quality and power-frequency field stress. “Ahead of the arrester” means the line/transformer side where the surge is still approaching; “behind the arrester” means the protected side where the arrester has already influenced the wave — and the arrester normally limits voltage better behind it than ahead of it.

Conclusions on configuration, GIS and arrester location

Multiple lines have two competing effects — extra surge paths reduce some voltages, but more collected surges can demand a steeper design surge; transformer voltages tend to rise slightly, others to fall.
All-lines-in-service is often critical — but not universally; contingency probability must be checked. The worst case is not obvious from the physical layout alone.
GIS is usually easier to protect against ordinary lightning — compact, low surge impedance, short distances — but needs special attention to particles, power-frequency field stress and very fast front transients.
Voltage ahead of the arrester is generally higher than behind it — so protection is usually better behind the arrester; place protected equipment electrically close to it, and still check open-end reflections.
Tools such as the SIMP program automate the simplified method to find voltages and BILs quickly — though for critical substations a detailed EMTP® model remains preferred.

Section 15

Final Engineering Lessons

The key takeaway

Station lightning insulation coordination is not only an arrester-selection exercise. It is a travelling-wave study in which the arrester location, the station layout, the transformer capacitance, the incoming-surge severity, the insulation type and the available standard BILs must all be considered together.

The lessons that carry across the whole topic

BIL selection is not a table exercise — calculate the stress, evaluate the strength, then select an available standard BIL for that equipment and voltage.
Arrester rating alone is not enough — equipment voltage depends on separation, lead length, layout, steepness, reflections and capacitance.
Transformer protection usually governs — it is expensive, non-self-restoring and slow to replace; include crest, waveshape, time to crest, the long-tail check and the safety factor.
External insulation needs atmospheric correction — bushings, bus supports, disconnectors, breaker externals and clearances all need altitude correction.
Nonstandard waveshapes must be treated carefully — severity-index methods for self-restoring insulation, test-wave comparison and judgement for transformers; a surge is not judged by crest alone.

Section 16

Summary of Key Equations (Whole Series)

Equation Summary

Arrester surge voltage

\( E_A = E_d + V_{pv} \)

Open-circuit maximum

\( E_{T,\max} = 2K_2 E_A \)

Transformer parameter

\( K_1 = \dfrac{S(T_T+T_A)}{E_A} \)

Breaker voltage

\( E_B = E_A + 2S(T_B+T_A) \)

Transformer BIL (fast)

\( \text{BIL} = \dfrac{1.20\,E_t}{1.10} \)

Transformer BIL (long tail)

\( \text{BIL} = \dfrac{1.20\,E_d}{0.83} \)

External BIL

\( \text{BIL} = \dfrac{E_b}{1.15\,\delta} \)

Relative air density

\( \delta = e^{-0.121A} \)

Air clearance

\( d = \dfrac{E_{\max}}{605} \)

Severity index

\( SI = \dfrac{E_c}{E_{\max}} \)

Section 17

Reader Should Remember

Station lightning insulation coordination ensures that equipment insulation strength is adequate for the lightning surges entering from connected lines. The calculation is \(\text{stress} \le \text{strength}\), but both sides must be interpreted correctly. Stress depends on the incoming-surge severity, line lightning performance, arrester rating, location and lead length, station layout, number of lines, transformer capacitance, reflections, open ends and GIS/AIS configuration. Strength depends on the BIL, chopped-wave and switching-impulse withstand, insulation type (self- or non-self-restoring), altitude, clearance and waveshape duration.

The headline conclusions: EMTP® is recommended for high-voltage stations; the simplified method is conservative and best for lower-voltage stations, early design and sanity checks; transformer protection usually governs because its insulation is non-self-restoring; more lines can both reduce voltage and raise exposure; all-lines-in-service is often — but not always — critical; GIS is compact and usually easier to protect, with VFFT and particle contamination as special concerns; arrester location and lead length are critical; BIL is chosen from the calculated stress, not a table alone; and nonstandard waveshapes need careful interpretation.

The one message that ties it all together

Station lightning insulation coordination is a layout-dependent, waveshape-dependent and reliability-dependent study. It cannot be reduced to selecting an arrester and reading a BIL from a table — a sound design considers the incoming surge, arrester behaviour, station travelling waves, equipment strength, safety margins, altitude and practical equipment availability together.

Reader should remember

The arrester protective level is not automatically the voltage at every item of equipment — surge waves travel, reflect and interact with transformer capacitance and open points. Transformer insulation is non-self-restoring and needs conservative coordination; external insulation is affected by altitude; and GIS is compact but has its own special issues. For important stations, the final design should be verified using EMTP®.

This is the final part of the eight-part self-study series on station lightning insulation coordination. Earlier parts cover the procedure and modelling, the voltage behind the arrester, insulation strength and BIL selection, worked station examples, gas-insulated stations and the IEEE/IEC comparison, and the evaluation of nonstandard waveshapes.

Section 18

Key Symbols

Table 2 — Key symbols used across the series.

Symbol	Meaning
BIL / BSL	Basic Lightning / Switching Impulse Level (\(\text{BSL} \approx 0.83\,\text{BIL}\))
\(E,\ S\)	Incoming-surge crest voltage; incoming-surge steepness
\(E_d,\ E_A\)	Arrester discharge voltage to ground; surge voltage at the arrester
\(E_T,\ E_t\)	Surge / voltage-to-ground at the transformer
\(E_B,\ E_b,\ E_J\)	Surge / voltage-to-ground at the breaker; at the arrester-bus junction
\(V_{pv}\)	Power-frequency voltage of opposite polarity
\(T_T,\ T_B,\ T_A,\ t_A,\ t_f,\ t_c\)	Travel times (transformer, breaker, lead); arrester operating, surge crest, long-tail times
\(C_T,\ K_1,\ K_2\)	Transformer surge capacitance; transformer parameter; arrester correction factor
\(\delta,\ A,\ d\)	Relative air density; altitude (km); clearance (m)
\(SF,\ SI\)	Safety factor; Severity Index

Part Eight Reading now

Summary, Rules & Conclusions

The whole series drawn together — the key voltage and coordination equations, and the ten main conclusions and final engineering lessons.

Series progress 8 of 8

Part Seven

Comparing the Severity-Index Methods

Comparing the methods on linear-front and oscillatory surges, the 75% duration rule, the GIS treatment and severity-index margins.