Station Lightning Insulation Coord. Part 5

Section 1

Extending the Topic to Gas-Insulated Stations

The earlier parts dealt mainly with air-insulated stations. This part extends station lightning insulation coordination to gas-insulated stations (GIS), the very fast front transients from disconnecting-switch operation, GIS insulation strength and BIL coordination, and a comparison of the chapter method with the IEEE and IEC guide methods.

GIS behaves differently from AIS because the bus geometry is compact, the surge impedance is much lower, travelling-wave travel times are very short, the SF₆ field is more uniform, and disconnecting-switch operation can create very fast front transients (VFFTs). AIS intuition does not always transfer directly.

From Part Four to Part Five

Part Four applied the simplified method to air-insulated station examples. Part Five extends the discussion to gas-insulated stations. GIS has shorter distances, lower surge impedance and more compact geometry than AIS, so the surge behaviour and the interpretation of insulation strength are different.

Why GIS is a separate topic

GIS cannot be treated as just a smaller version of AIS. The low surge impedance of the GIS bus, the short travel times, the compact enclosed geometry and the possibility of very fast front transients from disconnecting-switch operation make GIS insulation coordination a topic in its own right.

Two different GIS stresses to distinguish

Two GIS stresses should be kept separate. The first is the lightning surge entering the GIS from an overhead line or cable transition. The second is the very fast front transient (VFFT) produced inside the GIS by disconnecting-switch operation. They have different origins, different frequency content and different mitigation methods.

What Part Five covers

GIS arrangements and arrester application (conventional and in-gas);
lightning stress in GIS and the open-end \(2E_A\) reflection;
very fast front transients and why arresters do not limit them;
GIS insulation strength, particle sensitivity and \(BIL = 1.20\,E_t\);
GIS bus-length limits from the BIL coordination;
how the chapter, IEEE and IEC methods compare.

Section 2

Why GIS Is Used

Gas-insulated stations are used mainly where space is limited or where air-insulated equipment would be hard to apply — urban and high-density networks, sites with little land, high altitude, heavily contaminated environments, and severe weather or pollution. The insulation medium is normally SF₆ (or an equivalent gas technology). The main advantage is compactness and environmental protection.

Historically there were reliability concerns at high voltage: at 500 kV, AIS was once estimated to be about 2–3 times more reliable than GIS, and some utilities used an opened disconnecting-switch bypass around the GIS. Modern GIS technology has improved greatly, and GIS is now widely accepted at many voltage levels worldwide.

Why GIS is often easier to protect against lightning

Because GIS is compact, the distance between arrester and protected equipment is usually short, which reduces travelling-wave separation effects. The GIS bus also has a low surge impedance, which changes surge transmission and reflection. For ordinary lightning surges these features normally make GIS easier to protect than a large AIS layout.

Section 3

Total and Partial GIS Arrangements

Total GIS versus partial GIS

In a total GIS arrangement the transformer is connected very directly to the GIS bus, normally through a compact throat connection. In a partial GIS arrangement the transformer is connected through an air-insulated bus section and an air-to-gas bushing, which introduces an AIS/GIS transition and a higher-impedance open-air connection.

Table 1 — The two principal GIS arrangements.

Arrangement	Transformer Connection	Surge Effect
Total GIS	Throat-connected directly to the GIS bus (very short)	Compact integrated station
Partial GIS	Connected via an open-air bus (air-to-gas bushing)	Mix of GIS and higher-impedance air bus; extra transmitted surge usually not significant

The open-air bus in a partial GIS has a higher surge impedance than the GIS bus, which affects transmission and reflection — but the increase in surge transmitted to the transformer is normally not significant.

Section 4

Arrester Application in GIS

For coordination, the GIS is often treated as a single item of apparatus, with an arrester at each line entrance so every incoming line is protected before the surge enters the GIS bus.

Conventional arresters

Normally the line-entrance arrester is the same type used in an AIS, connected externally with physical leads and some separation distance — which add travel time and can increase the surge voltage seen inside the GIS.

In-gas arresters

In-gas arresters connect directly to the GIS bus, giving \(\text{lead length} \approx 0\) and \(\text{separation} \approx 0\), so protection is improved.

In-gas arresters: an economic decision

In-gas arresters improve protection because they remove the external arrester lead length and the separation distance between the arrester and the GIS bus. But that improvement must be justified against cost — in most ordinary lightning coordination studies, conventional line-entrance arresters provide sufficient protection.

In-gas arresters: rarely essential

In-gas arresters cost roughly 4 to 5 times a normal arrester and are technically seldom required, so their use is limited and normally justified only for special protection requirements.

Section 5

The GIS Lightning Surge Model

Why the GIS surge impedance matters

The surge impedance of a GIS bus is much lower than that of an incoming overhead line. So when a surge enters the GIS, part of the wave is transmitted and part reflected at the line-to-GIS transition. This impedance mismatch controls the voltage that develops inside the GIS.

To estimate the lightning surge inside GIS, consider an incoming line connected to a GIS bus that is open at the far end (representing an opened disconnecting switch). At an open circuit the reflection is positive, so the open-end voltage can increase. The incoming line has surge impedance \(Z\) and the GIS bus \(Z_c\), with normally \(Z_c < Z\) — e.g. \(Z = 450\ \Omega\) and \(Z_c = 60\ \Omega\). At the line-to-GIS transition the impedance changes, so part of the surge is transmitted and part reflected; because \(Z_c\) is much lower than \(Z\), the transmitted surge is significantly modified.

Section 6

Maximum Open-End Voltage — \(2E_A\)

Open-end reflection, in words

An open GIS bus end — for example an opened disconnector — reflects the voltage wave with the same polarity, so the reflected wave adds to the incoming wave at the open end. Even though the arrester limits voltage at the line entrance, the open end of the GIS bus can therefore experience a higher voltage.

\(2E_A\) is a limiting case, not a typical value

The value \(2E_A\) is the limiting open-end condition — the maximum theoretical voltage when the reflected voltage adds fully to the incident voltage. In practical GIS arrangements the actual value depends on the GIS bus length, the surge steepness, the arrester operation and the incoming-surge crest.

With a linear rising incoming surge of steepness \(S\) and a constant-voltage arrester, and a GIS bus travel time \(T\), the maximum voltage at the open end occurs when the arrester voltage is reached before a reflection returns from the open end:

\[ E_{T,\max} = 2E_A \]

\(E_A\): surge voltage at the arrester
\(E_T\): surge voltage at the open end of the GIS bus
\(T\): GIS bus travel time = bus length / propagation velocity

This is the same open-end reflection concept as in AIS: the arrester limits voltage at its own terminal, but the open end can see double. For a finite-crest incoming surge, the maximum open-end voltage depends on \(E\), \(S\), \(E_A\), \(T\) and the impedance ratio — estimated from general \(E_T/E_A\) curves without a full simulation.

The practical meaning

An arrester at the line entrance does not automatically make the voltage at an open GIS bus end equal to the arrester voltage — \(E_T\) can exceed \(E_A\).

Section 7

Worked Example — 230 kV GIS Lightning Stress

What the example shows

The 230 kV example shows how the open-end voltage in a GIS bus is estimated from the arrester voltage, the incoming-surge steepness, the GIS bus surge impedance and the bus travel time. The point is not just one number — it shows how GIS bus length and arrester location affect the protected voltage.

A 230 kV system with a 140 kV MCOV arrester (\(E_d = 446\ \text{kV}\)), \(E = 1400\ \text{kV}\), \(S = 2000\ \text{kV}/\mu\text{s}\), \(Z = 450\ \Omega\), \(Z_c = 60\ \Omega\), \(V_{pv} = 138\ \text{kV}\), a 12 m GIS bus (\(T = 0.04\ \mu\text{s}\)):

\[ E_A = E_d + V_{pv} = 446 + 138 = 584\ \text{kV} \]

\[ \frac{E_T}{E_A} = 1.1934 \;\Rightarrow\; E_T = 697\ \text{kV} \;\Rightarrow\; E_t = E_T - V_{pv} = 559\ \text{kV} \]

\[ E_{T,\max} = 879\ \text{kV} \;\Rightarrow\; E_{t,\max} = 741\ \text{kV} \]

The maximum open-end voltage is useful when checking whether a maximum permissible GIS bus length is reached.

Section 8

Short GIS Bus as a Lumped Capacitance

An interpretation, not the main design method

For very short GIS bus sections the bus can also be viewed approximately as a lumped capacitance. This helps explain the early voltage rise before the arrester operates — but for detailed GIS design a travelling-wave or EMTP® model should still be preferred.

For very short GIS bus lengths, the GIS can be treated approximately as a lumped capacitance. Before the arrester operates, the voltage follows a capacitor-charging expression:

\[ e = 2St - 2ZCS\left(1 - e^{-t/ZC}\right) \]

\(C\): total capacitance of the GIS bus
\(Z\): incoming-line surge impedance
\(S\): surge steepness
\(ZC\): time constant

Example: a 6 m bus (\(T = 0.02\ \mu\text{s}\)) with \(E_A = 620\ \text{kV}\), \(S = 5000\ \text{kV}/\mu\text{s}\), \(Z = 450\ \Omega\), \(Z_c = 60\ \Omega\), \(C = 333\ \text{pF}\) so \(ZC = 0.15\ \mu\text{s}\). The arrester operates at \(t_A = 0.16\ \mu\text{s}\); the open-end voltage occurs about one travel time later (\(0.18\ \mu\text{s}\)), giving \(E_T = 752\ \text{kV}\) — matching the detailed result. For short buses, the lumped-capacitance picture explains the voltage development.

Section 9

Arrester Lead Length in GIS

The GIS equations above assume no arrester lead. In practice an external arrester has leads, adding a voltage to the arrester voltage:

\[ E_A = E_d + V_{pv} + V_{\text{lead}} \]

The lead voltage depends on the current steepness and the lead inductance. The implication is the same as for AIS — a shorter lead gives a lower protected-equipment voltage — and it is one reason in-gas arresters (which eliminate external lead and separation) improve protection.

Section 10

Transformer Connected by an Open-Air Bus

If the transformer is connected to the GIS by an open-air bus, the surge transmitted to the transformer may increase, because the air bus has a higher surge impedance than the GIS bus. But this increase is normally not significant. Partial GIS arrangements should still be checked, but the open-air transformer connection does not usually dominate the lightning surge stress.

Section 11

Switching Stress — Very Fast Front Transients

Why VFFT is different from lightning stress

Very fast front transients differ from ordinary lightning surges. Their crest may be lower than a lightning impulse, but their front time is extremely short and their frequency content is very high — so they can excite internal resonances in transformers, bushings or GIS components.

A very important GIS stress comes from disconnecting-switch operation, which can create an almost vertical-front surge that is transmitted and reflected inside the GIS. Because GIS has low attenuation, the very fast front propagates through the station — unlike AIS, where fast components attenuate quickly.

Table 2 — Typical GIS disconnector switching overvoltages.

Disconnector	Overvoltage
Normal / slow-speed	≈ 1.7 pu (up to 2.0 pu)
High-speed	up to 2.5 pu

These magnitudes are not high compared with lightning BIL — but their fronts are extremely fast.

Magnitude is not the whole story

VFFT should not be judged by per-unit overvoltage magnitude alone. A 2.0 pu VFFT can still matter, because its very short front time and high-frequency content stress insulation differently from a conventional lightning impulse.

Section 12

Why Very Fast Front Surges Matter

VFFTs contain very high-frequency components that can excite transformer natural frequencies — in the early days of GIS, some transformer failures were attributed to this. The concern is not only crest voltage, but the very short front time, the high-frequency content, transformer resonance, the internal voltage distribution and the interaction with winding natural frequencies. The recent IEC standard recognises this as a very fast front surge: time to crest \(3\ \text{ns}\) to \(300\ \text{ns}\), with frequency ranges \(0.3\ \text{MHz}\) to \(100\ \text{MHz}\) and \(30\ \text{kHz}\) to \(300\ \text{kHz}\). Standardised test waveshapes and magnitudes are not yet fully established, so practical assessment depends on detailed modelling and manufacturer experience.

Section 13

Why Arresters Do Not Limit VFFTs

Why arresters cannot limit VFFT

Arresters are effective at ordinary lightning and switching-surge frequencies, but VFFT contains very high-frequency components and develops within nanoseconds. Arrester connection inductance and response behaviour limit its effectiveness for these extremely fast components, so VFFT mitigation depends mainly on GIS disconnector design and detailed high-frequency layout design.

Because VFFTs are low in magnitude (vs lightning levels), extremely short in front time and very high in frequency, surge arresters cannot significantly limit them — the arrester response and connection inductance are not effective for such fast components.

The role of disconnector design

The magnitude and frequency content of VFFT are strongly influenced by the GIS disconnecting-switch design. Improved contact design, controlled operation or pre-insertion resistors can reduce the severity of these transients.

Mitigation is by design, not by arresters

The main mitigation is disconnecting-switch design — plus pre-insertion resistors, improved contact design, controlled disconnector operation, GIS layout optimisation and manufacturer-specific measures. Accurate VFFT assessment requires a detailed high-frequency model of the bus sections, switches, breakers, spacers, bushings, transformer connection, terminations and enclosure (IEEE and CIGRE provide modelling guides). VFFT cannot be assessed with the simplified lightning equations.

Section 14

GIS Insulation Strength — and Its Weakness

Why a uniform field is also a weakness

GIS insulation is designed with a relatively uniform electric field, which gives high strength — but it also means small conducting particles or protrusions can locally distort the field. A small particle can therefore produce a disproportionate reduction in withstand strength.

GIS gets its strength from SF₆ insulation, a compact coaxial geometry, the metallic enclosure and a relatively uniform electric field — the conductor inside a circular enclosure gives a more uniform field than many AIS gaps, which improves withstand.

But the uniform field is also a weakness: small disturbances strongly reduce strength. The text especially highlights free conducting particles — small particles introduced during manufacturing or field assembly — along with metallic debris, sharp protrusions, surface and spacer defects and contamination. These distort the field and can reduce insulation strength drastically.

Particles are a quality-control issue

Free conducting particles are not a lightning insulation coordination issue in the usual travelling-wave sense, but they can strongly reduce GIS reliability. They are mainly a manufacturing, assembly and quality-control issue.

Section 15

Manufacturing, Assembly and Reduced BIL

To reduce particle contamination, some manufacturers ship fully assembled GIS modules — limiting field assembly and contamination risk, though full transport is difficult and expensive for large installations. Where field assembly is needed, quality control is critical: cleanliness, particle control, gas handling, enclosure and spacer inspection, high-voltage testing and partial-discharge monitoring.

Some GIS failures at higher voltages were associated not with transient overvoltages but with an increased power-frequency gradient, where the BIL was reduced below comparable lower-voltage practice. A reduced BIL implies a more compact design and higher operating stress, increasing sensitivity to particles and reducing reliability.

Reduced BIL raises the operating gradient

When the GIS BIL is reduced, the physical clearances and insulation dimensions may also be reduced, which can increase the normal power-frequency electric-field gradient. Some GIS reliability problems are therefore related not to lightning overvoltage, but to normal operating stress combined with particles or field distortion.

An important lesson

GIS insulation coordination must consider not only transient overvoltages but also the normal power-frequency electric-field stress.

Section 16

GIS Strength Is Nearly Constant with Waveform

Why crest is compared directly with BIL

For GIS, insulation strength does not increase significantly for shorter impulse durations the way external air gaps do. So the crest voltage inside the GIS is compared directly with the GIS BIL, rather than using a chopped-wave allowance.

Because the GIS field is approximately uniform, the strength to different waveforms is approximately constant, so:

\[ \text{BIL} \approx \text{chopped-wave strength}, \qquad \text{BSL} \approx \text{BIL} \]

This differs from external insulation, where chopped-wave strength may exceed full-wave BIL. For GIS, the crest surge voltage inside the GIS is compared directly with BIL — and although standards specify \(\text{BSL} < \text{BIL}\), the actual GIS BSL may be about equal to BIL, reflecting the uniform-field behaviour.

Section 17

GIS BIL Coordination

Why a 20% margin for GIS

The 20% margin reflects the importance of a conservative insulation level in a compact, enclosed, non-visible insulation system. Unlike an external air flashover, a GIS internal insulation problem may be hard to locate and may require major outage work.

GIS safety margins are usually at least 20%, so the required GIS BIL is a direct crest-voltage comparison — with no chopped-wave multiplier (unlike transformer coordination):

\[ \text{BIL} = 1.20\,E_t \qquad\Leftrightarrow\qquad E_{t,\max} = \frac{\text{BIL}}{1.20} \]

\(E_t\): crest surge voltage inside the GIS

Section 18

Example — 230 kV GIS with 950 kV BIL

Why the selected BIL controls bus length

This example shows that the selected GIS BIL directly affects the acceptable arrester separation or bus length. A higher BIL can provide enough margin for any practical bus length, while a lower BIL may require the arrester to be placed very close to the open end or the protected equipment.

Continuing the 230 kV GIS example with a selected BIL of 950 kV:

\[ E_{t,\max} = \frac{950}{1.20} = 792\ \text{kV} \qquad\text{vs calculated}\qquad E_{t,\max} = 741\ \text{kV} \]

Since \(741 < 792\), coordination is acceptable and no maximum bus-length limitation is reached — the permissible GIS bus length effectively approaches infinity for this case.

Section 19

Example — 230 kV GIS with 750 kV BIL

Restrictive, not unacceptable

The 750 kV BIL case is not automatically unacceptable, but it becomes much more restrictive. It may require shorter protected lengths, closer arrester placement, a different arrester arrangement or a more detailed transient study.

With a lower BIL of 750 kV:

\[ E_{t,\max} = \frac{750}{1.20} = 626\ \text{kV} \;\Rightarrow\; E_T = 764\ \text{kV} \;\Rightarrow\; \frac{E_T}{E_A} = 1.3079 \;\Rightarrow\; \ell_{\max} = 5.7\ \text{m} \]

Lower BIL ⇒ restrictive bus length

Reducing the GIS BIL from 950 kV to 750 kV cuts the maximum permissible bus length to about 5.7 m. A lower GIS BIL may require much closer arrester placement or a much shorter protected GIS bus length.

Section 20

Comparison with the IEEE Method

Why the IEEE method is compared

The IEEE method is included because it is a commonly referenced engineering guide. It reduces the station to a simplified transformer-protection equivalent and estimates the transformer voltage — practical, but it can become conservative for non-symmetrical layouts.

The IEEE guide method first reduces the station layout to a single-line transformer circuit, then calculates the transformer voltage, reducing the incoming-surge steepness depending on the number of lines after reduction (to \(S_p\)). The arrester voltage includes the lead drop:

\[ E_A = (\text{10 kA, 0.5-}\mu\text{s discharge voltage}) + L\frac{di}{dt} \]

\(L\): arrester lead inductance
\(di/dt\): current steepness through the arrester

The IEEE transformer-voltage equation includes the power-frequency voltage (appearing to assume it equals the line-to-ground voltage). The IEEE method focuses on transformer protection and does not give the station-wide equipment voltages of the chapter method.

Section 21

IEEE Conservatism in Non-Symmetrical Layouts

What circuit reduction loses

When a complex station is reduced to a single-line equivalent, some physical layout information is lost. Depending on the actual station geometry, this can overestimate or underestimate the local travelling-wave behaviour.

For the non-symmetrical station of Part Four, the IEEE method was very conservative — mainly because of the circuit reduction. Reducing a complex layout to a single-line transformer circuit can lose physical layout effects and overestimate the transformer voltage. So the IEEE reduced method can be conservative, especially for non-symmetrical layouts.

Section 22

138 kV Comparison and the IEEE Steepness Rule

A 138 kV system with \(BIL = 550\ \text{kV}\), \(MCOV = 84\ \text{kV}\), \(E_d = 267\ \text{kV}\), \(SF = 1.20\), \(S = 1000\ \text{kV}/\mu\text{s}\), \(V_{pv} = 113\ \text{kV}\). The maximum permissible transformer voltage is:

\[ E_t = \left(\frac{550}{1.20}\right)(1.10) = 504\ \text{kV} \]

With a 6 m arrester lead, the IEEE method gives a maximum arrester-to-transformer separation of \(17.8 + 6 = 23.8\ \text{m}\); the chapter method gives \(26.5\ \text{m}\). With no lead assumed, both agree at \(26.5\ \text{m}\) — the remaining difference is mainly the treatment of arrester lead length and power-frequency voltage. The IEEE method sets the incoming steepness from the arrester rating: \(11\ \text{kV}/\mu\text{s}\) per kV of MCOV, up to a maximum of \(2000\ \text{kV}/\mu\text{s}\) — a simplified rule versus the reliability-based calculation.

Section 23

Comparison with the IEC Method

Why the IEC method is compared

The IEC method uses a simple distance-based equation. It estimates the equipment voltage from the arrester discharge voltage, the incoming-surge steepness, the number of connected lines and the maximum travel time from the arrester to the equipment.

The IEC application guide calculates the voltage at any equipment with a simple expression based on the maximum distance to the closest arrester:

\[ E = E_d + 2\,\frac{S}{n}\,T \]

\(E_d\): arrester discharge voltage
\(S\): incoming-surge steepness
\(n\): number of connected lines
\(T\): maximum travel time arrester–equipment (incl. lead)

IEC sets the steepness as \(S = K_c/(d_m + S_L)\), where \(K_c\) is the corona constant, \(d_m\) the reliability distance and \(S_L\) the span length; unlike the chapter method, IEC adds the span length to \(d_m\) (rather than rounding up to the next tower). IEC uses a 15% margin for internal insulation (\(SF = 1.15\)) and 5% for external (\(SF = 1.05\)), comparing the calculated voltage directly with BIL (IEC does not specify a chopped-wave test in the same way).

Section 24

IEC Example — 145 kV Maximum System Voltage

With \(V_{\max} = 145\ \text{kV}\), \(E_d = 500\ \text{kV}\), \(S_L = 300\ \text{m}\), \(BFR = 1/100\) km-years, \(MTBF = 400\ \text{years}\), \(K_c = 675\ \text{kV-km}/\mu\text{s}\), \(n = 2\), separation 30 m (internal) and 60 m (external), IEC gives internal \(E_t = 622\ \text{kV}\) and external \(E_i = 745\ \text{kV}\):

\[ \text{internal: } \text{BIL} = 1.15 \times 622 = 715 \to 750\ \text{kV} \qquad \text{external: } \text{BIL} = 1.05 \times 745 = 782 \to 850\ \text{kV} \]

Section 25

The Chapter Method on the Same Example

Applying the chapter method (arrester lead 6 m so \(T_A = 0.02\), \(T_T = 0.08\), \(T_B = 0.18\ \mu\text{s}\); \(V_{pv} = 130\ \text{kV}\), \(E = 1200\ \text{kV}\)): the transformer voltage is \(E_t = 792\ \text{kV}\) and the breaker voltage \(E_b = 671\ \text{kV}\), giving required BILs of 864 kV (transformer) and 583 kV (breaker), so 900 kV is selected for both.

The chapter method gives a higher transformer voltage than IEC because it includes the transformer surge capacitance and the power-frequency voltage — effects not treated the same way in the IEC equation. Conversely, the breaker voltage can be lower in the chapter method, because the transformer capacitance can reduce the voltage behind the arrester. So the chapter method may increase transformer stress but reduce some behind-arrester stresses.

Why IEC can give a lower transformer voltage

The IEC method may give a lower transformer voltage because it does not treat the transformer surge capacitance and the power-frequency voltage in the same detailed way as the chapter method. This does not make IEC wrong — it shows that the assumptions behind each method must be understood.

Section 26

IEC Terminology and the Corona Constant

IEC terminology, side by side

IEC uses different names. The calculated equipment voltage is the coordination withstand voltage; the required BIL is the required withstand voltage; and the selected standard BIL is the standard withstand voltage.

IEC uses different terms: the calculated voltage at equipment is the coordination withstand voltage; the required BIL is the required withstand voltage; the selected BIL is the standard withstand voltage. The corona constant \(K_c\) increases with conductor bundling:

Table 3 — IEC corona constant by line type.

Line Type	\(K_c\) (kV-km/µs)
Single-conductor transmission line	675
Two-conductor bundle	1050
Four-conductor bundle	1650
Six- to eight-conductor bundle	2550

The conductor bundle configuration affects the surge steepness through corona effects.

Section 27

IEC Air Clearances and Switching Translation

Why IEC clearances can be larger

IEC tabulated clearances can be more conservative than clearances calculated directly from the simplified voltage-gradient method, because the IEC tables include standardised insulation practice and additional practical margins.

IEC tabulates air clearances directly — e.g. \(850\ \text{kV BIL} \to 1.7\ \text{m}\), versus the chapter method's \(792/605 = 1.3\ \text{m}\), so IEC tabulated clearances may be more conservative. Above 450 kV BIL the IEC clearances use a withstand gradient of about \(500\ \text{kV/m}\), reducing toward \(333\ \text{kV/m}\) at 20 kV BIL (IEEE has no comparable suggested clearances). Clearances from switching overvoltages may exceed those from lightning — important at higher voltages where switching impulse may govern.

For \(V_{\max} \le 245\ \text{kV}\), BSL is not normally provided, so IEC calculates switching surges and translates them to BIL using insulation-dependent ratios:

Table 4 — IEC switching-to-lightning ratios.

Insulation	\(BSL/BIL\)
Wet insulators	0.77
GIS (internal)	0.80
Liquid-immersed insulation	0.91
Solid insulation	1.00

Section 28

Chapter, IEEE and IEC Methods Compared

Table 5 — The three methods at a glance.

Aspect	Chapter Method	IEEE Method	IEC Method
Scope	Station-wide, detailed	Transformer-focused	Simple, systematic
Key effects	Capacitance, lead, PF voltage, waveshape, GIS, layout	Single-line reduction; lead drop	\(E = E_d + 2(S/n)T\)
Margins	\(SF=1.20\) (internal); small/none (external)	\(SF=1.20\)	15% internal / 5% external
Tendency	Balanced vs ATP; good sanity check	Conservative (non-symmetrical)	May underestimate transformer voltage

The methods differ because they model different physical effects and assumptions — so the engineer must understand each method's basis. The IEC method also offers tabulated air clearances, which the chapter and IEEE methods do not.

Table 6 — The methods compared — use, strength and limitation.

Method	Main Use	Strength	Limitation
Chapter method	engineering estimate / sanity check	includes transformer capacitance and travelling-wave effects	still simplified
IEEE method	transformer-protection estimate	practical and established	reduction may be conservative for non-symmetrical layouts
IEC method	standardised equipment-voltage estimate	simple and systematic	may not capture transformer capacitance in detail
EMTP® / ATP	detailed transient study	most flexible and accurate	needs detailed data and modelling effort

Section 29

Engineering Lessons

The key lessons

GIS is not just compact AIS — low surge impedance, short travel times and enclosed geometry change the surge behaviour.
Open GIS ends can see reflected overvoltages (up to \(2E_A\)) — always check open ends.
In-gas arresters improve protection but are rarely essential — high cost limits their use.
VFFT is a separate issue — not controlled by arresters; needs detailed modelling and good disconnector design.
GIS insulation is particle-sensitive — clean manufacturing and assembly are critical.
GIS BIL is compared directly with the surge crest — typically \(BIL = 1.20\,E_t\).
IEEE, IEC and the chapter method differ — understand the assumptions behind each.

Section 30

Summary of Key Equations

Equation Summary

Arrester surge voltage

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

Max GIS open-end voltage

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

GIS lumped-capacitance

\( e = 2St - 2ZCS(1 - e^{-t/ZC}) \)

GIS BIL coordination

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

Max GIS voltage for a BIL

\( E_{t,\max} = \dfrac{\text{BIL}}{1.20} \)

IEC equipment voltage

\( E = E_d + 2\dfrac{S}{n}T \)

IEC surge steepness

\( S = \dfrac{K_c}{d_m + S_L} \)

IEC internal BIL

\( \text{BIL} = 1.15\,E \)

IEC external BIL

\( \text{BIL} = 1.05\,E \)

IEC wet-insulator ratio

\( \dfrac{BSL}{BIL} = 0.77 \)

Section 31

Reader Should Remember

GIS is used mainly where space is limited or atmospheric conditions make AIS difficult. Its low surge impedance, short travel times and compact geometry mean travelling-wave behaviour differs from AIS. For lightning surges, an open GIS bus end can experience reflected overvoltages, in the limit \(E_{T,\max} = 2E_A\); and GIS BIL coordination is a direct crest comparison with a typical 20% margin, \(BIL = 1.20\,E_t\).

Very fast front transients from disconnector operation are a separate concern — only 1.7–2.5 pu in magnitude but with nanosecond fronts that arresters do not limit, so disconnector design and detailed high-frequency modelling matter. GIS strength is strongly affected by field uniformity and free conducting particles, so clean manufacturing and assembly are critical. And the IEEE, IEC and chapter methods do not always agree — IEEE can be conservative for non-symmetrical layouts, IEC is simple but may underestimate transformer voltage where capacitance and power-frequency effects matter, and the chapter method is a good sanity check against EMTP®.

The single most important message

GIS coordination needs both ordinary lightning checks and special attention to very fast front switching transients. The selected BIL depends not only on arrester rating but also on GIS bus length, arrester location, open-end reflections, safety margin and the calculation method used.

Reader should remember

GIS is compact and normally easier to protect against ordinary lightning surges than AIS, but it has special issues. Open bus ends can reflect voltage waves, VFFT from disconnecting switches is not effectively limited by arresters, and GIS insulation is sensitive to free conducting particles. For GIS, the crest voltage is usually compared directly with BIL using a safety margin. The IEEE and IEC guide methods are useful, but their assumptions must be understood.

This is Part Five of the station lightning insulation coordination series, covering gas-insulated stations, GIS stress and strength, and the comparison with IEEE and IEC methods. Later parts continue the series.

Section 32

Key Symbols

Table 7 — Key symbols used on this page.

Symbol	Meaning
GIS / AIS	Gas-insulated / air-insulated station (switchgear)
VFFT	Very fast front transient (disconnector operation)
\(E,\ S\)	Incoming-surge crest voltage; incoming-surge steepness
\(E_d,\ E_A\)	Arrester discharge voltage to ground; surge voltage at arrester
\(E_T,\ E_t\)	Surge voltage / voltage to ground at the GIS open end or equipment
\(Z,\ Z_c\)	Incoming-line surge impedance; GIS bus surge impedance
\(T,\ C\)	GIS bus travel time; GIS bus capacitance
BIL / BSL	Basic Lightning / Switching Impulse Level
\(K_c,\ n\)	Corona constant (IEC steepness); number of connected lines
\(T_A,\ T_B\)	Arrester lead travel time; travel time arrester-to-equipment

Station Lightning Coordination: Gas-Insulated Stations & Guide Methods