Insulation Coordination

Station Lightning Coordination: Standard BILs & Worked Examples

Part Four of the series. The simplified method applied to real stations: selecting standard equipment BILs (IEEE 1313.1, IEC 71.1), a full single-line 230 kV worked example across every equipment type, how a second line changes transformer and equipment BILs, contingency operation with one line out and the reliability probability adjustment, a non-symmetrical 115 kV layout with line-by-line checks, and validation against ATP — with the limits of the simplified method.

Reading time ≈ 45 min · Part Four of the series

Section 1

Applying the Simplified Method

The earlier parts built the calculation of station surge voltages, the voltage behind the arrester, and the insulation-strength criteria. This part applies all of it to practical worked stations — comparing the calculated stress with the available strength to select real, standard equipment BILs.

The whole exercise is stress versus strength: the stress is the calculated surge voltage at the equipment, and the strength is the BIL, chopped-wave strength, switching-impulse level or air-clearance strength. The section first looks at standard BILs, then works through a single-line 230 kV station, a two-line 230 kV station, a one-line-out contingency, and a non-symmetrical 115 kV layout.

From Part Three to Part Four

Part Three explained how calculated surge voltages are compared with equipment insulation strength. Part Four now applies that process to practical station examples — showing how required BILs are calculated, rounded up to standard values, and then checked against the ratings actually available for the equipment.

What Part Four covers

standard BIL / BSL values and how they are selected;
a full single-line 230 kV worked example (every equipment type);
how a second line changes transformer and equipment BILs;
contingency operation with one line out and the probability adjustment;
a non-symmetrical station and the line-by-line check;
comparison with ATP and the limits of the simplified method.

Section 2

Standard BILs

Why standard BILs appear here

The simplified method usually produces a non-standard required BIL. But equipment is purchased with standard insulation levels, so once the required BIL is calculated, the next available standard BIL must be selected.

The calculated required BIL is rarely exactly a standard equipment value — a calculation may give \(BIL_{\text{required}} = 711\ \text{kV}\), but equipment is only made at standard insulation levels. So the process is: calculate the required BIL, find the next-highest standard BIL, confirm it is available for that equipment type and system voltage, and select it. The selected BIL must be a real equipment insulation level, not just a mathematical value.

Standard BIL and BSL values come from IEEE 1313.1 and IEC 71.1. In IEEE these are suggested values for equipment standards to use; equipment standards may adopt them or specify others, but in practice the standard values are generally used.

Required, standard and selected BIL

The required BIL is the value calculated from the surge stress and the insulation-strength criterion. The standard BIL is the next-higher value from the standard BIL table. The selected BIL is the practical rating actually available for that voltage class and apparatus type.

The BIL-selection process, in four steps

calculate the crest voltage at the equipment;
calculate the required BIL using the correct insulation criterion;
round up to the next standard BIL;
check whether that BIL is available for the equipment at the system voltage.

Section 3

BIL and BSL Are Linked by Equipment Type

For a given equipment type, BIL and BSL are related — for transformers, \(BSL \approx 0.83\,BIL\). So even a standard BIL may have an associated BSL that does not exactly match a general table value; selection must consider the specific apparatus standard.

Two cautions on availability

Do not select BIL and BSL independently without checking the equipment standard. And for a given system voltage, not every BIL is available for every item: a transformer may have 1–3 candidate BILs, a circuit breaker often a single standard BIL, with bus supports and disconnectors following their own standards. The final selection must be checked against available equipment ratings.

Section 4

The Application Procedure

The simplified method compares the calculated voltage stress with the equipment insulation strength, in this order:

The procedure

define the incoming surge;
estimate the arrester current and discharge voltage;
calculate the equipment voltages;
compare with the insulation-strength criteria and calculate required BILs;
select the next available standard BIL;
check the air clearances.

The examples use transformer surge capacitances of \(2\ \text{nF}\) and \(4\ \text{nF}\), because that capacitance affects the calculated surge voltage.

Section 5

Example 1 — Single-Line 230 kV Station

Why the single-line station first

The single-line station is used first because it is the simplest case. With only one incoming surge path, the example clearly shows how the incoming surge, arrester voltage, transformer voltage, breaker voltage and bus-support voltage are each converted into a selected BIL.

Why \(1.2 \times CFO\)

The crest of the incoming surge is taken as \(1.2\,CFO\) as a conservative estimate. It does not mean that every incoming surge has this value — it is a design assumption used to keep the simplified calculation on the safe side.

A 230 kV single-line station designed for \(MTBF = 100\ \text{years}\), with a line backflashover rate \(BFR = 2.0\) flashovers/100 km-years, a 300 m span and a line CFO of 1300 kV. The incoming-surge crest is taken conservatively as \(1.2\,CFO\):

\[ E = 1.2 \times 1300 = 1560\ \text{kV} \qquad V_{pv} = 130\ \text{kV} \]

Section 6

Incoming-Surge Steepness

The steepness follows from the reliability criterion: the controlling distance is obtained from the MTBF, and because the span is 300 m the distance is increased to 600 m, giving \(S = 1167\ \text{kV}/\mu\text{s}\). In other words, a higher MTBF requirement gives a more severe (steeper) incoming surge, and a shorter controlling distance gives a steeper surge.

How the incoming surge is chosen

The incoming surge follows from the required station reliability. The line BFR and span length set how often a severe surge is expected to reach the station; the line CFO then defines a conservative incoming-surge crest, while the reliability calculation defines the incoming-surge steepness.

Section 7

Arrester Current and Voltage

The arrester voltage, in plain terms

The arrester discharge voltage is current-dependent, so the arrester current must first be estimated and the matching protective voltage read or interpolated from the arrester characteristic. The 8/20 µs discharge values are adjusted upward to represent the faster 0.5 µs front used in the lightning coordination calculation.

A 140 kV MCOV metal-oxide arrester is selected. Its 0.5 µs discharge voltage at 10 kA is 446 kV and its 8/20 µs value at 10 kA is 404 kV, so a multiplier of 1.10 is applied to the 8/20 values to represent the 0.5 µs discharge voltage (giving 418 kV at 5 kA and 446 kV at 10 kA). The characteristic is approximated by a slope resistance and intercept, and the current and discharge voltage estimated with \(E_d = f(I_A)\). The surge voltage at the arrester is:

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

The distinction matters: the arrester calculation uses the surge voltage, while equipment insulation is compared with the voltage to ground.

Why the power-frequency voltage is included

The lightning surge is assumed to sit on an opposite-polarity power-frequency voltage, so the insulation stress depends on the surge component plus the power-frequency component. This is why the voltage to ground used for BIL selection is not always the same as the travelling-wave surge component.

Section 8

Transformer Voltage and BIL

The transformer voltage is computed for \(C_T = 2\ \text{nF}\) and \(4\ \text{nF}\) and compared with the transformer criterion. Treated as a fast crest coordinated with the chopped-wave level, with \(SF = 1.20\) and \(E_t = 652\ \text{kV}\):

\[ \text{BIL} = \frac{SF\cdot E_t}{1.10} = \frac{1.20 \times 652}{1.10} = 711\ \text{kV} \;\;\Rightarrow\;\; \text{next standard} = 750\ \text{kV} \]

So the selected transformer BIL is 750 kV.

Section 9

Transformer Bushing and the Altitude Effect

Why the bushing is checked twice

The transformer bushing contains both internal and external insulation. The internal insulation is coordinated like transformer insulation; the external insulation is coordinated like external air insulation, including altitude correction where it applies. The selected external bushing BIL should not be lower than the internal bushing BIL.

The bushing has internal and external insulation. The internal is treated like transformer insulation, so its required BIL is the same — 711 kV → standard 750 kV. The external is treated as self-restoring; at sea level \(\text{BIL} = E_t/1.15 = 652/1.15 = 566\ \text{kV}\) → standard 600 kV. But the external must not be lower than the internal, so it is raised to 750 kV.

Altitude changes this. At 1600 m, \(\delta = 0.830\), so the external requirement becomes \(566/0.830 = 681\ \text{kV}\) (still standard 750 kV). At higher altitudes the external insulation can require a higher BIL than the internal — because external strength falls with air density while internal insulation is largely unaffected.

Section 10

Breaker, Disconnectors and Bus Supports

Why the breaker's selected BIL can exceed the requirement

The calculated required BIL can be lower than the selected breaker BIL because circuit breakers are normally available only at fixed standard insulation levels for each voltage class. So even when the required BIL is lower, the selected BIL is the standard breaker rating.

Circuit breaker: with \(E_b = 803\ \text{kV}\), \(\text{BIL} = 803/1.15 = 698\ \text{kV}\) → standard 750 kV — but the standard breaker BIL at 230 kV is 900 kV, so 900 kV is selected. If the required BIL ever exceeded the only available breaker BIL, the stress would have to be reduced (arrester location, more arresters, shorter leads, layout, or a less severe incoming surge).

Disconnecting switches: assumed at the breaker, so the same voltage and BIL — 900 kV.

Bus supports: located throughout the station, so the highest station voltage is used, \(943\ \text{kV}\): \(\text{BIL} = 943/1.15 = 820\ \text{kV}\) → standard 825 kV, but the selected available value is 900 kV.

Selecting bus-support insulators

Bus support insulators may be located anywhere on the station bus, so the maximum calculated voltage anywhere in the station is used when selecting their insulation level.

What the summary table shows

The table shows that different equipment items need different BIL checks. The transformer and internal bushing insulation are non-self-restoring; the breaker, disconnecting switch, bus supports and external bushing insulation are self-restoring or external. So the same calculated voltage does not always lead to the same BIL rule.

Section 11

Air Clearances and the Example 1 Summary

Air clearance, in words

Air clearance is selected by converting the maximum station voltage into a required physical distance using a conservative air-withstand gradient. The highest voltage in the station is used because the clearance must be adequate at the most stressed location.

Using the highest station voltage \(943\ \text{kV}\) and a conservative gradient \(605\ \text{kV/m}\):

\[ S = \frac{943}{605} = 1.56\ \text{m} \qquad (S_{\text{phase-ground}} = S_{\text{phase-phase}}) \]

Table 1 — Example 1 (single-line 230 kV) BIL selection.

Equipment	Voltage (kV)	Required BIL (kV)	Selected BIL (kV)
Transformer	\(E_t = 652\)	711	750
Internal bushing	652	711	750
External bushing	652	566	750 (raised to internal)
Circuit breaker	803	698	900 (available)
Disconnecting switch	803	698	900
Bus support	943	820	900
Air clearance	943	—	1.56 m

Section 12

Comparison with ATP

Why compare with ATP

The ATP comparison shows whether the simplified method is reasonable. Here the simplified method gives conservative voltages, while the transformer voltage agrees reasonably well with ATP — which supports using the simplified method for early estimates and sanity checks.

All simplified voltages are higher than the ATP results by about 1% to 25%, and for transformer voltages only about 3% to 6% higher — a good result, conservative yet close where it matters most. The ATP plots show initial voltage spikes, decay toward the arrester voltage, and oscillatory transformer voltage — confirming that real station waveshapes are not simple standard \(1.2/50\ \mu\text{s}\) impulses.

Section 13

Example 2 — Two-Line 230 kV Station

A second line is not automatically less severe

Adding a second line does not automatically make the station less severe. Extra lines provide additional surge paths, which can reduce some voltages, but they also increase the number of possible incoming surges and can require a steeper design surge for the transformer reliability check. These two effects compete.

The second example adds a line. The number of connected lines affects the incoming-surge selection, the arrester current, and the transformer, breaker and bus-support voltages, as well as the station reliability calculation.

With all lines in service, the transformer is evaluated on a different reliability basis: \(MTBS = 200\ \text{years}\), twice the \(MTBF = 100\ \text{years}\) — because a surge may enter from either line, exposing the transformer to both. With \(BFR = 2/100\) km-years and a 300 m span, the transformer-bus steepness becomes \(S = 2333\ \text{kV}/\mu\text{s}\) (crest still \(1560\ \text{kV}\)), while equipment not on the transformer bus uses a 100-year surge, \(S = 1167\ \text{kV}/\mu\text{s}\). Different locations can be evaluated with different reliability-exposure logic.

MTBF versus MTBS

\(MTBF\) is the target mean time between insulation failures. \(MTBS\) is the mean time between surges of a defined severity reaching the equipment. In multiline stations \(MTBS\) may be adjusted, because the same equipment can be exposed to surges arriving from more than one line.

Section 14

Transformer BIL Increases with the Second Line

Why the transformer BIL increases

In the two-line example the transformer BIL increases because the transformer is exposed to surges from more than one incoming line. The reliability-based surge used for the transformer check becomes more severe, even though the additional line may reduce some local voltages.

The arrester voltage is recalculated (the line count changes the surge-current sharing). The transformer crest voltage is \(E_t = 710\ \text{kV}\) for both capacitance cases, so:

\[ \text{BIL} = \frac{1.20 \times 710}{1.10} = 775\ \text{kV} \;\;\Rightarrow\;\; \text{next standard} = 825\ \text{kV} \]

A counterintuitive result

The transformer BIL rises from 750 kV (single line) to 825 kV (two lines), because the transformer is evaluated against a more severe surge tied to the reliability basis. Adding a line does not reduce every equipment stress.

Section 15

Other Equipment and Clearances (Two-Line)

Why other equipment can see lower voltage

Other equipment may experience lower voltages in the two-line case because the extra connected line provides another path for the surge energy and modifies the reflections. This is why the transformer and other equipment do not necessarily follow the same trend.

For equipment not on the transformer bus, the second line provides another surge path, so most voltages are about 3% lower than the single-line case (except one). But because breakers and bus supports have fixed available BILs, the selected values often do not change — breaker and bus support remain 900 kV. The highest two-line voltage is \(916\ \text{kV}\), giving a clearance \(S = 916/605 = 1.51\ \text{m}\) — slightly below the single-line 1.56 m, because the maximum voltage is lower.

Section 16

Why a 100-Year Surge Is Used for Other Equipment

For the two-line station, a surge on line A and a surge on line B each produce a set of voltages. At one disconnecting switch the values are \(776\ \text{kV}\) and \(543\ \text{kV}\), once in 100 years — the higher value sets the BIL. At another switch the values are \(916\ \text{kV}\) and \(543\ \text{kV}\), so the BIL is based on \(916\ \text{kV}\). This confirms that the 100-year surge is acceptable for these locations.

The reliability principle

The reliability basis must be linked to how often a given voltage appears at a given equipment location — not applied uniformly across the station.

Section 17

Contingency — One Line Out of Service

A contingency is not automatically the worst case

A contingency condition should not be assumed to be the most severe. If one line is out of service only part of the time, that configuration is less probable, so its incoming surge is selected using a different return period. This can make the contingency case less severe than the all-lines-in-service case.

Now the two-line station has one line out (the disconnectors on each side of one breaker opened), so fewer surge paths exist. The probabilities are \(P(\text{all lines}) = 75\%\) and \(P(\text{one line}) = 25\%\). To keep the 100-year MTBF, the contingency surge return period is adjusted to \(100 \times 0.25 = 25\ \text{years}\); and because either line could be the one out, the transformer is evaluated with \(MTBS = 50\ \text{years}\). The severity of the design surge changes when the probability of the station state changes.

The probability adjustment

If the one-line-out condition exists for 25% of the time, the surge return period used for that condition is adjusted accordingly. The study must consider not only the electrical configuration but also how often that configuration is expected to exist.

Section 18

Contingency Surge and Results

For a 50-year surge the distance is \(d_m = 1.2\ \text{km}\) and \(S = 483\ \text{kV}/\mu\text{s}\); for a 25-year surge, \(d_m = 2.1\ \text{km}\) and \(S = 333\ \text{kV}/\mu\text{s}\) (crest still \(1560\ \text{kV}\)). The station now behaves as single-line, so \(E_d = 441\ \text{kV}\) and \(E_A = 571\ \text{kV}\) as in the original single-line case.

The 50-year surge gives a transformer voltage of \(570\ \text{kV}\) — well below the \(710\ \text{kV}\) with all lines in service, so for the transformer the all-lines case is more critical. For other equipment, applying a 25-year surge to lines A and B gives switch voltages of \(552\ \text{kV}\) and \(462\ \text{kV}\); the BIL is based on \(552\ \text{kV}\), giving a required \(480\ \text{kV}\) → standard \(500\ \text{kV}\) — much lower than the all-lines case. So in this example, all-lines-in-service is the worst case.

Section 19

Contingency Cautions

Not a universal conclusion

In this example the all-lines-in-service case governs, but this is not a universal rule. In another station a contingency case may govern if the layout, arrester location, line exposure or probability assumptions are different.

That all-lines-in-service is most critical is specific to this example — it is not always true. The outcome depends on the contingency probability, the surge return period, the line arrangement and layout, the arrester location, the distance to equipment, the number of surge paths and the surge steepness. Contingencies should be evaluated whenever they are likely or operationally important.

Combining equivalent events

If the voltage at breaker A with line B open equals that at breaker B with line A open — say both \(700\ \text{kV}\) — then two \(700\ \text{kV}\) surges occur once in 25 years, equivalent to one \(700\ \text{kV}\) surge once in 12.5 years. The surge should then be based on \(MTBS = 50\ \text{years}\) and the calculation repeated. When multiple equivalent events produce the same stress, their occurrence rates must be combined.

Section 20

Example 3 — Non-Symmetrical 115 kV Station

Why surge direction matters

The non-symmetrical example shows that surge direction matters. If the station layout is not symmetrical, a surge entering from different lines can produce different voltages at the same transformer or breaker.

In a non-symmetrical station, surges on different lines produce different voltages, because the electrical distances from the arrester to equipment differ. This 115 kV station uses an 84 kV MCOV arrester (10 kA discharge voltage 273 kV) at the end of the bus, with incoming lines A, B and C and two transformers — TR2 being more distant from the arrester than TR1, so it is expected to see a higher surge voltage.

Section 21

Line-by-Line Evaluation (ATP)

A 100-year surge (\(E = 1080\ \text{kV}\), \(S = 1000\ \text{kV}/\mu\text{s}\), \(V_{pv} = 65\ \text{kV}\), \(Z = 450\ \Omega\)) is applied to each line. For TR2 with \(C_T = 4\ \text{nF}\), the crest voltages are:

\[ \text{line A: }394\ \text{kV} \qquad \text{line B: }417\ \text{kV} \qquad \text{line C: }454\ \text{kV} \]

The non-symmetry lesson

Line C produces the highest voltage at TR2. For non-symmetrical layouts, each incoming line must be checked — the worst line is not obvious without calculation.

Section 22

The Maximum-Distance Method, and IEEE vs IEC

Why the maximum-distance method is a simplification

The maximum-distance method is a practical simplification: it estimates the equipment voltage using the longest travel time from the equipment to the nearest arrester. It is useful for screening, but it may be conservative or unconservative depending on the actual station layout and reflections.

A simple IEC-style approach selects the maximum distance from the equipment to the closest arrester — here 36 m for TR2 (including the arrester ground lead) and 39 m for the breaker. It suits symmetrical and non-symmetrical layouts alike. In this example the transformer voltages come within about 3% of ATP, but the breaker voltages are more conservative, exceeding ATP by about 6% to 30%.

Comparing ATP, this chapter's simplified method, the IEEE method and the IEC method: the IEEE method tends to overestimate voltages, while the IEC method tends to underestimate them — so both should be used with awareness of their conservatism or limitations.

Section 23

When a Non-Symmetrical Station Can Be Treated as Symmetrical

When the station can be treated as symmetrical

If the calculated transformer voltages for surges entering from different lines are within about 2%, the station may be treated approximately as symmetrical. If the difference is larger, each incoming line should be checked separately and the controlling case used.

A practical rule: if the transformer voltages from surges on alternate lines are within about 2% of each other, the station may be treated as symmetrical and an incoming surge based on \(n\) lines used. If they differ by more than about 2%, the non-symmetry is significant and the calculation should use \(n = 1\) for the controlling line. Similar voltages ⇒ symmetrical approximation acceptable; different voltages ⇒ line-by-line evaluation required.

Section 24

Practical Lessons from the Examples

The key lessons

Standard BIL selection is a rounding-up process — \(711 \to 750\), \(775 \to 825\); a small voltage rise can push to the next BIL class.
Transformer BIL can increase with more lines — here \(750 \to 825\ \text{kV}\), via a more severe reliability basis.
Other equipment may see lower stress with more lines — extra surge paths reduce some voltages.
Contingencies are probability-dependent — a low-probability one-line-out case need not be the worst.
Non-symmetrical layouts need line-by-line checks — surge direction matters.
ATP validates the method — simplified results can be up to ~25% high (more for some equipment); use EMTP® for final accuracy.

Section 25

Summary of Key Equations

Equation Summary

Incoming-surge crest

\( E = 1.2\,CFO \)

Arrester surge voltage

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

Transformer BIL (fast crest)

\( \text{BIL} = \dfrac{SF\cdot E_t}{1.10} \)

External SR BIL (altitude)

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

Breaker / apparatus BIL

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

Bus support BIL

\( \text{BIL} = \dfrac{E_{\max}}{1.15} \)

Air clearance

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

Contingency adjustment

\( MTBF_{\text{cont}} = MTBF \times P \)

Section 26

Reader Should Remember

The application process is always the same: calculate the stress, calculate the required BIL, select the next available standard BIL, then check clearances. The selected BIL must be a real, available equipment level — not just the calculated value.

For the single-line 230 kV example the selections were: transformer 750 kV, transformer bushing 750 kV, breaker 900 kV, disconnecting switch 900 kV, bus support 900 kV, air clearance 1.56 m. For the two-line example the transformer BIL rose to 825 kV (more severe reliability basis) while some other voltages fell. Contingencies must be checked when likely or important, but are not automatically the worst case. For non-symmetrical layouts, check each incoming line — within ~2% a symmetrical approximation may be used, otherwise evaluate the controlling line.

The single most important message

Station insulation coordination is a stress-versus-strength process — but the stress depends on layout, line configuration, arrester location, reliability target and contingency probability, not on a table value alone.

Reader should remember

Part Four shows how calculated surge voltages become practical equipment BILs. The required BIL is calculated from the stress; the standard BIL is the next-higher standard value; and the selected BIL must be available for the equipment type and voltage class. Adding lines can reduce some local voltages but can also increase surge exposure. Contingency cases must be weighted by probability, and non-symmetrical stations should be checked line by line.

This is Part Four of the station lightning insulation coordination series, applying the simplified method to worked stations and standard-BIL selection. Later parts continue with further coordination topics and detailed studies.

Section 27

Key Symbols

Table 2 — Key symbols used on this page.

Symbol	Meaning
BIL / BSL	Basic Lightning / Switching Impulse Level (\(BSL \approx 0.83\,BIL\) for transformers)
\(BIL_{\text{req}}\) / \(BIL_{\text{std}}\) / \(BIL_{\text{sel}}\)	Calculated required BIL; next-higher standard BIL; practical equipment BIL actually selected
MTBF / MTBS	Mean Time Between Failures / Mean Time Between Surges
BFR	Backflashover rate (flashovers/100 km-years)
CFO	Critical Flashover Voltage (line insulation)
\(E,\ S\)	Incoming-surge crest voltage; incoming-surge steepness
\(V_{pv}\)	Power-frequency voltage of opposite polarity
\(E_d,\ E_A\)	Arrester discharge voltage to ground; surge voltage at the arrester
\(E_t,\ E_b\)	Transformer / breaker voltage to ground
\(C_T,\ \delta,\ n\)	Transformer surge capacitance; relative air density; number of connected lines

Part Four Reading now

Standard BILs & Worked Examples

Applying the simplified method — standard BIL selection and worked 230 kV single-line, two-line, contingency and non-symmetrical 115 kV examples.

Series progress 4 of 8

Part Five

Gas-Insulated Stations & Guide Methods

Gas-insulated stations — GIS lightning stress and the open-end reflection, very fast front transients, GIS strength, and the chapter/IEEE/IEC comparison.