Lightning Protection and Insulation Coordination

© State of NSW through Transport for NSW 2020

Lightning Protection and Insulation Coordination

T HR EL 21001 ST

Standard

Version 1.0

Issue date: 07 December 2020

T HR EL 21001 ST Lightning Protection and Insulation Coordination

Version 1.0 Issue date: 07 December 2020

© State of NSW through Transport for NSW 2020

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© State of NSW through Transport for NSW 2020 of 21

Standard governance

Owner: Lead Electrical Engineer, Asset Standards Authority

Authoriser: Chief Engineer, Asset Standards Authority

Approver: Executive Director, Asset Standards Authority on behalf of the ASA Configuration Control Board

Document history

Version Summary of changes

1.0 First issue




Preface The Asset Standards Authority (ASA) is a key strategic branch of Transport for NSW (TfNSW).

As the network design and standards authority for NSW Transport Assets, as specified in the

ASA Charter, the ASA identifies, selects, develops, publishes, maintains and controls a suite of

requirements documents on behalf of TfNSW, the asset owner.

The ASA deploys TfNSW requirements for asset and safety assurance by creating and

managing TfNSW's governance models, documents and processes. To achieve this, the ASA

focuses on four primary tasks:

• publishing and managing TfNSW's process and requirements documents including TfNSW

plans, standards, manuals and guides

• deploying TfNSW's Authorised Engineering Organisation (AEO) framework

• continuously improving TfNSW’s Asset Management Framework

• collaborating with the Transport cluster and industry through open engagement

The AEO framework authorises engineering organisations to supply and provide asset related

products and services to TfNSW. It works to assure the safety, quality and fitness for purpose of

those products and services over the asset's whole-of-life. AEOs are expected to demonstrate

how they have applied the requirements of ASA documents, including TfNSW plans, standards

and guides, when delivering assets and related services for TfNSW.

Compliance with ASA requirements by itself is not sufficient to ensure satisfactory outcomes for

NSW Transport Assets. The ASA expects that professional judgement be used by competent

personnel when using ASA requirements to produce those outcomes.

About this document

This document provides details for the selection of surge arresters, lightning protection and

insulation coordination requirements for the RailCorp high voltage (HV) network as well as

surge arrester requirements for the 1500 V dc traction system.

This standard supersedes RailCorp document EP 21 00 00 01 SP Insulation Co-ordination and

Surge Arrester Selection, version 3.1.

The changes from previous content include:

• requirements for when a lightning protection study is required

• update of terminology to align with IEC standards

• change of maximum continuous system overvoltage to 10%

• addition of pollution and altitude requirements for surge arresters

• addition of requirements for surge arresters used in the 1500 V dc traction system




• update of example on assessing if a surge arrester is technically suitable for use in the

RailCorp HV network

• inclusion of type approval requirement for surge arresters

This document is a first issue.




Table of contents 1. Introduction .............................................................................................................................................. 7

2. Purpose .................................................................................................................................................... 7 2.1. Scope ..................................................................................................................................................... 7 2.2. Application ............................................................................................................................................. 8

3. Reference documents ............................................................................................................................. 8

4. Terms and definitions ............................................................................................................................. 9

5. Type approval .......................................................................................................................................... 9

6. Lightning protection ................................................................................................................................ 9 6.1. Substations that require a lightning protection study ........................................................................... 10 6.2. Design requirements ............................................................................................................................ 10 6.3. Installation requirements ...................................................................................................................... 11

7. Aerial line lightning protection ............................................................................................................. 11 7.1. Overhead earth wire ............................................................................................................................ 11 7.2. Aerial line surge impedance ................................................................................................................ 12 7.3. Lightning strike rate ............................................................................................................................. 12 7.4. Aerial line outage rates ........................................................................................................................ 12

8. Aerial line characteristics and insulation levels ................................................................................ 12

9. Substation equipment insulation levels .............................................................................................. 12

10. HV ac surge arrester requirements...................................................................................................... 13 10.1. Type of surge arrester ..................................................................................................................... 13 10.2. Location of HV ac surge arresters ................................................................................................... 13 10.3. Electrical parameters for arrester selection ..................................................................................... 14 10.4. Pollution level ................................................................................................................................... 16 10.5. Altitude ............................................................................................................................................. 16 10.6. Mechanical requirements ................................................................................................................. 16 10.7. Physical orientation .......................................................................................................................... 16

11. Selection and specification of HV ac surge arresters ....................................................................... 17 11.1. Procedure for selecting surge arresters .......................................................................................... 17 11.2. Margin of protection ......................................................................................................................... 17 11.3. Inductive voltage drop of arrester leads. ......................................................................................... 17

12. 1500 V dc surge arrester requirements ............................................................................................... 18 12.1. Type of surge arrester ..................................................................................................................... 18 12.2. Location of 1500 V dc surge arresters ............................................................................................. 18 12.3. Electrical parameters for 1500 V dc arrester selection .................................................................... 19 12.4. Altitude and pollution level ............................................................................................................... 19 12.5. Mechanical requirements ................................................................................................................. 19 12.6. Physical orientation .......................................................................................................................... 19

Appendix A Surge arrester selection example .................................................................................... 20




1. Introduction The RailCorp high voltage (HV) alternating current (ac) distribution network has nominal

voltages of 11 kV, 33 kV, 66 kV and 132 kV.

The direct current (dc) traction system is a nominal voltage of 1500 V dc.

The HV ac distribution network includes equipment such as HV switchboards, HV feeders, air

break switches, system transformers, rectifier transformers and harmonic filters. The 1500 V dc

traction system includes equipment such as metalclad switchboards, feeders, sectioning

switches and isolating and rail connecting switches.

These types of equipment are found in a variety of installations such as the following:

• traction substations with indoor HV switchgear

• traction substations with outdoor HV yards

• traction substations with indoor HV switchgear and outdoor yards for HV harmonic filters

• HV switching stations

• standalone 1500 V dc switchyards

Lightning protection assessment and installation is critical to ensure the electrical locations and

associated infrastructure are protected from lightning strikes and that the reliability of the HV

networks and corresponding traction system is maintained.

2. Purpose This standard specifies the lightning protection requirements, insulation coordination and surge

arrester requirements for the RailCorp HV distribution network.

Surge arrester requirements for the 1500 V dc traction system are also specified.

2.1. Scope This document covers the lightning protection requirements for electrical installations,

associated infrastructure and surge arrester requirements for the RailCorp HV distribution

network and 1500 V dc traction system.

This standard does not include requirements for the 1500 V dc surge arresters on rolling stock.




2.2. Application The lightning protection requirements specified in this document apply to substations and aerial

lines within the RailCorp HV ac distribution networks.

The following are locations within the RailCorp HV ac distribution network that are classified as

substations for the purpose of this document:

• all new traction substations and HV switching stations

• sectioning huts with HV switchgear

• existing traction substations and HV switching stations that are being modified

Lightning protection is not applied to the 1500 V dc overhead wiring system,

The surge arrester requirements specified in this document apply to all HV surge arresters and

1500 V dc surge arresters that are to be installed in the RailCorp HV distribution network and

1500 V dc traction system.

3. Reference documents The following documents are cited in the text. For dated references, only the cited edition

applies. For undated references, the latest edition of the referenced document applies.

International standards

IEC 60071-1 Insulation co-ordination - Part 1: Definitions, principles and rules

IEC 60071-2 Edition 4.0 2018-03 Insulation co-ordination - Part 2: Application guidelines

IEC 60099-4 Surge arresters – Part 4: Metal-oxide surge arresters without gaps for a.c. systems

IEC 62497-1 Railway applications – Insulation coordination – Part 1: Basic requirements –

Clearances and creepage distances for all electrical and electronic equipment

IEC 62848-1 Railway applications - DC surge arresters and voltage limiting devices – Part 1:

Metal-oxide surge arresters without gaps

Australian standards

AS 1768 Lightning protection

AS 4436 Guide for the selection of insulators in respect of polluted conditions

AS/NZS 1429.1 Electric cables – Polymeric insulated – For working voltages 1.9/3.3 (3.6) kV up

to and including 19/33 (36) kV

Transport for NSW standards

T HR EL 10001 ST HV Aerial Line Standards for Design and Construction

T HR EL 90002 ST Heavy Rail Traction System-Voltage Ratings




T MU MD 00005 GU Type Approval of Products

T MU MD 00006 ST Engineering Drawings and CAD Requirements

4. Terms and definitions The following terms and definitions apply in this document:

ASA Asset Standards Authority

EFF earth fault factor EFF

LIWV lightning impulse withstand voltage

LPS lightning protection system

MCOV maximum continuous operating voltage

MOP margin of protection

OHEW overhead earth wire

OHW overhead wiring

RSM rolling sphere method

TOV temporary overvoltage

5. Type approval All HV and 1500 V dc surge arresters shall be type approved for installation in the RailCorp HV

distribution network and 1500 V dc traction system, by the Asset Standards Authority (ASA)

prior to being connected.

Refer to T MU MD 00005 GU Type Approval of Products for the type approval process.

6. Lightning protection The substations in the RailCorp HV distribution network provide electrical supply to varied

critical services such as the following:

• traction supply for train running

• signalling systems

• fire and life safety services

• operating centres

The assessment, design and installation of the lightning protection system (LPS) of substations

is a critical element in ensuring the reliability and availability of supply to these services.




Similarly the lightning protection design associated with aerial lines and associated structures is

critical to ensure a very low outage rate and a high availability of supply.

6.1. Substations that require a lightning protection study A lightning protection study shall be completed for all new substations.

A lighting protection study for existing substations shall be completed in the following

circumstances:

• modification to existing structures (for example, aerial line termination structures) where the

structure is part of the LPS.

Note: The majority of existing substations with an outdoor yard, the aerial line

termination structure is used for lightning protection of the substation.

• extension of substation buildings or outdoor yards

A review of the LPS and associated design and installation shall be conducted where a lightning

strike has resulted in damage to substation equipment. An updated LPS, associated design and

installation shall be completed by the operator and maintainer if the review determines a

deficiency in the lightning protection.

6.2. Design requirements The LPS for all new substation buildings and associated outdoor equipment areas shall be a

lightning protection level of class 1 in accordance with AS 1768 Lightning protection.

The Lead Electrical Engineer, ASA shall be consulted where the review of the LPS of an

existing substation is required and the lightning protection rating of the existing substation is

less than 1 (in accordance with AS 1768).

The rolling sphere method (RSM) as detailed in AS 1768 shall be used to assess the proposed

LPS.

The design report shall include the following:

• Written report with all assumptions, criteria, calculations, software simulation graphs and

plots, compliance and reference to standards. The report shall include all drawings

produced for the assessment and design and construction of the LPS.

• Drawings documenting the assessment of the LPS using the RSM. Drawings shall be to

scale with the substation building outline, outdoor yard and other substation equipment

locations (for example, transformer bays) shown with the proposed LPS. All major

equipment located outdoors, fences, lighting poles and aerial line structures shall be

shown. The spheres (or portion of) with relevant dimensions shall be shown to demonstrate

compliance.




• Design drawings to enable the construction and installation of the LPS. Drawings shall

include detailed parts list and comprehensive construction notes.

All drawings shall comply with the T MU MD 00006 ST Engineering Drawings and CAD

Requirements.

6.3. Installation requirements The correct installation of the LPS is critical to ensure it is effective and mitigates the hazards

that lightning poses to both infrastructure and human life.

The installation shall be in accordance with the approved design and any deviations shall be

pre-approved by the design AEO.

7. Aerial line lightning protection The design of aerial lines and associated structures is critical to ensure the reliability of the HV

distribution networks.

As a high degree of reliability is required, all new aerial lines that are 33 kV and above shall

have overhead earthing wires (OHEW) installed to provide shielding of the conductors from

lightning strikes.

See Section 7.1 for details regarding OHEW.

7.1. Overhead earth wire OHEWs shall be installed on RailCorp aerial lines as in T HR EL 10001 ST HV Aerial Line

Standards for Design and Construction.

The shielding angle of existing aerial lines is typically 30 degrees, however the shielding angle

for a specific aerial line shall be sourced from the design documentation.

An insulation coordination study shall be completed where a proposed modification to an

existing aerial line reduces the OHEW to less than 800 m or a new OHEW is less than 800 m is

proposed to be installed. The results of the study shall demonstrate that all connected

substations and equipment are suitably protected from lightning over-voltages. The study shall

take into account all operating modes (for example, circuit breakers, air break switches in both

open and closed positions).

The insulation coordination study shall be documented in a report with all assumptions, criteria,

calculations, software simulation graphs and plots, compliance and reference to standards. The

report shall include all drawings produced for the assessment and subsequent design and

construction recommendations.




7.2. Aerial line surge impedance The approximate surge impedance of RailCorp aerial lines is 500 Ω to 600 Ω. This means the

voltage generated on the aerial line due to a lightning strike will almost certainly result in a

flashover at the first pole (cross-arm will flashover) as the generated voltage (lightning current

times surge impedance) is greater than the typical insulation level at the cross-arm.

7.3. Lightning strike rate The RailCorp network covers a diverse geographical area. Certain areas such as the Blue

Mountains will have a different historical strike rate than other areas.

To ensure that the most accurate data is used for a certain area to calculate the annual number

of lightning strikes to ground the number of thunder days (TD) shall be sourced from the

following:

• the Bureau of Meteorology

• electricity distributors

• Sydney Trains database

• commercially available databases

7.4. Aerial line outage rates Shielded aerial line outage rates due to lightning shall be less than one outage per 100 km per

year for shielded aerial lines and two to eight outages per 100 km per year for unshielded lines

to ensure a high reliability of supply to critical services.

8. Aerial line characteristics and insulation levels Refer to T HR EL 10001 ST for details of the aerial lines installed in the RailCorp HV distribution

network.

The exact configuration of existing aerial lines shall be sourced from the design documentation

and the insulation levels shall then be determined by the design AEO.

9. Substation equipment insulation levels The insulation levels for substation equipment are standard values as prescribed by

IEC 60071-1 Insulation co-ordination – Part 1: Definitions, principles and rules and are detailed

in Table 1.




Table 1 – Equipment insulation levels

Nominal system voltage Rated short duration power frequency withstand voltage (kV rms)

LIWV (kV peak)

1500 V dc cable 15 (see note) 60 (See note)

1500 V dc metalclad switchgear

6.9 15

11 kV 28 95

33 kV 70 170

66 kV 140 325

132 kV 230 550

Note: 1500 V dc cable is nominally 3.8/6.6 kV rated. The rated short duration power

frequency withstand voltage and lightning impulse withstand voltage is the value as

prescribed in AS/NZS 1429.1 Electric cables – Polymeric insulated – For working

voltages 1.9/3.3 (3.6) kV up to and including 19/33 (36) kV.

At some substations with a nominal system voltage of 33 kV the LIWV of substation equipment

is 200 kV, however the lower value of 170 kV shall be used for the selection of surge arrestors.

10. HV ac surge arrester requirements The correct selection of HV ac surge arresters is critical to ensure that the arrester is suitable for

continuous operation and provides protection to equipment from overvoltage.

Section 10.1 to section 10.7 provide details on the requirements for surge arresters.

10.1. Type of surge arrester All surge arresters are required to be type approved, see Section 5 for type approval process.

Surge arresters shall meet the following requirements:

• be the gapless, metal oxide type

• be shatter proof

• comply with the requirements of IEC 60099-4 Surge arresters – Part 4: Metal-oxide surge

arresters without gaps for a.c. systems

10.2. Location of HV ac surge arresters HV ac surge arresters shall be installed in the following locations:

• at the termination structure where the aerial line interfaces with a substation outdoor

busbar

• on outdoor transformers with exposed terminals




• in the feeder panel of indoor 33 kV and 66 kV ac switchgear where the feeder is not cable

for its entire length

• at underground to overhead (UGOH) terminations

• at transformers connected to aerial lines, including pole top transformers

An AEO shall prepare a transient impulse study to demonstrate that surge arresters are not

required where installation of surge arrestors in indoor switchgear are not proposed. The

transient impulse study shall be submitted to the Lead Electrical Engineer, ASA.

10.3. Electrical parameters for arrester selection Section 10.3.1 to Section 10.3.6 provide details on the electrical parameters of surge arresters

that shall be determined and compared with the manufacturers surge arrester data sheet for

suitability in order to correctly specify an arrester to protect equipment and be operationally

reliable.

10.3.1. Maximum continuous operating voltage Maximum continuous operating voltage (MCOV) is the continuous operating voltage during

normal operating conditions with a maximum of 10% overvoltage (1.1 pu) to allow for variation

provided by supply authorities.

Table 2 details the MCOV for the individual HV ac networks in the RailCorp network.

Table 2 – MCOV

Nominal system volts MCOV (rms) applied to arrester (Ø-E)

11 kV 7.0 kV

33 kV 21.0 kV

66 kV 41.9 kV

132 kV 83.8 kV

10.3.2. Temporary overvoltage The system earthing configuration determines the temporary overvoltage (TOV) on a

non-faulted arrester during a phase to earth fault.

As the majority of RailCorp 33 kV and 66 kV systems originate from resistance earthed

supplies, these circuits shall be considered to be non-effectively earthed.

The RailCorp 11 kV system has a mix of solidly earthed neutrals and resistance earthed,

however to minimise system spares and to allow for the future conversion of a solidly earthed

system to resistance earthed, all 11 kV arresters shall be selected based on a non-effectively

earthed system for selecting the TOV.




The worst case is that non-faulted phase arresters are subjected to phase to phase volts for two

seconds maximum. The 2 seconds is the protection clearing time.

Table 3 details the TOV that shall be used for selecting surge arresters. The non-effectively

earthed values are the worst case and are based on an earth fault factor (EFF) of 1.73

The 11 kV solidly earthed has an EFF of 1.25 applied.

Table 3 - TOV for RailCorp lines.

Nominal system voltage TOV (rms), 2s duration

11 kV solidly earthed 8.7 kV

11 kV non-effectively earthed 12.1 kV




Where an appropriate surge arrester cannot be selected based on values in Table 3 then a

lower EFF can be determined for a specific location by calculation and interpreting the curves

detailed in Appendix A of IEC 60071-2:2018 Insulation coordination – Part 2: Application

guidelines. The reduced TOV can be used for surge arresters located at the specific location.

10.3.3. Switching impulse withstand voltage At voltage levels of 11 kV, 33 kV, 66 kV and 132 kV, the overvoltage caused by switching does

not have to be considered for insulation coordination as the standard short duration power

frequency or the standard LIWV covers this.

10.3.4. Arrester class Surge arresters for protection of substation equipment shall be a minimum substation class of

SL in accordance with IEC 60099-4 which was previously referred to as line discharge class 2.

10.3.5. Nominal discharge current The minimum nominal discharge current (8/20 µs) rating for all HV ac arresters installed in the

RailCorp distribution network is 10 kA.

10.3.6. Short circuit withstand capability The surge arrester short circuit rating (at 50 hz) shall be greater than the maximum fault current

for the location the surge arrester is installed and for a minimum duration of 200 ms. To

minimise system spares the maximum system fault current in accordance with Table 4 shall be

used for surge arrester selection.




Table 4 - Maximum system fault current

Network voltage Maximum system fault current (kA rms)

11 kV 16

33 kV 31.5

66 kV 16

132 kV 16

10.4. Pollution level The pollution level for surge arresters is defined in AS 4436 Guide for the selection of insulators

in respect of polluted conditions and is applicable to locations as detailed in Table 5.

Table 5 - Surge arrester pollution level

Pollution level Location

Light Not to be used.

Medium Applicable to the majority of the greater Sydney network except in the heavy pollution level area.

Heavy Applicable on the Illawarra line starting from Helensburgh and going south and locations that are subject to heavy pollution (for example, sea spray and industrial pollution).

10.5. Altitude All arresters shall be suitable for a minimum altitude of 1000 m except for arresters to be

installed west of Penrith on the Blue Mountains line which shall be suitable for a minimum

altitude of 1200 m.

The altitude correction factors as per IEC 60071-2 should be applied for altitudes greater than

1000 m.

10.6. Mechanical requirements The mechanical strength of the surge arrester shall be checked to ensure it is suitable for the

application.

10.7. Physical orientation The proposed installed orientation of the surge arrester shall be checked with the

manufacturers’ recommendation to ensure it is suitable.




11. Selection and specification of HV ac surge arresters The correct determination of electrical parameters and subsequent selection of a surge arrester

is critical to ensure the protection of equipment from damage due to a lightning strike.

11.1. Procedure for selecting surge arresters To determine whether an arrester provides protection for equipment the following shall be

checked:

• That the MCOV of the arrester is suitable. See Table 2

• That the TOV is suitable. See Table 3. The allowable TOV for a specific time of an arrester

is available from the manufacturer’s data sheet.

• The short circuit withstand capability is greater than the maximum system fault current. See

Table 4.

• The rated discharge current complies with this standard.

• The total residual voltage has the required margin of protection (MOP) to ensure protection

of the equipment.

• The non-electrical properties of the surge arrester are suitable for the application (altitude,

mechanical strength, orientation).

11.2. Margin of protection The MOP refers to the difference between the LIWV of the equipment and the total calculated

residual voltage of the protective device (surge arrester plus leads) as a ratio of the LIWV of the

equipment it is protecting.

The MOP shall be a minimum of 0.2 (20%).

11.3. Inductive voltage drop of arrester leads. The minimum surge arrester lead size shall be 70 mm2 copper for all surge arresters.

The length of the surge arrester leads is a critical parameter in determining if the selected surge

arrester will protect the equipment.

The inductive voltage drop of the surge arrester leads shall be calculated and added to the

surge arrester residual voltage. See Appendix A for an example of calculating the arrester lead

voltage drop.




12. 1500 V dc surge arrester requirements The correct selection of 1500 V dc surge arresters is critical to ensure that the arrester is

suitable for continuous operation and provides protection to equipment from overvoltage.

Section 12.1 to section 12.6 provide details on the requirements for surge arresters.

12.1. Type of surge arrester All surge arresters are required to be type approved, see Section 5 for type approval process.

1500 V dc surge arresters shall meet the following requirements:

• be the gapless, metal oxide type

• be shatter proof

• comply with the requirements of IEC 62848-1 Railway applications - DC surge arresters

and voltage limiting devices – Part 1: Metal-oxide surge arresters without gaps

12.2. Location of 1500 V dc surge arresters 1500 V dc surge arresters shall be fitted in the following locations:

• stand-alone 1500 V dc isolating and rail connecting switches

• at feeder termination plates located on 1500 V dc overhead wiring (OHW) feeding

structures (cable to OHW)

• on cable connected 1500 V dc sectioning switches

• on feeder panels of metal enclosed 1500 V dc switchgear unless a transient study

determines they are not required

• on 1500 V dc positive busbars in traction substations and sectioning huts

Surge arresters are connected between the 1500 V dc positive and earth.




12.3. Electrical parameters for 1500 V dc arrester selection Table 6 provides details of the electrical parameters for arrester selection which are defined in

IEC 62848-1.

Table 6 – 1500 V dc Surge arrester electrical parameters

Parameter Rating

Continuous operating voltage (Uc) ≥ 2000 V dc

Nominal discharge current (In) ≥ 10 kA (8/20 µs)

Arrester class DC-B

Short circuit rating (Is) ≥ 40 kA for 0.2 s

The continuous operating voltage of dc arresters are aligned with the 1500 V dc system voltage

as detailed in T HR EL 90002 ST Heavy Rail Traction System-Voltage Ratings.

The total residual voltage shall be calculated to ensure that the connected equipment is

protected.

12.4. Altitude and pollution level Surge arresters are required to be compliant with IEC 62848-1 which specifies:

• Altitude up to 1400 m which is suitable for the entire 1500 V dc traction system.

• Pollution level not exceeding PD4 in accordance with IEC 62497-1 Railway applications –

Insulation coordination – Part 1: Basic requirements – Clearances and creepage distances

for all electrical and electronic equipment for outdoor arresters which is suitable for the

entire 1500 V dc traction system.

12.5. Mechanical requirements The mechanical strength of the surge arrester shall be checked to ensure it is suitable for the

application.

12.6. Physical orientation The proposed installed orientation of the surge arrester shall be checked with the

manufacturers’ recommendation to ensure it is suitable.




Appendix A Surge arrester selection example This example, using the deterministic method, is for the selection of a 33 kV arrester to protect

equipment in a substation.

The surge arrester datasheet has the electrical parameters for the specific arrester and shall be

checked against the required 33 kV network parameters which are as follows:

• MCOV (see Table 2) ≥ 21 kV

• TOV (see Table 3) ≥ 36.3 kV for 2s

• Maximum system fault current (see Table 4) ≥ 31.5 kA

• MOP (see Section 11.2) ≥ 0.2

• Nominal discharge current (8/20 µs) ≥ 10 kA

• Arrester class ≥ SL (IEC 60099-4)

The non-electrical parameters also need to be checked for suitability.

Calculations:

Max allowable total residual voltage = 170 / (1+0.2) = 142 kV

Interpreting the manufacturers TOV graph (Figure 1) gives a factor = 1.36 for 2s for Uc

The minimum continuous operating voltage Uc = 36.3/1.36 = 26.7 kV

Therefore from Table 7:

• choose an arrester with (higher rating than 26.7 kV) continuous operating voltage = 27 kV

• corresponding residual voltage of arrester (1/…µs, 10 kA) = 90.4 kV

The arrester leads have an inductance of 1.25 µH/m (for 70 mm2 copper, stranded cable). A

waveform with 1 µs rise time, 10 kA peak corresponds to a lead voltage drop of:

V = l(length of lead) x L(inductance of lead) x di/dt = 1 x 1.25 x (10/1) = 12.5 kV/m.

Lead voltage drop for 2 m length = 2 x 12.5 = 25 kV

Total calculated residual voltage = 90.4 + 25 = 115 kV

Conclusions:

With 2 m of lead the arrester will protect the transformer as 115 < 142 kV.

The total arrester lead length can be (142 - 90.4) / 12.5 = 4.1 m, this will still comply with a 20%

MOP.




Figure 1 - Typical manufacturers’ TOV graph

Table 7 - Typical manufacturers’ voltage data

Rated voltage Ur (kV rms)

Continuous operating voltage Uc (kV rms)

Residual voltage (10 kA, 1/…µs) (kV rms)

Residual voltage (10 kA, 8/20µs) (kV rms)

31.3 25 83.8 76.8

32.5 26 87.1 79.9

33.8 27 90.4 82.9

35.0 28 93.8 86.0

36.3 29 97.2 89.1

37.5 30 100.4 92.1

Lightning Protection and Insulation Coordination

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