Understanding Insulation Coordination 2014 2z6111

Alberta Power Industry Consortium & University of Alberta Professional Development Course

Understanding Insulation Coordination

Organized By Alberta Power Industry Consortium & University of Alberta AESO  AltaLink  ATCO  Enmax  Epcor  FortisAlberta

Instructed By Dr. David Peelo

May 13 & 15, 2014 Calgary & Edmonton, Alberta, Canada

Abstract Insulation coordination is fundamental to the design and operation of power systems. In turn, overvoltage limitation is an important part of insulation coordination. Means for overvoltage limitation have evolved from rod gaps through gapped type surge arresters to the metal oxide surge arresters of today. This course will discuss the basic principles of insulation coordination and will then cover arrester application and selection. The course is intended for APIC company staffs who are not specialized in insulation coordination but want to gain an adequate understanding on the basics of insulation coordination and surge arrester application. It is hoped that the knowledge will help them to better appreciate the challenges and requirements of insulation coordination and equipment protection, which, in turn, will facilitate the execution of engineering projects involving multiple technical subjects including insulation coordination for T&D equipment.

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Confidentiality Requirement This course material was prepared by the University of Alberta for the ultimate benefit of the Alberta Power Industry Consortium (hereinafter called “SPONSORS”). It may contain confidential research findings, trade secrets, proprietary materials (collectively called “Proprietary Information”). The term Proprietary Information includes, but is not limited to, plans, drawings, designs, specifications, new teaching materials, trade secrets, processes, systems, manufacturing techniques, model and mock-ups, and financial or cost data. The document is made available to the sponsors only. The Sponsors will use all reasonable efforts to treat and keep confidential, and cause its officers, , directors, employees, agents, contractors and students, if any, (“Representatives”) to treat and keep confidential, and Proprietary Information in the document and the document itself. This course material shall not be disclosed to any third party without the consent of the Alberta Power Industry Consortium.

Disclaimer This document may contain reports, guidelines, practices that are developed by the University of Alberta and the of the Alberta Power Industry Consortium (APIC). Neither the APIC , the University of Alberta, nor any of other person acting on his/her behalf makes any warranty or implied, or assumes any legal responsibility for the accuracy of any information or for the completeness or usefulness of any apparatus, product or process disclosed, or accept liability for the use, or damages resulting from the use, thereof. Neither do they represent that their use would not infringe upon privately owned rights. Furthermore, the APIC companies and the University of Alberta hereby disclaim any and all warranties, expressed or implied, including the warranties of merchantability and fitness for a particular purpose, whether arising by law, custom, or conduct, with respect to any of the information contained in this document. In no event shall the APIC companies and the University of Alberta be liable for incidental or consequential damages because of use or any information contained in this document. Any reference in this document to any specific commercial product, process or service by trade name, trademark, manufacture, or otherwise does not necessarily constitute or imply its endorsement or recommendation by the University of Alberta and/or the APIC companies.

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About the Alberta Power Industry Consortium: The Alberta Power Industry Consortium consists of six Alberta utility companies (AESO, AltaLink, ATCO, Enmax, Epcor and FortisAlberta) and the University of Alberta. Established in the fall of 2007, its goal is to bring Alberta power companies together, with the University of Alberta as the coordinating organization, to solve technical problems of common interest, to produce more power engineering graduates, to the professional development of current employees, and to promote technical cooperation and exchange in Alberta’s power utility community.

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About the instructor:

Dr. David Peelo, P.Eng., is an independent consultant. He graduated from University College Dublin in 1965 and worked first for the ASEA Power Transmission Products Division in Sweden. He ed BC Hydro in 1973 where he rose to the position of specialist engineer for switchgear and switching and took early retirement in 2001 to pursue a second career as a consultant. In 2004 the Eindhoven University of Technology awarded him a PhD for original research on current interruption using air-break disconnect switches. He has published more than 60 papers on switching and surge arrester application and is actively involved with IEEE, CIGRE and IEC. He is a past convener of IEC Maintenance Team 32 Inductive Load Switching and IEC Maintenance Team 42 Capacitive Current Interrupting Capability of Disconnectors and a member of the Canadian IEC National Committees for switching equipment and for surge arresters. He is the author of a textbook on current interruption transients calculation and a coauthor of a textbook on switching in power systems both due for publication in 2014.

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Course Outline 1. Basic principles of insulation coordination 

Overvoltages in power systems



The concept of insulation levels



The role of surge arresters



Simplified approach to insulation coordination

2. Power system overvoltages 

Origins of overvoltages: lightning, switching and temporary overvoltages



Characteristics of overvoltages in power systems



Traveling waves

3. Metal oxide surge arresters 

Evolution of overvoltage protection devices



The design of metal oxide surge arrester



Characteristics and applications of metal oxide surge arrester

4. Surge arrester standards 

Evolution of surge arrester standards



IEC versus IEEE standards



Which is the recommended standard to follow?

5. Arrester application in substations 

Types and characteristics of incoming surges



Distance effects



Arrester selection

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APIC 2014

Understanding Insulation Coordination David Peelo DF Peelo & Associates Ltd.

© DF Peelo & Associates 2014

APIC 2014

Basic principles of insulation coordination • • • •

Overvoltages in power systems Concept of insulation levels Role of surge arresters Simplified approach to insulation coordination


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What is insulation coordination? IEEE definition: The selection of insulation strength consistent with expected overvoltages to obtain an acceptable risk of failure IEC definition: The selection of dielectric strength of equipment in relation to voltages which can appear on systems for which the equipment is intended and taking into the service environment and the characteristics of available protective devices


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Types of overvoltages and their causes:

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Insulation types and strengths: • External insulation • Internal insulation • Self-restoring insulation • Non self-restoring insulation

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Internal insulation - transformers

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Insulation Coordination Course

Insulation strengths: • Atmospheric air: statistical based on overvoltage type and insulation terminal configuration • Liquids and gases: deterministic or statistical based on type of equipment

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Overvoltage limitation: • Air gaps: dependent on electrode configuration and type of overvoltage • Silicon carbide surge arresters: voltage limited rating basis • Metal oxide surge arresters: energy absorption limited rating basis

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Basic definitions: that we will encounter throughout the various part of the course

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Nominal voltage of a system: A suitable approximate value of voltage used to designate or identify a system: example 230 kV system. Highest voltage of a system: The highest value of operating voltage which occurs under normal operating conditions at any time and at any point in the system: 245 kV on 230 kV system. Highest voltage for equipment (Um): The highest rms value of phase-to-phase voltage for which the equipment is designed in respect of its insulation as well as other characteristics which relate to this voltage in the relevant equipment Standards

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Isolated neutral system: A system where the neutral point is not intentionally connected to earth (ground) Solidly earthed (grounded) neutral system: A system where the neutral point or points are directly connected to earth (ground) Impedance earthed (grounded) system: A system whose neutral points are earthed (grounded) through impedances to limit fault current

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Earth (ground) fault factor: The ratio of the highest power frequency voltage on an unfaulted phase during a line-to-earth (ground) fault to the phase-to-earth (ground) power frequency voltage without the fault Overvoltage: Voltage, between one phase and earth (ground) or between two phases, having a crest value exceeding the corresponding crest of the highest voltage of the system. Overvoltages may be classified by shape or duration as either temporary or transient

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Performance criterion: The basis on which the insulation is selected so as to reduce to an economically and operationally acceptable level the probability that the resulting voltage stresses imposed on the equipment will cause damage to the equipment insulation or affect continuity of service. This criterion is usually expressed in of an acceptable failure rate of the insulation configuration

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Examples of performance criteria Transmission lines: 2 lightning flashovers per 100 km-years exposure 1 switching surge flashover per 100 switching operations Substations: Generally, station reliability criteria is 10 times line criteria Also, transformers and other non-self restoring insulation equipment arrester protected due to failure consequences Air insulated stations: MTBF of 50 to 200 years Gas insulated stations: MTBF up to 800 years due to failure consequences 16

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Withstand voltage: The value of the test voltage to be applied under specified conditions during which a specified number of disruptive discharges may be tolerated: • conventional assumed withstand voltage: number of disruptive discharges tolerated is zero and corresponds to a probability of withstand Pw = 100% • statistical withstand voltage: number of disruptive discharges tolerated is related to a specified withstand probability and is specified at Pw = 90%

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Representative overvoltages (Urp): Overvoltages assumed to produce the same dielectric effect on the insulation as overvoltages of a given class occurring in service due to various origins Coordination withstand voltage (Ucw): For each class of voltage, the value of the withstand voltage of the insulation configuration, in actual service conditions, that meets the performance criterion

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Insulation coordination procedure

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Required withstand voltage (Urw): The test voltage that the insulation must withstand in a standard withstand test to ensure that the insulation will meet the performance criterion when subjected to a given class of overvoltages in actual service conditions and for the whole service duration Standard withstand voltage (Uw): The standard value of the test voltage applied in a standard withstand test. It is the rated value of the insulation and proves that the insulation complies with one or more required withstand voltages

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Coordination factor (Kc): The factor by which the value of the representative overvoltage must be multiplied in order to obtain the value of the coordination withstand voltage Atmospheric correction factor (Ka): The factor to be applied to the coordination withstand voltage to for the difference between the average atmospheric conditions in service and the standard reference atmospheric conditions. The factor applies only to external insulation

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Standard reference atmospheric conditions: The standard reference atmospheric conditions are: • temperature

to

= 20°C

• pressure

bo

= 101.3 kPA (1013 mbar)

• absolute humidity

hao = 11 g/m³

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Safety factor (Ks): The factor to be applied to the coordination withstand voltage, after application of the atmospheric correction factor (if required), to obtain the required withstand voltage, ing for all the differences between the conditions in service and those in the standard withstand test Test conversion factor (Kt): The factor to be applied to the required withstand voltage, in the case where the standard withstand voltage is selected of different shape, so as to obtain the lower limit of the standard withstand test voltage that can be assumed to prove it

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Insulation coordination procedure

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Insulation coordination simplified approach

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Source IEC 62271-1

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Source IEC 62271-1

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References: 1. IEC and IEEE insulation coordination standards noted earlier. 2. IEC 60099-4 Surge arresters – Part 4: Metal-oxide surge arresters without gaps for a.c. systems. 3. IEC 60099-5 Surge arresters – Part 5: Selection and application recommendations. 4. Insulation Coordination for Power Systems (Book), A.R. Hileman, Marcel Dekker, Inc 1999. 5. Insulation Coordination for High-Voltage Electric Power Systems (Book), Diesendorf, Butterworth & Co. 1974.

W.

6. Surge arrester manufacturer websites: ABB and Siemens in particular have comprehensive selection and application guides.

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Power system overvoltages

• • • •

Lightning, switching and temporary overvoltages Characteristics of overvoltages in power systems Travelling wave basics How do surge arresters work?

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Spectrum of power system overvoltages:

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Overvoltage class: transient

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Overvoltage class: low frequency continuous and temporary

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Temporary overvoltages: causes and characteristics 1.

Ferranti effect: Steady state voltage (V2) at the open receiving end of an uncompensated line (no shunt reactors) is always higher than the voltage (V1) at the sending end; occurs because the (leading) capacitive charging current flows through the series inductance of the line

V2 1 = V1 cos(BL ) where L is the line length in km and B the phase constant (7.2 degrees/100 km at 60 Hz) 34

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Ferranti effect

V1 and V2 sending and receiving end voltages. 1: no compensation 2: 50% series capacitor compensation 3: 50% series capacitor and 70% shunt reactor compensation.

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2. Faults: A line to ground fault represents a typical example of a temporary undamped overvoltage that may be sustained on the unfaulted phases for up to hundreds of milliseconds The magnitude of the overvoltages on the unfaulted phases depends on the shift of the electrical neutral caused by the fault – earth fault factors

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SLG EFFs:

Maximum line to ground voltage at any fault location and under any fault condition for effectively grounded system. Numbers on curves are maximum line to ground voltages on any phase in percent of line-to-line voltage.

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Earth fault TOVs: Category

EFF (pu)

Duration

Grounded • Network, high SC* • Line radial line, low SC

1.2 – 1.4 1.2 – 1.5

1s 1s

Resonant grounded • Network or mesh • Long radial line

1.73 1.2 – 1.5

8 hours – 2 days 1s

1.73

1–2s (with fault clearing)

Isolated/Ungrounded • Distribution with O/H lines • Industrial with cable SC – short circuit

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3.

Load rejection: Load rejection occurs when a remote circuit breaker on a transmission line carrying a substantial load is opened due to system condition or an error. A voltage rise follows because: • the reduced current means a lower voltage drop across the internal system impedance • generators tend to overspeed to produce higher voltages

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Load rejection overvoltages: Category Load rejection • in a system • generator-transformer − steam turbine − hydro

Magnitude

Duration (s)

1.05

> 10

1.1 – 1.4 1.15 – 1.5

1 1

Note: Load rejection may be combined with the occurrence of a fault producing a combined effect

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4.

Resonance: Resonance in power systems can take two forms: • linear resonance when inductive and capacitive elements in series form a series resonant circuit; for example, an unsaturated transformer and a shunt capacitor bank • ferroresonance when saturated iron-cored inductive and capacitive elements in series form a series circuit; for example, the grading capacitors on an open circuit breaker in series with a magnetic PT

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Overvoltages associated resonance situations can be very significant and rapid intervention is necessary Resonance

Magnitude (pu)

Duration (s)

<2

≤ 0.5

Saturated phenomena

2 – 3+

0.5 – 10

Coupled circuits

3 – 5+

0.5 – 10

Unsaturated phenomena

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5.

Other TOV situations: TOVs due to

Magnitude (pu)

Duration

Line energization and re-energization

≤ 1.5

<1s

Stuck breaker pole

≤2

Steady state

Backfeeding

≤2

Seconds

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Interesting facts about TOVs: 1. TOVs DO NOT CONTRIBUTE TO INSULATION DESIGN…HOWEVER 2. TOVs DETERMINE THE VOLTAGE RATINGS OF THE METAL OXIDE ARRESTERS TO BE APPLIED ON THE SYSTEM

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Slow front switching surges: • Line switching • Making and breaking reactive currents • TRVs across circuit breakers

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Line switching: Due to travelling wave effects, line switching can result in overvoltages of significant magnitude. Worst case is re-energization of a line with a full DC trapped charge The resulting switching surges are slow-front with times to peak in the order of hundreds of microseconds

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Switching surge overvoltage distribution with various control measures

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Voltage profile on line

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Typical switching overvoltage magnitudes: Condition Energizing discharged line Re-energizing with no overvoltage control Re-energizing with pre-insertion resistors Re-energizing with controlled closing

Magnitude (pu) 1.5 – 2.0 3 – 3.4 2 – 2.2 1.2 – 1.7 49


Making and breaking reactive currents: • Inductive current switching Æ transformer magnetizing current Æ shunt reactors at EHV, HV and MV • Capacitive current switching Æ shunt capacitor banks Æ lines and cables 50

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Fast front overvoltages: Fast front overvoltages are those due to lightning and have the dimensioning role at system voltages below EHV levels (at EHV levels switching surges have the dimensioning role) The overvoltages are characterized by fast rise times of 0.1 to 20 µs and associated in the range 5 to 200 kA.

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Very fast front overvoltages: Very fast front overvoltages are generally associated with gas insulated switchgear (GIS) applications and in particular with the live operation of disconnect switches The overvoltage are not dimensioning with respect to the insulation coordination of GIS; however, present standards for disconnect switches now incorporate very severe tests to demonstrate that live operation of the switches does not compromise the GIS insulation structure

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What is lightning? • Lightning is the breakdown of atmospheric air; in dry air at sea level breakdown voltage is 30 kV/cm but, at higher altitudes in a region filled with water droplets, it is about onethird this value • Lightning strikes are not instantaneous starting as ‘local’ events and progressing in steps to the other electrode which could be earth (ground) or another cloud • Lightning strikes are very statistical in of current magnitude, number of strokes per strike and striking point

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Global electric circuit

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Travelling wave basics: Fundamental circuit for travelling waves

Vi Zi Zt Vr Vt

= = = = =

incident voltage surge impedance of Vi surge impedance of Vt reflected voltage = αVi where α = (Zt – Zi)/(Zt + Zi) transmitted voltage = βVi where β = 2Zt/(Zt + Zi) 62

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Voltage waves: Surge Impedance

Vr Reflected Voltage

Vt Transmitted Voltage

No change

Zi = Zt

0

Vi

Open circuit

Zt >> Zi

Vi

2 Vi

Zt = 0.5 Zi

-0.33 Vi

0.67 Vi

Zt = 0

-Vi

0

Case

50% reduction Short circuit

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Surge current (directional): I = V/Z Surge Impedance

Ir Reflected Current

Inet (Zi side)

No change

Zi = Zt

0

Ii

Open circuit

Zt >> Zi

Ii

0

Zt = 0.5 Zi

-0.33 Ii

0.67 Ii

Zt = 0

-Ii

2 Ii

Case

50% reduction Short circuit

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CO operation on 100 km unloaded transmission line

Sending end

Receiving end

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We can now consider the principle of overvoltage surge protection using arresters:

V1

V2

Z1

Z2

I3 V2 = V3

V3

Z3

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We can write:

V2 = V3  2Z T   V1 =  Z + Z T   1

where Z T =

Z2Z3 Z2 + Z3

 1 (1 + Z 2 Z 3 )  V2 = 2V1Z 2    Z1 + Z 2 (1 + Z 2 Z 3 ) 68

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If no arrester, Z3 = ∞

V2 = V3 =

2V1Z 2 Z1 + Z 2

If Z2 also ∞ as at open-ended line or transformer

V2 = V3 = 2V1 (voltage doubling)

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We can also write:

I3 = V2 Z3 = V3 Z3  Z 2   = 2V1  + Z Z 2 3   

 Z Z   Z1 + 2 3  Z 2 + Z3  

If ideal arrester (short-circuit), Z3 = 0 and

I3 = 2V1 Z 1 70

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We now have the two extreme points on the circuit load line:  Z2  (2V1 − Z1 I3 ) V2 =   Z1 + Z 2 

2V1*Z2 Z1+Z2

V2

I3

2V1 Z1

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Add the arrester VI-characteristic: 2V1*Z2 Z1+Z2

 Z2  (2V1 − Z1 Iar ) Var =   Z1 + Z 2  V2 Var Arrester VI-characteristic

I3

Iar

2V1 Z1

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Load line for transformer case: Z2 = ∞ 2V1

Var = 2V1 − Z1 Iar

V2 Var Arrester VI-characteristic

I3

Iar

2V1 Z1

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Question: how does travelling wave theory treat the effect of a surge arrester in clamping the voltage to the protective level at its location?

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Metal oxide surge arresters

• Evolution of overvoltage protective devices • Design of metal oxide surge arresters • Characteristics and application of metal oxide surge arresters


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History of surge arresters: • 1898 – rod gap design • 1908 – electrolyte arrester with non-linear resistive element to limit follow current and enable arc interruption • 1930 – silicon carbide arresters with silicon carbide resistive elements in series with gaps • 1957 – silicon carbide arresters with current limiting gaps • 1976 – metal oxide surge arresters with extreme non-linear characteristics

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Structure of metal oxide surge arresters: • Consist simply of zinc oxide varistor disks of varying sizes connected in series • Zinc oxide varistors are ceramic semiconductor devices with very non-linear voltage-current characteristics and very good energy absorption capability


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Zinc oxide disks are about 90% zinc oxide (ZnO) by weight plus various other metal oxides to provide specific properties – for example: Æ bismuth + CaO, CoO, BaO, SrO, MnO: non-linearity of the VI characteristic Æ K2O: inhibits grain growth Æ Cr2O3: enhances thermal stability Æ Ga2O3: increases exponent (α) of VI characteristic Æ Sb2O3: grain grown enhancer © DF Peelo & Associates 2014

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Varistor microstructure key elements: • Zinc oxide grains • Bismuth-rich intergranular layer • Spinel grains


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Varistor microstructure:

Source: ABB

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VI characteristic: three conductive regions

P er unit peak rated voltage

2.5

2

Region 1

Region 2

Region 3 LIPL

1.5 SIPL Rated voltage

1

0.5

MCOV

25 C

150 C

Resistive leakage current

0 1E-05 0.0001 0.001

0.01

0.1

1

10

100

1000

10000 100000

Current (A)

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Region 1:

pre-breakdown region is the low current region associated with steady state operation; material resistivity temperature dependent with negative temperature coefficient

Region 2:

breakdown region is the highly non-linear region associated with TOVs and switching surges; very small temperature dependence and exponent α = 30 to 50

Region 3:

high current region is the region associated impulse currents > 1 kA due to lightning; non-linearity much less than in the breakdown region


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Current conduction: equivalent circuit

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Pre-breakdown:

in this region current conduction is determined by the high “resistance” associated with grain boundaries with a significant temperature dependence

Breakdown:

at a certain applied voltage across grain boundaries, the intergranular layer “resistance” drops allowing a large increase in current; energy is therefore being absorbed in the intergranular layer

High-current:

the zinc oxide grain resistance dominates in this region


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Comparison: metal oxide versus gapped silicon carbide arresters – fundamentally different: • Metal oxide arresters are rated on the basis of ability to absorb energy and maintain thermal stability at rated voltage followed by MCOV • Gapped silicon carbide arresters are rated on the basis of ability to reseal – interrupt follow current – after discharging a lightning or switching surge


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Active gaps

SiC resistors

Grading resistors and capacitors ZnO arrester SiC gapped arrester Courtesy of ABB 86

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Rated voltage and energy considerations: metal oxide arresters are energy (temperature) limited

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Effects of aging: over lifetime

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IEC 60099-4 specifies five (5) line classes: Specific energy (kJ/kV U r )

7 Line class 1

6

Line class 2

5

Line class 3 Line class 4

4

Line class 5

3 2 1 0 1

1.5

2

2.5

3

3.5

Ures/Ur

Former IEC rating basis now changed to a thermal energy rating and a repetitive charge transfer rating

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Former IEC rating basis now changed to a thermal energy rating and a repetitive charge transfer rating

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Thermal Energy Rating Wth • Maximum specified energy given in kJ/kV of Ur that may be injected into an arrester within 3 minutes time duration without causing thermal runaway. • This rating is verified in the revised operating duty test – above injection preceded by conditioning consisting of two high current impulses only. • The rating is strictly thermal and no longer relates rated energy to protective levels; temperature coefficient for metal oxide material 0.33 ⁰C/J/cm3.


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Repetitive Charge Transfer Rating Qrs • Maximum specified charge transfer capability of an arrester in the form of a single event or group of surges that may transferred through an arrester without causing mechanical failure or unacceptable electrical degradation to the MO resistors. • The charge is calculated as the absolute value of current integrated over time. This is the charge that is accumulated in a single event or a group of surges lasting for no more than 2 seconds and which may be followed by a subsequent event at a time interval not shorter than 60 seconds.

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TOV versus time curves: 1.3

With prior energy

TOV factor k tov

1.2

Without prior energy MCOV

1.1 1 0.9 0.8 0.7 0.1

1

10

100

1000

10000

Time (s)

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Overvoltages: types and shapes

Source: IEC 60071-1 95

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Energy absorption: • TOVs: system is a voltage source • lightning and switching surges: system is a current source • For energy absorption studies, the minimum VI characteristics should be used


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Overvoltages and associated energies: Energy kJ/kV rated voltage

Type/Cause

Magnitude

Waveshape

Atmospheric Overvoltages: Lightning strikes or induced by lightning (multiple strikes may occur)

> 5 pu

Front: 1 – 6 µs Tail @ 50%: 50 µs

0.5

Switching Overvoltages: Line autoreclosing, switching capacitor banks, shunt reactors, issue more at EHV

2 – 4 pu

Front: 30 – 300 µs Tail @ 50%: 100 – 2000 µs

3–5

1 – 1.5 pu

Power frequency (may be distorted)

Very high (fast remedial action required)

Temporary Overvoltages: SLG faults in ungrounded systems, Ferranti effect, loading shedding++

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Protective and related characteristics: 192 kV arrester 700

Voltage (kV peak)

600 LIPL 470 kV peak

500

SIPL 384 kV peak

400 300 Rated voltage 272 kV peak

200

MCOV 218 kV peak

100 0 1E-05 1E-04 0.001

0.01

0.1

1

10

100

1000 10000 1E+05

Current (A)

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Because we are dealing with a VI characteristic, the respective protective levels are defined at standard normal discharge currents. For lightning protection:

System voltage

Standard nominal discharge current* (8/20 µs)

Distribution Sub-transmission up to 72.5 kV

5 kA 5 or 10 kA

72.5 to 245 kV 245 kV and up ≥ 500 kV * values also define so-called Arrester Classification

10 kA 10 kA or 20 kA 20 kA


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For switching surges: Arrester classification 20 kA

500 and 2000

10 kA

250 and 1000

10 kA distribution class *

Peak currents* (A)

125 and 500

in switching impulse residual voltage test, higher numbers used to define the protective level


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Voltage (pu of 10 kA value)

VI characteristic: switching and lightning currents

1.5 1.4

4/10micros

1.3

8/20micros

1.2

36/90micros

1.1 1 0.9 0.8 0.7 0.6 1

10

100

1000

10000

100000

Current (A)

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Extract from ABB surge arrester catalog: Guaranteed protective characteristics Recommended for system voltage

kVrms

Rated voltage

kVrms

Max. cont. operating voltage (COV)

kVrms

Maximum residual voltage with current wave MCOV as per ANSI tests

kVrms

TOV capability for 1s kVrms

10 s kVrms

Switching surges

8/20 µs

1 kA kVcrest

2 kA kVcrest

3 kA kVcrest

5 kA kVcrest

10 kA kVcrest

20 kA kVcrest

40 kA kVcrest

EXLIM P-A and P-B 245

180

144

144

209

198

351

362

371

392

414

452

497

192

154

154

223

211

374

386

396

418

442

482

530

198

156

160

230

218

386

398

408

431

456

497

547

210

156

170

244

231

409

423

433

457

483

528

580

219

156

177

254

241

426

441

452

477

504

550

605

228

156

180

264

251

444

459

470

496

525

573

630

102

51

3/6/2014


Energy absorption: • TOVs: system is a voltage source • lightning and switching surges: system is a current source • For energy absorption studies, the minimum VI characteristics should be used

103


Selection procedure: • Electrical characteristics Æ Rated voltage and MCOV Æ Nominal discharge current Æ Thermal energy rating Wth Æ Repetitive charge transfer rating Qrs Æ Lightning and switching surge protection levels • Mechanical characteristics Æ Strike and creepage distances Æ Short-circuit withstand Æ Seismic and tensile loads 104

52

3/6/2014


Selecting the housing: briefly the following applies • Arresters are self-protecting and the housings do not require the same withstand capability as other station equipment • Creepage distances should be the same as for all station equipment • Pollution or exposure to salt contamination can require longer creepage distances but consideration should also be given to using higher rated voltage arresters or more appropriate arrester designs

105


• Short-circuit testing is required on all arrester types over a range of current; for example, an arrester with a rated short-circuit current rating of 50 kA would also be tested at 25 kA, 12 kA and 600 kA • Selection of the short-circuit current rating should be based on the maximum expected short-circuit current magnitude at the arrester location

106

53

3/6/2014


Insulation coordination – iterative process

107


Station layout considerations: • Location of the arrester relative to the protected equipment is important and will be discussed later • Location of the arrester relative to other energized equipment on the same phase also requires attention • Question: how does the protected equipment ‘know’ that the arrester is ahead of it?

108

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3/6/2014


Testing: • Type testing is covered in great detail in IEC 60099-4 as is basic routine testing • As noted earlier routine testing is at least as important as type testing; in making comparisons between arresters from different manufacturers, compare the tests performed on each block before considering award

109


Surge arrester standards

• Evolution of surge arrester standards • IEC versus IEEE standards • Which is the recommended standard to use

110

55

3/6/2014


IEEE and IEC differ in approaches: • IEEE:

rated voltage is the duty cycle rating which is a holdover from gapped silicon carbide arresters and has no relationship to absorbed energy

• IEC:

rated voltage is a TOV to be withstood following absorption of defined energy under specific circumstances

111


IEEE rating basis: from IEEE C62.11-2005

3.29 duty cycle voltage rating: The designated maximum permissible voltage between its terminals at which an arrester is designed to perform its duty cycle. 8.14 Duty-cycle test The purpose of the duty-cycle test is that the arrester can withstand multiple lightning type impulses without causing thermal instability or dielectric failure.

112

56

3/6/2014


113


114

57

3/6/2014


Former IEC rating basis now changed to a thermal energy absorption rating

115


Importance of routine testing: • Absolutely no redundancy in metal oxide surge arresters: one bad block and the arrester will fail – guaranteed! • Routine testing has therefore equal or arguably greater importance than type testing

116

58

3/6/2014

APIC 2014

Arrester application in stations

• Type and characteristic of incoming surges • Distance effects • Arrester selection


117


Stations: insulation coordination • Transmission lines are normally shielded except where the soil resistivity is such that it is not possible to achieve low tower footing impedances at a justifiable cost • Lines, however, are typically shielded about 1 km or more out from the station in order to limit the occurrence, magnitude and steepness of incoming surges • Two types of lightning related failures are of interest: shielding failures where a lightning strike occurs directly to the line conductor and back flashover (also known as a backflash) where the tower or ground wire is struck and a flashover then occurs to a phase conductor. 118

59

3/6/2014


119


Lightning strike to a phase conductor

Only close-in strikes are of interest because of the damping effect of corona on the traveling wave. 120

60

3/6/2014


Travelling wave damping due to corona

Conductors

Zo(ohms)

Kc (km-kV/us)

Single

450

700

2 Conductor Bundle

350

1000

3 or 4 Conductor Bundle

320

1700

6 or 8 Conductor Bundle

300

2500

121


Voltage Surge Distortion Due to Corona

1500000

0.5 km

1.0 km 2.0 km

1000000 V oltage (V )

3.0 km 4.0 km

500000

0

-500000 0 V@ 3.0 km Electrotek Concepts®

10

20 V@ 4.0 km

Time (us) V@ 0.5 km

30

40 V@ 1.0 km

V@ 2.0 km TOP, The Output Pro

122

61

3/6/2014


Back flashover

123


Lightning currents Shielding failure: 4 kA or greater at 89% probability: 20 kA or greater at 80% probability. Back flashover: 20 kA or greater at probability of 80%; 90 kA or greater at probability of 5% Median current: 32 kA

124

62

3/6/2014


Station insulation coordination: determine the representative incoming lightning overvoltage.

Two approaches are possible: 1. Comprehensive approach: requires a computer study because of the complexity of the calculations. 2. Deterministic approach: a more general approach but does take into the lightning stress severity of the station under study.

125


Comprehensive approach flow diagram:

126

63

3/6/2014


Comprehensive approach - need to consider: • Statistical (stochastic) variation of the lightning flash and stroke parameters • Dependence of the flash strike point on these parameters • Response of the line to the lightning flash including the tower footing impedance current dependence • Propagation of the surge from the strike point to the station and the deformation effect of corona

127


Station insulation coordination: deterministic approach 1. Determine the lightning performance of incoming transmission line based on parameters discussed earlier – ground flash density, line height etc. 2. Determine steepness of incoming surge based on the above and system fault tolerance. 3. Determine the protective zones based on selected arrester locations; may need a number of iterations and this type of study is usually done using EMTP simulations. 4. Transformer protection tends to get most attention but open breaker protection is also a consideration 128

64

3/6/2014


129


Insulation Coordination 101

130

65

3/6/2014


Basic case: arrester at transformer only CASE1>B-B1 (Type 1)

1000000

800000

Voltage (V)

600000

400000

200000

VB-B1 V-CB V-JTN VSA-1 VTX

0

-200000 0.03300

0.03400

0.03500

0.03600

Time (mS) 131


Entrance Bus Voltages : Normal, Wih Ccvt, With SA2 & Ccvt

1000000

800000

Voltage (V)

600000

400000

200000

0

-200000 33

35 Vb: normal

Electrotek Concepts®

37 Time (us) Vb: With Ccvt

39

41

Vb: With SA2 & Ccvt TOP, The Output Processor®

132

66

3/6/2014


Transformer Voltages: Normal, With Ccvt, With SA2 & CCVT

1000000

800000

Voltage (V)

600000

400000

200000

0

-200000 33

35


Vtxc: normal

37 Time (us) Vtx: With Ccvt

39

41

Vtx: With SA2 & Ccvt TOP, The Output Processor®

133


600000

Vsa : Normal, With Ccvt, With SA2 & Ccvt

Voltage (V)

400000

200000

0 33

35 Vsa: normal


37 Time (us) Vsa: With Ccvt

39

41

Vsa: With SA2 & Ccvt TOP, The Output Processor®

134

67

3/6/2014


10000

SA Currents: Normal, With Ccvt, With SA2 & Ccvt

Current (A)

8000

6000

4000

2000

0 33

35

Isa: normal Electrotek Concepts®

37 Time (us) Isa: Ccvt

39

41

Isa: SA2 & Ccvt TOP, The Output Processor®

135


Surge arrester application: • Arrester selection – recap • Arrester location relative to protected equipment • Clearances associated with arresters

136

68

3/6/2014


Arrester selection based on: • system voltage and grounding • temporary overvoltages • desired safety factor relative to the equipment withstand voltage but its physical location relative to the protected equipment is also a consideration (why?)

137


Simple estimation of protective distance:

Urp = Upl + 2

SL c

where Upl = lightning impulse protective level of the arrester (kV) S = steepness of incoming surge (kV/µs) L = d1 + dA + d2 + d (m) c = velocity of light (300 m/µs) 138

69

3/6/2014


Example: 245 kV transformer, LIWL 850 kV 192 kV rated surge arrester, Upl 440 kV, 1000 kV/µs

= d1 + dA + d2 + d = 2 + 2.5 + 2.6 + d = 7.1 + d

L

Urp =

2 ⋅ 1000 (7.1 + d) 440 + 300 = 440 + 6.67 (7.1 + d) 139


900 800 700

U rp (kV)

600 500 400

Voltage at transformer 850*0.85

300 200

850*0.6375

100 0 0

10

20

30

40

50

Distance d (m)

140

70

3/6/2014


Let’s take a look at what is really happening when a surge approaches a transformer protected at a certain distance by an arrester: •Incoming surge steepness 1000 kV/μs •Arrester with protective level of 800 kV installed at distance of 75 m from the transformer • the questions: how does traveling wave deal with arresters and their voltage limiting effect and how does the transformer know there is an arrester ahead of it?

141


Arrester at distance 75 m and surge 1000 kV/µs

142

71

3/6/2014

APIC 2014

EMTP study result same case


143


What conclusions can we draw?

Urp = Upl + 2ST

for Upl > or = 2ST

Urp = 2Upl

for Upl < 2ST

where T is the travel time of the lightning surge L/c. Actual voltage at the transformer oscillates due the ‘arrival’ and ‘departure’ of the traveling waves but the maximum value is Urp.

144

72

3/6/2014


Disadvantages of simplified method: 1. Does not take the capacitance of transformers or reactors into 2. Real VI characteristic of the arrester is not used 3. Does not take the effect of the power frequency voltage into

145


Switching surges within stations: • Are ‘man-made’ events and generally more an issue for lines than for stations; however, station equipment and clearances must be capable of withstanding the applicable rated switching surge withstand voltages and surge arresters must provide the corresponding overvoltage protection. Equipment standards fix a certain ratio between rated lightning and switching impulse voltages as we have seen earlier. • Surge arresters are applied to limit switching overvoltages within stations for the switching of shunt capacitor banks. This is a special case usually requiring a computer study to determine energy requirements in particular but some generalization is possible. Key parameters are discussed in the following.

146

73

3/6/2014


Clearances: two types of clearances to consider • Station air clearances • Surge arrester clearances Æ to ground Æ to energized equipment on same phase

147


Statistical nature of electrical breakdown

148

74

3/6/2014


Statistical nature of electrical breakdown (cont/d)

• Applied voltage must exceed critical value (field strength) where cumulative ionization is possible • Statistical time lag ts from application of voltage to the time of creation of the first free electron – time lag decreases with increasing voltage • Once electron found must further generate a streamer, then streamer converts to a spark – formative time lag tf

149


Statistical nature of electrical breakdown (cont/d)

• Vs must be high enough for ionization to occur and sustained for duration longer than the total time lag • Total time lag ts+tf not the same for each voltage application – same voltage waveform may or may not cause breakdown and therefore it is a probabilistic event • Question: what does this mean for lightning impulses and switching surges?

150

75

3/6/2014


Influencing factors on breakdown: • Waveshape: breakdown voltage dependent on impulse voltage and its profile over time • Gap configuration: more of an influence on switching impulses than lightning impulses • Polarity: breakdown voltage lowest for positive impulses

151


Influence of waveshape: lightning impulses: • Lightning impulses represented by a standard 1.2/50 µs waveshape • Duration of voltage around the peak does not give enough time for leaders to develop and breakdown is dependent on streamers only • Breakdown (aka sparkover) voltage Vs given by: Vs = Esd where Es is the electric field gradient and d is the gap length 152

76

3/6/2014


• Rod-plane gaps have the lowest breakdown voltages for all gap configurations; for positive standard lightning impulses for such gaps 1 to 10 m (IEC 60071-2): U50RP = 530d U50RP is the 50% probability of flashover voltage in kV crest d is the gap spacing in m

153


• For other gap configurations such as rod-rod, U50 can be as high as 700d (rod-plane and rod-rod gaps tend to represent two extremes) • For rod-plane gaps up to 6 m and negative standard lightning impulses: U50RP = 950d0.8 and can be as low as 700d0.8 for other gaps

154

77

3/6/2014

U 50 flashover value (kV crest)


8000 Rod-plane positive

7000

Rod-rod positive

6000

Rod-plane negative

5000 4000 3000 2000 1000 0 0

2

4

6

8

10

Gap length (m)

155


Important notes for lightning impulses: 1.

Gap factors (to be discussed for switching impulses) are generally not directly applicable

2.

Because positive impulses give the lowest breakdown values does not mean that negative impulses can be ignored; most lightning surges have negative polarity and internal insulation (e.g. in a transformer) has lower withstand for negative impulses

3.

Above-noted equations apply at sea level

156

78

3/6/2014


Influence of waveshape: switching impulses • Switching impulses represented by a standard rise time of 250 µs and a time to half-value of 2500 µs • Breakdown is dependent on gap configuration and hence the use of gap factor • Breakdown also dependent on voltage rise time giving socalled U-curves

157


U50 flashover value (kV crest)

Rod-plane gaps: K = 1 4000 3500 3000 2500 2000 E dF

1500

H a rbe c - M e ne m e niis

1000

IE C 6 0 0 7 1

500

M e e k - C ra ggs

0 0

5

10

15

20

25

Gap length (m) 158

79

3/6/2014


So what is the gap factor? The gap factor represents the influence of the electrode configuration

Configuration

Gap factor

Rod-plane

1

Conductor-plane

1.15

Vertical rod-rod

1.4*

* dependent on length of grounded rod (5 m in this case)

159


Gap factors for phase-to-phase configurations*: α = 0.5

α = 0.33

Ring-ring

1.8

1.7

Crossed conductors

1.65

1.53

Conductor-conductor

1.62

1.52

ed busbars (including fittings)

1.5

1.4

Configuration

* IEC 60071-2 160

80

3/6/2014


U50 flashover values (kV crest)

Lightning versus switching impulse: rod-plane and rodrod gaps: 2500 IEC 60071 Meek-Craggs

2000

IEC*1.4 Meek-Craggs*1.4

1500

Lightning

1000 500 0 0

1

2

3

4

5

Gap length (m) 161


U 5 0 fla s h o v e r v a lu e s ( k V c r e s t )

2500 IEC 60071 Meek-Craggs

2000

IEC*1.4 Meek-Craggs*1.4

1500

Lightning

1000 500 0 0

0.5

1

1.5

2

2.5

3

Gap length (m) 162

81

3/6/2014


Arrester minimum clearances phase to ground: • Because arresters are self-protecting lower minimum clearances than station minimum clearances can be used • Consult the appropriate manufacturer catalog or instruction manual for their recommended minimum clearances

163


Surge Arrester Manufacturer Recommended Minimum Clearances Phase-to-Ground Arrester rated voltage (kV rms)

Minimum clearance phase-to-ground (mm) for Mfr 1 Mfr 2 Mfr 3 Mfr 4 Mfr 5

12

92

115

216

127

92

15 21 24 60 66 72 120 132 144 192 216 258 396

118 168 168 448 499 549 829 905 981 1270 1473 1599 -

115 165 165 505 505 505 870 1140 1140 1345 1465 1925 2270

216 229 279 483 508 914 965 1168 1448 1854 2337 3530

147 190 216 407 457 508 813 889 991 1334 1611 1842 2921

122 154 182 416 458 500 832 916 998 1198 1292 1612 2475

164

82

3/6/2014


Surge arrester clearances to energized equipment on the same phase: • Energized equipment on the same phase includes transformer bushings (arrester on an outrigger), CVTs or PTs, post insulators and so on • Arrester voltage distribution determined by grading rings and proximity to other equipment on same phase really only an issue where pollution is a consideration; however what about tight fit scenarios?

165


Distribution system arrester practices: • On overhead systems arresters applied at the cable riser pole from substation and possibly on pole-top transformers in very lightning prone areas • On underground systems, arresters applied at the cable riser pole and at open points in the underground circuit; other measures may include paralleling arresters at the riser pole or applying a surge arrester at next pole with or without a groundwire

166

83

3/6/2014


TWENTY-ONE QUESTIONS 1.

What are the voltages considered in insulation coordination?

2.

Which overvoltage determines the rated voltage of the surge arrester to be used?

3.

Metal oxide material consists of zinc oxide plus various additives: which additive gives the non-linear VI characteristic?

167


4.

Which four points are used to define the VI characteristic of the required surge arrester?

5.

Which quantities define arrester energy handling capability?

6.

Which test defines the relationship between arrester rated voltage and rated thermal energy?

168

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3/6/2014


7.

What determines the withstand capability of the arrester housing?

8.

How is the required creepage distance for the arrester housing selected?

9.

How should the required pressure relief capability for the arrester be selected?

169


10.

With respect to lightning, what representative quantity is used for station insulation coordination?

11.

If the required protective levels are not met, what additional measures can be applied to resolve it?

12.

Why is there no distance effect for switching surges?

170

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3/6/2014


13. What determines the required clearances for surge arrester installation? 14. Why is the separation distance arrester to transformer so important? 15. What is the effect on added capacitance on incoming surges?

171


16. What are the characteristic waveshapes for lightning and switching surges? 17. What is a gap factor? 18. Which electrode configuration has the lowest switching surge withstand capability?

172

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3/6/2014


19.

Rated lightning withstand capability for a transformer is deterministic: what is the expected probability of withstand?

20.

Rated lightning withstand capability for a circuit breaker is probabilistic: what is the expected probability of failure?

21.

More questions?

173


Thanks for listening!

174

87

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