Interruption indexes

This Technical Report provides guidance on how to calculate continuity of supply indices. These
recommended indices are more particularly given for European benchmarking of distribution network performance. For transmission network performance, more representative indices 2) may be used.
It presents
– an overview of practices in Europe on long and short interruptions,
– definition of physical interruptions in a harmonized way,
– philosophy and criteria for recommending indices,
– a suggested common approach to continuity indices.
The fact that the networks in different parts of any particular country will be subject to different conditions (e.g. weather and customer density) mean that it is not viable to apply common performance standards to all networks within any one country or any group of countries without
making these targets so weak that there is a good prospect of them being achieved in all areas. The
present situation where national regulators set performance targets within their own countries is widely regarded as being the most effective mechanism for achieving optimal socio-economic performance.
For these reasons this Technical Report does not provide common targets for the number and duration of interruptions that should not be exceeded.
This Technical Report is designed to be a first step towards benchmarking the interruption performance of European countries. Rules on the aggregation of interruptions, in particular short
interruptions, have not been considered in this Technical Report, however it is recognised that it might be necessary to describe aggregation rules in a second version of the Technical Report.

Unterbrechungsindizes

Indicateurs d'interruption

Prekinitveni indeksi

To tehnično poročilo podaja vodilo, kako izračunati neprekinjenost indeksov preskrbe. Priporočeni indeksi so natančneje navedeni za evropsko primerjalno analizo učinkovitosti distribucijskih omrežij. Za učinkovitost prenosnih omrežij se lahko uporabijo značilnejši indeksi.
Podaja
– pregled praks v Evropi glede dolgo- in kratkotrajnih prekinitev,
– definicijo fizične prekinitve na usklajen način,
– filozofijo in merila za priporočanje indeksov,
– priporočen skupni pristop za indekse neprekinjenosti.
Ker so omrežja v različnih delih določene države zavezana različnim razmeram (npr. vremenskim razmeram in gostoti odjemalcev), ni smiselno uporabiti skupnih standardov učinkovitosti za vsa omrežja v eni državi ali skupini držav, ne da bi ti cilji postali tako nizki, da bi bili najverjetneje doseženi v vseh območjih.
Trenutna situacija, ko nacionalni regulativni organi postavljajo cilje učinkovitosti v svojih državah, v splošnem velja za najučinkovitejši mehanizem za doseganje optimalne družbenoekonomske učinkovitosti.
Zato to tehnično poročilo ne podaja skupnih ciljev za število in trajanje prekinitev, ki ne bi smeli biti preseženi.
To tehnično poročilo je prvi korak k primerjalni analizi prekinitvene učinkovitosti v evropskih državah.
To tehnično poročilo ne obravnava pravil za združevanje prekinitev, še zlasti kratkotrajnih, priznava pa se, da bo morda treba opisati pravila združevanja v drugi različici tehničnega poročila.

General Information

Status
Published
Publication Date
15-Sep-2011
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
15-Sep-2011
Due Date
20-Nov-2011
Completion Date
16-Sep-2011

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SLOVENSKI STANDARD
SIST-TP CLC/TR 50555:2011
01-oktober-2011
Prekinitveni indeksi
Interruption indexes
Ta slovenski standard je istoveten z: CLC/TR 50555:2010
ICS:
27.010 Prenos energije in toplote na Energy and heat transfer
splošno engineering in general
29.240.01 2PUHåMD]DSUHQRVLQ Power transmission and
GLVWULEXFLMRHOHNWULþQHHQHUJLMH distribution networks in
QDVSORãQR general
SIST-TP CLC/TR 50555:2011 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST-TP CLC/TR 50555:2011
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SIST-TP CLC/TR 50555:2011
TECHNICAL REPORT
CLC/TR 50555
RAPPORT TECHNIQUE
May 2010
TECHNISCHER BERICHT
ICS 27.010;29.240.01
English version
Interruption indexes
Indicateurs d'interruption Unterbrechungsindizes
This Technical Report was approved by CENELEC on 2010-05-07.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus,

the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy,

Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia,

Spain, Sweden, Switzerland and the United Kingdom.
CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
Management Centre: Avenue Marnix 17, B - 1000 Brussels

© 2010 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.

Ref. No. CLC/TR 50555:2010 E
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Foreword

This Technical Report was prepared by Task Force 4, (Interruption definitions and continuity indices)

of Working Group 1 (Physical characteristics of electrical energy), of Technical Committee CENELEC

TC 8X, System aspects of electrical energy supply.

It was circulated for voting in accordance with the Internal Regulations, Part 2, Subclause 11.4.3.3

(simple majority).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. CEN and CENELEC shall not be held responsible for identifying any or all such patent

rights.
__________
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Contents

1 Introduction and scope ...................................................................................................................5

1.1 Introduction .............................................................................................................................5

1.2 Scope ......................................................................................................................................6

1.3 Continuity indices – Needs and applications ..........................................................................6

2 Terms and definitions .....................................................................................................................8

3 Continuity of supply – Interruptions .......................................................................................... 10

3.1 Background information on interruptions ............................................................................. 10

3.2 Interruptions as defined by EN 50160.................................................................................. 10

3.3 Interruption sources ............................................................................................................. 11

3.4 Classification of the interruptions ......................................................................................... 12

4 Explanation of continuity ............................................................................................................ 14

4.1 Long interruption continuity indices...................................................................................... 15

4.2 Short interruption continuity indices ..................................................................................... 18

4.3 Worst Served Customers ..................................................................................................... 19

4.4 Practices for recording short interruptions ........................................................................... 19

4.5 Rules for recording long interruptions .................................................................................. 20

4.6 Interruption indices in use across Europe ............................................................................ 24

5 Recommended continuity indices .............................................................................................. 29

5.1 Philosophy and criteria for recommending indices .............................................................. 29

5.2 Recommended continuity indices ........................................................................................ 29

6 Recommended data collection and aggregation ...................................................................... 30

6.1 General ................................................................................................................................ 30

6.2 Parameter uncertainty impact on continuity indices ............................................................ 31

6.3 Classification of reported indices ......................................................................................... 32

6.4 Example of continuity indices estimation ............................................................................. 32

7 Information helpful when comparing continuity indices ......................................................... 34

7.1 Important differences between countries and a single recommendation ............................ 34

7.2 Observation period for continuity indices ............................................................................. 35

8 Trends, new and future developments ...................................................................................... 36

8.1 Development / improvement of new tools (technology) ....................................................... 36

8.2 Recording data considerations ............................................................................................ 36

8.3 Development in telecommunication system s ....................................................................... 36

8.4 Smart grids and metering aspects ....................................................................................... 37

8.5 Simulation techniques .......................................................................................................... 37

8.6 Standardization considerations ............................................................................................ 37

8.7 Particular incidence in LV networks ..................................................................................... 37

8.8 Cause and effect considerations .......................................................................................... 37

Bibliography ......................................................................................................................................... 38

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Figures

Figure 1 – Network operator organizational levels and stakeholders ......................................................7

Figure 2 – Short interruption within a long interruption ......................................................................... 22

Figure 3 – Short interruption after a long interruption ........................................................................... 23

Figure 4 – Two consecutive short interruptions..................................................................................... 24

Figure 5 – Interruption scenarios ........................................................................................................... 33

Figure 6 – Risk performance versus risk exposure ............................................................................... 35

Tables

Table 1 – Indices for quantifying long interruptions used in European countries ................................. 25

Table 2 – Monitoring and indices for short and transient interruptions in European countries ............ 28

Table 3 – System characteristics recommended .................................................................................. 35

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1 Introduction and scope
1.1 Introduction

As a result of the liberalization of electricity markets, System Operators are being increasingly

encouraged to report the performance of their electric power systems to other parties, in particular the

network users and the national regulation authorities (NRA). While in the past, quality of supply was

generally considered as an implicit duty on System Operators, today quality objectives have become

more and more definite objectives agreed with the Regulator and/or part of the contracts negotiated

with the Network Users. Indeed a number of European Regulators have already defined, or planned to

define, quality of supply objectives (addressing continuity of supply and/or voltage quality) to be met

by electric distribution systems. In some countries, quality of supply objectives form part of the

incentive-based regulation.

Quality of supply limits can be seen as the outer envelope of performance for each quality of supply

parameter. Specific continuity of supply Indices are established by particular Regulators in order to

facilitate benchmarking the performance of the System Operators under their jurisdiction. The indices

allow System Operators to meet their obligation to routinely report continuity of supply performance. It

is important that the objectives are seen not only as achievable but also as being cost effective

considering the needs of all the network users.

As customers expect a high continuity of supply for a reasonable price, one of the roles of a System

Operator is to optimise the continuity performance of the electric system in a cost effective manner;

the role of the Regulator being to ensure that this is carried out in a correct way taking into account the

customers’ expectations and their willingness to pay. It needs to be recognized that historically the

electrical systems in different countries have been designed in different ways based on different

technological choices, commercial approaches or climatic conditions.

There is a great variety of reliability indices used within the different European countries. Each country

has its own indices, some are system orientated and others are customer oriented. Some countries

measure separately the frequency and the duration of interruptions, others combine them into a single

value. In addition, not all the countries use the same definitions for interruptions and their

classification. For all of these reasons it is currently very difficult to compare the continuity of supply

indices between countries.

Quality of electricity supply is a collective effect of all aspects of performance in the electricity supply. The quality of the

electricity supply includes as a prerequisite reliability of the electric power system, power quality and customer

relationships. For the purpose of this Technical Report the term continuity of supply is used for the availability of the

electricity.
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1.2 Scope

This Technical Report provides guidance on how to calculate continuity of supply indices. These

recommended indices are more particularly given for European benchmarking of distribution network

performance. For transmission network performance, more representative indices may be used.

It presents
– an overview of practices in Europe on long and short interruptions,
– definition of physical interruptions in a harmonized way,
– philosophy and criteria for recommending indices,
– a suggested common approach to continuity indices.

The fact that the networks in different parts of any particular country will be subject to different

conditions (e.g. weather and customer density) mean that it is not viable to apply common

performance standards to all networks within any one country or any group of countries without

making these targets so weak that there is a good prospect of them being achieved in all areas. The

present situation where national regulators set performance targets within their own countries is widely

regarded as being the most effective mechanism for achieving optimal socio-economic performance.

For these reasons this Technical Report does not provide common targets for the number and

duration of interruptions that should not be exceeded.

This Technical Report is designed to be a first step towards benchmarking the interruption

performance of European countries. Rules on the aggregation of interruptions, in particular short

interruptions, have not been considered in this Technical Report, however it is recognised that it might

be necessary to describe aggregation rules in a second version of the Technical Report.

1.3 Continuity indices – Needs and applications

Performance indices in general are important tools in decision making for transmission and distribution

system asset management. Such indices can be used to translate issues, which might be rather

vaguely expressed, into formalized parameters to be used in decision-making. As the reliability of the

power system is a key element in power system management, continuity indices are useful to

translate objectives such as
• to maximize power system reliability and

• to provide our customers with a supply that has the minimum number of interruptions.

into more formalized objectives and targets aimed to support asset management and stakeholder

communication.

In power system asset management, decisions must be taken at different organizational levels within

companies. Figure 1 illustrates the main decision levels as well as the most important stakeholders

that may influence decisions at different levels.

For example, "Average Interruption Time" is commonly used by TSO (AIT = T x ENS / E ).

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Figure 1 – Network operator organizational levels and stakeholders

The stakeholders set the general requirements, for the business, that are further translated into

internal business values and criteria. For example, compliance with the rules and regulations

concerning continuity of supply are strategic criteria important for the utilities, authorities and

regulators. Obtaining a sufficient profit or return on the assets is an important criterion for company

owners while low cost and high reliability of supply are requirements of the customers.

In order to be able to take into consideration all continuity aspects from different stakeholder

perspectives, continuity indices are needed.
Continuity indices have several applications:
a) they might be used to measure overall developments and trends;

b) they might be used in benchmarking to identify best practices and learn from others;

c) they might be included in planning objectives and/or planning restrictions;
d) they might be used in contractual arrangements;
e) they might be used by regulatory authorities;
f) they might be used in stakeholder communication.

The list above is rather general and covers company levels, national issues and international issues.

As this Technical Report focuses on the international level (European issues), the main applications

and criteria for recommending indices are given in Clause 5.
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2 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
2.1
high voltage (HV)
voltage whose nominal r.m.s. value is 36 kV < U ≤ 150 kV
[EN 50160:2010]

NOTE Because of existing network structures, in some countries the boundary between MV and HV can be different.

2.2
interruption threshold

voltage magnitude specified for the purpose of detecting the start and the end of a voltage interruption

[EN 61000-4-30]
2.3
low voltage (LV)
voltage whose nominal r.m.s. value is U ≤ 1 kV
[EN 50160:2010]
2.4
medium voltage (MV)
voltage whose nominal r.m.s. value is 1 kV < U ≤ 36 kV

NOTE Because of existing network structures, in some countries the boundary between MV and HV can be different.

[EN 50160:2010]
2.5
network operator

party responsible for operating, ensuring the maintenance of, and if necessary developing, the supply

network in a given area and responsible for ensuring the long term ability of the network to meet

reasonable demands for electricity supply
[EN 50160:2010]
2.6
network user
party being supplied by or supplying to an electricity supply network

NOTE 1 In several countries, the term network user includes network operators connected to a supply network with the same

or higher voltage level.
[EN 50160:2010]

NOTE 2 For the purposes of this Technical Report, the terms Network User and Customer are considered to have the same

meaning and are therefore interchangeable.
2.7
normal operating condition

operating condition for an electricity network, where load and generation demands are met, system-

switching operations are made and faults are cleared by automatic protection systems, and in the

absence of exceptional circumstances, i.e.:
• temporary supply arrangement;

• in the case of non-compliance of a network user's installation or equipment with the relevant

standards or with the technical requirements for connection;
• exceptional situations such as
– exceptional weather conditions and other natural disasters,
– third party interference,
– acts by public authorities,
– industrial actions (subject to legal requirements),
– force majeure,
– power shortages resulting from external events
[EN 50160:2010]
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2.8
planned outage
outage scheduled in advance, for maintenance or other purposes
[IEV 191-24-01]
2.9

reference voltage (for interruptions, voltage dips and voltage swells evaluation)

value specified as the base on which residual voltage, thresholds and other values are expressed in

per unit or percentage terms

NOTE For the purpose of this document, the reference voltage is the nominal or declared voltage of the supply system.

[EN 50160:2010]
2.10
residual voltage (U )
res
minimum value of U or U recorded during a voltage dip or interruption
rms(1/2) rms(1)

NOTE 1 The residual voltage is expressed as a value in volts, or as percentage or pu value of U . U is used for Class A.

din rms(1/2)

Either Urms(1/2) or Urms(1) may be used for Class S. See EN 61000-4-30:2009, 5.4.1.

[EN 61000-4-30]
NOTE 2 The terms U and referent voltage are considered to have the same meaning.
din
2.11
r.m.s. value

square root of the mean of the squares of the instantaneous values of a quantity taken over a

specified time interval
[EN 61000-4-30]
2.12
SCADA
Supervisory Control and Data Acquisition
2.13
supply interruption

condition in which the voltage at the supply terminals is lower than 5 % of the reference voltage. A

supply interruption can be classified as
• notified, when network users are informed in advance, or

• non-notified, caused by permanent or transient faults, mostly related to external events,

equipment failures or interference. An accidental interruption is classified as:
– a long interruption (longer than 3 min);
– a short interruption (up to and including 3 min)

NOTE 1 Normally, interruptions are caused by the operation of switches or protective devices.

NOTE 2 The effect of a notified interruption can be minimized by network users by taking appropriate measures.

NOTE 3 Notified interruptions are typically due to the execution of scheduled works on the electricity network.

NOTE 4 Non-notified supply interruptions are unpredictable, largely random events.

NOTE 5 For polyphase systems, an interruption occurs when the voltage falls below 5 % of the reference voltage on all

phases (otherwise, it is considered to be a dip).

NOTE 6 In some countries, the term Very Short Interruptions (VSI) or transitory interruptions are used to classify interruptions

with duration shorter than 1 s to 5 s. Such interruptions are related to automatic reclosing device operation.

[EN 50160:2010]
2.14
unplanned outage
outage that is not a planned outage
[IEV 191-24-02]
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3 Continuity of supply – Interruptions

An interruption occurs when the electricity supply is not available to one or more customers.

An interruption can be the result of system incidents caused by a wide variety of possible events, such

as humans, animals, weather, insulator degradation and other natural phenomena or can be caused

by the operation of switches to allow the execution of works on the distribution network.

More commonly interruptions are caused by the operation of switches or protection devices (circuit

breakers, fuses etc). In the absence of local generation that can provide an alternative supply to a

customer’s installation, the voltage at the customer’s supply terminals will rapidly drop to zero. From a

customer perspective, the main indication of a power interruption is that the lights go out and that

other electrical equipment stops functioning. From a network operator perspective, an interruption is

indicated by alarm signals from control centres (circuit breaker opening, low voltage alarms etc), by

the knowledge of planned disconnection operations or by notification from customers that are off

supply.
3)
3.1 Background information on interruptions

When considering the continuity of supply of an electric power system, it is very important to consider

the difference between “component outages” (outages) and “supply interruptions” (interruptions). An

interruption is a situation when a customer is without electricity. An outage is a situation when a

component in the power network (e.g. a cable or a transformer) is disconnected from the rest of the

network, either manually or as a result of a protection operation.

Supply interruptions are very often caused by component outages. However not all component

outages result in supply interruptions. The start of an interruption is typically due to the start of an

outage (a “component failure”). The end of an interruption may be due to a switching operation or due

to the end of a component outage (component restoration, repair or replacement).

An outage that results in an interruption for one or more customers can have its origin at various

locations in the network. The majority of customers are connected to the low-voltage network, but a

substantial part of the interruptions experienced by low-voltage customers is due to outages that occur

at higher voltage levels. For most low-voltage and medium-voltage customers, the majority of

interruptions are due to outages that occur at the medium-voltage level.

In radial networks (typically at low or medium voltage at remote locations) there is only one supply

path to the customers. The outage of a component will immediately result in an interruption and the

interruption will only end when the component is restored, or when an alternative supply is provided,

e.g. from another part of the network or from generation. In the first case the interruption exactly

corresponds to the outage. The duration of the interruption is equal to the restore, repair or

replacement time of the outaged component. In the case of an alternative supply, the duration is equal

to the time for first notification to the time when the alternative supply was provided. Those low-voltage

and medium-voltage networks where redundancy exists are sometimes referred to as “radically

operated meshed networks”. The start of an interruption corresponds with the start of an outage, but

the interruption can be ended (electricity restored) through a switching action (“back feeding”). This is

referred to as “redundancy through switching”.

In transmission networks and in important medium-voltage networks, the alternative path not only

exists but is also used during routine operation, i.e. electricity is supplied via parallel circuits (paths). In

the event of an outage one path is disconnected, but there is no interruption because supply is

maintained via the remaining healthy path. The customers will not experience any interruption. This is

referred to as “redundancy through parallel operation”.
3.2 Interruptions as defined by EN 50160

EN 50160 takes into account the characteristics of protection and automatic reclosing systems in use

in supply networks.
Source: 4 CEER Benchmarking Report on Quality of Electricity Supply [7].
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On medium voltage overhead networks it is common practice to perform an automatic reclosure after

the initial tripping of a line circuit breaker on a fault. The time delay between tripping and reclosing

depends on local conditions. The time delay between the circuit breaker tripping and reclosing is often

referred to as the “dead-time”, dead-times can vary between a minimum of 0,5 s up to about 180 s.

For lines controlled by circuit breakers set with multiple reclosures, the customers supplied by the line

will experience another short interruption followed by restoration of supply or a final tripping of the

circuit breaker, depending on whether the fault has cleared spontaneously or persists. Where multiple

reclosures are not used the circuit breaker will trip and “lock-out” i.e. not reclose, under these

conditions the customers supplied by the line will suffer a long interruption until the fault is cleared and

the circuit breaker is reclosed.

In this latter case, the supply will be interrupted until the fault is located and the faulted section of line

is isolated for inspection and repair. If an alternative source of supply is provided, this can be brought

into service either by manual or automatic switching. For networks with automatic switching, the delay

is commonly ranging between 30 s and 3 min.
3.3 Interruption sources

At present in all countries, most electrical power is generated at large power plants and stepped up to

high voltage for long distance transmission. At substations, power is stepped down to medium voltage

for shorter distance distribution.

For most consumers of electricity, whether industrial, commercial or residential, the power is

converted to its final, usable voltage by transformers at or very near to the consumer's location.

Taken together, the entire system of generating plants, transmission and distribution networks can be

seen as a “grid” that interconnects the electrical supplies.

As a whole, the “grid” is very reliable and resilient and can continue to operate in the presence of

individual component failures, even where these are very large generating plants or substations.

However, the networks are still susceptible to events that could lead to interruptions, for example:

• components’ failures caused by
– lightning strikes,
– animals coming into contact with power lines,

– wind or ice can force tree limbs onto power lines and occasionally blow down power lines,

– equipment malfunction,
– equipment aging degradation,
– overload
– bushfires,
– insulator damage due to acts of vandalism
– damage to underground power lines by contractors,
– substation transformer fires or explosions,
– result of outages on customer owned facilities;
• loss of supply from another network operator;

• operation of switches to allow the execution of works on the distribution network or the result of

some request of a third party or a public authority.

In addition to these very common and local causes, there can be other wide area power failures

caused by significant disturbances to the Transmission Network where lots of people lose supply for

long periods of time. These long interruptions are sometimes referred to as “blackouts”. Although,

these events are low probability, they have been experienced in a number of developed countries.

Storms can exaggerate local vulnerabilities by disabling numerous electricity lines at the same time

and reduce repair capacity.
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Because of the complexity of the electricity system the network operator needs the best available

information to identify “what-where-when and why an interruption has happened“, in order to have the

best description of the state of the distribution network and its performance.
3.4 Classification of the interruptions
Interruptions ca
...

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