Notes Power Quality

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Universidad Pontificia Comillas. Doctoral Course on Analysis of the electricity quality of service.Power quality.: basics and definitions

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DOCTORAL COURSE ON ANALYSIS OF THE

ELECTRICITY QUALITY OF SERVICE

POWER QUALITY: BASICS AND DEFINITIONS

Author: Toms Gmez San Romn

November 2006


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UPCO

Universidad Pontificia de Comillas. C/ Alberto Aguilera, 23, 28015 Madrid (Espaa)

La propiedad intelectual de este material pertenece ala Universidad Pontificia de Comillas.

Reservados todos los derechos.

No est permitida la reproduccin total o parcial de

este manual, ni su tratamiento informtico, ni la

transmisin de ninguna forma o por cualquier medio,

ya sea electrnico, mecnico, por fotocopia, por

registro y otros mtodos, sin el permiso previo y por

escrito de los titulares del Copyright.


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CONTENTS

1 QUALITY OF SUPPLY ..............................................................................................4

2 POWER QUALITY .....................................................................................................4

3 VOLTAGE DISTURBANCE DEFINITIONS...............................................................8

4 VOLTAGE SUPPLY CHARACTERISTICS .............................................................11

4.1 Supply voltage variations................................................................................. 11

4.2 Rapid voltage changes...................................................................................... 12

4.3 Supply voltage dips ............................................................................................ 12

4.4 Short interruptions of the supply voltage..................................................... 13

4.5 Harmonic voltage ................................................................................................ 13

5 LIMITS TO DISTURBANCE EMISSIONS................................................................14

6 CONCLUSIONS.......................................................................................................17

7 REFERENCES.........................................................................................................17


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1 Quality of supply

Regulated utilities should strike an optimal balance between their investment and

operation and maintenance costs on the one hand and the quality of supply provided to

consumers on the other. In the event of electricity distributors, there is a clear relationship

between investment and maintenance costs and quality of supply. The higher the cost and

investment the better the service quality, and vice-versa.

From the standpoint of electric power supply, quality of service is characterized by

three different properties:

Continuity of supply, measured in number and duration of outages or supply

interruptions.

Quality of the product or voltage quality, power quality, measured in terms of

the disturbances affecting the ideal voltage wave parameters: variations in voltage

magnitude, periodic oscillations in voltage, harmonics, voltage dips, brief or micro-

outages (lasting less than 3 minutes) and so on.

Customer support, measured by indicators such as the time taken to process and

act on customer applications for service, time taken to respond to complaints about

poor quality, number of bills based on estimated - in lieu of actual - readings, and

so on.

The rest of this chapter addresses power quality issues in greater detail.

2 Power Quality

The other technical property of electricity associated with the quality of the power

supplied is known as power quality or quality of the product. The voltage wave delivered

by the distributor to each customer at their supply points should meet certain ideal

requirements, defined with respect to the following characteristics: magnituderelative to

the nominal supply voltage, for instance 230 V for residential consumers in Europe;


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frequency, 50 Hz in Europe; wave shape conformity to sine wave; and symmetry

between the three phases in the event of three-phase supply. These characteristics that

define the product are subject to disturbance which, if substantial, may cause the system

or the equipment connected to it to malfunction. The most common types of

electromagnetic disturbance appearing in electric power distribution grids include:

harmonics, periodic or non-periodic voltage oscillations, voltage dips and overvoltages.

The objective pursued with respect to power quality is known as electromagnetic

compatibi lity (EMC). EMC is defined to be the ability of a device, apparatus or system to

satisfactorily operate in its electromagnetic environment without causing intolerableelectromagnetic disturbance. In this regard, there are two sides to any device or system:

EMISSION: of voltage, currents or electromagnetic fields that may potentially

cause disturbance.

SUSCEPTIBILITY: to the adverse effects of electromagnetic disturbance.

The EMC levelfor each type of disturbance is defined as the level of disturbance at which

there is an acceptable probability of EMC. This level may be exceeded during only a small

percentage of the time (usually under 5%).

The following figure shows how, for a given type of disturbance, compatibility can be

achieved between the emissions from the devices and equipment causing the disturbance

(curve on the left of the figure) and the susceptibility levels that ensure proper system

operation (curve on the right of the figure), by setting the EMC level to standards

endorsed by international bodies, in particular the International Electrotechnical

Commission or IEC.


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Internationalbodiess

Standards definingwave quality

Distributors

Control of userswith disturbance

equipment

Equipment

Manufacturers

Product quality.Verification tests

Probabilityfunction

"Emision ceiling"for disturbanceequipment

"ECM level"

"Immunity ceiling"

Disturbancelevel

Level ofsystemdisturbance

Equipment

Immunity levels

Planning levels(Company in-house

quality targets)

IECCENELECUNIPEDE

Figure 1. EMC levels and emission and susceptibility limits for one type of disturbance.

The characteristics associated with power quality and disturbances are therefore

regulated at the following levels:

Co-ordination committees define the EMC levels laid down in international

standards.

Product committees establish equipment susceptibility requirements to be

met by manufacturers.

Distribution companies set emission limits and divide them among users

running electric facilities and appliances.


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Industrial users specify and design their facilities to comply with emission

and susceptibility limits.

Manufacturers of small electrical appliances for residential or commercial

use must meet standards that limit the grid disturbance caused by their

machinery.

In the European Union, CENELEC (European Committee for Electrotechnical

Standardisation) standard EN-50.160, compliance with which is mandatory in the MemberStates, seeks to standardise the definition of the electricity product. Among others, it

specifies disturbance ceilings at customers service connections. For most disturbances,

such ceilings generally concur with the EMC levels established by IEC standards. Due to

the random nature of disturbance, the compliance criterion is established in terms of

testing or measurement time, which is usually one week. The established ceilings may not

be exceeded during 95% of that time. One important exception to this rule has to do with

voltage dips: the standard contains mere recommendations on the acceptable frequency

for this sort of disturbance in grids.

IEC standard 61000-4-30, in turn, specifies the procedures and instruments required

to measure disturbance levels in grids and at points of supply. In general, the IEC 61000-

X-XX family of standards regulates the many different aspects of electromagnetic

compatibility.

From the regulatory standpoint, distributors are required to comply with these

standards and solve any problems arising within a certain time frame; otherwise, financial

penalties are imposed to prevent problems from becoming perennial. In another vein,

certain kinds of disturbance, such as flicker or harmonics, are directly caused by customer

equipment. Distributors may impose certain disturbance emission limits in such cases and

fine customers failing to honour these limits, or, if the problem persists, interrupt their

power supply.


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3 Voltage disturbance definitions

In this section, the voltage disturbances more relevant to characterize power quality

aspects are defined following the European Standard EN 50160.

voltage variation: increase or decrease of voltage normally due to load variations

rapid voltage change: single rapid variation of the r.m.s. value of a voltage between two

consecutive levels which aresustained for definite but unspecified durations

voltage fluctuation: series of voltage changes or a cyclic variation of the voltage

envelope

flicker: impression of unsteadiness of visual sensation induced by a light stimulus whose

luminance orspectral distribution fluctuates with time

NOTE Voltage fluctuation causes changes of the luminance of lamps which can create the visual

phenomenon called flicker. Above a certain threshold flicker becomes annoying. The annoyance

grows very rapidly with the amplitude of the fluctuation. At certain repetition rates even very small

amplitudes can be annoying.

flicker severity: intensity of flicker annoyance defined by the UIE-IEC flicker measuring

method and evaluatedby the following quantities:

short term severity (Pst) measured over a period of ten minutes;

long term severity (Plt) calculated from a sequence of 12 Pst-values over a two

hour interval, according to the following expression:

The following curve shows the level of short term flicker severity corresponding to a value

of Pst = 1. This value is considered as the limit of acceptable disturbance (compatibility

level).


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Figure 2- ECM level, Pst=1, in LV distribution networks (Standard IEC 61000-2-2).

supply voltage dip: sudden reduction of the supply voltage to a value between 90 % and

1 % of the declaredvoltage Uc followed by a voltage recovery after a short period of time.

Conventionally theduration of a voltage dip is between 10 ms and 1 min. The depth of a

voltage dip is defined as the difference between the minimum r.m.s. voltage during the

voltage dip and the declaredvoltage. Voltage changes which do not reduce the supply

voltage to less than 90 % of thedeclared voltage Uc are not considered to be dips

supply interruption: condition in which the voltage at the supply terminals is lower than 1

% of the declared voltage,Uc. A supply interruption can be classified as

prearranged, when network users are informed in advance, to allow the execution

of scheduled works on the distribution network, or


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accidental, caused by permanent or transient faults, mostly related to external

events, equipment failures or interference. An accidental interruption is classified

as:

o a long interruption (longer than three minutes);

o a short interruption (up to three minutes).

NOTE 1 The effect of a prearranged interruption can be minimized by the network users by taking

appropriate measures.

NOTE 2 Accidental supply interruptions are unpredictable, largely random events.

harmonic voltage: sinusoidal voltage with a frequency equal to an integer multiple of the

fundamental frequency ofthe supply voltage. Harmonic voltages can be evaluated

individually by their relative amplitude (Uh) related to the fundamental voltage U1,

where h is the order of the harmonic,

globally, for example by the total harmonic distortion factor THD, calculated using

the following expression:

NOTE Harmonics of the supply voltage are caused mainly by network users non-linear loads

connected to all voltage levels of the supply network. Harmonic currents flowing through the network

impedance give rise to harmonic voltages. Harmonic currents and network impedances and thus the

harmonic voltages at the supply terminals vary in time.


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4 Voltage supply characteristics

According to the European Standard EN 50.160, Distribution network operators

(DNO) should supply the voltage at the network users supply terminals in public low and

medium voltage electricity distribution networks, meeting the limits or values within which

the voltage characteristics can be expected to remain.

In this section, the admissible variation ranges for those voltage characteristics more

representative are set.

4.1 Supply voltage variations

For low-voltage networks, the voltage variation should not exceed 10 %.

Situations like those arising from faults or voltage interruptions, the circumstances of

which are beyond the reasonable control of the parties, are excluded.

Under normal operating conditions,

during each period of one week 95 % of the 10 min mean r.m.s. values of the

supply voltage shall be within the range of Un 10 %, and

all 10 min mean r.m.s. values of the supply voltage shall be within the range of Un

+ 10 % / - 15 %.

NOTE In cases of electricity supplies in remote areas with long lines or not connected to a large

interconnected network, the voltage could be outside the range of Un + 10 % / - 15 %. Network users

should be informed of the conditions.

For medium voltage networks, during each period of one week, 95 % of the 10 min mean

r.m.s. values of the supply voltage shall be within the range of Uc 10 %. Where the

declared supply voltage Uc is normally the nominal voltage Un of the distribution


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network. If by agreement between the DNO and the network user a voltage different from

the nominal voltage is applied to the terminal, then this voltage is the declared supply

voltage Uc.

4.2 Rapid voltage changes

Single rapid voltage change. A rapid voltage change of the supply voltage is mainly

caused either by load changes in network users installations or by switching in the

system.

Under normal operating conditions, in LV networks, a rapid voltage change generally

does not exceed 5 % Un but a change of up to 10 % Un with a short duration might occur

some times per day in some circumstances.

NOTE A negative voltage change resulting in a voltage less than 90 % Un is considered a supply

voltage dip.

Under normal operating conditions, in MV networks, rapid voltage changes generally do

not exceed 4 % Uc but changes of up to 6 % Uc with a short duration might occur some

times per day in some circumstances.

Flicker severity. Under normal operating conditions, in any period of one week the long

term flicker severity caused by voltage fluctuation should be Plt 1 for 95 % of the time.

NOTE Reaction to flicker is subjective and can vary depending on the perceived cause of the flicker

and the period over which it persists. In some cases Plt = 1 gives rise to annoyance, whereas in other

cases higher levels of Plt are found without annoyance.

4.3 Supply voltage dips

Voltage dips are generally caused by faults occurring in the network users installations or

in the public distribution network. They are unpredictable, largely random events. The

annual frequency varies greatly depending on the type of supply system and on the point

of observation. Moreover, the distribution over the year can be very irregular.


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Indicative values:

Under normal operating conditions the expected number of voltage dips in a year may be

from up to a few tens to up to one thousand. The majority of voltage dips have a duration

less than 1 s and a retained voltage greater than 40 %. However, voltage dips with greater

depth and duration can occur infrequently. In some areas voltage dips with a retained

voltage between 85 % and 90 % of Un can occur very frequently as a result of the

switching of loads in network users installations.

4.4 Short interruptions of the supply voltage

Indicative values:

Under normal operating conditions the annual occurrence of short interruptions of the

supply voltage ranges from up to a few tens to up to several hundreds. The duration of

approximately 70 % of the short interruptions may be less than one second.

NOTE In some documents short interruptions are considered as having durations not exceeding one

minute. But sometimes control schemes are applied which need operating times of up to three minutes

in order to avoid long voltage interruptions.

4.5 Harmonic voltage

Under normal operating conditions, during each period of one week, 95 % of the 10 min

mean r.m.s. values of each individual harmonic voltage shall be less than or equal to the

value given in Table 1. Resonances may cause higher voltages for an individual

harmonic.

Moreover, the THD of the supply voltage (including all harmonics up to the order 40) shall

be less than or equal to 8 %.

NOTE The limitation to order 40 is conventional.


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5 Limits to disturbance emissions

The aim of imposing limits to the disturbances emitted by apparatus, equipment andinstallations is to guarantee that the specified ECM levels are verified, therefore facilitating

that DNO can ensure the adequate voltage characteristics at the users network supply

terminals as it is required by EN 50160.

For apparatus and electrical devices connected to LV networks the emission limits are set

at individual basis. Manufacturers of such type of equipment must meet the specific

standard requirements. The IEC standards 61000-3-(2, 3, 4 y 5) establish the emission

limits for harmonics and flicker that can be emitted by domestic appliances and similar

electrical equipment with the following characteristics:

A.C. single-phase or three-phase supply

Connected to A.C. low voltage networks (single-phase Un 240 V, or three-

phase Un 415 V, f=50 Hz).

Heating and cooking appliances

Appliances motor driven or with magnetic motion


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Tools and other electrical hardware

TV and radio sets

In case of users with industrial installations or special electrical equipment connected to

public distribution networks, DNO apply guidelines in order to evaluate if the particular

user meet the emission limits requirements. These guidelines and recommendations

usually are elaborated by DSO as internal guides that are mainly based on international

IEC standards and reports. For instance, standards IEC 61000-3-6 and 7 set guidelines

and procedures to impose emission limits for harmonics and flicker to industrial

installations. These guidelines follow an evaluation process to evaluate the application for

connection of a new customer similar to the one shown in next Figure.


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Compute the

allowanceemission limit

< ?

CONNECTION

ACCEPTANCE

CORRECTIVEACTIONS TO

LIMIT EMISSIONS

AFTER CONNECTIONMeasurement

control to verifyactual emissions

YES

NO

ASSESSMENTOF THE IMPACT

ON VOLTAGECHARACTERISTICS

Compute the

disturbanceemission

EMC planning

levels

Criteria to shareglobal emission limits

among network users

Disturbance

equipmentdata

Supply

Networkdata

Figure 3. Evaluation of new connection applications of potential disturbance installations


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6 Conclusions

Regulation of power quality aims to ensure adequate EMC levels in electricity

networks together with apparatus, devices and equipment connected to them.

Power quality is defined by a set of voltage characteristics that should be within a

set of admissible range variations in normal operating conditions. The most relevant

voltage disturbances that affect voltage characteristics are voltage variations, voltage dips

and short supply interruptions, and harmonic voltages.

The European standard EN 50.160 establishes the voltage characteristics that

should be met by DNOs when supplying electricity to users connected to public LV and

MV distribution networks. For voltage variations, flicker, and harmonic voltages, the

standard sets maximum ranges under specific measurement conditions that should be

met by DNOs. However, for voltage dips and short interruptions the standard only

provides indicative values that in normal operating conditions can be found in supply

terminals, with no specification of admissible maximum ranges.

Distribution network operators (DNO) in order to control voltage characteristics at

supply connection terminals should check that disturbance emission limits are complied

by disturbance customers or installations. In case a particular installation emits higher

emissions than the allowance, the disturbance customer should implement corrective

actions, such as harmonic filters or dynamic voltage compensators, to decrease the

disturbance emission level before the connection to the network can be approved by the

DNO.

In case of electrical appliances, emission limits should be verified by manufacturers

in order to meet the requirements imposed by corresponding IEC standards before those

devices can be approved to be marketed and sold in the European Union market.

7 References

International Electrotechnical Commission. International Standard - IEC 61000-2-2,

"Electromagnetic compatibility (EMC)- Part 2: Environment - Section 2: Compatibility


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levels for low-frequency conducted disturbances and signalling in public low-voltage

power supply systems", 1990. (Also EN 61000-2-2, 2002)

International Electrotechnical Commission. Report IEC-CEI 868, "Flickermeter.

Functional and design specifications", 1986

EN 61000-4-15, "Electromagnetic compatibility (EMC)- Part 4-15: Testing and

measurement techniques Flickermeter Functional and design specifications ",

2003. (IEC 61000-4-15:1997 + A1:2003)

International Electrotechnical Commission. International Standard - CEI 61000-4-7,

"Electromagnetic compatibility (EMC)- Part 4: Testing and measurement techniques

Section 7: General guide on harmonics and interharmonics measurements and

instrumentation, for power supply systems and equipment connected thereto", 1991.

EN 61000-4-30, "Electromagnetic compatibility (EMC)- Part 4-30: Testing and

measurement techniques Power quality measurement methods (IEC 61000-4-

30:2003).

International Electrotechnical Commission. International Standard - IEC 61000-3-3,

"Electromagnetic compatibility - Part 3: Limits - Section 3: Limitation of voltage

fluctuations and flicker in low-voltage supply systems for equipment with rated current


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CENELEC, Norma Europea EN50160. November 94. Also as Spanish Standard UNE-

EN 50.160. (Version final draft prEN 50160, October 2006 will supersede EN

50160:1999)

Notes Power Quality

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