power quality in low voltage

Copper Development Association

Power Quality in Low Voltage Installations

Power Quality Partnership

David Chapman, CDA

Ken West, Fluke UK

David Bradley, Rhopoint Systems

Shri Karve, MGE UPS Systems


Programme

Introduction to the Power Quality Partnership & LPQI

David Chapman, CDA

Introduction to power Quality

David Chapman, CDA

Power Quality Measurement

Ken West, Fluke UK and David Bradley, Rhopoint Systems Ltd

Power Quality Improvement Techniques

Shri Karve, MGE UPS Sysytems

Earthing Issues in Power Quality

David Chapman, CDA


An Introduction to Power Quality

David Chapman, CDA


LPQI..a supply that is always available, always within voltage and frequency tolerance, with a pure, noise free, sinusoidal wave shape

IEEE..the concept of powering and grounding sensitive electronic equipment in a manner suitable for the equipment

Sankaran (modified)..a set of boundaries that allow electrical appliances and systems to function as intended without significant loss of lifetime or performance

What is Power Quality ?


Power Quality Issues

Electricity is a raw material but unlike any other

consumed at the instant of production

many miles from producer

transported over a shared (and vulnerable) network

no chance for ‘goods-in’ inspection

The epitome of ‘Just in Time’ - but without a

close customer/supplier relationship



USWSJ, 1992 $ 13 B/yrCEIDS, 2002 $ 119 - 188 B/yr

Eurelectric 2002Canada $ 650 mFrance $ 25 BGermany $ 20-25 BSpain $ 6-7 BWorld several 100 B$/yr

ECI 2000Europe € 15 - 20 B/yr



The Cost of (poor) Power Quality

Electricity supply chain

Loss of revenue for electricity not supplied

Loss of customer confidence - but so what?

Customer can only choose who sends the bill

- not who carries electricity to the plant




ConsumerProcess disruption

Glass production

Paper making

Data processing

Financial losses

Idle labour

Waste of raw materials

Value of work in progress destroyed




Consumer

Shortened equipment lifetime

e.g. Transformers

Unexpected early failure

Overloading of equipment

Potential catastrophic failure

Chain reaction


Power Quality IssuesCompatibility concept

Disturbance Level

Pro

bab

ility

Den

sity

Compatibility Level

Susceptibility of local equipment

Immunity (test) levels

Total supply network disturbance

Emission limits for individual sources



The Cost of Power Quality Solutions



Voltage stability

Harmonics

Resilience

Earth ing and EMC


Voltage stability

Tolerance (long term)

Unbalance (long or short term)

Disturbances (shorter term)

Dips and surges

Outages, Blackouts

Transients

Flicker


Voltage stability - tolerance

Standard EN50160

Supply voltage variations:

Under normal operation conditions, excluding situations arising from faults or voltage interruptions:

• during each one week period 95% of 10 minute average rms values of the supply voltage shall be within the range of Un 10%

• all 10 minute averages of the supply voltage shall be within the range of Un +10%/-15%


Voltage stability - Unbalance

Causes of unbalance:

Generators, Transformers

Unbalanced impedancelong, non-transposed low voltage lines

Unbalanced load currentssingle-phase loads on three-phase systems, e.g. railways

Embedded generation



Background:



Standard EN50160

Supply voltage unbalance

Under normal operating conditions, during each period of one week, 95% of the 10 minute mean rms values of the negative phase sequence component of the supply voltage shall be within the range 0 to 2% of the positive phase sequence component

In some areas with partly single phase or two phase connected customers’ installations, unbalances up to about 3% at three-phase supply terminals occur



Effects of unbalance:

Motors

reduced torque

bearing wear

excessive heating -> lower efficiency

Transformers

homopolar components cause excess heat in delta windings

negative sequence transformed normally



Effects of unbalance:

Equipment capacity

RMS current includes ‘useless’ negative sequence currents

additional losses in cables etc

affects protection settings

Electronic power converters

generate uncharacteristic harmonics - problem for passive filtering



Mitigation of unbalance

Careful distribution of loadsdon’t just put everything on the red phase!

Transpose lines on long routesespecially LV feeds to remote equipment (pumping stations)

Keep impedance lowdon’t convert current unbalance to voltage unbalance

Connect at highest possible voltage levellower current and lower impedance (eg: railways)

Connect heavy single phase loads via special transformers

Scott & Steinmetz transformers


Voltage stability - Dips

Voltage dips - nomenclature

e.g:

‘..a 120 ms dip to 32%..’


Voltage stability - dips

Standard EN50160

Supply voltage dips

Under normal operation conditions the expected

number of voltage dips may be from a few tens up

to one thousand in a year

The majority of voltage dips have a duration less than 1 second and depth less than 60%. (Retained voltage >40%)


Voltage stability - interruptions

Standard EN50160

Short interruptions (up to 3 min)

Under normal operation 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


Voltage stability - interruptions

Standard EN50160

Long interruption (> than 3 min)

Under normal operation conditions the annual frequency of voltage interruptions longer than 3

minutes may be less than 10 or up to 50 depending on the area

This does not include interruptions announced in advance


Voltage disturbance statistics

2890Total

146160Interruptions

7,121900Sags

6,7141640Swells

164190Transients

WorstAvgBestType of disturbance

Source: Dorr, US ° Canada 1992 - 1994


Voltage stability

Causes of Voltage Dips

Installation issues

starting of heavy loads

Distribution and transmission

Faults on the distribution network

line damage

bird strikes

vandalism

equipment failure

weather


Area of Vulnerability

Plant


Area of Vulnerability

Plant

Plant


Network faults - auto-reclosers

• Auto-reclosers reclose a circuit breaker a few hundred milliseconds after it has opened

• Often, the fault has been cleared by the fault current - tree branches burnt away for example - so the duration of the fault is minimised


• If the fault is still present, the re-closer may operate again perhaps up to 10 times

• This is a good thing - – for the customers on branch in question

fault duration is minimised

– for the supplier no call-out is needed



• But – customers see multiple dips

• The trade off is between power failures and dips

• Supplier’s interests favour dips!– Keep customer minutes lost to a minimum



Voltage stability

Responsibility

Network can never be fault free

Reducing number of dips would require enormous investment to reduce number and impact of faults

Relatively few customers (by number, not load) would benefit



The Cost of Power Quality Solutions



CBEMA Voltage tolerance curve



ITIC Voltage tolerance curve



ANSI Voltage tolerance curve


Voltage stability - DipsNetwork performance

ITIC

Network performance

Required immunity


Voltage stability - DipsEquipment performance PC

1998

2001

2002

19972000

1999


Voltage stabilityVSD - Dip in a single phase


Voltage stabilityVSD - dip in all three phases


Voltage stability - DipsAC Contactor construction


Voltage stability - DipsAC Contactor


Voltage stability - DipsMitigation of voltage dips

Small UPSCVTVoltage regulator

Large static UPSRotary UPS

Dynamic voltage restorer (DVR)Grid upgrade


Transients

• Transients are– high speed

• microseconds – large magnitude

• few hundred to several thousand volts


Transients

• Transients originate from– network switching

• capacitor compensation– load switching

• dynamic power factor correction• arc welding

– lightning - not direct strike


Transients

• Transients are high frequency signals

– magnitude reduces quickly as they travel

across network

• close transients important

• distant transients less important


Effects of Transients

• Transients can cause

– instantaneous equipment damage

– equipment degradation

– temporary loss of functionality

• communications blackout

• permanent data loss


Transients - mitigation

• Switching effects

– reduced by proper equipment design

• Lightning effects

– reduced by surge arrestors on network, incoming

services, etc.


Voltage stability - Flicker

Defined by Symptoms

variation in light intensity from tungsten filament lamps

defined in terms of perception

Causes

rapidly fluctuating cyclic loads

e.g. spot welders



Flicker mitigation

reduce supply source impedance for problem load

fast-acting power electronics solutions



Standard EN50160

Rapid voltage changes

Magnitude of rapid voltage changes

Under normal conditions a rapid voltage change generally will not exceed 5% Un, but a change of up to 10% Un with a short duration might occur several times per day

Flicker severity

Under normal operation conditions, in any period of one week the long term flicker severity caused by fluctuation should be Plt 1 for 95% of the time


Harmonic currents

Harmonic currents are caused by non-linear loads

Switched mode power supplies (SMPS)

Electronic fluorescent lighting ballasts

Variable speed drives

Un-interruptible power supplies (UPS)


Harmonic currentsTypical Switched mode power supply current waveform


Harmonic currentsHarmonic spectrum of SMPS current


Harmonic currentsHarmonic profile of a three phase 6-pulse load

Six pulse bridge - harmonic current

0

5

10

15

20

25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Harmonic number

%


Harmonic currentsEquivalent circuit of a non-linear load


Harmonic currents

Problems caused by harmonic currents:

currents within the installation overloading of neutrals

overheating of transformers

nuisance tripping of circuit breakers

over-stressing of power factor correction capacitors

skin effect

voltages within the installation voltage distortion & zero-crossing noise

overheating of induction motors

currents in the supply


Neutral conductor current


Harmonic currentsNeutral conductor sizing IEC standard 60364-52


Harmonic currents

Effect on transformers

Transformers supplying harmonic loads must be appropriately de-rated

Harmonic currents, being of higher frequency, cause increased magnetic losses in the core and increased eddy current and skin effect losses in the windings

Triple-n harmonic currents circulate in delta windings, increasing resistive losses, operating temperature and reducing effective load capacity


Harmonic currents

Harmonic currents in the supply

Harmonic currents flowing back into the supply cause harmonic voltages that spread around the network

Suppliers limit the level of harmonic current that a user can allow back onto the supply network

G5/4 covers these limits


Harmonic compatibility

Disturbance Level

Total supply network disturbance

Pro

bab

ility

Den

sity

Compatibility Level

Susceptibility of local equipment

Immunity (test) levels

Planning levels

Emission limits for individual sources


Harmonics

Standard EN50160

Harmonic voltage

Under normal operation conditions, during each period of one week, 95% of the 10 minute mean rms values of each individual harmonic voltage shall be less than or equal to the value given...

Moreover, the THD of the supply voltage (including all harmonics up to the order 40) shall be less than or equal to 8%.


Harmonics

The Challenge:

to keep harmonic currents below levels

that cause equipment overload and damage within the

installation

that are permitted by G5/4

to keep the harmonic voltage distortion at the point of common

coupling below levels permitted by G5/4


Harmonic standards

Electricity Association Engineering Recommendation G 5/4 (2001)

BS EN 61000

IEEE Std 519-1992 Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems

ISBN 1 - 55937 - 239 - 7


Harmonic mitigation

Steps to be taken to reduce voltage distortion on the

supply include, for example:

Passive harmonic filters

Isolation transformers

Active harmonic conditioners


Harmonic mitigation

Passive filters are useful when

the harmonic profile is well defined – such as motor controllers

the lowest harmonic is well above the fundamental frequency

- but filter design can be difficult and, especially for lower harmonics, the filters can be bulky and expensive


Harmonic mitigation


Harmonic mitigation

R S T

N

N

R S T

Delta - Zigzag transformers


Harmonic mitigation

Delta - Zig zag transformers

Load

Interconnected Star Transformer sized for

harmonic currents only

I3


Harmonic mitigationDelta-star transformers


Harmonic mitigation

Active conditioners

Where the harmonic profile is unpredictable or contains a high level of lower harmonics, active filters are useful

Active harmonic conditioners operate by injecting a compensating current to cancel the harmonic current


Harmonic mitigation

AHC

CT

Is + IhIs

Ih


Introduction to Reliability

• Reliability

• Availability

• Resilience

• Redundancy


Reliability

3 5 7 9 11 13 15 17 19 21 23 25 27

0.1

0.2

0.3

0.4

0.5

0.6

0.8

0.7

29 31 33 35 37

0.9

1.0

1.1

(t)

Failure Rate (t)

E lapsed Tim e (days)


Reliability

• Reliability is measured in terms of ‘Mean time to failure’ or MTTF.

timethatatfailuresofNo

componentsallfortimeoperatingTotalMTTF


What is Availability?

• Availability is the proportion time for which a system is serviceable. It is defined as:

MTTRMTBF

MTBFtyAvailabili

• In most circumstances it is the availability of the system that is of most concern


Availability

• Availability, expressed as a simple percentage, tells us nothing!

• e.g. Availability of 99.9% may mean that the system is unavailable for:

• ~9 hours every year• 1.5 minutes every day• 3.6 seconds every hour

• Which is least disruptive?


• Availability depends on both MTTF and MTTR

• MTTF is a function of original design and manufacturing quality, but MTTR can be controlled by the user by:

– Setting up on-site servicing and support contracts

– Holding key spares on-site or regionally

– Planning maintenance to reduce failures

Availability


• Very high availability is difficult to achieve

– Repair times cannot be further reduced:• engineers’ travel time• restart (or re-boot) time• process clean-up time

– Mean times to failure are statistics, not fact!

– Long MTTF figures are difficult to verify

Availability


What is Resilience?

• Resilience is the ability a system to withstand a failure in any individual component of the system

• It is achieved by a combination of:– high reliability– redundancy

• In effect, redundancy is used to improve availability


Redundancy

• In a redundant system key elements are duplicated so that, in the event of a single failure, the system as a whole can still operate

– Reliability - MTBF - is greatly increased

– MTTR is potentially reduced to zero

– Availability is potentially 100%

- if the redundancy is correctly chosen!


How redundancy improves reliability

• Overall reliability is:

1 - { [1-R1(t)] x [1 - R2(t)] }

If R1 and R2 are 0.90, the overall reliability is 0.99

- equivalent to increasing the MTBF by a factor of 10

Reliability R1(t)

Reliability R2(t)


System reliability

• System reliability can be calculated from the

known reliabilities of the individual components

• If the failure of any component causes the system

to fail, then, as far as reliability is concerned, the

components are in series otherwise they are in

parallel


• For example, if these two switches must be closed for a process to operate, they are in series as far as reliability of the process is concerned

System reliability

- but if either must be open to ensure safety, then they are in parallel as far as safety is concerned


Types of Redundancy

• In many cases, such as communications links or ventilation units, it is not necessary to provide 100% redundancy

• For example, installing three links each with 50% capacity provides high reliability for full traffic capacity and very high reliability for lower demand periods where one link would be sufficient


• In this example, each component is capable of supporting 100% of the demand

Types of Redundancy


• In this example, each component is capable of supporting only 50% of the demand

• Any two units must be functional at any time

Types of Redundancy


Objectives

• Identify and remove all single points of failure

• Balance reliability throughout the system - the system can be no more reliable than the least reliable link

• Prefer active redundancy

• Verify that the stand-by systems can be switched in as required


Power supply20 kV

End Use End Use

Couplingsystem

45 % Load

Couplingsystem

(Lot of power electronic with high harmonics level)

45 % load

1 250 kVA Transformers

Case study industrial glass plant


Power supply20 kV

(Lot of power electronic with high harmonics level)

End Use End Use

Couplingsystem

90 %Load

Case study industrial glass plant

First Event : Breakdown of one transformer due to over-voltage

Supply of all end used by one transformer

Second Event: Breakdown of the second transformer; over-heating due to harmonics


Curative solution

Replace 1 250 kVA transformers by 1600 kVA

transformers.

Cost : Cost of 2 transformers

Cost of emergency leasing of transformers

Cost of 3 days of no production

Total cost : 600 k EURO

Case study - Industrial glass plant


Preventive solution

Use 1 600 kVA transformers instead of 1 250 kVA

(at initial construction)

Extra cost of 1 600 kVA compared to 1 250 KVA

< 10 k EURO

Extra cost for two 1 600 kVA transformers

< 20 k EURO

Case study - Industrial glass plant


An Introduction to Power Quality

David Chapman, CDA

power quality in low voltage

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