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## Understanding Harmonics

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Presentation Transcript

Agenda

- Introduction
- Definition of ‘Power Quality’
- Identification of power quality problems
- Harmonics – causes and effects
- Mitigation techniques
- Conclusion

The cost of poor power quality

- Cost of power quality problems to European industry & commerce is estimated at €10 billion per annum
- Expenditure on preventative measures is less than 5% of this

Source – Leonardo Power Quality InitiativeDefinition of Power Quality

- ‘A supply that is always available, always within voltage and frequency tolerance, with a pure, noise free, sinusoidal wave shape’

How good is good enough?

- No definitive answer – entirely dependant on compatibility of equipment and supply

Power standards

- Power standards are defined by the electricity regulator OFGEM
- Standard EN 50 160
- ‘Voltage characteristics of electricity supplied by public distribution systems’

Long term interruptions 10 to 50

Short term interruptions 30 to 1000

Dips 30 to 1000

Short-term over-voltage <1.5kV

Steady state voltage 230V +/- 10% for 95% of time

Voltage unbalance <2% for 95% of time

EN 50 160Total harmonic distortion </= 8% for 95% of time

Transient over-voltages Majority <6kV

Frequency 50Hz +/- 1% for 99.5% of time

Frequency 50Hz +/- 2% for 100% of time

EN 50 160Identification of problems

- Harmonic distortion
- Voltage sags (‘dips’, ‘brownouts’)
- Voltage swells (‘surges’)
- Outages (‘power cuts’, ‘blackouts’)
- Transient voltage surges (‘spikes’)
- Earthing (‘grounding’)
- Poor power factor

Definition

- Waveforms with frequencies that are multiples of the fundamental frequency (50Hz UK & Europe, 60Hz North America)

Waveforms - Fundamental

Fundamental Wave, 50Hz

Waveforms - Fundamental, 2nd and 3rd harmonic

Fundamental Wave, 50Hz

2nd Harmonic, 100Hz

3rd Harmonic, 150 Hz

All wave-shapes can be reduced to a sine wave plus harmonics

- Even a square wave
- Square wave equation

Causes of harmonics

- Harmonic currents are caused by the use of non-linear loads:
- Switched mode power supplies
- HF fluorescent ballasts
- Compact fluorescent lamps
- Inverters
- Variable frequency drives
- UPS systems

Effects of harmonics

- Erroneous operation of control systems
- Excessive heating in rotating machines
- Overloading of transformers
- Overloading of switchgear and cables
- Nuisance tripping of circuit breakers

Effects of harmonics

- Overloading of capacitors
- Damage to sensitive electronic equipment
- Excessive currents in neutral conductor

Effects of Triple-N harmonics

- Triple-N harmonics are odd multiples of 3 times fundamental frequency, i.e., 3rd, 9th, 15th etc.
- They are all in phase and sum in the neutral conductor
- Switched Mode Power Supplies (SMPS) produce a lot of 3rd harmonic - this is especially problematic in commercial buildings due to the vast number of computers, office equipment etc.

Effects of Triple-N harmonics

- A 3-phase star connected system with a balanced linear load has no current flowing in the neutral
- Where a lot of 3rd (or other triple-N) harmonics are present, neutral currents can be considerably in excess of phase currents
- This causes overheating of neutral conductors. Note these may only be 50% rated in older buildings
- Neutrals do not normally have over-current protection

Limits on Harmonic Distortion

- Harmonic currents flowing back to the supply cause harmonic voltage distortion due to the supply impedance
- Governed by Engineering Recommendation G5/4
- Title : ‘Limits for Harmonics in the U.K. Electricity Supply System’.
- Guidance ONLY

Mitigation measures

- Neutral up-sizing
- Passive filters
- Active harmonic conditioners
- Transformer based solutions

Neutral up-sizing

- All neutrals in the system, including switchgear etc., must be rated for the neutral current as well as phase currents
- A 4 or 5 core 3 phase cable is rated for current flowing in the phase conductors only. Current in the neutral can cause overheating of the cable
- Above 7th harmonic (350 Hz), skin effect should be considered
- Cables should be de-rated in accordance with IEC 60364-5-523 / BS 7671 (Appendix 4)

Passive filters

- Capacitor and reactor combination
- Tuned to specific frequency
- Requires higher voltage capacitors
- Designed for a fixed system requirement

Avoiding resonance with PFC capacitors

- Calculate the Resonant Frequency

Effect of adding reactors

Current flowing into supply in A

Series Reactor Tuned to the frequency shown below

Single Frequency Filter

Double Tuned Filter

2nd Order High Pass Filter

Filters|z|

f (Hz)

|z|

f (Hz)

|z|

f (Hz)

Harmonics In Practice

Sub-Station

When others add to your system

Sub-Station

Active harmonic conditioner

- Harmonic current compensation, 2nd to 25th
- Harmonic neutral current compensation
- Global or selective harmonic current compensation
- Site adjustable compensation parameters

AHC points of connection

INCOMING SUPPLY

SUB BOARD 1

SUB BOARD 2

AHC GLOBAL

DIS BOARD

AHC PARTIAL

DIS BOARD

AHC points of connection

INCOMING SUPPLY

SUB BOARD 1

SUB BOARD 2

AHC GLOBAL

DIS BOARD

AHC PARTIAL

AHC LOCAL

DIS BOARD

AHC advantages

- Continued guaranteed effective harmonic compensation
- Easy to use and install
- Auto configures
- NOT susceptible to harmonic overload
- Expandable
- Compatible with electric generators
- Connected anywhere

Transformer based solutions

- 3rd Harmonic rejection transformers
- Phase shifting transformers
- Isolation or harmonic suppression transformers

Conclusions

- As more electronic equipment is used in industry and commerce, harmonics have become a major power quality problem – more harmonics are generated, and more equipment is adversely affected by these harmonics
- A combination of good design practice and effective harmonic mitigation measures is required

Conclusions

- The power quality required will be dependant upon the equipment to be operated at any given location
- A holistic approach to power quality is required – one solution is unlikely to address all the problems – a combination of equipment will be required to achieve the quality required.

Power quality measurement

- Most power quality problems can be measured or monitored – if you suspect a problem, we can conduct a PQ survey to identify:
- Harmonic distortion
- Transient voltage disturbance
- Power factor
- Load survey
- Unbalance
- Flicker

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