Harmonics – Commercial Electrical Power Impediments

Harmonics – Non Linear Loads

Harmonics are currents, usually in multiples of the supply fundamental frequency, produced by ‘non-linear’ loads such as the AC to DC power conversion circuits.
For example, on a 50Hz supply, the 5th harmonic is 250 Hz, 7th harmonic is 350 Hz, etc. These are called ‘integer harmonics’ – i.e. exact multiples of the supply frequency. These power conversion circuits draw only short pulses of current from the supply network and combine with the source impedance resulting in distortion of the supply voltage.

Harmonic effects

The main effects of voltage and current harmonics within the power system are:-

  • Amplification of harmonic levels resulting from series and parallel resonance;
  • Reduction of efficiency of power generation, transmission, and utilization;
  • Aging of the installation of electrical plant components and as a consequence the shortening of their useful life;
  • Plant Mal-operation;
  • Malfunctioning and failure of electronic equipment;
  • Overheating and failure of electric motors;
  • Overloading, overheating and failure of power factor correction capacitors. Resonance due to interaction of capacitors with harmonics;
  • Overloading and overheating of distribution transformers and neutral conductors;
  • Excessive measurement errors in metering equipment;
  • Spurious operation of fuses, circuit-breakers and other protective equipment;
  • Voltage glitches in computers systems resulting in lost data. Excessive flicker on VDU’s;
  • Electromagnetic interference with TV, radio, communication & telephone systems;
  • Damage and disruption to standby generators and associated AVR control equipment;
  • Interference with ripple control systems.

Harmonic sources

The main sources of voltage and current harmonics within the power system are:-

Single-Phase:

  • Computers, fax machines, photocopiers, UPS’s, TV’s, VCR’s, etc.;
  • Lighting dimmers & electronic ballasts for high efficiency lighting;
  • Single-phase AC & DC drives;
  • Ultra-violet disinfection systems;

Three- Phase:

  • Variable speed AC & DC drives;
  • UPS systems;
  • Arc furnaces & SCR temperature controllers;
  • Battery chargers;
    In fact any single or three phase electrical power conversion equipment which converts from AC to DC!

Methods of harmonic mitigation include:-

  • Phase Shifting;
  • Passive Filters;
  • Phase Staggering;
  • Active Filters.

Phase Shifting

  • Most common is a phase shift transformer to create two supplies displaced in phase by 30º from the original supply. By feeding two 6-pulse diode rectifiers to create a 12-pulse rectifier, certain harmonics will be opposite in phase and magnitude, and will cancel each other out.
  • Solutions up to quasi 48-pulse can be created.

Passive Filters

  • Passive filters are series capacitor & reactor resonant circuits ‘tuned’ to present a low impedance path to a specific frequency (i.e. 5th – 250Hz, 7th – 350Hz).
  • They are more commonly used at the PCC to prevent harmonics ‘escaping’ into the utility power supply, but can be connected to individual loads in the plant.
  • Due to possible system resonance and interaction problems, a harmonic study of the power system is often necessary. If multiple filters are installed anti-resonance is a problem. Install with caution!
  • Typical total harmonic attenuation to ~ 10-15%

Phase Staggering

  • Phase staggering is simply the phase shifting of individual loads such that the harmonics produced by one or more loads cancels the harmonics produced by others.
  • For phase staggering to be successful at least two balanced loads of similar ratings are required.

Active Filters

AIM (active injection mode) Filter

  • AIM filters are the most technically advanced and effective solution in reducing the total harmonic current distortion to below 5%, in line with IEEE 519-1992 or UK Engineering Recommendation G5/4 harmonics standards.
  • The AIM filter continuously monitors the harmonic current demanded by the load and generates an adaptive waveform which matches exactly the shape of the non linear portion of the load current.
  • AIM injects this adaptive current into the load at the point of connection. Only fundamental (50Hz) current is drawn from the source by the load.
  • AIM compensates from 2nd to 51st harmonic. Response time ~ 100 micro-seconds, especially suitable for dynamic loads.

Harmonics and Poor Power Supply – Discussion and Solutions

In the world of Electrical Engineering and Electrical Contracting, we run into problems with businesses experiencing poor power supply.  This has been an ongoing problem, but we are seeing the problems with poor power supply affecting machinery and sensitive equipment.  A lot of these problems can point back to harmonics.

Harmonics have been around for a long time. Power systems were affected by harmonics when the first AC generator was created which dates back to 1832.   So how does this term that seems to be so harmless have such a large impact impact in our business and industrial sector?  Lets take a look at what harmonics are and discuss some mitigation techniques.

 What are Harmonics?

A pure sinusoidal voltage is a theoretical quantity that could be generated only under ideal conditions like (an ideal AC generator, finely distributed windings and uniform magnetic field)However, in real-time applications, it is impossible to create ideal conditions. Hence, the voltage waveform would be distorted and the voltage-time relationship would deviate from the pure sine function. This distortion in voltage waveform is negligible and appears in the form of a periodic function. In other words, it creates harmonics. Harmonics are created as an integral multiple of fundamental power line frequency.

Loads can be classified broadly into two types:

  • Linear loads – here output current is directly proportional to input voltage and there is no distortion. Example : Incandescent lamp
  • Non-linear loads – here current is not directly proportional to input voltage and results in distortion. Example : SMPS

When a non-linear load is applied to the circuit, it could result in the superimposition of waveforms. These multiple frequencies are also termed as harmonics of the fundamental frequency.

These harmonics are different from the transient distortions that occur in power units like spikes. Harmonics repeat after cycle and are steady-state distortions. Harmonics can seriously affect the efficiency and quality of operation in both industrial and commercial applications.

Voltage harmonics – Current harmonics result in voltage harmonics. Voltage harmonics created by current harmonics is in direct proportion to source impedance of voltage source.

Impact of harmonics on industrial equipment and commercial equipment

Harmonics related problems are here to stay and could result in a galore of detrimental consequences in both commercial and industrial scenarios. In case of resonance, probability of such detrimental effects are higher. Resonance condition is broadly classified into two: parallel resonance and series resonance.

Parallel resonance – Inductor and capacitor circuit elements are connected in parallel. If the natural frequency of this parallel combination is equivalent to harmonic frequency, heavy current can flow through the circuit and result in transformer overheating.

Series resonance – Inductor and capacitor circuit elements are connected in series. If the natural frequency of this series combination is equivalent to harmonic frequency, it would result in voltage distortion.

Overloading Neutral Conductors

Three individual phase conductors and a neutral conductor is included in a three-phase system. If individual phase conductors carry the same current, phase currents would be cancelled (given that the load is balanced). Hence, the size of neutral conductor can be decreased. However, a very high third-harmonic current is often found in SMPS (used in computers). If there are too much of PCs in buildings, very high current can flow through neutral wires. This current can go higher than the maximum acceptable limits and result in a potential fire hazard. Harmonics could also result in interference or failures in computers and telephones.

Transformer Losses

Harmonic-producing loads in transformers can result in eddy current loss. This loss is directly proportional to the second power of product of harmonic current and frequency.  This results in overheating of transformer and affects the insulation materials as well. This paves the way for transformer failure.

Nuisance Tripping

Multiple resonant frequencies would be produced in circuits which contain capacitance and inductance elements. If harmonic frequency created by nonlinear loads, tally with anyone of the resonant frequencies, it could result in harmonic resonance. This could result in distortion of voltage and current waveforms. This results in nuisance tripping in circuit breakers which paves the way for production losses.

Affect life span and affects quality of operation – Harmonics can damage the equipment and reduce the life span. Harmonics can overheat the wires and build up stress on equipment and associated cables.

Increased power consumption – Harmonics result in increased power consumption which paves the way for higher electricity bills.

Driver and power supplies – Harmonics could result in breakdown of commutation circuits present in AC/DC drives.

Harmonic distortion could also result in poor power factor. Some of the other problems caused by harmonics are metering inaccuracies, generator breakdowns etc.

Harmonic mitigation – Ways to Reduce Harmonics

There are different methods to reduce the effect of harmonics in power systems and each of them corresponds to different levels of efficiency.

Harmonic filtering

Multiple tuned series LC circuits are used to remove the harmonic current from the system. They also help in correcting power factor. There are 3 different types of harmonic filters:

  • Passive filters
  • Active filters
  • Hybrid filters

Passive Harmonic Filter

Passive filters are usually used in

  1. applications that need power factor correction
  2. circuits which contain group of non-linear loads with power more than 500kVA .
  3. Current/voltage distortion has to be decreased.

An LC filter is provided in parallel to the non-linear load which generates harmonics. If there is high harmonic current in the circuit, it could be decreased by connecting several LC filters in parallel to the load.

Active filter

Active filters are usually used in following cases:

  1. Current distortion has to be decreased.
  2. Circuits which contain group of non-linear loads with power less than 500kVA .

Active Harmonic Filter

 

 

Here, filters are installed either series on parallel with the load. Here, current produced by the filter circuit compensates the harmonic current produced by the non-linear load.

Hybrid filters

Hybrid filters are usually used in following cases:

  1. Current or voltage distortion has to be decreased.
  2. Circuits which contain group of non-linear loads with power more than 500kVA .
  3. Power-factor correction is required
  4. Harmonic emissions has to be regulated precisely.

Hybrid Harmonic Filter

Hybrid filter comprises of both passive and active filters. Hence, this type of filter is able to combine the benefits of both these types of filters.

Increase the length of neutral wire

In most of the cases, neutral wire is designed to have similar capacity as power wiring. In buildings that consist of plenty of PCs, it is better to design the wiring in such a way that the neutral wire is bigger than the phase wire by almost 200%. However, this method does not offer any protection to the transformer and it safeguards only the building wiring.

Separate neutral conductors

Separate neutral conductors can be installed for each phase conductor. This helps to improve the efficiency of branch circuit to deal with harmonic loads and to get rid of harmonic currents.

K-rated transformers

A standard transformer is unable to handle harmonic currents and would be overheated. This could pave the way for early breakdown. K- rated transformers can cope with the heat produced by harmonics. For a standard transformer, K-factor is one. Transformers with higher value of K-factor can deal with more heat energy.

Power system design

While designing the circuit, special care should be given to reduce the non-linear load to 30% of total transformer capacity.

Harmonic trap filter

This type of filters are used in circuits that have high non-linear ratio. Filters are designed in such a way to  remove particular harmonic like 3rd, 4th etc. They also offer power factor correction.