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