Harmonics is defined as the content of the signal whose frequency is an integral multiple of the system frequency of the fundamentals. Harmonics current generated by any non-linear load flows from the load into the power system. These harmonic currents degrade the power system performance and reliability and can also cause safety problem. Harmonics need to be clearly located, sources identified and corrective measures taken to prevent them.
Electrical load is categorised under two categories
- Linear load:Such load draws voltage and current in essentially sine wave shape but at varied phase shift (power factor). Example: resistors, inductors, capacitors and their combinations are classified as linear load. Linear loads have smooth, straight and predictable response.
- Non-linear load:Power supplies in non-linear load draw current in abrupt pulses rather than in smooth sinusoidal wave. It indicates distorted or suddenly changing response. Example-modern electronic/electrical equipments consisting of rectifying, charging /discharging and phase control circuits.
Harmonics: The distortion in a sinusoidal wave is generally defined in terms of various harmonics components. Harmonics are defined as the content of signal whose frequency is an integral multiple of the system frequency of the fundamental. Typical harmonics for a 50 Hz system (fundamental frequency) are the 5th (250 Hz), 7th (350 Hz), 9th (450 Hz).
The harmonics of a periodic wave can be represented by a Fourier series:
f(wt) = AO + A1coswt + A2 cos2wt + B1sinwt + B2 sin2wt + ——-
f(wt) = Given non sinusoidal periodic wave form with angular velocity w = 2 Σ f
A0 = Const.
A1, A2, A3 ———- An coefficient of cosine terms, nth is the order of harmonic.
B1, B2, B3, ——– Bn coefficient of sine terms, nth is the order of harmonic.
Effects of harmonics: Harmonics current generated by any non-linear load flows from the load into the power system. These harmonics currents degrade the power system performance and reliability and could also cause safety problem. Harmonics need to be clearly located, sources identified and corrective measures taken to prevent these problems. THD (Total Harmonic Distortion) can be computed as per IEE-519 standard as:
Where hn is the individual harmonics of nth order.
Source of harmonics: (1) Transformers under no load and light loads (2) Saturated Reactors (3) Thyrister controlled motor drives (4) Arc Furnaces (5) Arc Welders (6) Conduction Furnaces (7) Gas discharging lighting-low pressure/ high pressure Sodium vapour lamps (8) High-pressure Mercury vapor lamps (9) CFL/fluorescent tube lights (10) Energy conservation devices e.g. soft starters, electronics ballast and fan regulators (11) Rectifiers (12) UPS (13) Static VAR compensator (14) HVDC transmission system (15) Solar power conversion.
Why to worry for harmonics: Voltage distortion is generally very harmful because it can increase the effective peak value and also the RMS current in some devices connected to the network. For a capacitor, impedance decreases drastically as it is inversely proportional to the frequency. Under normal circumstances the voltage distortion in primary electrical distribution network is minimal and can usually be ignored from a practical point of view. On the other hand distortion of current wave shape is common particularly when electronic equipment is connected to the network or when non-linear loads are connected. Current distortion, in general, causes overheating due to increase in the losses and affects all electrical machines, transformers etc. This causes derating of equipment. The amount of derating will depend upon which harmonics are present and the magnitude of the individual current and resistance.
Positive sequence harmonic component would generate a magnetic field, which rotates in the same direction as the fundamental. A negative sequence harmonic would generate the rotating magnetic field in the reverse direction. The zero sequence harmonic would not rotate the magnetic field in any direction.
Limits of harmonic levels: Depending upon the system network, various countries have adopted different limits for deciding the tolerance levels of harmonic distortion. The ranges of limits generally adopted are indicated below.
It is necessary to fix the limits of the harmonics generation levels and make mandatory to the users. However, in our country still no regulations have been made in this regard. The regulation is only for variation of rated voltage which is ± 10% and ± 2% of frequency.
Theoretical value of the harmonic current = I/h
I = fundamental value of the current
h = Order of harmonics
Electrical waveform with harmonic distortion…
Harmonic effects on various components
- Transformers:Harmonics in transformers cause an increase in the iron and copper losses. Voltage distortion increase losses due to hysteresis and eddy currents and causes overstressing of the insulation material used. The primary effect of power line harmonics in transformer is, thus the additional heat generated. Other problems include possible resonance between the transformer inductance and the system capacitance, thermal fatigue due to temperature cycling and possible core vibrations.
- Motor and generators:Harmonic voltage and current cause increased heating in rotating machines due to additional iron and copper losses at harmonic frequencies. This lowers the machine efficiency and affects the torque developed. The flow of harmonic currents in the stator induces current flow in the rotor. This results in rotor heating and pulsating or reduced torque. Rotor heating reduces the efficiency and life of the machinery whereas pulsating or reduced torque results in mechanical oscillation causing shaft fatigue and increased ageing of mechanical parts.
iii. Thyrister drives: AC variable frequency drives with thyrister converter when operated at slow speed, generally result in poor power factor.
- Power cable:Normal level of harmonics currents cause heating in cables. However, cables involved under system resonance condition may be subjected to voltage stress and corona, which can lead to insulation failure.
- Metering equipments:In general, harmonics flowing in induction type metering equipment will generate additional coupling paths thereby increasing the speed of the disc and hence an apparent increase of costs.
- Switchgear and relay:Harmonics current increases heating and losses in switchgear there by lowering its normal current capacity and shortening the life due to voltage stress fuses require derating due to the heat generated by harmonics.
vii. Earthing system and computer performance: In a 3 phase and neutral system- when 3rd harmonics and multiples are expected, the neutral conductor size should be the same size as the phase conductor size.
Computer hanging up, loosing instructions, data or misbehaving can be as much attributed to poor quality of power. Eearthing of computer equipment should be independent and be fixed into the mains earthing at one point – preferably at the entry point only. Multipoint earthing introduces coupling to various other equipments.
viii. Communication network: The induction coupling between the AC power transmission lines containing harmonics and the neighbouring communication network causing high noise levels.
- Capacitor:Capacitors for power factor correction are always present in industrial installations and are worst affected if harmonics are present. Capacitors do not generate harmonics, but provide network loop for the possible resonance. Capacitive reactance decreases with frequency whereas inductive reactance increases directly with frequency. At the resonant frequency of any inductive capacitance (LC) circuit, the inductive reactance will equal the capacitive reactance. In an actual electrical system utilising power factor correction capacitor, both series and parrelel resonance and a combination of the two can occur. In the case of a series circuit, the total impedance at the resonant frequency reduces to only the resistive component of the system. If this component is small, high current magnitudes will result at the resonant frequency. In the case of a parallel circuit, the total impedance at the resonant frequency is very high (approaching hypothetically infinity) thus, when excited from even a small source at the resonant frequency; a high circulating current will flow between the parallel capacitor and inductor. The voltage across the parallel combination could be quite high. Consequently, if the resonant point of either or both these type of circuits happens to be close to one of the frequencies generated by the harmonic sources in the system, the result may the flow of excessive amount of harmonic current and/ or the appearance of excessive harmonic voltage. These occurrences may cause such problems as capacitor bank failures; excessive capacitor fuse operation and dielectric break down of insulated cables. In most low voltage installations, the following guidelines may be followed:
1. If the KVA of the harmonic generating loads is less than 10% of the transformer KVA rating capacitor can be installed without concern for the resonance.
2. If the KVA of the harmonic generating load is less than 30% of the KVA rating and the capacitor KVAR is less than 20% of the transformer KVA rating, capacitor can be installed without concern for the resonance.
3. If the KVA of the harmonic generating load is more than 30% of the transformer KVA rating capacitors should be applied as filters.
The above guidelines are applicable when transformers with 5 to 6% impedance are used and the system impedance is less than 1% at the transformer base.
Filters for harmonics
For healthy operation of power system, two things serve as guidelines:
- The consumer is responsible for maintaining current distortion within permissible/acceptable levels.
2. The electricity board is responsible for maintaining voltage distortion within permissible/acceptable levels.
There are different types of filters:
– Single tuned filters.
– High Pass (first, 2nd or third order etc.)
A capacitor with a series reactance can be so designed as to tune to a given harmonic. It offers almost a zero impedance parallel path and absorbs a particular harmonic. At the fundamental frequency, it also helps in power factor correction. Thus, wherever filters are required, a portion of the P.F. capacitor bank is converted into a filter or filters. A filter bank increases the cost of capacitor installation because of extra circuit breakers and reactors.
Undesirable harmonic current is prevented from flowing into power system by use of high series impedance to block them or direct them by means of low impedance shunt path.
Series filters should be designed to carry full load current and should be insulated to full rated voltage of the system, while shunt filters are less expensive and provide reactive compensation in fundamental frequency. Therefore, it is generally preferred to use shunt filters.