HARMONICS CAUSES THE FOLLOWING ADVERSE EFFECTS:
Overheating of transformers and rotating equipment
Motors and generators can be adversely affected by the presence of harmonic voltage and current due to increased heating caused by iron and copper loss. In addition to this harmonic current can increase an audible noise emission and reduce machine efficiency.
All these effects combined together to increase energy consumption, and reduce machine life considerably. Iron loss of a machine for the fifth harmonics can be calculated with the following formula.
Iron Loss KW5 = KW1 X (V5/ V1)2 X (f5 / f1)2
Iron losses increase as harmonic frequency increases.
Calculation of Iron loss
An induction motor of rating 275 HP,415 V, 3 Ph, 50 Hz is fed by a 1000 KVA,11/0.433 KV transformer. No load loss at 50 Hz is 3 KW. In the harmonic voltage spectrum the individual harmonic distortion values are as follows : V5 = 7 % V7 = 6 % V11 = 4.2 % Calculation of iron loss at 5th harmonic
KW5 = KW1 X (V5/ V1)2 X (f5 / f1)2
= 3 X (0.07) 2X (5)2
= 0.3675 KW
Similarly, the losses for other order of harmonic frequencies can be calculated.
KW7 = KW1 X (V7/ V1)2 X (f7 / f1)2
= 3 X (0.06)2 X (7)2
= 0.52 KW
KW11 = KW1 X (V11/ V1)2 X (f11 / f1)2
= 3 X (0.042) 2X (11)2
= 0.64 KW
The total iron loss is equal to the sum of iron losses due to the fundamental and harmonic voltages.
KW total = 3 + 1.5367
= 4.5367 KW
Increase in iron loss due to presence of harmonics = (4.5367 – 3) /3 X 100 % = 51.23 %
In induction motor additional losses occur because of harmonic generated fields. Each harmonic has a sequence +, -, and 0 sequence which indicates the direction of rotation that would result if it were to be applied to an induction motor with respect to the fundamental.
Third and multiples of third produce a stationary field, but since the harmonic field frequencies are higher the magnetic losses are greatly increased and the harmonic energy is dissipated as heat.
Negative sequence harmonics result in a counter-rotating field (with respect to fundamental) which causes reduced torque. Positive sequence harmonic produced forward rotating field that adds to torque. Due to the interaction of positive and negative sequence harmonic components motor vibrates and reduces the service life of the motor.
Over heating of neutral conductor
Under balanced load conditions without harmonics, the phase currents cancel each other in neutral and the resultant neutral current is zero. However, in a 4 wire system with single-phase nonlinear loads, odd-numbered multiples of the third harmonics ( 3,6,9, etc ) do not cancel, rather add together in the neutral conductor.
In a system with a substantial amount of nonlinear single-phase loads, the neutral current may rise to a dangerously high level. There is a possibility of excessive heating of the neutral conductor since there are no circuit breakers in the neutral conductors like in the phase conductors. It is important to take care of the size of the neutral conductor if harmonics are prevalent in the system. A recent case study found that the neutral currents as 150 Amp while the phase current were only 100 amps. The neutral sizing thus becomes very critical.
Nuisance Tripping Of Circuit Breakers and Blowing Of fuses
Several protective relays see the neutral current and act accordingly. Since the neutral current increases due to harmonic such relay malfunctions.
Similarly, relays that see crest voltage/current or voltage zero for their operation are affected by harmonic distortion. Due to the resonance effect, the current levels may rise to manifold levels which result in tripping of circuit breakers and melting fuses. This situation results in serious problems in industries that rely on the quality of power for the continuous operation of their sensitive processes.
Overstressing Of Power factor Correction Capacitors
The impedance of a capacitor is inversely proportional to frequency so the impedance to harmonic frequency is very low and the capacitor tends to hog the harmonic current. This causes undue heating and reduces the service life of the capacitor.
The second problem is that the capacitor along with line and transformer inductance can resonate at near or one of the harmonic frequencies resulting in a very high current. In such a case, the capacitor will act as a harmonic current amplifier.
Higher I2R Loss
The resistance of conductors increases at higher harmonic frequencies due to the skin effect. Due to the phenomenon of skin effect all current-carrying conductors exhibit higher I2R loss.
Further, due to the presence of harmonics, the RMS current gets increases which result in a further increase in I2R losses.
Overloading/decrease of life of transformers
Transformers are designed to deliver power at network frequency (50 Hz). The iron losses are composed of the eddy current loss (Which increases with the square of the frequency) and hysteresis losses (which increase linearly with the frequency). With increasing frequencies the losses also increase, causing additional heating in the transformer.
Losses in distribution equipment
Harmonics in addition to the fundamental current cause additional losses in the cable, fuses, and also in the bus bars.
Malfunctioning of the electronic control and computers
Electronic controls and computers rely on power quality for their reliable operation. Harmonics result in a distorted waveform, neutral currents, and voltage which affect the performance of these gadgets. Due to excessive current in the neutral conductor voltage between neutral and ground rises above 3 volt. In this condition reliability of electronic equipment is questionable.
Measurement error in the metering systems
The accuracy of the metering system is affected by the presence of harmonics. Watt-hour meters accurately register the direction of the power flow at harmonic frequencies, but they have magnitude error which increases with frequency. The accuracy of demand meters and VAR meters is even less in the presence of harmonics. The solution lies with the use of True RMS meters.
Zero crossing noise
In order to reduce the generation of transients and EMI when on the inductive loads many electronic controllers detect the points at which supply voltage crosses the zero point. Due to the presence of harmonic, the rate of change of voltage at zero crossing becomes very fast and difficult to identify, leading to the erratic operation.
Electrostatic interference with communication circuits
Higher-order harmonics frequency interface with neighboring communication circuits and it affects the performance of the communication system.
The resonance between the inductance of the transformer winding and the capacitance of the Feeder to which they are connected. There are two types of resonance.
- Series Resonance
- Parallel Resonance
Series resonance :
Series resonant circuit is formed by a series connection of inductive and capacitive loads. The reactance of the inductor is proportional to the frequency. The reactance of the capacitor is inversely proportional to the frequency which can be shown as below:
It is seen that at resonant frequency the impedance reduces to a minimal value. At the resonant frequency, the impedance is very low result in a high current. The primary side of the transformer along with the capacitor on the LV side acts as a series resonating circuit and provides a low impedance path for harmonics close to resonating frequency.
Thus the circuit offers very low impedance at the input signal at this frequency which results in a multiple-fold increase in the current. The voltage drop on the individual component increase moving closer to the resonant frequency.
The LV side of the transformer along with the power factor correction capacitor behaves as a parallel resonating circuit at the resonating frequency the impedance offered is very high consequently the harmonic current causes an increased harmonic drop which may be accompanied by distortion of the fundamental. Transformers and capacitors are additionally loaded.