Understanding Harmonics

Harmonics are undesirable components in the sinusoidal waveform of the AC Power supply. Harmonics occur as integral multiples of the fundamental frequency. That is, the third order harmonic will have a frequency of 3 times the fundamental frequency; 150 Hz which is 3 times the fundamental 50 Hz frequency. Harmonics affect power quality and equipment life and efficiency.

It is therefore necessary that Harmonics in any power system be monitored. Should Harmonics be present, they can be rectified by using suitable methods such as filters.

Causes of Harmonics

Harmonics are caused by Non-Linear Loads. The majority of electrical loads are linear meaning that the current varies sinusoidally with the voltage, though it may have a phase displacement.

However, of late, the proliferation of electronic devices such as Variable frequency drives, chopper circuits, inverters, etc cause non-linear loading of the power system. The current does not vary sinusoidally with the voltage. This leads to harmonics in the power system. The fundamental frequency will have many other frequencies superimposed on itself. This causes distortion of the waveform.

Using a mathematical technique known as Fast Fourier Transforms, the distorted AC waveform can be resolved into its component waveforms. Of the measured harmonics, the even harmonics(harmonics whose frequency are the fundamental frequency multiplied by even numbers such as 100Hz(2 *50) or 200Hz(4*50) get cancelled out and have no effect. For the study and management of Harmonics, only the odd harmonics are considered.


Effects of Harmonics

Harmonics have a wide range of effects such as heating of conductors, motors etc which can affect equipment efficiency. Besides, they can cause transient over/under voltages and can cause equipment failure.

Harmonic Analysis

If the problem of Harmonics is suspected, a harmonic analysis needs to be conducted. Harmonic analyzers are dedicated equipment to study the harmonics in a power supply. Typical Analyzers can resolve harmonics upto the 25th order.

Harmonics can be neutralized by means of Harmonic filters. Harmonics filters are usually LC circuits tuned to the frequency of the particular order of harmonics to be neutralized.

Voltage Classification - LV, MV and EHV

AC voltages have been classified in various manners.  In earlier times, there were just two categories LV and HV.  As the level of voltages increases, there was a need for more levels.  However, there was ambiguity as to where each band ended and the other began.  For instance, 11kV can be MV in some systems and HV in another. 

The International Electrotechnical Commission has classified the voltages into the following levels(IEC 60038).  This classification system is fast gaining acceptance. 

Low Voltage           - upto 1000V

Medium Voltage     - 1000V to 35kV

High Voltage           - 35kV to 230 kV

Extra High Voltage  - above 230 kV.


In some situations, the term Ultra High Voltage is used to denote voltages above 800 kV.

In addition, the IEC defines a voltage band known as the Extra Low Voltage with a AC voltage less than 70 V.  See article here.

Braking methods in Induction motors

Braking in induction motors refers to quickly bringing the speed of the motor to zero.  Braking can be categorized into two broad categories viz. mechanical braking and electrical braking.
Mechanical braking involves stopping the shaft by means of a braking shoe.  When the braking is to be done, the supply to the motor is cut off and the brake is applied to bring the motor to a halt.
Mechanical braking used in cranes and hoists.  It is also used in elevators when the elevator has to stop at a specific floor of the building.

Electrical braking involves stopping the motor using electrical means.  Most electrical braking systems have a mechanical brake to hold the shaft in position once the machine has been stopped.
There are two main types of Electrical braking.

1. Plugging
2. Dynamic braking
3. Regenerative braking

Plugging
Plugging involves reversing the supply in two of the phases.  For instance, R and Y can be interchanged.  This leads to a torque being developed in the opposite direction to the rotation of the motor.  This causes the motor to stop at once.  Once the motor stops, the reverse supply is cut off (to prevent the motor from running in the opposite direction).  The rotor is secured by a mechanical brake.
Dynamic Braking can be classified into DC injection braking, AC dynamic Braking and Capacitor Braking.
AC dynamic Braking
In AC dynamic braking, the supply to one of the phases is cut off.  Thus the motor runs as a single phase motor.  This induces negative phase sequence components in the supply and the motor stops.  Another method is to give the remove one phase and give the same phase to two terminals.  For instance, two terminals will have 'Y' phase and one will have 'B' phase.
DC injection braking
In DC injection braking, a separate rectifier circuit produces a dc supply.  When the brake is to be applied, the ac supply to the stator is disconnected and a dc supply is given to two of the phases.  The dc voltage in the stator sets up its own magnetic field.  The conductors of the rotor which is rotating will cut the magnetic field.  As the conductors are short circuited, a high current is produced.  This causes a braking torque to be produced in the rotor.  The current produced in the rotor is dissipated as heat.  This system can be used only when the rotor can withstand the heat which will be produced when the brake is applied.
Capacitive Braking
Here the AC supply to the stator terminals is cut off and the terminals are connected to a three phase capacitor bank.  The capacitors will excite the induction generator.  This sets up a magnetic field which will cut the rotor bars.  The rotor energy is thus converted into heat and the motor is stopped.
Regenerative Braking
In Regenerative braking,  the supply frequency to the stator is reduced.  This is possible with VFDs where the frequency can be varied.  When the supply frequency is reduced, the synchronous speed of the motor is reduced. When the synchronous speed falls below the rotor speed, the induction motor works as an induction generator and power is supplied back to the terminals.  The energy in the rotor is thus recovered.  Due to the loss of energy, the rotor slows down and stops.