Interharmonics in Power System

The power electronics is being widely used nowadays to improve the system efficiency,flexibility and reliability. The semiconductor devices used in power electronics are non linear devices and the current drawn by these devices is not linear to the voltage. This non linearity introduces harmonics in the electrical network. The harmonics are categorized into even and odd order harmonics. Even and odd order harmonics has frequency which is integral multiples of the fundamental frequency. 

Apart from the even and odd order harmonics, interharmonics are generated when power electronics devices are used. The interharmonics frequency is not equal to the integral frequency of the fundamental frequency . The interharmonic order frequency is non integral of the fundamental frequency.

Definition of Interharmonics

The IEEE defines interharmonics as: 

“A frequency component of a periodic quantity that is not an integer multiple of the frequency at which the supply system is operating (e.g., 50 Hz or 60 Hz).” 

The IEC defines interharmonics as: 

“Between the harmonics of the power frequency voltage and current, further frequencies can be observed which are not an integer of the fundamental. They can appear as discrete frequencies or as a wide-band spectrum.”  

If f₁ is the fundamental frequency and m is any positive non integer n, mf₁ is an interharmonics of f₁ . When m is greater than zero and less than one, mf₁ is sometimes referred to as a subharmonic of f₁. 

INTERHARMONIC SOURCES 

The interharmonics are genearted in the system when the rapid change in voltage and current caused by loads operating in a transient state. When voltage or current modulation is done for control purpose, it also generates interharmonics. Changes in current magnitude or phase angle can also create sidebar components of the fundamental frequency and its harmonics at interharmonic frequencies.

When the static converter is  switched in non synchronization to the power frequency, the static converter generates harmonics. In line commutated converters and inverters the thyristor are triggered into forward conducting mode and keep conducting until their current falls below the holding current. The thyristors get switched off at the zero crosssing of the line voltage. The synchronous switching of the thyristors in line commutated inverters and converters does not produce interharmonics. The thyristors are prone to malfunction with variation or surges in the ac supply and the new devices insulated gate bipolar transistor(IGBTs), which can be turned on and off at any time. This asynchronous switching of converters or inverters using IGBTs produce interharmonics. 

Arc furnace are used for melting application.The load varies throughout the melt cycle. The interharmonics are produced due to load variability in the arc furnace. The welding machines which use IGBTs for voltage controlling produce interharmonics. 

The  induction motor can produce interharmonics when it is operated through variable frequency drive. The motor is to be operated at rated flux density in the air gap length. If the V/f ratio  does not remain constant and if over fluxing state happens, the rotor and stator slots can  generates interharmonics. If the motor delivers varying torque this can lead production of interharmonics. PWM technique is used in variable frequency drive and PWM waveform is as shown below. 

Electronic frequency converters use a dc link to convert one frequency to another. The filtering on DC bus decouples the voltage and current on each side of the link. However, this filtering is not perfect and generates interharmonics in the system.

The most important feature of the IGBT over thyristor is that IGBTs can be switched on or switched off at any instant. The IGBTs are widely used for variety of applications including HVDC transmission, static compensators (STATCOM), variable frequency Drives(VFDs). The use of IGBTs in VSC produce interharmonics. The pulse width modulation used for voltage source converter produces interharmonics. The frequency and level of interharmonics produced in VSC depends on the number of design factors(pulse number,levels etc.).

Variable Load Dives such as traction system power supplies that use IGBTs or experience sudden load changes can produce interharmonics, usually at fixed frequencies.

INTERHARMONIC EFFECTS

Harmonics and interharmonics add additioanl signal to fundamental power supply.The additional signal cause adverse effect on the power system. The condition becomes more severe if the additional signal gets amplified by resonance. The wider range of frequencies may cause greater risk of resonance. The some of the adverse effect of interharmonic are similar to what caused by harmonic frequencies.

Effect of interharmonics similar to those of harmonics

Effects of interharmonics similar to those of harmonics caused by an additional signal superimposed on the fundamental can be categorized into : overloads, oscillations and distortion.The effect of overloads include additional heat loss,oscillation in mechanical system,acoustic disturbance etc. The distortion of the fundamental frequency can cause interference with the communication system. The distorted fundamental frequency can also cause shifting of the zero crossing of the waveform and adversely affect the operational performance of the equipment.

The following are the adverse effect which are caused by the interharmonics and not  caused by the harmonic distortion are as floows.

Light Flicker

The interharmonics cause variation in magnitude of the rms value. The perceptibility of flicker varies with the frequency and magnitude of voltage variations.Virtually all types of lighting can be susceptible to flicker, but flicker intensity can vary for different types of lighting when they are exposed to voltage deviations caused by interharmonics.

Power LIne Communication
  

Power Line Communications problem occurs due to interharmonics. Protection and ripple control signals usually consist of a single interharmonic frequency. They are usually not affected by other interharmonics that are of lower magnitude or do not match their frequency. 

Two way communications based on voltage  or current step changes send bits of information consist of multiple interharmonic frequencies. If the interharmonics frequency range  is the same as that of power line communication signal, the interharmonics frequencies  interfere with the communication signal and adversely affects the power line communications.

With interference of the interharmonics frequencies with the communication signals, the interfered communication signals with interharmonics frequencies cause flicker. 

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