Harmonics and its effects on electrical systems

Harmonics in Electrical Systems: Causes, Effects, and Impact on Equipment

What Are Harmonics?

Harmonics are unwanted voltages and currents in electrical systems that distort the fundamental waveform (50 Hz in India). They arise due to non-linear loads that draw current in abrupt pulses rather than smooth sinusoidal waves.

In simple terms, harmonics are like pollution in electricity—they degrade power quality without necessarily affecting power availability.

Classification of Harmonics

Harmonics are integer multiples of the fundamental frequency (50 Hz):

• 2nd harmonic (100 Hz)

• 3rd harmonic (150 Hz)

• 4th harmonic (200 Hz)

• … up to the 11th harmonic (550 Hz) and beyond.

Depending on their order:

• Negative Sequence Currents: 2nd, 5th, 8th, 11th

• Zero Sequence Currents: 3rd, 6th, 9th

• Positive Sequence Currents: 4th, 7th, 10th

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Why Are Harmonics Increasing?

The rise in harmonics is directly linked to the increasing use of electronic and power conversion devices.

Major Sources of Harmonics

1. Non-linear loads such as Variable Frequency Drives (VFDs), UPS, SMPS, rectifiers, and inverters.

2. Arcing devices (arc furnaces, welding machines).

3. Ferromagnetic devices (transformers operating near saturation).

4. Electronic switching power converters.

5. Household and commercial appliances with embedded power electronics.

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Effects of Harmonics on Electrical Systems

General System Effects

• Overheating of electrical equipment.

• Reduction in equipment life.

• Malfunctioning and premature failure of devices.

• Higher system losses and reduced efficiency.

• Interference in communication systems.

• Nuisance tripping of circuit breakers and fuses.

• Motor vibrations and noise.

• Computer screen flickering and data errors.

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Harmonics and Power Factor

Traditionally, power factor (PF) is:

$$PF = \frac{\text{Real Power (kW)}}{\text{Apparent Power (kVA)}}$$

However, with harmonics, we must consider Distortion Power Factor (DPF):

$$DPF = \frac{1}{\sqrt{1 + THDi^2 + THDv^2}}$$

Where:

• THDi = Total Harmonic Distortion in Current

• THDv = Total Harmonic Distortion in Voltage

Thus, the Total Power Factor = Displacement PF × Distortion PF, which means PF will never be unity in the presence of harmonics, even with capacitor banks.

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Effects of Harmonics on Different Equipment

1. Transformers

• Increased eddy current losses.

• Additional heating in windings.

• Higher skin effect losses.

• Premature insulation failure.

2. Motors

• Increased hysteresis losses (∝ frequency).

• Eddy current losses (∝ frequency²).

• High rotor & stator losses.

• Tooth pulsations leading to vibration.

• Overheating and shortened lifespan.

3. Cables

• Higher proximity and skin effects.

• Increased resistance and power losses.

• Overheating leading to insulation degradation.

• Derating of cable capacity.

• Higher neutral currents causing imbalance.

4. Capacitor Banks

• Resonance with 7th harmonic (risk of overvoltage).

• Reduced capacitive reactance.

• Premature failure due to overheating.

• Increased KVA demand and electricity bills.

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Why Should We Care About Harmonics?

• Reduced efficiency → Higher operating costs.

• Premature equipment failure → Expensive replacements.

• Poor power factor → Higher utility penalties.

• System instability → Risk of downtime.

When harmonics are present in a system, they increase KVA demand and ultimately raise electricity bills. Here’s why:

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1. Relationship Between kW, kVA, and Power Factor

• kW (kilowatt) = Useful (real) power that does actual work.

• kVA (kilovolt-ampere) = Total apparent power supplied.

• Power Factor (PF) = kW ÷ kVA.

When harmonics distort the waveform, they:

• Increase the RMS current in the system.

• Cause a drop in power factor (due to distortion PF).

• This means: For the same useful power (kW), the required kVA (apparent power) increases.

👉 Utilities often charge based on maximum kVA demand (not just kW). So, higher kVA = higher demand charges.

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2. How Harmonics Increase System Losses

• Harmonics introduce extra current components (2nd, 3rd, 5th, etc.) that do not contribute to useful power.

• These harmonic currents cause:

  • I²R losses in cables and transformers.

  • Higher eddy current and hysteresis losses in transformers/motors.

  • Extra heating and derating of equipment.

👉 This wasted energy still flows through the meter, showing up as increased kWh consumption.

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3. Capacitor Bank Issue

Many plants use capacitor banks to improve PF. But with harmonics:

• Capacitors may resonate at certain harmonic frequencies (e.g., 5th, 7th).

• This amplifies harmonic currents instead of compensating them.

• The plant ends up drawing more reactive power from the grid, worsening kVA demand.

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4. Direct Impact on Electricity Bill

1. Higher kVA demand charges

   • Utilities charge based on peak demand (kVA).

   • Harmonics inflate apparent power → demand charges increase.

2. Higher energy (kWh) consumption

   • Extra losses caused by harmonics (heating, eddy currents, neutral currents) are billed as real energy consumed.

3. Penalty for low power factor

• Some utilities impose penalties if PF drops below a threshold (say 0.9).

• Harmonics reduce PF even if displacement PF is corrected.

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✅ Example:

• A factory needs 500 kW of real power.

• Without harmonics: PF ≈ 0.95 → kVA = 526.

• With harmonics: PF drops to 0.8 → kVA = 625.

That’s nearly 100 kVA extra demand for the same work → more demand charges + higher losses.

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⚡ In short:
Harmonics → Higher RMS currents → Higher apparent power (kVA) → Higher demand charges + more losses → Bigger electricity bills.

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✅ Next Step: For practical methods to mitigate harmonics, check out Methods for Reducing Harmonics in System.

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⚡ Final Note: Harmonics are unavoidable in modern power systems, but their impact can be minimized through proper design, filtering, and load management.

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