What are the effects of harmonics on power transformers?
- Harmonic currents increase copper losses in transformers, leading to localized overheating, vibration, excessive noise, and additional winding heating.
- Harmonic voltage causes additional losses, increasing hysteresis and eddy current losses. When system voltage is high or unbalanced, the harmonic components in the excitation current rise, amplifying electrical stress on insulation materials and increasing partial discharge risks. For delta-connected windings, zero-sequence harmonics create circulating currents, raising winding temperatures.
- Harmonic currents in transformer excitation currents amplify inrush currents during switching. Under resonant conditions, these harmonics threaten transformer safety.
How do harmonics affect power surge arresters?
In substations, large-capacity, high-voltage transformers with prolonged inrush currents (lasting seconds or more) may trigger resonant overvoltages. This can extend the discharge time of surge arresters, causing damage. Selecting arrester parameters for protecting inductors or capacitors in high-voltage filters becomes challenging.
Impact of harmonics on transmission lines:
- Harmonics increase line losses. Skin effect and harmonic currents generate additional losses, especially in unbalanced three-phase systems, significantly raising neutral-ground line losses.
- Harmonics amplify neutral line currents, causing neutral point drift. In low-voltage networks, zero-sequence harmonics (3rd, 6th, 9th...) overload neutral lines, increase losses, and degrade power quality through voltage drops.
Effects of harmonics on power capacitors:
Nonlinear loads combined with parallel capacitor banks lead to harmonic current injection into capacitors (due to their low harmonic impedance). Resonant conditions between capacitor banks and system inductance amplify harmonic currents, causing overheating and capacitor failure. Voltage/current harmonics exceeding limits raise capacitor currents beyond safe thresholds (e.g., 1.3x rated current for self-healing capacitors). Distorted waveforms induce partial discharges, accelerating insulation aging.
Harmonic impact on power cables:
Harmonics increase dielectric losses, leakage currents, temperature rise, and partial discharges in dry-type cables, raising single-phase fault risks. Cable capacitance amplifies harmonics, especially during low-load periods. Higher-voltage cables face greater instability risks due to harmonic-induced dielectric stress.
Harmonic effects on other power system equipment:
- Synchronous generators: Harmonic/negative-sequence currents cause additional losses, localized heating, and torsional rotor vibrations.
- Circuit breakers: Harmonics reduce interrupting capacity, induce re-ignition during inductance current interruption, and damage contacts.
- Arc suppression coils: Severe harmonics impair compensation during single-phase faults, escalating system failures.
- Power-line communication: Harmonics introduce noise in voice communication, distort data transmission, and disrupt EMS/DAS systems.
Impact on relay protection and automation:
- Environmental influence: Areas with high harmonics (e.g., arc furnaces, railways) or resonant risks disrupt protection systems.
- Sensitivity: Negative-sequence or zero-sequence relays are highly vulnerable to harmonics.
- Device errors: Digital relays and microprocessor-based protections malfunction due to waveform distortion or zero-crossing interference.
Harmonic interference with differential protection:
Digital differential protection, despite its speed and simplicity, may misoperate under distorted current waveforms in poor power quality conditions.
Harmonic influence on energy metering:
Harmonics reduce accuracy in induction and electronic energy meters, leading to measurement errors.
User-side harmonic effects on the grid:
Nonlinear loads (e.g., inverters, arc furnaces) are major pollution sources. Switching devices and large-scale industrial converters distort voltage/current waveforms.
Impact on end-users:
- Motors: Harmonics cause overheating, vibration, noise, and torque reduction. Negative-sequence harmonics (5th, 7th, 11th...) mimic unbalanced voltage effects, damaging windings.
- Capacitors: Poorly designed capacitor banks face resonance, overcurrent, and explosion risks.
- Automation systems: Harmonics distort sensor signals, damage hardware, and trigger false protections.
- Residential users: Light flicker, appliance stress, TV/radio interference, and billing inaccuracies occur.
Safety and computer system risks:
Harmonics increase fire hazards, equipment damage, and communication failures. For computers, voltage fluctuations, transients, and nonlinear loads disrupt operations, causing data errors or hardware damage.
