Harmonic THD Impact: From Grid to Equipment

The impact of harmonic THD errors on power systems must be analyzed from two aspects: "actual grid THD exceeding limits (excessive harmonic content)" and "THD measurement errors (inaccurate monitoring)" — the former directly damages system equipment and stability, while the latter leads to improper mitigation due to "false or missed alarms." When combined, these two factors amplify system risks. The impacts span the entire power chain — generation → transmission → distribution → consumption — affecting safety, stability, and economy.

Core Impact 1: Direct Harm of Excessive Actual THD (High Harmonic Content)

When grid THDv (voltage total harmonic distortion) exceeds national standards (≤5% for public grids) or THDi (current total harmonic distortion) surpasses equipment tolerance (e.g., transformers ≤10%), it causes physical damage to system hardware, operational stability, and end-user equipment.

• Transmission Systems: Increased Losses and Overheating

• Increased Copper Losses: Harmonic currents cause "skin effect" in transmission lines (e.g., 110kV cables), concentrating high-frequency currents on the conductor surface, increasing resistance and copper losses with harmonic order.
  Example: When THDi rises from 5% to 10%, line copper losses increase by 20%-30% (calculated via I²R). Prolonged operation raises conductor temperature (e.g., from 70°C to 90°C), accelerating insulation aging and shortening line life (from 30 to 20 years).

• Worsened Voltage Sag: Harmonic voltages superimpose on fundamental voltage, distorting waveforms at load ends. Sensitive users (e.g., semiconductor plants) may experience equipment shutdowns due to irregular voltage, with single incidents costing hundreds of thousands.

• Distribution Equipment: Overheating, Damage, and Reduced Lifespan

• Transformer Failure Risks:
  Harmonic currents increase "additional iron losses" (eddy current losses rise with the square of harmonic frequency). At THDv=8%, transformer iron losses increase by 15%-20% compared to rated conditions, raising core temperature (e.g., from 100°C to 120°C), accelerating insulation oil degradation, potentially causing partial discharge or burnout (e.g., a substation lost a 10kV transformer due to excessive 5th harmonic, with direct losses over one million).
  Unbalanced three-phase harmonics also increase neutral wire current (up to 1.5× phase current), risking neutral overheating and breakage, leading to three-phase voltage imbalance.

• Capacitor Bank Resonance Damage:
  Capacitors have low impedance to harmonics, easily forming "harmonic resonance" with grid inductance (e.g., 5th harmonic resonance can cause capacitor current to reach 3–5× rated value), resulting in insulation breakdown or explosion. One industrial workshop damaged three 10kV capacitor banks within a month due to 7th harmonic resonance, with repair costs exceeding 500,000.

• Generation Equipment: Output Fluctuations and Efficiency Drop

• Synchronous Generator Output Limitation:
  Grid harmonics back-feed into generator stator windings, creating "harmonic torque," increasing vibration (speed fluctuation ±0.5%), reducing output (e.g., a 300MW unit drops to 280MW at THDv=6%), and raising stator temperature, affecting generator lifespan.

• Renewable Inverter Grid-Connection Failure:
  PV/wind inverters are sensitive to grid THD. If point-of-connection THDv > 5%, inverters trigger "harmonic protection" and disconnect (per GB/T 19964-2012), causing renewable curtailment (e.g., a wind farm lost over 100,000 kWh in one day due to excessive 3rd harmonic).

• Control Systems: Maloperation Leading to System Faults

• Relay Protection Misoperation:
  Harmonic currents cause transient saturation in current transformers (CTs), leading to inaccurate sampling in overcurrent or differential protection. For example, superimposed 5th harmonic current distorts secondary CT current, causing overcurrent protection to falsely detect "line short circuit" and trip, resulting in widespread outages (e.g., a distribution network experienced 10 feeder trips due to THDi=12%, affecting 20,000 households).

• Automation System Communication Interference:
  Harmonics couple electromagnetically into control communication lines (e.g., RS485, fiber), increasing data error rates (from 10⁻⁶ to 10⁻³), delaying or corrupting dispatch commands (e.g., a "trip fault line" command fails to deliver, expanding the fault).

• End-User Equipment: Performance Degradation and Frequent Failures

• Industrial Motor Overheating and Burnout:
  Asynchronous motors under harmonic voltage generate "negative sequence torque," causing speed fluctuations, increased vibration, and higher stator copper losses. At THDv=7%, motor efficiency drops by 5%-8%, temperature rises by 20–30°C, and lifespan is halved (e.g., a steel plant burned two rolling mill motors within six months due to 7th harmonic, with repair costs over 2 million).

• Precision Equipment Accuracy Loss:
  Sensitive equipment like semiconductor lithography machines and medical MRI systems require extremely clean voltage (THDv≤2%). Excessive THDv increases measurement errors — e.g., a lithography machine’s etching precision drops from 0.1μm to 0.3μm due to voltage harmonics, reducing product yield from 95% to 80%.

Core Impact 2: Indirect Risks of THD Measurement Errors (Inaccurate Monitoring)

THD measurement errors (e.g., actual THDv=6%, measured as 4%, error = -2%) lead to "false compliance" or "over-treatment," exacerbating risks or causing economic waste — essentially, "data distortion leading to poor decisions."

• Missed Detection of Excess: Delayed Mitigation, Escalated Harm
  If measured THD is lower than actual (e.g., actual THDv=6%, measurement error -1%, displayed as 5%), it falsely indicates "harmonic compliance," delaying filter installation (e.g., APF). This allows long-term harmonic accumulation:

• Short-term: Accelerated aging and higher failure rates of transformers, capacitors, etc.

• Long-term: Risk of system resonance, potentially causing regional grid collapse (e.g., a regional grid experienced resonance after two years due to missed 3rd harmonic detection, resulting in 5 substations offline).

• False Alarm of Excess: Over-Investment, Wasted Costs
  If measured THD is higher than actual (e.g., actual THDv=4%, measurement error +1%, displayed as 5%), it falsely indicates "harmonic excess," leading to unnecessary filter installation:

• Economic waste: A 10kV/100A APF costs ~500,000; if no mitigation is needed, the equipment sits idle (with annual maintenance of 20,000).

• System disturbance: Excess filters may create new resonance points (e.g., installing a 5th harmonic filter triggers 7th harmonic resonance), introducing new risks.

• Data Distortion: Affects Grid Planning and Dispatch
  THD measurement errors distort harmonic distribution data, impacting long-term planning:

• Example: A region’s monitoring shows average THDi=8% (actual 6%), leading to over-provisioning of harmonic mitigation capacity (building 2 extra filter stations, investment over 10 million).

• In dispatch, inaccurate THD data prevents precise harmonic source identification (e.g., wrongly blaming a PV plant, limiting its output), affecting renewable energy integration.

Core Impact 3: Economic Loss — From Direct Costs to Indirect Losses

Harmonic THD errors (including excess and measurement inaccuracies) cause significant economic losses through equipment damage, increased energy consumption, and production downtime, quantifiable in three cost categories:

Summary: The Core Impact Chain of THD Errors on Power Systems

The fundamental impact of harmonic THD errors follows a cascading chain: "waveform distortion → equipment damage → system instability → economic loss." Measurement errors act to amplify or misjudge this chain:

• Excessive actual THD is the "primary hazard", directly damaging power system hardware and compromising stability;

• THD measurement error is the "decision interference", leading to improper mitigation—either worsening risks or wasting resources;

• Ultimately, both lead to safety risks (equipment burnout, system collapse) and economic losses (repair costs, energy waste, production downtime).

Therefore, power systems must adopt a dual approach: "precise monitoring (controlling THD measurement error ≤ ±0.5%) + effective mitigation (keeping actual THDv below 5%)" to comprehensively avoid these risks.