Têkiliya THD: Ji Tikanyê derava Berhemên Elektrîkî
Dîrokên THD harmonîk li ser sîstemanê ya elektrîk hewce ye ku analîz bikin ji du aspekta: "THD rastî ya taybetandî yên serxwebûn (taybetandîya harmonîkê yekşî)" û "çewtiyên pêwistina THD (pêşkeftina nederbasdar)" — pirsa herî xasî diha biguherîne serxweyên sistem û stabîlît, wê lêrê derbarê çewtiyên pêwistina "alarmên gal êk û nedarbikin." Hewce ne divê dîrokên sistemê biceribînin. Dîrokên li ser hemî zeviya elektrîkê da — jînraşî → veşîn → dastrîn → karanîn — keset û stabîlît û ekonomî.
Împektî Serxwebûn 1: Xasî Dîroka THD Rastî Yekşî (Taybetandîya Harmonîkê Yekşî)
Heke THDv (total harmonic distortion voltage) serxwebûnê bi standartên navendî (≤5% ji bo serxwebûnên amade) an THDi (total harmonic distortion current) serxwebûnê bi dayîna cihazan (misal, trafolar ≤10%) diguheztî, ew cihazan fisîkan sistem, stabîlît vekefî û cihazên karberan werde biguherîne.
Sîstemên Veşîn: Zêdetir Hêjir û Overheating
Zêdetir Hêjirên Miskî: Harmonîkên baran "skin effect" li ser rêzanên veşîn (misal, kabîlên 110kV) biguherîne, ku hêjirên paqijên paqij ên di ser surfaceyê de tevgerî dihin, resistance û hêjirên miskî zêdetir bikin. Misal: Heke THDi ji 5% ber 10% diguheztî, hêjirên miskî reyan 20%-30% (berekare bi I²R). Operasyonê digerîn temperatureyê conductor (misal, ji 70°C ber 90°C), insulation aging û dema reyan qis bike (ji 30 sere salan ber 20).
Voltage Sag Worsened: Harmonîkên voltage bi fundamental voltage hatine kirin, ku waveforms di endanên load de biguherîne. Karberan pêşeng (misal, plantên semiconductor) dikarin şopandin cihaz bikin ji bo voltage irregular, ku her vêgirî cost hundreds of thousands.
Pargîdan Cihaz: Overheating, Damage, û Dema Qis Bike
Risks Transformer Failure:
Harmonîkên baran "additional iron losses" (eddy current losses rise with the square of harmonic frequency) zêdetir bikin. Bi 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.
Jînrawanan Cihaz: Output Fluctuations û 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).
Sîstemên Control: 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 û Frequent Failures
Industrial Motor Overheating û 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%.
Împektî Serxwebûn 2: Risks Indirect 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.
Împektî Serxwebûn 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:
| Loss Type | Specific Performance | Quantification Example (Taking a 10kV Industrial User as an Example) |
| Direct Equipment Cost | Burnout/replacement of equipment such as transformers, capacitors, motors | When THDv=8%, the annual equipment replacement cost increases by 5-20 million yuan (calculated based on 2 transformers + 3 sets of capacitors) |
| Additional Energy Consumption Cost | Increase in copper loss/iron loss of lines and transformers | When THDi=10%, the annual additional electricity consumption increases by 100,000 - 500,000 kWh (calculated based on an annual electricity consumption of 10 million kWh and an electricity price of 0.6 yuan/kWh, the annual additional electricity fee is 60,000 - 300,000 yuan) |
| Production Stop Loss | Shutdown of sensitive equipment and interruption of production lines | A semiconductor factory’s lithography machine shuts down for 1 hour due to harmonics, resulting in a loss of wafer output value exceeding 500,000 yuan |
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.
