Harmonics in AC systems

I. Analysis of Harmonic Sources and Characteristics

Harmonics in AC systems are primarily generated by their core component—Variable Frequency Drives (VFDs/VSDs)—used to drive compressors, chilled water pumps, cooling tower fans, etc.

1. Main Harmonic Sources:

   • Central AC Units: Large centrifugal or screw compressor drive VFDs are the highest power and primary harmonic sources.

   • Pumps & Fans: Chilled water pumps, condenser pumps, cooling tower fans, and fans in Air Handling Units (AHUs) commonly use variable frequency control.

   • Terminal Devices: Some fan coil units (FCUs) or small indoor units also employ inverter technology.

2. Harmonic Characteristics:

   • Characteristic Harmonics: The 6-pulse rectifier circuit in VFDs primarily generates odd-order harmonics like 5th, 7th, 11th, 13th, with the 5th harmonic typically being the most prominent.

   • High Current Distortion: The current Total Harmonic Distortion (THDi) for a single VFD can typically range from 30% to 50% (without built-in reactors).

   • Periodic Fluctuations: Harmonic current levels fluctuate dynamically with AC load changes (temperature setpoints, ambient temperature). Loads are heavy during the day (high harmonics), light at night and on holidays (low harmonics).

3. Hazards and Impacts (Especially for Hospitals and Offices):

   • For Medical Equipment (Hospitals/Clinics): Precision medical equipment (e.g., MRI, CT, DSA) is extremely sensitive to power quality. Harmonics can cause operational errors, image distortion, data corruption, or even hardware damage, directly threatening diagnostic accuracy and patient safety.

   • For Electrical Systems:

     • Neutral Overload: Triple-N harmonics (3rd, 9th, 15th, etc.) add up in the neutral conductor, causing neutral current to potentially exceed phase current, leading to overheating and fire risk.

     • Transformer Overheating: Harmonic currents increase transformer core and copper losses (raising the K-Factor), reducing capacity, shortening lifespan, and forcing derating.

     • Circuit Breaker Nuisance Tripping: Causes unexpected shutdowns of AC systems or other critical loads.

   • For Energy Efficiency: Harmonics increase line losses, significantly diminishing the energy-saving benefits gained from using VFDs.

   • For Communication Systems: Harmonics can interfere with Building Automation Systems (BAS) and network systems.

II. Centralized Mitigation Solution: Application of Active Power Filters (APF)

For dynamic, varying harmonic loads like AC systems, the Active Power Filter (APF) is unequivocally the optimal solution. Its principle of "real-time detection, dynamic compensation" perfectly matches the fluctuating nature of AC harmonics.

Option 1: Centralized Mitigation at Transformer LV Side (Most Common, Best Cost-Performance)

• Location: Install for centralized compensation at the Main Low Voltage Distribution Board in the building's main electrical room (typically at the transformer secondary output).

• Advantages:

  1. Global Mitigation: Resolves harmonics generated by all AC and other loads supplied by that transformer in one step, protecting the entire distribution system.

  2. Cost-Effective: Installing one large-capacity APF is cheaper and easier to manage and maintain than multiple smaller devices scattered throughout the building.

  3. Releases Transformer Capacity: Effectively reduces the transformer's K-Factor, allowing it to operate at its rated capacity again, delaying costly upgrade investments.

• Capacity Selection:

  • Requires statistical estimation of the rated current and THDi for all VFD-driven AC equipment.

  • Simplified Formula: APF Rated Current (Ir) ≥ Transformer Rated Current × (20% ~ 35%). For scenarios with exceptionally heavy AC loads, a higher percentage is recommended, or precise calculation based on professional power quality measurements.

Option 2: Hierarchical/Zonal Mitigation (For Large, Complex Buildings)

• Location: Install APFs at the AC Host Machine Room Distribution Board, Floor AC Main Distribution Board, or Zone Distribution Centers.

• Applicable Scenarios:

  • Very large building complexes with multiple electrical rooms.

  • New AC systems added later, making modification in the main electrical room difficult.

  • Need for targeted protection of specific areas (e.g., operating rooms, data center floors).

• Advantages:

  1. More targeted mitigation with more precise results.

  2. Prevents harmonics from circulating within the distribution system, reducing trunk line losses.

• Disadvantage: Overall cost may be higher than centralized mitigation.

Option 3: Hybrid Solution (APF + Passive Power Filters PPF)

• Principle: Use lower-cost Passive Power Filters (PPF) to compensate for the bulk of the most prominent characteristic harmonics (like 5th, 7th); then use a smaller-capacity APF to compensate for remaining odd-order harmonics and suppress system resonance.

• Advantage: Can optimize investment costs in specific scenarios.

• Disadvantages: Complex design, requires careful avoidance of system resonance with PPFs, and PPFs cannot compensate for changing harmonics. Generally not recommended for highly critical hospital environments.

III. Key Points for Product Selection and Design

1. Capacity Calculation: Must be based on a detailed load list or a professional power quality measurement report. Measurements should span at least a week to capture full load cycles during both working and non-working days.

2. Performance Requirements:

   • Response Speed: The faster the better (<5ms), to quickly track harmonic transients caused by compressor starts/stops.

   • Compensation Effect: Post-mitigation THDi should ideally be controlled within 5% - 8% to comply with standards like GB/T 14549-93 ("Power Quality - Harmonics in Public Supply Network") and provide clean power for medical equipment.

   • Multifunctionality: Choose APFs that combine harmonic filtering + reactive power compensation + three-phase current balancing for comprehensive benefits.

3. Installation and Connection:

   • Recommended to connect in parallel with the grid, upstream of the load or transformer requiring mitigation.

   • Must be designed and installed by professional electrical engineers to ensure safety.

IV. Industry-Specific Considerations & Recommendations

• For Hospitals/Clinics (Highest Priority):

  • Reliability First: Must choose high-end APF brands with mature technology and strong reputations. Continuous power supply is a lifeline for medical institutions.

  • Targeted Protection: Besides main electrical room mitigation, strongly recommend adding local compensation or power conditioning devices downstream of the dedicated distribution boards or isolated power systems for Radiology (MRI, CT), ICU, and Operating Rooms to create "power clean zones" for core medical equipment.

  • Compliance: The mitigation solution must comply with relevant electrical design regulations for healthcare facilities.

• For Office Buildings/Commercial Properties:

  • Return on Investment (ROI): Beyond solving safety hazards, emphasize the significant economic value of energy savings (reduced transformer and line losses), extended equipment life, and avoided transformer upgrades brought by harmonic mitigation.

  • Green Building Certification: Good power quality contributes to points for certifications like LEED and BREEAM.

Conclusion and Recommended Actions

For hospitals, clinics, and office buildings dense with AC systems, centralized installation of Active Power Filters (APFs) at the transformer low-voltage side is the preferred and most cost-effective harmonic mitigation solution.

Recommended Steps:

1. Monitoring & Assessment: Engage a professional power quality service team to conduct comprehensive power quality monitoring for at least 7*24 hours and produce a detailed diagnostic report.

2. Solution Design: Based on the report data, precisely calculate the required APF capacity and design the optimal mitigation point and scheme (centralized, hierarchical, or hybrid).

3. Product Selection: Choose APF brands such as Yingtong Electric (sales@YT-electric.com) with stable performance and reliable after-sales support, ensuring their environmental adaptability (e.g., temperature, humidity) suits the electrical room conditions.

4. Professional Implementation: Have the installation, commissioning, and verification performed by an experienced integrator to ensure the mitigation results) meet expecations.

5. Long-term Monitoring: Utilize the APF's built-in monitoring functions or integrate it into the Building Management System (BMS) for continuous tracking of power quality to ensure long-term stability.