Active Harmonic Filters (AHF)
Harmonic currents are caused by non-linear loads connected to the distribution system. A load is said to be non-linear when current it draws does not have same wave shape as the supply voltage. The flow of harmonic current through the system impedances causes voltage distortion in the distribution system. Following are the major harmonic generators :
- Industrial Processes viz Welding, Impact Loads, Induction Furnaces, Battery Charger, DC Power Supplies.
- Variable Speed Drives for AC & DC motors.
- Uninterrupted Power Supplies.
- Office Equipment (PCs, Printers, Servers, Displays etc.)
- Harmonic currents increase the RMS current in electrical systems and deteriorate the quality. They stress electrical network and potentially damage the equipment. They may disrupt normal operation of devices and increase operating costs.
- Office buildings, commercial establishments having predominant non-linear loads such as data servers, UPS systems and mix of singe phase loads leads to harmonic generation in the supply network which create several problems such as overheating of cables, tripping of breakers, blowing of fuses, additional heating in the transformers etc. High levels of triplex (3rdorder and multiple) harmonics specifically overload the neutral conductor and often result in unacceptable neutral to earth potential. Therefore the load currents in such installations are highly polluted due to the presence of high level of harmonic current
- Improvement upto the extent of 95% can be achieved in the harmonic current distortion (THDI) at certain feeders after installation of active harmonic filters. Excessive THDI at high load current is the main cause of higher heating losses, tripping of breakers and other problems. The active harmonic filters not only reduce the high harmonic distortion but also improve the power factor close to unity (0.99 lag). This has resulted in to 20% reduction in KVA consumption by the UPS system. Low power factor is characterized by high load current which in turn results in additional line losses, overheating and in some cases penalties causing direct financial losses. A leading power factor is not desirable and may require derating of UPS systems. The active harmonic filters correct the leading power factor to unity. The combined effect of harmonic filtering and power factor improvement brings additional benefits to the customer. Reduced losses lower the operating temperature of the equipment which indirectly increases the life of the electrical equipment.
- Active harmonic filters are designed to measure the load current, calculate variance from objectives set and inject the right amount of current to meet the required level of harmonics, displacement power factor or load balancing.
- When harmonic mitigation is required, the logic measures the load current and calculates the amplitude and phase angle for every harmonic to as high as 50th order. The control signal is generated and semiconductors are directed to duplicate the control signal as injected into the supply. This reduces the supply side harmonies.