Harmonic Filter Powerful with Super Tuned Specification
When super tuned, the electrical harmonic filter turns a multitasker that eliminates harmonics, maintains power system stability by mitigating power surge, voltage fluctuation and resonance. A super tuned harmonic filter is an engineering tool that prevents power plant blackout by stabilizing power system parameters for a successful grid de-synchronization, and islanding power generators on utility power failure. Besides, it relives station generators from momentary voltage fluctuations and power surges over a grid or internal power disturbance which helps in eliminating premature winding insulation failure of the generator.
A quality harmonic filter panel is custom-built for the project needs with incremental costs and complexities. In its basic specification, it provides a low impedance parallel path which enables current harmonics from its source at downstream loads to flow into it and allows a tolerable level of current harmonics, within IEEE-519 norm, to flow into the upstream utility grid. However, basic harmonic filters can’t control high harmonic levels. Whereas, a top-end harmonic filter mitigates both current and voltage harmonic distortion from any higher level to within limits.
Harmonic Filter – a tool that prevents Arc Flash
A mandate that is increasingly becoming noticeable among electrical engineers and safety regulators is the NEPA 70E standard. Besides, the Occupational Safety and Health Administration (OSHA) issues citations based on the requirement NEPA 70E. It stipulates employers to protect employees from arc flash hazard while working in an electrical system by wearing Switching Arc-Flash Suit. We are using non-linear-loads (NLL) since the 1970s, but the arc flash issue came into prominence much later since the year 2005. What has changed since then?
Arc flash and electrical blast – why it’s happening now?
By now, we have expanded our use of variable frequency drive (VFD) also called variable speed drive (VSD). Then, we used speed control for critical process load that required matching speed variation. We then preferred DC drive over VFD drive which did not make use of high-frequency switching, the IGBTs. Today, we use variable-speed drives engaging high-frequency IGBT at every other motor load. We save energy using VFD drives for energy conservation projects. We achieve conditions for our comfort by utilising it in HVAC and air-conditioning projects.
Arc flash – what’s the Root Cause?
The switching frequency (also called carrier frequency) of a VFD drive is the rate at which it’s DC bus voltage is switched on and off during pulse width modulation (PWM) process. VFD manufacturers increase switching frequency apparently to reduce audible noise from motor and harmonic generation. VFDs typically operate in between 4-20khz frequency range.
At higher switching frequency, the noise pitch from stator lamination moves higher beyond the human hearing range, so we hear a lower audible noise from the motor. Similarly, at the higher switching frequency, the measurable harmonic generation, typically up to 50th order are lesser, which matters for the sake of meeting regulatory harmonic-norms which do not measure higher and kilo Hz order harmonics.
Harmonics are high-frequency currents and voltages. All frequencies which are odd multiple of three (like 3x50hz, 9x50hz, 27x50hz etc.) are called triplen harmonic currents or voltages. Unlike the fundamental-frequency (50 or 60hz) and other harmonic frequencies; triplen harmonics don’t cancel at three-phase neutral but add-up and grow three-fold. The lower order triplen frequencies add-up and increase the neutral current in the electrical power circuit. The arc-flash problem happens with the higher-order triplen-frequencies which add-up to a charged voltage forming an electrical field (potential gradient) around electrical conductors in its power system.
Arc flash – how encon harmonic filter eliminates it
Arc flash phenomenon is similar to the corona effect on high-voltage transmission power lines. It happens when the air around a high voltage electrical conductor ionise to a sufficient potential gradient (electrical field) to form a conductive region. It generates a bluish glow around the air of the current-carrying metal conductor. When the strength of the electrical field is high enough to cause an electrical breakdown, arcing happens to a nearby object at ground potential.
Adding-up of high-frequency triplen-harmonics from the variable frequency drives and other IGBT-based NLL is the precursor of an arc flash. Once it attends sufficient field strength, it arcs to a nearby object at ground potential causing electrical blast involving electrical equipment.
We have troubleshot many dangerous arc flash blasts wherein we found the frequent arcing objects to be switchgear (air circuit breaker, MCCB, Fuse), the body of electrical panel, Bus Bar. The electrical bus bar affects in several ways. Pitting along the bus bar is a common effect. In one incident a portion of bus bar at the junction of horizontal and vertical joint evaporated and vanished in an arc with its body. We built our super tuned harmonic filters with special zero-sequence winding which provides a path to the high-frequency electric field which prevents arcing.
Arc flash solution – a case study
We have written a separate case study for electrical arc flash – read it at Mitigate VFD Harmonics.
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