3 Phase Power Measurement, Harmonics

How is a wattmeter connected to a load?

Has two sets of leads, one to go in series with load (current) and one to go in parallel with load (voltage)

How is 3-PH power accurately measured with one wattmeter if it’s a balanced load?

How can one wattmeter be used if no neutral is available? (star)

This creates a “floating” star point, as long as both resistors have the same resistance as the wattmeter’s voltage coil to provide balanced voltage

What is the formula for power factor?

P / S (ratio of true power / apparent power

How is 3-PH power accurately measured with three wattmeters?

All in series with phases for current, all in parallel to single point (floating star point) for voltage

How is 3-PH power accurately measured with two wattmeters?

Both in series with phases for current, both in parallel to third phase for voltage (Note: only works if NO neutral)

When measuring power with two wattmeters, how are the readings interpreted?

If both values positive, add together for total power. If one is negative, minus negative from positive.

What is a linear system vs a non-linear system?

PICTURED: NON-LINEAR SYSTEM. Sinusoidal current (sin wave) used in linear systems (heaters, motors etc.). Non-linear systems have fluctuating resistance (computers, variable frequency drives etc.)

In a non-linear system, what is critical firing voltage?

When a certain voltage is reached in the period of a sine wave, it allows current to flow (firing edge) and scales respectively until it reaches the “downward side” of the sine wave and stops conducting current (stopping edge). Current conducted in pulses.

What is a harmonic?

A multiple of the fundamental frequency (50Hz) e.g. 1st harmonic = 50Hz, 2nd = 100Hz, 3rd = 150Hz, etc.

What happens to even and odd harmonics in 3-PH systems?

Even (2nd, 4th, 6th etc.) harmonics cancel out, meaning we only have to deal with odd (3rd, 5th, 7th etc.) harmonics

What damage can harmonic voltage distortion do to a device?

Harmonic current distortion can begin to distort voltage waveforms in the supply system. Voltage distortion causes premature failure

How does the 3rd harmonic affect 3-PH neutral conductors?

3rd harmonic of each of 3 phase conductors are exactly in phase, which adds in the neutral to above any of the phases’ currents, meaning the neutral conductor is no longer protected

What damage can 3-PH harmonic current distortion do and how can it be fixed?

raises temp of neutral, overheats supply transformer, UPS + generator + computers malfunction, etc. Solutions include oversizing transformers + generators, neutral conductors (up to 200%), using separate neutrals for each phase, filtering, subcircuit segregation etc.

In a 3-PH induction motor, what different harmonics can be observed?

positive sequence (1st, 4th, 7th): rotates in same direction as fundamental. negative sequence (2nd, 5th, 8th): rotates in reverse direction. zero sequence: odd multiples (3rd, 6th, 9th) don’t rotate

What are three symptoms of harmonics in a 3-PH supply system?

overheating neutrals, circuit breakers tripping without obvious overloads, 3-PH motors overheating and performing poorly