Resolved, never to do anything, which I should be afraid to do, if it were the last hour of my life.

Resolution Number 7.
Jonathan Edwards

Understanding phase rotation is vital when connecting two systems together because the results can be catastrophic if someone doesn’t understand how to interpret phase rotation drawings. You would think something as important as phase rotation would have consistent terms across the entire industry. Unfortunately, you’d be wrong. //

You can’t determine phase rotation with a phasor diagram unless you know the one universal rule in the relay testing world. ALL PHASORS ROTATE COUNTER-CLOCKWISE. //

If you want to be sure you understand phase rotation correctly, put your finger anywhere on the phasor diagram and imagine that the phasors are spinning counter-clockwise. Start paying attention when your reference phasor crosses your finger. Which phasor crosses your finger next? Which is the last phasor to cross your finger?

Have you ever wondered why the square root of three shows up in so many three-phase power calculations?

Where does this number come from, and why is it so special?

While the long answer to these questions comes from trigonometry, the good news is that we can use phasor diagrams to make explaining it very simple to understand.

Understanding phasor diagrams is an important skill for relay testing and working through the examples in this article will give you a much deeper understanding of and appreciation for the phasor quantities in phasor diagrams. Regardless of which part of the industry you work in, this will greatly benefit your career in electrical power and relay testing.

What happens when you shoot a full oxygen tank with a large-caliber gun?

The following chart includes the values generated for a three-phase balanced offline-meter test into an SEL-351 relay using a Megger Test-Set or RTS.

Test-Set
Relay Magnitude Angle
Voltage Channel V1 VA 69.28V 0°
Voltage Channel V2 VB 69.28V 120°
Voltage Channel V3 VC 69.28V 240°
Current Channel I1 IA 1.000A 0°
Current Channel I2 IB 1.000A 120°
Current Channel I3 IC 1.000A 240°

The relay is connected to 300:5 CTs and 35:1 PTs. Is everything correct in the following meter test?

Here’s what we know so far:

• The metering results aren’t zero, which means the relay’s analog to digital converters are working.
• The CT and PT ratio settings in the relay are correct (notice that we don’t need to look at the actual settings to determine this). We’re injecting 1A in all three phases and the relay is reporting approximately 60A. The worst-case accuracy is -0.355% error, which is consistent with the 60:1 CT ratio. The relay is reporting approximately 2.42kV in all three-phases with a maximum percent error of -.07%, which matches the PT ratio.
  • The relay is looking in the correct direction because the currents and voltages are in-phase.
  • We are injecting A-B-C, or 1-2-3, rotation because the following pattern exists in the relay and test-set: A-Phase is 0°, B-Phase lags A-Phase by 120°, and C-Phase lags A-Phase by 240°.
  • The relay is programmed with the WRONG phase rotation because the sequence components show 0% positive sequence, 100% negative sequence, and 0% zero-sequence.