Why can't circuit breakers close across a large phase angle difference?

Question

In power systems I understand that breakers have mechanisms preventing their closing if they detect the phase angle (and voltage) across them to be too large. This is usually called a 'sync check'. But assuming the voltage magnitude was the same and only the phase was different why is this required?

Answer 1

Short answer here.

If you imagine connecting two DC sources like batteries together that have different voltages, you would end up with current flowing from the high voltage source to the lower voltage source. So in the circuit the result is current flow in an undesired direction (short) and in some cases a large current spike.

Now imagine the same example in a power system where you are connecting two AC generators (or breakers to match your question). Remember that in a three phase AC waveform, if the two waveforms you want to sync have a different phase angle, this means that at that instance of time, the voltage magnitude is in fact different, identical to the DC scenario provided above. So if the phase angles of the AC sources were not equal, you would end up with undesired circulating currents between the breakers. Now with the power system the result is a bit more drastic.

• Huge currents from the high voltage ranges can flow.
• As mentioned in the comments by Andy and Brian above, these high currents cause substantial wear and damage to switch gear, and the magnetic mechanical force stresses placed on rotating machines (generators) is not ideal.

When the two sources to be synced are identical (or very close), the synchronizing will appear transient free.

If you are still unsure and this response doesn't answer your question, I would recommend reading up on synchronizing AC generators and it should provide clarity.

Answer 2

If the circuit breaker closes while we have a large phase difference the instant voltages on both sides of each phase will be different and this will be equivalent to a short circuit.

This must not happen even if we know that the circuit breaker will trip since depending on the type of the generator providing the enegry, different unwanted consequences will result.

For instance if we have a diesel genset then not only the excessive current will affect the altenator but also a back-torque will be applied on the diesel. If we have an inverter the power circuit will be defected.

So either we close the circuit breaker on a dead bus or we must synchronize.

After closing the breaker the game is not over we continue to apply what we call load sharing to make sure that each energy provider is providing the convinient power to its capacity and to avoid what we call reverse power.

Answer 3

This is to prevent short circuit, because, you can have 90° from phase to phase and ca short.I work in a power station and the sync sees frequency, voltage, and phase to close the power interruptor in 500 kV.