Transformers - Instantaneous Loss of Load on Secondary

[SailorMan]

Can you provide some information regarding the instantaneous loss of a load on the secondary of a transformer? What impact does this have on the voltage, reflected to the primary, if any? The flux should remain but the inductance will not want to allow a change of current - to a now open circuit.

 

[electricpete]

I posted a response but it seems to have disappeared.

There is capacitive coupling between the primary and secondary of a transformer. At power frequencies it is very high capacitive reactance and insignificant to the power frequency modle. But as frequency increases (fast-rising transients), the capacitive reactance decrease and becomes more signficant.

Bottom line... a high-frequency voltage transient will be capacitively-coupled from secondary to primary (or vice versa). The higher the frequency (steeper rise time etc) the better the coupling.

The nature of the voltage spike on the low side will depend on the interrupting device. Also cabling between the spike and the low side will provide inductance which will lower the magnitude and lower the frequency content of the spike at the low-voltage side => lowering the spike at high side.

 

[jbartos]

Suggestion: Essentially, there are three types of loads that potentially may be on the transformer secondary:
1. Prevailing resistive load. If abruptly lost, there will be some voltage increase transient on the transformer primary.
2. Prevailing inductive load. If abruptly lost, there will be a noticeable transient voltage increase on the transformer primary, e.g. it will start with a voltage spike.
3. Prevailing capacitive load. If abruptly lost, there will be an elevated voltage oscillatory transient on the transformer primary.
Also, the system impedance will impact the voltage transients. If the system impedance is very low (stiff voltage source), there will be smaller voltage transients.