What is Open delta:
Two single phase voltage transformer uses in this configuration. The transformer’s one end is connected with the another end of the transformer creates series connection with two transformer and primary is connected in star connection. The secondary output voltage of the open delta configuration is equal to 1/root (3) of that delta configuration. That’s why, The open delta will only deliver 57.7% of the power of a conventional transformer.
Broken Delta configuration:
Three number of potential transformers are connected in delta connection and the one end of the delta connection will be in open condition. It protects the winding against earth faults & used to find out the vector sum of the phase voltages. Under balanced condition, the vector sum of the output voltage is equal to zero. Mostly, this kind of configuration is used in Natural displacement relays to protect the delta winding against earth faults.