The connection generally comes off of the grounding transformer into a separate input on the relay. This is for high impedance grounding of gen neutral through a distribution transformer.
27G usually refers to a 3rd harmonic voltage sensing relay that monitors the generator's neutral to ground voltage. The relay is connected to the grounding trasnformer/resistor as smallGreek says.
In "normal" operation a 3rd harmonic voltage exists between the neutral and ground and can be measured by the relay. When a ground fault occurs near the neutral point of the winding, the transformer primary is shorted out and the 3rd harmonic voltge disappears, picking up the 27G element. The 59G relay does not react to faults close to the generator neutral, there is not enough voltage.
Problem: 3rd harmonic N-G voltage varies from machine to machine and varies with load. You usually cannot calculate the setting. Have to measure the values and select a safe setting.
Some 27G relays take input from the generator VT's and/or CT's to develope blocking/permissive signals.
