1. PT is a special transformer used to change voltage. PT is kind of transformer whose capacity is small and it can change different primary voltage to lower standard voltage through the configuration of the number of primary and secondary winding. Normally, voltage is 100V or 100√3V, open delta is 100/3.
2. PT has ungrounding PT (fully insulated) and grounding PT (Half insulated). Fully insulated means that two outgoing terminals of primary winding is to earth and insulated.
3. Residual winding of PT: secondary side of PT has one residual winding. For PT whose central point has non-effective grounding system, rated voltage of residual voltage winding is 100/3V. Standard grade is 3P or 6P. Connect to open delta when using residual winding. When system is in normal operation, the total sum of voltage phasor of three residual winding is zero--- U△=0. When single phase grounding fault or serious imbalance of three phase happened, voltage will appear at open delta end.
4. Precautions for PT secondary wiring
a. Secondary circuit should have short-circuit protection, for example, using air switches or fuse, but no need for central line, secondary circuit of open delta winding and PT of auto regulating excitation device of generator.
b. Secondary winding should have reliable security grounding point.
c. There are measures to prevent voltage feedback from secondary circuit to primary circuit.
5. Classification of PT
a. According to phase: single phase and three-phase
b. According to winding: double winding, three winding, four winding
c. According to insulation medium: oil insulation, casting resin insulation, dry-gas insulation
d. According to the kind of device: indoor type and outdoor type
e. According to structure: magnetic type and capacitor type