Introduction: YDTW series AC test transformers are of module type and oil insulated, inner insulating materials are transformer oil, shells are insulating cylinders, which are made of epoxy resin. The high voltage can directly feed to test object without bushing, and modules are allowed to stack in cascade to reach very high voltage levels by keeping reasonable floor space. Rated voltage of single module can be up to 750kV, the highest voltage can reach 2250kV by stacking three of these transformer modules. Application: AC test transformer systems are conventional AC high voltage test equipments, mainly designed for medium capacitance testing objects, particularly for tests requiring stable voltage even if the load changes under voltage (corona, wet & pollution tests). Especially when the load is inductive (like inductive voltage transformers), tests need resistive current, AC test transformer systems are the only choice. Configuration: Essential: Regulating transformer AC test transformer Protect resistor Capacitor divider Control and measure system Optional: Isolation transformer Compensation reactor Power filter HV filter Electronic fast protect device Principle: Test transformer steps up the voltage coming from the regulating transformer. The controls act on the regulating transformer to adjust precisely the input voltage of the transformer and thus the test voltage. If needed, a compensating reactor can be connected between the regulating and the test transformers and compensates the capacitive power required for the test object. This way, the regulating transformer and power line filter ratings can be kept small, and the requirements for the power supply are reduced.