In accordance with the relevant provisions of the Code for Handover and Preventive Tests of Electrical Equipment, power plant maintenance units shall conduct regular maintenance on CTs and PTs in accordance with the code to avoid short-circuit faults in PT circuits and open-circuit faults in CT circuits.

The neutral point CTs and enclosed bus PTs of power plant generators are installed in special positions with complicated wiring, which may become blind spots in the electrical maintenance work of power plants. This can lead to unstable performance of PTs and CTs at the generator terminals and trigger a series of accidents.

Test Project

Special maintenance and testing of the generator neutral point CTs, outlet PTs and enclosed bus PTs for Unit 1 and Unit 2 of the 600MW coal-fired generator set.(After the commissioning and power generation of this generator set, faults in the primary circuits such as PTs on the 20kV side of the generator set outlet have occurred for many times, resulting in abnormal operation of the generator set.)

Test Equipment

CTP Series Transformer Tester

△ CTP Series Transformer Tester

△ Outlet Bus of Steam Turbine Generator

A 600MW generator is usually equipped with three sets of outlet PTs, which are respectively used for measurement, protection, excitation and other circuits. Faults in the primary and secondary circuits of the generator and PTs can all cause voltage abnormalities. Therefore, it is crucial to accurately test the transformation ratio, polarity and other parameters, quickly identify the fault points and analyze the fault causes.

△ Phases A, B and C of the Enclosed Bus PT at the Generator Outlet

△ Wiring is carried out simultaneously on the outside and inside sides of the wall during the test of the generator enclosed bus PT

The CTP Series Transformer Tester is a new generation of high-precision tester for transformer excitation characteristics, transformation ratio and polarity based on low-frequency frequency conversion technology. It combines the low-frequency frequency conversion method with the voltage method. Compared with the power frequency indirect test method for excitation characteristics, it does not affect the insulation of CT windings, and features low output voltage, low output capacity, higher safety in the test process, and a compact and lightweight design.

Traditional transformer testers usually apply power frequency voltage and current for measurement. A relatively high voltage, even up to several thousand volts, is required for the volt-ampere characteristic test; and a current of several thousand amperes is sometimes needed for the transformation ratio, ratio error and phase error tests. These tests demand high-power voltage and current boosters, resulting in bulky and heavy equipment. Such testers are incompetent for testing CTs with large current and high voltage, and cannot complete the testing of generator neutral point CTs and enclosed bus PTs.

△ Key CT parts of the generator with narrow space and high testing difficulty

△ Inconvenient wiring at the enclosed bus PT end on the outer wall, requiring high automation of test equipment

The CTP Series Transformer Tester only needs to output a small voltage and power, and is small in size and light in weight, making it highly suitable for the maintenance and testing of complex working positions such as generator neutral point CTs, enclosed bus PTs, transformer bushing CTs and pole-mounted switch transformers. For transformers with an extremely large transformation ratio, it can also measure and verify TP-class transient CTs with an excitation saturation voltage of up to 45kV.

The equipment features simple wiring, high safety and high testing efficiency, and is equipped with a fully automatic test function. It can automatically complete the steady-state or transient characteristic tests of CTs, including volt-ampere characteristics, 10%/5% error curves, transformation ratio, phase, polarity, winding resistance, secondary load, as well as ratio error and phase error under different operating currents and loads. The test accuracy reaches 0.05%; the measurement results are processed automatically, with test data displayed in real time and curves drawn; inflection points are calculated, and the results are automatically stored, printed and uploaded to the computer for archiving. This effectively eliminates measurement errors caused by manual reading and manual processing.

△ Wiring Schematic Diagram

△ Printing and Verification of Test Results