The variable frequency transformer tester is an advanced testing device that employs variable frequency power technology and digital signal processing technology. By changing the frequency of the test power supply (typically adjustable within the range of 1Hz - 200Hz, with 40Hz - 70Hz being the most commonly used), it measures key parameters such as the ratio difference (deviation ratio), phase difference (angle difference), excitation characteristic curve, volt-ampere characteristic, error curve, and secondary winding DC resistance of the transformer, and is particularly suitable for on-site testing.
Why is "variable frequency" needed? The core principle and advantages of the variable frequency transformer tester
The traditional testing method for transformers uses an AC power supply (50/60Hz). However, when testing large-capacity and high-voltage-level transformers (especially protective CTs) on-site, there are two major challenges:
Large power capacity: To test the excitation characteristics, a high voltage is required. If a 50Hz power supply is used, the power equipment (voltage regulator, booster) needed would be extremely bulky and unable to be brought to the site.
Core saturation: Under the power frequency, the core is prone to enter the saturation zone, making it difficult to accurately measure the characteristic curve before the saturation point.
Solution based on frequency conversion technology:
According to the law of electromagnetic induction, the induced electromotive force E = 4.44fNBmS. Under the condition that the magnetic flux density Bm remains constant, the voltage is directly proportional to the frequency. Therefore:
Reducing the frequency (for example, to 45Hz), the required test voltage for generating the same magnetic flux will also decrease accordingly. This means that smaller and lighter power supply equipment can be used to easily generate a sufficient high test voltage on-site to obtain the complete excitation curve up to saturation.
This is its most crucial advantage: "frequency reduction and voltage boosting", which enables the device to be portable and ensures efficient testing.
The main functions of the frequency conversion transformer tester
Excitation (volt-ampere) characteristic test:
This is the most important test for protective CTs. By measuring and plotting the voltage-current curves of the secondary windings, it is possible to calculate:
Turning point voltage (saturation point): This is the key factor for determining whether the CT meets the 10% or 5% error curve requirements.
Accurate Limiting Factor (ALF): Evaluates whether the CT can accurately transmit signals during a short-circuit fault without saturation.
Secondary limit voltage: Verify whether the CT performance meets the requirements of the protection system.
Ratio and polarity test:
Automatically measure the actual transformation ratio and compare it with the nameplate.
Check whether the polarity is correct (negative polarity or positive polarity).
Difference and angle difference test:
This is the core test for the measurement of CT/PT. It measures the proportional error and phase error between the actual secondary current/voltage and the ideal value, and determines whether it meets accuracy levels such as 0.2S, 0.5, 5P, and 10P.
Automatically calculate and plot the 10% error curve:
The instrument can automatically calculate and draw the 10% error curve of the CT based on the excitation characteristic data, and directly judge whether the error of the CT exceeds the standard under various short-circuit currents.
Measurement of DC resistance of the secondary winding:
Check the connection quality of the winding wires, as well as for any broken wires or inter-turn short circuits.
Demagnetization function:
After testing or in case of a malfunction, the CT core may retain residual magnetism, which can affect performance. The instrument can automatically perform demagnetization.
Load (impedance) test:
Measure the actual load of the secondary circuit of the transformer, and determine whether it exceeds its rated load.
Application scenarios of the frequency conversion transformer tester
Relay protection acceptance and verification:
Verify whether the peak voltage and 10% error curve of the newly installed or overhauled protective CT meet the requirements of the differential protection, overcurrent protection, etc. of the line or main transformer.
Measurement verification:
Carry out ratio and phase difference tests on the CT/PT used for metering at the关口 or for important billing purposes, to ensure the fairness and accuracy of trade settlements.
Preventive test:
Regularly check the characteristic changes of the operating transformers to prevent protection maloperation or failure to operate due to CT saturation.
Fault Diagnosis:
When a protection malfunction occurs, check whether the performance of the relevant CT has deteriorated or become saturated.
Engineering commissioning and handover: Essential testing items for new or renovated substations.
