The CTP Series Transformer Tester is a new generation of high-precision transformer excitation characteristic, transformation ratio and polarity tester developed by Wuhan Haomai Electric Power based on low-frequency frequency conversion technology. It has more comprehensive test functions and greatly improved measurement accuracy, bringing convenience to on-site operations. At the same time, it requires a certain understanding of transformer parameters.

△ CTP Series Transformer Tester

At present, the number of transformer test parameters has increased, and the analysis of test results has become more diversified. It is necessary to understand the formation, purpose and impact on performance of each parameter. The following is a brief analysis based on the test result parameters of the CTP Series Transformer Tester.

△ Test Results of P-Class Transformer

△ Parameter Description of Test Results

Resistance (25°C) (Secondary DC Resistance): Unit: Ω, the resistance of the CT secondary winding at the current temperature. It mainly detects the welding quality of the transformer secondary winding joints, whether there is an inter-turn short circuit or open circuit in the winding, and whether there is poor contact in the lead wires. The measurement error range of the secondary DC resistance of transformers in the same batch shall not exceed 10%.

Knee Point Voltage and Knee Point Current: Unit: V and A respectively. According to the standard definition, when the knee point voltage increases by 10%, the knee point current increases by 50%. The knee point is the saturation point of the volt-ampere characteristic curve (magnetization curve of the iron core).

Significance of Testing the Volt-Ampere Characteristic Curve:

1. Detect the iron core quality of newly commissioned transformers and retain original data;
2. Determine whether the commissioned transformers have defects such as inter-turn short circuits;
3. Verify the 10% error curve of the differential protection CT accuracy as required.

Limit Electromotive Force: Unit: V, the limit electromotive force calculated according to the CT nameplate and 75°C resistance. It is mainly used for capacity calculation to verify whether the actual parameters of the transformer meet the nameplate requirements. The rated knee point electromotive force shall be less than the measured limit electromotive force.

Composite Error: The composite error at the limit electromotive force or rated knee point electromotive force Ek. In a steady state, it is the root mean square value of the difference between the instantaneous value of the primary current and the instantaneous value of the secondary current multiplied by the rated current ratio (transformation ratio). When a CT operates under overcurrent conditions, due to the distortion of the excitation current waveform, the secondary current is not a sine wave, so the error characteristics of the CT cannot be specified directly by current error and phase difference, and composite error is required.

Accuracy Limit Factor ALF (Protection Class CT): In a steady state, the ratio of the maximum primary current value (Ipal) that the CT can meet the composite error requirement to the rated primary current (Ipn). Kal=Ipal/Ipn. When a short-circuit current much larger than the rated current flows through the primary winding, the protection class CT starts to work effectively, transmitting information to the secondary side to ensure the operation of relay protection, and the composite error shall not exceed the limit.

If the measured ALF ≥ rated value, the CT is judged to be qualified; if the measured ALF is less than the rated value, the CT is judged to be unqualified, and protection refusal to operate may occur.

Instrument Security Factor FS (Metering Class CT): The ratio of the rated instrument limit primary current to the rated primary current. For metering CTs, under normal operating conditions, small error and high accuracy are required. Under primary overcurrent conditions, large error is required, and the secondary current no longer increases proportionally with the increase of the primary current to avoid damage to the instruments connected to the secondary circuit due to large current impact.

Unsaturated Inductance: Unit: H, the average inductance of the linear segment of the excitation curve. During the test, attention should be paid to whether the unsaturated inductance is too large.

Residual Magnetic Coefficient: The ratio of residual magnetic flux to saturated magnetic flux. For commonly used 0.2(S) and 0.5(S) CTs, residual magnetism may cause the error to shift to the negative direction by 0.1%~0.2%, resulting in smaller measurement and metering values. When the system short-circuit fault is in the transient process, if the direction of residual magnetic flux is consistent with the direction of magnetic flux generated by the transient non-periodic component, the performance of the current transformer will deteriorate seriously. Therefore, residual magnetism is one of the causes of protection maloperation and refusal to operate.

Secondary Time Constant: Unit: S, the time constant of the CT secondary when connected to the rated load. It is derived from the ratio of the sum of excitation inductance and leakage inductance to the secondary circuit resistance.

Symmetrical Short-Circuit Current Multiple Kssc (TP Class CT): Used to calculate the limit electromotive force and its corresponding peak instantaneous error.

Transient Area Coefficient Ktd (TP Class CT): Short-circuit current generally contains non-periodic components, which will seriously deteriorate the transmission characteristics of the current transformer. Because the excitation characteristic of the current transformer is designed according to power frequency, when transmitting non-periodic components with very low equivalent frequency, the iron core magnetic flux (i.e., excitation current) needs to be greatly increased. The time and value when the magnetic flux reaches the maximum are related to Tp, Ts, etc. According to whether the transient process of short-circuit current is considered, current transformers are divided into two categories: P and TP. P class current transformers are required to be unsaturated under ΦAV conditions, while TP class current transformers are required to be unsaturated under the total magnetic flux Φ＝ΦAC＋ΦDC in the entire working cycle. Therefore, the iron core of TP class current transformers is required to be much larger than that of P class current transformers.

Calculation Coefficient Kx (TP Class CT): A coefficient given by the user, representing the multiple of the rated secondary current that the transformer appears under power system fault conditions, including safety factors. It is required that the transformer meets the required performance at this value.

Polarity: The polarity relationship between the primary and secondary of the CT, including same polarity/-(subtractive polarity) and opposite polarity/+(additive polarity).

Turns Ratio: The actual turn ratio of the tested secondary winding to the primary winding.

Transformation Ratio: The actual current ratio under rated load.

Ratio Error: The current error under rated load, caused by the inconsistency between the actual current ratio and the rated current ratio.

Phase Difference: The phase difference under rated load. The vector direction is determined based on the phase difference of an ideal transformer being zero. If the secondary current vector leads the primary current vector, the phase difference is positive.