Transformer comprehensive testerTraditional electromagnetic CT from manufacturers is mainly divided into protective CT and metrological CT. Protective CT mainly tests its volt ampere characteristics, 10% error curve, transformation ratio, polarity, etc; Metrological CT mainly tests its transformation ratio, polarity, ratio difference, angle difference, and other indicators under various primary currents and secondary loads.
1、 The traditional method of testing CT volt ampere characteristics and transformation ratio using power frequency voltage current method
The traditional testing method for protecting the volt ampere characteristics of CT in China is mainly manual voltage regulation measurement. The testing principle is shown in the following figure. The output voltage of the self coupling boost regulator is manually adjusted and applied to the CT transformer. The readings are taken using a regular voltage and current meter. Two people operate, one operates the regulator and the other reads. When the manual voltage regulator reaches a certain value, both voltage and current data are read simultaneously, and then the volt ampere characteristic curve is manually drawn.
The principle of variable ratio measurement is shown in the following figure. By manually adjusting the output voltage of the voltage regulator and converting it into a large current through the current regulator, the transformer can be applied to the primary side of the CT. The secondary side current can be measured and the ratio of the primary to secondary side currents can be calculated to determine the transformation ratio.
The main disadvantages of traditional methods are complex wiring, high manual labor consumption, slow speed, poor measurement accuracy, and error tolerance. Sometimes it is necessary to apply a voltage of up to 2000V when testing the volt ampere characteristics, which makes manual operation extremely unsafe.
The traditional methods are obviously very outdated and urgently need improvement.
2、 The testing methods and instruments for CT have undergone three main stages of change
1. * Instrument: Semi automatic (manual voltage regulation, automatic measurement) CT volt ampere characteristics
A digital recording method measurement device developed based on the basic principles of traditional direct measurement methods. Install the manual voltage regulator, digital voltage and current measurement part, and microcontroller circuit in the same device, manually adjust the voltage rise, and the microcontroller records the data and depicts the volt ampere characteristic curve. This method has made some technological progress compared to fully manual testing methods, but it is inconvenient to operate and still lacks safety and reliability.
Transformer comprehensive testerMain drawbacks:
● Manual operation, slow speed
● Poor measurement accuracy
Due to direct manual adjustment, the operational safety is extremely poor
Directly generating high voltage and current for measurement, using large voltage boosters and current boosters, therefore clumsy and heavy.
2. Second generation instrument: fully automatic type, automatic boost method for testing CT volt ampere characteristics, automatic boost method for testing CT variable polarity
A fully automatic testing device controlled by a microprocessor. The fully automatic booster with built-in stepper motor control generates voltage, automatically measures and depicts the volt ampere characteristic curve, and calculates the 10% error curve; Built in automatic current regulator to measure CT transformation ratio and polarity. The testing principle is shown in the following figure.
Key Features:
● Fully automatic testing: It can automatically complete the testing of CT's volt ampere characteristics, transformation ratio, polarity, secondary load, and 10% error curve, simplifying the test wiring and achieving high testing efficiency.
High precision measurement: eliminates measurement errors caused by manual reading and processing.
High safety: Simple wiring, fully automated testing process, no need for manual contact, ensuring high safety.
Automatic processing of measurement results: Real time display of test data and depiction of curves, calculation of inflection points, automatic storage and printing, uploading to computer for archiving, etc.
Main drawbacks:
It still belongs to the direct measurement method of power frequency and requires the generation of high voltage and high current for testing
Due to the use of high voltage and current amplifiers, the volume and weight are large
Due to the wide range of voltage and current to be measured, the measurement accuracy cannot be very high, so it can only be used to test protective CT and cannot be used to test measurement CT
Typical representative:
3. Third generation instruments: low-frequency method for testing volt ampere characteristics, voltage method for testing transformation ratio and polarity, which can cover complete universal testing of protective CT and metering CT
The principle of low-frequency method for testing volt ampere characteristics: Under the same excitation inductance of CT, the excitation impedance value is proportional to the frequency. Therefore, in order to achieve the same saturation of the excitation inductance, the lower the frequency of the applied voltage, the lower the amplitude of the voltage. This is the basic principle of low-frequency testing of volt ampere characteristics. Low frequency measurement can reduce the amplitude of the applied voltage (i.e. reduce the power of the applied voltage), thereby achieving small size, light weight, and greatly improving measurement accuracy. It can be used to protect the volt ampere characteristics of CT and to measure the specific angle difference of CT. This method can achieve comprehensive and complete testing of all CT scans.
The principle of measuring transformation ratio using voltage method: Applying AC voltage to the CT secondary winding will generate induced voltage on the primary side. The ratio of the AC voltage on the secondary winding iron core to the amplitude of the induced voltage on the primary side is theoretically equal to the turn ratio. Compared with the direct method of passing large current on the primary side, this transformation ratio testing method does not require large current and has the characteristics of small testing equipment capacity, safety and reliability. This CT ratio testing method is also known as the indirect method.
Transformer comprehensive testerKey Features:
● Comprehensive function: It can be used for testing the volt ampere characteristics, 10% error curve, transformation ratio, phase, polarity, secondary winding resistance, secondary circuit load, ratio difference and angle difference of various types of CT (including TP), as well as stable or transient characteristics such as ratio difference and angle difference under different working currents and loads
Measure and verify various types of CT, including protective CT, metering CT, TP level transient CT, CT with excitation saturation voltage up to 30KV, and transformer bushing CT
● Comprehensive universal testing covering protection CT and measurement CT, including TP level transient CT, CT with saturation voltage up to tens of thousands of volts, transformer bushing CT, etc. This is the technical and application advantage of the power frequency direct method *
● High precision measurement: Due to the small measurement voltage range, high measurement accuracy can be achieved, with an accuracy of 0.05%
Only requires a small output voltage and power, making it compact, lightweight, and portable
Automatic processing of measurement results: Real time display of test data and depiction of curves, automatic storage and printing. And it can be uploaded for computer management and archiving.
Main drawbacks:
Due to the use of direct measurement method, it does not output high current, so it cannot be used as a high current source for on-site output of high current
Some types of instruments do not have high-precision measurement functions, so they can only be used for relatively simple tests such as protecting the volt ampere characteristics and transformation polarity of CT, and cannot be used for comparison and angle difference tests, let alone for measuring CT
Typical representative: CT parameter analyzer
3、 Basic principles of frequency conversion measurement method
The principle circuit of CT volt ampere characteristic measurement is as follows: the CT primary side is open circuit, voltage is applied from the secondary side, and the relationship curve between the applied voltage V and the input current I is measured. This curve approximates the relationship between the excitation potential E and excitation current I of CT.
If the excitation inductance of the CT excitation winding is L and the excitation impedance is Z at a certain excitation current I, then:
V = I·Z
The relationship between inductance L and impedance Z is as follows:
Z = ω·L = 2 π f L
Then: V=I · 2 π f L
From the formula, it can be seen that the voltage V applied to a certain excitation inductance L is proportional to the frequency f.
Assuming that when f=50hz, the required voltage Vx to achieve the excitation current Ix is 2000V
Vx = I·2 π f L= Ix·2 π f L = 2000V，
If different frequencies are applied:
f = 50hz，Vx = 2000V
f = 5hz， Vx ≌ 200V
f = 0.5hz，Vx ≌ 20V
It can be seen that the CT needs to enter the same saturation level, and applying a lower frequency signal can significantly reduce the voltage required. This is the basic principle of the frequency conversion method.
It must be strictly noted that the required voltage is not linearly proportional to frequency, nor does it decrease proportionally with frequency. It is necessary to strictly follow the mathematical model of the transformer for complete theoretical calculations.