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Current transformer test of transformer

Transformer comprehensive tester is a new generation of CT and Pt testing instrument of our company. CTP-1000 series products are based on the testing principle of frequency conversion method, which can complete the comprehensive test of CT and PT on site without voltage regulator, voltage booster and current converter. The output voltage is low, and the safety of the tested equipment and test personnel can be ensured no matter whether there is wrong operation or not. The device adopts high-performance DSP and FPGA, and advanced manufacturing technology to ensure stable and reliable product performance, complete function, high degree of automation, high test efficiency and is in the leading level in China. It is a professional test instrument for transformer in power industry.
 
The test wiring steps are as follows:
Step 1: according to the description of CT test items described in Table 2.1, conduct wiring according to figure 2.1 or Figure 2.2 (for CT of various structures, refer to the actual wiring mode described in Appendix D).
 
Table 2.1 Description of CT test items

Resistance excitation transformation ratio load description wiring diagram
      Measurement of CT secondary winding resistance and excitation Figure 2.1
      Figure 2.1
      Figure 2.1
    Figure 2.1
        Figure 2.2
 


Figure 2.1 connection mode of CT direct resistance, excitation and transformation ratio test
Figure 2.2 wiring mode of CT secondary load test
 
 
Step 2: other windings of the same CT are open circuit, the primary side of CT should be grounded, and the equipment should also be grounded.
Step 3: turn on the power and prepare for parameter setting.
2.1.2 Parameter setting
The test parameter setting interface is shown in Figure 2.3.

Fig 2.3 Basic parameter setting interface
The parameter setting steps are as follows:
Rotate the mouse to switch the cursor and select the test item to be carried out. When the cursor stays at a certain test item, the screen displays the parameter settings related to the test item; when the cursor leaves the test item, the screen displays the wiring diagram corresponding to the selected test item.
The parameters that can be set are as follows:
(1) Number: input the number of this test, which is convenient for printing, saving, management and searching.
(2) Rated secondary current: the rated current at the secondary side of current transformer, generally 1a and 5A.
(3) Grade: the level of the tested winding. For CT, there are 8 options, such as P, TPY, metering, PR, PX, TPS, TPX, TPZ.
(4) Current temperature: the winding temperature during the test. Generally, the air temperature during the test can be input.
(5) Rated frequency: optional values are: 50 Hz or 60 Hz.
(6) Maximum test current: generally, it can be set as the rated secondary current value. For TPY class CT, it can be set to 2 times of the rated secondary current value. For class P CT, if it is 5p40 and rated secondary current is 1a, then the maximum test current should be set as 5% * 40 * 1A = 2A; if it is 10p15 and rated secondary current is 5a, then the maximum test current should be set as 10% * 15 * 5A = 7.5A.
 
If you want to see the following results, you need to set the basic parameters accurately (it is recommended that you set them).
(1) Turn ratio error, ratio difference and phase difference
(2) Accurate calculation of limit electromotive force and its corresponding compound error
(3) The measured accurate limit coefficient, instrument safety factor and symmetrical short circuit current multiple are obtained
(4) Measured transient area coefficient, peak instantaneous error and secondary time constant
 
For different levels of CT, the parameter settings are also different, see table 2.2.
Table 2.2 Parameter Description
Parameter Description P TPY Metering PR PX TPS TPX TPZ
Rated primary current Used to calculate the exact actual current ratio
Rated load power factor Rated load on the rating plate with power factor of 0.8
Rated accuracy limit factor K The default value on the name plate is 10. It is used to calculate the limit electromotive force and its corresponding compound error              
Rated symmetrical short circuit current coefficient Kssc The default value on the name plate is 10. It is used to calculate the limit electromotive force and the corresponding peak instantaneous error        
Primary time constant default: 100ms          
Secondary time constant default: 300ms            
Work cycle C-t1-0 or C-t1-0-tfr-C-t2-0,
Default : C-t1-0 cycle
           
t1 First current passing time, Default: 100 ms            
ta11 Time for primary flow to maintain accurate limits, Default: 40ms                
tfr Time delay for first opening and reclosing, Default: 500ms. Select C-t1-0-tfr-C-t2-0 cycle to display            
t2 Second current passing time, default: 100. Select C-t1-0-tfr-C-t2-0  cycle to display          
ta12 Time for secondary flow to maintain accurate limits, Default: 40ms.
Select C-t1-0-tfr-C-t2-0  cycle to display.
           
Safety factor of rated instrument The default value on the name plate is 10. It is used to calculate the limit electromotive force and its corresponding compound error              
Rated calculation coefficient                
Rated inflection point potential Ek                
le corresponding to EK                
Area coefficient                
Rated Ual Rated equivalent secondary limiting voltage              
lal corresponding to UAL                
 
Step 5: select the start button on the right to test.
 
2.1.3 Test results
The interface of test result page is shown in Fig. 2.4

Figure 2.4 Test result interface
 
For different levels of CT and selected test items, the test results are also different, as shown in Table 2.3.
Table 2.3
Test Result Discription P TPY Metering PR PX TPS TPX TPZ
Load Measured load Unit: VA, CT secondary side measured load
Power factor measured load power factor
Impedance Unit: Ω, measured impedance of CT secondary side
resistance Resistance(25℃) Unit: Ω, resistance at current temperature
Resistance(75℃) R, Unit: Ω, Converted to the resistance at 75℃
Excitation Inflection point voltage and inflection point current Unit: V and A respectively. According to the standard definition, when the inflection point voltage increases by 10%, the inflection point current increases by 50%
Unsaturated inductance L Unit: H, average inductance of linear section of excitation curve
Remanence coefficient K ratio of residual flux to saturation flux
Quadratic time constant T Unit: s, time constant of CT secondary connection with rated load
Limit electromotive force E Unit: V, limit electromotive force calculated according to CT rating plate and 75 ℃ resistance    
Composite error ε Composite error under the limit electromotive force E or rated inflection point potential EK        
Peak instantaneous error ε Peak instantaneous error under the limit electromotive force E          
Accuracy limit coefficient Accurate limit coefficient measured            
Instrument safety factor Measured instrument safety factor              
Symmetrical short circuit current multiple Kssc Measured symmetrical short circuit current multiple        
Transient area coefficient Actual transient area factor          
Calculation coefficient Kx Actual calculation coefficient              
Rated inflection point potential Ek Measured excitation dia corresponding to rated inflection point potential              
Ie corresponding to Ek                
Rated Ual rated equivalent secondary limiting voltage              
Ial corresponding to Ual Measured excitation current corresponding to rated equivalent secondary limiting voltage              
Error curve 5% (10%) error curve  
Transformer Ratio Transformer Ratio Actual current ratio at rated load
Turn ratio Actual turn ratio of secondary winding to primary winding under test
Ratio difference Current error under rated load
phase difference Phase difference at rated load
Polarity There are two kinds of polarity relations between CT primary and secondary, i.e., homopolarity/ -(reduced polarity) and reverse polarity/+ (additive polarity)
Turn ratio error Relative error between measured turn ratio and rated turn ratio            
Standard error Table of current error and phase error of international inspection current point under rated load and lower limit load