CVT Auto-excitation Measurement with HCL2840 Type C/D Dielectric Loss Tester

HIMALAYAL - SHANGHAI - CHINA

I. Basic principles for CVT measurement
Capacitor voltage transformer (CVT) applies stacked structure, on account that capacitive voltage divider and electromagnetic unit of CVT with stacked structure are inseparable, causing difficulty in measurement of capacitive voltage divider capacitance and dielectric loss. Separate measurement of capacitive voltage divider and dielectric loss has been conducted before stacked at CVT manufactory. Whereas for stacked CVT with medium voltage terminal (point A) extracted, applies tangent direct measurement, auto-excitation is the exclusive way of measurement for CVT without tapping.

                                                                                                      
                                                                                                                       Fig1. 500kV CVT diagram
II. Protection for CVT auto-excitation measurement
During dielectric loss test applying CVT auto-excitation, measurement of primary side voltage cannot simply base on turn ratio due to “Circuit Resonant” and “Capacitive Rise Effect”. Capacity for CVT voltage-dividing capacitor is large (>40000P), volts of secondary side voltage rise can cause primary side voltage rise by thousands of volt. Make sure to monitor strictly that primary side voltage (point A) below 3kV, secondary side current below 6A. Thus HCL2840 type C and D are equipped with protection control for high voltage, high voltage current, low voltage and low voltage current for CVT measurement, HCL2840 adjust voltage with accuracy according to these four protection control to ensure safety of test instrument and device.

III. Introduction on CVT Auto-excitation
1. Traditional CVT auto-excitation:
Require external connection to standard capacitor, low voltage excitation power supply and monitoring voltmeter, see high voltage test procedures for wiring details.
2. Non-grounded CVT auto-excitation measurement use HCL2840C as generatrix

    
                Fig2. C₁ measurement                                                   Fig3. C₂ measurement
                                                 
                                                                                   
                                                                                 Fig4.
Please note that high voltage line ought to be suspended instead of touching the ground, otherwise additional dielectric loss to the ground can cause error, use thin cable to connect high voltage socket and CVT test object and suspend it. Plus in consideration of voltage dividing effect caused by C2 or C1 connected in serial to internal Cn, capacitance can be calibrated based on formula below:
                                                         
                                                           Fig5.
Cc represents calibration empirical value, which include impact of capacitance of Cn and high voltage line against the ground, value can be taken as 110pF.
3. CVT auto-excitation measurement with generatrix grounded
If generatrix is grounded, measurement below is recommended:
First, perform wiring based on figure 3, 10, 11, 1, apply “Internal Cn” to measure C13:

                        Fig6. Wiring for C₁₃ measurement           Fig7. C₁₃ as standard capacitor in measurement of C₂
                                                                     
                                                                        
                                                                              Fig8.
Please note that high voltage line ought to be suspended instead of touching the ground, otherwise additional dielectric loss to the ground can cause error (seen as C13 dielectric loss increase). Use thin cable to connect high voltage socket and CVT test object and suspend it.
In order to compensate impact of C2 connected in series to standard circuit to C13, C13 capacitance can be calibrated based on the formula below:

                                           Fig9.
Cc represents calibration empirical value, which include impact of capacitance of Cn and high voltage line against the ground, value can be taken as 110pF.

The next step is to perform wiring according to figure 3, 10, 11, 2, use C13 as standard capacitor to measure C2, input C13 capacitance and dielectric loss to the instrument external standard capacitance item before measurement (see 3.3), and select “External Cn” mode.
This wiring can be used on condition that generatrix is not grounded.

4. CVT auto-excitation measurement with HCL2840D
Generatrix cannot be grounded if C1 capacitor is single stage; if C1 capacitor is multiple stage, it is not necessary to dismantle high voltage lead, generatrix can be grounded, measurement of C11 and C12 can apply both regular tangent and inversed wiring, C13 and C2 measurement apply auto-excitation.

                                            Fig10.
During CVT auto-excitation measurement, the instrument measures C1 first, then automatically conduct inversed winding and measure C2, along with automatic calibration for impact of voltage dividing.
During measurement of C13, high voltage is added to high voltage line core wire and shielding, low voltage is added to CX line core wire and shielding.
During measurement of C2, low voltage is added to high voltage line core wire and shielding as well as CX line core wire and shielding.
Please note that high voltage line ought to be suspended instead of touching the ground, otherwise additional dielectric loss to the ground can cause error, use thin cable to connect high voltage socket and CVT test object and suspend it.

IV. On condition of tangent direct measurement of C1C2 cascade, dielectric loss is a minus value or very small, whereas during direct measurement of C1C2 cascade with inversed wiring, dielectric loss is a major value
For tangent direct measurement of C1C2 cascade, capacitance is measurable, yet stray impedance of electromagnetic unit can be stimulated as an RC cascade model, advancing point A voltage, causing Ix deflecting forward to the second quadrant, therefore reducing angleδtoδ' and resulting to a negative value if the deflection exceeds 90 degrees. Similarly, during direct measurement of C1C2 cascade with inversed wiring, inversed wiring <3kV atδend, dielectric loss is a major value.

Fig11. Tangent direct measurement of C₁C₂ cascade equivalent circuit

               Fig12. Ix shift phase to Ix’
Note: nameplate of some CVT indicate capacitance as C₁, C₂; whereas some indicate C₁, C₁C₂ cascade, calculation based on: C_cascade=(C₁C₂)/(C₁+C₂).