A.C. Voltage Source
The A.C. High Voltage Source supplies the required voltages for the accuracy test up to 120% of the highest rated voltages. Two high voltage sources are available in the lab., one is 50kV; 5000VA with digital indication and the other source is 150kV, 10,000VA with analogue indication.
A.C. Current Source
The A.C. High Current Source supplies the required current for the accuracy test up to 120% of the highest rated current. Since the rated primary current includes a range of 5A to 5000A, therefore, the current source should be provided with tappings to allow the current to be adjusted to the required value.
Voltage Transformer Burden
Standard burdens are required for the calibration of voltage transformers as per IS 3156 and IEC 60044-2:1997. The rated burdens, which are normally required, are 2.5VA, 5VA, 7.5VA, 10VA, 15VA, 20VA, 40VA for the rated secondary voltage of 110V and 110/√3V at a power factor of 0.8P.F Lag.
Current Transformer Burden
Standard burdens are required for the calibration of current transformers as per IS 2705 and IEC 60044-1:1996 The rated burdens which are normally required are 2.5VA, 5VA, 7.5VA, 10VA, 15VA, 20VA, 30VA, 40VA for the rated secondary currents of 1A and 5A at a power factor of 0.8P.F. Lag.
The temperature of the lab is maintained at (25 ± 2)°C and humidity (50 ± 10)%.
In order to ensure that the measurement traceability of the equipments being used as Reference Standards and other auxiliary instruments, they are linked up with higher accuracy standards at the International Level. Our measurement traceability is with PTB, Germany.
The calibration facilities for AC High Current Ratios are from 5A-5000/1A, 5A and for Voltage Transformers of any ratio with 100V – 100kV/100V, 110Vand 110/√3 V secondary at 50 Hz at the required burdens.
Calibration of Voltage transformers:
The standard used for the calibration of voltage transformer is Standard Voltage Divider. It comprises of a Capacitive Voltage Divider and an Electronic Device. The uncertainty of this reference standard is ± 50ppm and phase displacement uncertainty is ± 0.2 minutes. The Standard Voltage Divider is traceable to PTB, Germany.
The schematic block diagram of the calibration set–up in its simplest form is shown in Fig. below for voltage transformers.
Calibration of Current Transformers:
The accurate and precise calibration of Current Transformers (CT) is accomplished by comparison method i.e. by comparing the customer’s current transformer to a Reference Standard Current Transformer (Std CT). The Comparison Method compares a calibrated Standard CT of the same ratio as that of the current transformer under calibration.
The Current Transformer Test Set (CTTS) is used for this purpose to compare the output of the current transformer under calibration to that of the Standard CT.
The calibration of Voltage Transformers (VT) it is also accomplished by comparison method i.e. by comparing the customer’s Voltage Transformer to a High Voltage Ratio Measuring System (HVRMS) which basically comprises the Capacitive Voltage Divider (CVD) C1/C2 and High Precision Electronic Voltage Divider (EVD). The Comparison Method compares a standard voltage transformer or HVRMS of the same ratio as that of the voltage transformer under calibration. A Voltage Transformer Test Set (VTTS) is used for this purpose to compare the output of the voltage transformer under calibration to that of the Standard Voltage Transformer.
Standards Used For Calibration
The standard used for the calibration of current transformer is a Current Comparator having 155 standard ratios right from the lowest 5A/1A, 5A to the highest 5000A/1A, 5A. The uncertainty of this reference standard is ± 30 ppm and phase displacement uncertainty is ± 0.1 min. The Current Comparator is traceable to PTB, Germany.
The uncertainty of the AITTS used as CTTS is 20ppm in ratio and ± 0.1 minutes in phase displacement. The AITTS is traceable to PTB, Germany.
The schematic block diagram of the calibration set–up in its simplest forms is shown in Fig. below for current transformers.
Calibration of Capacitance and dissipation factor (C & Tan δ):
The Capacitance and Tan δ standards has been established at NPL. The National Standard for Capacitance and Tan δ is to check the Electrical properties of insulating systems, which change due to aging and continuous electrical stress. Tan-Delta measurement (also called Loss Angle or Dissipation Factor) is a diagnostic method of testing electrical equipment for integrity of the insulation. By measuring electrical properties such as Capacitance and Tan-Delta regularly it is possible to ensure the operational reliability of HV insulating systems and also to avoid costly insulation breakdown. This is particularly important for High-Voltage Bushings, Power Transformers, Generators, Power Capacitors, H.T. cables etc. This is needed for power companies (Generating Stations, Transmission, Distribution, Sub-stations and Industries etc.)
Changes in normal Capacitance of electrical apparatus insulation indicates the presence of moisture layer, short circuits & open circuits in capacitance network. Dissipation factor measurements indicate the following condition of the insulation of electrical instruments:
- Chemical detoriation due to time & temperature
- Contamination by water, carbon deposits, dirt & other chemicals
- Leakage through cracks & over surfaces
The Capacitance & Tanδ measurement at NPL uses Automated High precision Capacitance & Tan δ Bridge which performs the measurements through automatic balancing. This is better In comparison to the other method of measuring Capacitance & Tan δ through Schering Bridge which must be balanced manually and the balance is observed in a null indicator.
Dissipation Factor (Tan δ): In an Ideal capacitor, the resistance of the insulation material (dielectric) is infinitely large and when AC voltage is applied, the current leads the voltage by exactly 90°. But every insulation material contains single free electrons and under AC conditions dielectric hysteresis loss occurs which is analogous to hysteresis loss in iron cores. An equivalent circuit and vector diagram of a real (lossy) capacitance is given below::
Fig 1: Equivalent circuit and vector diagram of a real capacitor
UTest Applied test voltage
IC Current through capacitance
IR Current through resistance (insulating material)
C Ideal capacitance
R Ideal resistance
Status of Capacitance and Tanδ in NPL: For the measurements of Capacitance & Tan δ, using Capacitance & Tan δ measuring bridge, we can calibrate the Insulation of Transformers, Cables, Bushings, Circuit Breakers, insulators and Capacitors upto 200kV with current capacity upto 15Amps. Apart from this we can also calibrate C & Tanδ bridges and Schering Bridges by comparison method. The measurement uncertainties in the Capacitance & Tanδ measurement is 0.011%.
Methodology of Calibration:
The accurate and precise calibration of High Voltage Capacitors & Tan δ measurements are accomplished by using standard reference capacitor with high precision C and Tan δ measuring system . The capacitance value Cx (under test) can be measured online along with number of pre-decided parameters chosen from the library on display of the bridge, as required (L, C, Tan δ, V,I & PF etc.).
For measurement of C and Tan δ, the voltage according to the requirement of the capacitor under test is applied to both reference and under test capacitor.
C and Tan δ Bridge with HV capacitors
Standards Used for Calibration:
The standards used for the calibration of High Voltage Capacitors & Tan δ measurements are Standard Capacitors with any rating, 200kV/100pF; 30kV/1000pF; 2kV/100pF or 2kV/1000pF and Standard High Precision C & Tan δ Measuring Bridge. The uncertainty of this system is 110 ppm.
Experimental Setup of C & Tan δ Measuring System