7070A-UI AC Ratio Bridge

  • Calibration of Voltage and Current Transformers from a Single Instrument
  • Calibration of Instrument Transformers with Different Rated Ratios Using One Single Reference Standard
  • Calibration of Voltage Transformers (VT) Up to 800 kV When Used with MI 2500 Series of HV Dividers and Standard Capacitors
  • Calibration of Current Transformers (CT) Up to 10000 A When Used With MI 7020 and/or 7200 Series of Precision Current Transformers and/or Current Comparators
  • Short Measurement Times with High Accuracy and Low Inherent Burdens
  • Instrument Applications are Valid for Single Phase Measurements. Three-Phase Transformers Measurements Performed by Testing Each Phase Separately
  • Test Frequency from 15 to 100 Hz
Features & Benefits

The accurate measurement of electric energy in high-voltage, high current distribution systems relies on 3 key components: a watt-hour metre, a high-voltage transformer and a high current transformer to step down the high-voltage and high current to accurate low levels for input to the kWh metres or other electrical measuring devices. However, if the instrument transformers are not calibrated with similar accuracy, then the precise measurements made by the measuring devices will be with high uncertainty and misleading. For high-voltages and currents, this inaccuracy can have a significant impact on the result. Therefore, calibration of the instrument transformers has become increasingly important.

Measurements International (MI) has many years of experience in high-voltage and high current engineering. Beginning with single instruments such as automated high-voltage capacitance bridge, high-voltage dividers and wattmeter to high complex systems like power calibration system, Automated Load Loss Measurement System (ALMS) and isolating current transformer (ICT) system. MI is also 17025 accredited for both in-house and on-site calibration of these measurements. This capability authenticates not only our credentials as a industry-leading instrument developer but also as calibration service provider for instrument transformers.

The model 7070A series of AC Ratio Bridges developed at MI are fully automated for fast and accurate measurements of instrument transformer errors. The 7070A is easily integrated in complex system to achieve calibration of voltage transformers up to 800 kV and current transformers to 10,000 Amps. The 7070A will set new standards of measurement in the calibration of instrument transformers.

The 7070A multi-range design allows to cover a wide range of measurements ratio with minimum number of reference standard when calibrating current transformer.

Model 7070A-UI

The model 7070-UI is an AC Ratio Bridge that combines AC current and voltage ratio measurement capabilities. It provides the user with the convenience of one single measuring unit to calibrate VTs and CTs without losing accuracy. Its main application is for VT and CT test sets. However, the combination of voltage and current inputs can also be used to perform power measurements. The flexibility and high accuracy of this design can be a single instrument solution for multiple measurements at any AC lab. The block diagram for the 7070A-UI is shown in Figure 1.

AC Ratio Bridge Block Diagram
Figure 1. Model 7070A-UI for 4 inputs

Measurement Systems

MI designs and supplies system solution for current and voltage transformer measurements customized for specific requirement. Some of the system components are listed below:
1. 7020 series of Standard Current Transformers
2. 7200 series of Current Comparators
3. 2500 series of Voltage Dividers
4. Model CG series of High-Voltage Capacitors
5. Electronic burdens
6. Computer and printer for external control and data recovery.

Specifications
Model No. 7070A-UI
Input Channels 4
Current Measurements
Input Ranges per Channel (A) 0.005, 0.01 ,0.02, 0.05, 0.1,
0.2, 0.5, 1, 2, 5
Current Accuracy (ppm of FS)
@ 50/60 Hz (23 ° C ± 5 °C)
± 50
Ratio Accuracy (ppm)
(23 °C ± 5 °C)
± 75
For 0.5 ≤ Ix/Is ≤ 2.0
± 100
For 0.5 ≥ Ix/Is ≥ 2.0
Phase shift Accuracy (min)
(23 °C ± 5 °C)
± 0.2
For 0.5 ≤ Ix/Is ≤ 2.0
  ± 0.4
For 0.5 ≥ Ix/Is ≥ 2.0
Linearity (ppm) ≤ 20
Input Impedance (Ω) ≤ 3.5
Isolation (Vpp) 600
Frequency Range (Hz) 15 to 100
Voltage Measurements
Input Ranges per Channel (V) 120 and 240
Voltage Accuracy (ppm of FS)
@ 50/60 Hz (23 °C ± 5 °C)
± 50
Ratio Accuracy (ppm)
(23 °C ± 5 °C)
± 75
For 0.5 ≤ Vx/Vs ≤ 2.0
± 100
For 0.5 ≥ Vx/Vs ≥ 2.0
Phase Shift Accuracy (min)
(23 °C ± 5 °C)
± 0.2
For 0.5 ≤ Vx/Vs ≤ 2.0
± 0.4
For 0.5 ≥ Vx/Vs ≥ 2.0
Linearity (ppm) ≤ 20
Input Impedance (kΩ) 500 @ 120 V Range
1000 @ 240 V Range
Frequency Range (Hz) 15 to 100
Power Measurements
Line-to-Neutral (ppm of FS)
@ 50 or 60 Hz (23 °C ± 5 °C)
± 100
For PF ≤ 0.5
± 150
For PF > 0.5
Line-to-Line (ppm of FS)
@ 50 or 60 Hz (23 °C ± 5 °C)
± 125
For PF ≤ 0.5
± 150
For PF > 0.5
Operating Environmental Conditions
Temperature (°C) 15 to 40
Relative Humidity 10 to 80
Non-Condensing

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