7070A-UI AC Ratio Bridge

  • Calibrate voltage and current transformers with a single instrument
  • Calibrate instrument transformers with differently-rated ratios using a single reference standard
  • Calibrate voltage transformers (VT) up to 800 kV (when used with MI 2500 Series of HV dividers and standard capacitors)
  • Calibrate current transformers (CT) up to 10,000 A (when used With MI 7020 / 72XX series of precision current transformers/current comparators
  • Test frequency: 15 Hz to 100 Hz
Features & Benefits

Calibrate Voltage and Current Transformers:

Accurately measuring electric energy in high-voltage, high-current distribution systems rely on three key components:

  • a watt-hour meter;
  • a high-voltage transformer, and;
  • a high-current transformer (to step down high-voltage and high current to accurate low levels for input to the kWh meters or other electrical measuring devices).

If the instrument transformers are not calibrated with similar accuracy, then the measuring device’s measurements will be with high uncertainties. For high voltages and currents, this inaccuracy can have a significant impact on the results. Ensure precision from the start—MI’s 17025-accredited lab can calibrate these instruments with NMI traceability. This means accuracy  from the first measurement you take.

Beginning with single instruments, such as an automated, high-voltage capacitance tan delta bridge, high-voltage dividers, wattmeters/power analyzers, and systems, like the MI Power Calibration System (PCS), Automated Load Loss Measurement System (ALMS), and Isolating Current Transformer (ICT) system.

MI’s 7070A series of fully-automated AC ratio bridges were developed for fast and accurate measurements of instrument transformer errors. Integrate the 7070A into complex systems to calibrate voltage transformers up to 800 kV and current transformers to 10,000 A. The 7070A sets new measurement standards in instrument transformers calibration.

Model 7070A-UI

The model 7070-UI is an AC Ratio Bridge that combines current and voltage ratio measurement capabilities. It provides the user with the convenience of one measuring unit to calibrate VTs and CTs without compromise. Its main application is for VT and CT test sets. The unit is designed with flexibility and high accuracy, creating a single solution for multiple measurements in any AC lab.

* Optional Power Measurement Function available upon request.

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

Measurement Systems

MI designs and supplies system solutions for current and voltage transformer measurements customized for specific requirements. 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 7070A-UI
Input Channels 4
                                                                                 Current Measurements
Input Ranges / Channel (A) 0.004, 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
Reading Resolution                           1 ppm
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 Hz
                                                                                 Voltage Measurements
Input Ranges per Channel (V) 6, 15, 30, 60, 150, 300
Voltage Accuracy (ppm of FS)
@ 50/60 Hz (23 °C ± 5 °C)		 ± 50
Ratio Accuracy
(23 °C ± 5 °C)
± 75 ppm
for 0.5 ≤ Vx/Vs ≤ 2.0
± 100
for 0.5 ≥ Vx/Vs ≥ 2.0
Reading Resolution                                        1 ppm
Phase Shift Accuracy

(23 °C ± 5 °C)
± 0.2 min
for 0.5 ≤ Vx/Vs ≤ 2.0
± 0.4
for 0.5 ≥ Vx/Vs ≥ 2.0
Linearity ≤ 20 ppm
Input Impedance 5 MΩ
Frequency Range 15 to 100Hz
                                                                                   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 15°C to 40°C
Relative Humidity  10% – 80% RH (non-condensing)

Information Request