7070A-I AC Current Ratio Bridge

Accurately measuring electric energy in high-voltage, high-current distribution systems relies 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 result. 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-I

The MI 7070A-I is an AC Current Ratio Bridge designed for the precise calibration of current transformers using a reference CT, such as the MI 7020 series or another standard reference current transformer. The 7070A-I provides 10 selectable input current ranges from 5 A down to 0.005 A (5, 2, 1, 0.5, 0.2, 0.1, 0.05, 0.02, 0.01, and 0.005 A), providing the flexibility needed for a wide range of testing applications. Higher test currents can be achieved by incorporating external precision CTs. With its broad range of input currents, a single ratio reference CT can be used to calibrate current transformers with different rated ratios, simplifying test setups and reducing the need for multiple reference standards.

Figure 1. Model 7070A-I Current Transformer Configuration

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 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.