Energy Efficiency

High-Accuracy ±0.5% Current and Isolated Voltage Measurement Ref Design

The TIDA-00835 reference design from Texas Instruments allows for accurate voltage and current measurement using a bipolar input configuration by incorporating a four- channel, 24-bit simultaneously-sampling differential input Delta-Sigma ADC over wide dynamic range.

Most of the current generation of multifunction protection relays provides power-measurement features. Protection relays are specified to measure wide input voltage and currents within a specified range of accuracy. To achieve wide dynamic input measurement within specified accuracy, ADC with PGA or a high-resolution ADC are used. In this design a 24-bit delta-sigma ADC is used.

The ADC is configured to measure ±2.5 V bipolar input.  The inputs are scaled to ADC measurement range of ±2.5V using a fixed gain amplifier. This AFE chains two ADCs together using a common external clock to expand the number of input channels to eight while simultaneously sampling all of the input channels. By having more measurement channels per module, overall system cost is reduced.

Depending on the protection relay configuration and application, the number of current and voltage channels varies from 4 to 16 channels. These 4 to 16 channels are realized by using multiple ADCs. If each ADC has separate interfaces, the complexity increases. Also, it may be necessary to synchronize multiple ADCs to ensure accuracy. The simplest way to interface multiple ADCs are daisy-chaining and using a common clock. In this design two ADCs are used. Each ADC has four channels. The ADCs are chained as a single interface and share a common clock.

TIDA-00835 block diagram (click on diagram to enlarge)

Protection relay is used in HV, MV, and LV applications. Depending on the application the voltage levels are high, and during fault conditions high voltage levels damage the protection relay. The solution to this problem is isolating the voltage and current inputs. Voltages are isolated to provide user safety. The inputs can be isolated using analog isolation amplifiers or digitally using modulators. In this design, isolation amplifiers are used and the output of the isolation amplifiers is interfaced to the delta-sigma ADC.

Primary current is reduced to measurable secondary current by an external CT and applied to the protection relay. The protection relay has an internal CT to transform the secondary current to a measurable input current level. The advantage to using CTs is that they provide isolation, and no additional isolation is required on the protection relay when an internal CT is used.

High-voltage inputs are connected to a PT and the secondary is connected to the input of the protection relay. Internally the protection relays have a potential transformer that can transform the input AC voltage into a measurable value. Potential transformers are large and have inherent nonlinearity, which is overcome by using a resistor divider. Resistor dividers do not provide isolation, as provided by the PT.

Isolation amplifiers provide the required isolation when resistor dividers are used. Isolation amplifiers with either basic or reinforced isolation can be used based on the application. The solution using the resistor divider and isolation amplifier can be considered an alternative to the conventional potential transformer.

TIDA-00835 High Accuracy ±0.5% Current and Isolated Voltage Measurement Reference Design Using 24-Bit Delta-Sigma ADC Board

Summary of features

  • Precision data acquisition over wide input range using 4-channel, simultaneous sampling, 24-Bit ∑-∆ ADCs
  • Channel expansion by chaining multiple devices and simultaneous sampling by synchronizing with a buffered common clock
  • Current measurement – performance verified using current transformer or Rogowski coil with hardware integrator
  • Voltage measurement – performance verified using potential divider (PD) without and with an isolation amplifier (basic or reinforced isolation)
Texas Instruments Incorporated
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