Tripping Point: Simplifying circuit breaker design using delta-sigma ADCs


In circuit-breaker applications, successive approximation register analog-to-digital converters (SAR ADCs), internal to the microcontroller (MCU) or external, are preferable to delta-sigma ADCs due to their faster startup times. Yet delta-sigma ADCs provide higher dynamic range and higher resolution, an internal PGA (programmable gain amplifier) and consume lower power.

Delta-sigma ADCs have a startup time of ~100ms during power up, which limits their application in circuit breakers. A delta-sigma ADC with a faster startup time is the solution. Using higher-resolution delta-sigma ADCs reduces the need to have multiple hardware for different models of air circuit breakers (ACBs) and reduces design cycle time.

The High Resolution Fast Startup Analog Front End for Air Circuit Breaker reference design (TIDA-00661) focuses on solving critical circuit breaker requirements. You can also use part or all of the solution in molded-case circuit breaker (MCCB) designs.

ACBs for low-voltage applications

Figure 1: SACE Emax 2 air circuit breakers up to 6300A

Domestic, commercial and industrial power-distribution systems use ACBs with integrated electronic trip units (ETUs) for load-side protection.

The ETU in ACB

ETUs provide measurement information also offer functions such as:

  • Fast startup – the trip unit must be operational and begin sampling analog inputs in <5ms.
  • MCU-based true RMS measurement.
  • Current (X 5) and Voltage (X 3) measurement.
  • Making current release.
  • Power measurements.
  • Self-powered when the phase current is >20% to 25% of the nominal current (In) or auxiliary powered (DC input).

For a more detailed description of ETU features, see Section 1.3.1 of the reference design’s design guide.

ACB features : Making current release

ACBs protect power distribution systems/loads against overload, short circuits or earth faults. ACBs are specified in terms of their rated current-carrying capacity; the rated current varies from 200A to 6,300A. The pickup current for the breaker to trip during overload or short-circuit conditions can be set to 10-15 times the rated current, with a maximum rating up to 100kA. Since breakers carry large currents, they have to be protected against closing with faults. This protection feature is called making current release (MCR).

The MCR function trips the ACB if the current exceeds the short circuit or instantaneous pickup-current setting during closing operation. MCR also trips the ACB should an attempt be made to close onto a short-circuit fault current greater than the rated making capacity (50kA, 63kA or 100kA depending on applications, makes and models).

Depending on manufacturer, the breaking time specified is between 30-50ms. Breaking time includes sampling the input current and processing the samples to provide trip commands to the solenoid, which breaks the fault current. This requirement limits your DC/DC converter, MCU and ADC selection.

Reference design features

The reference design contains an AFE board with the ADC and an interface board with MCU and power. It also has a signal-processing front end for an ETU for use with an ACB. This design uses a high-resolution delta-sigma ADC for measuring wide current and voltage inputs within a specified accuracy; the ADC can measure up to eight simultaneous inputs with 24-bit resolution. The ADC interfaces with an MSP430™ MCU for analog input processing.

The design is powered with rectified current input or auxiliary DC input power supplies. It offers two options based on power requirements to generate positive and negative power supplies, one using the LM5017 and the other with the LM5160 configured in Fly-Buck™ mode.

Design features include:

  • Three voltage and five current inputs interfaced to an eight-channel, simultaneous sampling, 24-bit ADS131E08S delta-sigma ADC with fast startup (<3ms).
  • Measurement of AC current input with dynamic range of ≤500 within ±1%, with fixed PGA gain.
  • Measurement of AC voltage input from 10-900V within ±1%, with fixed PGA gain.
  •  DC/DC converters in the Fly-Buck converter configuration to generate supply outputs.
  • Subsystem configurable for 2W or 8W power outputs.

Reference design advantages

The ADS131E08S delta-sigma ADC-based reference design fulfills some of the critical requirements of ACBs, such as:

  • Fast startup: ACBs are specified to trip within 30-50ms when powered with a fault. The  time includes system power up, AC input current measurement and breaking of the fault current.
  • Wide-input measurement: The fault current input range varies from 0.3-15 In (nominal current) or more for a given current-breaker rating. The circuit breakers are available in multiple current ratings. An ADC with high resolution ensures the use of the same ETU for multiple current ratings or all current ratings with no hardware changes. Similarly same ETU can be used for multiple Voltage rating simplifying the hardware design and testing.  The wide input measurement is achieved by high resolution ADC, PGA and Reference that can be set to 2.4V or 4V resulting in increased dynamic range.
  • Accurate measurement of voltage and current inputs: The accurate measurement of input current ensures a repeatable trip-time performance for protection and accurate measurement of different parameters for metering. Accurate measurement is achieved by the High resolution ADC, internal PG and internal reference.
  • Increased reliability and temperature performance: The integration of reference and PGA reduces external component requirements, improves temperature performance, and increases reliability.

Table 1 lists the startup delay for the reference design’s various subsystems.

Table 1: Startup Delay for ACB ETU TI design

The reference design has been tested for voltage and current accuracy performance over a wide range of current and voltage inputs.  Accuracy of ±0.5% was achieved for 1 cycle measurement (80 samples @ 4000 samples per cycle for 50Hz input) See Section 8 of the reference design’s design guide.

Summary

The ADS131E08S delta-sigma ADC-based reference design TIDA-00661 solves critical system design challenges for ACBs using delta-sigma ADCs and improves measurement accuracy for half- or one-cycle measurements. It comes with the design guide, Altium schematics and printed circuit board (PCB) files, assembly files, and a bill of materials (BOM) for accelerating your design.

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