Development and technological advantages of DC energy metering

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Driven by the advancement of efficient and cost-effective power conversion technologies based on wide-bandgap semiconductors such as GaN and SiC devices, many applications are now recognizing the benefits of transitioning to direct current (DC) energy. Therefore, precise DC energy metering is becoming increasingly important, particularly in areas involving energy billing. This article will discuss the opportunities for DC metering in applications such as electric vehicle charging stations, data centers, microgrids, and the relevant solutions introduced by ADI.

DC energy metering enhances accuracy in energy billing

Governments worldwide are currently implementing action plans to address the long-term and complex challenge of reducing CO2 emissions. CO2 emissions have been identified as a major contributor to climate change consequences, driving rapid demand for new, efficient energy conversion technologies and improved battery chemistry. 

Today, there is a growing demand for more efficient and environmentally friendly energy solutions. While early grid developers found it easier to supply the world with alternating current (AC), direct current (DC) offers significantly improved efficiency in many areas. DC energy metering applications are diverse, with electric vehicle DC charging stations poised to become a critical development direction.

In recent years, significant efforts have been made to increase battery capacity and lifetime, alongside the deployment of a widespread electric vehicle charging network. This network is essential to eliminate concerns about driving range or charging times, enabling comfortable long-distance travel. Many energy suppliers and private enterprises are deploying fast chargers with capacities of up to 150 kW, with public interest also piqued by ultra-fast chargers capable of up to 500 kW per charging station. Given the local charging peak power of megawatts at ultra-fast charging stations and associated rapid charging energy premium rates, electric vehicle charging is set to become a substantial market for electrical energy exchange, necessitating accurate energy metering for billing purposes.

Another crucial application of direct current distribution is microgrids, essentially smaller versions of public utility systems that require safe, reliable, and efficient power sources. Microgrids are utilized in settings such as hospitals and military bases and may even function as part of public systems where renewable energy generation, fuel generators, and energy storage collectively create a dependable energy distribution system.

Microgrids are also utilized in building structures, where widespread use of renewable energy generators allows buildings to self-sufficient electricity. With energy generated by rooftop solar panels and small wind turbines being sufficient for standalone operation while still providing support from the public grid.

Data centers powered by direct current supply are another significant application. Data center operators are actively considering various technologies and solutions to enhance facility power efficiency, given that power is one of their major costs.

Operators of data centers are recognizing the benefits associated with direct current distribution, which not only minimizes the need for minimal conversions between AC and DC but also facilitates easier and more efficient integration with renewable energy sources. Achieving energy savings of 5% to 25% can improve transmission and conversion efficiency, reduce heat generation, and enhance reliability and availability twofold while reducing the required floor space by 33%. With many operators adopting measurement approaches based on electricity consumption for billing customers, precise DC energy metering is becoming increasingly important.

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Electric energy metering requires the capability to detect faults and electricity tamper

In the early 20th century, traditional AC meters were entirely electromechanical. They utilized a combination of voltage and current coils to induce eddy currents in a rotating aluminum disc. The torque produced on the aluminum disc, due to the product of the magnetic flux generated by the voltage and current coils, was proportional to the electricity consumed. Finally, a braking magnet was added to the aluminum disc to ensure that the rotational speed was directly proportional to the actual power consumed. By counting the number of rotations over a period of time, the electricity consumption could be measured.

Modern AC meters are much more complex and accurate, and they can also prevent electricity theft. Advanced smart meters can monitor their absolute accuracy and detect signs of theft on-site around the clock. Whether they are modern meters, traditional meters, AC meters, or DC meters, they are classified based on their kilowatt-hour impulses constants and percentage accuracy levels.

To measure the power consumed by a load (P = V × I), at least one current sensor and one voltage sensor are required. Typically, the current flowing through the meter is measured on the high-voltage side when the low-voltage side is at ground potential. This configuration minimizes the risk of unmeasured leakage current. However, current can also be measured on the low-voltage side if the design architecture demands it, or both sides can be measured. The technique often involves measuring and comparing the current on both sides of the load to enable fault detection and tamper detection capabilities in the meter. When measuring current on both sides, at least one current sensor with isolation is necessary to handle the high potential between conductors.

Voltage is usually measured using resistive potential dividers, where a series of resistors are used to reduce the voltage proportionally to a level compatible with the system's ADC input. Precise voltage measurement can be easily achieved using standard components due to the large amplitude of the input signal. However, it's important to consider the temperature coefficient and voltage coefficient of the selected components to ensure the required accuracy across the entire temperature range.

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Provide a high-speed ADC with ultra-low input current

In DC energy metering applications, ADI's AD7779, AD8629, and ADA4528-1 play significant roles. Among them, the AD7779 is an 8-channel simultaneous sampling ADC that integrates 8 full Σ-Δ ADCs on-chip. The AD7779 features ultra-low input current, allowing direct connection to sensors. Each input channel includes a programmable gain stage with gains of 1, 2, 4, and 8, enabling lower amplitude sensor outputs to be mapped to the full-scale ADC input range, maximizing the dynamic range of the signal chain. The AD7779 accepts VREF from 1 V to 3.6 V. The analog inputs accept unipolar (0 V to VREF/GAIN) or true bipolar (±VREF/GAIN/2 V) analog input signals, with analog supply voltages of 3.3 V or ±1.65 V. The analog input can be configured to accept true differential, pseudo-differential, or single-ended signals to match different sensor output configurations.

Each channel includes an ADC modulator and a sinc3 low-latency digital filter. The AD7779 utilizes SRC for fine resolution control of the Output Data Rate (ODR). This control is useful for applications where ODR resolution needs to maintain coherence when line frequency changes by 0.01 Hz. The SRC can be programmed via a Serial Peripheral Interface (SPI). The AD7779 supports two different interfaces: a Data Output Interface and an SPI Control Interface. The ADC Data Output Interface is dedicated to sending ADC conversion results from the AD7779 to the processor. The SPI interface is used to configure AD7779 configuration registers for read/write operations and to control and read data from the SAR ADC. The SPI interface can also be configured to output Σ-Δ conversion data.

The AD7779 features a 12-bit SAR ADC that can be used for diagnostics within the AD7779 itself, eliminating the need to dedicate a Σ-Δ ADC channel specifically for system measurement functions. Through external multiplexers (controlled using 3 general-purpose input/output, GPIO pins) and signal conditioning, the SAR ADC can be utilized for verifying Σ-Δ ADC measurement results in applications requiring functional safety. Additionally, the AD7779 SAR ADC includes a multiplexer that can be used to sense internal nodes.

The AD7779 incorporates a 2.5 V reference voltage source and reference buffer. The temperature coefficient of the reference voltage source is 10 ppm/°C (typical). The AD7779 operates in two modes: high-resolution mode and low-power mode. High-resolution mode provides higher dynamic range at a power consumption of 10.75 mW per channel, whereas low-power mode operates at a lower dynamic range specification with a power consumption of 3.37 mW per channel. The rated operating temperature range for the AD7779 is -40°C to +105°C, with a maximum device operating temperature of +125°C.

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An amplifier with ultra-low noise, drift, and current characteristics

The AD8629 amplifier from ADI features ultra-low offset, drift, and bias currents, making it an ideal choice for precision applications. It is a wide-bandwidth, auto-zero amplifier with rail-to-rail input and output swing capabilities, as well as low noise characteristics. The AD8629 operates from a single supply voltage of 2.7 V to 5 V (or dual supply voltage of ±1.35 V to ±2.5 V).

The AD8629 offers advantages previously found only in expensive auto-zero or chopper-stabilized amplifiers. These zero-drift amplifiers utilize ADI's circuit topology to combine low cost with high precision and low noise performance, all without the need for external capacitors. Additionally, the AD8629 significantly reduces the digital switching noise present in many chopper-stabilized amplifiers.

AD8629 features an offset voltage of just 1 µV, offset voltage drift less than 0.005 µV/°C, and noise of only 0.5 µV peak-to-peak (0 Hz to 10 Hz), making it suitable for applications where error sources are not tolerated. These devices exhibit near-zero drift within their operating temperature range, making them highly advantageous for applications such as position and pressure sensors, medical equipment, and strain gage amplifiers. Many systems can benefit from the AD8629's rail-to-rail input and output swing capabilities to reduce input biasing complexity and achieve higher signal-to-noise ratios.

The AD8629 has a rated temperature range of -40°C to +125°C, extending into industrial temperature ranges. It is available in standard 8-lead narrow SOIC and MSOP plastic packages.

Another amplifier from ADI, the ADA4528, is an ultra-low noise, zero-drift operational amplifier with rail-to-rail input and output swing capabilities. It features an offset voltage of 2.5 µV, offset voltage drift of 0.015 µV/°C, and noise of 97 µV peak-to-peak (0.1 Hz to 10 Hz, AV = +100), making it highly suitable for applications where error sources are not permitted.

The ADA4528 operates over a wide supply voltage range of 2.2 V to 5.5 V and offers high gain, excellent CMRR, and PSRR specifications, making it an ideal choice for precision amplification of low-level signals in applications such as position and pressure sensors, strain gages, medical instrumentation, and more.

The ADA4528 has a rated temperature range of -40°C to +125°C, extending into industrial temperature ranges. The ADA4528-1 is available in 8-lead MSOP and 8-lead LFCSP packages, while the ADA4528-2 comes in an 8-lead MSOP package.

With a maximum offset voltage of 2.5 µV and maximum offset voltage drift of 0.015 µV/°C, the ADA4528 is well-suited for providing ultra-low drift, 100 V/V amplification for small current signals. Therefore, it can be directly connected to the amplification stage of a synchronous sampling, 24-bit ADC like the AD7779, which exhibits a 5 nV/℃ input reference drift. Using a resistor potential divider with a 1000:1 ratio directly connected to the input of the AD7779 ADC, high DC voltages can be accurately measured.

Conclusion

DC energy metering offers higher accuracy compared to AC energy metering. In rapidly growing markets such as charging stations, microgrids, data centers, and other applications, DC energy metering provides fair billing and reduces the need for conversions between AC and DC, thereby minimizing energy losses. Integrating with renewable energy sources is also easier and more efficient with DC metering, making it a significant trend in development. ADI is a leading industry expert in precision sensing technology, offering complete signal chains for precise current and voltage measurements to meet stringent standards. The products discussed in this article represent some of the best choices for DC energy metering applications.

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