DC Energy Metering Applications

Why is DC energy metering important?

In the 21st century, governments around the world are developing action plans to meet the long-term and complex challenge of reducing CO2 emissions, which are proven to be the cause of serious consequences of climate change, while the demand for new and efficient energy conversion technologies and improved battery chemistries is growing rapidly.

Including both renewable and non-renewable sources, the world’s population consumed nearly 18 trillion kWh last year alone, and this demand continues to grow; in fact, more than half of all existing energy has been consumed in the past 15 years.

To do this, our electrical grids and generators continue to grow; today, the need for more efficient, greener energy sources is growing. Early grid developers used alternating current (ac) to power the world because it was easier to use, but in many areas, direct current (dc) is significantly more efficient.

Driven by the development of efficient and economical power conversion technology based on wide bandgap semiconductors (such as GaN and SiC devices), many applications now see the benefits of converting to DC power. As a result, accurate DC power measurement has become increasingly important, especially where energy billing is involved. This article will discuss the development opportunities of DC metering in electric vehicle charging stations, renewable energy generation, server farms, microgrids and peer-to-peer energy sharing, and introduce a DC meter design.

DC Energy Metering Applications

Electric Vehicle DC Charging Station

Plug-in electric vehicles (EVs) are expected to grow at a CAGR of +70% by 20181 and are expected to grow at a CAGR of +25% from 2017 to 2024.2 The charging station market will grow at a CAGR of 41.8% from 2018 to 2023.3 However, in order to accelerate the reduction of CO2 emissions caused by private transportation, the demand for electric vehicles has become the first choice of the automotive market.

In recent years, a lot of work has been done to improve battery capacity and lifespan, but at the same time, a widespread electric vehicle charging network must be provided so that long-distance travel can be achieved without worrying about driving range or charging time. Many energy suppliers and private companies are deploying fast chargers up to 150 kW, and ultra-fast chargers with power of up to 500 kW per charging pile have also aroused public interest. Considering ultra-fast charging stations with local charging peak power of up to megawatts and the associated fast charging energy premium rate, electric vehicle charging will become a huge energy exchange market, which will require accurate energy billing.

Currently, standard electric vehicle chargers are metered on the AC side, with the disadvantage that the energy lost during the AC-DC conversion process cannot be measured, and therefore, the billing is inaccurate for the end customer. Since 2019, new EU regulations require energy suppliers to only charge customers for the energy transmitted to the electric vehicle, making both power conversion and distribution losses borne by the energy supplier.

While advanced SiC EV converters can achieve efficiencies of over 97%, when fast and ultrafast chargers are connected directly to the car battery, the energy is transferred in DC, and in this case, accurate billing on the DC side is clearly required. In addition to the public interest in EV charging metering, private and residential point-to-point EV charging schemes may have greater incentives for accurate energy billing on the DC side.

Figure 1. DC energy metering for future electric vehicle charging stations

Figure 2. DC energy metering for sustainable microgrid infrastructure

DC Power Distribution - Microgrid

What is a microgrid? Essentially, a microgrid is a smaller version of the utility power system. As such, it requires a safe, reliable, and efficient power source. Microgrids may be used in hospitals, military bases, or even as part of a utility system where renewable energy generation, fuel generators, and energy storage work together to form a reliable energy distribution system.

Microgrids are also being used in buildings. With the widespread use of renewable energy generators, buildings can even power themselves, with rooftop solar panels and small wind turbines producing enough power to operate independently but still provide support from the public grid.

Additionally, up to 50% of a building’s electrical load is DC. Currently, every electronic device must convert AC to DC, losing up to 20% of the power in the process, resulting in an estimated total energy savings of up to 28% compared to traditional AC distribution.

In buildings where DC power is deployed, energy consumption can be reduced by converting AC power into DC power once and feeding the DC power directly into the required equipment (such as LED lights and computers).

As interest in DC microgrids grows, the need for standardization is also increasing.

IEC 62053-41 is an upcoming standard that will specify requirements and nominal levels for residential DC systems and enclosed meters (AC equivalents to DC energy metering).

As of May 2017, the DC microgrid sector was valued at approximately $7 billion and is expected to grow further with the emerging trend of DC distribution.

DC Powered Data Center

Data center operators are actively considering different technologies and solutions to improve the power efficiency of their facilities, as electricity is one of their largest costs.

Data center operators see benefits associated with DC power distribution, not only in terms of reducing the minimum number of conversions required between AC and DC, but also in making integration with renewable energy sources easier and more efficient. The reduction in the number of conversion stages is estimated as follows:

•     Energy saving of 5% to 25%: Improve transmission and conversion efficiency and reduce heat generation

•     Double the reliability and availability

•     33% less floor space

Figure 3. DC power delivery in data centers requires fewer components and has lower losses than traditional AC power distribution.

Figure 4. Renewable energy integration in a DC-powered data center

Distribution bus voltages range up to around 380 VDC, and accurate DC energy metering is becoming more and more of a concern as many operators begin to adopt metering methods to charge colocation customers based on electricity usage.

There are two common ways to bill colocation customers for electricity:

•     Each time (fixed fee per export)

•     Electricity consumed (metered outlet - charged per kWh consumed)

To encourage greater power efficiency, metered output methods are becoming more popular, with customer pricing involving the following components:

Recurring costs = Space costs + (IT equipment meter reading × PUE)

•     Space Fee: Fixed, includes security and all building operating costs

•     IT equipment meter reading: kWh consumed by IT equipment multiplied by the cost of electricity

•     Power usage effectiveness (PUE): Considers the efficiency of the IT infrastructure behind it, such as heat dissipation and cooling.

A typical modern rack consumes up to 40 kW of DC power. Therefore, a revenue-grade DC meter is required to monitor currents up to 100 A.

Challenges of Precision DC Energy Measurement

In the early 20th century, conventional AC meters were entirely electromechanical. A combination of voltage and current coils was used to induce eddy currents in a rotating aluminum disk. The torque produced on the aluminum disk was proportional to the product of the magnetic flux produced by the voltage and current coils. Finally, a broken magnet was added to the aluminum disk so that the rotation speed was proportional to the actual power consumed by the load. At this point, the power consumption could be measured by simply counting the number of rotations over a period of time.

Modern AC meters are more sophisticated, more accurate, and prevent electricity theft. Advanced smart meters can now even monitor their absolute accuracy and detect signs of electricity theft 24/7 when installed in the field. This capability is available in the ADE9153B metering IC from Analog Devices, which uses mSure® technology.

Whether modern, traditional, AC or DC, meters are classified based on their pulse constant per kilowatt-hour and percentage grade accuracy. The number of pulses per kilowatt-hour indicates the energy update rate, i.e., the resolution. The grade accuracy indicates the maximum measurement error of the energy.

Similar to old-fashioned mechanical meters, energy is measured in a given time interval by counting these pulses; the higher the pulse frequency, the higher the instantaneous power, and vice versa.

DC Meter Architecture

The basic architecture of a DC meter is shown in Figure 5. To measure the power consumed by the load (P = V × I), at least one current sensor and one voltage sensor are required. The current flowing through the meter is usually measured on the high voltage side when the low voltage side is at ground potential to minimize the risk of unmeasured leakage, but the current can also be measured on the low voltage side or on both sides if the design architecture requires it. The technique of measuring and comparing the current on both sides of the load is usually used to enable the meter to have fault and power theft detection capabilities. However, when measuring the current on both sides, at least one of the current sensors needs to be isolated to handle the high potential between the conductors.

Voltage measurement

Voltage is often measured using a resistor divider, where a ladder of resistors is used to scale the potential down to a level compatible with the system ADC input.

Due to the large amplitude of the input signal, accurate voltage measurements are easily achieved using standard components. However, attention must be paid to the temperature and voltage coefficients of the selected components to ensure the required accuracy over the entire temperature range.