What factors need to be considered when designing the power measurement of a smart meter

Smart meters are advanced devices that can identify energy consumption more accurately than traditional meters. They are designed to obtain information about when energy is used, not just how much energy is used, and communicate this information to local utility companies for power monitoring, billing, and other purposes (Figure 1).

In order to meet the demand for advanced billing and energy management services, the MCUs of today's smart meters and sub-metering systems use high-precision analog front-end (AFE) circuits and advanced data processing software. But how much precision is needed for a specific application? Can the new generation MCU with integrated AFE provide enough accuracy for your application? Do you compensate for line disturbances, otherwise hardware, software, or a combination of the two will be used to distort the MCU's measurement? In this two-part series, we will share the answers to these questions and provide power measurement solutions.

What factors need to be considered when designing the power measurement of a smart meter
Figure 1: Modern smart meters rely on complex energy measurement ICs to measure power consumption, provide tamper detection, and in some cases sense power factor in real time (provided by STMicroelectronics).

Smart grid needs smarter electricity meters

In addition to the direct benefits of automatic meter reading (AMR), utility companies are turning to smart meters to implement advanced features that allow them to operate their power generation and distribution networks more efficiently. The detailed data generated by the smart meter can realize fine-grained client load management, better use of distributed energy, and so-called smart billing based on time, peak consumption level, and peak load management messages, and the so-called smart billing caused by reactance load interference. At the same time, commercial and industrial customers are increasingly using smart sub-metering systems to help manage their loads and control their utility bills.

In both cases, a new energy measurement technology is needed to provide accurate time-based images, including the power and quality delivered to the customer's premises, as well as any interference caused by the customer's load. On the grid.

Until recently, most smart power meter designs were based on a separate AFE, configured and read by a separate MCU (Figure 2). In order to ensure accuracy and minimize the possibility of tampering, the MCU that reads the AFE is dedicated to this task, and the result is passed to the more powerful host MCU to handle the remaining data processing and communication tasks of the smart meter.

What factors need to be considered when designing the power measurement of a smart meter
Figure 2: Microchip Technology’s MCP3901 dual-channel independent AFE includes two simultaneous sampling Delta-Sigma analog-to-digital converters (ADC), two PGAs, phase delay compensation module internal reference voltage, modulator output module and high-speed 20 MHz SPI compatible Serial interface.

Since most of the major players in the smart meter IC market (including ADI, Maxim, Microchip and Texas Instruments) have introduced dedicated metering processors, such as Texas Instruments’ equipment, this has happened in the past year or so. Variety. ‘MSP430AFE series (Figure 3), integrated analog front end and dedicated MCU. Mixed-signal equipment is usually affected by some degree of digital noise, which affects its analog sensitivity, accuracy, and overall accuracy. In applications that do not require the performance that a two-chip solution can provide, there is often a trade-off in reducing cost and simplifying implementation.

At the same time, the ability of IC manufacturers to integrate high-precision analog metering continues to improve, and meter performance requirements become more and more clear. These trends will make single-chip metering solutions more widely accepted. Therefore, although applications that require extremely high precision will still use independent AFEs, the lower solution cost and higher reliability provided by integrated AFE/MCU solutions will make it the "leading variety" in the mainstream smart meter market.

What factors need to be considered when designing the power measurement of a smart meter
Figure 3: TI’s MSP430AFE series integrates a 16-bit RISC CPU and three independent 24-bit sigma-delta converters and all necessary peripherals for a metrological analog front end with tamper-proof function and an error of less than 0.1% ( AFE) Energy accuracy in a wide (2400:1) dynamic range (provided by Texas Instruments).

Although these new single-chip and dual-chip metering solutions (and the reference designs that support them) greatly simplify the design of smart meters, there are still many issues that need to be resolved. In addition, the solution you choose also depends on how much precision/precision your application requires. In addition to basic power consumption, you also need the type of measuring instrument and the overall architecture that your design will adopt.

decision making

The accuracy required for utility and sub-metering applications are quite different, but in general, utility-level solutions require higher accuracy (a serious error of 0.1%) and require compliance with existing requirements IEC or ANSI standards. In contrast, sub-metering systems that are used for internal energy management and do not involve billing can provide good results with lower accuracy, usually about 2%.

However, the precision or accuracy in smart grid applications is much more complicated than the number of bits equipped with AFE's A/D converter. To some extent, this is because the AC voltage leaving the grid is not a pure 50 Hz or 60 Hz sine wave, and contains a treasure trove of harmonics, phase and impulse noise introduced during transmission, usually from consumer equipment. In order to accurately measure the customer's power consumption and any other impact they have on the grid, the AFE's ADC must maintain linearity between half of its line frequency and around kHz above it. At least, regardless of the specified resolution, it is necessary to understand the effective number of bits (ENOB), signal-to-noise ratio and distortion ratio (SINAD) or total harmonic (THD) value that the ADC can actually provide.

For example, Microchip's PIC18F87J72 smart meter MCU has a 16/24-bit ADC, which can provide up to 90 dB of SINAD and -101 dBc THD (to the 35th harmonic), enabling it to meet IEC Class 0.5 specifications. For applications that require a separate AFE to provide higher accuracy, Microchip's MCP3901 (single-phase) and MCP3903 (3-phase) can provide up to 91 dB SINAD/-104 dBc THD with an accuracy of 0.2%.

There are other issues that affect accuracy, including line disturbances that can distort the MCU measurement. Filtering or compensating for this noise is accomplished through a certain combination of hardware and software, and these combinations vary according to your design architecture. In all cases, the PCB around the AFE must be carefully arranged, and its passive components must be selected to resist noise and line disturbances. Software also plays a role in this work, especially to break down small spikes that would otherwise destroy the measurement data. The type of analog converter used by the AFE also affects how much additional compensation it needs. For example, the delta-sigma-based ADC used in the AFE of Microchip's smart meter products and Maxim's 71M654x and 71M6x01 smart meter chipsets can provide a high degree of filtering for high-frequency noise and other EMC-related issues (Figure 4). Maxim's design also includes on-chip AFE hardware features, such as jitter, which can improve the overall noise immunity of the system and eliminate the need for software-based cancellation.

What factors need to be considered when designing the power measurement of a smart meter
Figure 4: Maxim's 71M654x and 71M6x01 chipsets use Delta-Sigma AFE, which can reduce the impact of high-frequency components and line noise on power measurement accuracy. It also supports the optional interface of Teridian 71M6x01 series isolation sensor, which can provide BOM cost reduction, anti-magnetic tampering function and enhanced reliability.

Other matters needing attention

Precision, accuracy and noise immunity are just a small part of the issues involved in energy measurement in smart meters. For example, it is also a key issue that the metering element can provide this degree of dynamic range. When manufacturers want to use a single design on multiple platforms, wide dynamic range is another issue, each platform has its own set of different sensors and system parameters. In these cases, a metering element with a wide dynamic range increases the flexibility of subsystem design.

More importantly, unlike most consumer products, these meters can maintain acceptable levels throughout their service life, which can be 20 to 40 years or more. Another consideration is that for some applications, customers may wish to use the lowest possible current shunt value or use transformer-based current monitoring technology to further reduce losses.

As we have seen, as the demand for utilities grows, they are driving more and more intelligent functions such as load demand response, tariff management, communications and other "must-have." In response, most smart meter architectures usually use a separate metering module that partitions the data collection and processing functions from the MCU that handles meter management and management functions. Until recently, most smart power meter designs were based on an independent AFE, configured and read by a separate MCU. Now the integrated AFE/MCU solutions are available for selection, usually used for degree/cost/performance decisions.