Electric Meters

Overview
To an increasing extent, modern electric meters provide both metrology and communication functions. Each of these is addressed below:

Metrology
Developers of electricity meters face a number of challenging design objectives. First, the system must measure a range of electrical parameters and it must do so with sufficient guard band so that it meets utilities accuracy requirements across all field operating conditions. A typical list of measurement parameters include total active power, total active energy, total reactive power, total reactive energy, RMS current, RMS voltage, apparent power and apparent energy. Electricity meters must function properly and survive in the field for years so product quality standards are critical and agency certification tests must be passed. Some of these tests include areas like system-level electrostatic discharge (ESD), electrical fast transient (EFT), and immunity to radio frequency interference.

When developing an electric meter, there are numerous factors that need to be considered and inevitably the designer must make some tradeoffs. One important factor is cost - both from the standpoint of product bill-of-material cost and also R&D development cost. Some meter architectures are very low cost from a product and development standpoint but compromise along other important dimensions. Flexibility and scalability are also important concerns in meter design. The designer must determine whether the underlying architecture and resulting hardware platform can address a range of current and future technical requirements. Another important item to address is system architecture and whether metrology functions are largely implemented in dedicated hardware or in a general purpose DSP or MCU. A high-performance, flexible architecture may consist of a two-chip solution that includes an A/D converter and a separate DSP or MCU. With this type of architecture, the designer must invest significant time programming all of the metrology functions in the external processor. A different approach still uses two chips but employs an analog front end and an external MCU. The AFE is a combination of A/D converters and a dedicated hardware metrology block. In this case, the designer can usually select a smaller, less expensive processor and concentrate on managing the AFE and the external interfaces as the AFE manufacturer guarantees that all the energy metering quantities are computed within the right specifications. A third type of meter architecture is essentially a system-on-chip where all of the conversion, metrology computations and control are implemented in a single device. This approach can yield a very low cost design however it comes at the expense of less flexibility. For example, with an SOC architecture it is more challenging to incorporate other system functions such as communication for AMI or smart metering.

In the last several years, the electric meter market has evolved in some significant ways. For example, utilities have increased their focus on maintaining the overall health of grid transmission and distribution networks. In turn, they are asking meter manufacturers to measure new parameters related to detecting fault conditions, monitoring power quality and identifying loads that can cause disturbances or outages on the grid. A partial list of new parameters includes peak voltage, peak current, zero-crossing detection and harmonic content. The need for measuring more harmonic content is driving up bandwidth requirements and compute resources needed for metrology. Other important trends are the need to directly measure neutral current channel in order to avoid electricity theft and meter tampering.

Communications
Electric utilities worldwide are aggressively rolling out smart meters with a wide range of communications technologies: PLC/RF, narrow/wide bandwidths and mesh/star topologies.  Irrespective of chosen technology, all smart meter networks bring unique challenges.  Utilities will deploy millions of meters, often in locations that have very poor signal conditions, and they expect 100% network coverage of all meters.

Meter communications systems operate in extremely noisy and increasingly crowded spectrum.  Unlicensed RF bands are particularly challenging with no ability to control noisy neighboring networks.  PLC networks face similar challenges with a wide variety of noise sources on the power line. In both networks, the spectrum becomes increasingly crowded as more meters are deployed.  Successful deployment depends on cost driven components designed specifically for these unique challenges.  Network robustness will depend on providing optimal link margin with devices with high sensitivity.  In addition these systems must be designed with excellent interferer and adjacent channel blocking performance. 

Utilities expect decades of useful life from their meters and networks.  Flexibility to anticipate future requirements is therefore critical to the architecture and system specifications.  These future requirements span new interference sources, new communications standards and new utility applications.  In addition, with unique country requirements, no one design can meet all worldwide requirements.  To reduce the effort to create multiple solution, smart meter communications engineers need a family of components that enable highly integrated or highly modular designs.  This includes processor solutions spanning low power microcontrollers to cost optimized DSP's, standalone RF transceivers to single chip RF-SOC's, and discrete converters and amplifiers to full PLC modules.