Power Management
Embedded silicon for
smart meter designs
As the take up of smart meter technology accelerates so the challenges associated with embedded smart meter design increase. Andreas Kohl considers a number of new technologies that could aid in the design process
D
emands to reduce global power consumption, simplify the collection of usage and billing data and help consumers and businesses better manage their energy are fuelling growth of the ‘smart grid’. And while smart grid definitions vary all have in common the deployment of smart meters to simplify energy monitoring and control. Currently there are different rates of ‘take up’ for smart meter technology around the world. A small number of ‘progressive’ energy metering countries such as Italy and Sweden are already claiming 90% smart meter implementation, while Germany and the UK saw smart meter penetration at just 5% by the end of 2011. However, evolving environmental legislation – in Germany, for example, the law now demands that every new house has a smart electricity meter – is fuelling significant growth.
Hierarchy of metering Smart grid implementations are typically built around three levels of metering
30 March 2013
systems with varying capabilities and performance.
consumer premises supports remote collection of billing data, the ability to update tariffs and provides a mechanism for alerting consumers to peak and low- cost periods for demand response initiatives. Commercial and industrial (C&I) or mid- range meters are higher performance versions of the residential meter that can monitor three-phase supplies, offer advanced programming capabilities and have more options for measurement, control and communication with the utility.
At their highest level, smart meters with ‘gateway’ capabilities take functionality beyond the physical metering layer. These meters may incorporate local area and wide area networking capabilities to support remote switch on, shut off and control of appliances, heating, lighting and other home comforts via a remote PC or other Internet-enabled device.
Figure 1: Smart Meter – Key functional elements
The entry level in this metering hierarchy is the single-phase residential meter. This meter raises consumer awareness of energy use and cost by measuring and displaying real-time energy use. Two-way communication between suppliers and
Components in Electronics
Embedded design challenges The heart of all of these meters is the embedded system - and how this system is implemented is critical to successful meter design, scalability for the future and, ultimately, the overall vision of the smart grid. Among the challenges that the embedded system designer has to address include metrology implementation, power consumption, display capabilities and the integration of relevant connectivity (see Figure 1). The metrology design, for example, must meet legal obligations that demand power measurement accuracy to within 0.1% for billing purposes. And the key challenge in terms of power is that smart meters, during operation, consume more energy than conventional electromechanical units. Clearly careful attention to power reduction techniques is required if the benefits of the smart grid are not to be outweighed by increased energy consumption. Providing an intuitive user interface, typically via an LCD screen, is essential - especially for domestic meters - while connectivity may include wireless communication,
powerline communication (PLC) and home area networking alongside web server capabilities. And, in all cases, all of these challenges must be addressed with the minimum component count, at very low cost and in short development timescales.
Microcontrollers for smart meters It is with these challenges in mind that semiconductor manufacturers such as Toshiba have begun to offer microcontrollers with performance and functionality optimised for smart metering. The TMPM061 microcontroller for example - the first to feature the ARM Cortex-M0 32-bit core - is dedicated to entry-level residential smart meter applications. A block diagram of this device is shown in Figure 2. With its small silicon area and minimal code footprint the ARM Cortex-M0 core supports ultra-low-power operation, while the microcontroller has four standby modes – IDLE, SLOW, SLEEP, STOP – giving flexible options for designers to minimise or eliminate power consumption depending on smart meter status. A key benefit of this new device is that it allows designers to replace a more traditional two-chip analogue front end (AFE) and processor implementation with a single chip. This has been achieved through an on-board DSP power calculation engine and integrated three- channel, high-precision 24-bit Delta-Sigma (∅∑) analogue-to-digital converter (ADC). The ∅∑ ADC supports simultaneous sampling at up to 6kHz and has a SINAD (signal-to-noise and distortion ratio) that is suitable for a residential meter, while the power calculation engine can calculate active energy, reactive energy, and power factor as well as monitoring voltage and frequency fluctuation and pulse outputs. Figure 3 illustrates active power calculation in a smart meter design.
Figure 2: 32-bit ARM Cortex-M0 microcontroller for residential smart meters
Gain and offset correction help to meet specified measurement accuracy targets and phase correction can be independently configured for different ADCs. An in-built unbalanced current detection function allows OEMs to implement anti-tampering features. Furthermore, to address the broadest possible range of meter applications, the basic energy use calculation can be updated, modified and enhanced by the designer as necessary. Other on-board functions that minimise components needed for the smart meter design include an additional 10-bit ADC, a temperature-compensated real-time clock (RTC), Flash ROM (128Kbytes) and RAM (8Kbytes), a 9-channel, 16-bit timer, a temperature sensor, a voltage detection circuit and a controller for directly driving an LCD display. Integrated connectivity comprises a 5-channel general-purpose serial interface (selectable between UART
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