Data Concentrators Combine AFEs, MCUs, and Radios to Key Smart Grid Efficiency

Data concentrators provide the communications lynchpin in automated metering and information systems within smart grid architectures. Typically located at the transformer or substation level in the power grid, data concentrators ensure data integrity and security in communicating energy measurement information from smart meters and usage analysis information from power utilities. Typically, these systems use sophisticated MCU-based designs that rely on power line communications (PLC) in the last mile to consumer meters and energy management systems. To meet a wide range of requirements for these devices, engineers can design sophisticated PLC-based data concentrators using available AFE ICs and MCUs from manufacturers that include Freescale Semiconductor, Maxim Integrated, STMicroelectronics, and Texas Instruments, among others.

Data concentrators serve as the interface between the utility-controlled smart grid distribution network and end user, managing the data exchange between the utility and multiple smart meters in a particular geographical area (Figure 1). In both automated metering infrastructure (AMI) and automated meter reading (AMR) systems, data concentrators – also called data aggregators – provide the core functionality required to measure, analyze, and collect energy usage. They then communicate that data to a central database for billing, troubleshooting, and analyzing.

Figure 1: An Automated Metering and Information System comprises multiple levels of data management and communications, starting with hardware data concentrators that provide data acquisition, processing, and security between the utility and end-user energy systems including smart meters. (Courtesy of Siemens.)

Data concentrators enable utilities to use information from smart meters to gain a better understanding of grid status, including immediate knowledge of outages. With this detailed information about the power distribution grid, power companies can evaluate overloads and unbalances in the grid, as well as more effectively detect faults. This information also allows utilities to evaluate more static losses in the grid by comparing measured power at higher levels of the grid with that measured by smart meters at the periphery of the grid. Ultimately, the combination of detailed smart meter power measurements and upstream monitoring and analysis will provide utility customers with better knowledge of their consumption and power saving opportunities, and inform them about network situation disturbances.

By concentrating data across multiple smart meters and end-user networks, data concentrators help simplify the design of smart meters, enabling those systems to focus on the critical tasks of energy measurement. In turn, data concentrators are responsible for providing the necessary communications and networking functionality needed to link several utility meters to the utility's central servers. Among its tasks, the data concentrator synchronizes data obtained across several meters and ensures secure data transfer of user authentication and encryption information.

Consequently, data concentrators are sophisticated systems that combine powerful processing capabilities with a broad range of highly flexible communications options needed to support the broad range of data transmission capabilities needed to serve the widest possible range of end systems and operating environments (Figure 2).

Figure 2: Among connectivity options, data concentrator designs typically need to support power line communications and wireless communications capabilities – along with varying degrees of support for display options. (Courtesy of Texas Instruments.)

Data concentrators typically support both wireless and wired communications options for both end-user and utility-side communications. On the user side, wired communications typically includes power line communications (PLC), Ethernet, or even serial communications, depending the specific needs and limitations of a particular operating environment. Wireless communications typically includes low-power RF using sub-GHz, 2.4 GHz, or even cellular networks. On the utility side, concentrators need the flexibility to connect to the utility's servers through a variety of different methods, so they need to support a broad range of communications options including Ethernet, GSM, GPRS, WiMAX, or telecom networks.

MCU-based designs

At the heart of these systems, applications processors serve as the central host, providing both data analytical capabilities as well as data communications. To support design of these systems, silicon manufacturers offer a wide range of MCU options. For example, Texas Instruments offers a number of MCU solutions including its AM18xx, AM335x, TMS320C674x, and OMAP-L1x applications processors.

Based on the ARM926EJ-S 32-bit RISC processor core, TI Sitara AM18xx microprocessors address low-power applications with a variety of on-chip peripherals, including Ethernet MAC, MDIO, UART, USB, I²C, and SPI, among others. The TI device combines memory stores associated with the ARM core with 128 Kbytes of on-chip memory, enhanced DMA controller 3, and external memory interfaces for DDR2 and EMIFA.

The TI Sitara AM335x processor is based on an ARM® Cortex™-A8 core, which includes a SIMD parallel processing coprocessor. The device combines this sophisticated processing core with an on-chip graphics acceleration engine and controllers for LCD and touchscreen devices. This provides a comprehensive platform for data concentrator designs requiring more advanced data and graphics processing and display requirements. Along with multiple on-chip connectivity peripheral options, the device provides hardware crypto accelerators to support heightened security in data concentrator designs.

For data concentrator designs faced with more demanding data and signal processing requirements, the TI TMS320C674x family is a DSP that supports both fixed and floating-point processing. The device's VLIW DSP core features 64 32-bit registers, six ALUs, and two multiply units. Along with a broad of range of connectivity peripherals found on the previously mentioned devices, the TMS320C674x family offers a secure boot feature that allows engineers to protect their code and prevents third parties from modifying secured algorithms.

For data concentrator applications facing particularly demanding processing requirements, TI's OMAP-L1x family combines the ARM ARM926EJS core and a TI C674x DSP core. Along with the dual cores, the OMAP-L1x family combines multiple on-chip peripherals of the type supported by the AM18xx and TMS320C674x family devices. As with the TMS320C674x, the OMAP-L1x family offers the secure boot feature to protect code in Flash or EEPROM.

The ARM926EJ-S™ core also provides the processing engine for data-concentrator solutions in MCUs from STMicroelectronics and Freescale Semiconductor. The STMicroelectronics SPEAr300, SPEAr310, and SPEAr320 MCUs combine the ARM core with a broad array of memory interfaces and connectivity options. An integrated crypto coprocessor can run autonomously from the host processor to execute AES, DES, SHA-1, and other crypto algorithms.

Freescale Semiconductor addresses data concentrator designs with its ARM926EJ-S-core-based i.MX28 MCU family, as well as with its MPC8308 MCU based on its PowerQUICC® II Pro processor. Along with these general-purpose host MCUs, Freescale offers its dual-core P1025 communications processor for data concentrator applications (Figure 3). The P1025 is a QorIQ device built on Power Architecture technology, offering a dual-core solution to allow applications and communications to coexist. Besides supporting a fully featured operating system, the P1025 offers hypervisor support for a high-level application such as a data concentrator.

Figure 3: The Freescale P1025 dual-core communications processor combines dual e500 cores with a full complement of on-chip interfaces needed to create an efficient data concentrator design solution. (Courtesy of Freescale Semiconductor.)

Power line communications

On the downstream side, communications with smart meters and customer energy management systems can use wired or wireless communications. Wired communications typically relies on PLC, although the nature of those communications can vary widely.

Power line communications offer a convenient method for data transmission by using the same medium used for power transmission to convey data about power usage. Power lines, however, are noise environments, with much of the energy in the noise concentrated in short time-domain bursts that occur at the same period as the zero crossing of the mains AC cycle – 8.33 ms in the U.S. Although the specific noise profile can vary significantly from one site to the next, energy distribution tends to fall off at higher frequencies (Figure 4).

Figure 4: Along with non-impulsive noise from different sources, power line signals face impulsive noise associated with zero crossing of the mains AC cycle. (Courtesy of Texas Instruments.)

Further difficulty arises in dealing with the topology of distribution networks on the customer side. In the U.S. and in Japan, for example, individual distribution transformers support only a few houses, particularly in areas of low population density such as rural areas. To minimize the cost in those situations, the concentrator would best reside on the medium-voltage (MV) side, requiring signals to cross each distribution transformer to establish communication between meters on the low-voltage (LV) side and the concentrator.

Noise on the MV side follows much of the characteristics of noise on the LV side, but passing a signal across a transformer from MV to LV can significantly attenuate the signal on a frequency-selective basis that varies from site to site. As a result, to operate across an MV–LV transformer, an effective method should offer communications spanning the 30 to 450 kHz band.

Designed to support communications across the MV–LV boundary, G3-PLC, the worldwide standard for PLC, supports 10 to 490 kHz frequency bands to provide a long-range communications method that can cross MV–LV transformers, reducing the number of data concentrators required in a particular geographical area.

The standard specifies support for addressing both overall attenuation and frequency-dependent attenuation at higher frequencies in passing through a MV–LV transformer. Here, the transmitter is expected to adjust its overall signal level and shape its power spectrum, while the receiver provides both an analog and digital AGC (automatic gain control) to provide sufficient gain to compensate for the overall attenuation. For environments with particularly severe attenuation, a PLC–G3 system can operate as a repeater on the LV side – decoding received frames from the MV side and retransmitting them at a higher signal level on the LV side in an effort to compensate for the attenuation introduced by the transformer.

G3-PLC incorporates 802.15.4-based MAC layers with AES-128 security to enable interoperability without compromising security. Furthermore, this standard can coexist with technologies such as S-FSK and BPL, while also providing more robust communications capabilities able to operate in noisy environments as low as –1 dB SNR.

PLC solutions

Engineers can create a PLC solution using available analog components such as TI OPA365 CMOS power amps, TI PGA112 programmable gain amplifiers (PGAs), and C28x Delfino or Piccolo MCUs from TI's C2000 32-bit MCU families (Figure 2).

TI integrates a complete analog signal chain needed for G3 PLC in its AFE031 IC. Designed specifically as a PLC AFE, the AFE031 includes an integrated receiver able to detect signals down to 20 μVrms while providing programmable gain control needed to adapt to changing input signal conditions associated with changes in the power line and power line noise level. The device's power amp operates from a single supply ranging from 7 to 24 V, while the analog and digital signal circuitry operates from a single 3.3 V supply. Along with a TI C2000 MCU, the device needs only minimal external circuitry to provide complete PLC system solution (Figure 5).

Figure 5: Designed for PLC applications, the Texas Instruments AFE031 AFE integrates a complete analog signal chain for PLC Tx and Rx functionality, communicating with a host TI C2000 MCU through its serial interface. (Courtesy of Texas Instruments.)

An original author of the PLC-G3 specification, Maxim offers its own PLC solution through a pair of chips – MAX2991 AFE and MAX2990 baseband modem. The MAX2991 AFE transceiver IC integrates a dedicated PLC analog signal chain, providing both transmit and receive paths (Figure 6).

Figure 6: In the Maxim Integrated MAX2991 AFE transceiver IC, a transmit path injects an OFDM-modulated signal into the power line, while the receive path provides signal enhancement, filtering and digitization of the received signal. (Courtesy of Maxim Integrated.)

Specifically designed for OFDM (orthogonal frequency division multiplexing) modulated signal transmission over power lines, the MAX2991 AFE IC operates in the 10 to 490 kHz band and provides programmable filters that allow engineers to ensure compliance with CENELEC, FCC, and ARIB standards using the same device.

Paired with the MAX2991 AFE, the MAX2990 PLC modem IC provides a complete PLC solution. The MAX2990 combines Maxim's MAXQ 16-bit RISC core with PHY functionality, serial interfaces including SPI, I²C, and UART. Along with Jammer cancellation features, the device includes a security engine that DES encryption/decryption.

Summary

Data concentrator designs face challenges in addressing a combination of regulatory standards and functional requirements that can vary widely across different regions and power distribution utilities. At the same time, a wide array of sophisticated MCUs and flexible AFE devices offer capabilities uniquely targeted to these design applications. Using these available devices, engineers can create data concentrator designs able to address the widest possible range of processing, security, and display requirements.