By Michael Navid
Fast, secure, and reliable communications are critical for the “energy internet,” an entire interconnected network of interactive smart devices. Therefore, the selection of the right network infrastructure and communications technology is crucial to achieving any smart grid vision.
One of the biggest challenges facing implementers is how to meet both current and future smart grid requirements, while ensuring interoperability and open-endedness among grid elements.
CHOOSING A TECHNOLOGY PLATFORM FOR THE FUTURE When evaluating communications platforms, it is important to look for a solution that:
- Provides a cost effective system architecture
- Includes security mechanisms to protect grid assets
- Is standards-based to ensure interoperability and open-endedness
- Capacity to enable new distribution switches, sensors, and new home area network (HAN) applications
- Coexists with older technologies such as S-FSK.
MAXIM’S PLC SOLUTION MINIMISES COSTS FOR MEDIUM AND LOW VOLTAGE INFRASTRUCTURES
Power line communications (PLC) technology provides the best performance and cost efficiency for medium and low voltage power grids. By communicating on the very lines it measures and controls, PLC minimises infrastructure costs, installation costs, and maintenance costs.
Traditionally, it has been difficult to achieve fast, reliable communications in the severe conditions that characterise power lines. The 10 kHz to 500 kHz frequency region allocated worldwide (e.g. FCC, ARIB, CENELEC A) for power line signalling is particularly susceptible to interference, background noise, impulsive noise, and group delays.
An additional difficulty for power line in the past has been in crossing transformers. This capability is needed in certain topologies, especially in the US, to minimise the amount of concentrators or repeaters required to move data.
Maxim’s PLC technology overcomes these challenges by using orthogonal frequency-division multiplexing (OFDM). This approach maximises bandwidth utilisation, thus allowing advanced channel coding techniques for robust communications as well as higher data rates.
THE BUSINESS CASE FOR USING OFDM-BASED PLC
The obvious advantage of PLC is that one does not need to build a separate communications network to supplement one’s grid infrastructure. Beyond this, Maxim’s OFDM-based PLC technology offers additional benefits not available with other PLC schemes.
The robustness of Maxim’s technology allows long distance (over 10 km) data transmission over medium voltage lines, thus enabling the use of fewer repeaters. This technology also achieves reliable communication across medium to low voltage transformers, which can reduce the number of concentrators required by a factor of 3:1. This capability can substantially cut costs in rural areas by allowing fewer concentrators to support isolated meters.
Additionally, OFDM delivers a much faster application data rate than single-carrier schemes: 4 s to read 3,300 bytes versus 28 s for S-FSK 2400 (or 56 s for S-FSK 1200). This accelerated load profile reading offers significant advantages to applications that demand real time visibility into grid conditions and energy usage.
G3-PLC SPECIFICATION PROVIDES AN OPEN STANDARD FOR GLOBAL SMART GRID IMPLEMENTATIONS
An open standard using OFDM-based PLC is available to all smart grid architects. In partnership with Sagem Communications and Electricité Réseau Distribution France (ERDF), Maxim developed the G3-PLC specification to promote interoperability and open-endedness among smart grid implementations.
The G3-PLC specification includes an OFDM-based PHY to ensure robust operation in severe environments; an IEEE 802.15.4-based MAC layer well suited to low data rates; and a 6LoWPAN adaptation layer to transmit IPv6 packets over power line channels.
A closer look at OFDM-based PLC
Orthogonal frequency-division multiplexing (OFDM) is a modulation technique that utilises the frequency band very efficiently, thus allowing the use of advanced channel coding techniques. This PLC technology enables very robust communication in the presence of narrowband interference, impulsive noise, and frequency selective attenuation.
The figure illustrates why OFDM is so much better than single carrier techniques for data communications. In this example, eight tones between 10 kHz and 95 kHz are used, providing a usable channel bandwidth of 85 kHz. In contrast, a single carrier solution uses only two tones to transmit data in that bandwidth.
In both cases, four data bits and four error correction bits are sent. In the top half, OFDM transmits all eight bits with a single symbol. In the lower, FSK needs four symbols to transmit the same payload. Because OFDM uses the spectrum more efficiently, it opens the channel for more data and, therefore, a higher data rate.
Maxim’s PLC solutions use convolutional and Reed-Solomon coding to provide redundancy bits. This approach allows the receiver to recover bits lost due to background and impulsive noise. An interleaving scheme is also used to decrease the correlation of received noise at the input of the decoder.
As an example, a typical single carrier FSK modem in the CENELEC A band (10 kHz to 95 kHz) can only transmit 2 kbps at 12 dB SNR with a bit error rate (BER) of 10-4, meaning that one bit in 10,000 transmitted is lost. In contrast, an OFDM system can transmit up to 32 kbps at around 4 dB SNR. Thus, OFDM modulation with error correction techniques enables an 8 dB improvement in performance at much higher data rates in this band. Effective data rates of >100 kbps can be achieved when operating in the FCC (10 kHz to 490 kHz) and ARIB (10 kHz to 450 kHz) bands due to the wider bandwidths.
To further improve performance, the MAX2990 automatically switches to robust mode when input signal variations exceed predefined thresholds, such as SNR levels, input fluctuation levels, and potential in-band tone reductions. As a result, this mode achieves ~5 dB improvement in SNR.
Application data rates for PLC technologies*
|Technology||Time (s) to read 3,300 bytes|
*S-FSK data rate calculated by DLMS-UA. OFDM data rate measured in the field.
Independent test results for PLC technologies*
|Technology||Master - Slave||Slave - Master|
|Data rate (bps)||Frame error rate (%)||Data rate (bps)||Frame error rate (%)|
|Communication over LV networks|
|Communication across MV to LV transformers|
|Long distance (6.4 km) communication over MV lines without repeaters|
*Test conditions: 32 kHz to 95 kHz frequency band; 116 dB/μV injected power; data rate measured at the application layer.
The specification also includes innovations such as:
- MAC-level security using an AES-128 cryptographic engine
- Coexistence with older S-FSK systems
- Adaptive tone mapping for optimal bandwidth utilisation
- A robust mode of operation to improve communication under noisy channel conditions
- Two layers of forward error correction (FEC)
- Channel estimation to select the optimal modulation scheme between neighbouring nodes
- Mesh routing protocol to determine the best path between remote network nodes.
The complete G3-PLC profile specification as well as specifications for the PHY and MAC layers can be downloaded from ERDF’s website www.erdfdistribution.fr.
A FULLY TESTED, FIELD PROVEN SOLUTION
During the evaluation period, ERDF tested the G3-PLC technology against competing technologies under real world conditions and scenarios. Test results are provided left. These advanced field trials demonstrated that G3-PLC offers the most robust data communication over the PLC network, as compared to other technologies.
The G3-PLC open standard approach and global support enable developers to seamlessly adapt this next generation PLC platform.
Customers interested in deploying this technology in largescale projects can contact us at: G3-PLC@maxim-ic.com.