IEC 61850 critical tosmart grid designs

Driven by the adoption of the Ethernet IEC 61850 protocol, substation automation is becoming much more sophisticated, combining network communications, measurement and control into more potent solutions. The challenge for embedded system designers is how to create real-time targets that can also incorporate power quality monitoring, phasor measurements and other smart grid-related analysis capabilities. Al Presher

ETHERNET NETWORKING, MEASUREMENT and control technologies are pushing into smart grid applications with much higher and more sophisticated solutions. Driven by the impetus that has resulted from implementing IEC 61850, power, utility and smart grid designers are leveraging new digital technology that will enhance two-way communication between the utility and its customers, power monitoring and analysis, plus control and sensing along transmission lines.

“New protocols coming online such as IEC 61850 based on Ethernet communications are serving as new international standards for the smart grid,” said Brian MacCleery, Principal Product Manager for Clean Energy Technology at National Instruments in a recent interview. “Engineers are trying to figure out how to implement the technology on their embedded systems.”

Emergence of IEC 61850The major trend with IEC 61850 is that the complexity of the protocols has increased dramatically, and is driving the need for better system-level tools.

“Moving to smart grid applications combines network communications and measurement with a much higher level of computer control than in the past, along with pushing all of those vectors at the same time,” MacCleery said. “There is a need for more sophisticated control, more sophisticated networking, plus more sophisticated measurements and analysis on the embedded system.”

The goal is to embed all of these capabili-ties on cost-effective systems that can be

deployed in large volume out on the smart grid. It is pushing Moore’s Law on a number of fronts because of the processing required just to do the networking protocols; the device becomes a small server.

The IEC 61850 standard is a set of open protocols based on Ethernet communications commonly used in electrical utilities. SCADA systems use IEC 61850 to communicate between a master station, remote terminal units (RTUs), and intelligent electronic devices (IEDs). Real-time targets may be programmed as IEC 61850 outstation devices with advanced functionality such as power quality monitoring, phasor measurement and other smart grid-related analysis.

DNP3 is the protocol that has been histori-cally used in these types of applications, and is a simpler protocol more similar to Modbus that engineers are replacing with IEC 61850 and, to a lesser extent, IEC 60870-5.

“The task for the embedded developer is daunting to develop these protocols on their own,” said MacCleery. “DNP3 is less complex and Modbus-like, so the development process wasn’t too complicated. IEC 61850, on the other hand, is extremely complex so there is significant value is getting a toolkit and being able to implement that easily on the control system without re-writing the driver from scratch.”

While part of the puzzle for developing smart grid applications is supporting these modern protocols, one engineering challenge is keeping up-to-date since the standards have still been evolving over the past year. To create an embedded system for the smart

grid, it needs to be as future proof as possible.MacCleery said that one of the ways

developers are doing this is by putting the control in the FPGA, which leaves the processor free to handle the networking requirements of the system. These tasks tend to be processor-intensive as the protocols become more complex. If the processor is free to do the networking, the control can run on another target such as an FPGA. This is a common architecture for this type of system.

Because the FPGA is now a hybrid device that contains multiple DSP cores, modern embedded systems use a hybrid FPGA that has the DSP cores and the FPGA fabric plus a real-time processor that can be used to run these communications protocols.

There are subsections of 61850 that, for example, define if you are a wind turbine what classes of data you will report. There are other classes which provide a useful framework for how data is organized to provide structure. This fits into the ongoing trend toward programmable automation controllers (PACs).

“You have a control system doing Ethernet network communications in a very sophis-ticated way, combined with measurement, analysis and control,” said MacCleery. “Older systems used a PLC which was a control-only device, so this becomes an extension of that approach and a way for embedded systems to do more than just the control.”

Any embedded system from National Instruments that runs LabVIEW real-time can use 61850 for smart grid applications including the Compact RIO, the single-board RIO and the general purpose inverter controller.

Applications

11

Critical to the reliable delivery of energy is the capability to remotely monitor, coordinate and operate distribution components in a real-time mode using IEC 61850 networks.

Phot

o: N

ati

oN

al

iNst

rum

eNts

02.2013 i ndus t r i a l e the rne t book

IEB74.indd 11 1/25/2013 11:27:16 AM

Toolkits are targeting the needs of the developer. With a SmartGrid rear-closer appli-cation, for example, that monitors electric voltage and current to trip a vacuum breaker, it is also communicating with the grid using 61850 to provide power analysis data at the same time. The customer needs to make a custom device that has a certain protocol and data classes. But the bigger challenge is bringing all of the pieces together.

The value of an integrated platform such as LabVIEW is that can provide the control and measurement capabilities plus analysis, networking and visualization tools to build this type of systems. Developing these types of systems from scratch is becoming more and more difficult and impractical for most companies.

“Smart grid ready” invertersDesigning a smart grid ready inverter means incorporating features such as utility network protocol communication and local power analysis to make an inverter that is “grid aware”. This new type of device acts as an active contributor to grid stability and help ensure stable, reliable operation and interop-erability, particularly in power networks with a high penetration of inverters. These smart grid ready inverters act as distributed energy resources (DERs) with features and capabilities

that enable the inverter to be an asset that enables the utility to improve grid operation.

The hybrid architecture of the General Purpose Inverter Controller (GPIC) from National Instruments (NI sbRIO) facilitates the design of grid aware, smart grid ready inverters since it contains both a Xilinx Spartan-6 field programmable gate array (FPGA) and VxWorks/PowerPC real-time processor.

Typically the FPGA is completely focused on the inverter control, fast monitoring protection and acquisition/resampling of the grid measurements. This leaves some clock cycles free on the Real-Time processor for supervisory control, electrical power analysis, data logging, and communication with the utility network and/or cloud based monitoring service for end-user monitoring.

In most applications, all of the fast, time-critical inverter control and interlock protection loops are executed in the FPGA. The FPGA also performs resampling of the 50 or 60Hz grid voltage and current waveforms. This is a digital signal processing (DSP) intensive operation which realigns the power signals so they are synchronous to the line frequency with a constant number of samples per cycle (typically 192). For example, in 50Hz systems, IEC 61000-4-30:2008 (methods for measurement and interpretation of results for power quality parameters) requires that the

frequency of signaling voltage must be at most 3,000Hz.

“By offloading the compute-intensive resampling operation to the FPGA, more processing cycles are available on the real-time processor for analysis and supervi-sory control,” said MacCleery. “A feature of the NI RIO FPGA driver software, called direct memory access (DMA), enables the FPGA to stream the resampled waveforms into the memory space of the real-time processor with minimal processor overhead.”

Usually a smart grid aware inverter will perform only the most important power quality measurements necessary to assess the inverter performance and/or local grid conditions. This may include some power quality parameters such as power frequency, magnitude of the supply voltage, flicker, supply voltage dips and swells, voltage interruptions, transient voltages, supply voltage unbalance, voltage harmonics and interharmonics, mains signaling on the supply voltage and rapid voltage changes.

The goal is to enable the inverter to be grid aware and act as a smart grid analyzer and while producing power for the grid and responding to the needs of the utility. Pending suitable modifications to grid interconnection standards, this may include features such as:• The ability to respond to disconnect or real

12

Appl

icatio

ns

i n dus t r i a l e the rne t book 02 .2013

Developing embedded systems for the smart grid using IEC 61850 is expanding the ability of devices to combine advanced communications with measurement and control.

IEB74.indd 12 1/25/2013 11:27:16 AM

power curtailment commands from the utility.• The ability to provide active voltage regulation Volt-VAR support implemented automatically or based on scheduling commands from the utility. • The ability to ride through faults and stay online during sag, swell and grid frequency disturbances.

A leader in this area is the Electric Power Research Institute. For more information, see their web page Demonstration of Inverters with Smart-Grid Functionality (www.epri.com).

Embedded system designA challenge to developing embedded smart grid systems is an ability for designers to take a very comprehen-sive view of the problem.

The process starts with simulation tools that enable the system designer to look at the coupled interaction between the control software that runs on the inverter and the analog circuitry of the switching power system such as the power transistors, capacitors, inductors, transmission lines and so on. The goal is for the designer to be able to see the interaction between the embedded system they are designing and the rest of the power grid on day one when they start writing the control software.

Because any technology that is grid-tied

needs to be out in the field for a very long time, FPGAs provide a way to rewire the hardware down at the silicon level. That’s a

competitive advantage for companies when customers want to know how their systems are going to keep up with the new communi-cation protocols and control algorithms and take advantage of updates to comply with new standards that get passed a year from now.

So in terms of providing an improved tool chain, the FPGA is really being designed as the heart of new developments for smart grid and clean renewable energy systems because re-configurability is such a big advantage. But another reason they are increasingly popular is

that they are actually now hybrid devices with embedded DSPs inside. The Spartan-6 LX45 FPGA from Xilinx, for example, offers 58 DSP

cores inside the device, so rather than buying a multicore DSP, designers can use an FPGA with an array of DSPs inside that are distributed throughout the fabric itself. Particularly for switching power applications like grid-tied power converters, that combina-tion of programmable logic and DSP computing is appealing.

For specific functions such as pulse-width modulation (PWM) algorithms, the combination of precisely controlled timing logic with fast digital signal processing provides an ability to reduce the EMI noise coming out of the inverter, and simultane-

ously extend the lifetime of components that are the common failure modes such as the power transistors by reducing the temperature fluctuations and the capacitors by reducing current ripple.

Al Presher is a U.S. industry commentator and editor with the Industrial Ethernet Book.

Applications

Heart of the digital world

Deutsche Messe AG • Messegelände • Hannover, Germany • Tel. +49 511 89-0 • info@messe.de

Be where the digital future is revealed: at CeBIT 2013! Plug into the hottest trends, topics and innovations the digital industry has to offer

Get the inside track – straight from experts from around the globe

See all the right solution providers thanks to targeted matchmaking

Reserve your admission ticket today: www.cebit.de/en/ticket

DE_EN_CeBIT_210_140_3330.indd 1 15.01.13 12:55

Phot

o: N

ati

oN

al

iNst

rum

eNts

Single-board general-purpose inverter controllers that support IEC 61850 Ethernet communications provide an off-the-shelf option for inverter control applications.

Circle 71

FOR MORE INFORMATION CIRCLE 01

IEB74.indd 13 1/25/2013 11:27:24 AM