Implementing 61850 7-420 to Enable PV Inverter Interoperability
Written by Kumaraguru Prabakar and Deepthi Vaidhynathan
Interoperability is the ability of two or more intelligent electronic devices (IEDs)—from the same vendor or from different vendors—to exchange information and to use that information for the correct execution of specified functions . Device interoperability prevents additional spending on data concentration or protocol translation. Interoperability is essential for enabling communication between different assets in the power system at a reasonable cost. The interoperability requirement in IEDs started as a standard requirement for substation automation and is now a requirement for most standard equipment used in the power system space. Multiple protocols are available in the industry to enable interoperability in photovoltaic (PV) inverters, including International Electrotechnical Commission (IEC) 61850 , Distributed Network Protocol 3 (DNP3) , SunSpec Modbus , and OpenFMB .
Interoperability protocols started as substation automation protocols supporting communication between the equipment in the substation. In the past decade, protocols enabling interoperability started to cover IEDs in the field beyond those in the substation. Increased penetrations of distributed energy resources (DERs) and the advanced grid support functions provided by DERs has increased the need to add communication functionalities. These communication functionalities in DERs have pushed the need for interoperability that can enable DERs to communicate with the rest of the IEDs and management systems. For example, as more DERs are installed in the system, IEDs and distribution management systems sensing and operating the system should be to communicate with DERs to sense and control the state of the DERs.
In our research work, we developed and implemented an interoperability code for PV inverters using IEC 61850. IEC 61850 contains more than 14 parts that cover decades of development of the protocol, as shown in Figure 1. These different parts are critical to the successful implementation of IEC 61850. Under IEC 61850, part 7-420  covers the logical nodes for DERs. To implement IEC 61850 for a PV inverter, the domain-specific logical node and data object classes defined under IEC 61850 7-420 need to be used. We used IEC 61850 7-420 and other necessary parts as prescribed by IEC 61850 and as shown in Fig. 1 to implement the interoperability code for a PV inverter. Our team used a simple BeagleBone board microcontroller and C programming language to implement the interoperability code. This developed code was evaluated in a controlled lab environment by exchanging different messages between a test IEC 61850 client and the developed IEC 61850 server.
Fig. 1. Using different parts under IEC 61850 for implementation
IEC 61850 supports client-server architecture and publisher-subscriber architecture. Client-server architecture can be used for client-initiated “request-response” architecture and event-based reporting to a client. The publisher-subscriber architecture enables high-speed peer-to-peer communication based on events (Generic Object-Oriented Substation Event) or peer-to-peer communication on regular time intervals (sampled values) that enables fast communication between assets. Figure 2 shows the different types of communication architecture available under IEC 61850.
Fig. 2. Different communication architectures available under IEC 61850 for data exchange between assets: (a) client-server architecture and (b) publisher-subscriber architecture
The first step in enabling and implementing IEC 61850 is to build the IED Capability Description (ICD) files in extensible markup language (XML) file format that contain the information about the IED that is needed by the IEC 61850 device to communicate. This XML file contains information on inverter grid support functions, such as volt-volt ampere reactive (VAR), volt-watt, and others. Figure 3 shows an MMXU logical node, which is commonly used to define the measurements in the IEDs. In the example shown, the XML file defines the frequency, line voltage, and current measured under the MMXU logical node. To develop this XML file, IEC 61850 part 7-420 was used to define the information models and logical nodes to be used in the exchange of information between DERs and an IEC 61850 client. We also programmed logical nodes—including ZINV (inverter characteristics), XCBR (AC circuit breakers), and MMXU—in the XML file. This developed ICD is available in GitHub under an open-source software license from the National Renewable Energy Laboratory.
Fig. 3. XML section defining the MMXU logical node for the PV inverter
Figure 4. Different layers programmed in the ICD file detailing the information available in the PV inverter
Figure 4 shows the implementation of the MMXU in the ICD file and how the different layers of the information being communicated are programmed in the ICD file. We programmed three logical devices in the ICD file: a control logical device, a protection logical device, and a measurement logical device. Under the control logical device, we programmed volt-VAR, volt-watt, and frequency-watt. Under the protection logical device, we programmed voltage ride-through and frequency ride-through functions. And under the measurement logical device, we exchanged measurements such as voltage, current, frequency, and power.
Figure 5. IEC client-server test setup to evaluate the interoperability code
Figure 5 shows the client-server setup used to evaluate the interoperability code. To show the communication exchange between the client and the server, we used Wireshark to capture the communication exchange between the devices. Figure 6 shows the request and response from a client through Manufacturing Message Specification (MMS) MMS protocol.
Figure 6. Wireshark screen capture of MMS exchange between client and server: (a) client read request and (b) server response to the read request
Enabling interoperability in PV Inverters is a critical step in sensing and controlling of the state of DERs in the distribution system. In the project, we developed and implemented IEC 61850-based communication for PV inverters. We developed ICD files for a PV inverter supporting the exchange of advanced grid support function curves between the client and server. Advanced grid support functions and communication requirements are changing dynamically as the requirements of distribution system operators are changing to meet resilience requirements through appropriate sensing and control of PV inverters. The open-source ICD file developed in this project can be leveraged to enable interoperability for a PV inverter using a simple microcontroller and can be improved upon based on the needs of the user.
- IEC 61850-1:2013, “Communication networks and systems for power utility automation – Part 1: Introduction and overview.”
- IEC 61850, “Communication networks and systems for power utility automation.”
- “IEEE Standard for Electric Power Systems Communications-Distributed Network Protocol (DNP3),” IEEE P1815/D11, April 2012 - Approved Draft, pp. 1–866, Oct. 2012.
- “Specifications - SunSpec Alliance.” https://sunspec.org/specifications/ (accessed Jun. 01, 2021).
- M. Bartock and R. Herold, “OpenFMB Proof of Concept Implementation Research,” National Institute of Standard and Technology, 2020.
- IEC 61850-7-420, Communication networks and systems for power utility automation – Part 7-420: Basic communication structure – Distributed energy resources logical nodes.
This article edited by Gabriel Ordoñez