Dhanabal Mani - Applications development engineer, Jason Buneo - Applications development manager, Rene Aguilar - Senior application engineer
IEC 61850 enabled substations are becoming increasingly common. Despite the many benefits that result from implementing the IEC 61850 standard, significant challenges remain. This article explores some of those challenges by investigating and discussing the lessons learned from misoperation of 400 kV line reactor protection in a breaker-and a-half scheme.
In 2005, the first edition of the IEC 61850 standard was released and was adopted by utilities across the world as a viable solution for substation protection. The standard first came about as separate initiatives. These included UCA 2.0 from the Electric Power Research Institute (EPRI), as well as IEC 60870-5-101, IEC 60870-5-103, and IEC 60870-5-104 from the International Electrotechnical Commission. In 1997, the initiatives were combined as part of an internationally agreed process to become a full standard.
One of the aims of that process was to allow communication in substations between intelligent electronic devices (IEDs) originating from different manufacturers. Prior to the release of the standard, proprietary network protocols hampered communications between competing IEDs, with the result that such communication was reduced to either direct wiring via input/output contacts or the use of low-speed serial connections.
In contrast, the implementation of the IEC 61850 standard now allows substation devices to be part of a local area network (LAN) based substation, with high-speed peer-to-peer communications. The advantage of the LAN based approach is that simple or complex control schemes can be easily implemented without increasing the complexity of the physical wiring. For communications, IEDs can be connected to the LAN by a single cable, and this can either be an optical fibre or a copper Ethernet cable. Dual redundancy connections are also possible.
In a LAN based protection scheme using the IEC 61850 standard, the primary form of communication between protective devices is the Generic Object Oriented Substation Event, or GOOSE message. GOOSE messages contain parameters that form the self-description of the message and include such items as GOOSE control block name, Application ID, MAC address, Dataset name and items, GOOSE ID, Priority Tag, Virtual LAN (VLAN) Tag, VLAN ID, etc. A GOOSE message can be sent from one IED and be selectively received from several other IEDs. Some of the most common applications of GOOSE messages are monitoring breaker open/ close status and sending trip commands.
The GOOSE messages pass from IEDs to an Ethernet switch and then to other IEDs or a station controller. Depending on how complex the protection scheme is, the GOOSE messages can pass through several switches before the destination IED is reached. These switches are critical components in the substation network as they keep the data flowing between the IEDs. However, there are some key differences between switches and IEDs that protection engineers need to be aware of when designing and testing their protection schemes. In particular, even though IEDs can be programmed to adhere to the IEC 61850 standard, Ethernet switches follow different IEEE standards that may be in conflict with certain GOOSE parameters.
Substation automation and protection
In the first quarter of 2006, one of the first substations in India that used the IEC 61850 standard was commissioned. The substation has two 400 kV buses with more than 12 breaker and-a-half schemes. One of the breaker-and-a-half schemes has an oil-immersed shunt reactor, as shown in Figure 1. The reactor is provided with temperature monitoring elements for overload protection, as well as with sudden pressure detection and release valves. As shown in Figure 2, the substation automation protection scheme is broken into two levels in accordance with the IEC 61850 standard: station level and bay level.
The station level design consists of a communication network composed of industrial-grade Ethernet switches and controllers that manage the bay-to-bay and bay-to-station communications. The Ethernet switches are configured in a redundant ring network that, in the event of switch failures, uses the Rapid Spanning Tree Protocol in line with the IEEE 802.1W standard.
The bay level consists of the circuit breaker , associated disconnector switches, earth switches, instrument transformers and protective IEDs. The IEDs perform all of protective functions, control logic, and status monitoring. GOOSE messaging is used for the communications between the IEDs, and the protective functions are modelled as logical nodes.
Commissioning a substation with automation that includes IEDs communicating via GOOSE messaging proved to be a challenging task. One of the most challenging aspects was adjusting to the concept of the protective logic being implemented with GOOSE messages transmitted via fibre optic and Ethernet cables rather than being implemented with the external inputs and outputs on the relays. The IED manufacturers’ configuration software was used to configure the protective functions as well as to map the GOOSE messages that carry the virtual inputs and outputs over the communication network.
The IED configuration was exported to the IED configuration description (ICD) file. The ICD files from individual IEDs were then integrated to work together in a substation configuration language (SCL) file. Each of the IEDs has its own configuration, but shares a common SCL file. The engineering process is shown in Figure 3.
During the commissioning of the substation, the correct operation of the protective functions and blocking capabilities of the logic were verified. All functions were found to operate as expected, and the substation was brought online.
After commissioning, it was found that an additional circuit breaker needed to be added as a logical node in one of the ICD files protecting the shunt reactor. The new ICD file was uploaded to the IED. Shortly after, there was an incident
with the shunt reactor. During the hottest part of the day and at peak load, a fault occurred that triggered the sudden pressure alarm. The pressure relief valve operated, spraying oil over a wide area, and a pressure relief valve trip signal from the reactor IED was sent to the bay control IED. However, the bay control failed to send a GOOSE message to the breakers responsible for isolating the line, and it was necessary to manually open the breakers to clear the fault.
An investigation was conducted to determine why the breakers did not operate properly to clear the fault. It was found that the change made to the bay control IED protecting the reactor meant that this did not have the same SCL file as the other IEDs on the system. Because of this discrepancy, all of the trip signals from
this IED were ignored. Since the SCL file from the reactor IED did not match the file in the other IEDs, as far as the rest of the network was concerned, GOOSE messages sent from the reactor IED should be ignored.
Figure 4 shows improper procedure in updating an IEC 61850 system. The green box indicates where the change to the logical node should have taken place in the engineering process. The red box is where the change actually took place, resulting in the reactor IED not containing the same SCL file as the other IEDs on the network.
Corrective action was taken and all of the IEDs were updated to the same SCL configuration. The logic was retested and the substation was brought back online.
As the acceptance of the IEC 61850 standard becomes more and more widespread, it is important to be mindful of previous oversights so that potential catastrophic failures can be avoided. Strict procedures in the design and commissioning stages can greatly help in mitigating the risk associated with operating complex systems.
Reference . First Experiences with Design and Engineering of IEC 61850 Based Substation Automation Systems in India, Rajiv Krishnan, Bapuji Palki, CEPSI 2006
Conference, Mumbai, India, November 6-10, 2006.
* Icons used in Figure 2 are made by Freepik from www.flaticon.com and licensed by CC BY 3.0