GOOSE protocol

The GOOSE protocol, described in the International Electrotechnical Commission (IEC) 61850-8-1 clause, is one of the most widely known protocols provided by the IEC 61850 standard. Literally, the expansion of the GOOSE (Generic Object-Oriented Substation Event) acronym can be translated as "general object-oriented substation event". However, in practice, one should not place much importance on the original name, as it provides no insight into the protocol itself. It is much more convenient to understand the GOOSE protocol as a service designed for digital signal exchange between Relay Protection and Automation (RPA) devices. GOOSE Message Formation

In the previous publication [1], we examined the device information model and data organization, and concluded with the formation of datasets—Datasets. Datasets are used to group data that will be transmitted by a device using the GOOSE message mechanism. Subsequently, in the GOOSE transmission control block, a reference to the created dataset is specified; in this case, the device knows exactly which data to send (see Fig. 1). It should be noted that a single GOOSE message can transmit both a single value (e.g., a Maximum Current Protection (MCP) trip signal) and multiple values simultaneously (e.g., an MCP trip signal and an MCP operation signal, etc.). The receiving device, in turn, can extract only the data it requires from the packet. The transmitted GOOSE message packet contains all current values of the data attributes included in the dataset. Upon a change in any attribute value, the device immediately initiates the transmission of a new GOOSE message with updated data (see Fig. 2).

By design, a GOOSE (Generic Object Oriented Substation Event) message is intended to replace the transmission of discrete signals via auxiliary DC (direct current) networks. Let us consider the requirements imposed on the data transmission protocol in this context. Digital Communications Replacing Analog To develop an alternative to signal transmission circuits between relay protection and automation (RPA) devices, the properties of information transmitted between RPA devices via discrete signals were analyzed:

• Small volume of information — essentially "true" and "false" values (or logical "zero" and "one") are transmitted between terminals;
• High transmission speed required — a large portion of the discrete signals transmitted between RPA devices directly or indirectly affects the speed of abnormal mode clearance; therefore, signal transmission must be carried out with minimal latency;
• High message delivery probability required — to implement critical functions, such as issuing a circuit breaker trip command from an RPA device or exchanging signals between RPA devices when performing distributed functions, guaranteed message delivery must be ensured both during normal operation of the digital data transmission network and in the event of its short-term failures;
• Ability to transmit messages to multiple recipients simultaneously — when implementing certain distributed RPA functions, data transmission from one device to several others is required;
• Data transmission channel integrity monitoring is necessary — the presence of a communication channel status diagnostic function allows for an increased availability factor during signal transmission, thereby increasing the reliability of the function performed by transmitting the specified message.

These requirements led to the development of the GOOSE message mechanism, which meets all the stated criteria.

Ensuring Data Transmission Speed

In analog signal transmission circuits, the primary delay in signal transmission is caused by the response time of the device's discrete output and the contact bounce filtering time at the receiving device's discrete input. Compared to this, the signal propagation time through the conductor is negligible. Similarly, in digital data transmission networks, the main delay is caused not so much by signal transmission through the physical medium as by its processing within the device. In data network theory, it is customary to segment data transmission services according to the levels of the OSI (Open Systems Interconnection) model [2], typically descending from the "Application" layer (the application presentation layer) to the "Physical" layer (the physical interaction layer of devices). In its classical representation, the OSI model has only seven layers: physical, data link, network, transport, session, presentation, and application. However, implemented protocols may not include all of the specified layers; that is, some layers may be bypassed. Table 1. Standard seven-layer OSI model.

OSI Model
Data Type	Layer	Functions
Data	7. Application	Access to network services
	6. Presentation	Data representation and encryption
	5. Session	Session management
Segments	4. Transport	End-to-end communication and reliability
Packets	3. Network	Routing and logical addressing
Frames	2. Data Link	Physical addressing
Bits	1. Physical	Handling of transmission media, signals, and binary data

The mechanism of the OSI (Open Systems Interconnection) model operation can be clearly illustrated by the example of web page data transmission over the Internet on a personal computer. The transmission of web page content over the Internet is carried out using the HTTP (Hypertext Transfer Protocol), which is an application layer protocol. HTTP data transmission is typically performed by the TCP (Transmission Control Protocol) transport protocol. TCP protocol segments are encapsulated into network protocol packets, which in this case is IP (Internet Protocol). TCP protocol packets form Ethernet data link layer frames, which, depending on the network interface, can be transmitted using different physical layers. Thus, the data of a web page being viewed on the Internet undergoes at least four transformation layers during the formation of a bit sequence at the physical layer, and subsequently the same number of reverse transformation steps. Such a number of transformations leads to delays both during the formation of the bit sequence for transmission and during the reverse transformation to retrieve the transmitted data. Accordingly, to reduce latency, the number of transformations should be minimized. This is precisely why GOOSE (Generic Object Oriented Substation Event) data (application layer) is assigned directly to the Ethernet data link layer, bypassing the other layers. In general, IEC 61850-8-1 provides for two communication profiles that describe all data transfer protocols provided by the standard:

• "MMS" profile;
• "Non-MMS" profile (i.e., non-MMS).

Accordingly, data transfer services can be implemented using one of these specified profiles. The GOOSE (Generic Object Oriented Substation Event) protocol (as well as the Sampled Values protocol) belongs specifically to the second profile. Using a "shortened" stack with a minimum number of transformations is an important, but not the only, way to accelerate data transmission. The use of data prioritization mechanisms also contributes to accelerating data transmission via the GOOSE protocol. Specifically, for the GOOSE protocol, a separate Ethernet frame identifier is used — the Ethertype — which has a deliberately higher priority compared to the rest of the traffic, such as that transmitted using the Internet Protocol (IP) network layer. In addition to the mechanisms discussed, a GOOSE message Ethernet frame can also be equipped with IEEE 802.1Q protocol priority tags, as well as ISO/IEC 8802-3 protocol Virtual Local Area Network (VLAN) tags. Such tags allow for increasing the priority of frames during processing by network switches. These priority enhancement mechanisms will be discussed in more detail in subsequent publications. The use of all considered methods allows for significantly increasing the priority of data transmitted via the GOOSE protocol compared to other data transmitted over the same network using different protocols, thereby minimizing delays both during data processing within source and receiver devices and during processing by network switches.

Sending information to multiple recipients

Physical addresses of network devices — MAC (Media Access Control) addresses — are used for addressing frames at the data link layer. In this regard, Ethernet allows for so-called multicast. In such a case, a multicast address is specified in the destination MAC address field. A specific range of addresses is used for multicast transmissions via the GOOSE protocol (see Fig. 3).

Byte	Hex	Description
1	01	Multicast identifier
2–3	0C CD	Address range reserved for IEC TC 57
4	01	GOOSE protocol identifier
5	xx	Message ID in range 00–01
6	xx	Message ID in range 00–FF

Messages having the value "01" in the first octet of the address are sent to all physical interfaces in the network; therefore, multicast transmission effectively has no fixed recipients, and its MAC address serves more as an identifier for the multicast itself rather than directly pointing to its recipients. Thus, the MAC address of a GOOSE (Generic Object Oriented Substation Event) message can be used, for example, when organizing message filtering on network switches (MAC filtering), and this address can also serve as an identifier to which receiving devices can be configured. Thus, the transmission of GOOSE messages can be compared to radio broadcasting: the message is broadcast to all devices in the network, but to receive and subsequently process the message, the receiving device must be configured to receive that specific message (see Fig. 4).

Guaranteed Message Delivery and Channel Status Monitoring

The transmission of messages to multiple recipients in Multicast mode, as well as the requirements for high data transfer rates, do not allow for the implementation of delivery acknowledgments from receivers during GOOSE message transmission. The procedure of sending data, generating an acknowledgment by the receiving device, its reception and processing by the sending device, and subsequent retransmission in case of a failed attempt would take too much time, which could lead to excessively large delays in the transmission of critical signals. Instead, a special mechanism has been implemented for GOOSE messages to ensure a high probability of data delivery. Firstly, in the absence of changes in the transmitted data attributes, GOOSE message packets are transmitted cyclically through a user-defined interval (see Fig. 5 a). The cyclic transmission of GOOSE messages allows for continuous diagnostics of the communication network. A device configured to receive a message expects its arrival at specified time intervals. If a message does not arrive within the waiting period, the receiving device can generate a fault signal in the communication network, thereby notifying the dispatcher of any occurring malfunctions. Secondly, when one of the attributes of the transmitted dataset changes, regardless of how much time has passed since the previous message was sent, a new packet containing the updated data is formed. After this, the transmission of this packet is repeated several times with minimal time delay (see Fig. 5 b), and then the interval between messages (in the absence of changes in the transmitted data) increases back to the maximum value.

Thirdly, the Generic Object Oriented Substation Event (GOOSE) message packet provides several counter fields that can also be used to monitor communication channel integrity. Such counters include, for example, the cyclic sequence number (sqNum), whose value changes from 0 to 4,294,967,295 or until the transmitted data changes. Every time the data transmitted in a GOOSE message changes, the sqNum counter will reset, and another counter—the status number (stNum), which also changes cyclically in the range from 0 to 4,294,967,295—will increase by 1. Thus, if several packets are lost during transmission, this loss can be tracked using these two specified counters. Finally, fourthly, it is also important to note that a GOOSE transmission may contain, in addition to the discrete signal value itself, a quality attribute (quality flag) that identifies a specific hardware failure of the information source device, the presence of the information source device in test mode, and several other abnormal modes. Thus, before processing the received data according to the prescribed algorithms, the receiving device can perform a check of this quality attribute. This can prevent incorrect operation of information receiving devices (for example, false operations). It should be kept in mind that some of the built-in mechanisms for ensuring data transmission reliability can lead to negative effects if used incorrectly. For instance, if a maximum interval between messages is chosen to be too short, the network load increases, even though, from the perspective of communication channel availability, the effect of reducing the transmission interval will be extremely negligible. When data attributes change, transmitting packets with minimum time delays causes increased network load (an "information storm" mode), which theoretically can lead to data transmission delays. This mode is the most complex and should be taken as the design basis when engineering the communication network. However, it should be understood that peak load is very short-lived; according to experiments conducted in our laboratory for interoperability testing of devices operating under IEC 6 61850 standards at the Department of Relay Protection and Automation of Nuclear Power Plants (RP&A NPP) of the National Research University of Electric Power Engineering (NRUP EE), such loads are observed over a 10 ms interval.

Commissioning and Testing of Relay Protection and Automation (RPA) Systems Using GOOSE Communications

When constructing RPA systems based on the GOOSE protocol, their commissioning and testing procedures change. Now, the commissioning stage consists of organizing the Ethernet network of the power facility, which will include all RPA devices that require data exchange. To verify that the system is configured and connected in accordance with project requirements, it becomes possible to use a personal computer with specialized pre-installed software (Wireshark, GOOSE Monitor, etc.) or specialized test equipment supporting the GOOSE protocol (RETOM 61850, Omicron CMC). It is important to note that all checks can be performed without disrupting the pre-established connections between secondary equipment (RPA devices, switches, etc.), since data exchange occurs over the Ethernet network. In contrast, when exchanging discrete signals between RPA devices in the traditional manner (by applying voltage to the discrete input of the receiving device when the output contact of the transmitting device closes), it is often necessary to break connections between secondary equipment to include them in test set circuits for verifying the correctness of electrical connections and the transmission of relevant discrete signals. Conclusions The GOOSE protocol provides a whole complex of measures aimed at ensuring the necessary performance and reliability characteristics during the transmission of critical signals. The use of this protocol, combined with proper design and parameterization of the communication network and RPA devices, allows in several cases to abandon the use of copper circuits for signal transmission while providing the required level of reliability and speed. References

1. Anoshin A.O., Golovin A.V. IEC 61850 STANDARD. Device Information Model // Electrical Engineering News No. 5 (77).
2. Information and Computing Networks: a study guide Kapustin, V. E. Dementiev. — Ulyanovsk: UlGTU, 2011. — 141p.