IEC 61850 GOOSE Messaging

GOOSE operates on a publisher-subscriber model, fundamentally different from client-server SCADA protocols. A single Intelligent Electronic Device (IED) publishes a dataset—such as a trip signal or breaker status—onto the substation LAN. Multiple subscribing IEDs simultaneously receive and process this data. This eliminates the need for point-to-point copper wiring between every relay and bay controller, drastically reducing design complexity and commissioning time. The multicast addressing ensures that critical state changes reach all logical nodes subscribed to the specific GOOSE control block within the required transfer time.

To guarantee delivery without TCP overhead, GOOSE employs a rapid retransmission strategy directly over Ethernet Layer 2. Upon a state change event, the IED publishes the GOOSE message immediately, then retransmits it at exponentially increasing intervals:

• T1 (First Retransmission): Typically 2-4 ms after the initial event.
• T2, T3... Tn: Intervals double until reaching a steady-state heartbeat period (e.g., 5-10 seconds). This mechanism ensures that even if the initial frame is lost due to a network microburst, subsequent frames arrive within the stringent 3 ms transfer time required for tripping applications, matching the performance of hardwired schemes.

GOOSE frames are tagged with IEEE 802.1Q VLAN headers to enforce traffic segregation and quality of service on the substation bus. Key parameters include:

• Priority Code Point (PCP): Set to 4 or higher to ensure GOOSE traffic is placed in the highest priority egress queue on substation switches, ahead of SCADA or file transfer traffic.
• VLAN ID (VID): Isolates protection traffic into a dedicated virtual network, preventing broadcast storms from non-critical devices from impacting relay-to-relay communication. This strict prioritization is essential for meeting the deterministic latency requirements of IEC 61850-5 performance classes during fault conditions when network congestion peaks.

Each GOOSE message carries a structured dataset—a predefined collection of data attributes like circuit breaker position, protection start, and quality bits. Integrity is maintained through strict numbering:

• StNum (State Number): Increments each time a data attribute within the dataset changes value. A subscriber detects a state change by comparing the received StNum to the last stored value.
• SqNum (Sequence Number): Increments with every retransmission of the same state, resetting to 1 when StNum changes. This allows subscribers to detect missing retransmission frames.
• TimeAllowedtoLive: A holdover timer that informs the subscriber how long to wait for the next expected retransmission before declaring a communication loss.

The logical connections between publishing and subscribing IEDs are defined using the Substation Configuration Language (SCL) in XML format. The System Configuration Description (SCD) file binds GOOSE publishers to subscribers, replacing physical wiring diagrams with software-defined signal mapping. This enables:

• Late Binding: Protection engineers can modify trip logic and interlocking schemes without pulling new cables.
• Automated Testing: GOOSE subscriptions can be simulated and tested using software tools before commissioning.
• Full Traceability: Every signal path is documented in the SCD file, eliminating the ambiguity of undocumented field wiring modifications.

While both are IEC 61850 Layer 2 protocols, they serve distinct purposes:

• GOOSE: Transmits binary status and analog setpoints (e.g., trip, close, interlock) on an event-driven basis. Data is published only when a change occurs, with a background heartbeat.
• Sampled Values (SV): Streams raw digitized current and voltage waveforms from merging units at high rates (80 samples/cycle for protection, 256 for metering). SV is a continuous, high-bandwidth stream. A modern process bus architecture uses SV to digitize the instrument transformer outputs and GOOSE to transmit the resulting trip commands, fully eliminating copper from the switchyard to the relay room.