The IEC 61850-5 transfer time is the time a value takes from the logic of the IED that produces it to the logic of the IED that uses it, including the communication processing in both devices and the trip through the network. Edition 2 classifies it from TT0 (over 1 s) to TT6 (3 ms); GOOSE trips and blocking signals require TT6. In the field the whole chain is measured, not each link separately.
What transfer time is
IEC 61850-5 defines transfer time as the time it takes a value to travel from the processing logic of one device to the processing logic of another. It is neither the network time nor the relay time alone: it is application-to-application time.
It breaks down into three links:
- t_a: communication processing in the publishing IED, from the moment the function hands over the value until the frame leaves the port.
- t_b: the trip through the network, including switches, their queues and the links.
- t_c: communication processing in the receiving IED, from the moment the frame arrives until the logic has the value.
The transfer time is t = t_a + t_b + t_c.
Transfer time classes
In edition 2 of IEC 61850-5, the requirements are expressed as classes:
| Class | Transfer time | Typical use |
|---|---|---|
| TT0 | Over 1000 ms | Files, records, configuration data |
| TT1 | 1000 ms | Events and alarms |
| TT2 | 500 ms | Operator commands |
| TT3 | 100 ms | Slow automatic interactions |
| TT4 | 20 ms | Fast automatic interactions |
| TT5 | 10 ms | Releases and status changes |
| TT6 | 3 ms | Trips and blocking |
In edition 1, the same trip requirement corresponded to message type 1A with performance classes P2 or P3, at 3 ms, while P1 allowed around 10 ms.
Why a GOOSE trip needs the most demanding class
When a protection function in one IED sends a trip to another over GOOSE, that time adds to the total fault clearing time. If a breaker failure trip or a bus blocking signal arrives late, the fault lasts longer than the study assumed. A hardwired trip takes as long as the relay contact takes; so that communication does not make things worse, the standard asks for the message to arrive within 3 ms.
For functions that still use contacts, the physical output and breaker times add up just the same. What changes with GOOSE is that the copper wire is replaced by the network.
What each link adds
In the publishing IED (t_a). It depends on the relay architecture: how often it runs its logic, how it hands the value to the communication processor and how long it takes to encode the frame. The manufacturer declares it.
In the network (t_b). Each switch receives the whole frame before forwarding it, and every hop adds that time; at 100 Mbit/s a 1500-byte frame takes about 120 µs to be received. If the trip frame waits in a queue behind low-priority traffic, the delay grows. That is why IEEE 802.1Q priority, fast links between switches and few hops are used. If the network reconfigures with RSTP at that very moment, the message can be lost and arrive with the next retransmission.
In the receiving IED (t_c). Decoding the frame, checking that it belongs to a configured subscription and delivering the value to the logic. The manufacturer declares this too.
- 01Publishing IED logicThe protection function decides to trip
- 02t_aCommunication processing and frame output
- 03t_bNetwork switches, queues and links
- 04t_cReception, decoding and delivery to the receiving IED logic
- 05Receiving IED logicThe value is available to trip
How it is verified, in general terms
In the field, t_a, t_b and t_c cannot be measured separately without special equipment. The whole chain is measured and compared with the required class. There are two general approaches:
- Round trip (ping-pong). A test set publishes a GOOSE message; the IED receives it and, through its logic, publishes a reply. The test set measures the round-trip time. That time includes two transfers plus the IED logic time, so it has to be interpreted with judgment.
- Time-stamped capture. Frames are captured at points of the network with synchronized time stamps, and the departure instant is compared with the arrival instant, or with a change of a physical output.
In both cases the test is repeated with the network loaded with real traffic and with each path of the redundancy disconnected. IEC 61850-10 conformance testing covers IED performance; the complete system is checked as part of commissioning.
Transfer time and network design
Transfer time is taken care of from the design stage. For functions that require TT6, the network is sized with few hops, 1 Gbit/s links between switches when there are many bays, VLANs that separate the traffic and redundancy with zero recovery time, such as PRP or HSR. The time budget is split between the IEDs, declared by the manufacturer, and the network, calculated by the designer.
Common mistakes
- Assuming the IED class is enough, without counting the real network.
- GOOSE trips without priority in the same queue as engineering or video traffic.
- Measuring with an empty network instead of with operating traffic.
- Confusing transfer time with clearing time, which also includes detection, the output and the breaker.
- Not testing with degraded redundancy: one PRP LAN disconnected or a ring opened.
Engineering and configuration with Tiza Group
Checking that a GOOSE trip arrives on time starts in the design. Our team does the communications engineering of IEC 61850 substations: network architecture, priorities, VLANs, redundancy and the time budget of each function, for medium, high and extra-high voltage, in substations and industry. Publication, subscription and relay logic are configured with our remote relay configuration, and end-to-end tests are done in Colombia, or elsewhere on request. Contact us with your project architecture.
Related guides
- IEC 61850 GOOSE
- GOOSE not arriving or arriving late
- VLANs and IEEE 802.1Q priority
- PRP and HSR (IEC 62439-3)
- Station bus
- IEC 61850 digital substation
- IEC 61850 overview
- Substation network topologies: star, ring and more
- Sampled Values and GOOSE bandwidth: how to estimate it
- Substation communications by voltage level
- GOOSE between substations: TR 61850-90-1 and R-GOOSE
References
- IEC 61850-5, Communication requirements for functions and device models: definition and classes of transfer time.
- IEC 61850-10: conformance testing.
- IEC TR 61850-90-4:2020, Network engineering guidelines.
- IEC 61850-8-1: GOOSE mapping to Ethernet.
- IEEE 802.1Q: VLANs and priority.
Frequently asked questions
Why is a GOOSE message expected to arrive within 3 ms?
Because IEC 61850-5 assigns trips and blocking signals the most demanding transfer time class, TT6, which is 3 ms. That time runs from the logic of the publishing IED to the logic of the receiving one. The relay's physical output and the breaker opening time are added afterwards.
Does the transfer time include the protection operating time?
No. It starts when the protection function has already decided and hands the value to the communication stack, and it ends when the receiver's logic has it. Fault detection time, the physical output and the breaker are counted separately, in the total fault clearing time.
How do I know whether a relay meets class TT6?
The manufacturer declares it, and some laboratories test it as part of GOOSE performance conformance testing. On the project the real system is checked, with the network loaded and with redundancy in different states, because the switches and their queues count too.