IEC 61869-9 describes the variants of a merging unit on its nameplate with the notation F f S s I i U u: samples per second, ASDUs per message, currents and voltages per ASDU. F4800S2I8U0 is 4,800 samples per second, two ASDUs per message, 8 currents and no voltages. Table 902 sets the standardized rates and marks 4,800 with 2 ASDUs, 14,400 with 6 and 96,000 with 1 as preferred.
Short answer
A merging unit can publish many combinations of rate, quantities and frames. So that two manufacturers understand each other, IEC 61869-9, 2016 edition, limits the variants and describes them on the nameplate with a short notation. Reading it avoids buying a merging unit whose relays cannot subscribe.
The F f S s I i U u notation
Clause 6.903.2 presents it as a simple way to describe in readable text what a merging unit can do. The capital letters are separators, and the lowercase ones are the numbers:
| Field | Means |
|---|---|
| F f | Sampling rate of the digital output, in samples per second |
| S s | Number of ASDUs (samples) in each sampled values message |
| I i | Number of currents in each ASDU |
| U u | Number of voltages in each ASDU |
An ASDU is an application service data unit: it carries one sampling instant with all its quantities. A message can carry one or several.
The nameplate documents the capability of the equipment, that is, the range of variants it supports. The active configuration is specified separately, in the configuration file of the merging unit.
Three examples from the standard
- F4000S1I4U4: 4,000 samples per second, one ASDU per message, four currents and four voltages. The standard identifies it with the MSVCB01 stream of the 9-2LE guideline in 50 Hz systems.
- F12800S8I4U4: 12,800 samples per second, eight ASDUs per message, four currents and four voltages. It is the MSVCB02 stream of 9-2LE in 50 Hz systems.
- F4800S2I8U0: 4,800 samples per second, two ASDUs per message, eight currents and no voltages.
The 9-2LE guideline is a document of the UCA International Users Group on the digital interface of instrument transformers. IEC 61869-9 keeps several of its rates for compatibility.
With the third example you can do a calculation. With two ASDUs per message, the merging unit publishes 2,400 messages per second and each one carries 8 quantities per sample.
Standardized rates (Table 902)
| Sampling rate | ASDUs per frame | Frames per second | Standard’s remark |
|---|---|---|---|
| 4,000 | 1 | 4,000 | 50 Hz systems, compatible with 9-2LE |
| 4,800 | 1 | 4,800 | 60 Hz systems compatible with 9-2LE, or 50 Hz with 96 samples per cycle |
| 4,800 | 2 | 2,400 | Preferred for metering and protection, at any system frequency |
| 5,760 | 1 | 5,760 | 60 Hz systems with 96 samples per cycle |
| 12,800 | 8 | 1,600 | Deprecated, 50 Hz only |
| 14,400 | 6 | 2,400 | Preferred for power quality and for low-bandwidth DC control |
| 15,360 | 8 | 1,920 | Deprecated, 60 Hz only |
| 96,000 | 1 | 96,000 | Preferred for high-bandwidth DC control |
The rates of F4000S1I4U4, F4800S1I4U4, F12800S8I4U4 and F15360S8I4U4 match those of 9-2LE and are kept for compatibility. From now on, the standard gives preference to 4,800, 14,400 and 96,000 with 2, 6 and 1 ASDUs.
The rates are constant and are normally synchronized with an external time source. If the application needs to follow the system frequency, the subscriber does it, not the merging unit.
- What will you do with the samples?
- Protection and metering: 4,800 samples per second with 2 ASDUs (preferred); 4,000 or 4,800 with 1 ASDU if there are 9-2LE devices
- Power quality: 14,400 samples per second with 6 ASDUs
- DC control: Low or high bandwidth?
- Low bandwidth: 14,400 samples per second with 6 ASDUs
- High bandwidth: 96,000 samples per second with 1 ASDU
Mandatory configurations
An instrument or a SAMU that declares compliance with IEC 61869-9 must be configurable with one of the preferred rates of Table 902 and with at least one of these backward-compatible configurations:
- F4000S1I4U4
- F4800S1I4U4
- F5760S1I4U4
Variants with another number of currents and voltages may exist. The minimum is one quantity among currents and voltages.
How many quantities fit on the network
The standard limits the number of quantities a merging unit can publish on a 100 Mbit/s network:
- metering and protection in general: 24 at most;
- power quality: 8 at most;
- DC control: 24 at most.
The aim is to guarantee fair access to the network and prevent it from being blocked by Ethernet frames that are too long. For networks of 1 Gbit/s or more there are no specific limits. DC transformers may require point-to-point connection and Gigabit links.
The bandwidth that results from a variant on your specific network is another calculation. It is studied in the guide on sampled values and GOOSE bandwidth, and you can check it with our free GOOSE and SV addresses tool.
How to choose the variant for a project
- List the devices that will subscribe: relays, recorders, meters.
- Check which variants each subscriber supports. Some accept only those of 9-2LE.
- Pick the preferred rate of Table 902 if all of them support it.
- Count the quantities per merging unit and compare them with the network limit.
- Write the active variant in the specification and in the configuration file.
What not to do
- Do not pick the highest rate without counting the quantities. Each variant takes up bandwidth on the network.
- Do not mix variants in one subscriber without checking that it accepts them.
- Do not assume 12,800 or 15,360 are still recommended. The standard marks them deprecated.
- Do not confuse the nameplate notation with the active configuration.
Related topics
The sampling rate relates to synchronization, delay and conformance classes. Network load is explained in bandwidth and the message format in 9-2. The overview is in merging unit.
When to call us
At Tiza Group we define the sampled values variant with you, check that each relay subscribes to it and verify the process bus network. We offer IEC 61850 engineering and communications engineering. We quote the work according to scope.
Related guides
- Merging unit synchronization, holdover and free-running mode
- Merging unit maximum processing delay
- Conformance classes a, b, c and d of a merging unit
- Merging unit: IEC 61869-9, 9-2LE and testing
- IEC 61850 Sampled Values (SV): 9-2, 9-2LE and IEC 61869-9
References
- IEC 61869-9, Instrument transformers - Part 9: Digital interface for instrument transformers, edition 1.0 of 2016, clauses 6.903.2 and 6.903.3 (Table 902).
- UCA International Users Group, Implementation Guideline for Digital Interface to Instrument Transformers using IEC 61850-9-2 (9-2LE), cited by IEC 61869-9.
Frequently asked questions
What does F4800S2I8U0 mean on the nameplate of a merging unit?
Four fields: F4800 is 4,800 samples per second, S2 is two ASDUs (samples) per message, I8 is eight currents in each ASDU and U0 indicates there are no voltages. IEC 61869-9 introduced this notation to describe in readable text what a merging unit can publish.
Which sampling rate is preferred for protection and metering?
4,800 samples per second with two ASDUs per frame, whether the network is 50 or 60 Hz. For power quality, the preferred one is 14,400 with six ASDUs, and for high-bandwidth DC control, 96,000 with one ASDU.
How many quantities can a merging unit publish on a 100 Mbit/s network?
At most 24 for metering and protection, 8 for power quality and 24 for DC control. The minimum is a single quantity among currents and voltages. For networks of 1 Gbit/s or more, the standard sets no limits.