Sampled Values (SMV)

Basics

The service Sampled Value is meant to replace copper wiring between current transformers (CT) and voltage transformers (VT). As such, the data it is carrying digitalized current and voltage measurement information (sin waves). Depending on the frequency, the sampling rate must be high enough to not lose information (see Nyquist–Shannon sampling theorem ).

The SMV is mapped to the Ethernet packet “only“, which means there is no Transmission Control Protocol (TCP) or Internet Protocol

(IP) it is utilizing. With that there are three important implication:

1. No IP: GOOSE does not allow for one-to-one association. (except R-GOOSE) Instead, GOOSE is utilizing the Ethernet multicast

2. No TCP: GOOSE publisher (sending IED) and GOOSE subscriber (receiving IED) are completely decoupled. There is no mechanism that allow to resend a GOOSE on a receiving IEDs request. to compensate for that, GOOSE has a so-called repetition strategy.

3. No TCP: The GOOSE payload is limited to an Ethernet frame.

Ethernet multicasting

Instead of utilizing the IP protocol to define the destination, GOOSE is utilizing the Ethernet multicasting. Any IED that is connected to the same switch as the sending IED is receiving the GOOSE packet and then can decide to use this information or drop it. It is doing so by fixing the destination MAC-address range to 01-0C-CD-04-00 to 01-0C-CD-04-1F-FF. With a destination address starting with 01 defining the multicast range.

Sampling rates

The sampling rate of a sampled value stream is the implementation choice of the sending IED (Sampled value publisher). Practically certain sampling rates became a de facto standard.

SMV control block (MSVCB)

The service Sample Values can be found in the data model of the IED. It is represented there with a very specific data object called MSVCB. It can only be allocated in the logical node LLN0. A MSVCB is like a data object structured data. Instead of data attributes the control block hold configuration setting for the Sampled Value such as:

• MsvCBName: the name of the MSVCB

• MsvCBRef: the reference of the MSVCB in the data model

• SvEna: whether the GOOSE is active

• MsvID: the GOOSE ID

• DataSet: the reference to the data set

• ConfRef: a integer number indicating changes in the GOOSE data set

• NdsCom: whether the GOOSE still to be commitioned

• SmpMod: Sampling mode (SamplingPerSecond or SamplingPerCycle)

• SmpRate: the sample rate depending of the sample mode

• OptFields whether optional information is included in the sample value massage

  • refresh-time

  • reserved

  • sample-rate

  • data-set-name

  • sample-mode

  • synch-source-identity

• DstAddress: the destination MAC-address of the GOOSE

The MSVCB can be configured through an SCL file. There it is represended with the element SampledValueControl