IEC 61850 Sampled Values protocol

IEC 61850 (IEC 61850 – Communication Networks and Systems in Substations) standard defines Sampled Values (SV) protocol as a publisher/subscriber type communication. This protocol is used for information exchange between Merging Units and IEDs (IED – Intelligent Electronic Device) in a Substation over the Ethernet.

The "Implementation Guidance for Digital Interface to Instrument Transformers Using IEC 61850-9-2", or IEC 61850-9-2LE (LE - Light Edition), guide is used as a reference in the implementation of the SV protocol. The guide can be found at this link: IEC 61850-9-2LE.

The concept of SV communication is that the publisher periodically sends messages with exactly defined time intervals. The time interval depends on two factors: measured signal frequency and Samples Per Period (SPP). IEC 61850-9-2LE defines two SPP values of 80 and 256. So, for example, if the measured signal frequency is 50 Hz and SPP is 80, the sending time interval will be 1/50/80, or 250 µs.

All messages are published under a topic. The subscriber receives all messages from the system, but filters and parses only the messages sent with the subscribed topic.

Since the SV protocol is publisher/subscriber based, communication is possible only inside the local network (LAN).

The protocol defines the SV message as shown in Figure 2. The field definition is presented in Table 1.

The structure of Reserved 1 is defined in Figure 3.

S: Simulate. This flag indicates that the SV message is sent by a test device. This flag helps the Subscriber to distinguish whether the received values are real Voltage and Current values coming from the Merging Units or simulated test values.

R: Reserved. These bits are reserved for future use and are set to 0 by default.

Reserved Security: these four bits and the Reserved 2 field form a 28 bit word defined by the security standard IEC/TS 62351-6. It is used as defined when an SV message with security is transmitted, otherwise it is set to 0.

The APDU field contains the payload of the SV messages. Every APDU contains up to 8 ASDU (Application Specific Data Unit), where each ASDU contains one three phase current and voltage measurements and each ASDU has a unique SV identification value. The content of an APDU can be found on Figure 4.

The guide also defines the scaling factors for current and voltage measurements. Current measurements should be scaled with the factor of 1000 and the voltage with the factor of 100 and written in integer format.

Sampled Values protocol in Typhoon HIL toolchain

As mentioned, SV protocol is implemented using IEC 61850-9-2LE. It is supported by the following Typhoon HIL devices: HIL402, HIL404, HIL602+, HIL604, and HIL606.

The whole SV protocol is defined by using following components from Schematic library: SV Setup, SV Publisher and SV Subscriber.

SV Setup

The SV Setup component window is shown in Table 2.

The SV Setup component is used to define SV parameters. If SV protocol is used in the model, there must be exactly one SV Setup component for each HIL device that uses SV protocol.

The component is shown in Table 2.

Table 2. SV Setup component

Component	Component dialog window	Component properties
SV Setup		Ethernet port Override last digit in MAC address Source device MAC address Signal frequency Samples per period Enable message quick parse Execution rate

SV Publisher

The SV Publisher component is used for specifying parameters for one ASDU field inside the APDU (Figure 4). With each SV Publisher component, you can specify if the component creates a new SV message or if it will add an ASDU in an existing SV message. If the component creates a new SV message, you can specify the destination address, user priority, VLAN ID, and APP ID values.

Table 4. SV Publisher component

Component	Component dialog window	Component properties
SV Publisher		General: Create Destination MAC address User priority VLAN ID APP ID Simulate source ASDU index ASDU: svID confRev Synchronization value is I scaling factor I type V scaling factor V type Signal Quality Generate quality Current signal quality Voltage signal quality Va quality Vb quality Vc quality Vn quality

An example using SV Publisher components is illustrated in Figure 6. Only one SV Publisher component exists in the model with the parameters defined as shown on the right of the figure. The component creates a new SV message with one ASDU entry where the values are not scaled and are written in real format. The signal quality is fixed and predefined.

All input signals to the SV Publisher component must be vectors of length 4. The vector should be encoded as [Ia, Ib, Ic, In], [IQa, IQb, IQc, IQn], [Va, Vb, Vc, Vn], and [VQa, VQb, VQc, VQn]

Figure 7 illustrates the message captured with the network sniffing tool.

To add an extra ASDU in the existing SV message, just add another SV Publisher component and choose the Create: Sequence ASDU option. This is illustrated in Figure 8. The SV Publisher component is the same as in the example above and creates a new SV message. The SV Publisher1 component is defined to add an ASDU to the message with APP ID 0x4000 as shown on the right of the figure.

A capture of the newly created message is illustrated in Figure 9.

PTP synchronization

The PTP synchronization mechanism ensures that all devices on the network have the same time reference. This time reference is dictated by the Master device on the network, usually called the Grandmaster clock.

Time synchronization of the SV Publishers ensure that they all start counting samples from the same moment in time. In other words, the SV Publishers reset their sample count (smpCnt) value to 0 each time the seconds value is changed (i.e. seconds rollover). This way, a SV Receiver that receives two SV data streams can easily compare the current/voltage values and detect phase shifts.

The PTP Slave runs automatically alongside the SV application and enables the synchronization of SV Publishers to the PTP Master clock.

PTP synchronization is available through the same Ethernet port used for the SV protocol.

IRIG-B synchronization

The use of this synchronization method is limited to devices with an IRIG-B port. The list of compatible devices can be found in the General Specifications.

Each time the IRIG-B signal is received it resets the sample count (smpCnt) value and updates the internal clock of the HIL device which allows for accurate timestamps in SV Publisher messages.

If both PTP and IRIG-B synchronization methods are active, PTP will have higher priority.

More information about the IRIG-B signal can be found in the Time synchronization page.

SV Subscriber

The SV Subscriber component is used for subscribing to a certain ASDU field from a certain SV message on the network. To subscribe to an SV message, the APP ID and SV ID must be specified for message filtering. The APP ID value is used to filter the SV message from the network, and the SV ID is used to extract the corresponding ASDU from the message.

All output terminals of SV Subscriber component are vectors of length 4. The vectors are encoded as [Ia, Ib, Ic, In], [IQa, IQb, IQc, IQn], [Va, Vb, Vc, Vn], and [VQa, VQb, VQc, VQn].

Table 6. SV Subscriber component

Component	Component dialog window	Component properties
SV Subscriber		APP ID SV ID I scaling factor I out type V scaling factor V out type

Virtual HIL support

Virtual HIL currently does not support this protocol. When using a Virtual HIL environment (e.g. when running the model on a local computer), inputs to this component will be discarded and outputs from this component will be zeroed.