Redundancy

WinCC OA Redundancy

A report control block can be used only by one client at once. Both WinCC OA redundancy partners share the same RCB, however only the active client receives values from the device. In case of a redundancy switch-over the client which is going to be passive has to release the RCB before the other client can use it.

Depending on the used RCB type the behavior is different in case of a redundancy switch-over:

• URCB - is released immediately
• BRCB - is not available for the other client till the time period defined at the ReserveTms data point element expires. Therefore, the value for ReserveTms must be defined as low as possible (1 second).

The actual switch-over time depends on the reason for the connection loss. E.g. both partners must detect the connection loss. Whereas the WinCC OA client detects a connection failure immediately, how fast a IED detects the connection loss and releases the RCB depends on the device type.

Therefore, it is recommended to use BRCBs for transmitting important data. The IED stores BRCBs in a buffer. In case of a connection loss, the reports are transmitted in chronological order after connection reestablishment. Depending on the configuration of ReadCompleteBuffer the complete buffer or only reports accumulated during downtime are retrieved.

Dynamic datasets must not be used in a redundant system.

Device redundancy

The WinCC OA IEC 61850 client allows to configure two IEDs as one redundant pair of servers. In this case both IEDs are handled as one device with two connections. The redundant devices must be identical insofar, as the respective *.cid file must be based on the same *.icd file

Any kind of synchronization between the redundant IEDs is in the domain of the IEDs.

If the created WinCC OA device (see Configuration of the IEC 61850 client) is enabled, the client tries to establish the connection to both IEDs. The primary IED will be active, the secondary IED passive. If the WinCC OA device is deactivated, the connections to both IEDs will be terminated. The connection state (_IEC61850_IED.Config.Active) applies to both IEDs.

The single connections of the redundant pair can be disabled via the respective checkboxes or by setting the data point elements _IEC61850_IED.DeviceRedundancy.Config.ConnectPrimary or _IEC61850_IED.DeviceRedundancy.Config.ConnectSecondary to FALSE.

The outcome of a switchover is written to the internal data point element _IEC61850_IED.DeviceRedundancy.State.ActiveDevice. The appropriate value is generated as follows:

Switchover

A switchover can be triggered by following events:

• Connection to active IED is lost
• Tag-based (triggered by IED logic, see switch tag for further information)
• Manually (see Device redundancy) or application-based (see health tag)

This list shows the priority (highest to lowest) for the switchover. This means that there is no switchover if e.g. the switch tag is FALSE but the other IED is not connected.

The internal data point element _IEC61850_IED.DeviceRedundancy.State.ActiveDevice shows which IED is the active one.

IED name in reference strings

The IEC client automatically replaces the IED name of the primary device in address strings with the name of the secondary device. This applies to all IEC 61850 address strings like peripheral address, dataset names or RCB names.

When using device redundancy, the ldName attribute must not be defined for logical devices (<LDevice> tag) in the SCL file.

Further notes & restrictions

User bit 6 of user byte QualityMode is used to identify which IED has sent a specific data value. The client sets (=primary device) or resets (=secondary device) the user bit. This behavior can be enabled with the config entry [iec61850] userByteQualityMode.

Using the config entry "[iec61850] userBitReduSourceDevice" allows you to define which user bit should be used to distinguish between the redundancy partners.

If the tags contained inside the message do not support quality information (see Details of the IEC 61850 client) or are received via poll requests all user bits except for redundancy information are set to 0.

Following services are only available for the currently active device:

• Write data
• Read data
• File transfer

Online-Browsing

Every browsing action overwrites the results of the previous one. Possible differences between SCD file configuration and device configuration, or configuration differences between the two IEDs are not indicated.

Synchronization

The synchronization between the IEDs forming the redundant pair is in the domain of the IEDs.