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IEC 61850 controlling MV and LV power systems

Although protocols and architectures have been in place to deal with device and system monitoring, control and automation for years, there has been a demand for a standardise protocol. This drove the International Electrotechnical Commission (IEC) to develop a means of communication for Intelligent Electronic Devices (IEDs) from different vendors. IEC 61850 is a single international standard protocol that has been implemented in many High Voltage (HV) substation which results in an increase in performance and reducing the complexity and cost. Until now, any Medium or Low Voltage (MV or LV) applications were not considered due to the restricted number of IEDs being compatible.

How is it done today?

An IED, such as a Programmable Logic Controller (PLC), shared its inputs and outputs (I/Os), often 100s or 1000s, to a central Supervisory Control and Data Acquisition (SCADA) system to allow automation and supervision of the power system. Sometimes meters and other devices are added for an Energy Monitoring Systems (EMS) or BMS for creating energy usage models and interactive control. However, these would be separate systems as it would require the use of different types, protocols, networks and cabling depending on the IED which need to be independently tested and commissioned.

What is IEC 61850?

IEC 61850 is the result of a combination of the newly available information technology/computer science and the power engineering field. It is a combination of protocols in different layers of the communication process depending on usage and defined in the standard document with ten major sections.

IEC 61850 treats every IED as a physical device by assigning it a network address. Each physical device can then have multiple logical devices associated to it, acting as a gateway. Each logical device has one or multiple logical nodes which represent a specific function related to the device. The nodes contain elements of data with a standard unique name which falls into functional categories for reading a status, measuring or controlling a certain value in this data set. This can be compared to object oriented programming.

Data Transmission

Once data has been assigned to an object, IEC 61850 maps this to protocols for standardised communication methods such as GOOSE, SMV and MMS.

For example, the current DC control architecture allows only vertical communication and requires a PLC for the logic, using the GOOSE protocols allows for horizontal communication between IEDs on what is referred to as the ‘station bus’ and removes the need for a PLC. Effectively, Device A will subscribe to a certain data set from Device B which will publish this data on the network for all subscribers to use when there is a change of state.

SMV (Sampled Value Multicast)

Similarly to GOOSE, SMV operates through horizontal communication on what is referred to by the ‘process bus’. This protocol offers the ability to digitalise sampled values (voltage, current, status) and transfer them in the network for the devices that require this information, thus, for example, metering data no longer must be on a different system such as an EMS.

Redundancy?

IEC 61850 now covers redundancy standards for the station bus and process bus; these are the two standards offered by IEC 62439, HSR (High-availability Seamless Redundancy) and PRP (Parallel Redundancy Protocol). Effectively, PRP being a radial 2N arrangement, while HSR corresponding to a ring design. Unlike hot-standby PLC formats, both these protocols offer ‘zero switchover time’ (hot-hot) in the event of an IED or path failure.

For data centres/critical facilities, good resilience can be achieved by IEC 61850. While PRP offers a clear 2N independent system, it is also possible to have two (2) loops in the HSR system to achieve the same result.

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