The Business Case for IEC 61850

Author: Alexander Apostolov, USA

Substation Costs

The cost of building new substations or expanding existing substations, especially in densely populated area can be significantly reduced by using IEC 61850 based digital substations. This cost reduction is most significant when the conventional current and voltage instrument transformers are replaced with non-conventional sensors and the combination of disconnector-breaker-disconnector is replaced by a Disconnecting Circuit Breaker (DCB) available from several major suppliers. The substation footprint in this case may be reduced to about 50% of the footprint of a conventional substation.
Reducing the number of primary devices also reduces the construction costs due to the smaller number of foundations needed.

Replacing the traditional hardwired analog and binary circuits between the primary substation equipment and instrument transformers (Figure 1) by the optical fibers of the IEC 61850 process bus results in a significant reduction in copper cables. When we include also the replacement of the hard wired interfaces between IEDs it can reach up to 80% in transmission level air insulated substations. This will lead to significant cost savings not only by reducing the cost of the copper cables, but also by limiting the transportation costs. Copper cables remain for power supply and short connections between primary equipment and marshalling kiosks in the switchyard.

The replacement of conventional instrument transformers with non-conventional sensors also reduces the transportation costs because of the significant difference in their weight. For example a non-conventional current sensor weights less than 15% of the weight of a conventional current transformer and is also smaller in size. (Figure 2)
Further construction cost reductions are achieved by the smaller size of the control house due to the reduced number of panels. This is the result of several factors, such as the smaller size of the limited number of IEDs which do not have terminal blocks for the traditional hardwired interfaces, very limited number of terminal blocks, and high levels of functional integration. In the future centralized IEC 61850 based digital substations the control house as we know it may disappear.

IED Interfaces Installation and Commissioning
The wiring of conventional substations (Figure 4) requires a significant amount of time by skilled technicians in order to provide all required interfaces:

  • Between the substation equipment in the yard and the panels in the control house
  • Between the panels in the control house
  • Between the panel terminals and the IEDs in the panel
  • Between the IEDs in the panel

This time is further extended by the preparation time of the copper wires and their labeling. Another issue is the risk for human errors and the requirements for extensive testing in order to ensure the quality of the interfaces.
In the IEC 61850 based digital substations the thousands of hardwires carrying individual analog and binary signals are replaced by a limited number of fiber optic cables transmitting sampled values, GOOSE messages or client-server communications. (Figure 3)

Mapping IED Data to HMI
One of the most time consuming engineering tasks when building a new substation and engineering its protection, automation and control system is the development of the substation Human Machine Interface (HMI). In conventional substations using communication protocols which do not use semantics based data naming the mapping of the data from the registers of each IED to the HMI is an extremely time consuming work that is also prone to human errors.

The use of a standard, semantics oriented data model in IEC 61850 allows the development of engineering tools that can generate automatically the substation HMI based on extensions to the model that support the visualization of the substation equipment on the HMI in a user customizable manner. This will lead to significant reduction of the HMI development time - from months to minutes, and will also improve its quality.

Limited Number of Inputs and Outputs
Multifunctional IEDs interface with other IEDs based on hardwired relay outputs and opto inputs.  Some protection schemes may require a large number of interfaces.
The limited number of opto inputs and relay outputs is another challenge for the development and implementation of some more advanced distributed protection, automation and control schemes. In some cases this problem is solved by using auxiliary relays, however the price is the reduced reliability of the schemes due to the increased number of devices and the hardwired interfaces between them. The addition of the auxiliary relays also increases the operating time of the scheme, which may not be acceptable. This also leads to increased installation, commissioning and maintenance costs.

Replacing the hardwired interfaces with GOOSE messages eliminates this problem due to the fact that a single fiber connection can carry the equivalent of a practically unlimited number of signals between the IEDs, resulting in significant time and costs savings.

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