How to convert an old substation into a digital substation?

Substation digital communications

Typically, substations comprise power elements like power transformers, switches, busbars, etc. and control elements like protection units, metering units, RTUs, SCADA, etc. While the power elements are mandatory requirements, the control elements vary as per the function of a substation and the capital investment available.

In between all these, digital communication, data acquisition, automation, intuitive features and, information sharing via Ethernet or any other modern communication protocol separates a conventional substation with a modern digital substation. During the initial days of power system development, substations served the sole purpose of power distribution with few automation and digital data acquisition provisions.

Advanced communication protocols were not ready to be used in the power sector back then. To bring it in perspective, IEC 61850, an international standard that defines the modern communication protocol for digital electronic devices within substations, was introduced only in 2003.

This article will discuss the fundamentals of the up-gradation of old substations into digital substations capable of full automation, real-time data acquisition, and monitoring, along with lots of additional features.

Table of contents:

1. Typical operation and communication mechanism of old substations
2. Digital substation and communication protocol in a nutshell
3. Why convert old substations into digital?
4. Architecture and essentials of conversion
   1. Equipment selection
      1. Primary sensing elements and interfacing devices
      2. Integrating existing digital devices and new IEDs
      3. Provision for synchronization and switches
      4. SCADA and HMI
   2. Selecting bus architecture
   3. Final integration with the existing components

1. Typical operation and communication mechanism of old substations

The primary components of a substation have not changed much over the decades, and the basic functionality remains more or less the same. Thus the focus of advancement and change shifts to secondary setup. Concisely, most of the substations built before or during the 1990s were hardwired for any level of the substation automation system and remote control available.

The electromechanical and static protection systems were dominant before and during the 1990s. Static protection developed during the early 1990s was a boost in the field of substation automation, but still way short of modern-day advancement and flexibility of the digital system.

As illustrated in Figure 2, conventional substations have a vast network of physical copper wiring, creating a network of all primary sensors like CTs, VTs from the switchyard, or RTU to the control room. Likewise, connections to relays, switchgear, or existing HMIs are hardwired, with individual copper wires too, with a bare minimum possibility of automation and limited data acquisition.

Even today, the number of substations in operation with very little or no automation is very high compared to a modern digital substation. Barring some minor maintenances, once commissioned, power substations last for 30-40 years or even more.

These substations would still be considered old and outdated for their lack of flexibility in connectivity within devices, a vast tiresome network of hardwired copper cabling, lack of extensive data acquisition, and lack of full automation to the potential of installed numeric relay and so on.

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2. Digital substation and communication protocol in a nutshell

IEC 61850 is the core of any modern-day digital substation. Rather than emphasizing the perks of a digital substation, we will focus more on the bus architect used in a digital substation related to the up-gradation of older substations, which we will discuss later in this article.

The digital substation comprises three levels in terms of communication:

1. the station level,
2. the bay level, and
3. the process level.

In a digital substation, only a few fibers and Ethernet cables connect all these components and levels mentioned above, which makes the physical realization much easier and also much reliable at the same time.

In a conventional substation, a vast network of copper wiring makes the system both unreliable and cumbersome.

The digital substation comprises some key elements. IEDs, Generic Object Oriented Substation Event (GOOSE), GPS, station and process bus, merging units, and non-conventional transducers are integral parts of a digital substation.

An IED is any device within a substation that consists of one or more processors and can control data from external sources like sensors and transducers. Digital relays, controllers, power analyzers, etc. used for protection and control fall in this category.

Merging units act as an interface between the conventional transducers and the IED like bay controllers and protection relays. Their primary function is to digitize the analog signals and transmit them to IEDs as per the adopted protocol. Likewise, the trend of using optical current and voltage transducers is evergrowing.

These modern transducers are capable of communication to IED via optical fiber with no intermediate units.

Recommended reading:

IED (Intelligent Electronic Device) advanced functions that make our life better

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3. Why convert old substations into digital?

While the primary components like transformers, switches, busbars, etc. of a substation can operate on their own when connected in proper sequence, it is the secondary components that guide, control, and regulate the designated operation. In an older substation, those operations are manual or with minimum automation.

Older substations perform their primary functions but lack digital features like real-time monitoring, data logging, and automation. Upgrading the old control network into a digital network revives the whole substation.

Upgrading requires good planning and replacement and addition of some key elements that can be done with pretty low expenses compared to the construction of an entirely new digital substation.

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