​Digital Substation

​Digital Substation

The term "digital substation" lacks a standardized definition, but it generally refers to substations where the majority of primary process data is converted into digital format directly within the substation, close to the point of measurement. These digital substations facilitate the exchange of measured values, commands, and signals between secondary and primary devices through a communication network adhering to the IEC 61850 standard. This communication network is commonly referred to as the "process bus."
The process bus replaces the traditional copper cables that were utilized for measuring and control circuits, which previously ran from the control building to separate high voltage bays in conventional substations. In digital substations, the only remaining copper cables between the control building and the high voltage bays are those used for AC and DC power supply circuits.
A digital substation comprises various components and technologies that work together to modernize and enhance the performance, efficiency, and reliability of the power grid. The key components of a digital substation include:
Non-Conventional Instrument Transformers (NCITs) have had limited utilization in substations thus far. Generally, NCITs do not offer the standard analog 1 A/5 A or 110 V output required by traditional protection, control, and monitoring devices. However, with the introduction of a standardized process bus, the integration of NCITs into substations becomes more straightforward. Given that NCITs typically rely on digital signal processing, a digital signal is the logical output choice for such sensors.
What sets NCITs apart from Conventional Instrument Transformers (CITs) is that they do not operate on the principle of an iron core transformer. Consequently, the limitations stemming from iron core-based CITs can be overcome through the use of NCITs. NCITs are founded on either electrical or non-electrical measuring principles. Some examples of NCITs that employ electrical measuring principles include Rogowski coil current sensors and capacitive voltage dividers. On the other hand, Optical Current Transformers (OCTs) utilize a non-electrical measuring principle based on the Faraday effect.
Merging Units (MUs) and Stand-Alone Merging Units (SAMUs) serve as interfaces for connecting Non-Conventional Instrument Transformers (NCITs) or Conventional Instrument Transformers (CITs) to the process bus. These units sample current and voltage measurements from instrument transformers and distribute the digital output as Sampled Values (SV) on the process bus, following the IEC 61850-9-2 LE standard. Secondary devices connected to the process bus can subscribe to these SV. To align SV from different merging units, they are time-stamped and synchronized.
 There are two types of merging units: MUs, which work with NCITs and are typically integrated with the sensor electronics, and SAMUs, which interface with CITs.
The use of SAMUs is expected to be prevalent in future digital substations for several reasons. Firstly, transmission networks also include voltage levels below 200 kV. In these lower voltage parts of transmission substations, the continuous use of CITs might be necessary due to the unavailability of OCTs rated for these voltage levels. This situation may arise, for example, on the Low Voltage (LV) side of a power transformer. Currently, OCTs offered by various vendors do not cover the lowest voltage levels in the transmission network. Additionally, the transition from conventional substations to digital substations will occur gradually, with CITs being replaced step by step. SAMUs enable the integration of existing CITs into digital substations, facilitating a gradual transition.
In digital substations, conventional switching equipment is integrated using Switchgear Control Units (SCUs). These SCUs provide a binary interface to connect conventional switching equipment to the process bus, allowing monitoring and control of the switchgear. SCUs gather information about the position and status of the switching equipment and distribute this data on the process bus in compliance with the IEC 61850-8-1 standard. Additionally, SCUs actuate the switchgear based on commands and signals received from protection and control Intelligent Electronic Devices (IEDs) over the process bus.
The Process Bus serves as a communication network that replaces the traditional hardwired point-to-point connections between primary equipment and secondary devices in conventional substations. This communication network facilitates the exchange of time-critical information between process-level devices and bay-level devices bidirectionally. Unlike copper cables used in conventional substations for measuring and control circuits, the process bus utilizes fiber optic cables.
On the process bus, primary process data is distributed and made accessible to all secondary devices connected to this communication network. There are three primary types of communication on the process bus: SV streams, Generic Object-Oriented Substation Event (GOOSE) messages, and client-server communication based on the Manufacturing Message Specification (MMS) protocol.
Sampled current and voltage measurements from Stand-Alone Merging Units (SAMUs) and associated Merging Units (MUs) of Non-Conventional Instrument Transformers (NCITs) are published to the process bus and sent as Sampled Values (SV) to subscribing secondary devices, following the IEC 61850-9-2 standard. Using multicast messages, the same SV can be simultaneously sent to multiple secondary devices.
Moreover, status data and alarm information from the primary process are transmitted to secondary devices, typically as GOOSE messages, adhering to the IEC 61850-8-1 standard. Trip signals and open/close commands are also sent from protection and control Intelligent Electronic Devices (IEDs) to the Switchgear Control Units (SCUs) as GOOSE messages, following the IEC 61850-8-1 standard.
Digital substations enable real-time monitoring, control, and automation, leading to improved grid management and reduced maintenance costs. They replace conventional hardwired connections with a process bus, allowing for faster and more reliable data exchange between devices. Digital substations are a key component of the smart grid, providing the foundation for a more intelligent and interconnected power system.