A Substation (SS) acts as a strategic node in a transmission and distribution (T&D) network. Most substations operating at high voltage (HV) or extra-high voltage (EHV) perform switching and routing of AC power. So, the operation of substation equipment which is aimed at the routing/switching of power (operation of switchgear etc.) is termed as the “primary process”. A substation is called digital in which the data related to protection, control and monitoring of the primary processes is digitized immediately after the measurement. Technically, digital substation refers to a substation that employs both IEC 61850 Process Bus and Station Bus in its protection and control architecture. Before going into the details of digital substations, it’s crucial to summarize the governing communication standard of digital substations, IEC 61850.
What is IEC 61850?
With the introduction of powerful micro-processing tools and multi-functional relays (known as Intelligent Electronic Devices or IEDs), the power systems industry entered the digital age. Initially, the use of communication technologies in a substation was limited to offline data collection only. However, the advancements in communication IEDs has enabled utilities to perform real-time control and protection operation. In the early 90s, various manufacturers introduced communicable IEDs, however a lack of common communication protocol resulted in a chaotic situation. As a result, utilities ended up spending huge amounts of money on device integration from different manufacturers. At that time, international organizations started looking into the possibility of standardizing the communication protocol for power systems industry. In 1997, both IEC and IEEE partnered together to develop a common standard for substation communication which is now popularly known as IEC61850. Naturally, the standardization process involved all the major stakeholders including system manufacturers and key utilities.
Traditionally, process equipment was connected with bay level devices through copper wiring. With the introduction of IEC 61850 process bus, the process equipment is connected with the rest of the system (IEDs) through a digital interface. Two crucial parts of IEC 61850 series of standards, IEC 61850-8-1 and IEC 61850-9-2 are defined as Generic Object-Oriented System Event (GOOSE) and Sampled Measured Values (SMVs).
Hardware for a Digital SS:
A standardized communication mechanism laid a great foundation for digitally enabled substations. Additionally, evolution of hardware equipment, especially sensing and measurement equipment, has revolutionized the field of digital SS as well. The most crucial enablers of digital substations are the non-conventional Instrument Transformers (NCITs) and standalone merging units (MUs) which utilize IEC 61850 process bus communication.
Non-Conventional Instrument Transformers (NCIT):
At its core (pun intended), the working principal of a traditional instrument transformer (current [CT] & voltage [VT] transformer) is based on ferromagnetic circuit tightly coupling primary and secondary coils. So, much like power transformers, conventional instrument transformers also transfer power from its primary to secondary. NCITs on the other hand have a very low energy output (typically do not transfer power to the secondary) which eradicates the risks typically linked with a conventional instrument transformer including open (short) secondary in CT (VT), saturation and non-linearity. Additionally, the inherent signal processing nature of NCITs removes the need for the analog-to-digital conversion which makes them a perfect building-block for a digital substation. These are the different technologies used in NCITs:
- Fiber Optical Current sensor (FOCS)
- Rogowski Coil CT
- Electronic Voltage Transformer
Merging Unit (MU):
Merging unit is defined in IEC 60044 and its functions include simultaneous sampling and relaying of current and voltage measurements. It converts the analog signals from CT and VT into IEC 61850 SMVs. Merging unit itself does not include any protection functionality but it acts as an interface for the primary process equipment.
Benefits of a digital substation:
In order to make a measurable impact in the market, any new technology must be able to present tangible benefits not only in terms of cost but also in performance over the existing technology. Additionally, for power grid, reliability and security also play an extremely crucial role in the adoption of a technology. Here are some of the benefits presented by IEC 61850 process bus based digital substations over conventional substations.
Design Simplicity and Flexibility:
As described earlier, in the traditional substation design several point-to-point copper wire connections exist from the switchyard to the bay level protection and control devices. Due to the hardwired connections, output from an instrument transformer cannot be shared with several bay level devices. In case of process bus, any IED at bay level can speak with any process level device. Figure 1 and figure 2 depict (in a simplified form) the difference between a conventional and a digital substation.
Conventional Substation with multiple comm. connections.
Digital Substation with process bus.
Implementation of process bus in a substation can reduce the total cost of installation which include engineering, drafting, construction and commissioning. Complex copper wiring is a big contributor to the engineering cost of a substation design.
Protection Scheme Enhancements:
As already explained, the flexibility achieved through IEC 61850 process bus in terms of information exchange plays a crucial rule in providing enhanced protection coordination among the bay level IEDs. Trip, blocking, status or any logical signals are transferred through the same network using GOOSE, and can support peer-to peer applications in a substation. In addition to that, reference presents other opportunities, such as failure of any current transformer or an MU can be easily compensated by estimating lost signal from other MUs, and installing centralized back-up protection system instead of providing individual duplication.
Different manufacturers can provide full integration of protection functions on all bay level IEDs. A simplified representation of interoperability is shown in the figure below showing how IEDs from various manufacturers can interact with each other in a substation environment.
IEC 61850 revolutionized substation automation enabling utilities to implement a cost effective ‘copper-less’ (reduced capital cost) solution while creating an interoperable (reduced O&M cost) communication network throughout their substations. More utilities around the globe have tendency to incorporate IEC 61850 process bus enabled digital substations in their network. Process Bus technology offers a variety of new and exciting possibilities in designing the next generation of Ethernet-based protection and control systems.