Modernized Grids Require Substation Virtualization
The developed world’s electrical power transmission and distribution systems—often referred to simply as the grid—deliver admirable reliability and uptime, which is notable considering the large proportion of decades-old operating assets. Today’s grid is being asked to perform tasks it was not originally designed for, such as handling bi-directional power flows due to power supplied from distributed energy resources (DERs), many of which are small-scale renewable power generation sites supplying power on an intermittent basis. Aging systems must be upgraded, and any new work or rework must accommodate the latest service and supply demands. The failure to do so results in such catastrophes like what we have recently seen in the state of Texas.
As legacy assets approach end-of-life and need replacement, and as the grid expands to support new construction and generation sites, designers are looking to equal or better the reliability of traditional technologies. Applying modern computing techniques, like hypervisors and redundant HSR/PRP networking architectures, will become necessary to deliver the required automation and data communication, while making deployment, operation, maintenance, and grid resilience optimized beyond the current state.
Substations are fundamental to grid operations, whether stepping up voltages to transmission levels, or stepping down voltages to sub-transmission and distribution levels. This article will focus on the distribution substations used to stepdown sub-transmission voltages to levels suitable for supply to pole- and pad-mounted distribution transformers(Figure 1), which in turn step down local distribution voltages to 120/240/480 VAC for industrial, commercial, and residential use.
Figure 1: Distribution substations step down sub-transmission voltages to local distribution levels, and perform many other automation, protection, and control functions.
In addition to traditional functions associated with outgoing power flows, distribution substations must now accommodate incoming power flows from DERs, greatly complicating operation. To address these and other issues, virtualization is now being used.
This technology allows multiple operating systems and associated applications to run simultaneously on a single hardware platform. Recent and forthcoming advances will allow these virtual machines (VMs) to provide the sub-millisecond response times required for even the most critical substation functions, such as protective relaying.
Well over 90% of the need is for modernizing and refurbishing existing assets, with new substations typically designed and built using modern technology.
Distribution Substation Devices
Many different types of devices are utilized to perform all the functions required in modern distribution substations(Figure 2).In addition to traditional functions associated with outgoing power flows, distribution substations must now accommodate incoming power flows from DERs, greatly complicating operation.
Figure 2: Distribution substation Level 0 devices are linked to a control center via a process and a station bus.
Figure 2 depicts some of these devices in a typical substation automation system (SAS), but there are many others, each of which performs one of two main functions:
- Protection & Control: 100% real-time autonomous elements
- Automation: command and control, with some real-time elements
Protection & control devices include intelligent electronic devices (IEDs) installed with circuit breakers to monitor their operation. Merging unit (MUs) collect multi-channel digital signal outputs from current and voltage transformers synchronously, and then transmit these signals using the IEC 61850-9 protocol to protective, metering, and control devices.
IEDs perform various functions, such as protective relaying, remote fault indication, monitoring and regulating power and voltage levels, automated switching of power from one source to another for bus transfers, and reclosing feeder lines after temporary faults.
Automation devices—such as PCs, routers, unmanaged/managed switches, programmable logic controllers, and human machine interfaces—are typically used to coordinate operation among protection and control devices, and to provide other required functionality, such as networking. All of these devices contain electronic hardware to control operation, and often host software to perform operations.
Ideally, all of these devices comply with the IEC 61850 standard protocols, a communication service that connects all the devices at Level 0, the servers at Level 1, and the clients at Level 2. Although this may be ideal, it is often not the case; nd even when it is, other communication and integration problems remain.
A typical distribution substation contains many of these devices supplied by a variety of vendors—each with proprietary electronic hardware, and often with their own operating systems and hosted software—raising a variety of operational and management issues.