This piece was originally published in the November 2018 issue of electroindustry.
Andre Smit, Engineering Manager, Siemens Industry, Inc., Suraj Chanda, Application Engineer, Siemens Industry, Inc., Alexandr Stinskiy, PhD, Application Engineer, Siemens Industry, Inc., and Farel Becker, Product Manager, Siemens Industry, Inc.
Many utilities are looking for better and improved ways to optimize distribution feeder automation by exploring cellular communication services.
Distribution feeder automation systems provide many different types of solutions. The primary reliability improvement functions provided by these systems include fault locate, isolate and service restoration, and automatic transfer functionality. These functions greatly improve the reliability indicators of distribution feeders and can be centralized, decentralized, or a combination of both.
New Possibilities for Reliability
Centralized systems have traditionally been the solution of choice to automate distribution feeder networks. However, these systems react slowly, as they have to wait for the protection systems to disconnect faults in the network before they can take action to locate the faulted feeder segment, reconfigure the feeder, and supply alternate power to unaffected areas.
Decentralized systems, by contrast, provide the capability to synchronize protection and automation functionality in field devices to provide faster fault isolation and system configuration actions. The combination of decentralized distribution automation (DA) systems and cellular communications provides new possibilities for greater reliability of the distribution network.
Traditionally, low-powered unlicensed radio devices, and more recently, direct fiber-optic cable connections, have been common methods of data communications in DA applications. For electrical utilities, using cellular communication is not new; in fact, it is usually an IT service used to communicate field device information data back to the utility for use in various systems, such as advanced metering infrastructure (AMI).
The use of cellular communication to transmit data in time-critical applications such as direct transfer trip (DTT), fault location isolation and service restoration (FLISR), and Automatic Transfer System (ATS) is now possible by securing a reliable communication link between the field device controllers. To accomplish this, an operational technology (OT) type of service to support these unique requirements is needed. OT systems are deterministic in nature and able to produce actions based on information received. The system requires security, dependable latency, and reliability in accordance with previously established substation protection Standards.
To further explore this, let’s take a look at the use of a private network and how a DA solution provider, such as Siemens, establishes a private network within the cellular provider’s network for DA applications.
Unlike a public plan, the basic premise of a private network is that there is no connection to the internet. On public networks, unsolicited internet traffic is data sent to a wireless device that was not solicited by the wireless device owner. This data could be a result of random queries from unknown third parties or could be malicious attempts to cause a service disruption.
On a private network, no unauthorized traffic can travel over the network, eliminating the risk of unsolicited traffic from external sources. All data is sent to and from devices configured for a specific private network and is segregated from all other traffic. In a private network, customers can acquire a pool of allocated machine-to-machine (M2M) data plans that is required for the DA application.
To support these applications, cellular modems and routers are preconfigured by the solution supplier and are used with IPsec tunnels, firewalls, and special filtering to securely transmit data between field devices. M2M data plans for DA solutions are well defined and tested to support the applications.
For example, a manufacturer’s systems may use small Ethernet IEC 61850 Generic Object Oriented Substation Event (GOOSE) messages to communicate information peer to peer between pole-mounted field controller devices/servers on cost-effective M2M data plans that are designed to support applications such as DTT, FLISR, and ATS within the private cellular network.
These DA systems rely on communications availability to perform as designed. As a failsafe, if the system detects that a modem was abnormally disconnected from the cellular network, the system will automatically notify the operator that a link is down. This notification service ensures that our customers can better maintain their communications systems that support important OT applications.
For the progressive electrical utility, the combination of cellular communications and the new approaches to distribution feeder protection and automation is yielding improved system reliability. In contrast, the traditional approach consists of two distinctly different functions: protection first and then automation.
The new protection system disconnects the fault from the network through coordinated overcurrent tripping and auto-reclosing actions. The automation system then locates the faulted segment of the feeder, isolates this faulted segment, and executes automatic closing of the field switch to provide alternate power from a different power source to unaffected feeder segments. This is often referred to as self-healing.
The new approach eliminates the complexity of adapting the coordinated overcurrent settings on devices when topology changes are executed by the automation system. In order to accomplish this, the protection and automation systems must be closely coordinated in actions for both protection and automation functions to operate effectively and the protection and automation actions must be executed synchronously.
Today, most digital protection devices have powerful logic programming capability; therefore, it is possible to move the automation functions to the protection devices. This logic is used to isolate the exact section of the distribution network where a fault has occurred. A jump differential (jDiff) is detected in the feeder current on both sides of the faulted line segment. It was developed to gain the benefits of a selective differential protection function on distribution feeders using various forms of wireless communications systems.
The jDiff method makes it possible for each device in the field to communicate in multiple upstream and downstream devices across the communications network. The jDiff algorithm converts magnitude changes in the phase currents to logical signals called “Positive Jump” and “Negative Jump.” These signals are transmitted through the communications network as binary signals to the upstream and downstream devices. The cellular network provides communications infrastructure that makes it possible to both enable and simplify this new approach. The jDiff locates the fault location and can block and unblock a simplified coordinated overcurrent protection system.
Another advantage that can be gained by this type of decentralized system operated in combination with a cellular network is the speed of operation of the automation system. If one considers adapting protection systems to incorporate all feeder automation actions, it is possible to greatly improve the combined system performance while maximizing the reliability by exposing less of the system to protection and automation operational interruptions.
To get this approach to work, it is essential that the field and substation devices communicate and share information in real time. GOOSE messages are likely the best possible platform to communicate this information between protection devices.
What makes GOOSE ideal for this application?
- It is a small data packet, ideal for wireless systems.
- Analog and binary information can be shared for processing by the protection and automation controllers.
- Data traffic can be managed using set time intervals of the GOOSE packets.
- The GOOSE packets contain quality information. Therefore, devices can filter and discard GOOSE packets with incorrect quality information.
- An additional layer of security is added to normal IT cybersecurity requirements.
The implementation of cellular networks for peer-to-peer communications of the pole-mounted DA controllers has the potential to increase the reliability and speed of DA networks.
jDiff is a proven method to locate a faulted line segment by comparing positive and negative phase current measured on both sides of the fault. When a fault is located, the logic within the DA controllers executes a series of commands to automatically isolate the fault and restore power to the unaffected line segments.
Although several types of communications methods may be used, an M2M data transfer across a private cellular network provides a secure, cost-effective, and easy to deploy solution.