A Migration Path for Legacy Distribution Protection and Control Systems
- Written by Nirmal-Kumar C. Nair and Momen Bahadornejad
In the journey towards a smarter and more dynamic operational future, distribution utilities will have to significantly modify traditional practices of protection and control. Fortunately, existing and emerging standards offer a clearly demarcated pathway to the new world of automated power delivery.
Present-day protection and control methods for distribution networks are relatively simple. They consist mainly of coordinating overcurrent relays, fuses and sometimes re-closers, in systems with radial topologies that connect high-voltage substations with end users. In some selected distribution substations there may be a need for under-voltage or under-frequency relaying, for load shedding or demand response, but such protection settings are made in collaboration with the transmission grid operator.
From the viewpoint of control objectives, distribution utilities are mandated to deliver power within a prescribed voltage band as well as an acceptable power quality (in terms, for example, of total harmonic distortion). In many jurisdictions, maintenance of acceptable supply reliability—as measured by the System Average Interruption Disruption Index or the System Average Interruption Frequency Index—is also closely monitored. The control hierarchy, put in place to satisfy reliability requirements, consists of voltage control devices (tap changers, capacitor banks and voltage regulators) and does not involve coordination or communications interfaces to a significant degree.
In a smart grid world, distribution networks become more active to accommodate distributed generation, microgrids, storage, and electric vehicles. Rather than radial topologies, meshed topologies will be necessary resulting in the need for bidirectional protection and control schemes, which cannot be satisfied by current practices. Getting to this world will require a standardized migration pathway for protection and automation.
With voltage control and automation, means will be sought to leverage greater efficiencies through coordinated operation of existing devices with newer ones, such as STATCOMs and EVs. At the same time, more communications will be needed in protection and control automation to deliver "digital-quality" power supplies sensitive to the voltage requirements of loads and processes. Distribution systems as a whole will have to be more adaptive and responsive.
The journey towards a smarter distribution utility operation will depend on standards-based solutions. The IEC 61850 suite of standards provides an attractive pathway forward, inasmuch as protection and local control functions can be integrated into a single, microprocessor-based device, which can exchange information via a high- speed data network.
IEC 61850 was originally developed exclusively for inter-substation automation and protection applications. It has since been extended to intra-substation and now incorporates protection and control of distributed energy resources. It has been identified by EPRI and NIST as one of the smart grid standards of key interest.
Particularly noteworthy is the introduction in by IEC 61850 of various elements for substation automation system (SAS) architecture. SAS can be represented in three layers with the lowest, physical layer incorporating intelligent end devices, such as circuit breakers, remotely operated switches, current and voltage sensors, and condition monitoring units for switchgear, transformers, and so on.
A capsule of data objects and functionality is called a logical node (LN) in IEC 61850. Several LNs can form a logical device, which can be implemented on a single physical device. Generic Object Oriented Substation Event (GOOSE) messages are used to model the transmission of high priority information like trip commands or, in keeping with sub part 61850-8-1 of the standards, can be used for signal interchange between intelligent electronic devices to obtain required interlock information. Status and control information can also be exchanged through other forms of lower priority messages, like GSSE (Generic Substation State Events).
The authors have proposed in print and demonstrated, on the basis of field experiences, how 61850-enabled features can be used to implement remedial actions or coordinated control scenario for distribution networks. GOOSE-enabled automatic bus transfer was implemented in the protection and control scheme of a substation where a new transformer was installed to parallel the existing transformers, demonstrating a smart strategy to manage the increased fault level.
Ongoing work includes schemes to implement real time distribution state estimation, enabling remedial actions to prevent component overloads, reconfiguring a distribution network to maximize usefulness of installed distributed generators, proposing new demand side management pricing algorithms, automated fault location and isolation, and voltage/ reactive power asset optimization.
One aspect has been intentionally left unaddressed in IEC 61850: it does not include any standardized provisions for representing complex substation protection or control schemes that involve several intelligent electronics devices. We have identified the IEC 61499 standard as one option that can fill this gap. The IEC 61499 standard describes general purpose Function Block (FB) architecture for industrial measurement and distributed control systems. The use of FBs makes the control and monitoring devices open, programmable and easily reconfigurable. IEC 61850 lacks the specification of functions, and IEC 61499 lacks "standard" communication services. However each standard can satisfy the needs of the other, creating architecture for truly flexible and adaptable power system monitoring and automation.
Implementing standards based distribution system protection and control can provide a robust implementation pathway for distribution utilities and significantly improve performance and reliability, thus delivering value on smart grid investments. By enabling both direct and indirect benefits in distribution automation, smarter protection and control will provide the stimulus for the journey from legacy to future distribution utility operation.