Remedial Action Schemes (RAS, SPS): How They Work

What a remedial action scheme is

Some contingencies cannot be handled by equipment protection alone. If a heavily loaded line trips, power redistributes and may overload other lines, drive the system into an unstable swing or depress voltages. Building the network to withstand all of those contingencies can take years.

A remedial action scheme detects predetermined system conditions and automatically takes the corrective actions defined by studies, such as tripping a generating unit or shedding a block of load. NERC kept the term RAS (Remedial Action Scheme) and retired SPS (Special Protection System). IEEE uses SIPS (System Integrity Protection Schemes), which also includes underfrequency load shedding and undervoltage load shedding, and covers them in guide IEEE C37.250. Grid codes in other regions use their own names, such as system protection schemes or supplementary schemes, for the same idea. It is the last group of substation protection schemes.

Its parts and how they are coordinated

• Detection: breaker and disconnector status, protection trips, measurements of power flow, voltage, frequency or angle.
• Arming: logic that enables the scheme only when the pre-contingency conditions require it, for example when the flow through a corridor exceeds the studied level.
• Decision: a logic controller, in a relay, a dedicated PLC or a wide-area system.
• Communications: channels between substations, through teleprotection, transfer trip, GOOSE or wide-area networks.
• Action: generation tripping, load shedding, line opening, switching reactors and capacitors in or out, changes in HVDC links.

Logic and sequence

An example of an event-based scheme:

1. The flow through a two-line corridor exceeds the level at which losing one line would overload the other; the scheme is armed.
2. A fault takes one of the two lines out; its protection clears it normally.
3. The scheme receives the status of the open breakers and confirms the contingency.
4. It sends a trip to a predefined generating unit at the exporting end.
5. The flow through the remaining line stays within its rating.

Other schemes are response-based: they act on a system measurement (frequency, voltage, angle) rather than on a specific event.

The two types complement each other. An event-based scheme is fast, because it acts as soon as it confirms the contingency, but it only covers the contingencies that were studied. A response-based scheme covers unforeseen situations, but it has to wait for the measurement to deviate, and by then the disturbance has progressed. Many systems combine both: an event-based scheme for known critical contingencies and frequency or voltage load shedding as the last line of defence.

Relation to protection

A remedial action scheme does not replace the protection of the faulted equipment: the faulted line is cleared by its own protection, for example the main 1 and main 2 scheme of a line. The scheme acts afterwards, on the rest of the system, so that the redistribution of power does not cause further outages.

Variants by voltage level

• Extra-high voltage: wide-area schemes on interconnections and transmission corridors, with generation tripping and area separation. See area separation and islanding.
• High voltage: local schemes to evacuate generation through a limited network, such as plants connected by a single circuit.
• Subtransmission and industry: load shedding on loss of a supply, transfers and internal load shedding.

Design criteria

• Contingencies and actions come from load-flow and stability studies, reviewed whenever the network changes.
• Dependability and security: it must operate when needed and not operate when not; many are designed with redundancy and voting.
• Total time from the contingency to the action, including the channels.
• Interaction with other schemes and with the protection.
• Life-cycle management: documentation, periodic testing and change control, a central topic of IEEE C37.250.

How each manufacturer implements it

Simple schemes are built with protective relays and programmable logic from Siemens SIPROTEC (DIGSI), SEL (AcSELerator QuickSet), Hitachi Energy and ABB Relion (PCM600), GE Vernova Multilin (EnerVista) or MiCOM (MiCOM S1 Agile, Easergy Studio), with signals over teleprotection or GOOSE. Wide-area schemes usually rely on dedicated controllers, synchrophasors and their own communication networks. Each platform has its own way of programming the arming, the voting and the channel supervision.

Why it fails or misoperates

• Wrong arming: the scheme was not armed when it should have been, or it was armed on conditions that no longer apply.
• Wrong status information: auxiliary contacts that do not reflect the actual position.
• Channel down with no alarm or no redundant path.
• Network changed without reviewing the scheme: new lines or plants that alter the contingencies.
• Unforeseen interaction with another scheme or with load shedding.

How it is tested

Each relay and controller is tested by secondary injection and status simulation. The scheme is verified end to end, from detection to the action at each substation involved, with GPS-synchronised end-to-end tests, and with periodic tests as required by the system operator.

Do you need to implement or review a remedial action scheme at your substation? At Tiza Group we configure the logic, the relays and the communications remotely. See our remote relay configuration service or contact us.

References

• IEEE C37.250-2020, Guide for Engineering, Implementation, and Management of System Integrity Protection Schemes.
• NERC, Glossary of Terms Used in NERC Reliability Standards, definition of Remedial Action Scheme.
• CIGRE TB 187 (2001), System protection schemes in power networks.
• P. Kundur, Power System Stability and Control.

Frequently asked questions