STR-P Time Relay: Wide-Range Auxiliary Supply for Reliable Protection Timing

TonyZhang
10 hours ago
5 min read

Why Secondary Protection Circuits Need a “Durable Clock”

In secondary protection and control circuits, time coordination is not a luxury – it is part of the protection function itself.

Breaker failure logic, block / unblock sequences
Signal latching and release
Power-on delays and power-off delays
Pulse extension for relay outputs or interlocking

If the timing element is unstable, sensitive to supply fluctuations, or loses its settings, the entire scheme becomes harder to commission and harder to trust.

The STR-P time relay with auxiliary working power is designed as a long-life timing element for protection and control circuits. It stays energised, maintains its settings, and provides on-delay, off-delay and pulse-extension functions in one device – giving the secondary system a robust “clock” that is always ready.

STR-P at a Glance: A Time Relay Built for Secondary Systems

The STR-P is intended to be used as a time-delay element in secondary protection and control circuits, performing:

Power-on delay (on-delay)
Power-off delay (off-delay)
Pulse extension / output prolongation

The key design principle is:

The timing input is a galvanically isolated switching signal; the auxiliary working power is a wide-range, long-term control supply.

This separation allows the STR-P to:

Be permanently energised from the station AC or DC system
Accept start signals flexibly from protection outputs or logic contacts
Maintain its configuration and readiness even during long periods without timing operations

Wide-Range Auxiliary Working Power: Always Energised, No “Memory Loss”

A distinctive feature of STR-P is its wide-range auxiliary working power:

DC 88–370 V
AC 85–265 V, 47–63 Hz

This allows the relay to be supplied from:

Station DC systems (e.g., 110 V DC, 220 V DC)
Station service AC (e.g., 110 V AC, 220/230 V AC)

Engineering benefits:

One part number covers multiple supply scenarios – useful for mixed DC/AC environments, retrofits, and projects with varied station standards.
Continuous energisation – the relay can remain powered for long periods, keeping its settings intact and ready to respond instantly when the start signal arrives.
Better tolerance to supply variation – wide control voltage range and defined pickup thresholds reduce sensitivity to normal voltage deviations on AC/DC systems.

In other words, STR-P is designed as a permanently energised timing module that behaves predictably under real substation supply conditions.

Three Timing Modes in One Device

Rather than using different relays for different timing functions, STR-P integrates three commonly used timing modes in a single device:

Power-on delay (on-delay)
Power-off delay (off-delay)
Pulse extension / output prolongation

Having these modes available in one device allows engineers to:

Implement the majority of protection timing needs using one standard relay type
Reduce the number of different timing devices and spare parts
Simplify schematics and material lists

Hard Engineering Parameters: Contacts, Ratings and Pickup Behaviour

STR-P is built with parameters that match secondary protection practice.

Contact arrangement and rating

Two changeover contacts (2PDT / 2CO)
Contact rating: 8 A at 250 V AC or 30 V DC

This allows the relay to:

Directly drive most interposing relays and signal circuits
Provide both normally-open and normally-closed contact functions without additional devices

Timing range

Typical setting range: 0.1 s to 10.9 h

This wide span means one relay can cover:

Fast, sub-second interlocking and blocking
Medium-range delays for coordination and sequence control
Long holding functions on the order of hours for special logic requirements

Pickup voltage behaviour

The relay is designed to operate correctly in the range of approximately 55–65 % of rated control voltage (Ue)

Engineering implications:

The timing function remains reliable even when the control supply experiences moderate voltage dips or ripple.
The relay is less susceptible to nuisance dropout during transient supply variations.

Start Input: Galvanically Isolated Switching Signal

STR-P uses a switching input that is galvanically isolated via an internal relay.

Benefits:

Flexible front-end adaptation – the start signal can be taken from protection relay contacts, logic outputs, or manual switches without directly exposing them to the internal electronics.
Reduced impact on upstream circuits – the STR-P start input presents a well-defined load and isolation barrier, minimising interaction with the upper-level logic.

This architecture separates:

Auxiliary working power (long-term, wide-range supply)
Start signal (short-term, logical trigger for timing)

which is good practice for secondary system timing functions.

Typical Applications and Pain Points Addressed

1. “Metronome” for Secondary Protection Logic

In complex schemes, timing elements are used to:

Sequence protection outputs and interlocks
Manage block / unblock windows
Maintain signals for defined durations

STR-P helps by:

Providing deterministic timing behaviour with wide setting range
Keeping its auxiliary power on permanently, so it is always ready to act when a protection event occurs
Reducing the number of separate timing devices needed in a panel

2. Stable Operation Under Complex Station Supply Conditions

Substation and plant auxiliary supplies may exhibit:

AC/DC voltage fluctuations
Short-duration dips and disturbances

The combination of:

Wide-range auxiliary supply
Defined pickup characteristics (55–65 % Ue)

means STR-P is better suited to staying in service and maintaining timing accuracy under typical station conditions. This reduces:

The need to “re-teach” or frequently recheck timing during commissioning and maintenance
Nuisance behaviour caused by marginal supply conditions

3. Direct Load Driving in Secondary Circuits

With 8 A contact rating, STR-P can:

Drive many interposing relays directly
Handle typical secondary loads without an extra output stage

This keeps the timing section structurally simple, avoiding unnecessary cascade of auxiliary contacts.

Implementation Notes for Protection and Control Engineers

When integrating STR-P into a design, good practice includes:

Choosing the appropriate timing mode (on-delay, off-delay, pulse extension) based on the protection or control logic requirement.
Using both changeover contacts intelligently – one for the main controlled circuit, another for signalling, blocking or feedback to a bay controller.
Setting the timing value with margin – ensure the set time accounts for breaker operating times, communication delays and coordination requirements.
Documenting the auxiliary power and start-input wiring clearly in schematics, so maintenance teams can test and troubleshoot easily.

Conclusion: A Long-Life, Engineering-Grade Time Relay for Secondary Circuits

In secondary protection and control systems, time coordination depends on a timing element that is:

Energised and ready at all times
Tolerant of realistic AC/DC supply conditions
Flexible enough to provide on-delay, off-delay and pulse extension in one device
Mechanically and electrically robust for long-term operation

The STR-P time relay with auxiliary working power is designed to meet these needs:

Wide-range AC/DC auxiliary supply for permanent energisation
Three timing modes integrated in one relay
2PDT contacts rated 8 A at 250 V AC / 30 V DC
Timing range from 0.1 s to 10.9 h

For designers of non-electrical protection circuits and secondary logic that require stable, reliable time control, STR-P is a durable, easy-to-set, engineering-grade choice.

You can find more information on STR-P and related time relays at www.odes-electric.com.

If you are designing or upgrading secondary protection and control panels and need a long-life time relay with wide-range auxiliary supply, ODES can help you consult on STR-P application scenarios, request data sheets and wiring examples, and learn how to standardise timing functions across your projects.