Reliable Dual DC Control Power for Protection Panels

TonyZhang
Dec 3, 2025
5 min read

Dual DC Control Power as a Non-Negotiable Baseline

In 220 kV and above substations and power plants, it is now standard practice to provide two fully independent DC systems for protection, measurement and control, communication, and bay controllers. Yet in many projects, the weakest link is not the station battery itself, but the way A/B DC is distributed and switched at the panel level. A single DC disturbance or maintenance outage on one section must never drop a protection IED or gateway offline.

To address this, ODES has developed the RUS-32B series EMC-enhanced auxiliary relay specifically for “dual DC supply + automatic transfer” at the cabinet level. It accepts two DC inputs and transparently transfers the load when one side fails, while reporting input and output status clearly to the station control system. For more information on ODES solutions across secondary systems, visit www.odes-electric.com.

Instead of ad-hoc diode OR’ing or improvised selector switches, the RUS-32B offers a deterministic, testable transfer scheme that fits cleanly into standard control circuit designs.

Typical Application Architectures

A. Protection / Control Panels with Dual A/B DC

In most 220 kV and above yards, the protection and control panels are fed from “A” and “B” DC sections. The engineering requirement is simple but strict:

Any fluctuation, fault, or maintenance on one DC section must not cause loss of power to protection, bay controllers, or merging units.
Operations and maintenance personnel must still be able to see clearly which source is healthy and when a transfer has occurred.

A common architecture is:

The RUS-32B supervises both DC inputs, performs automatic transfer when one side falls below its pickup/reset thresholds, and reports:

Input presence/failure
Output energized/de-energized state
Transfer events toward the station SCADA or to an “accident annunciation” bus.

In this way, “loss of one DC section” becomes “safe, reliable transfer + visible event,” rather than a hidden vulnerability inside the panel.

B. Applications with Explicit Main / Backup Source Priority

Some projects require that one DC source be treated as the preferred main source, with automatic re-transfer when it recovers. Examples include:

One primary station battery plus an industrial DC supply
A local DC converter used as backup only during maintenance
Segregation between “control” and “communication” DC buses with defined priority.

For this, the RUS-32B-H variant provides configurable logic such as:

Main-source priority
Maintain current source during short-term dips
Automatic or inhibited re-transfer when the main recovers.

The result is a clearly documented main/backup strategy that supports both day-to-day operation and planned DC maintenance without compromising the stability of downstream devices.

C. Safe Transfer for Higher DC Loads

In some cabinets, the DC load is significantly higher:

Multiple IEDs and communication devices on one feed
Downstream interposing relay groups
Energy storage modules or other buffered loads.

For these “heavier” DC circuits, the transfer device must offer higher making and breaking capacity, as well as terminals sized for larger conductors and defined tightening torque.

The RUS-32B-H is designed for such applications, extending:

DC breaking and continuous current ratings
Terminal cross-section range and mechanical robustness
Coverage across a wider range of primary voltage levels and load types.

This allows engineers to standardize on one auxiliary relay family from light control circuits up to more demanding DC feeders within the same panel.

Transfer Logic That Stays Stable Under Real DC Disturbances

Simply switching between two DC sources is not enough; the logic must behave well under realistic substation conditions: transient dips, brief ripple, and slow recovery of station batteries. The RUS-32B transfer logic is engineered for this environment.

1. Automatic Source Supervision with Hysteresis and Delay The relay continuously supervises both DC inputs and declares a source “failed” when its voltage falls below a defined undervoltage threshold. Recovery is not based on a single crossing; instead, the circuit applies:

Hysteresis between pickup and reset levels to avoid oscillation
Time delay to ride through short-duration dips and charging transients.

This combination provides effective transient ride-through and prevents nuisance operation while still guaranteeing fast transfer during genuine loss of DC.

2. Break-Before-Make Transfer for Safety When one source fails and the other must assume the load, the internal mechanism uses break-before-make transfer:

First disconnect the failing source from the load
Then connect the healthy source.

This prevents the two DC systems from being paralleled even for a short instant and eliminates the risk of backfeed or unintended current paths between A and B DC buses. In substations where each DC section is individually fused and monitored, this is a critical safety requirement.

3. Clear Local Indication and Remote Signalling On the output side, the RUS-32B provides:

Visual indication of which source is in service and whether the output is energized
Signalling contacts for loss of input, loss of output, and transfer events.

These contacts can be wired into the station HMI, bay controller, or an annunciation relay, enabling operators to correlate DC disturbances with protection or communication behaviour during event analysis.

4. Panel-Friendly Mechanics and Wiring

From an installation perspective, the device is designed as a standard DIN-rail mounted module:

DIN 35 mm rail mounting
Front-accessible terminals with clear, consistent terminal numbering
Documentation that includes terminal schematics and typical application diagrams.

This reduces drawing effort for design engineers and makes testing and commissioning straightforward for site teams.

Selecting Between RUS-32B and RUS-32B-H

From a practical engineering standpoint, selection can be guided by two main questions:

Is the load a typical protection/control/communication panel, or a heavier DC feeder?
Is there a defined main/backup source strategy?

Standardizing around these two variants allows EPC contractors and utilities to simplify their bill of materials while still matching the requirements of individual panels and projects.

Proven in Long-Term Utility and Industrial Service

Dual DC transfer is not a theoretical feature; it has been engineered into thousands of cabinets in transmission, generation, and industrial distribution projects over more than two decades.

Projects range from large hydropower stations and major 220 kV/500 kV substations to industrial plants and infrastructure facilities where control power continuity is critical. In these installations, the RUS-32B family has been applied as a panel-level building block to achieve:

Consistent transfer behaviour across different vendors’ panels
Repeatable testing and acceptance criteria
Easier troubleshooting when DC disturbances occur.

By treating dual DC transfer as a standardized function — rather than a custom wiring exercise — utilities gain higher system visibility and more predictable performance over the lifecycle of the asset.

Conclusion: Engineering Reliability into Every Panel

In modern protection and control systems, it is no longer sufficient to specify “two DC systems” at substation level. Reliability is decided cabinet by cabinet, where each relay, merging unit, and communication device depends on how its control power is engineered.

An EMC-enhanced auxiliary relay such as the RUS-32B/RUS-32B-H family provides a disciplined, verifiable way to:

Supervise dual DC sources with defined thresholds and delays
Perform safe break-before-make transfer between A/B DC sections
Report input, output, and transfer events to the wider automation system
Scale from light IED loads to heavier DC feeders within the same design philosophy.

By embedding this function directly into the panel design, engineers can substantially improve reliability, maintainability, and fault transparency across the secondary system.

If you are reviewing your control power concept or upgrading existing protection and control panels, ODES can help you consult on per-cabinet transfer schemes, request ready-to-use terminal maps, and learn how to standardize dual DC architectures across sites.

To contact our engineering team or request a tailored RUS-32B / RUS-32B-H application proposal (including main/backup strategy, threshold settings, and test cases), please write to:

📩 tonyzhang@odes-electric.com

You can also learn more about our solutions across protection, control, and auxiliary functions at www.odes-electric.com.