Designing Intelligent Load Prioritisation in DC Systems
Introduction
In critical infrastructure such as utilities, transport, water and wastewater, power generation, and industrial facilities, the reliability of the DC system often defines the reliability of the entire site. During normal operation everything appears stable, but extended outages reveal the system’s true design strength. When battery autonomy begins to fall after a long fault, severe weather event, or generator failure, it becomes immediately clear whether the system was engineered with proper load shedding and prioritisation in mind.
For many operators, the challenge isn’t that the DC system lacks capacity; it’s that every load has been wired and treated as if it is equally important. In reality, this is rarely the case. A well-designed DC system recognises that some devices are essential to safety, network visibility, communications, and protection, while others support convenience or efficiency but are not necessary for survival during prolonged events.
Intelligent load prioritisation, using structured disconnect logic, voltage thresholds, and staged reconnection, can dramatically extend uptime for critical loads when battery levels decline. This approach ensures limited stored energy is used strategically rather than being consumed by non-essential devices.
This article provides a practical, engineer-focused guide to designing effective load shedding strategies. It applies across critical infrastructure including substations, treatment plants, industrial sites, and remote facilities, and reflects the type of applied engineering we regularly support at Zyntec Energy.
Why Load Shedding Matters More Today
Extended outages are no longer theoretical edge cases. Utilities and critical service providers are facing increasing threats:
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Storms and extreme weather events
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Ageing infrastructure
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Difficulty refuelling generators during emergencies
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Higher load on backup systems
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Growing reliance on digital communications and automation
When a DC system is forced to run exclusively from batteries for an extended time, two questions become central:
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Which loads must stay alive at all costs?
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How do we ensure those loads run as long as possible?
Without a load-shedding framework, everything runs until everything dies. For critical services, this is unacceptable.
Understanding Load Categories: Critical, Semi-Critical, Non-Critical
The foundation of any load-shedding strategy is proper load grouping. A simple and consistent hierarchy is:
Critical Loads
These must remain operational as long as physically possible. They typically include:
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Protection relays
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Control systems
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Communications equipment
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SCADA / RTU
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Essential alarms
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Trip and close circuits
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Safety-related instrumentation
Loss of these loads compromises the system’s ability to monitor, control, and protect.
Semi-Critical Loads
These contribute to performance or convenience but can be sacrificed to extend autonomy. Examples:
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Redundant comms hardware
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Non-essential lighting
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Secondary monitoring
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Low-priority instrumentation
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Tech support devices
Shedding these loads early has minimal operational impact while meaningfully increasing battery run time.
Non-Critical Loads
If the system is running on battery alone, these loads do not need to remain energised. Common examples include:
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HVAC for control rooms
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Non-essential lighting
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Auxiliary power sockets
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Charging stations
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Peripheral IT equipment
These loads must be the first to disconnect automatically.
Establishing these groups ensures the system has a clear roadmap for voltage-based or time-based shedding.
Key Load Shedding Mechanisms
1. Battery Low Voltage Disconnect (BLVD)
BLVD protects the battery from deep discharge. It is essential for battery health and longevity.
In a well-engineered system, BLVD is the final threshold not the only trigger.
When designing BLVD:
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Ensure thresholds match battery chemistry
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Confirm BLVD does not drop essential control power too early
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Verify BLVD logic is compatible with upstream charger behaviour
BLVD protects the asset; it is not a load-prioritisation tool by itself.
2. Load Low Voltage Disconnect (LLVD)
LLVD modules are where true load prioritisation begins. These devices automatically disconnect specific load groups when voltage declines to preset levels.
Best practice for LLVD:
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Assign LLVD channels to non-critical and semi-critical loads
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Ensure critical loads bypass LLVD entirely or sit on final-stage LLVD
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Set clear disconnect and reconnect hysteresis to avoid chatter
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Test thresholds during commissioning, not just design
LLVD is the most cost-effective way to stretch autonomy.
3. Staggered / Tiered Disconnect Logic
Rather than dropping everything at once, the system should shed loads progressively.
Example sequence:
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Non-critical loads drop early (e.g., at 48.0V on a nominal 48V system if not at mains fail.)
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Semi-critical loads drop later (e.g., at 46.5V)
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Critical loads run until BLVD, which might be set at 44.5V
This structured approach creates a steady, controlled reduction in demand that greatly extends run time for critical equipment.
Engineers should always model the estimated runtime extension achieved by removing each tier. Even small reductions in load early in the discharge cycle can yield large runtime gains later.
4. Intelligent Reconnect Strategy
Reconnecting loads after a generator restart or grid recovery is just as important as managing the disconnects.
Without proper sequencing, all loads may reconnect simultaneously, causing:
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Voltage dips
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Breaker trips
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Charger overload
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System instability
Best practice:
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Reconnect critical loads first
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Wait for system voltage to stabilise
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Reconnect semi-critical loads after a defined voltage or time delay
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Reconnect non-critical loads last, and only when full stability is achieved
An intelligent reconnect strategy ensures a graceful return to full operation.
Examples of Good and Poor Load Shedding Design
Poor Example: Everything on a Single Bus
A facility wires all DC loads to the same distribution bus with no LLVD. The system experiences a prolonged outage.
All loads stay on full draw until battery voltage collapses.
Outcome: Total loss of communications, control, and protection.
Improved Example: Basic LLVD
The same facility implements LLVD to shed non-essential loads at 48V.
Battery life increases by 20–30%.
However, semi-critical loads remain online too long, still draining autonomy.
Best Practice Example: Fully Tiered System
A three-tier grouping is implemented:
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Non-critical at 48.0V
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Semi-critical at 46.5V
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Critical loads only disconnect at BLVD
Reconnection is sequenced after recovery.
Outcome: Critical loads remain operational for hours longer, ensuring visibility and control through the entire fault window.
This approach reflects the engineering philosophy applied frequently by Zyntec Energy when helping operators optimise autonomy.
Design Considerations for Engineers
When implementing or reviewing load shedding:
1. Identify Every Load Early in Design
Never wait until commissioning to label loads as critical or non-critical.
2. Validate Real Current Draw
Actual loads often differ from theoretical values—sometimes significantly.
3. Check the Impact of Temperature
Battery performance can vary by up to 30% based on ambient temperature.
4. Consider Worst-Case Scenarios
Assume generator failure or delayed refuelling.
5. Build in Testing Capability
Engineers should be able to simulate tiered disconnect events.
These principles form the backbone of robust DC system engineering across the industry.
Conclusion / Final Thoughts
Intelligent load prioritisation is one of the simplest and most powerful ways to increase system resilience during prolonged outages. By grouping loads thoughtfully, applying staged disconnect logic, and ensuring controlled recovery, engineers can dramatically extend battery autonomy while protecting system integrity.
Across substations, treatment plants, industrial sites, and other critical infrastructure, these strategies provide operators with the visibility and control they need at the very moment reliability matters most. This type of disciplined design work is increasingly vital as networks face greater demand, weather volatility, and operational complexity.
At Zyntec Energy, we regularly help utilities and operators design and optimise tiered load shedding frameworks that match their operational priorities and risk profiles. Thoughtful engineering at the design stage can be the difference between full visibility during an event or complete loss of situational awareness.
If you're reviewing your DC architecture or want to improve your load shedding strategy, now is the perfect time to assess how your critical, semi-critical, and non-critical loads are prioritised.
Reach out to Zyntec Energy, we’re always happy to share insights, review designs, or support your next upgrade project.
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