Current/OS Set of Rules: DC Distribution Redefined
Current/OS is a nonprofit, open partnership of electricity stakeholders and manufacturers promoting Direct Current (DC) technology. Our mission is to empower a sustainable future with reliable electricity access for all.
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Current/OS Safetyfor Seamless Operations
The Current/OS solution makes it easy, safe and cost-effective to build and control DC microgrids. It’s designed to be implemented seamlessly by any professional in the field.
at the core of the
Set of Rules
The rules specify EMC requirements, current profiles, pre-charge, disconnection for black starts, and criteria for detecting faults and avoiding nuisance tripping. They also ensure safe de-energization of microgrid sections for maintenance.
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Current/OS Certification for Universal Interoperability
Current/OS integrates distributed control to ensure seamless interoperability across different vendors and devices.
Any Current/OS certified load added to your electrical system operates intelligently and effortlessly.
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Current/OS Convergencefor Easy Global Deployment
Thanks to the Set of Rules convergence, international deployment of DC devices, is simplified, lowering costs, improving efficiency for organizations, and reducing implementation challenges.
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Current/OS Intellectual Propertyfor Flexibility and Simplicity
and Self-Regulating
Proprietary software using a Modbus-based data model enables the microgrid to optimize electrical resources and prioritize loads.
While communication can collect device data, it’s not essential for managing sources and loads, ensuring resilience to communication losses.
Developing compliant products is truly plug-and-play: Current/OS-certified loads work intelligently out of the box.
Semiconductor protection streamlines the design, and with no central control setup, installation is simple and requires no IT or automation expertise.
Current/OS DC microgrids complement the main AC power grid.
Current/OS smooths bidirectional power exchanges by reducing sharp power surges and returns typically seen with photovoltaic systems.
Current/OS Set of Rules
Current/OS Voltages: Using Band Limits as Signals
Current/OS supports multiple voltage bands, where voltage serves as a meaningful signal across all circuits. The signal informs the system about power availability, prompting circuits and devices to react accordingly.
Benefit: Standardized voltages bring interoperability within the DC community worldwide.
Voltage bands
The chart illustrates example of several voltage bands. Nominal voltage is simply a label to identify the installation. The most popular labels are 350VDC and 700VDC. Other systems with higher voltages (1400VDC) or lower (175VDC, 48VDC) are also possible.
Voltage band details
With a traditional approach, voltage is defined by a nominal value and a tolerance. With Current/OS microgrids, the nominal voltage is just a label. The key values are the voltage band limits inside which the system operates. Within these limits, voltage serves as a meaningful signal for all circuits, indicating power availability and prompting appropriate responses.
For instance, the so-called 350VDC “nominal” system will typically operate between 320VDC and 380VDC. The range of 250VDC to 320VDC is considered an emergency range, while 380VDC to 540VDC is an overvoltage range.
Current/OS devices
Core principle
A core principle of Current/OS is that the voltage level triggers the operating modes of loads and sources. Consequently, no communication system or central control is required to maintain application stability.
Voltage bands details
Current/OS Voltage: Using Voltages as Signals
Current/OS uses the voltage as a shared signal that reflects the power availability in the application to ensure the DC grid stability and energy management.
Benefit: The installation is self-regulating. Droops bring resilience, as well as convergence.
Voltage droop bands principles
Circuits respond differently to voltage changes, and these responses are defined by droop curves. Droop curves describe the power response (horizontal axis) to the voltage (vertical axis).
During commissioning, all droop curves can be adjusted to define source and load priorities. They can also be adjusted in real-time to influence the behavior of different circuits.
Oversupplied situation
Undersupply situation
Conversely, when the voltage falls below the nominal value (350VDC or 700VDC), it indicates an “undersupplied” situation. This voltage level will signal:
– Loads to deactivate according to priority settings
– Thermal storage to become inactive
– Electrical storage to supply power to electrical loads
Current/OS Electrical Protections: Defining DC Risk Categories
In order to ease design and operation of DC installations, circuits or group of circuits are classified according to the available fault energy and the protection means in 6 DC Zones, labeled as 0, 1, 2.1, 2.2, 3 and 4, described in the following sections.
IEC 60364 standards apply to all DC Zones.
Benefit: the solution is arc-free and easy to design, saving on cables and electrical equipment.
DC Zone 0 Unprotected source
Main characteristics of the circuits:
High short-circuit current source with no overcurrent protection.
Main consequences:
· High fault current.
· High incident energy in case of arc flash.
Circuits cannot be distributed but must be segregated in panels or in rooms with restricted access.
DC Zone 1 Protected source with high short-circuit power
Main characteristics of the circuits:
· Most sources have high prospective short-circuit current.
· Overcurrent protection devices (OCPD) on the DC bus have breaking time < 50ms (e.g.: EMCBs and fuses).
· Multiple sources, all connected to a common distribution board.
Main consequences:
· High fault current.
· High incident energy in case of arc flash.
· Circuits should not be distributed but segregated in panels or in spaces with restricted access.
· Design cannot be replicated and easily upgraded.
DC Zone 2.1 Protected source with low (bounded) short-circuit power
Main characteristics of the circuits:
· Sources cannot deliver a current significantly higher than its nominal current, by design.
· OCPD with breaking time < 50ms (e.g.: EMCBs and fuses) on the DC bus.
· Energy stored in capacitors does not exceed 600 J.
· Multiple sources connected to a single distribution board.
Main consequences:
· Design of protection only by skilled engineers.
· Designs difficult to be replicated and upgraded.
· Arc-flash incident energy lower than in DC Zone 1 but requiring
assessment.
· Limited system availability as selectivity is generally not possible.
· Limited design flexibility as sources cannot be distributed.
· Suitable for installations with limited complexity.
DC Zone 2.2 Protected source with low (bounded) short-circuit power
Main characteristics of the circuits:
· Multiple distributed sources.
· Sources (or inductor) that limit the fault current rise and peak.
· OCPD with breaking time < 1ms (e.g.: SCHCBs) on the DC bus.
Main consequences:
· Capacitor discharge is limited.
· Sources can be distributed.
· Reduced arc-flash incident energy.
· Selectivity is possible but difficult to design.
· Designs are replicable.
· Design upgrades require care.
· Design by skilled design offices.
· Suitable for complex installations (after design by skilled engineers).
· Additional protection possible with RCD
DC Zone 3 Electronic source(s)
Main characteristics of the circuits
· Multiple distributed sources.
· OCPD with breaking time < 10 µs (e.g.: SSCBs) on the DC bus.
Main consequences
· Ultra-limited fault currents on the DC bus
· Sources can be distributed.
· Incident energy is always very small (<<1.2 cal/cm²). Assessment is
typically non required.
· Design is limited to nominal currents. Fault calculations is not needed.
· Loop or mesh topologies are possible (and easy).
· Additional protection possible with RCD
· Mid-point earthing makes maintenance simpler.
· Designs are replicable and upgradable. Calculation of fault energy and current are not needed.
· Suitable for complex installations.
· Design at nominal is sufficient, fault conditions design is not needed.
DC Zone 4 Single electronic source
Main characteristics of the circuits:
Same as DC Zone 3 with a Single source
Main consequences
Same as DC Zone 3.
Current/OS Intertripping Wire: a simple intertripping solution
· Intertripping wire is a general fault protective solution.
· Intertripping wire can also shut down all sources around for
maintenance.
· 48V high impedence signal turns down the connected source when
shorted to 0V. It also disconnects the power feed to the grid.
Benefit: Safety and control with a simple, effective intertripping mechanism.
Current/OS Earthing/Grounding: Isolating DC microgrid from the AC grid for Stability
The founding principle of Current/OS earthing is to isolate the DC installation from the AC grid, preventing disturbances and faults from propagating between systems.
Benefit: This isolated TN-S configuration makes DC applications safer, simpler to design and more immune to external issues. By eliminating the need for continuous specialized maintenance, it is particularly well-suited for commercial and industrial applications.
Note: Earthing is commonly used in IEC standards, while Grounding is used in North American Standards
Earthing principles
The IEC 60364 standard has defined three types of Earthing Systems, namely TT, IT, and TN systems which are also recognized in many national standards.
The TN system is further subdivided into TN-C, TN-S and TN-C-S and thus we will refer to 5 types of Earthing Systems prevalent worldwide.
Current/OS Recommendation
1. Current/OS requires to separate or isolate the AC system and DC system
- To avoid faults on the DC side to be transmitted to the AC side, and vice versa.
- To select the type of system earthing on the DC side independently of the AC type.
2. Current/OS requires to apply a TN-S type of system on the DC part.
- To mitigate the risk of corrosion (versus TT or TNC mainly)
- To simplify protection against electric shock by automatic disconnection in case of line to earth fault. (versus IT system mainly)
Available Earthing Systems
for Current/OS
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