DC Distribution Board: Rectifier, Battery Bank and 110/220 VDC Supply
DC distribution board: A DC distribution board, together with the rectifier and battery bank, forms the uninterrupted auxiliary supply that feeds breaker tripping, protection relays, control and monitoring circuits even when the AC network fails. Sizing follows continuous load, momentary tripping surge and the required autonomy.
Contents
When the main supply of a substation is lost, protection relays must still operate, breakers must trip and events must be recorded. The system that secures these duties is the direct current auxiliary supply, working independently of the AC network. If the trio of rectifier, battery bank and DC distribution board is not sized correctly, the protection chain silently goes out of service exactly when a fault occurs.
Why a DC auxiliary supply is required
Protection relays, breaker trip and closing coils, interlocking circuits, indication lamps, remote monitoring units and communication devices all need a dependable auxiliary supply. If that supply is taken from the AC network, the voltage can collapse during a fault and the trip command may never reach the breaker. A DC system provides energy independently through the battery bank; 24, 48, 110 and 220 VDC are the most common levels, selected according to distance and load structure.
Sizing the rectifier and the battery bank
Sizing consists of three components: continuous load, short-time surge load and autonomy. The continuous load covers relay, monitoring and indication consumption, while the surge load covers the coil current of breakers tripping simultaneously and is very high for a few seconds. Autonomy is usually selected between one and eight hours and guarantees a safe shutdown before the network returns. The rectifier must feed the continuous load while charging the battery, so its rated current follows the sum of both components.
Battery technologies and selection criteria
VRLA lead-acid batteries offer low initial cost and easy availability, but they are temperature sensitive and have a shorter service life. Nickel-cadmium batteries tolerate wide temperature ranges and deep discharges, last longer and are preferred in critical substations. Lithium-ion solutions save space and weight, but the battery management system, the fire scenario and the communication protocol must be part of the design. The table below compares the three technologies.
DC board design and selectivity
| Criterion | Lead-acid (VRLA) | Nickel-cadmium | Lithium-ion |
|---|---|---|---|
| Typical service life | 5 – 10 years | 15 – 20 years | 10 – 15 years |
| Temperature tolerance | Narrow, life shortens with heat | Stable over a wide range | Moderate, climate control preferred |
| Initial investment | Low | High | Medium to high |
| Space and weight | High | High | Low |
| Maintenance and monitoring | Annual capacity test | Electrolyte and level checks | Battery management system required |
The first point in a DC board is that direct current has no natural zero crossing, which makes arc quenching difficult; protective devices are therefore selected for their DC rating and connected with the number of poles given in the manufacturer table. Circuits are separated by criticality: tripping, control and monitoring circuits are fed through separate fuses. Selectivity is built by combining device curves with cable resistance, and the prospective short-circuit current is derived from the internal resistance of the battery.
Insulation monitoring, maintenance and testing
Because DC systems usually run unearthed, a single earth fault does not cause an immediate interruption, but a second fault creates a short circuit. An insulation monitoring device is therefore kept permanently in service and its alarm is routed to the monitoring system. Maintenance covers terminal torque checks, internal resistance measurements and an annual capacity test; the charging voltage of the rectifier is verified together with temperature compensation and alarm contacts are proven under real conditions.
Key takeaways: DC distribution board
- Sizing — Continuous load, breaker trip surge and autonomy are calculated together.
- Rectifier capacity — Rated current follows the sum of continuous load and battery charging current.
- DC protection — Devices are chosen for their DC rating and wired with the specified pole arrangement.
- Insulation monitoring — In an unearthed DC system the first earth fault is caught early by the monitor.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
How is the choice made between 110 VDC and 220 VDC?
220 VDC is advantageous on long tripping circuits because it reduces voltage drop. In smaller installations with short cable runs, 110 VDC is sufficient and reduces the number of battery cells.
How many hours of battery autonomy are needed?
It depends on the criticality of the site. One to two hours is enough where the network returns quickly, while substations without a generator or with difficult access often require up to eight hours.
Can an AC circuit breaker be used in a DC board?
Only if the manufacturer also publishes DC ratings and the specified pole connection is used. Because direct current has no zero crossing, selecting a device by its AC label leads to insufficient arc quenching.
Related articles
- UPS and Critical Power Distribution: Bypass, STS, Dual Feed
- Protection Relays and ANSI Codes: 50/51, 27/59 and 87 Functions
- What is a Medium Voltage (MV) Switchgear? Metal-Clad and RMU Switchgear.
- Battery Energy Storage (BESS) Panel: DC Protection, PCS Interface and Safety
- All technical articles
Let us size the DC auxiliary system of your substation together with the load list, autonomy target and battery technology; Devpan builds rectifier and DC distribution boards project by project. Related Devpan solution: DevPanel main distribution board.
