With modernisation of plants, facilities and equipment, our expectations are for increased availability and reliability. Much of the control power required for these applications is often sourced via DC power supplies.

Backup plan
When uptime is critical, we need to consider the consequences of a temporary loss or extended failure of either the DC power supply or the AC power that feeds it.

Typical terms used when designing redundant power solutions are 1+1, N+1, N+N redundancy. This simply means that for N quantity of power supplies for normal operation 1 or 2 or N additional (suitably sized) power supplies are available as backup and to ensure continuous operation.

For temporary backup, buffering or uninterruptible power supplies (UPS)are used. Typically in a redundant system, two or more power supplies are switched in parallel and decoupled by one or more redundancy modules. This module prevents short-circuiting of the bus voltage if a short circuit occurs in the output side of a power supply.

Designing your redundant network
To minimise the impact of a failure, which solution best suits your requirements? The following 5 questions should assist with designing a fault tolerant DC supply network.

1. Are power disturbances common i.e. Remote location running on solar power?If so, how often and for what duration is a backup required?
2. Are alternative AC power sources available i.e. generator, AC UPDS, solar, etc.?
3. Do you require device status or alarms for monitoring, alarming and/or logging?
4. Determine how long the system needs to continue operating during a power disturbance i.e. Short term (<15 seconds) to allow a system shut down or possibly send alarm status or long term whereby full system operation is required.
5. Calculate total power requirements for critical equipment i.e. PLC, HMI, cellular router etc.

With this in mind, you can now select from one or more of the following solutions below.

Option A - Redundancy Modules

Long term bridging of two or more (with additional modules) power supplies fed from the same or ideally separate sources i.e. Mains and Generator. Select Mosfet modules for higher efficiency over diode modules. Suits loads of up to 80A or add additional modules for greater capacity.

Option B - Buffer Modules

Short-term buffering of loads up to 20A. Energy is sourced from capacitors or ultra-capacitors. Up to 15 second backup at full load. Multiple modules can be added to increase duration.

Option C - DC UPS

Long-term backup with energy sourced from batteries. Used when the alternative power source is not available. Suitable for loads up to 20A and includes various relay outputs for status/alarms.

You can also combine more than one solution to give uninterrupted power to critical devices