Industrial power supplies are electrical devices that convert a particular type and level of power (typically ac) to another — typically dc. These devices receive ac power through their input terminals and (through output terminals) supply dc power to connected electrical loads. Power supplies execute this task efficiently and reliably. However, they can’t independently guarantee a constant and problem-free dc power supply. Case in point: During a power failure, the power supply (if left on its own) will cease to supply dc power to its connected loads. Any fault in the power supply (or even a faulty load) can have a similarly detrimental result.
In critical industrial applications, any power failure or power-supply interruption can have severe implications. For example, the depowering of monitoring and control equipment in an oil refinery can imperil the safety of personnel. No wonder in such critical setups — commonplace in industrial settings — power supplies are often complemented by redundancy modules and buffer modules to avoid issues.
Redundancy power-supply modules
Redundant power-supply systems consist of two or more identical power supplies connected in parallel to the attached electrical load. The power supplies are decoupled (prevented from concurrently supplying current) with supplementary redundancy modules on each power supply. These redundancy modules:
- Continuously monitor all the systems’ power supplies
• Instantaneously detect power-supply faults
• Immediately bypass the faulty unit and shift to using the other supply or supplies
Such operation ensures attached loads receive constant error-free dc power.
Redundant systems may be 1+1 or n+1. 1+1 redundancy is with twin power supplies connected in parallel through a redundancy module to the load. Here, redundancy modules let power flow from only one power supply at a time — so rated current is never exceeded. So twin 5-A power supplies are needed to create a 5-A redundant system. The off-duty power supply stays off until the redundancy module detects a fault in the on-duty power supply — and then swaps their roles.
Redundancy modules are intermediary components — installing between power supplies and their connected loads.
An n+1 system follows the same working principle but use three or more identical power supplies. These n+1 systems excel where a particular amperage is required but two power supplies of the required rating are unavailable. To illustrate: Consider a system for which a 40-A redundant system is required but there aren’t 40-A supplies in the installation. Here, five 10-A power supplies can deliver that 40 A. The quintuplet connects in parallel to the redundancy module to let current flow from four of the power supplies — and leaving one redundant. Should a fault arise in any of the four active supplies, the redundancy module immediately spurs the redundant power supply into action to maintain output. Note that it’s rare that any redundancy module provision for more than two inputs — so n+1 systems typically rely on multiple electrically connected redundancy modules.
Decoupling is the process of isolating power supplies while keeping one or more supplies redundant. Redundancy modules typically perform decoupling and switching using diodes or metal oxide semiconductor field effect transistors (MOSFETs). Diode technology has been used for some time in redundancy modules and is more ubiquitous … but MOSFETS are increasingly common in new modules. The latter practically eliminate voltage drops and power losses.
Benefits of redundancy modules and redundant power systems
Redundancy modules ensure that variations in mains voltage, faulty loads, or even total ac-power failure for one power supply doesn’t totally take down the system’s ability to supply dc power. Redundant systems also improve the operational life of connected devices. In addition, using multiple redundancy modules with multiple power systems is a flexible solution. For example, an engineer can create a redundant system capable of satisfying needs for 50, 40, and 30 A using just six 10-A power supplies and three dual-input redundancy modules. Finally: Redundancy power systems allow a power supply to be taken out for repair or replacement without shutting down the entire system. This process is called hot swapping.
Limitations of power-redundancy modules and redundant systems
Redundant systems require at least twice the cost, energy, and space of using one power supply. (This is in addition to the resources required by each redundancy module for its own operation.) Where faults simultaneously arise in multiple power supplies, connected loads can be left unpowered. In addition, voltage drops and power losses (albeit minimal) may occur as power from the power supply passes through the redundancy module to the load.
Buffer modules in power-supply systems
While power failures can cause expensive damage and downtime, most only last for a fraction of a second. Buffer modules are supplementary electrical devices that address these failures by providing “bridge” dc power. In typical installations, the buffer module directly connects to the power supply and (during normal operation) stores “standby” energy in its electrolytic capacitors. Whenever the module detects a primary power failure, it releases the required power to the attached electrical load — and extending operating time for periods typically measured in milliseconds. In the case of longer power shutdowns, sufficiently sized buffer modules supporting machine controls might report the loss … and let the system save critical data — and complete processes before complete shutdown.
Buffer modules are rated for set voltages and set buffer times that depend in part on the attached load’s amperage. For example, a 24-V buffer module may last 300 msec at 20 A and as long as 650 msec at 10 A — and even to 20 sec if it only needs to supply 0.2 A. The ampacity and buffering time of a buffer module are proportional to its capacitors’ capacities. Buffer-module gangs can connect in parallel to boost output ampacity and buffer time.
To be clear: Buffer modules don’t supply continuous backup power. Rather, they provide instantaneous momentary backup in power “hiccups” lasting milliseconds.
To handle overloads, buffer modules can also deliver extra short-term peak current above the industrial power supply’s current rating. This function eliminates the need to over-dimension the power supply.
Benefits of buffer modules abound. Buffer modules ensure constant dc power supply to critical components. Plus they provide extra power in load spike conditions — eliminating the need for over-sizing of power supplies. Finally, buffer modules can for many settings eliminate the need for costlier, bulkier, and more complex UPS systems.
There are two caveats about the limitations of buffer modules, though. Buffer modules can only operate for extremely short periods. These periods are mostly sufficient but can be the origin of grievous downtime if power shutdowns occur. In addition, some buffer modules supply power at voltages slightly below their rated power. For example, a 24-V buffer module may supply power at 22.5 V … though in fact, this difference usually isn’t an issue. ⚙️ Contribution by Etiido Uko • Mechanical engineer
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Filed Under: Power supplies