UPS parallel operation — N+1 redundancy and circulating current between modules

The guide on the static bypass switch covers the transfer within a single UPS unit between the inverter and the bypass mains. This guide covers a different, complementary redundancy concept that is often applied alongside it in critical installations: multiple UPS modules operating in parallel on the same output bus, so that the failure of one module does not affect the load.

What N+1 redundancy means

In an N+1 configuration, more UPS modules (or frames) are installed than strictly necessary to carry the load. If N is the number of modules that together deliver exactly enough power for the full load, one extra module (+1) stands by. If one module fails, malfunctions, or is taken out for planned maintenance, the remaining N modules continue to carry the full load without any interruption to the load. This differs from the internal redundancy of a single large UPS unit (for example with duplicated power stages): with N+1, the units are fully independent, parallel-connected UPS units.

Common configurations include, for example, 3 modules of 100 kVA each, of which 2 (N=2) are already sufficient for a 180 kVA load, with the third as a redundant spare — or larger data-centre installations with multiple frames connected via a shared, parallel output bus (often referred to as the "system bus" or parallel output bus).

Why parallel UPS modules can develop a circulating current

As soon as two or more inverters are electrically connected in parallel on the same bus, their output voltages must be exactly equal in amplitude, phase, and frequency to share purely active power. In practice there are always small, unavoidable differences between modules — due to component tolerances, measurement-chain offsets in the voltage regulation, or a slight phase error in the PWM control. Any small difference in the instantaneous value between the output voltages of two parallel-connected inverters drives a current between those inverters that does not go to the load, but circulates between the modules: the circulating current.

This current delivers no useful power to the load, but it does increase the load on the power semiconductors and internal wiring of the modules involved, and — at a significant amplitude — can reduce the effective available power capacity of each module and negatively affect component lifetime through extra losses and heating.

How modern UPS control manages circulating current

Modern, modular parallel UPS systems use a shared communication bus (often a redundant, fibre- or CAN-bus-based link between the modules) to:

• distribute active power sharing evenly between modules (load sharing), so each module carries a proportional share of the total load current instead of one module structurally carrying more than the others;
• actively suppress the circulating-current component by continuously aligning the instantaneous voltage and phase regulation of each module, rather than relying on coincidental agreement;
• fall back, if the communication bus fails, to a degraded, autonomous control mode (droop control) that shares less precisely but remains stable without the circulating current growing unchecked.

In older or simpler parallel systems without an active communication bus between the modules, droop control alone is often used: the output voltage of each module drops slightly as the current it delivers increases, which naturally forces a certain balance but is less precise than communication-bus-based control.

Typical mistakes

1. Paralleling modules from different vendors or firmware versions without explicit manufacturer approval — small differences in control algorithms between vendors increase the risk of a persistent circulating current.
2. Not recognising a defective communication bus between modules as a fault — when the bus fails, the system falls back to less precise droop control; if this goes unnoticed and unresolved, load-sharing accuracy can degrade without being directly visible on the load side.
3. Confusing N+1 with full load reduction upon failure of a module — N+1 means the remaining N modules can carry the full load; if the actual load exceeds the N capacity (for example due to load growth after the original sizing), redundancy has effectively disappeared even though the +1 module is still physically present.
4. Not keeping the output impedance and cable length between modules and the shared bus equal — asymmetric wiring between modules can by itself cause a voltage difference at the bus connection points and thereby introduce circulating current, independent of the internal regulation of the modules.

Related

• UPS static bypass switch — thyristor switching and make-before-break transfer
• DRUPS — diesel rotary UPS systems and short-circuit capacity
• Periodic inspection of a UPS installation
• Synchronising check (25) — synchronism-check relay when paralleling