DRUPS — diesel rotary UPS systems and why their short-circuit capacity differs from a static UPS

The guide on UPS battery types and the guide on UPS backup-power continuity cover the common static UPS: a rectifier, a battery bank and an inverter, built entirely from power electronics. This article covers a fundamentally different type: the DRUPS (Diesel Rotary UPS), also called a rotary or dynamic UPS — a system without batteries, with properties that matter for the protection coordination of the installation.

Operating principle: flywheel, coupling and diesel engine on one shaft

A DRUPS consists of a flywheel (kinetic energy storage), an electrical machine (acting as motor/generator) and a diesel engine, mechanically coupled on a single shaft. Under normal grid supply, the shaft rotates at synchronous speed and the electrical machine delivers filtered, uninterrupted power to the load. On grid loss, the flywheel's moment of inertia supplies enough energy for typically 10 to 30 seconds to keep the shaft at speed while the diesel engine starts and comes up to speed — after which the diesel engine drives the shaft and the supply continues without interruption.

Because there is no battery bank in the power path, a DRUPS has no battery ageing, no periodic battery replacement, and none of the thermal runaway risk that applies to lithium-ion battery banks (see the guide on thermal runaway in lithium-ion BESS) — a frequently cited advantage in high-power data-centre applications.

Short-circuit capacity: the key difference from a static UPS

This is where a DRUPS behaves fundamentally differently from a static UPS, with direct consequences for protection coordination:

A static UPS is built from power electronics (IGBTs) that, to protect themselves, typically limit their output current under a short circuit to roughly 150-300% of rated current. That can be insufficient to trip a downstream circuit breaker quickly and selectively (see the [guide on prospective short-circuit current and Icu sizing](/guides/nen-1010/prospectieve-kortsluitstroom-icu-dimensionering) for why enough short-circuit current matters for magnetic tripping to operate correctly).
A DRUPS behaves under short circuit like a rotating synchronous machine — comparable to a backup generator (see the guide on generator short-circuit capacity and protection selectivity) — and can therefore deliver a multiple of rated current, usually enough to trip conventional thermal-magnetic protective devices (see the guide on LSI settings) in the usual way.

When designing protection coordination behind a static UPS, it must therefore be explicitly checked whether the UPS's limited short-circuit current is sufficient to trip downstream breakers within the required time — a check that is generally less critical for a DRUPS because of its generator-like short-circuit behaviour.

Bypass and maintenance

Like a static UPS, a DRUPS typically has a bypass path to supply the load directly from the grid during maintenance. Some DRUPS designs use an electronic (thyristor) bypass comparable to a static UPS, others a mechanical decoupling of the shaft. Periodic maintenance, in addition to the regular NEN 3140 inspection points, focuses on the flywheel's bearings and coupling, and on a load test of the diesel engine (see the guide on backup generator load testing for the principle, also applicable to the diesel-engine part of a DRUPS).

Note: DRUPS systems are typically applied at large power ratings (from several hundred kVA upward) such as data centres and industrial continuous processes; for smaller backup-power applications, a static UPS with a battery bank is nearly always the common and more economical choice.

Practical relevance

When assessing the protection coordination behind a UPS, it is essential to first establish whether it is a static or a rotary (DRUPS) UPS — the limited short-circuit current of a static UPS can create a selectivity problem that generally does not occur with a DRUPS, due to its generator-like behaviour.

Common mistakes

Equating the short-circuit current of a static UPS with that of a grid connection or a generator when sizing downstream protective devices — the limited output current of power electronics is fundamentally lower.
Assuming a DRUPS requires the same maintenance cycle as a battery-based UPS — for a DRUPS, the points of attention are mechanical components (bearings, coupling, diesel engine) rather than battery ageing.
Not performing a load test of the diesel-engine part because the system is called "a UPS" — the diesel-engine part of a DRUPS requires the same periodic load test as a conventional backup generator.