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HITEC DRUPS

Knowledge page · Dynamic UPS

Rotary UPS and DRUPS

How a rotary UPS and a diesel rotary UPS (DRUPS) work, what to examine before you choose, and all current models from HITEC, Piller and Active Power.

3,600 kVAper machine
97%efficiency
2–3 suntil the engine runs
See all modelsFAQ

Updated · Norwerk

01

Understand the technology

What a rotary UPS is, how a DRUPS works and how much time the rotating energy gives.

In short

What is a rotary UPS and a DRUPS?

A rotary UPS stores energy in a rotating mass – typically a flywheel – instead of in batteries. When the grid fails, the rotating energy keeps the load supplied without interruption.

A DRUPS is a rotary UPS with a built-in diesel engine. The engine starts while the flywheel carries the load and then takes over the supply as long as there is fuel.

The technology is used mainly in data centres, hospitals, airports and industry, where UPS and standby power are combined in one machine. Designs vary between manufacturers, so clutch, energy store, controls and cooling must be assessed for the specific system.

Energy storeFlywheel – some models also battery
Rating per machineApprox. 500 kW to 3,600 kVA
Ride-throughSeconds to a couple of minutes
EfficiencyUp to 97–98% according to manufacturers

Types

Three types of rotary UPS

The term rotary UPS covers three basic types. The difference lies in where the energy comes from when the grid is lost, and what takes over if the outage lasts.

Active Power CleanSource HD

Flywheel UPS

Active Power CleanSource HD

A flywheel supplies power for up to approx. 2 minutes while a separate generator starts.

2 minride-through, load dependent
HITEC PowerPRO3600

DRUPS

HITEC PowerPRO3600

Flywheel, diesel engine and generator on one shaft. The flywheel bridges until the engine runs.

3,600 kVAper machine at 60 Hz
Piller UNIBLOCK UBTD+

Separate energy store

Piller UNIBLOCK UBTD+

Flywheel or battery connected electrically to the machine, with built-in or external engine.

40 MWin parallel systems

Operation

How a DRUPS works

In normal operation the grid drives the machine as a motor and keeps the energy store spinning. Together with the choke, the machine stabilises the supply and damps voltage dips and harmonic currents.

When the voltage is lost or leaves its limits, the stored energy keeps the generator running while the diesel engine starts and engages. With mtu, the kinetic energy also helps the engine up to speed quickly, so the load does not notice the transfer.

  1. 1Normal operationSupplies the load: Grid

    The grid drives the synchronous machine as a motor. The energy store is kept up to speed, and the choke filters the supply.

  2. 2Mains failureSupplies the load: Kinetic energy

    The system detects the fault, disconnects the grid and supplies power from the rotating mass. The load sees no interruption.

  3. 3Engine startSupplies the load: Kinetic energy

    The diesel engine starts, accelerates and engages via the clutch while the energy store still carries the load.

  4. 4Island modeSupplies the load: Diesel

    The engine delivers the mechanical power. The system runs as long as there is fuel and recharges the energy store at the same time.

  5. 5Mains returnSupplies the load: Grid

    When the grid is stable, the system synchronises and transfers the load back. The engine cools down and stops.

GridKinetic energyDiesel
The operating sequence in five phases. The colour shows which energy source supplies the load.
0 %25 %50 %75 %100 %125 %0 %25 %50 %75 %100 %From 100% to 90% speed:19% of the energy is releasedRotational speed in % of nominalStored energy
0 %25 %50 %75 %100 %125 %0 %25 %50 %75 %100 %From 100% to 90% speed:19% of the energy is releasedRotational speed in % of nominalStored energy
Stored energy rises with the square of the speed. The band shows the energy from 100% to 90% speed.

Kinetic energy

How much ride-through do you get?

The rotating mass stores kinetic energy. The amount depends on the moment of inertia J and the angular velocity ω:

E = ½ · J · ω²

Because speed is squared, small changes in speed matter a lot. HITEC, for example, states 25% more energy in the latest module for the PowerPRO3. But not all the energy can be used: the generator must keep voltage and frequency within the load’s limits. If speed falls from 100% to 90%, only 19% of the energy is released.

Ride-through depends on

  1. 01The load’s power and power factorThe larger the load, the faster the speed drops.
  2. 02Size and speed range of the energy storeDetermines the energy that can actually be used.
  3. 03Load steps during engine startDraw extra energy during the transfer.
  4. 04Engine start and engagement timeThe time the energy store carries the load alone.
  5. 05Controls and settingsE.g. delayed engine start for short outages.
Ride-through is not one numberAsk for documentation for the specific configuration and load – including outages during a load step.
Handled by the energy store when the diesel start is delayed20810–100 ms1820.1–0.5 s620.5–1 s181–3 s93–20 s820–60 sDuration of mains failure
Failures per year10–100 ms2080.1–0.5 s1820.5–1 s621–3 s183–20 s920–60 s8
Mains failures per year by duration in an overhead-line network. Source: Piller, UNIBLOCK UBTD+ brochure. Figures vary between networks.

Mains disturbances

Fewer diesel starts with longer ride-through

Most mains failures last less than one second. If the diesel engine starts on every disturbance, it runs many short, unnecessary starts – with wear, fuel use and emissions as a result.

A DRUPS with a large energy store can delay the engine start and let the store handle the short failures. Piller states that the diesel start delay on the UNIBLOCK UBTD+ can be set from seconds to minutes.

So don’t just ask how fast the engine starts – ask how often it has to.

93 %of the failures in the example last under 1 second
sec–minadjustable start delay (Piller UBTD+)

Energy storage

Flywheel or battery?

The ride-through energy can come from a flywheel or – on some designs, such as Piller UNIBLOCK UBTD+ – from batteries. The choice shifts space, efficiency and the number of diesel starts.

FlywheelBattery
SpaceCompact; no air-conditioned battery roomRequires a battery room with cooling
Electrical efficiencyLower than battery – the flywheel has running lossesHigher than an equivalent kinetic store (Piller)
AutonomySeconds to a few minutesCan be sized longer – even fewer diesel starts
RechargeCan absorb energy as fast as it releases it (POWERBRIDGE)Slower; limited by the charger
Life and environmentNo battery replacement or disposalReplaced during the plant life; must be disposed of
02

Assess the design

The technical questions that separate a good DRUPS project from a datasheet.

Design

Mechanically and electrically coupled designs

In some systems the diesel engine, clutch, energy store and generator sit on one shaft. In others the energy store is a separate unit connected electrically to the machine. The difference decides how energy is transferred, how the engine engages and how the system responds to load steps and faults.

HITEC PowerPRO3600HITEC PowerPRO3600

01

Mechanically coupled

Everything sits on one shaft. Examples: HITEC PowerPRO, mtu Kinetic PowerPack.

  1. DieselEngine
  2. ClutchFreewheel or electromagnetic
  3. Kinetic energy moduleFlywheel on the shaft
  4. Synchronous machineMotor/generator
  5. LoadVia choke
Piller UNIBLOCK UBTD+Piller UNIBLOCK UBTD+

02

Electrically coupled

The energy store is a separate unit. Example: Piller UNIBLOCK UBTD+.

  1. DieselEngine
  2. ClutchFreewheel
  3. Motor-generatorThe UNIBLOCK machine
  4. LoadVia choke
Energy storeFlywheel (POWERBRIDGE) or battery, connected electrically to the machine

Mechanical shaftElectrical connection

The two basic ways to build a DRUPS.

What the project must clarify

  1. 1Which machine architecture is used?
  2. 2How is energy transferred from store to generator?
  3. 3What happens if the engine fails to start?
  4. 4Can the system provide UPS function during engine service?
  5. 5Which operating modes and bypass options exist?
  6. 6How are mains return and parallel operation handled?

Load

Load steps, harmonic currents and fault current

The load in a data centre or factory changes quickly when UPS modules, chillers, pumps and motors start and stop. Three issues need particular attention.

46 Hz47 Hz48 Hz49 Hz50 Hz51 Hz0 s1 s2 s3 s4 s5 s6 sLoad stepLarge step at onceSame load in smaller stepsTime after mains failure in island mode
46 Hz47 Hz48 Hz49 Hz50 Hz51 Hz0 s1 s2 s3 s4 s5 s6 sLoad stepLarge step at onceSame load in smaller stepsTime after mains failure in island mode
Illustration: a large load step in island mode causes a deeper frequency dip than the same load in smaller steps. Figures are illustrative.
  1. 01

    Load steps

    A large, sudden load step makes voltage and frequency dip before the engine and controls have stabilised.

    Ask for: Largest single load, motor starts, transformer energisation and simultaneous restart.

  2. 02

    Harmonic currents

    Non-linear loads stress generator, cables and components. The choke and rotating machine can damp them.

    Ask for: Measured or calculated performance for your load.

  3. 03

    Fault current

    The generator’s fault current differs from the grid’s. Protection and selectivity must work in both grid and island mode.

    Ask for: A protection study for both operating modes.

Power quality

A filter that works both ways

The load is supplied through a choke and a synchronous machine. This damps sags, spikes and harmonics from the mains – and keeps the load’s own harmonics away from the mains. Piller points out that the UBTD+ has no power capacitors in the supply path.

MAINSCRITICAL LOADChoke+ synchronous machineThe load’s harmonics are kept away from the mainsSags and harmonicsClean sine
MAINSChoke+ synchronous machineCRITICAL LOAD
down to 50%voltage sags handled without disconnection
−30 %brownout handled without using the energy store
approx. 99%harmonic isolation between mains and load
near unitypower factor seen from the mains

Figures: Piller for UNIBLOCK UBTD+. Require the values documented for the specific machine.

Selectivity

Fault current that makes the breaker work

When a feeder short-circuits, the nearest breaker must trip while the rest of the plant keeps running. That requires enough fault current. A synchronous machine has low sub-transient reactance and can deliver fault current similar to a normal supply transformer. A static UPS inverter limits the current to protect itself and typically transfers to bypass on large faults.

Selectivity must still be documented in a selectivity study for the specific plant.

DRUPSStatic UPSFeeder 1RunningVoltage dipFeeder 2RunningVoltage dipFeeder 3TrippedVoltage dip
High fault current: the breaker trips only the faulty feeder.The inverter limits the current – the breaker may not trip in time.

Case · Piller

Toulouse-Blagnac Airport

Runway lighting is supplied through several step-up and step-down transformers that the UPS alone must energise. The system must therefore deliver high inrush currents and a high short-circuit current so selectivity holds in all operating modes. The airport chose Piller’s electrically coupled UBTD+ over mechanically coupled alternatives for a larger energy reserve, better efficiency and lower maintenance cost, according to Piller.

1.0 MWPiller UNIBLOCK UBTD+
EASArunway lighting requirements
0batteries in the energy store
Piller UNIBLOCK UBTD+

Transfers

Mains failure and re-connection

The system must detect the fault, isolate the installation from the grid and keep the load supplied while the engine starts. Mains return is equally important: the system must check grid quality, synchronise and manage the load transfer.

12345The load is suppliedwithout interruption
  1. 1
    Grid operationThe grid is within limits and supplies the load.
  2. 2
    DisconnectionThe fault is detected and the grid disconnected.
  3. 3
    Island modeThe diesel engine carries the load.
  4. 4
    Grid checkVoltage and frequency must be stable for the set time.
  5. 5
    SynchronisationPhase, frequency and voltage are matched and the load transferred back.

What the design must describe

  1. 1Which deviations trigger the transfer to standby power?
  2. 2How is back-feeding into the disconnected grid prevented?
  3. 3Which deviations can the load see during the transfer?
  4. 4How do synchronisation and re-connection work?
  5. 5What happens with repeated or unstable outages?
Delayed engine startHITEC and Piller can delay engine start for short disturbances, so the engine does not start at every small dip. Settings must match your distribution system and load pattern – a datasheet cannot stand alone.

Redundancy

Where are the common failure points?

Several DRUPS units give capacity and redundancy – but only if a fault in a shared component cannot hit them all. Otherwise the component is a single point of failure.

N+1 only gives the intended reserve if the remaining units can carry the entire load without exceeding their limits. Test it in both functional testing and scenario analysis.

DRUPS 11,000 kWDRUPS 21,000 kWDRUPS 31,000 kWDRUPS 4ReserveShared dependency
DRUPS 11,000 kWDRUPS 21,000 kWDRUPS 31,000 kWDRUPS 4ReserveShared dependency

One fault here can take out all four units – the reserve does not help:

Shared fuel tankShared busbarShared controlsSame room and fire compartment
N+1 with four 1,000 kW units.
Shared dependencyFuelContaminated fuel stops every engine.
Shared dependencyCooling and ventilationOverheating during a long outage.
Shared dependencySwitchboards and busbarsOne fault can interrupt the whole system.
Shared dependencyControls and communicationA software fault hits every unit.
Shared dependencyRoom and fire compartmentFire or water takes out everything.
Shared dependencyMaintenance bypassNo protection during service.

Configurations

Four ways to connect several machines

One DRUPS is rarely enough. The configuration decides how redundancy, fault current and maintenance fit together – and how large the plant can become.

  1. Parallel redundant

    The units share a common bus. At low voltage, simple paralleling is limited to approx. 5 MW (Piller).

  2. Isolated redundant

    Each unit has its own load. A reserve unit normally runs without load and takes over automatically if a unit fails – without going to bypass.

  3. Isolated parallel (IP)

    The units are connected to a common IP bus through chokes. Gives large N+1 systems without high fault currents and can be maintained in operation. Piller: up to 40 MW.

  4. Dual output

    The same machine supplies both no-break to the critical load and short-break to e.g. cooling and lighting – like a built-in standby generator.

HITEC PowerPRO3

Building

Ventilation, cooling, exhaust and fuel

A DRUPS gives off heat all the time because machines and energy store rotate. In standby operation, engine heat, combustion air and exhaust are added. Ventilation is therefore part of security of supply: if the room gets too hot, performance can drop precisely during a long outage.

HVO HITEC PowerPRO3 and mtu Kinetic PowerPack are approved for HVO to EN 15940 with no loss of performance, according to the manufacturers.

12345
  1. 1
    Cooling air inNormal and diesel operation
  2. 2
    Combustion airDiesel operation only
  3. 3
    Warm air outLosses from machines and engine
  4. 4
    ExhaustExhaust pipe and silencer
  5. 5
    FuelDay tank, main tank, filtration, pumps, alarms and refuelling
NeedNormal operationDiesel operation
CoolingLosses from machines and energy storeFar more heat from engine and generator
AirCooling airCooling and combustion air
ExhaustNone – the engine is at restCorrectly sized exhaust routing
NoiseLowHigh – silencing and vibration isolation
FuelFuel determines actual reliability: quality, water and particles, filtration, pumps, alarms and refuelling access – not just tank size.
03

Size and compare

From critical load to installed power – and how DRUPS compares with a static UPS and generator.

Sizing

Example: 2.4 MW critical load

An illustrative example, not a specific project: an installation has a 2.4 MW critical load. With 25% room for growth the system must deliver 3.0 MW – also when one unit is out.

2.4 MWcritical load today
3.0 MWwith 25% growth
4 × 1.0 MWN+1 – 3.0 MW left on failure
0 MW1 MW2 MW3 MW4 MW2.4 MWCritical loadtoday3.0 MWWith 25% growth4.0 MWInstalled (4 ×1.0 MW)3.0 MWRemaining afterone fails
0 MW1 MW2 MW3 MW4 MW2.4 MWCriticalloadtoday3.0 MWWith 25%growth4.0 MWInstalled(4 × 1.0MW)3.0 MWRemainingafter onefails
Installed power versus the power left when one unit is out.
StepTopicQuestion
1Continuous loadWhat power must be supplied at all times?
2Load stepsWhich motors, chillers or transformers switch in suddenly?
3GrowthHow much capacity is expected to be added?
4RedundancyMust one unit be removable without interrupting the load?
5Run timeHow long must the system run on fuel?
6Failure scenariosLoss of a unit, switchboard section or shared auxiliary system?
A 2.4 MW load is not a 2.4 MW DRUPSInstalled power is not the same as the power left when the largest unit is out. The final choice requires a load analysis, single-line diagram, protection study, ventilation calculation and testing of the operating sequences.

Comparison

DRUPS or static UPS with diesel generator?

The alternative to DRUPS is typically a static UPS with batteries and a separate diesel generator. Read more about static systems in All about UPS systems at Bluecircle.

HITEC PowerPRO3600
One machineDRUPS
vs
Diesel generator
Several unitsStatic UPS + generator
Mains failureDRUPSFlywheelDieselStatic UPS + generatorBatteriesDiesel0SecondsMinutesHours
Mains failureDRUPSFlywheelDieselStatic UPS + generatorBatteriesDiesel0SecondsMinutesHours
What carries the load after a mains failure – illustration, not to scale.
DRUPSStatic UPS + generator
Seconds from flywheel, then dieselAutonomyMinutes from batteries, then diesel
One machine in the power chainComponentsUPS, batteries, generator and switch
Often none – check the configurationBatteriesMust be monitored and replaced
Approx. 200 kVA/m² (HITEC PowerPRO3)SpaceTypically more for UPS, batteries and generator
Rotating parts and engineMaintenanceElectronics, batteries and generator separately
Large ratings from approx. 500 kWPower rangeFrom small to large ratings

Manufacturers highlight high power, integrated standby power and fewer batteries. What matters is whether the documented properties fit your load, operating strategy and maintenance requirements.

Total cost of ownership

The losses cost more than the machine

Piller writes that in most TCO calculations the electricity cost of losses is the largest item – larger than purchase and maintenance. Losses must also be cooled away, which adds further consumption.

Loss = P × (1/η − 1)

Example: 2 MW load around the clock. The difference between 97% and 94% efficiency is approx. 576 MWh a year – before cooling. Multiply by your electricity price.

04008001,20054297 %73096 %92295 %1,11894 %MWh loss per yearEfficiency
MWh loss per year97 %54296 %73095 %92294 %1,118
Norwerk’s calculation. Efficiency must be measured at your actual load level.
04

Choose and operate

Where DRUPS makes sense, all current models – and how to keep the system ready.

Applications

When is a DRUPS relevant?

When the installation has high power, critical loads and a need for both seamless transfer and long standby power.

Data centres
Data centresLarge, continuous IT loads where space and efficiency count.
Industry
IndustryProcesses where even short voltage dips cost production.
Hospitals and airports
Hospitals and airportsSafety-critical systems such as runway lighting and operating theatres.

Go ahead with a DRUPS analysis if

  1. The load is large or has significant motor starts.
  2. An integrated rotary solution fits the building’s supply.
  3. Battery-free energy storage is a requirement or advantage.
  4. Space, cooling and maintenance can be designed realistically.
  5. Redundancy and fault-handling requirements can be met.
All five metDRUPS analysis

Products

All current models

Four selected systems followed by the full range from HITEC, Piller and Active Power. Filter by manufacturer and type.

Latest generation · HITEC

HITEC PowerPRO3

New kinetic energy module with 25% more energy, Cummins G-Drive engine and approved for HVO. Succeeds the PowerPRO2700 in the same footprint.

2,500 kVAat 50 Hz
97%efficiency
35 kVup to medium voltage
See PowerPRO3
HITEC PowerPRO3600

HITEC

PowerPRO3600

3,600 kVAat 60 Hz
View system →Piller UNIBLOCK UBTD+

Piller

UNIBLOCK UBTD+

500–2,700 kWper machine
View system →Active Power CleanSource HD

Active Power

CleanSource HD

98%efficiency
View system →

15 products

HITEC PowerPRO3
Price on request

HITEC PowerPRO3

DRUPS · Up to 2500 kVA (50 Hz) / 3000 kVA (60 Hz)

  • Power 50 Hz: 2500 kVA / 2000 kW (50 Hz)
  • Power 60 Hz: 3000 kVA / 2400 kW (60 Hz)
  • Efficiency: Up to 97 %
View system
HITEC PowerPRO3600
Price on request

HITEC PowerPRO3600

DRUPS · 2500–3000 kVA (50 Hz) / 2500–3600 kVA (60 Hz)

  • Power 50 Hz: 2500 / 2850 / 3000 kVA @ 400 V 50 Hz
  • Power 60 Hz: 2500 / 2850 / 3600 kVA @ 480 V 60 Hz
  • Efficiency: Up to 97 %
View system
Piller UNIBLOCK™ UBTD+
Price on request

Piller UNIBLOCK™ UBTD+

DRUPS · 500–2700 kW per machine

  • Power: 500–2700 kW
  • Parallel: Up to 40 MW
  • Efficiency: Up to 97 % with energy store connected
View system
Piller UNIBLOCK™ UBT+
Price on request

Piller UNIBLOCK™ UBT+

Rotary UPS · 500–2700 kW per machine

  • Power: 500–2700 kW
  • Parallel: Up to 40 MW
  • Efficiency: Up to 97 %
View system
Piller UNIBLOCK™ UB-V Series
Price on request

Piller UNIBLOCK™ UB-V Series

Rotary UPS (electrically coupled) · 1.0–3.24 MW (1.1–3.6 MVA) per unit

  • Power: 1.0–3.24 MW / 1.1–3.6 MVA
  • Parallel: Up to 100 MW / 115 MVA
  • Efficiency: Up to 98 % at 100 % load, 97 % at 50 % (IEC 62040-3)
View system
Piller CPM 300 / 360
Price on request

Piller CPM 300 / 360

Flywheel UPS · 300/360 kW modules, up to 2400/2880 kW

  • Power: 300 / 360 kW per module
  • Parallel: Up to 8 units (2400/2880 kW)
  • Efficiency: Above 96 % at 25 % load, up to 98.5 % (IEC 62040-3)
View system
Piller POWERBRIDGE™
Price on request

Piller POWERBRIDGE™

Energy store · Up to 60+ MJ per unit

  • Energy: Up to 60+ MJ per unit
  • Power / time: Above 3 MW; 1 MW for more than 60 seconds
  • Space: Up to 90 % less floor space than batteries
View system
Piller DeRUPS™
Price on request

Piller DeRUPS™

Configuration · UBT+ with external gensets

  • Genset: Free choice of engine and supplier
  • Count: Number of gensets ≠ number of UPS
  • Loads: Supplies both short-break and no-break loads
View system
Piller Isolated Parallel (IP) System
Price on request

Piller Isolated Parallel (IP) System

Configuration · Low voltage up to 20 MW

  • Certification: Tier IV Scheme Approved (Uptime Institute)
  • Efficiency: Modules near optimum efficiency (c. 97 %)
  • Size: Low voltage up to 20 MW
View system
Active Power CLEANSOURCE® HD
Price on request

Active Power CLEANSOURCE® HD

Flywheel UPS · 625 kW (50 Hz) / 675 kW (60 Hz), up to 5400 kW

  • Power: 625 kW @ 380/400/415 V 50 Hz
  • Parallel: Up to 5400 kW
  • Efficiency: Up to 98 %
View system
Active Power CLEANSOURCE® PLUS SMS
Price on request

Active Power CLEANSOURCE® PLUS SMS

Flywheel UPS · 300 kW (50 Hz) / 333 kW (60 Hz)

  • Power: 300 kW @ 380/400/415 V 50 Hz
  • Parallel: Up to 2667 kW
  • Efficiency: Up to 98 %
View system
Active Power CLEANSOURCE® PLUS MMS
Price on request

Active Power CLEANSOURCE® PLUS MMS

Flywheel UPS · 300–2400 kW (50 Hz) / 333–2667 kW (60 Hz)

  • Power: 300–2400 kW @ 50 Hz
  • Building blocks: 333 kW building blocks (60 Hz)
  • Efficiency: Up to 98 %
View system
Active Power CLEANSOURCE® XT
Price on request

Active Power CLEANSOURCE® XT

Flywheel UPS · 225–1800 kW (50 Hz) / 250–2000 kW (60 Hz)

  • Power: 225 kW (SMS) / 225–1800 kW (MMS) @ 50 Hz
  • Energy: 6.2 MJ per flywheel
  • Efficiency: Up to 98 %
View system
Active Power POWERHOUSE®
Price on request

Active Power POWERHOUSE®

Containerised solution · 333–2667 kW in a 40 ft container

  • Power: 333–2667 kW (standard 1333 kW)
  • Redundancy: N+1 up to 2333 kW
  • Ambient: −25 to 50 °C
View system
Active Power GenSTART™
Price on request

Active Power GenSTART™

Accessory · 1725 A cold cranking amps

  • Cranking: 1725 A cold cranking amps
  • Mounting: Floor or wall mounting
  • Disconnect: Integrated AC disconnect
View system

Images: manufacturers’ product photos.

6 MW load bank test

Service

Testing and maintenance

A short test that the engine starts does not prove that the standby function works. The test must show the whole sequence from mains failure to stable island mode – and back – under real load.

  1. 01Rotating machinesBearings, lubrication, vibration and temperature
  2. 02Diesel engineStart function, starting supply and preheating
  3. 03FuelQuality, water, filters, pumps and alarms
  4. 04ControlsAlarms, event log and remote monitoring
  5. 05Operating sequencesMains failure, island mode, synchronisation and return under load
  6. 06Load sharingSharing between units and response to load steps
  7. 07ProtectionSelectivity in both grid and island mode
  8. 08BuildingCooling, ventilation and exhaust in operation

Bluecircle carries out commissioning, load bank testing and UPS audits – and has a guide to DRUPS.

Standard

Specify to EN 88528-11

Static UPS systems are tested to IEC 62040-3, which does not cover rotating machines. For rotary UPS – including those with a diesel engine – IEC/EN 88528-11 applies, last confirmed in 2026. The standard describes what the manufacturer must declare and how it is tested.

Performance class
Limits for voltage and frequency deviations on load steps and mains failure.
Filter properties
Measured in both directions: from mains to load and from load to mains.
Stored energy
The time the energy store can carry the load.
Efficiency
Measured by the standard’s method – not just as a catalogue value.
Multiple modules
Performance when several machines work together.
Black start
That the plant can start without mains.
HITEC PowerPRO3

Conclusion

DRUPS must be assessed as a complete system

A DRUPS is more than a diesel engine coupled to a generator. It is an integrated power solution where energy store, machines, engine, controls, switchboards, protection and building must work together.

Ride-through, load steps, transfers, selectivity, redundancy, ventilation and fuel cannot be answered by a brochure. Start with the load profile, failure scenarios and operating requirements – and follow them through design, testing and maintenance.

FAQ

Questions and answers about rotary UPS

Can’t find the answer? Give us a call.

+45 50 53 13 17
What is the difference between a rotary UPS and a DRUPS?

A rotary UPS stores energy in a rotating mass and supplies power for seconds to a couple of minutes. A DRUPS is a rotary UPS with a built-in diesel engine, so the same machine also provides standby power for hours or days.

How long is the ride-through of a DRUPS?

It depends on the energy store, the load and how far the speed may drop. Manufacturers typically state seconds for systems with a flywheel on the shaft and up to minutes for flywheel UPS and systems with batteries. Ride-through must be documented for the specific configuration and load.

Does a DRUPS have batteries?

Many DRUPS systems are battery-free and use kinetic energy only. Some designs, such as Piller UNIBLOCK, can be ordered with batteries instead of or in addition to a flywheel, so check the specific configuration.

How efficient is a rotary UPS?

HITEC states up to 97% for the PowerPRO3, and Active Power states up to 98% for the CleanSource HD flywheel UPS. Actual efficiency depends on load level and operating mode and should be calculated for your load profile.

When is DRUPS better than a static UPS with a diesel generator?

DRUPS is especially relevant at large ratings – the machines start at approx. 500 kW – where space, battery replacement and component count weigh heavily. For smaller systems or long battery autonomy, a static UPS with a separate generator is often simpler. Compare the whole power chain, not just the machines.

Can a DRUPS run on HVO?

Yes. Several manufacturers have approved their DRUPS for HVO to EN 15940, including HITEC PowerPRO3 and mtu Kinetic PowerPack. Check the approval for the specific engine.

How is a DRUPS maintained?

Maintenance covers the rotating machines, bearings and lubrication, the diesel engine, fuel, cooling, ventilation, controls and protection. The whole sequence from mains failure to island mode and back should be tested under load.

Who manufactures DRUPS?

Manufacturers include HITEC Power Protection (PowerPRO), Piller (UNIBLOCK UBTD+) and Rolls-Royce (mtu Kinetic PowerPack). Piller also owns Active Power, maker of the CleanSource flywheel UPS.

How many diesel starts does a DRUPS make?

It depends on the grid, the energy store and the start delay. Most mains failures last under one second, so a plant that can delay the engine start handles many disturbances without a diesel start. Piller states that the start delay on the UNIBLOCK UBTD+ can be set from seconds to minutes.

Which standard applies to rotary UPS?

IEC/EN 88528-11 sets out performance requirements and test methods for rotary UPS, including diesel-coupled units. IEC 62040-3 applies to static UPS and does not cover rotating machines.

Can a DRUPS supply fault current for selectivity?

Yes. The synchronous machine has low sub-transient reactance and can deliver high fault current, so the breaker on the faulty feeder can trip. Selectivity must be documented with a study for the specific plant.

How long can a DRUPS run without mains?

As long as there is fuel and the engine is cooled and ventilated. The run time is set by the tank size and consumption at the actual load and must match the requirement in the contingency plan.

Read more at Bluecircle

Norwerk’s partner Bluecircle has in-depth pages on the topic.

DRUPS – diesel rotary UPS

All HITEC PowerPRO models reviewed

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Load bank rental

Testing UPS and generator plants

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Commissioning

Commissioning of data centres

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Service plans

Response time and preventive service

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Case: Google, Fredericia

Load bank test of generator plant

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Do you have a project or a system that needs service?

Send us the load, required autonomy and voltage level – or describe the task. We will come back with a solution sized for your operation.

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