Rotary UPS and Gensets

Totally flexible and high efficiency

UPS configurations

2

Contents

1 Introduction ................................................................................................................... 3

2 Characteristics of modern Rotary UPS ........................................................................ 3

3 Bridging time ................................................................................................................. 6

4 Various UPS and Diesel combinations ........................................................................ 7

5 Functional aspects of combining Rotary UPS with Gensets ...................................... 9

6 Arrangements of RUPS and Gensets ......................................................................... 12

Diesel generators located upstream of the UPS..........................................................12

Diesel Rotary UPS (DRUPS) ......................................................................................14

Rotary UPS with Diesel Generators connected downstream………………………….. 14

7 Summary ...................................................................................................................... 17

8 References ................................................................................................................... 17

3

1 Introduction

To protect mission critical facilities against power outages the installation of uninterruptible

powers supplies (UPS) is the standard method. Usually long and short term outages need to

be bridged and so UPS systems are complemented with external Diesel generators or

directly combined with a diesel engine in a Diesel Rotary UPS (DRUPS). The combination of

Rotary UPS (RUPS) and external Diesel generators is a tried and tested solution with an

installed base of approaching 1000 MVA. Because of the distinct advantages with this

combination, some of the most recent high profile and major Tier III and Tier IV data centres

use this technology. With the latest release of RUPS, the system designer can realise an

unprecedented multiple of configurations in conjunction with extremely high efficiencies even

at the lower loads often encountered in early-life data centres.

This paper focusses on the general requirements for a Diesel Generator to be combined with

Rotary UPS and shows a variety of combinations that provide an increased maintainability as

well as an extended choice of engines and can be realized in low voltage as well as in

medium voltage.

Additionally the paper describes how the Diesel generator can be integrated into the controls

of a Rotary UPS to build a powerful and multifunctional system, which is named a DeRUPS

(Diesel electrically coupled Rotary UPS). This integration allows the use of external Diesel

generators in more complex designs like IP-Systems[1] while keeping the advantages coming

with the physical separation of UPS and Diesel.

2 Characteristics of modern Rotary UPS

The modern Rotary UPS combines the highest possible reliability with extremely high

efficiencies and a choice of ride-through energy store. Their routine maintenance regime is

simple and relatively non-intrusive when compared with Static UPS solutions.

Reliability: Rotary UPS do have the most simple and most reliable method of power

generation using synchronous machines. This operating principle in combination with up-to-

date digital electronic controls and user interfaces results in the highest reliability for any

single UPS available on the market today.

4

Efficiency & Cost: The modern Rotary UPS uses advanced materials and designs to

achieve an operating efficiency of up to 97% with low load efficiencies as high as 95%. Due

to the low losses and robust design, RUPS often do not require any active cooling devices

like air conditioning which makes operation even more cost effective over the system life.

Fig. 1 Total Cost of Ownership comparison for high MW systems

(Discounted cash-flow)

Although the day 1 investment in a Rotary system can sometimes be more than that of the

static option, the reduced complexity of routine maintenance, extended life and high

operating efficiency of the Rotary all contribute to reduce the total ownership costs and

produce a break-even point within just a few years.

Flexibility: The topology allows the Rotary UPS system to be employed readily either at low

or at medium voltage and to utilise either batteries or flywheel as the energy store for small

and large unit sizes alike.

The combination of RUPS with kinetic energy storage devices like flywheels result in a

massively reduced footprint compared to any battery-based systems. This makes a

significant amount of additional floor space available that can be used for other equipment or

to increase net useable technical/production floor space.

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Rotary UPS can easily be paralleled to increase the total power or to achieve redundancy.

There are no ultra-fast and complex regulation structures necessary as the paralleling of

synchronous generators is based on strong, self-effective synchronizing powers.

Because of utilizing synchronous generators certain Rotary UPS are able to deliver full active

power from power factor 0.8 lagging to 0.8 leading without the necessity of de-rating at

leading power factors. Their short circuit capacity of 12x-14x nominal current allows easy

discrimination in the downstream distribution and eliminates the necessity of bypass

transfers to clear a downstream fault.

Customisedbattery forshort backup

Flywheelfor up to 39sshort backup

Totally flexible configurations,Medium and Low Voltage

High load-carrying generator with a robust, highly efficient design

Customisedbattery forshort backup

Flywheelfor up to 39sshort backup

Totally flexible configurations,Medium and Low Voltage

High load-carrying generator with a robust, highly efficient design

Fig. 2 Some features of modern Rotary UPS and their flexibility

All of this demonstrates a high suitability and benefit with Rotary UPS for high end

applications in which reliability balanced with ownership costs really matters. For that reason

many large international data centres, banks, continuous process facilities and hospitals rely

on Rotary UPS and stay with this technology through clear choice if new systems need to be

installed.

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3 Bridging time

According to the statistics more than 96% of the mains outages have durations of less than

three seconds.

Fig. 3 Mains failure statistic of a medium voltage grid

The practical bridging times of energy stores are:

� Flywheels: 0 s … 30 seconds

� Batteries: 0 s … 30 minutes.

� Diesel generators: 10s … Hours

Comparing these figures with the statistics about mains failures shows that pure flywheel

UPS are able to cover more than 96% of all mains outages. Battery UPS would – depending

on the battery size – be able to bridge approximately another 3%. A stand-alone Diesel

generator is not able to bridge the first 10 seconds of a mains failure so to be sure to cover

all kinds of mains failures a combination of one short term energy storage and a long term

bridging device like a Diesel generator is mandatory. The question of whether a combination

of a battery UPS and a Diesel generator really makes sense in considering that the bridging

time of a flywheel is sufficient to ensure the engine start, is a well known subject of

discussion and dealt with in another paper[2].

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4 Various UPS and Diesel combinations

Fig. 4 shows the typical arrangement of a UPS and a Diesel generator in which the Diesel

generator functions as a mains substitute after utility failure. This layout is typically chosen

for a Static UPS solution. Since the inverters normally used in Static UPS are not designed to

operate bi-directionally this is the simplest way to charge the energy storage during Diesel

operation. If the Static UPS is operated in double conversion mode1 frequency fluctuations of

the Diesel are not transferred to the load side of the UPS and harmonics produced by the

loads are not transferred to the generator. However, operating the Static UPS in ECO mode

will increase its efficiency but adds the drawbacks of unfiltered harmonics on the generator,

no power factor correction at the input of the UPS and frequency variations of the Diesel.

No Break LoadsMains

GD

No Break LoadsMains

GD GD

Fig. 4 A typical combination of a Static UPS with batteries and a Diesel generator

This mode of operation is something certain modern Rotary UPS do not have to struggle

with. In mains as well as in Diesel operation they are running with the same high efficiency,

guaranteeing power factor correction and harmonic filtering at the same time.

Additionally and in contrast to conventional Static UPS, a RUPS offers a much wider variety

of combinations with Diesel engines. One such combination is the Diesel Rotary UPS

(DRUPS), in which the Diesel engine, like it is shown in Fig. 5, directly drives the generator of

the Rotary UPS. During a mains failure the load is first supplied by the short term energy

storage, which can be a battery or a flywheel. Then the Diesel just needs to start and once

the Diesel reaches nominal speed the overrunning clutch between the engine and the

permanently rotating generator closes automatically.

rotating generator closes automatically.

1 In double conversion mode the power is transferred through rectifier and inverter.

8

Mains No Break Loads

M/GG

or

D

Short Break LoadsMains

Fig. 5 A Diesel Rotary UPS (DRUPS):

The Diesel directly drives the internal generator via an overrunning clutch. If

the Rotary UPS utilizes a combined motor/generator, the short breaks

loads can be supplied by the motor winding of the electrical machine,

alternatively the short-break loads can be supplied from the UPS output

bus.

But of course a Rotary UPS can also be combined with external Gensets. The combination

of Rotary UPS with electrically coupled Diesel generators is called DeRUPS and offers one

more interesting variant than simply using the Genset as a mains replacement. With Rotary

UPS it is also possible to connect the Genset to the output bus of the UPS. This offers some

advanced features regarding power sharing, frequency stabilization and maintainability.

Short Break Loads

Mains No Break Loads

GD GDM/GG

or

Mains

G D

Short Break Loads

Mains No Break Loads

M/GG

or

Mains

Fig. 6 DeRUPS: Rotary UPS in combination with Gensets, which can either be

located upstream or downstream.

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Each external Genset basically allows supplying the no-break loads as well as short-break

loads (Fig. 6). If the Genset is connected to the input of the UPS the no-break and the short-

break loads remain isolated also in Diesel operation, a feature that is quite welcome in the

world of data centres.

Since almost all Rotary UPS are also available in a medium voltage version, all combinations

with external Gensets can also be realized in medium voltage. In this case the Gensets can

be equipped with medium voltage generators if the voltage level does not exceed the

technical limits for these machines.

5 Functional aspects of combining Rotary UPS with Gensets

A general aspect that needs to be considered when combining any UPS with a Diesel

generator is the interaction between the two power sources, namely the energy storage of

the UPS and the Diesel engine. This interactions finally makes the difference between a

simple combination of any UPS with a Diesel Generator and a DeRUPS (Electrically coupled

Diesel Rotary UPS) in which the Genset is totally integrated in the functional behaviour of the

UPS. To understand this interaction it is helpful to differentiate between active power sources

and reactive power sources, as it is shown in Fig. 7.

BatteryPowerbridge

M/G

Active Power Reactive Power

BatteryPowerbridge

M/G

Active Power Reactive Power

Fig. 7 Relationships between power sources and power type

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Independent of whether the Diesel generator is connected upstream or downstream of the

UPS, the Diesel engine needs to be able to deliver the active power for the load, to charge

the energy store and to cover the losses of the UPS. The generator of an upstream Genset

needs to supply the current taken by the UPS. It also needs to be able to supply the reactive

power taken by the UPS and to deal with the harmonics of potential rectifier circuits in the

input of the UPS. Due to their natural sine wave generation, Rotary UPS do not create any

harmonics and their input power factor is close to 1.0.

Having the Genset connected downstream can be slightly more efficient because this avoids

forward losses in the UPS during Diesel operation. The exchange of reactive power between

the UPS and the generator needs to be avoided by utilizing reactive currents controls. It is

best practice that either the UPS or the Genset supplies reactive power to the load while the

other component regulates its reactive power to zero.

A UPS with a Diesel generator located upstream of the UPS normally behaves like a UPS in

mains operation, even if the supply is a Diesel generator. Therefore the UPS transfers any

load changes to the mains, which is not as stable as in the past. Load steps of between 80%

up to 100% can then easily cause the engine to collapse and shut down independent of

whether this is an on-load or an off-load situation.

Certain Rotary UPS with electrically coupled flywheels are able to handle both power

directions and can therefore stabilize the frequency of a Diesel generator. In this case it does

not matter whether the Genset is connected upstream or downstream of the RUPS.

However, since the frequency regulation is assigned to the load side of the DeRUPS, the

performance is better if the Genset is connected downstream.

The comparison of Fig. 8 and Fig. 9 as an example shows the influence of bidirectional

frequency stabilization on a network supplied by a Diesel generator.

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Fig. 8 Frequency response of a Diesel generator during 50% load disconnection

and reconnection, here without additional frequency stabilization.

Fig. 9 Frequency response of a DeRUPS in Diesel operation during 50% load

disconnection and reconnection, with bidirectional frequency stabilization

by a Piller UNIBLOCK UBT equipped with a POWERBRIDGE flywheel

energy storage device and an external Diesel generator.

Additionally an electrically coupled flywheel that can be charged and discharged at the same

rate which allows short re-charge times and cuts down Diesel run time with further savings in

running costs. This is not possible with battery UPS and so this kind of flywheel supported

system is well prepared against multiple utility disturbances.

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6 Arrangements of RUPS and Gensets

In the following different arrangements of UPS and Diesel generators are compared.

Diesel generators located upstream of the UPS.

Upstream Diesel generators are used to operate as a mains substitute during a mains

outage. They can either operate in a parallel manner feeding a common bus to supply

several UPS or, in the case of large UPS units, each can be assigned to a single UPS. If the

Diesel generators are paralleled it is best practice if they feed an isolated bus that can be

connected to the UPS input bus via a central breaker, like it is shown in

Fig. 11.

The separated Genset bus also allows starting the Gensets via start up synchronisation, also

known as dead bus synchronisation. As in this case all Gensets start while already being

connected to the bus, there is no extra synchronisation time for the individual Gensets to be

considered. Combined with high quality Genset controls containing this special functionality

this start method allows shortest start times and guarantees that the load can reliably be

supplied by the Gensets even if UPS with flywheel energy storage are used.

Mains

Genset breaker(s)

(Genset) Group BreakerMains Breaker

Gensets

to UPS

GG

GSM M M

MMM

MM

M

M M M

GS GS

Mains

Genset breaker(s)

(Genset) Group BreakerMains Breaker

Gensets

to UPS

Mains

Genset breaker(s)

(Genset) Group BreakerMains Breaker

Gensets

to UPS

GG

GSGSM M M

MMM

MM

M

M M M

GSGS GSGS

Fig. 10 Example of Genset controls suited to perform start up synchronisation

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Once all Gensets are started and feeding their collecting bus, the central breaker will be

closed to energize the UPS input bus. This prevents the UPS to return to ‘mains’ as longs as

not all Gensets are ready to feed the bus. This central breaker does have a counterpart in

the mains feeder both building a breaker pair which easily allows a synchronized changeover

to utility once mains is available again.

M/G

G

M/G

G

M/G

G

M/G

G

Loads

Incoming

powerGD GDGD

M/G

G

M/G

G

M/G

G

M/G

G

M/G

G

M/G

G

M/G

G

M/G

G

Loads

Incoming

powerGD GD GD GD GD GDGD GD GD

Bypass

Fig. 11 A set of Diesel generators feeding the incoming bus of a group of Rotary

UPS, which can be operated in a stand alone as well as in a parallel

configuration.

An advantage of this arrangement is that the number of Diesel engines can be different to

the number of UPS, even though redundancy of Diesel generators also needs to be

considered.

Maintenance on the Gensets can be done independently from UPS maintenance, both

separately taking one single unit out of a redundant arrangement.

As a fallback the Diesel generators are able to permanently supply the loads via the UPS

maintenance bypass which can be an interesting feature in countries with a really bad mains

quality.

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Diesel Rotary UPS (DRUPS)

The one big advantage of this system is that the Diesel engine is also mechanically

integrated into the UPS functionality. Therefore no extra controls for synchronization or

breaker operation are necessary, as it is for external Genset arrangements. The redundancy

is naturally the same for the UPS and the Diesel engines as the number of UPS and Diesel is

equal.

D M/G

G

D M/G

G

D M/G

G

D M/G

G

Loads

Incoming

power

D M/G

G

DD M/G

G

M/G

G

D M/G

G

DD M/G

G

M/G

G

D M/G

G

DD M/G

G

M/G

G

D M/G

G

DD M/G

G

M/G

G

Loads

Incoming

power

Fig. 12 A power supply arrangement containing Diesel Rotary UPS (DRUPS).

During maintenance always the whole UPS including its Diesel engine needs to be taken out

of service.

Rotary UPS with Diesel Generators connected downstream

Having Gensets connected to the load side of the UPS is a less familiar method since most

Static UPS are not able to handle this situation. Fig. 13 shows a possible configuration which

comes along with some advantages worth consideration.

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Loads

M/G

G

M/G

G

M/G

G

M/G

G

M/G

G

M/G

G

M/G

G

M/G

G

G

D

G

D

G

D

G

D

G

D

G

D

G

D

G

D

G

D

G

D

G

D

G

D

Fig. 13 A group of Rotary UPS each combined with a Genset connected to the load

side.

In this configuration it is necessary to control the power sharing between the energy storage

of the UPS and the Diesel generator. This power sharing capability is then used to shift the

power that the UPS feeds to the load right after the mains failure to the Diesel generator

once it is started and connected to the load bus. This functionality, which makes the

combination of RUPS and Genset being a real DeRUPS, is already implemented in certain

modern Rotary UPS and does also allow charging the energy store of the UPS during Diesel

operation. In case of a bi-directional operating energy storage device like an electrically

connected flywheel, the energy store can then be used to keep a stable output frequency.

This is realized by having the flywheel delivering power in the case of on-load events until the

Diesel has taken over as well as in the case of off-load events having the flywheel absorbing

superfluous power from the Diesel until it is throttled down. Because of this interaction the

DeRUPS is also able to control all synchronization processes like, for example, the return of

the DeRUPS into mains operation once utility is available. As for upstream connected

Gensets, it is not strictly necessary to have one Genset assigned to one RUPS as it is also

possible to have different numbers of RUPS and Gensets combined while keeping the same

functionality. The DeRUPS is also well suited to be implemented into an IP-System. Fig. 14

shows an example of such combination.

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M/G

G

M/G

G

G

D

G

D

Short-Break-LoadNo-Break-Load

M/G

G

M/G

G

G

D

G

D

Short-Break-LoadNo-Break-Load

Fig. 14 DeRUPS integrated in an IP-System.

Here the extra power of the Genset is also used to supply the short-break

loads.

Another advantage of having the Diesel generators connected downstream of the RUPS is

that they can be operated independently of the UPS if necessary. Therefore maintenance on

a Genset can be done separately from UPS maintenance based on the Gensets own

controls coming along with each individual Genset. During maintenance of the Genset the

load can still have short term protection by the RUPS which normally bridges almost all of the

mains outages without Diesel support.

Loads

Incoming

power

M/G

G

M/G

G

M/G

G

M/G

G

M/G

G

M/G

G

M/G

G

M/G

G

G

D

G

D

G

D

G

D

G

D

G

D

G

D

G

D

G

D

G

D

G

D

G

D

Fig. 15 Diesel generators separated from the UPS allow individual maintenance on

each type of power supply keeping the remaining system functional.

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7 Summary

A combination of UPS and Diesel generators can be much more than simply using the

Gensets as a mains substitute. Besides this variant which is typical for a combination of

Static UPS with Gensets, Rotary UPS offer a much wider variety of combinations with

superior load handling characteristics, reliability and ownership costs. Compared to the well-

known Diesel Rotary UPS the combination of a Rotary UPS with a separated Diesel

generator connected to the load side offers some interesting features. The possibility to

operate the Diesel generator and the RUPS independently offers enhanced maintainability

while the combination of both guarantees outstanding frequency stability even in Diesel

operation. The combination of high efficiency and low operating costs with configuration

flexibility and pre-eminent reliability makes the Rotary UPS and Genset pair a highly

desirable offering for large-scale project designs.

8 References

[1] Dipl.-Ing. Frank Herbener

Iso-Parallel UPS Configuration

Piller White Paper 051, 2009

[2] Dipl.-Ing. Frank Herbener

Batteries and Flywheels

Piller White Paper 056, 2011

Piller Group GmbH,

Frank.Herbener@piller.com, Germany

White Paper No. 0061-1 / Apr 2013