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Standby System for emergency electricity supply

, Using non-renewable energy source

By Donald Smith

This report was submitted as the Electrical Engineering Graded Unit 2 paper, a 2 Credit Unit, for the HND Electrical Engineering course. The report was submitted on the 29th of April 2016.

Faculty of Science and Technology,

Glasgow Kelvin College,



The contents of the project are as shown below it should be noted that all figures and tables are in appendix “A”.

1) Title

2) Brief

3) Introduction-Hospital

4) Project Objectives

5) Why is a UPS important?            

6) Design

7) Function of UPS

8) Other types of UPS

9) Generator

10) Sizing and selecting the correct UPS

11) Mains Supply Reliability

12) UPS Battery Autonomy

13) UPS maintenance and testing

Title (1)

Standby System for emergency electricity supply

, in a Hospital Data Centre

Brief (2)

In many aspects of modern life, it is critical to have a safe and secure electrical connection. For important locations such as hospitals and airports, a power failure can have serious consequences. Many of these institutes rely on information technology systems to obtain critical information or even just to help the building operate (in cases where the building has a BMS-Building Management System). This is why it is vital for these types of buildings to have an emergency electricity supply. The emergency electricity supply will come from the use of a UPS (Uninterruptible Power System). In this project I will discuss a UPS system powered by a diesel generator in a Police Station.

Introduction (3)

I will design a standby generator system for a fictional data centre in a hospital. I shall estimate the design load, assuming that there shall be 20 server units that require 5 KW of electricity to maintain operational integrity. This shall mean the load is 100 KW, giving a 20% allowance for future expansion. Therefore our total load is 120 KW. This is the figure we shall base our design around.

Design Load = 120 KW

Project Objectives (4)

Knowledge Base – I will research my topic and provide information in this report

Presentation- I will give an oral presentation of my chosen topic to my classmates

Log Book- I will keep track of my work by noting my work weekly.

Evaluation-I will discuss my project work and findings.

Verification-I will state how I decided my findings are correct

Source Material- I will give references of the sources I have used.

Why is a UPS Important? (5)

Serious disruption can be caused for most business operations if a sudden loss of power occurs, and in some cases lead to a total inability to trade. However, it is not just mains failures or “blackouts” that can cause devastating effects. Many electrical loads, for example computer systems, are equally susceptible to:

• power sags (A drop in the mains supply that can last for several cycle)

• brown outs (Identical to sags but more serious and last longer)

• black-outs (Complete Power Loss)

• power spikes (short duration voltage transitions applied to the mains waveform)

• surges (Sustained voltage increases above the normal mains value that last for more than one cycle)

• noise and radio frequency interference (result of disturbance between supply line and earth)

• Harmonics (Caused by non-linear loads which pull current from the mains supply in large peaks)

Such loads are referred to as “critical loads”. This is because their functioning is critical for the business to operate correctly, and also because they require a more stable and reliable power source than that generally offered by the utility mains supply in order to guarantee their correct function. This stable and reliable power can be offered by a UPS should a mains failure occur

Design (6)

I have chosen to use a diesel rotary UPS system.

• Diesel Rotary UPS

Diesel Rotary UPS systems contain a device often referred to as an induction coupler, which is an electro-mechanical flywheel that stores kinetic energy. In the event of a mains failure, the energy stored in the induction coupler is used to maintain the required motor or generator shaft speed while the diesel engine is brought up to speed so it can take over.

A diesel rotary uninterruptible power-supply system incorporates a motor or generator unit that operates continuously to produce a clean and stable power supply for critical loads. Under normal operating conditions the motor element, which is powered by the mains supply of the unit, drives the generator. In the event of a mains failure, the generator continues to be driven by an electro-mechanical flywheel (often referred to as an induction coupler), which is maintained at high speed in readiness for a mains failure. The induction coupler stores sufficient kinetic energy to maintain the generator output while the diesel engine is brought up to speed and is ready to take over.

An alternative to the rotary diesel UPS depicted is a system available from Piller (UK). Instead of an induction coupler, the system uses a proprietary device known as powerbridge. This also stores kinetic energy to bridge the time required for engine starting.

The difference between a powerbridge and an induction coupler is that it provides electrical power (rather than using mechanical power) to the motor generator unit, and consequently it can be located separately as it is not mounted on the common drive-shaft. This enables the bed-plate is not mounted on the common drive-shaft. This enables the bed-plate for the diesel engine and motor generator unit to be appreciatively shorter.

Generic System  

The basic components of a rotary diesel uninterruptible power supply system are listed below.

Induction coupler (kinetic energy store)

The induction coupler is an electro-mechanical device that stores kinetic energy by means of an inner rotor (flywheel) turning at 3000 rev/min. In the event of a mains failure the energy stored in the rotor is used to maintain the required generator shaft speed while the diesel engine is brought online. The transfer of energy to and from the rotor is carefully controlled by means of an ac and dc winding. During normal operation, the ac winding drives the rotor, and if there is a mains failure a dc winding acts a break and transfers kinetic energy from the rotor to the generator.


The generator provides clean, regulated power. In the event of a mains failure, the generator is initially driven by the induction coupler while the diesel engine is brought up to speed.

Diesel engine

In the event of a mains failure, the diesel engine automatically starts and takes approximately 2-4 seconds to reach its operating speed(1500 rev/min).


The clutch will automatically connect the diesel engine to the induction coupler, as soon as the diesel engine reaches 1500 rev/min.

Rotary Diesel UPS systems


• As there is no direct electrical link to the main supply, this prevents any spikes, or surges from reaching critical loads.

• For certain installations that need an extended back-up supply, the need for an isolated diesel generator plant is evaded and space is saved.

• The need for battery maintenance is almost completely avoided

• This type of UPS system is better in dealing with high load faults and fluctuating loads than static UPS systems.

• As some systems produce a dual output so that critical and non-critical loads can be provided from separate windings in the motor or generator unit (they are electrically secluded from each other). This avoids the requirement for a separate diesel generator unit for non-critical loads.


• This type of UPS system has a fairly high installation cost compared to other types of UPS system.

• This type of UPS system has to rely on the diesel engine(s) starting.

• Engine noise must be lessened, during periods of extended mains failure.

• A diesel fuel store is needed

• Routine mechanical maintenance is also needed.

A diesel rotary UPS

Diesel Rotary UPS systems can combine a standard diesel generator set and flywheel energy storage in a compact design, this saves a fair amount of space compared to traditional battery based static UPS systems. This space could quite possibly be used for an extra data floor.

Hitec Diesel Rotary UPS systems are line-interactive, with very good high end-to-end energy efficiency.

In utility mode it acts as a power conditioner and active filter, eliminating brief interruptions, spikes and sags from the utility supply. If the utility supply fails, the Hitec Diesel Rotary UPS system will then immediately take over the supply of power. It will manage to do this without problem, there will be no interruptions or disturbances.

Function of UPS (7)

There are two basic functions of UPS Systems:

• To ensure the continuity of the power supply irrespective of fluctuations interruptions in the mains supply. This is an essential requirement for critical services such as IT and communication systems, which can fail if there is a momentary break in the supply.

• To provide a clean and stable power supply and free from spikes, surges and harmonics. This is also important for critical, computer-based systems, which can fail if any of these irregularities occur in the mains supply at a significant level. All but the most basic UPS systems provide at least some protection from poor power quality.

Knowledge Base

Other Types of UPS include (8):

• Hybrid rotary UPS

Hybrid Rotary UPS systems do not incorporate a diesel engine or induction coupler. Instead, they use batteries and an inverter to provide the ac power needed to continue driving the motor or generator in the event of a mains failure. A standby diesel generator will be required for critical electrical loads that must have power at all times.

• Static UPS Systems

A Static UPS has no moving parts and incorporates batteries, a battery charger, and an inverter. Under normal operating conditions most UPS systems provide at least some protection against spikes and sags. The level of protection depends on the specific types of UPS. In the event of a mains failure; the batteries feed the critical loads via the inverter, which converts dc battery to an ac supply. The battery supply has sufficient capacity to enable critical computer systems to be backed up and shut down. For critical loads that need power at all times, a standby diesel generator is required to ensure a continuous electrical supply.

Static UPS-Offline Systems

Off-line UPS systems

Off-line systems are the cheapest types of UPS. When an off-line UPS system is in normal operation the mains supply will pass directly through the unit to the equipment served, which shall then be exposed to any minor fluctuations in the supply voltage that may arise. (major fluctuations will create a switch to the battery supply. A narrow amount of spike protection is usually provided, however overall protection from mains-borne disturbances is not very good.

When in normal operating conditions, the battery charger is always active and hence keeps the battery in full charge. When a mains failure occurs the static switch will function, allowing the battery to continue supplying the load through the inverter for a few minutes.

Normally the switching operation takes less than 4 milliseconds(ms), however some systems can take up to 25 ms which can lead to a computer crashing. As consequence, these types of off-line systems may well be better described as a back-up power supply instead of a proper UPS. Typical applications include most computers and non-critical, low-load applications.

Line Interactive Off-line systems

Line interactive systems work in the same way as the basic off-line systems. During normal operation the mains supply will pass directly through the unit to the equipment served. The difference between the two is that the line interactive unit UPS systems incorporate a special type of transformer which automatically reimburses for low or high mains- supply voltages.

This feature gives a big and clear advantage over the basic off-line system. In parts where there can be wide fluctuations in the mains-supply voltage, the basic-off-line system would often need to switch to the battery supply to maintain a satisfactory supply voltage. As battery operation can only be maintained for a certain time, it may prove in the event of a sustained drop or rise in the mains-supply voltage

Static UPS-On-line systems

On-line UPS systems main difference to off-line systems is that the batteries, battery charger and inverter within an on-line system operate at all times. In this system the incoming mains ac supply is changed to dc power at battery voltage and is then converted back into ac power at the voltage needed for the critical load. The batteries are also kept charged by the dc power supply. This is a process that is usually referred to as double conversion as power is being converted from ac to dc then from dc to ac. Although this process is inefficient, the rectifier and inverter give a very good voltage regulation and a good spike protection. Also if a mains failure occurs, it will avoid a break in the supply to critical loads, as the batteries are already connected and will continue to feed the inverter. In small to medium-sized systems, the batteries are usually kept inside one or more free-standing cabinets, with the UPS equipment found in another cabinet. Large on-line systems could possibly have rack-mounted batteries located in a dedicated battery room.

Standby Generators

In the event of a mains failure, the batteries in a static UPS will give sufficient power to back up computer to prevent losing data. However, for some very important business activities such as data processing in banks, systems shut down is unacceptable. It is therefore essential to install standby generators to take over from the UPS batteries and to continue the supply to the critical load until the mains supply is working again. The generators will automatically start up when a mains failure occurs. Once running, they are brought online as soon as they have reached the required speed and have stabilised.

Standby Generators installed into rotary diesel UPS System

On-line UPS System


• Cheaper than rotary UPS systems

• Useful for when critical load protection is needed

• Exceptional voltage regulation and spike protection

• No break in supply to critical loads

• As opposed to rotary diesel systems, the UPS system and generators can be found separately


• Higher capital cost than off-line systems

• Poor system efficiency (energy loss due to the double conversion process)

Hybrid Rotary UPS Systems

Similarly, to rotary diesel systems, the hybrid rotary system incorporates a generator. Consequently, it shares many of the same advantages over static systems. The main difference between the 2 is that the hybrid system will use battery power to continue driving the generator if a mains failure should occur. This means a diesel engine will not be required, but the restricted capacity of the batteries will decide the length of time that power is available in the event of a mains-supply failure.

For critical business activities where a shut-down of system is unacceptable, standby generators are needed to take over from the hybrid rotary UPS. Hybrid systems have the advantage that the diesel generator can be found separately from UPS system, which can be beneficial in optimising plant layout. Rotary diesel systems do not have this advantage.

Internal view of a hybrid rotary UPS cabinet

Hybrid Rotary UPS Systems


• As there is no direct electrical link to the mains supply, this prevents any spikes or surges from reaching critical loads.

• Reliance on diesel start up is avoided

• In diesel rotary UPS systems, associated environmental considerations are avoided, for example noise, and exhaust fumes and the storage of diesel fuel.

• Slightly cheaper than diesel UPS systems

• Better at dealing with high-load faults and fluctuating loads than static UPS systems

• As key components can be found separately in this type of UPS system, this allows the plant layout to be optimized.


• If a power failure is unacceptable and power is required at all times, standby generators will be required, beyond the capacity of the UPS batteries.

• Period of critical load protection is limited to approximately 10 minutes.

• Batteries must be maintained and periodically replaced

• Needs more space than diesel rotary UPS systems

• More expensive than static UPS systems

 Generator (9)

What is a Generator?

A generator is a machine that converts a stored energy source(fuel) into electrical energy. The stored energy could be either gas or diesel fuel. I have chosen to use a diesel generator as gas powered generators are only used in Combined Heat and Power(CHP)applications. Diesel generators however are usually used for both base load (continuous) and standby applications.

Diesel Engine Generator

Diesel generators use engines that are very similar to the ones found in trucks and lorries. The difference is that instead of the engine driving a gearbox and prop-shaft in a motor vehicle it drives an alternator in a generator. Majority of vehicle owners will know the basic requirements of keeping their car working well and keeping ready for use and the same basic principles apply to the diesel generator. It must:

• be well maintained

• have a supply of fuel

• have a healthy battery for starting

• have sufficient coolant and oil


Alternators convert the mechanical power of the engine into AC electrical power. The alternator will usually produce either single phase (230V) or three phase(400V) voltages in the UK and the value of the output voltage is decided by how the alternator is “wound”. The amplitude and stability of the alternator output voltage is controlled by Automatic Voltage Regulator(AVR). The engine speed decides the frequency of the output voltage (normally 50HZ in the UK). It is standard for an engine speed of 1500 RPM to give an output of 50 HZ although the speed/frequency relationship is a function of the design of the alternator. The engine speed and hence output frequency are controlled by a “governor” which regulates the amount of fuel that goes into the engine. (more fuel=higher engine speed=higher output frequency)


There are 2 basic types of generator governing: mechanical and electronic.

Springs and spinning weights are used by a mechanical governor to regulate the supply of fuel. Electronic governors are more expensive than mechanical governors. However, because of mechanical governors’ mechanical nature they are less responsive (slower)and provide less stable engine speed (frequency) regulation.

An electronic governor counts the teeth on the flywheel of the alternator as it rotates and regulates fuel flow accordingly. Electronic governing is very responsive and it offers a very stable engine speed regulation. This is why it is almost without exception a pre-requisite for any standby generator being used with a UPS system.

A Generator as a standby power source   

The generator must always be prepared to start and to support the load as mains power failures cannot be predicted. This is why the generator is referred to as a “standby generator”.

In order to make sure that the generator is always prepared to start it must be kept warm and it must also have a fully charged battery and sufficient fuel. It must also be able to “know” when the mains supply has failed and when it has returned so that it “knows” when it is to start and stop.

With the use of “engine water heaters” (sometimes referred to as jacket heaters), standby generators are kept warm. These “engine water heaters” are powered by the mains supply. And batteries are also trickle charged by a mains powered battery charger. The generator has an Automatic Mains Failure (AMF) panel which signals it to start and stop when required.  The alternative to an AMF panel is a manually started generator which relies on someone starting and stopping it whenever it is used.

As a minimum all generators used in UPS applications must have:

• an AMF panel

• an electronic governor

• an engine water heater

• a battery charger

Sizing and Selecting the Correct UPS (10)

Collating and Calculating Load Data

When a UPS installation is being planned it is quite important to gather as much information as possible about the proposed load and make allowances for future load expansion. Most items of electrical equipment have labels which carry information on the electrical characteristics of the item

It is important to collate information for items of the proposed load.

The information on the load that is required in order to correctly size the UPS includes:

• Supply voltage

• Supply frequency

• Number of phases

• Load current

• Power factor

• Power consumption

Supply Voltage

The ac supply voltage in Volts is usually found on the label or in the manufacturers literature. Single phase equipment should normally have a supply voltage of 230/240 Vac and three phase equipment should give 400/415 Vac in the UK. Labelling could give a range of operating voltage as some equipment is designed for international use.

Supply Frequency

The ac supply frequency in Hertz(Hz) is also usually itemised on the label or in the manufacturer’s literature. The UKs version of equipment will usually have a supply frequency of 50Hz. Similar to the Supply voltage the supply frequency may also be designed for international use and the labelling may give a range of operating frequencies.

Number of Phases

The equipment will either be single phase or 3 phase. If the label does not give a value for phases, then 230/240 Vac can be assumed as single phase and 400/415 Vac can be assumed to be 3 phase.

3 Phase UPS systems will be able to supply 3 phase and single phase loads. However single phase UPS systems can only supply single phase loads.

Load Current

The devices load current is usually found on the label or in the manufactures literature.

Power Factor

In some electrical loads like motors and computers, current will flow into the equipment without being usefully converted into energy. This occurs when current that is drawn by the equipment isn’t in phase with the applied voltage.

Current which is always in phase with the voltage is drawn by some equipment, however majority of the equipment that is expected to be connected to a UPS will draw additional current that is not in phase with the voltage.

Power factor states how much of the supply current is correctly in phase with the voltage and is effectively used.

Power Consumption

Power consumption may be stated in Watts (W) or Volt Amperes (VA) but not often both. VA or KVA is used by manufacturers. It useful to obtain the VA rating of all load items.

If the VA rating is not stated it can be obtained by:

• Multiplying the supply voltage (V) by the load current (A) or

• Dividing the power consumption (W) by the stated power factor (P.F)

Mains Supply Reliability (11)

The utility mains supply in the U.K is generally very reliable. However, figures regarding continued supply without disturbances are not available as factors such as critical load sensitivity and the proximity of other equipment cannot be determined on a national basis.

Statistically 95% of all mains disturbances last less than five minutes with anything longer likely to last for many hours.

Once a UPS has been established, the next considerations should be:

• Can a UPS with batteries fully protect the critical load? Or

• Is the load so “business critical” that a standby generator is essential to keep the load operational during extended periods without mains supply?

UPS Battery Autonomy (12)

UPS batteries are sized to provide emergency back-up power to the UPS in the event of a total loss of ac input supply.

As a minimum, a UPS battery must have enough capacity to allow time for a generator to start or for an orderly shutdown of the critical load of applications.

Most UPS suppliers can provide battery solutions which will allow the UPS to support the critical load for longer periods. For example, a 10kVA UPS may be supplied with a 10-minute battery as standard. However, by adding another, matching, cabinet it is possible to extend the autonomy time to 30 minutes. Some systems allow many external battery cabinets to be connected to achieve autonomy times extending to several hours.

Additional cabinets and batteries obviously take more space and will require both periodic maintenance and end-of-life replacement.   

UPS maintenance and testing (13)

Each UPS, battery and generator supplier will have its own planned maintenance (PM) programmes which will typically include the following:

• Inspect site log for incidents since previous PM visit

• Check and record equipment meter readings and verify accuracy

• Check that meters and instrumentation operate correctly

• Verify the correct status indications for the local and remote monitoring panels and communication facilities.

• Check that all indication lamps are operational

• Check that all switchgear operates correctly

• Check the air flow in and around the equipment

• Replace the air filters if fitted

• Remove any material and obstructions from around the equipment

• Check the environment for abnormal conditions

• Check for excessive heat, noise and vibration

• Visually inspect all components for signs of damage

• Visually inspect power and control wiring

• Check for loose connections

• Inspect the ac and dc capacitors for swelling and leakage

• Check the power supply voltages and the power waveforms

• Check the operation of the circuit breakers and associated trips and/or undervoltage releases

• Check the overall operation of the UPS

• Maintain the site log with written service reports.

Additional checks and testing will be required if a parallel UPS system is installed

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