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622 A. Nasiri

Static Switch

(Normally off)

Load

FIGURE 24.6 Conﬁguration of a three-phase on-line UPS system.

increasing the cost. When the AC input voltage is outside the

preset tolerance, the inverter and battery maintain continuity

of power to the load. The duration of this mode is the duration

of preset UPS backup time or till the AC line returns within

the preset tolerance.

The main advantages of on-line UPS are very wide tolerance

to the input voltage variation and very precise regulations of

output voltage. In addition, there is no transfer time during

the transition from normal to stored energy modes. It is also

possible to regulate or change the output frequency [8]. The

main disadvantages of this topology are low power factor, high

THD at the input, and low efﬁciency. The input current is

distorted by the rectiﬁer unless an extra PFC circuit is added;

but, this adds to the cost of the UPS system [9].

As mentioned for the standby UPS system, different topolo-

gies are employed for the AC/DC rectiﬁer and DC/AC inverter.

Unlike standby UPS system, in this system, these converters

provide power to the load continuously. Therefore, more care

should be given to the quality of the input current and out-

put voltage as well as the efﬁciency of the system. Figure 24.6

shows the conﬁguration of a three-phase on-line UPS system.

The proper switching method such as PWM is employed for

the AC/DC rectiﬁer to minimize the input current harmonics

and provide regulated DC bus voltage. A low pass ﬁlter at the

output of the system removes the switching frequency from

the output voltage.

24.2.3 Line-interactive UPS

Line-interactive UPS systems consist of a static switch, a

series inductor, a bi-directional converter, and a battery bank.

An optional passive ﬁlter can be added at the output of the

bi-directional converter or at the input side of the load. A line-

interactive UPS can operate either as an on-line UPS or as an

off-line UPS. For an off-line line-interactive UPS, the series

inductor is not required. However, most of the line-interactive

UPS systems operate on-line in order to either improve the

power factor of the load or regulate the output voltage for

the load. When the AC line is within the preset tolerance, it

feeds the load directly. The AC/DC converter is connected in

parallel with the load and charges the battery. This converter

may also be used to improve the power factor of the system

and compensate the load current harmonics. [10, 11]. Typical

conﬁguration of a line-interactive UPS is shown in Fig. 24.7.

When a power outage occurs or input voltage falls out-

side the preset tolerance, the system goes to bypass mode.

In this mode, the bi-directional converter operates as a DC/AC

inverter and supplies power to the load from the battery set.

The static switch disconnects the AC line in order to prevent

back feed from the inverter. The main advantages of the line-

interactive UPS systems are simple design and, as a result,

high reliability and lower cost compared to the on-line UPS

systems. They also have good harmonic suppression for the

input current. Since this is a single stage conversion topology,

the efﬁciency is higher than on-line UPS system. The main dis-

advantage is the lack of effective isolation of the load from the

AC line. Employing a transformer in the output can eliminate

this; but, it will add to the cost, size, and weight of the UPS

system. Furthermore, the output voltage conditioning is not

good because the inverter is not connected in series with the

load. In addition, since the AC line supplies the load directly

during the normal mode of operation, there is no possibility

for regulation of the output frequency.

24.2.4 Universal UPS

This type of UPS is also called “series-parallel” or “delta con-

version.” Its topology is derived from uniﬁed power quality

conditioner (UPQC) topology and combines the advantages of

both on-line and line-interactive UPS systems [12, 13]. It can

achieve unity power factor, precise regulation of the output

voltage, and high efﬁciency simultaneously. Its conﬁguration

is shown in Fig. 24.8. It consists of two bi-directional con-

verters connected to a common battery set, static switch, and

24 Uninterruptible Power Supplies 623

Static Switch

Line

Load

Battery Bank

Series

Inductor

Bidirectional

AC/DC

Converter

FIGURE 24.7 A typical conﬁguration of a line-interactive UPS system.

Series

Bi-directional

Converter

Battery Bank

Series Transformer

Static

Switch

Line

Load

DC Link

Parallel

Bi-directional

converter

FIGURE 24.8 Block diagram of a universal UPS system.

a series transformer. The series bi-directional converter is rated

at about 20% of the output power of the UPS system and it

is connected via a transformer in series with the AC line. The

second bi-directional converter is the usual inverter for a line-

interactive UPS connected in parallel to the load and rated at

100% of the output power.

When the input voltage is in the acceptable range, the sys-

tem is in the bypass mode. In this mode, parallel converter

deals with current-based distortions. It mitigates load cur-

rent harmonics and improves input power factor. At the same

time, it charges the battery pack. Series converter deals with

voltage-based distortions. It cancels input voltage harmonics

and compensates voltage sags and swells. Most of the power

is supplied directly from the AC line to the load. Only a

small percentage of the input power is absorbed by parallel

converter. This power is used to compensate the differences

between input and reference voltages and to charge the bat-

tery pack. On the other hand, when the input voltage shuts

down, the static switch separates the source and the load and

the system goes to backup mode. In this situation, the par-

allel inverter acts as a DC/AC inverter and supplies power to

the load. Since a large portion of the power ﬂows without

any conversion from the AC line to the load, the efﬁciency is

higher than that of an on-line UPS system. Having eliminated

the main drawback of double-conversion UPS systems, the

universal UPS topology appears to be a strong competitor of

on-line UPS systems in many applications. Figure 24.9 shows

the topology of a three-phase universal UPS system.

24.2.5 Rotary UPS

Rotary UPS systems use the stored kinetic energy in the elec-

trical machines to provide power to the load when a power

outage occurs. There are different conﬁgurations for rotary

UPS systems. The simplest topology consists of an AC motor

and an AC generator, which are mechanically coupled. A ﬂy-

wheel is also used on the shaft of the machines to store more

kinetic energy in the system. In normal operation, the input

AC line provides power to the AC motor and this AC motor

drives the AC generator. The conﬁguration of this system is

shown in Fig. 24.10a. In backup mode, the kinetic energy

stored in the motor, ﬂywheel, and generator is converted

to electric power and supplies the load. This simple topol-

ogy is designed to provide short time backup power to the

624 A. Nasiri

Non-linear

Load

Battery

L2a

P2a

P1a

L1a

′

C2a

C1a

FIGURE 24.9 A universal UPS topology based on two three-leg bi-directional converters.

Utility AC

Power

Utility AC

Power

Utility AC

Power

Load

AC Motor AC Generator

Flywheel

Static Switch

(a)

Load

DC Machine AC Generator

Flywheel

AC Motor

Battery

Static Switch

(b)

(c)

Load

AC Generator

Diesel/ Natural

Gas Standby

Engine

Static Switch

Normally On

Static Switch

Normally Off

FIGURE 24.10 Different conﬁguration of rotary UPS systems: (a) motor–generator set; (b) rotary UPS with battery backup; and (c) rotary UPS

with standby diesel/natural gas engine.

load (typically less than 2 s) in case of power interruption.

In another conﬁguration of rotary UPS system which is shown

in Fig. 24.10b, an AC motor, a DC machine, an AC genera-

tor, and a battery bank are used. During the normal mode of

operation, the AC line supplies the AC motor, which drives

the DC machine. The DC machine drives the AC generator,

which supplies the load. During the backup mode of oper-

ation, the battery bank supplies the DC machine, which, in

turn, drives the AC generator and the AC generator supplies

the load. This system can provide long time backup power to

24 Uninterruptible Power Supplies 625

the load depending on the capacity of the battery set. These two

rotary UPS systems are much more reliable than the static UPS

systems and provide complete electrical isolation between the

load and input AC line. Yet, they require more maintenance

and have much bigger size and weight. Therefore, they are

usually used for high power applications [14, 15]. The conﬁg-

uration of a standby rotary UPS system is shown in Fig. 24.10c.

This system does not provide electrical isolation between the

load and input AC. There is also a transition delay for switch-

ing from main AC to backup AC generator. However, it can

provide power to the load as long as needed.

Static Switch

(a)

Load

AC Motor AC Generator

Battery

(b)

Utility AC

Power

Load

AC Generator

Static Switch

Bi-directional

AC/DC

Converter

Fast Switch

AC Motor

Battery

Utility AC

Power

Utility AC

Power

Load

AC Machine

AC/DC

Rectifier

AC/DC

Rectifier

DC/AC

Inverter

DC/AC

Inverter

Diesel

Engine

Flywheel

Clutch

Series Inductor

Main Breaker

Gen. Breaker

Inverter

Breaker

(c)

FIGURE 24.11 Three conﬁgurations of hybrid rotary-static UPS systems: (a) motor–generator set with battery backup; (b) motor–generator set

with power conditioning at input side; and (c) battery-less hybrid UPS system.

24.2.6 Hybrid Static/Rotary UPS

Hybrid static/rotary UPS systems combine the main features

of both static and rotary UPS systems. They have low out-

put impedance, high reliability, excellent frequency stability,

and low maintenance requirements [6]. Typical conﬁgura-

tions of hybrid static/rotary UPS are depicted in Fig. 24.11.

They are usually used in high power applications. In the sys-

tem shown in Fig. 24.11a, during normal operation, the input

AC power feeds the AC motor. The power is provided to the

load from the AC generator, which is driven by the AC motor.

626 A. Nasiri

In case of low input power quality or power interruption, the

bidirectional AC/DC converter acts as an inverter and feeds

the AC motor from battery pack. Conﬁguration of a hybrid

UPS system with power conditioning at the input is shown in

Fig. 24.11b. Figure 24.11c shows the conﬁguration of a more

complicated hybrid UPS system. This system has three opera-

tion modes. In normal operation, the load is directly supplied

by the main AC input and the AC motor is rotated at no-

load. In the case of short power interruption, main breaker

and generator breaker are opened and the inverter breaker

is closed. The DC/AC inverter provides power to the load

from the kinetic energy stored in the AC machine. If power is

not restored in the short-term, the diesel engine is turned on,

which provides power to the load through the AC generator. In

this mode, the main breaker and inverter breaker remain open.

One of the advantages of this topology is operation without a

battery set to minimize cost, space, and required maintenance.

The second advantage is avoiding double power conversion in

long-term power interruption.

24.2.7 Comparison of UPS Conﬁgurations

Table 24.1 below provides the comparison between character-

istics of different types of UPS systems.

24.3 Performance Evaluation

There are four criteria for evaluating the performance of a UPS

system: quality of output voltage, input PFC and current har-

monic cancellation, transition time, and efﬁciency. The quality

of output voltage is the most important factor. The output

voltage of a UPS system should be sinusoidal with low THD

in different loading conditions even with non-linear loads. The

control system should have small transient responses to pro-

vide appropriate line conditioning in different loading proﬁles.

Typically, rotary UPS systems, which employ an AC genera-

tor at the load side, have better output voltage quality than

static UPS systems. In these systems, there is no converter

switching frequency present at the output voltage. Among the

TABLE 24.1 Performance comparison of different conﬁgurations of UPS systems

Parameter On-line Line interactive Off-line Universal Rotary Hybrid

Surge protection Excellent Good Good Good Excellent Excellent

Transition time Excellent Good Poor Good Excellent Excellent

Line conditioning Poor Good Poor Excellent Good Good

Backup duration Depends on

battery

Depends on

battery

Depends on

battery

Depends on

battery

Typically 0.1–0.5 s Depends on battery

Efﬁciency Low around 80% High up to 95% High High up to 95% High typically above

85%

High typically

around 95%

Input/Output

isolation

Poor Poor Poor Poor Perfect Perfect

Cost High Medium-high Low High Very high Very high

static UPS systems, on-line UPS conﬁguration provides better

output voltage quality. In this system, output voltage is pro-

vided by a DC/AC inverter regardless of input voltage quality.

Usually, a pulse width modulation (PWM) method is used

to regulate the output voltage. This kind of UPS should be

designed to have minimum switching frequency at the out-

put and provide pure sinusoidal voltage at different loading

conditions. Followed by on-line UPS system are universal and

line-interactive conﬁgurations. In universal topology, during

normal mode of operation, the series converter provides volt-

age conditioning and regulates output voltage. In the backup

mode, the parallel converter provides the load with sinusoidal

voltage. In the line-interactive topology, during normal oper-

ation mode, input voltage directly supplies the load and no

voltage conditioning is provided. In the backup mode, the

DC/AC inverter provides the load with sinusoidal voltage.

The second criterion is transition time from normal mode

of operation to stored energy mode. On-line rotary and static

UPS systems have superior performance in this regard. The

output voltage is always provided by the output generator

or output DC/AC inverter and there is no transition time

between operation modes of the systems. However, some of

the rotary and hybrid conﬁgurations shown in Figs. 24.10 and

24.11 can only provide power to the load for a limited time.

This time is determined by the amount of kinetic energy stored

in the mechanical system. The transfer time in universal and

line-interactive topologies depends on the time necessary for

converting the power ﬂow from the battery bank through the

inverter to the load. Improved performance is achieved by

choosing the DC bus capacitor voltage at the battery side to be

slightly higher than the ﬂoating voltage of the batteries. There-

fore, when the AC line fails, it is not necessary to sense the

failure because the DC bus voltage will immediately fall under

the ﬂoating voltage of the batteries and the power ﬂow will

naturally turn to the load. For off-line UPS systems, the trans-

fer time is the longest. It depends upon the speed of sensing

the failure of the AC line and starting the inverter.

The next important factor is the input power factor and

the ability of the system to provide conditioning for load

power. Universal UPS system has better performance followed

24 Uninterruptible Power Supplies 627

by line-interactive and on-line UPS in this regard. During

normal mode of operation, the parallel converter acts as an

active ﬁlter and compensates reactive current and current har-

monics generated by the load. In the line-interactive system,

the bi-directional AC/DC converter performs this task. In an

on-line UPS system, an additional system must be added to

improve PFC and mitigate current harmonics.

The last criterion for performance evaluation is efﬁciency.

To emphasize this factor, it should be noted that losses in UPS

systems represent about 5–12% of all the energy consumed

in data centers. Efﬁciency in rotary and hybrid conﬁgurations

depends on the topology of the system but typically for low

power application due to mechanical loss in the motor and

generator, the efﬁciency is not very high. Among the static UPS

systems, on-line UPS system has the poorest efﬁciency due to

double conversion. Line-interactive and universal topologies

provide higher efﬁciencies since most of the power directly

ﬂows from the input AC to the load during normal operation.

24.4 Applications

The UPS systems have wide applications in a variety of indus-

tries. Their common applications range from small power

rating for personal computer systems to medium power rat-

ing for medical facilities, life supporting systems, data storage,

and emergency equipment and high power rating for telecom-

munications, industrial processing, and on-line management

systems. Different considerations should be taken into account

for these applications. For emergency systems and lighting, the

UPS should support the system for at least 90 minutes. Except

for emergency systems, the UPS is designed to provide backup

power to sensitive loads for 15–20 minutes. After this time, if

the power is not restored, the system will be gracefully shut

down. If a longer backup period is considered, a larger battery

with higher cost and space is required. For process equipment

and high power applications, some UPS systems are designed

to provide enough time for the secondary power sources such

as diesel generators to start up.

Sensitive Loads

Network

Main Power

Distribution System

Utility Power

UPS

Static Bypass

Switch

Load

FIGURE 24.12 Typical conﬁguration of a distributed UPS network.

For industrial applications, it should be noted that UPS sys-

tems add to the complexity of the electrical system. They also

add installation and ongoing maintenance costs. They may

also add non-linearity to the system, decrease the efﬁciency,

and deteriorate the input PFC mechanism. The power rating

of the UPS should be appropriately selected considering the

existing load and future extensions. For many applications,

input voltage surges and spikes cause more damage than power

outages. For these systems, another device instead of UPS can

be utilized. Load characteristics should also be considered in

UPS selection. For motor loads, the inrush current, which is

sometimes 2.5 times of the rated current, should be consid-

ered. A good UPS for the motor loads is the one with higher

transient overloads. For non-linear loads such as switching

power supplies, the input current is not sinusoidal. There-

fore, the instantaneous current is higher than the RMS current.

This high instantaneous current should be considered in UPS

selection.

For a power distribution network, two different approaches

are taken to support sensitive loads. In a distributed approach,

which is more suitable for highly proliferated loads such as

medical equipment, data processing, and telecommunications

many separate UPS units operate in parallel to supply critical

loads. UPS units are placed ﬂexibly in the system to form a

critical load network. A typical on-line distributed UPS sys-

tem is shown in Fig. 24.12. High ﬂexibility and redundancy

are the main advantages of distributed systems. Individual load

increase can be supported by adding more UPS systems. Con-

sideration for future extension can also be delayed until the

loads are added. On the other side, this method has some dis-

advantages. The load sharing between different UPS units is a

difﬁcult task. Complicated digital control methods and com-

munication between units are required to perform optimal

load sharing. The second disadvantage is that the monitoring

of the whole system is difﬁcult and requires specially trained

staff.

The other method to support distributed loads is to use a

large UPS unit to supply all the critical loads in a centralized

approach. This approach is more desirable for industrial and

628 A. Nasiri

utility applications. The advantage of this method is easier

maintenance and troubleshooting. The disadvantages on the

other side are lack of redundancy and high installation cost. In

addition, consideration for system expansion should be taken

into account when the original UPS unit is selected.

24.5 Control Techniques

The main task of the control system in a UPS unit is to

minimize the output voltage total harmonic distortion in dif-

ferent loading proﬁles. In addition, it should provide the

proper mechanism to recharge the battery set and maintain

high input power factor and low total input current har-

monic distortion. Other factors considered for a good control

technique are nearly zero steady-state inverter output volt-

age error, good voltage regulation, robustness, fast transient

response, and protection of the inverter against overload under

linear/non-linear loads.

The most common switching technique is Sinusoidal PWM.

This method can be utilized for both single-phase and three-

phase systems. The advantage of this method is low output

voltage harmonic and robustness. This strategy uses a single

feedback loop to provide well-regulated output voltage with

low THD. The feedback control can be continuous or discon-

tinuous. Analog techniques are used in continuous approach.

The sinusoidal PWM (SPWM) can be of natural sampling type,

average type, or instantaneous type [17, 18].

Load

(a)

PID

Controller

Control

Triangle

Comparator

Gate of

and S

(b)

FIGURE 24.13 (a) Conﬁguration of a three-phase DC/AC inverter for UPS systems and (b) simple voltage controller using PWM technique.

In natural sampling type, the peak value of the output volt-

age is detected and compared with a reference voltage in order

to obtain the error, which is used to control the reference to

the modulator. The average approach is basically the same; but,

the sensed voltage is converted to an average value and after

that, is compared with a reference signal. These approaches

control only the amplitude of the output voltage and are good

only at high frequencies. In an instantaneous voltage feedback

SPWM control, the output voltage is continuously compared

with the reference signal improving the dynamic performance

of the UPS inverter.

A typical block diagram of a three-phase DC/AC inverter

for UPS systems and SPWM switching control technique is

shown in Fig. 24.13. The disadvantage of this method is lack

of ﬂexibility for non-linear loads. Other programmed PWM

techniques such as selective harmonic elimination, minimum

THD, minimum loss, minimum current ripple, and reduced

acoustic noise may be used for the inverter.

Better performance even with non-linear and step-changing

loads can be achieved by multiple control loop strategies [19].

As shown in Fig. 24.14, there are two control loops: an outer

and an inner. The outer control loop uses the output voltage

as a feedback signal, which is compared with a reference sig-

nal. The error is compensated by a PI-integrator to achieve

stable output voltage under steady-state operation. This error

is also used as a reference signal for the inner current regula-

tor loop, which uses the inductor or the capacitor output ﬁlter

current as the feedback signal. The minor current loop ensures

24 Uninterruptible Power Supplies 629

PID

Controller

Current

Regulator

Gate of

and S

FIGURE 24.14 Typical current and voltage control loops for UPS inverter.

fast dynamic responses enabling good performance with non-

linear or step-changing loads. The basic current regulators

employed as minor current loop are: hysteresis regulators,

sinusoidal PWM regulator, and predictive regulators. In a typ-

ical hysteresis regulator, the reference signal is compared with

the feedback signal. The sign and predetermined amplitude of

the error determine the output of the modulator. The dura-

tion between two successive levels is determined by the slope

of the reference signal. The output voltage tracks the reference

signal within the upper and lower boundary levels. This hys-

teresis control has fast transient response; but, the switching

frequency varies widely [20].

In SPWM control technique, the output voltage feedback

is compared with a sine reference signal and the error volt-

age is compensated by a PI-regulator to produce the current

reference. The current through the inductor or the capaci-

tor is sensed and compared with the reference signal. After

being compensated by a PI-regulator, the error signal is com-

pared with a triangular waveform to generate SPWM signal for

switching control. The SPWM current control has a constant

switching frequency and also provides fast dynamic responses.

In predictive current control method, the switching instants

are determined by suitable error boundaries. When the current

vector touches the boundary line, the next switching state vec-

tor is determined by prediction and optimization in order to

minimize the error. Predictive current control requires a good

knowledge of the load parameters. All these current regulators

are typically used as an inner loop to regulate the current in

the ﬁlter inductor. The current reference for the current reg-

ulator is obtained by summing together the error in an outer

voltage loop with the actual load current to yield the rated

output voltage.

With the increase of speed and reliability of digital proces-

sors and a decrease in their cost, digital processors have been

facing an enormous growth of popularity in control applica-

tions in the past few years. Many digital and discrete control

techniques such as dead-beat control [21], dissipativity-based

control [22], sliding-mode control [23], space vector-based

control [24], and multiple-feedback loop [25] have been

developed using digital signal processors (DSP).

In this section, fundamental analysis of a dead-beat control

method is explained for the three-phase UPS conﬁguration

shown in Fig. 24.13a. The state space equations of one phase

of this system in the continuous time-domain are as follows.

= i

−i

(24.1)

= V

−V

(24.2)

Considering V

and i

as state variables, the state space

equation of the system is as follows:





•



01/C

−1/L







1/L





−1/C



(24.3)

These continuous time-domain state space equations are con-

verted to the discontinuous time domain with a sampling

period of T

[26].



(k + 1)









cos ω

sinω

−

sinω

cos ω









(k)







1 −cos ω

sin ω





(k)





−

sin ω

1 −cos ω





(k)

(24.4)

Where ω

is the angular resonance frequency of L

and C

The sampling frequency of the system is always considered

much higher than the resonance frequency of L

and C

. With

this assumption, Eq. (24.4) is simpliﬁed to Eq. (24.5). This

conversion is valid for almost f

≥ 20f



(k +1)





1 T

−T



(k)







(k)+



−T



(k) (24.5)

630 A. Nasiri

The current equation according to Eq. (24.5) is given by:

(k + 1) = i

(k) +

(k) − V

(k)] (24.6)

Alternatively, this equation can be achieved by converting

Eq. (24.2) from a differential equation to a difference equa-

tion. The same suggestion of f

≥ 20f

has to be made for this

conversion as well. If V

and i

∗

are considered constant over

the next switching period, the output voltage of the inverter,

which corrects the error of i

after two sampling periods, is

described by:

(k + 1) = V

(k + 1) +

∗

(k + 1) − i

(k + 1)]

(24.7)

A linear estimation of V

(k +1) can be achieved from previous

values:

(k +1)=V

(k)+[V

(k)−V

(k −1)]=2V

(k)−V

(k −1)

(24.8)

By substituting Eqs. (24.8) and (24.10) in Eq. (24.9) and updat-

ing reference current for i

in every two sampling periods, the

dead-beat digital control for series converter is described by:

(k +1)=

∗

(k)−i

(k)]−V

(k)+3V

(k)−V

(k −1)

(24.9)

Equation (24.9) ensures that the current error between i

and

∗

at time k +2 goes to zero with a delay of two sampling peri-

ods. Avoiding interaction between voltage and current control

loops, load voltage, V

, is sampled at half of the current sam-

pling frequency. The voltage equation according to Eq. (24.5)

is as follows.

(k + 1) = V

(k) +

(k) (24.10)

(k + 2) = V

(k + 1) +

(k + 1)

= V

(k) +

(k + 1) (24.11)

PLL

A/D

Converter

(k)

Source

Voltage

sampling

frequency

(k– 2)i

–

(k+1

)

(k)

(k)i

(k)

–

(k) –V

(k–1)

–

FIGURE 24.15 Implementation of the current and voltage control for the inverter shown in Figure 24.13a.

As current control is suggested to be dead-beat with a delay of

two sampling periods, capacitor current at time k and (k+1)

are given by:

(k) = i

∗

(k − 2), i

(k + 1) = i

∗

(k − 1) (24.12)

Substituting Eq. (24.12) in Eq. (24.11) and updating the refer-

ence current at each of the two sampling periods, V

(k +2) is

given by:

(k + 2) = V

(k) +

∗

(k − 2) (24.13)

The current of i

∗

at time k which corrects the voltage error

of V

at time k + 4 is as follows.

∗

(k) =

∗

(k) − V

(k)]−i

∗

(k − 2) (24.14)

A block diagram of the implementation of voltage and current

control of the inverter is shown in Fig. 24.15. Block diagram

of the current and voltage controller for the inverter is also

shown in Fig. 24.16. Voltage regulator is a pure dead-beat

controller with a delay of two sampling periods including the

consumed time for calculation. G

is the time delay needed

for calculations and analog to digital conversions. G

is the

time delay caused by the PWM inverter and G

is the transfer

function of the low pass ﬁlter. Current regulator is also consid-

ered as a pure delay. The output voltage of the inverter follows

its reference with four sampling periods of delay. In practice,

the dynamics of the current regulator is not a pure delay and

shows some deviation from the dead-beat controller.

24.6 Energy Storage Devices

In this section, three dominant energy storage devices for the

existing and future UPS systems are described. These energy

storage devices are battery, ﬂywheel, and fuel cell.

24.6.1 Battery

Battery is the energy storage component of current static UPS

systems. It determines the capacity and run-time of the UPS.

24 Uninterruptible Power Supplies 631

= e

–sT

Current

Regulator

Equation (24.9)

Current

Regulator

Equation (24.14)

s +T

1+L

(k)

FIGURE 24.16 Block diagram of the current and voltage controller for the inverter shown in Figure 24.13a.

For small units, it is the size of battery that determines the

size of the UPS. Different types of batteries are used in UPS

systems but the most commonly used types are lead-acid,

nickel–cadmium, and lithium ion. The lead acid batteries used

in this application are the same as the ones used in the cars.

However, there is one small difference. Car batteries generate

electricity by the reaction of sulfuric acid on lead plates that

are drowned under the liquid. These types of battery cells are

not suitable for UPS applications because there is a chance

of acid spillage from them. In addition, during the charging

process, they release hydrogen that is explosive and dangerous

in a closed environment. Lead acid batteries used in UPS sys-

tems are a special kind called sealed or valve-regulated. The

nickel–cadmium batteries are another popular type of batter-

ies used in UPS systems. They usually provide higher energy

and power density compared to lead-acid batteries. The nom-

inal voltage of nickel–cadmium cells is 1.2 V, which is smaller

than 1.5 V of lead-acid batteries. However, the cell voltage

variation throughout different charge levels is less than lead-

acid batteries. These batteries also have less series resistance

and can provide higher surge currents. Lithium-ion batteries

have much higher energy density. This kind of battery can be

molded into different shapes. They have a nominal voltage

of 4.2 V. The main disadvantage of lithium-ion battery is that

they lose their capacity from the time of manufacturing regard-

less of their charge level and conditions of use. Table 24.2

shows a comparison between different kinds of batteries for

UPS application.

TABLE 24.2 A comparison between different types of batteries for

UPS systems

Battery type Energy density

(WH/kg)

Power density

(W/kg)

Commercial

availability

Lead-acid 35 300 Very mature and

readily available

Nickel–cadmium 40 200 Mature and

available

Lithium-ion 120 180 Available

Nickel hydride 70 200 Available

Zinc-air 350 60–225 Research stage

Aluminum-air 400 10 Research stage

Sodium chloride 110 150 Available

Sodium sulfur 170 260 Available

Zinc bromine 70 100 Available

The traditional method of charging batteries is to apply con-

stant current and constant voltage in two consecutive periods.

Constant current is applied at the beginning of a typical full-

charge cycle, when the battery voltage is low. When the battery

voltage rises to a speciﬁed limit, the charger switches to con-

stant voltage and continues in that mode until the charging

current declines to nearly zero. At that time, the battery is

fully charged. During the constant-voltage phase, the current

drops exponentially due to the sum of battery resistance and

any resistance in series with the battery (much like charg-

ing a capacitor through a resistor). Because current drops

exponentially, a complete, full charge takes a long time.

24.6.2 Flywheel

Flywheel is simply a mechanical mass that is placed on the shaft

of a motor–generator set and stores mechanical energy in the

form of kinetic energy. When the electrical power is required,

this kinetic energy is converted to electricity by the genera-

tor coupled with the ﬂywheel. Flywheels are the oldest type of

energy storage devices. The advantages of ﬂywheel energy stor-

age systems are high efﬁciency, high energy and power density,

and long life. On the other hand, ﬂywheels are more expen-

sive and require more space than batteries and fuel cells. There

are also some safety concerns about ﬂywheels rotating at high

speeds.

24.6.3 Fuel Cell

Due to high efﬁciency and low emissions, fuel cell systems

have been gaining popularity in recent years. A fuel cell uses

hydrogen as fuel and produces electricity, heat, and water from

the reaction between hydrogen and oxygen. Each cell consists

of an electrolyte and two electrodes as anode and cathode.

Figure 24.17 shows the conﬁguration of a typical fuel cell sys-

tem. There are different kinds of fuel cell system depending on

the types of electrolyte and hydrogen sources. Some fuel cell

systems have an on-board fuel reformer and generate hydrogen

from natural gas, methanol, and other hydrocarbons. Recent

technology development in this ﬁeld has made fuel cells a more

reliable and cost-effective alternative for batteries. Fuel cells

currently have a variety of applications in automotive, electric

utility, and portable power industries. Table 24.3 provides a

comparison between the most popular types of fuel cells.