Power electronic handbook

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714 C. V. Nayar et al.

Generator

Diesel Engine

Synchronous

Generator

Supervisory control

Load

Bus

DC/DC

Converter

DC/DC

Converter

DC/AC

Converter

DC/AC

Converter

Wind

Turbine 1

Wind

Turbine 2

FIGURE 27.68 Schematic diagram of isolated grid system having a wind park.

electronics equipment used today is based on industrial

motor drives technology. Having dedicated, high power

density, modular systems will provide ﬂexibility and efﬁ-

ciency in dealing with different energy sources and large

variation of generation systems architectures.

• There is a need for new packaging and cooling technolo-

gies, as well as integration with PV and fuel cell will have

to be addressed. The thermal issues in integrated systems

are complex, and new technologies such as direct ﬂuid

cooling or microchannel cooling may ﬁnd application in

future systems. There is large potential for advancement

in this area.

• There is a need for new switching devices with higher

temperature capability, higher switching speed, and

higher current density/voltage capability. The growth in

alternative energy markets will provide a stronger pull for

further development of these technologies.

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Fuel-cell Power Electronics for

Distributed Generation

S. K. Mazumder, Ph.D.

Department of Electrical and

Computer Engineering, Director

Laboratory for Energy and

Switching-Electronics Systems

(LESES), University of Illinois,

Chicago, Illinois, USA

28.1 Distributed Generation ............................................................................ 717

28.2 Fuel-cell Based Energy Systems for DG ....................................................... 719

28.3 Power-electronic Topologies for Residential Stationary Fuel-cell

Energy Systems....................................................................................... 720

28.4 Towards Power-electronic Topologies for High Power Fuel-cell Based DG........ 731

References ............................................................................................. 736

28.1 Distributed Generation

On-site power generation (often called as distributed

generation (DG) [1, 2]) using alternative/renewable-energy

technologies, as illustrated in Fig. 28.1, can minimize envi-

ronmental pollution and reduce our dependence on fossil

fuels. Distributed energy resources (DER) are parallel and

stand-alone electric generation units located within the elec-

tric distribution system near the end user. The distributed

energy resources, if properly integrated can be beneﬁcial to

electricity consumers and energy utilities, providing energy

independence and increased energy security. Each home and

commercial unit with DER equipments can be a micro-power

station, generating much of the electricity it needs on-site and

sell the excess power to the national grid.

Table 28.1 provides information regarding commercially

available equipment for DER [1, 2] and for those technolo-

gies still undergoing development. Some of these technologies

are listed in both categories because they are commercially

available and undergoing further research and development

as well. There are several customer DG applications includ-

ing (i) allowing customers to continuously generate their own

electricity, with or without grid backup; (ii) permitting cus-

tomers to generate power while serving their thermal and/or

cooling loads; (iii) generating a portion of electricity on-site to

reduce the amount of electricity purchased during peak-price

periods; (iv) licensing customers to sell excess generation back

onto the grid when their own demand is low, especially during

peak-pricing periods; (v) using standby or emergency power

to backup grid-based power; (vi) improving customer power

quality and reliability; (vii) serving niche applications, such as

“green” power or remote power; and (viii) meeting contin-

uous power, premium power, or cogeneration needs of the

residential market.

Customer Generation

Continuous customer generation applications produce power

on a nearly continuous basis, running at least 6,000 hours per

year. When evaluating the usage of DG technologies in this

capacity, customers consider competing grid price, as well as

the installed cost of the unit and fuel costs. Maintenance costs,

power quality, and reliability of grid power are other critical

components. In non-attainment areas, emissions can provide

a strong barrier to these applications.

Cogeneration

Cogeneration, also known as combined heat and power

(CHP), utilizes otherwise wasted exhaust heat as useful ther-

mal output, typically steam. The steam may be used either

for space heating or space cooling. Again, CHP applications

are driven by grid price and installed cost, but emissions can

provide a strong barrier to implementation, especially in non-

attainment areas. As with continuous power applications, these

717

718 S. K. Mazumder

Residential

Main

Generating

Station

Commercial Building

Chip Manufacturer

Substation

Photovoltaic

And Fuel Cell

Reciprocating Engine

Standby/Peak Shaving

Transmission

Fuel Cell

FIGURE 28.1 Illustration of a DG-based power generation.

TABLE 28.1 Attributes of commercially available and emerging DER

technologies

DER Technologies Commercially

Available

Emerging

Technology

Size Efﬁciency

(%)

Microturbines X X 10–300 kW 20–30

Reciprocating

engines X – 50 kW–6 MW 33–37

Stirling engines – X 10–200 kW <40

Solid-oxide

fuel cells (SOFC) X X 5 kW–3 MW 45–65

Proton-exchange-

membrane

fuel cells X – 1 kW–3 MW 34–36

Photovoltaic

systems X – 1 kW–1 MW <22

Wind systems X X <30

Hybrid systems – X <1 kW–1 MW 40–50

Combined heat and

power (CHP) X X 5 kW–25 MW 50–80

units will run on a nearly continuous basis, typically at least

6,000 hours per year.

Peak Shaving

Driven primarily by high utility demand charges, peak-shaving

applications (sometimes called peak clipping) are also affected

by installed cost, perceived unit reliability, and fuel prices.

These units operate much less frequently than do continuous

power or CHP applications, often running as few as several

hundred hours annually.

Selling Power to the Grid under Net Metering

Depending on what state they live in, some customers may be

eligible for net metering. If eligible, net metering effectively

allows a customer to sell its excess generation back to the

grid at the same retail price as the customer buys power

from the grid during other periods. With this ﬁnancial incen-

tive, the market for small commercial and residential DG

installations, especially those based on the renewable energy

technologies, should increase.

Standby/Emergency Generation

Applications providing standby or emergency power are typ-

ically driven by the reliability (perceived or real) of the grid

and the cost to the customer of outage. Although the DG unit

may only operate a few hours a year, it is used to power critical

devices whose failure would result in property damage and/or

threatened health and safety. Some customers like hospitals

and airports may be required by code to install and maintain

these units. For others, the high cost of outage drives the appli-

cation. The choice of DG technologies for these emergency

and standby applications is determined by installed cost, time

required to start (i.e. black-start response), fuel access/storage,

and size/weight of the unit.

Premium Power

On-site generation can improve both power quality and power

reliability, especially when backed up with grid-based power.

This application requires a DG technology that can oper-

ate continuously. In an era of both increasing power outages

and rising demand for premium power, many businesses may

install DG units to protect against the risk and cost of power

outages. These customers include banks, semiconductor man-

ufacturers, grocery stores, hospitals, and many other industrial

and commercial market sites.

Green Power

Many of the renewable technologies and fuel cells have

very low emissions. With the addition of emission-reducing

technologies, microturbines and miniturbines also emit low

levels of regulated gases. Customers, who are environmentally

inclined, may purchase these DG applications for this reason,

even if they have to pay a slight premium for green power

compared with grid-based power purchases.

Remote Power

Residences and small commercial establishments (such as

ranches, dairy farms, and ﬂower growers) that are located well

away from the transmission and distribution (T&D) system

may prefer to generate their power on-site. By doing so, they

eliminate both the cost of connecting to the grid and any prob-

lems associated with their position at the end of a long T&D

line. The elimination of these problems, which include power

outages and lower quality power, can produce the compelling

economics necessary to the further use of DG technologies in

a remote power capacity.

28 Fuel-cell Power Electronics for Distributed Generation 719

(

)(

)

2000

1600

1200

800

400

1970 1980 1990 2000 2010

1970 1980

1990 2002 2010 2025

2020

History

Projections

Industrial

Residential

Commercial

History

Projections

Coal

Petroleum

Nuclear

Nonhydro

Hydro

Natural gas

renewables

FIGURE 28.2 (a) Current and projected electricity sales in terawatt-hours (TWh) by sector and (b) energy production (quadrillion BTU) by fuel in

USA [3].

28.2 Fuel-cell Based Energy Systems

for DG

Hydrogen-based fuel-cell

energy is one of the two front-

runner alternative-energy solutions to address and alleviate

the imminent and critical problems of rapid depletion of

existing fossil-fuels (as illustrated in Fig. 28.2) and environ-

mental pollution due to high emission. The framework for

integrating these “zero-emission” alternative-energy sources

to the existing energy infrastructure has been provided by

the concept of DG, which provide an additional advantage:

reduced reliance on existing and new centralized power gen-

eration, thereby saving signiﬁcant capital cost. However, to

achieve the projected worldwide target of $50 billion by

2015, the fuel-cell energy systems (as illustrated in Fig. 28.5)

have to address cost, durability and reliability, and energy

efﬁciency.

Currently, the applications of fuel cells (especially SOFC)

for stationary applications are primarily for (i) lower power

residential with a power range of <5 kW and (ii) for very

high power reactive-power and harmonic compensations and

high temperature coal-power applications. In Sections 28.3

and 28.4, we outline some established and possible power-

electronic topologies for low- and high-power stationary

applications, respectively.

Table 28.1 shows that SOFC (electrical characteristics illustrated in

Fig. 28.3) is the most energy efﬁcient among the listed DG equipment. The

efﬁciencies of the SOFC systems can be further enhanced by utilizing the qual-

ity of waste heat derived from the fuel-cell reactions for CHP and combined

cycle applications (as illustrated in Fig. 28.4).

Stack Voltage (V)

Activation Region

Ohmic Region

Concentration Region

Maximum Power/Efficiency

Current (A)

Power (W)

Open Circuit Voltage

100

0 102030405060708090

1000

2000

3000

4000

5000

6000

FIGURE 28.3 Illustration of a SOFC operation in activation, ohmic, and

concentration/mass-transport regions. The simulated current–voltage

characteristics and the corresponding power output for a 100-cell planar

SOFC stack are shown.

100

Advanced

Gas Turbine

System

High

Temperature

Fuel Cell

Gas Turbine/

Fuel Cell

Combined Cycle

EFFICIENCY (%)

FIGURE 28.4 An illustration showing the vision of combined SOFC

and gas-turbine operation to improve energy efﬁciency.

720 S. K. Mazumder

FUEL

SUPPLY

FUEL

CELL

STACK

THERMAL

MANAGEMENT

AIR mgmt.

SENSORS

FUEL Mgmt.

AND

ELECTRONIC

CONTROLS

FUEL

PROCESSOR

(GASOLINE

OR METHANE)

BATTERY

DC/DC

CONVERTER

DC/AC

INVERTER

120V / 240V

60 HZ LOAD

WASTEHEAT

Mgmt.

CONTROL ELECTRONICS

FOR DC/DC CONVERTER,

INVERTER

CENTRAL POWER CONTROL UNIT

FIGURE 28.5 A typical fuel-cell energy system architecture comprising: (a) the fuel processor/balance-of-plant (BOP); (b) the fuel-cell stack (see

Fig. 28.3 for a typical characteristic); and (c) the power-electronic subsystem (PES). We note that single-stage PES design is also feasible.

28.3 Power-electronic Topologies

for Residential Stationary Fuel-cell

Energy Systems

The choice of power-electronic topologies can be broadly

categorized as push–pull and full-bridge based topologies.

In [4–7], low-cost push–pull topology for power levels

>1 kW is proposed. Push–pull based topology, owing to its

low part count, is a good candidate for a low-cost fuel-cell

converter and can achieve an efﬁciency of 90% for low and

medium power. However, at higher power it can suffer from

transformer ﬂux imbalance and core-saturation problems.

For current-fed topologies, the main limitation is encoun-

tered at greater than 3 kW, where, leakage inductance of the

transformer poses a problem, unless soft switching is used.

Further, the limited available range of switch duty cycle also

makes it difﬁcult to track variations in the input voltage.

In [8–14], full-bridge based fuel-cell inverter topologies are

discussed. Because of the symmetrical transformer ﬂux and

equal electrical stress distribution, several variations of full-

bridge inverter topologies have been found to be useful from

the cost and efﬁciency point of view, especially when imple-

mented for power levels greater than 3 kW. A 1.8 kW prototype

of a boost converter followed by a two-stage dc/ac converter

is discussed in [9]. The two-stage dc/ac converter comprises

a full-bridge high-frequency inverter and a cycloconverter,

both operating at the same switching frequency, is based on a

novel multi-carrier pulse-width modulation (PWM). A sim-

ilar two-stage dc/ac converter is discussed in [13], which

incorporates a zero-current switched cycloconverter; while the

full-bridge high-frequency inverter is switched with a ﬁxed

50% duty cycle. An improved version of the prototype in [13],

comprising a soft-switched sine-wave pulse-width modulation

(SPWM) full-bridge HF inverter followed by a cycloconverter

(operating at line frequency) is proposed in [14]. Among all

the low-cost fuel-cell inverters [4–14], which aim to achieve an

efﬁciency of greater than 90%, a maximum full-load efﬁciency

of 87 and 89% is demonstrated in [4] and [14], respec-

tively. Also, the power-electronic topologies in [4–14] are not

designed to improve efﬁciency of the stack.

In this section, we describe three low-power stationary

fuel-cell power-conversion schemes being pursued at the

University of Illinois, Chicago with focus low cost, energy

efﬁciency, and reliability. The ﬁrst PES comprises a zero-ripple

boost converter followed by a two-stage dc/ac converter com-

prising a soft-switched phase-shifted SPWM multilevel high-

frequency inverter and a line-frequency-switched cyclocon-

verter. The second PES comprises a front-end phase-shifted

high-frequency full-bridge inverter followed by a step-up high-

frequency transformer and a dual forced cycloconverter for

universal power conditioning at high efﬁciency. The ﬁnal PES

comprises a single-stage isolated differential

Cuk inverter [15]

and has very low switch count and reduced cost.

A. Ripple-mitigating Power Conditioner [16–18]

The power conditioner described in this section comprises a

fuel-cell powered dc/dc zero-ripple boost converter (ZRBC),

which generates a high voltage dc at its output, followed by a

soft-switched, transformer isolated dc/ac inverter, which gen-

erates a 110 V ac. The high-frequency (HF) inverter switches

are arranged in a multilevel fashion and are modulated by

a fully rectiﬁed sine wave to create a HF, three-level ac

voltage as shown in Fig. 28.6. Multilevel arrangement of

the switches is particularly useful when the intermediate

28 Fuel-cell Power Electronics for Distributed Generation 721

Fuel

Cell

Stack

Load

AC/AC

Filter

out

fly

HF Inverter

ZRBC

1:N:N

pri

0.5V

0.5NV

(a)

out

pri

Zero Ripple

Filter

pri

(b)

(

)

FIGURE 28.6 Schematic of the ripple-mitigating fuel cell power conditioner [16–18].

dc voltage >500 V. The HF inverter is followed by the ac/ac

cycloconverter which converts the three-level ac to a voltage

that carries the line-frequency sinusoidal information.

Zero Ripple Boost Converter (ZRBC)

The ZRBC is a standard non-isolated boost converter, with

the conventional inductor replaced by a hybrid zero-ripple

ﬁlter (ZRF). The ZRF (shown in Fig. 28.6) is viewed as a

combination of a coupled inductor (shown in Fig. 28.6) and

a half-bridge active power ﬁlter (APF) (shown in Fig. 28.6).

Such a hybrid structure serves the dual purpose of reduc-

ing the high- and the low-frequency current ripples. The

coupled inductor minimizes the high-frequency ripple from

the fuel-cell current (I

+ i

= i

) and the APF min-

imizes the low-frequency ripple from the fuel-cell current

+ i

= i

). I

is the dc current supplied by the fuel cell,

is the high-frequency ac current supplied by the series com-

bination of identical capacitors C

and C

(in Fig. 28.6), and

is the low-frequency ac current supplied by the APF stor-

age reactor L

. For effective reduction of the high-frequency

current from the fuel-cell output, the value of the capacitors

and C

should be as large as possible. However, the

series combination should be small enough to provide a high

impedance path to the low-frequency current i

. Therefore,

for a chosen value of capacitor, the values of the following

expression hold true [18]:

= C

= 2C (28.1a)

√

(28.1b)

√

(28.1c)

where

is the switching frequency of the converter,

is the lowest frequency component in i

Assuming the switching frequency is approximately twenty

times the lowest frequency component, the value of ZRF

722 S. K. Mazumder

Half-Bridge Active Power Filter

+ i

–

Coupled Inductor

Zero Ripple Filter

–

+ I

(a)

(b)

(c)

FIGURE 28.7 Schematic diagrams for: (a) coupled-inductor structure for reducing the HF current ripple; (b) half-bridge active ﬁlter, which

compensates for the low frequency harmonic current ripple demand by the inverter; and (c) the proposed hybrid zero ripple ﬁlter structure.

passive components L

and L

can be determined as

follows [18]:

≥ 20f

(28.1d)

√

≥

√

(28.1e)

≥ 100L

(28.1f)

Therefore, the value of L

should be small in order to limit the

value of L

and also to minimize the phase shift in the injected

low frequency current i

In the following sub-sections, the high- and low-frequency

ac-current reduction mechanisms (Fig. 28.7) and the condi-

tions to achieve it are discussed. In addition to this, the effect

of coupled inductor parameters on the bandwidth of the open

loop system will be discussed. For the purpose of analysis, the

value of the capacitors C

and C

are assumed to be large.

Hence, the dynamics of the APF is assumed to have minimal

effect on the coupled inductor analysis.

High-frequency Current-ripple Reduction

In this section, the inductance offered by the coupled induc-

tor and the ripple reduction achievable is discussed. For that

purpose we need to derive an expression for the effective

inductance of the coupled inductor. Because, the value of

the capacitors C

and C

are large and that of L

is small,

the dynamics of the APF is assumed to have minimal effect

on the coupled-inductor analysis. In the pi-model for the zero

ripple coupled inductor and the excitation voltage and the cur-

rent for the primary and the secondary windings are shown in

the Fig. 28.8. The currents i

1HF

and i

2HF

are, respectively, the

primary and the secondary AC currents shown in Fig. 28.8.

(

)

1HF

+nL

2HF

(28.2a)

(

)

1HF

(

+nL

)

2HF

(28.2b)

2HF

1HF

2HF

–

1:n

FIGURE 28.8 AC model for the coupled inductor shown in Fig. 28.7a.

n =

∼



(28.2c)

where L

is the self inductance of the primary winding with N

turns. Solving Eqs. (28.2a and b), the expressions for di

1HF

/dt

and di

2HF

/dt are obtained using

1HF

+nL

−nL

−v

)

(28.3a)

2HF

−v

−L

(28.3b)

By substituting Eq. (28.3a) in (28.3b), we obtain the following

expression:

1HF

+nL

−nL

)

2HF

(28.3c)

28 Fuel-cell Power Electronics for Distributed Generation 723

To reduce the AC component of the fuel-cell-stack current to

zero, the following condition should hold true:

1HF

2HF

(28.4)

Therefore, using the above condition and Eq. (28.3c), we

obtain

1HF



1 +

(1+n)



eff

(28.5)

The denominator of Eq. (28.5) is the effective inductance L

eff

offered by the coupled-inductor structure of the hybrid ﬁlter.

The effective inductance depends on the turns ratio n, the cou-

pling coefﬁcient k, and the self inductance L

of the primary

winding. For very small values of turns ratio (n 1), signif-

icantly large values of effective inductances can be obtained.

Figure 28.9 shows the effective inductance curves and the cor-

responding reduction in the ripple. Figure 28.9a shows the

dependence of normalized L

eff

on n as a function of k. For the

values of effective inductance shown in Fig. 28.9a, the cor-

responding values of achievable ripple current in both the

coupled-inductor windings are shown in Fig. 28.9b. Using

Fig. 28.9b, a designer can decide on a value of high-frequency

current ripple and using the corresponding values of n and k,

the normalized effective inductance can be chosen from

Fig. 28.9a. While deciding the value of high-frequency ripple,

one should choose a small value for n (<0.25), to ensure that

is small enough to prevent signiﬁcant variations in the

voltage across capacitors C

and C

. Also, the effective induc-

tance should be chosen to meet the bandwidth requirements

of the ZRBC. Increase in the effective input inductance has a

two-pronged effect on the open loop frequency response of

the ZRBC. First, the bandwidth is reduced and second, the

Turns Ratio (n)

Normalized Effective Inductance (L

eff

)

k=0.9

k=0.1

k=0.3

k=0.7

k=0.5

Normalized Ripple Current (di

1HF

/dt)

k=0.9

k=0.1

k=0.3

k=0.7

k=0.5

(a)

Turns Ratio (n)

(b)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.8 0.9 1

FIGURE 28.9 Normalized: (a) effective inductance and (b) ripple current of the coupled inductor.

RHP zero is drawn closer to the imaginary axis, resulting in a

reduction in the available phase margin and thereby the ZRBC

stability.

Active Power Filter

The input current of the inverter comprises as dc component

and a 120 Hz ac component and is expressed as

out

cos(θ) −

out

cos(2ωt −θ) (28.6)

where

out

inverter output voltage,

out

are inverter output current,

is the average value of voltage across the series capaci-

tors C

and C

θ is the load power factor angle.

Here, we derive the condition for low frequency current

ripple elimination from the PCS input current. For the APF

shown in Fig. 28.6, the voltage across the storage reactor L

the APF is expressed as

= V

−

= V



−



(28.7)

where S

is the modulating signal. The reactor current i

−1/2)

jωL

(28.8)

where

= 0.5 +

∞



n=1

sin n(ωt + φ)