Inversin R. Allen Micro-hydropower Sourcebook

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one of the few approaches that can be applied without

resorting to more conventional approaches.

For the sample iastallation,

a&he

that the village’

las avariable’load totaling a maximum”of 4 kW, and

tl-.at about 15 kW excess capacity is used on a contin-

Cal basis to heat water in vats for a village dyeing ‘,

aperation. The gearing ratio between. turbine and

I the previous

generator continues to be 2.5, as in

sxamples. In this case, with only tbe water heating

sloments on-line, the turbine inlet valve is adjusted

10 that the output frequency is about 52 Hz. As the

variable load increases from zero to the maximumof

4 kW during the day, the output frequency would

decrease to about 46 Hz. Consequently, the

operat-

ing point would remain along the solid line on the

curve in Fig. 8.10 during the plant’s operation.

as load increases,

operating point with

operating point

only a fixed load

moves to the left

(15 kW nominal)

1 I,

/ I\

46 50 52

Frequency (Hz)

Ffg. 8.10, The opemttng cur% assuming a fixed base

load of 15 kW (nomfnaV and a 4 kW variable user load

In this case, assume that the voltage is proportional

to generator speed and that the voltage at the gener

ator terminals at 52 Hz is thus correspondingly large

than nominal. Therefore, althougb a 15 kW heating

element is used, the actual power consumed by

the

elements is sl ghtly higher, approximately 15 kW

times (52/50) or 16 kW. After an additional load

with a nominal power rating of I. kW has been placed

on the system, frequency decreases to 50‘Hz and the

16 kW actually available is dissipated in both the

heater (15 kW) and the additional load (1 kW). As an

additional 3 kW

nominal load is imposed on the

system, the same 16 kW

(or

now slightly less, because

the operating,point has moved to the left, off tbe

peak) is distributed to all 19 kW

nominal

load.

Each device then actually receives slightly less than

its desiped rate+

pyer, ,beca+ the voltage is

decreesed

to ‘(46/50)

92% of its

nommal

value.

As noted in

Ccmstaat loads

(p. 207) and Manual

control

(p. 2071, these two approaches described in those sec-

tions have more flexibility than is first apparent,

because the turbine output need not be matched exactly

to the load. Although the systems in these cases are

designed to operate at the normal frequency-at the

peak of the power curve-they would still operate satis-

factorily if either the total load were somewhat below

(e.g., because some lights have burned out)

above

(e.g., because several bulbs are replaced with some of

higher capacity) its normal value.

Operation on the backside of power curve

Constraints: This approach requires a plant with an

impulse turbine large enough to generate more power

than is to be consumed. In addition, the flow must be

available to generate maximum power, even though this

power will not be generated or consumed. If a reaction

turbine

were

used with this approach, it would be

adversely affected by cavitation. This approach can

only be used where the turbine and generator are cou-

pled by a belt that permits a choice of gearing ratio

between the two. If a turbine is directly coupled to a

generator, it is conventionally designed to operate at

the point of peak turbine efficiency and this approach

cannot be used. In addition, the turbine, but not the

generator, must be capable of operating continually

near its runaway speed.

Although good speed regulation can be obtained using

this approach, it will usually lead to less than optimum

use of the installed capacity of the turbine. However,

plants implemented in rural areas

often

are oversized in

anticipation of a gradual increase in size and types of

load. By the time demand grows, the villagers should

have more familiarity with the plant and with electri-

city and be able to use more sophisticated approaches to

governing to make better use of the plant’s capacity.

This approach is a simple and cost-free way of providing

effective control over speed, and requires no overspeed

protection.

A turbine with a parabolic power output vs. speed curve

shown in Fig. 8.lla is again assumed, but the following

description is valid for any curve; only the actual values

may change. Whatever the curve, there are a number of

possible operating points along it.

Conventionally, a generator is coupled to the turbine so

that it generates the standard frequency of, for exam-

ple, 50 Hz at the turbine speed correspouding to peak

efficiency and power (Fig. 8.11b). If the electrical load

is removed, however, a turbine with no governor would

reach runaway and the generator would be driven at

approximately twice nominal speed. The generator and

voltage regulator would no longer function properly at

these high speeds.

If the gearing ratio between the generator and turbine is

changed so that the nominal operating frequency is

generated when turbine speed is halfway between nomi-

nal and runaway speed as shown in (Fig. 8.114 then

from zero to full load, speed variation would amount to

2 10 Governing

(a)

0.4

(cl

turbine power output

fli

Frequency (Hz)

(d)

PI2

Frequency (Hz)

w 0

*6%

nominal

operating

,/-

point

/ J

(f)

variable

load

-’ --7\- 1

/’

,‘nominal

‘W

/ operating

fixed

point 1

base

I I I

load\,

1 !

Fig. 8.11. Opemting on

the backside

the power curve:

(aI

the

assumed turbine power output vs. speed curve,

@I conventional opemting point, wfth large frequency

Increase at runaway, (cl decreasing the

gearing mtto

beet

ween the turbine and genemtor so as to opemte on the

about &30% of the nominal speed. This might still be

too large a speed (and frequency) variation, but it still

Fermits full use of plant capacity.

If the variations in the user load are less than the power

potential of the plant, it is possible to operate at any of

a number of points:

o With no constant base load and only a variable user

load imposed on the system, the generator could be

coupled to the turbine so that the nominal frequency

would be generated at either the turbine runaway

speed (Fig. 8.11d) or somewhat below that speed

(Fig. 8.1 le). The actual frequency variation depends

on what percentage of peak capacity the actual var-

iation in load represents; the percentage frequency

variation is about one-quarter this percentage. If

the electrical load varies from zero to half or 50%

of turbine capacity, as shown in the figure, the fre-

quency variation will be about -12% or *6% (or

Hz).

One drawback of this approach is that the bearings

supporting the turbine shaft turn at about twice the

usual speed and that their life is therefore reduced

by an average of 50%.

back of the power curve, (d1. a gearing ratio designed to

generate 50

at turbine runaway, (e) a gearing mtio

deslgned to genemte 50 Hz at slightly below runawuy, and

CfI

system

wfth a ffxed base

load

and additional WrlabZe

user load opemting farther up the curve.

o With a constant

base

load less than the peak power

available from the turbine and an additional variable

user load superimposed on this, the generator could

be coupled to the turbine to generate 50 Hz at some

lower speed (Fig. B.llf). Because the turbine’s

power curve flattens .as its peak is approached, a

given change in load will result in a somewhat higher

frequency variation than when no base load is

imposed.

The hydropower plant at Kibimba, Burundi, operates ic

this fashion (see WHY CONSIDER

VARIOUS

GOYERN-

ING OPTIONS?, p. 200). Because no governor was

available, the plant was operated near the end of the

power curve, with the turbine operating near runaway

speed. In addition, during periods when the large light-

ing load was off, switching on the water heater base

load brought the frequency to its nominal value during

the day when other uses

were

minimal.

Ope9ation in patallel with dissipative load

Constraints: This approach requires a second load,

preferably a mechanical load serving a productive pur-

pose, in addition to a primary variable load. This load

Governing 211

must be able to accommodate a range of speeds. This

approach is also applicable when the turbine is directly

coupled to the generator so that it operates at its most

efficient point. To the author’s knowledge, this

approach has not yet been tried.

Although operating on the backside of the turbine’s

power curve (p. 210) permits some control over speed

over a

range

of loads, it results in an inefficient use of

turbine

the power potentially available. The approach described

in this section permits productive use of this otherwise

wasted power.

Let the dotted line in Fig. 8.12a represent the total

power available from the turbine at different speeds.

From an engineer’s point of view, a curve illustrated by

(electrical generator)

the solid line has preferred characteristics-maximum

dissipative

power at nominal speed, with a very small change in

frequency or speed as the load varies from full load to

base load

zero. This can be accomplished by connecting a dissipa-

(heat generator)

tive load to the turbine, which dissipates the power

between the two curves as indicated. It would require a

Fig. 8.13.

One layout

for turbo-generating equipment

. . .

load that consumes no power at the nominal operating

speed--at the peak of the turbine’s power curve--but

quickly consumes a large amount of power as the speed

is increased slightly above this nominal operating speed

(Fig. 8.12b).

Finding a simple device with such a loading characteris-

tic is difficult. One device that only approximates this

desired characteristic is a mechanically driven heat

generator, basically a fan that generates heat by fric-

tion through agitation of a fluid (gas or liquid) (see

Mechanical

heat generator,

p. 241). The power charac-

teristic

this device is indicated by the dashed lines in

Fig. 8.12~; the magnitude of the power dissipated varies

approximately with the cube of the speed. The precise

when a

dissipative load

Well

electric genemtor

are

coupled to a

single

turbine.

in Fig. 8.14 again represents the power available from

the turbine, and the dashed line represents the portion

of that power consumed by the heat generator, the dis-

sipative load. The solid curve-obtained by graphically

subtracting the dashed from the dotted curve-repre-

sents the power remaining to drive the electrical gener-

ator. In this example, most of the power available from

the turbine is converted to electrical power (P,) under

normal operating conditions. A small remaining portion

(P,) is converted to heat, which can be used for some

productive purpose. As the electrical load decreases,

the speed increases and the dissipative load converts an

curve depends on the gearing ratio between the turbine

and this device.

If both this dissipative load and the generator are cou-

pled to a turbine, as shown in Fig. 8.13, the dotted line

increasing amount of power into heat. When the entire

electrical load is withdrawn, the turbine attains its

highest speed. This value depends on how the dissipa-

tive load is incorporated in the overall system. (See

EXAMPLE 8.6.)

(a)

(cl

power power

power that must be con- power dissipated by a

to be

available sumed by dissipative load to

dissipated

mechanically driven heat

from

obtain power characteristics

shown in (a)

generator Y /

: I

-----J

Frequency

change in frequency

from zero to full load

Fig. 8.12. Opemting In pamllel with

a dfssipatiw

load:

(a)

desimble

turbine power characteristics are

shown by

dissipative

load

to achieve the preferred net power chamc

the solid

line,

lb) the desired characteristics

the pamllel

teristics shown

in (a),

and Cc) the actual power

dissipation

chamcteristics of

a mechanically driven

heat

genemtor.

212 Governing

If coupling between the turbine and generator is

changed, this approach can be used, with the approach

described in Operation on the backside of power curve

(p. 210), for speed control. (See end of EXAMPLE 8.6.)

PT = P1 + P2

power consumed /

by dissipative I

load

l e

;

l a

.*a*

l .

. ’

. I

_A*

1 I I

Frequency (Hz)

Fig.

8.14

Power

available

for

electric power generation

when the excess power available

from the

turbine is

con-

sumed

by a heat generator.

Turbine flow modification

Constraints: Unlike other approaches discussed in this

chapter, this approach cannot simply be added onto an

existing system, because it involves phenomena that

occur within the turbine. In addition, although this

approach has been used to exercise some control over

the speed of Pelton turbines, some research and devel-

opment work would be required before this approach

could be used with other types of turbines. Finally, this

approach is better suited to persons who fabricate their

own turbines, because the flow modification devices can

then be integrated within the turbine during its

fabrication.

To control speed by turbine flow modification, a device

must be placed in the vicinity of the turbine runner. As

the imposed load on the system decreases and speed

increases above nominal speed, this device rechannels

the flow within the turbine to introduce disturbances

that effectively reduce the system efficiency. This

reduces the power available from the turbine to m. tch

the reduced user load. -4 system developed in Colombia

best illustrates such a design and its operation.

In field installations, Jaime Lobo Guerrero of the Uni-

versidad de 10s Andes (BogotB) has been using a device

he developed

to control

the

speed of a Pelton runner.

Although the initial studies were theoretical, the actual

device grew out of numerous laboratory trials using a

variety of designs and variations

on those designs that

were more a result of hunches based on experiment and

experience than of theoretical calculations.

LFsume that maximum

use is to be made of outpu

rom the 25 kW turbine.

Only a niaximum

of 5 kV

his capacity will be

used

to,

generate electricity;

emainitig 50 kW ,will

be used fof driving’, a

mehax

:al heat generator.. Under

tjaese con$itims, point

curve A and point

S on &roe B in Fig&: 8,15

WOI

epresent thy +pf$atixig points of the- +ect+a.l

ig points of the- &M+.l

generator an&dissipating lo&& respectively.

generator and -dissipating lo&& respectively.

rical load is removed‘ from the generator, the opa rical load is removed‘ from the generator, the opa

tting point for thb electrical generator descends tting point for thb electrical generator descends

:urve A to pOint R. The

freque

:urve A to pOint R. The frequency. increases to a

imit

abdtit 54 Hz, at which poi& all the energ! imit

abdtit 54 Hz, at which point all the energ!

:onsumed by the dissipating load ,(point T). :onsumed by the dissipating load ,(point T).

r r

I I

Frequency {Hz)

#fi”““““J

300

6Oil

900

1200

Generator Speed (rev/min)

l l

. + . . . *power avaiLable from turbine

-power consumed to generate eledtricity

- - ---power consumed by dissipating load

Fig. 8.15. The effqci of changfng the gearing

m$o bet-

ween

turbine a+ he+ genhmtor uxthe power odlatile

for

6?ectrtcitygenemtlIori’ _ I’ -’ ..;

..-I ,:.

Now consider Ghan&g the

power

dissipation char-

”

acteristics of the dissipatiye.l$ad. With :a ‘heat

generator, tl$is can bi$ done by changing the gearing

ratio between the turbbe’and the heat generator. If

this is reduced, this load will tu&e& at the same

nominal turbine speed, dissipating less

ehergy,

If the

(Continued on

p. 2141

Governing 2 13

To develop such a device requires d familiarity with the

speed increases. For example, Fig. 8.17 depicts in a

flow pattern within a turbine under normal operating

highly simplified manner the flow pattern emerging

conditions and with changes in t!lat pattern as runner

from a Pelton runner rotating at different speeds.

Pelton runner

normal speed

(design load)

increasing speed

(less than design load)

runaway speed

(no load)

free

runner

0.8

Fig. 8.18. A three-jet Pelton

runner fitted

with speed con-

trol sleeves. With this unit, orifice plates are used

as noz-

zles (SO).

0.2

0.4

0.6 0.8

(Runner speed)/(runaway speed)

The speed control device is designed to leave the flow

largely unaffected at the normal runner speed, but at

higher than normal speeds, it gathers a portion of that

flow and uses it to reduce the net efficiency and power

output to the runner. It thereby reduces the increase in

speed to below that which would have occurred without

the device.

The Colombian desigil that has proved to be the most

effective to date is illustrated in Fig. 8.18. Short sec-

Ftg. 8.19. Speed chamcteriatics of

the speed

control

devfce shown in

Fig. 8.18 (5Q).

tions of PVC pipe were curved to

form

casings or

sleeves through which the buckets of the Pelton runner

pass. A gate that leaves just enough clearance for the

buckets to escape is placed at the exit of each casing.

As the load on the turbine decreases and the runner

increases in speed, the flow leaving the buckets

becomes caught up within this casing and, in trying to

runner outside

-!- diameter

/ /

..I.........

*’

. . . . . . . . . . . .

jet

d- -

- runner

disk

Section AA

60 -

0.8

Bucket-tojet speed ratio

P lg. U.PU. Anouter attempt at speecl control by modifying

flow exiting a single-jet

Pelton runner: a control casing

with the shape and position

shown in

(aI results in reduced

overspeed (b) (63).

Governing 215

escape, applies a back pressure on the Pelton buckets.

In addition, when three jets are used, as shown in

Fig. 8.18, the water leaving one casing tends to spoil the

jet immediately behind it, further increasing the effec-

tiveness of this method of speed control. The speed

characteristics of a Pelton runner fitted with these

devices are shown in Fig. 8.19.

Because of the placement of the casings with this

design, some water is caught up even at normal speed,

resulting in a small drop in the peak power available.

Based on an awareness of how the pattern of the water

emerging from a Pelton runner changes with speed,

shifting the casings back slightly would gather less

water under normal operating conditions and should

improve the peak power. Tests undertaken at the ATDI

in Lae, Papua New Guinea, support this hypothesis (63).

Figure 8.20a shows the position of an improvised casing,

and Fig. 8.20b presents the associated power curve.

With this design, however, although peak power remains

unchanged with the addition of the casing, the speed

variation from full to zero load conditions is greater

than with the previous design. Tests with other designs

might identify one that leaves peak power unaffected

and, at the same time, decreases further the speed

increase on loss of load.

To illustrate some available design options, Fig. 8.21

presents the basic configuration of a number of designs

tested in the development of the Colombian design, as

well as curves representing the effect of each design on

the power output of a Pelton turbine.

One advantage of using flow modifications is that pro-

tection against runaway is intrinsic; no additional pro-

tective device is required. When the entire turbine load

is removed, the power is dissipated within the turbine

and the usual runaway speeds are not attained.

0.8

0.6

‘G 0.4

iii

0.2

- flow at design speed

-----+ flow at higher than design speed

Fig. 8.22. Will the addition of the deflector affect the

power chamcteristics of a crossflow turbine and reduce

the

speed

increase when load on the turbine is decreased?

It may also be possible to use a similar approach with

other types of turbines. For example, it might be possi-

ble to control the speed of a crossflow turbine by incor-

porating a flow deflector, as illustrated in Fig. 8.22.

Although the flow at normal operating speed (solid line)

would leave the turbine largely unaffected, perhaps the

flow at greater speeds (dotted line) can be caught and

redirected against the runner and used to have some

effect on controlling speed. There may be other

approaches to redirecting flow to control overspeeding,

but these need further research and development.

short sleeve around each of three jets

single short control sleeve

long control sleeve

modified re-entry

(see Fig. CL)

0.2

0.4 0.6

0.8

1.0

(runner speed)/(runaway speed)

Fig.

8.21.

Test results

usfng

casings

various shapes placed

around a

Pelton runner (51).

2 16 Governing

IX. ELECTRICAL ASPECTS

INTRODUCTION

In this chapter, it is assumed that at least a portion of

the power available from the turbine will be converted

to electricity. It is also assumed that the turbine speed

is adequately governed to meet the needs of the con-

sumer (see GOVERNING, p. 199). Now it is necessary to

specify the generator that will generate the electrical

power and the monitoring, control, and protection

equipment that will ensure protection of both the

equipment and the user.

This chapter first reviews several decisions which will

have to be made--whether alternating current (ac) or

direct current (dc) is to be generated and whether

single- or three-phase power most appropriately meets

the consumers’ needs. The following discussion assumes

that the hydropower plant will provide power to isolated

consumers and will not supplement power into an

already energized grid.

This chapter then describes the parameters required to

specify the generator and briefly describes the function

of the switchgear and the various monitoring and pro-

tection devices. It is assumed that the reader is famil-

iar with basic electrical terminology.

The design of the distribution, secondary service, and

housewiring for a system served by a micro-hydropower

plant does not differ from one served by any other type

of electric power generator. Because this task is best

addressed by an experienced electric power engineer

and electricians (except for very small schemes), this

publication does not discuss these areas in detail.

AC VS. DC

If a micro-hydropower installation is to generate elec-

trical energy, a basic decision to be made is whether ac

or dc generation is more appropriate in a specific situa-

tion. Selecting one

the other involves a trade-off

between the advantages of dc--storage of electrical

energy and a less sophisticated and costly overall

system--and those of ac-transmission over greater dis-

tances, ready availability of end-use appliances, and

convenience.

Magnitude

of power

generated

AC generators are available to generate power

over

the

entire power range, from the bottom of the micro-

hydropower range to the level of massive hydroelectric

powerplants. For a large hydropower plant that

serves

wide service area, ac generation is preferred; transmit-

ting a significant amount of low-voltage dc some dis-

tance would require heavy and costly cables to minimize

losses. With ac, relatively inexpensive transformers can

be used to step up transmission voltage and reduce

losses, even when small cable is used.

DC generators are available up through several kilo-

watts. They are frequently used at the

very

low end--up

to several hundred watts-where battery storage of dc is

required to make such an installation worthwhile

where the costs for governing an ac generator would

contribute significantly to the total cost

the plant.

Between this very low end and several kilowatts,

either

ac or dc generation can be used, depending on the speci-

fic circumstances--whether electrical storage is

required, the distance

from

the pcwerhouse to the con-

sumer, and the uses to which the power will be put.

Usually ac is generated.

Storage of energy

If the energy (kWh) available from a well-situated, iso-

lated installation is adequate on a daily basis but its

power (kW) potential is less than the peak electrical

power demand, energy available during low demand per-

iods must be stored to meet needs during peak demand

periods. This can be done two ways: by storing water

behind a dam or in a forebay during times of low

demand or by generating power and storing the excess in

batteries if small guantities of electrical energy are

envisioned. When the only option is the second one, dc

is often generated. However, if ac is generated and a

load controller is used to govern turbine speed, excess

power could be rectified and stored in batteries at the

powerhouse or anywhere along the ac system. In this

case, an inverter (see p. 218) would be needed to con-

vert dc back to ac if ac equipment is used by the con-

sumer. Though batteries provide a means of storing

power, they are heavy, relatively costly, and require

care for their proper operation and eventual disposal.

Cost and complerity of system

With ac systems, there is generally a need to maintain

frequency and therefore the speed of the turbo-generat-

Electrical aspects 217

ing unit within fixed limits. A governor or load control-

!er often is used for this purpose. These are sophisti-

cated and sometimes costly devices which, if well

designed and maintained, work effortlessly. However, if

a part fails, the plant may have to be shut down, possi-

bly for months, until the needed part and/or expertise is

obtained.

Because dc is obtained by rectifying ac generated at any

frequency, the precise speed

the turbine and genera-

tor is not critical. Thus, there is no need for sophisti-

cated equipment to control speed, and with less sophis-

tication, fewer problems can occur. Providing for the

storage of electrical power in the overall dc system

design does not so much increase system complexity as

it does the need for proper maintenance and use of bat-

teries.

If only small quantities of power are required, dc gener-

ation may be more accessible, because a dc generator or

alternator is a component found in all automobiles and

trucks and is therefore available worldwide. Discarded

cars often contain alternators in good working condi-

tion.

Convenience

Appliances, lights, and motors operating off either ac or

dc are available commercially. Because national power

systems around the world generate ac, however, ac

devices are much more common, generally less costly,

and available in a much wider variety. Most dc appli-

ances are relatively difficult to locate and are usually

available from suppliers that furnish equipment to own-

ers of trailers or caravans (where power comes from

automobile batteries), pleasure boats, and windmills

(which often generate dc to permit energy storage to

compensate for the variability of the wind).

Transmission of power

In transmitting power over any distance, losses inversely

proportional to the square of transmission voltage are

incurred. Minimizing the losses therefore requires max-

imizing, within economic limits, the voltage at which

the power is transmitted.

For the range of power being considered, ac generated

at the standard user voltage, such as 240 V, is rarely

transmitted more than a couple of kilometers. If power

is to be transmitted farther, a transformer can be used

to increase the voltage. With dc, there is no easy means

of changing voltage; therefore, dc losses can be kept

low either by ensuring that the load is near the power-

house or by generating at a higher dc voltage and ensur-

ing that the consumer can use this higher voltage.

Appliances that operate off this higher voltage must

then be found or a sufficient number of storage batter-

ies must be connected in series. The individual batter-

ies in the series can power low-voltage appliances. If

storage batteries are used, proper management of the

system is essential to ensure that the appropriate bat-

teries are being charged by the generator or discharged

by the consumer.

In rural areas, where small quantities of power would be

used by a dispersed population primarily for lighting,

another way of transmitting power might be consid-

ered-carrying energy not by power lines, but in a “box.”

The powerhouse can serve as a small, battery-charging

enterprise where persons who desire power can have

their batteries recharged. This method has several

advantages:

6 a dc generating system at reduced cost and complex-

ity can be used,

distribution costs are avoided, and

a resident can discontinue service

relocate with-

out leaving behind costly power poles and power

lines that may no longer serve any consumer.

However, if automotive storage batteries are used, they

are not

very

portable, can be damaged easily, and their

contents can spill.

The load factor--the ratio of energy actually consumed

to the energy potential if power were consumed contin-

ually at peak levels-in rural areas, especially in devel-

oping countries, tends to be very low; there is a large

demand for power several hours each night, primarily

for lighting, and little demand the rest

the day. This

demand profile requires a larger and costlier hydro-

power installation than is really necessary, and with a

low load factor, energy cost is high and usually has to be

subsidized. Another advantage of generating dc and

storing excess power in batteries is that a smaller and

less costly installation can meet peak lighting loads,

because energy generated during nonpeak times can be

stored and used to supply peak power. The cost of

energy is therefore reduced.

Another approach to reduce transmission costs while

keeping the turbo-generating system simple was pro-

posed in Indonesia (1031c In homes off the grid, families

commonly use motorcycle or automobile batteries to

power lights, radios, televisions, and small electrical

appliances. These are charged periodically at small

shops in electrified villages. Because the use of batter-

ies is well established, it was proposed that power from

a micro-hydropower plant, generating ungoverned ac,

would be transmitted to central locations in several

unelectrified villages. There, it would be rectified for

charging the batteries brought from homes in the vicin-

ity. This system would permit ac to be generated at a

higher voltage, such as 240 V, to minimize transmission

losses, yet would require no expensive governing

because power would be rectified. The turbine would be

operated under some load all the times and, with an

automatic voltage regulator incorporated in the genera-

tor, there would be no large voltage variations.

hlvtwters

It is possible to generate dc and still be able to use

lower-cost and readiiy available ac appliances and to

218 Electrical aspects

transmit power farther with reduced losses. This

requires using an inverter, an electronic device that

converts dc into ac. DC can be generated and stored at

the powerhouse; on demand, an,inverter

can convert dc

to ac, which is then transmitted to the user to power

conventional appliances. Unfortunately, although this

approach is possible, the advantages of transmitting and

using ac that are gained may be offset by the cost and

sophistication of the inverter that would be required.

However, it can still prove effective if storage is essen-

tial to a system’s operation. If flow is sufficient and

storage is not required, using a load controller to con-

trol frequency with an ac system can prove more cost-

effective.

SINGLE- VS. THREE-,PHASE

If an ac generating system is found to be more appro-

priate for an installation, as is usually the case, there is

still a need to determine whether single-

three-phase

power will be generated. The availability of equipment

and the type and size of the loads affect this choice.

Single-phase generators are available to cover the

entire micro-hydropower range, and three-phase

genera-

tors cover the range down to about 2-3 kW. Below this

rating, individual loads would probably be a more signi.

ficant percentage of the total generator capacity, and

balancing the phases would be more difficult. This is

one reason that single-phase generation is commonly

used with schemes less than lo-15 kW. Although both

single- and three-phase generators cover the entire

POWei

range of concern here, local availability of

generating equipment may also affect the final choice.

For small units, the costs for single- and three-phase

generators are similar; however, a three-phase system

requires costlier switchgear and monitoring and control

equipment. A single-phase generating system can be

less costly because it requires switchgear and monitor-

ing, control, and protection equipment for only one

rather than three phases.

The size of the larger anticipated system loads may

determine which of these options is most appropriate.

Single-phase motors are available for applications that

require no more than several horsepower. For larger

loads, three-phase motors are generally used.

The circuit for each phase of a three-phase generator is

designed to accommodate one-third of the generator’s

rated power output. Consequently, as this output is

approached, it becomes increasingly important that the

power in each phase be balanced; otherwise, excess

power may flow through one or two of the other cir-

cuits, overheating the generator windings and eventually

causing the insulation to break down and the generator

to fail. For example, a balanced 30 kVA generator

operating at full capacity would generate 10 kVA in

each phase. If the flow through the turbine remains

unchanged but the load through one phase drops signifi-

cantly, the two other phases would have to accommo-

date

than 10 kVA each. Adequate protection

equipment can avoid this potential problem, which is not

encountered when a single-phase, two-wire generator is

used. Thus, one advantage of a single-phase, two-wire

system is that no balancing of loads is necessary.

Whether single- or three-phase power is transmitted

more cost-effectively depends on the configuration

selected. Table 9.1 compares the quantity of conductor

material required for different configurations for lines

of equal length, with equal balanced electrical loads.

ELECTRj[cAL EQUIPMENT

In the following discussion, it is assumed that a micro-

hydropower plant will

generate ac

rather than dc-which

is generally the case in all but some very small

systems-and will operate in isolation of any other

power source, such as a diesel generating set or a

national or regional grid.

For micro-hydropower plants, a generator coupled to a

turbine and associated electrical equipment are often

furnished as part of a pre-engineered package. For a

supplier to assemble the most appropriate unit for a

specific site, he must have the necessary specifications.

TABLE

8.1.

Compiam of the lwlauw total c?oductor

materfaf requfred for uwfous

powr

tmnsmlsslon

Wffp

mtftm with bnlanced elect&al lotwb

Comparative total

conductor material

System

short Long

line*

single-phase,

two-wire,

240 v

three-phase,

three-wire,

240 v

(delta system)

single-phase,

three-wire,

4801240 v

three-phase,

four-wire,

4151240 v

(Y system)

100 100

*For a short line, the conductor size is based on

keeping heating of the conductor within acceptable

limits. For a long line, maintaining voltage drop

within acceptable limits determines conductor size.

Electrical aspects 219