DG 3 Voltage Rise

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Embedded generation

Voltage rise

the big issue when

c

nnecting

embedded generat on to long

11 k V

overhead lines

There has recently been much interest in embedding sma ll generator s deep within

distribution systems. The steady-state voltage rise resultingfrom the connection

of

these gener ators ca n be

a

ma jor obstacle to the ir connection

at

the lower voltage

levels. This article sum marises the results of some generic studies, explaining this

voltage rise issue and how it ma y be overcome.

by C. L. Masters

here has recently been much interest in

connecting small generators, between

200kW and lOMW, deep within distri-

T

ution systems. These networks are,

by tradition, passive networks. They were

designed to pass power from the national

grid system, down the voltage levels, to LV

customers. They were generally not designed

for the connection

of

generators. There are

many technical issues that must be considered

when connecting a generating scheme to the

distribution system, such as:

thermal rating of equipment

system fault levels

stability

reverse power

flow

capability of tap-changers

line-drop compensation

steady-state voltage rise

losses

power quality (such as flicker, harmonics)

protection.

This article concentrates on the steady-state

voltage rise that occurs when connecting

small generators to

l l k V

networks and often

seriously impacts on the technical feasibility of

such schemes.

Allowable voltage variations

The Electricity Supply Regulations’ stipulate

that, unless otherwise agreed, the steady-state

voltage

of

systems between l OOOV and 132kV

should be maintained within

+6%

of the

nominal voltage. For systems above 50V and

below lOOOV , variations of between +lo%

and -6% of nominal voltage are permitted.

Prior to the

1994

amendments, variations of

+6% were permitted. This change was a result

of proposals to harmonise the UK electricity

system with those in Europe.

The Electricity Supply Regulations are soon

to be replaced with the Electricity Safety,

Quality and Continuity Regulations.’ They

were due to come into force in October 2001,

but have been delayed due to the numerous

comments made during the consultation

process. The Electricity Safety, Quality and

Continuity Regulations do not propose to

make any immediate changes to the permitted

voltage variations. However, it is proposed

that, with effect from January 2003, the

permitted voltage variations for systems

between

50V

and lOOOV will change to

+lo .

It is the Distribution Network Operator’s

(DNO’s) responsibility to ensure that its

systems are operated within the limits

permitted by the Electricity Supply Regula-

tions. However, at the planning stage, the

l k V

system is often designed to maintain voltages

within *3% of nominal,

so

that the voltage

variations seen by the LV connected customers

remain within the permitted +lo and -6%

limits.

When a generator is to be connected to the

distribution system, the DNO will consider

POWER ENGINEERING JOUR NAL FEBRUARY 2002

5



Embedded generation

1 Voltage profile along

the heavily loaded

11kV

overhead line used

in

the example

primary

substation

IO

r

ominal voltage at primary substation

03 of nominal voltage at primary substation

06 of nominal voltage at primary substation

+6 voltage limit

1 6

1 4

102

1

98

96

94

92

I

I I

I

0 4 8 12 16 2

distance from the primary substation, km

the worst case operating scenarios and ensure

that their network and customers will not be

adversely affected. Typically, these scenarios

are:

no generation and maximum system demand

maximum generation and maximum system

maximum generation and minimum system

demand

demand.

Some

DNOs

take into account the diversity

of

the local load and consider the system with the

minimum expected demand. Others do not,

and assume no load

as

the worst case scenario.

Distribution system s with no embedded

generation

To transmit power from an l l k V primary

substation to

a

typical LV connected customer

some distance away will require the voltage

at the primary substation to be higher than

the voltage at the point of connection

of the customer to the 11 kV system. This

is

explained using Panel 1.

Generally the X/R ratio of an ll kV overhead

line tends to be low,so neither

of

the terms R

or X Q can be neglected. This, coupled with the

fact that the reactive power pushed down the

line is usually much lower in magnitude than

the power (assuming the customer imports

reactive power), leads to there being a voltage

drop along the line from the primary substation

to the point of connection

of

the customer.

To demonstrate this, consider the following

example (Fig. 1 : connected to a primary

substation is a 2Okm long, 1 l k V overhead line ,

comprising 16mm2 copper conductors . Every

4km along the line is a three-phase load of

lOOkW

and 20kvar. As the distance from the

primary substation increases the voltage falls.

With the primary substat ion at nominal voltage

(Il kV) , the far end

of

the line

fal ls

to 10.3kV

(6% below the nominal voltage). This is right

on the permitted limit. If the line had been

longer or the load greater, the voltage would

have fallen even further.

To maintain system voltages within permit-

ted limits, DNOs often maintain primary

substations above nominal voltage using

automatic voltage control (AVC), on-load

tap-changers and line-drop compensation.

POWER ENGINEERING JO URN AL FEBRUARY 2002



Embedded generat ion

where

VPS is the primary sub station voltage

VC is the vol tage at the customer connect ion p oint

R,

X

are the resistance and reactance

of

the overhead line

P, Q

are the power and r eactive power transm itted

from

the primary substation nto the overhead line

Controlling the primary substation, in this

example, to

103%

and

106

of

nominal voltage

(11.3 kV and 11.7 kV) maintains the end of the

l lkV line well within the permitted voltage

limits.

Although the Electricity Supply Regulations

allow voltage variations on the

11kV

system of

c6 , D N O s often impose limits of *3 at the

planning stage. This is in order to maintain

the LV connected customers within the

permitted + lo and -6% of nominal voltage.

In this generic study the +3 planning limit is

ignored. The

l l k V

system voltages are allowed

to

vary by +6% of nominal voltage, to more

clearly demonstrate the effect of connecting a

generator.

Effect of connecting generation to

distribution systems

Connecting a generator to the distribution

system will affect the flow of power and the

voltage profiles. To export its power, a genera-

tor is likely to have to operate at a higher

able to absorb a significant amount of reactive

power. This is explained using Panel

2.

As the

XIR

ratio of the l l kV line is small,

neither

R P

nor X Q is negligible. The XQ term

may be positive or negative, depending on

whether the generator is exporting or

importing reactive power. However, as the

magnitude of the reactive power will be small

compared to that of the power (unless some

form of compensation is used), the RP XQ

term will tend to be positive. Thus , the voltage

at the point

of

connection of the generator to

the l l k V system will rise above that of the

primary substation.

To demonstrate this, a 300kW generator

(operating at unity power factor) is connected

l2km from the primary substation (controlled

at 103% of nominal voltage). The output of the

generator is equal to the downstream demand,

so the direction of the power flow from the

primary substation is not altered. The voltage

falls as the distance from the primary sub-

station increases, as before. But the magnitude

voltage than the primary substation, unless it is

of the voltage drop

is

less profound (Fig. 2).

-- I _ _ _ _ _I I - . I ___

where

VG N

t

PS

VPS

is the primary substation voltage I

VG N

IS

the voltage at the generator connection point

R X are the resistance and reactance

of

the overhead line

P Q

are the power and reactive power transmitted from

the generator into the overhead line

I . . ..

....

POWER ENGINEERING JOURNA L FEBRUARY

2002



Embedded generation

o generation

00

kW

generator

MW

generator

/

g 109

<

107

._ 105

E

103

101

-

979 0 12 16 20

0 4 8

12 16 20

no demand on the lineull demand on the line

distance from the primary substation, km

2

Effect of connecting

a

Increasing the generation to 1MW reverses

- -

generator On thevoltage

profile along the 11kV

line

used

n the

the flow of power along the line, from the

generator towards the primary substation.

The voltage at the generator rises above that

elsewhere, thus allowing the power to be

exported in both directions. In this example,

the voltage in some parts of the system rises

above the permitted +6%voltage limit.

The voltage rise is more onerous when

there is no demand on the system,

as

all

the generation is exported back to the

primary substation. With 1MW of generation

connected, the voltage rises to 112% of

nominal. This suggests that it

is

the voltage rise

during periods of no/minimum demand that

limits how much generation can be connected.

When connecting

a

generator to the distri-

bution system, a

D N O

must consider whether

3

the power may be exported back through the

primary substation primary substation and must ensure that

ru r l

311 1kV

the transformer's tap-changers are capable of

operating with a reverse power flow.

How

can this voltage rise be counteracted?

If the connection of a generator to an 11 kV

overhead line causes an excessive voltage

rise, there are several techniques that can

be employed to alleviate the situation, for

example:

reduce the primary substat ion voltage

allow the generator to import reactive power

(reducing the RP+XQ term)

install auto transformers, or voltage regu-

lators as they are often called, along the line

(resetting the voltage along the line)

increase the conductor size (reducing the

resistance)

constrain the generator at times of low

demand (reducing the transmitted power)

a combination of the above.

Reduce the primary substation voltage

It is common practice for

DNOs

to maintain

llkV primary substations above nominal

voltage to ensure that system voltages remain

within the permitted

-6%

voltage limit. In the

previous example, the voltage at the 1MW

generator is 109% of nominal (under full-

load conditions). Lowering the voltage at the

primary substation from

103%

to 100% of

nominal reduces the voltage rise to just below

the permitted +6%voltage limit (Fig. 4). I t

also

reduces the voltage during periods of no

system demand to around 110% of nominal,

which is not sufficient.

Before lowering the voltage at

a

primary

substation, a D N O must ensure that it will not

adversely impact on any of its customers. If

there are other feeders connected to the

primary substation or teed off the l l k V line,

the voltage profile along these circuits may be

depressed. This may reduce the voltage of the

LV customers connected to these feeders below

the permitted

-6%

limit.

Also, if the generator is not exporting power,

the system voltages will be depressed. In this

example, the primary substa tion is maintained

at 103%of nominal to ensure that the voltage

2Okm away is satisfactory. If the primary

substation voltage is reduced to l l kV in order

to connect the generator, the voltage at the end

of the line will drop to 94% of nominal

whenever the generator is not export ing power.

The D N O must consider how it will correct this

voltage depression. One solution may involve

POWER ENG

N

EERlNG JOURNAL FEBRUARY

2002



Embedded generation

xample system

ffect of reducing primary substation to nominal voltage

ffect of the generator operating at

0

9

power factor leading

ffect of installing an auto transformer,

8 km

from the

ffect of upgrading he line with 70mm copper conductor

ffect of constraining the generator

primary substation

110

...

+6 volta

I I I I I I I I I

0 4 8 12

16

20 4 8 12

16

2

full demand on the line no demand on the line

distance from the primary substation,km

customer minutes lost while the off-circuit

tap-changers are reset on the 11/0-415kV

distribution transformers. However, this may

not be practical if there are long lines or many

distribution transformers involved.

Import reactive power

DNOs may stipulate that generators operate at

leading, lagging or even unity power factor,

depending on the

X / R

ratio of the system,

voltage regulation, local load etc. Generators

are typically operated at a power factor such

that if they trip, when at rated generation, the

disturbance to the system is minimised.

The amount of reactive power that can

be imported is generally governed by the

parameters of the generator. Typically a

synchronous generator can import reactive

power at a 0.95 power factor. Wind turbines,

with uncompensated induction generators, can

import reactive power at around a 0.9 power

factor.

In the initial example the 1MW generator

operates at unity power factor. The voltage

rises to almost 109% of nominal (under full

load conditions) and 113% of nominal (under

no load conditions). Allowing the generator to

operate at a leading power factor of 0.9 limits

the voltage rise to

104%

and 108%of nominal,

respectively (Fig. 4). With maximum demand

on the system, this brings the voltages within

the permitted +6% voltage limit. During

periods

of

no system demand, the voltage is not

Effect of using various

methods to r educe he

voltage rise on the 11kV

line used in the example

lowered sufficiently.

If a generator is to import significant levels

of reactive power, it may be necessary to agree

a charging mechanism with a supplier to cover

the costs involved with purchasing and

transporting

these extra kvars. The

DNO

must

also consider the effect that this additional

reactive power flow

will

have on system losses

and the loading on circuits. The effect

of

the

generator tripping must also be considered, as

this will cause a transient voltage rise. It may

take the transformer tap-changers at the

primary substation several seconds to respond

and restore the voltages. Under such circum-

stances a DNO may be able to use a switched

'capacitor bank or some other form of reactive

5

Typical rur l l k v

overhead line

POWER ENGINEERING JOURNA L FEBRUARY 2002

9



Embedded generation

6

Some

guidance as to

the

level

of generation

that can be accepted

onto

an 11kV overhead

line

6

mm2

conductor, 11 kV at primary substation

6 mm2conductor, 11.3 kV at primary subs tation

6 mm2 conductor, 11.6 kV at primary substation

70 mm2 conductor, 11 kV at primary substation

70 mm conductor, 11.3 kV at primary substation

70 mm2

conductor, 11.6 kV at primary substation

2 4 6 8 IO 12 14 16 18 20 22 24 26 28 30

dis tance, km

power compensation to restore the system

voltages.

Install auto transformers along the line

Auto transformers (voltage regulators or

voltage boosters) are simply transformers with

a voltage ratio of 1:land on-load tap-changers

for voltage regulation. Essentially, inserting an

auto transformer into a long circuit splits it into

two sections. The voltage along one section

will be regulated by the AVC, tap-changers

and line-drop compensation at the primary

substation. The auto transformer will regulate

the voltage along the other section.

Inserting an auto transformer 8km from the

primary substation, in the initial example, has

little effect on the voltage profile between itself

and the primary substation. Under full and

no load conditions the primary of the auto

transformer rises to 106% and 109% of

nominal voltage, respectively

(Fig. 4).

The on-

load tap-changer, in this example, is set to

control the voltage at the secondary of the auto

transformer to nominal voltage (using a tap

range of

~ 5

n five steps). Under both full and

no load conditions it operates to reduce the

voltage to 101%of nominal, thus maintaining

the voltage rise along the remainder of the

llkV line below the permitted +6% voltage

limit. In this example, the auto transformer

does not prevent this limit being exceeded

when there is no demand. However, by careful

positioning of either one or two auto

transformers, the voltages may be maintained

within limits.

Auto transformers have not traditionally

been used by

DNOs

in this manner because

there has been little generation connected to

the distribution system. However, as the levels

of embedded generation are set to increase

their use may become more common.

When installing an auto transformer into the

distribution system the

DNO

must consider its

effect' on the system voltages under all the

worst case operating scenarios to ensure that

no customers will be adversely affected. The

effect of the auto transformer on the line

loading must also be taken into account, as it

may increase the flow of reactive power along

the line. The

DNO

must also consider how the

presence of the auto transformer will affect

system security, as it will introduce another

factor of unreliabil ity into the sys tem.

Upgrade

the conductor

Small overhead line conductors have higher

impedance than large conductors. A 70mm2

copper conductor has approximately one-third

of the resistance and 90% of the reactance of

a 16mm2conductor. Thus, upgrading the con-

ductor on an llkV overhead line will signifi-

cantly reduce its resistance and will smooth the

voltage profile along the line.

In the initial example, the voltage at the

1MW generator was 109% of nominal (under

full load conditions) and 113% of nominal

10

POWER ENGINEERING JOUR NAL FEBRUARY

2002



Embedded generation

(under no load conditions). The voltage profile

along the line is improved by replacing the

16mm2conductor with 70mm2copper (Fig. 4).

It reduces the voltage at the generator to

less than 105% of nominal (under full load

conditions). With no demand on the line, it is

marginally above the permitted +6% voltage

limit.

This suggests that upgrading the conductors

is

a very effective method of counteracting the

voltage rise problem. However, replacing the

conductors can be expensive and may make a

scheme uneconomic.

Constrain the generation

The sophisticated control systems available

these days will allow a generator to control i ts

output in line with the system voltage. Thus if

the voltage is approaching the permitted +6%

voltage limit, a generator can reduce its output

in order to maintain the voltage below the

threshold. This will allow the generator to

continue operating, rather than being con-

strained off during periods of low system

demand. Conversely, should the system voltage

fall below nominal, a generator may be able to

respond by increasing its output.

The initial example suggests that the 1MW

generator cannot be accepted onto the llkV

line, even when it is fully loaded. Its output has

to be constrained to 750kW to maintain the

system voltages within the permitted +6% limit

(Fig. 4). It will have to be constrained further

as the system loading

is

reduced. Under no load

conditions the generator has to be constrained

to 300kW to maintain the voltages below the

permitted +6%threshold.

Constraining an embedded generator will

obviously affect the economic benefit of the

scheme. It is usually only-aviable option when

the constraints are expected to be infrequent

and where significant system reinforcement

costs are avoided.

How much generation can be connected to

an

kV

overhead line?

The level of generation that can be absorbed

onto the distribution system is determined by

many factors, such as:

voltage level

voltage at the primary substation

distance from the primary substation

size of conductor

demand on the system

other generation on the system

operating regime of the generation.

Fig. 6 gives some indication as to the amount

of generation that can be connected to an 11kV

overhead line. It is clear that, as the distance

from the primary substation increases, the

amount

of

generation that can be accepted

reduces.

Case studies

Three brief case studies are presented here to

show how Innogy plc has approached this

voltage rise issue when developing small

generating schemes.

ChiRex.CHP scheme

The ChiRex combined heat and power (CHP)

scheme in Northumberland (Fig. 7) comprises

a 4.5MW gas turbine. It has been operational

since June 1994, providing electricity and

steam to the ChiRex pharmaceutical plant.

Both are normally connected to the llkV

primary substation by a single ll k V cable.

During some periods, such as Christmas, the

demand at the pharmaceutical plant falls

dramatically, and the CHP scheme exports the

majority of its power into the distribution

system. This causes the voltage to rise and the

generator was once tripped off by the

overvoltage protection.

This problem was overcome by altering the

operating procedure of the CHP scheme. The

output and power factor of the generator are

7 ChiRexCHPscheme

a

POWER ENGINEERING JOUR NAL FEBRUARY 2002

1 1



Embedded generation

8 Typical small hydro

generating scheme

Blantyre, Scotland)

now manually adjusted by the operators who

monitor the local demand and the system

voltage.

9 Jenbacher gas

engine, produced by

Clarke, or small

embedded generation

schemes photo:

courtesyof Clarke

Energy, www.clarke-

energy.co.uk)

I

500k

W hydro-generating scheme

Innogy Hydro is in the early stages of

developing a 500kW hydro-generating scheme

in the north of Scotland (Fig. 8).The generator

is to be connected to an l l k V overhead

line, comprising 16 copper conduc tors,

approximately 15km from the primary

substation. Also connected to this 1lk V line

are numerous domestic customers fed by

individual 1U0.415 kV distribution trans-

formers with off-circuit tap-changers.

The DNO has stipulated that the voltage

along this l l kV line must not exceed 11.13 kV

(1.2 above nominal voltage) as th is will raise

customers' voltages above 53\3 the +lo%

tolerance specified in the Electricity Supply

Regulations.

Provisional studies have shown that the

existing llkV system cannot accept 500kW

of generation. Reducing the voltage at the

primary substation is not feasible, as there are

other l l k V circuits connected to the primary

subst ation. Upgrading the line with 70

copper conductors increases the amount of

generation that can be connected, but not

sufficiently.

I

The cost and feasibility of two methods of

overcoming the voltage rise problem are

currently being considered-installing reactive

power compensation at the generator, or an

auto transformer part way along the 11kV line.

1OMW mines gas generating scheme

Cogen, an Innogy subsidiary, is currently

developing a lOMW generating scheme to bu rn

methane gas from a disused coal mine. The

scheme will comprise two 5MW plants (Fig. 9)

connected separately to two existing llkV

cables that run along the edge of the proposed

site. Studies have shown that the generation

cannot be accepted onto the existing system

due to the excessive voltage rise.

The local DNO currently operates the

primary substation at 11.6kV. Following tests

on the system, the DNO has agreed to reduce

the primary substation voltage to 11.3kV

so

the

generation can be connected. However, should

the generating scheme be out of service, the

system voltages will be depressed and the

voltage of a few customers will fall below the

permitted limit. As the generating scheme is

expected to operate at base load, this scenario

will not occur frequently. It has been agreed

that, when this does occur, the DNO will

dispatch an engineer to manually alter the

distribution transformers' tap-changers and

'the DNO will be compensated appropriately by

Innogy.

Conclusions

In conclusion, there are many factors that

determine the level of generation that can be

connected to the distribution system at 1lkV

Thus every scheme will face different technical

and commercial issues and must be studied on

a site-by-site basis. One of the major technical

difficulties is the voltage rise resulting from the

reversed power flow. There are methods of

counteracting this voltage rise; however, a

developer must consider whether the addi-

tional costs are justified.

References

1 The Electricity Supply Regulationsl988: Regulation

30, paragraph 2, amended in 1994

2 The Electricity Safety, Quality and Continuity

Reg ulation s. 2001: Draft copy-available for con -

sulta tion purposes on the D TI wehsite

IEE: 2002

Dr C. L. Masters is a Power Systems Engineer in

Operations and Engineering, lnnogy plc. She is a

Member of the IEE.

12

POWER ENGINEERING JOURNAL FEBRUARY

2002

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DG 3 Voltage Rise