ConexionTransformadoresEnGD

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Abstract-- The interconnection of Distributed Generation(DG) into the power distribution system creates many challenges

for utilities. Many of these challenges are related to the type of

interconnection transformer (or lack thereof) and consequently

the grounding used for the DG interconnection. There remains

confusion about the various transformer types for DG

interfacing. This is a critical point because the selection of the

correct interface with the local utility will simplify feasibility

studies; therefore, decreasing the overall cost of the DG

interconnect investigation to benefit all. This paper discusses the

advantages and disadvantages of different interconnectingtransformers. Neutral reactor sizing for grounding transformer

connections is discussed.

Index Termsdistributed generation; grounding; neutral

reactor; isolation transformer

I. INTRODUCTION

ntroducing Distributed Generation (DG) into a distribution

system designed for radial power flow creates many issues

including but not limited to: harmonic concerns, system

overvoltages, fault coordination, increased fault currents,

insulation coordination, and islanding concerns. The addition

of DG may also alter the distribution of fault current affectingthe existing overcurrent protection coordination.

The added DG may expose the existing utility system to

temporary system overvoltages that are damaging to the line-

to-neutral-connected electrical equipment. For example, Fig.

1 illustrates typical phase voltages when a single line-to-

ground (SLG) fault is applied at Phase C. Fig. 1(a) illustrates

the pre-fault condition, (b) illustrates a system that is not

effectively grounded, and (c) illustrates a system that is

effectively grounded. As can be seen in Fig. 1 (b), there are

excessive voltages on the unfaulted phases that will cause

damage to surge arresters and other electrical equipment.

This condition may occur when a DG becomes islanded onthe faulted section.

This issue as well as other conditions yielding overvoltages

on the distribution system may be alleviated by choosing an

appropriate interconnection transformer that maintains an

effectively grounded system at all times. However, this may

R. F. Arritt is with EPRI, Knoxville, TN 37932 USA (email:

[email protected])

R. C. Dugan is with EPRI, Knoxville, TN 37932 USA (email:

[email protected]

change the flow of fault current effecting the existing ground

fault protection. The selection of DG interconnection

transformer will also have an effect on the insulation

coordination and the flow of third harmonics.

V a

V c

V b

V a G N D = 1 p u

V a G N D = 1 . 2 5 t o 1 . 7 3 p u

V a

V c

V b

V a G N D = < 1 . 2 5 p u

V a

V b

V c

P r e - F a u l t C o n d i t i o n

S L G - F a u l t C o n d i t i o n

N o t E f f e c t i v e l y G r o u n d e d S y s t e m

S L G - F a u l t C o n d i t i o n

E f f e c t i v e l y G r o u n d S y s t e m

1 . 2 1

1 . 2 5

1 . 7 3

1 . 7 3

1 . 0

1 . 0

1 . 0

( a )

( b ) ( c )

Fig. 1 Neutral Shift During Single Line to Grounded Fault (SLG) on Phase C

(a) Pre-Fault Condition (b) Not Effectively Grounded System (c) Effectively

Grounded System

II. DIRECT DGCONNECTION

While it is possible to obtain DG rated for the utility

medium voltage (MV) distribution system, there can beconsiderable risks in connecting such equipment directly to

utility distribution system. This is particularly risky for

installations fed by overhead lines. The insulation level of the

machines may not coordinate with the utility system.

Interconnecting DG through a transformer can eliminate many

of the difficulties that may occur with directly-connected DG

installations. Therefore, direct connection is often

discouraged.

Distributed Generation Interconnection Transformer

and Grounding SelectionR. F. Arritt,Member, IEEE, and R. C. Dugan, Fellow, IEEE

I

2008 IEEE.


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III. INTERCONNECTION TRANSFORMER CONFIGURATIONS

Several of the concerns with the interconnection of DG can

be addressed, or at least minimized, through means of an

appropriately-configured interconnection transformer. Three

interconnection transformer winding configurations will be

discussed here along with their advantages and disadvantages

with respect to system protection (overvoltages and fault

current), harmonic alteration, and other general system

considerations. The transformers are assumed to be sized

properly and have an appropriate insulation level for

coordination between the utility and load-side equipment.

A. Grounded-Wye (Utility) Grounded-Wye (DG)

This is a common connection applied in North American

distribution systems for three-phase loads. It is applied

because of its reduced susceptibility to ferroresonance on

cable-fed loads and fewer operating restrictions while being

switched for maintenance. It is also generally well-behaved

with respect to DG applications; however, a few issues do

exist. A summary of advantages and disadvantages are listed

below:

Advantages:

General

1) Less concern for ferroresonance in cable-fed

installations; some core designs may be more prone

than others if there is sufficient capacitance.

2) More economical than other connections in some

applications, particularly at 25 and 35 kV class

voltages.

3) More economic fusing than similarly-rated delta

connected primary winding. This may also translate

to a smaller-sized padmounted transformer.

System Protection4) No phase shift in system voltages (relaying); can

detect primary side voltages with low-voltage

relays.

Disadvantages:

General

1) DG sees same imbalance that utility system sees.

Harmonics

2) Will directly pass zero-sequence harmonic currents

(such as the 3rd harmonic).

System Protection

3) DG may feed into any type of fault that is on the

utility system.4) Utility will supply fault current for internal

generator ground faults, increasing fault damage.

5) Does not necessarily provide an effectively

grounded source when islanded despite the fact that

both windings are solidly grounded. Ground

reference will be provided by the generator and/or

load.

Because of the fact that this type of connection allows the

DG to feed into all fault types and also does not inhibit the

flow of zero-sequence harmonics, it may be difficult to

operate some generators in parallel with the utility system

using this transformer connection. Synchronous generators

may produce third harmonic voltage distortion, depending on

its winding pitch. This connection provides a very low

impedance path for the third harmonics when paralleled to the

utility system. This decreased impedance path may add or

sink neutral harmonic current, damaging the generator or

simply adding unwanted harmonic currents to the utilitysystem. A neutral reactor may be added to limit the flow of

zero-sequence harmonics and fault currents. Care must be

taken when sizing the reactor (see following paragraph). The

interconnection diagram shown in Fig. 2 shows a typical

grounded-wye/grounded-wye installation.

Despite the fact that both windings of the grounded-

wye/grounded-wye transformer are grounded, this connection

may not provide an effectively-grounded source to the utility

system when the DG becomes islanded. This is brought about

by the fact that some generators are not themselves effectively

grounded.

For example, the ground fault current from a solidlygrounded synchronous generator is typically larger than the

three-phase fault current due to the fact that the neutral zero-

sequence impedance of a synchronous generator is typically

smaller than the sub-transient positive-sequence reactance.

Therefore, a common approach to limiting the ground fault

current is to ground the synchronous generator through an

impedance between the neutral point and ground. This added

impedance may result in the DG not being effectively

grounded while in an islanded condition. Fig. 3 uses the

sequence diagram for the SLG fault situation to illustrate the

flow of zero-sequence current through an islanded DG with a

grounded-wye/grounded-wye transformer and a grounding

reactor.

Also, inverters used to connect such generation as

microturbines and photovoltaics need isolation and will

generally not be grounded at all. Likewise, induction

generators seldom have a connection to ground. Thus, DG

relying on these technologies will not provide an effectively-

grounded source to the utility when interconnected with a

grounded-wye/grounded-wye transformer. The characteristics

with respect to many power system faults and other

disturbance events are similar to the delta/wye connection

(discussed below).

Fig. 2. Grounded Wye/Wye Interconnection Transformer


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Z t r a n s f o r m e r + Z p o s g e n

Z e q u i v a l e n t



3 x Z N

D G S i d e

U t i l i t y S i d e

P o i n t o f C o m m o n

C o u p l i n g

Z t r a n s f o r m e r + Z n e g g e n

Z t r a n s f o r m e r + Z z e r o g e n

S y s t e m N e u t r a l R e f e r e n c e

a f t e r I s l a n d i n g

P o s i t i v e S e q u e n c e

N e g a t i v e S e q u e n c e

Z e r o S e q u e n c e

Fig. 3. Grounded Wye/Wye Interconnection Transformer Sequence Diagram

B. Delta (Utility) Grounded-Wye (DG) Interconnection

Transformer

This is another common connection for three-phase loads

in North America, and the most common connection in

European-style systems. It would probably be favored for

serving loads in most cases if it were not for the susceptibility

of the connection to ferroresonance in the types of cable-fed

systems common in North America. A summary of

advantages and disadvantages are listed below:

Advantages:

Harmonics

1) Triplen harmonics from the DG do not reach the

utility system.

System Protection

2) Provides some isolation from voltage sags due to

utility-side single line-ground (SLG) faults, allowing

the DG to better ride through voltage sags.

3) Does not feed directly into utility-side SLG faults (it

can contribute through other ground sources on the

utility system).

Disadvantages:

General

1) Very prone to ferroresonance in cable-fed

installations, especially during open conductor fault

conditions.

Harmonics

2) Depending on generator neutral grounding, 3rd

harmonics in the DG may cause excessive current in

the DG-side neutral.

System Protection

3) Cannot provide an effectively grounded system

during islanding or open conductor conditions i.e. If

islanded on SLG fault, can subject utility arresters

to overvoltages.

4) Difficult to detect some utility-side SLG faults from

the generator side by voltage relaying alone. The

delta winding tends to hold the generator side

voltage magnitudes up and in the proper phase

relationship. Primary-side relaying is often requiredto ensure rapid detection.

5) Due to the susceptibility to ferroresonance,

instantaneous overvoltage relaying (59I) is often

required to ensure prompt detection of this

condition.

This type of connection is not alone in being prone to

ferroresonance and subjecting arresters and loads to

overvoltages during single-line-to-ground faults. This concern

is shared with all transformers that have an ungrounded

primary connection.

While this type of connection prevents most third harmonic

currents that might emanate from the DG from reaching theutility, it does not prevent their flow on the DG side. As

discussed in the previous section, a neutral reactor is typically

used to limit the third harmonic current flow if the generator

is not of 2/3 pitch design which minimizes the 3rd

harmonic.

While the winding connection can be beneficial to the load

in reducing the impact of voltage sags due to SLG faults, it

also makes some SLG faults on the utility system more

difficult to detect. This increases the likelihood of islanding

because it will delay fault detection until the utility breaker

operates for many faults. As with all ungrounded primary

connections, there is danger in being islanded on a SLG fault

with very light load even briefly because this can result in asevere resonant condition.

Therefore, it is common to add other relaying functions to

aid in the early detection of utility-side faults when this

connection is used. A negative sequence relay can make the

detection more reliable and will also help detect single-

phasing conditions in which a fuse has blown. While the

voltage magnitudes seen on the secondary may not change

sufficiently, they will become unbalanced, resulting in

detectable negative sequence voltages and currents. Care must

be taken not to set these relays too sensitive, resulting in

nuisance generator tripping.

A common approach to more sensitive utility-side fault

protection is to add relaying on the primary side of the

transformer, such as a ground overvoltage (59N or 59G) relay

that can detect the presence of the SLG fault. This is

implemented by installing potential transformers (PTs) on the

primary system and then placing a voltage relay in the corner

of the delta winding on the instrumentation transformer. The

interconnection diagram shown in Fig. 4 shows a typical

delta/grounded-wye installation.


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Fig. 4. Delta/Grounded Wye Interconnection Transformer Connection

C. Grounded-wye (Utility) Delta (DG) Interconnection

Transformer

This connection is rarely used on utility power distribution

systems to serve loads, but is considered by many as the best

way to interconnect large three-phase DG. This transformer

connection is often referred to as a grounding bank and has

a number of special characteristics that must be considered in

its application on utility power systems. A summary of

advantages and disadvantages are listed below:

Advantages:

General

1) Protection schemes are well understood by both

vendors of larger DG equipment and utility

protection engineers.

Harmonics

2) Triplen harmonic currents that might be produced

by the generator are blocked by the delta winding

and cannot flow on the generator side and,

therefore, are not passed on to the utility power

system.

System Protection3) Utility-side faults are generally more readily

detected by the DG system protection because the

transformer itself actually participates in all ground

faults. This generally allows the DG to disconnect

more quickly. (This connection is also known as a

grounding transformer or a ground source.)

4) Should the DG become isolated from the utility

source (islanded), this connection helps the DG

system to present an effectively grounded source to

the utility distribution system and avoid the

resonance and overvoltage issues of other

connections.

Disadvantages:

Harmonics

1) Triplen harmonic currents already present in the

utility system from other sources will tend to flow

into transformers with this winding connection,

contributing to transformer heating.

2) The flow pattern for triplen harmonic currents on

the distribution system is altered, which could be

either beneficial or detrimental to

telecommunications interference and neutral-to-

ground voltages depending on the path taken by the

currents. This is difficult to predict prior to

construction and can require mitigation after

commissioning should complaints arise from

customers.

System Protection

3) Contributes strongly to all ground faults, which can

increase damage due to high fault currents.

4) The connection contributes to sympathetic trippingof the feeder breaker for faults on other feeders.

The transformer supplies current to other feeders

connected to the same substation bus. Ground trip

pickup levels must be increased to maintain

coordination, which results in less sensitive fault

protection.

5) Utilities may have to change relaying depending on

whether a transformer of this type is connected or

disconnected. This requires expensive

communications and control equipment that might

not be needed for the other connections.

6) The transformer itself is subject to short-circuit

failure when the fault occurs. This is particularlytrue for smaller transformer banks with impedances

less than 4-5%. A special transformer must

generally be ordered.

The fact that this transformer acts as a ground source is

undesirable on many distribution systems and few utilities will

allow this connection on the distribution system without

special study. Implementing this connection often implies

changes to the feeder overcurrent protection scheme that are

costly either in terms of having to replace equipment or of

inconvenience to other customers on the feeder.

Fig. 5 shows how the connection contributes to a SLG fault

on a 4-wire, multi-grounded neutral distribution system, the

most common in the U.S. Fig. 5(a) illustrates a ground fault

when the DG transformer is not connected. Fig. 5(b)

illustrates a ground fault when the DG transformer is

connected. The arrows show the paths of the current from the

grounded-wye/delta DG interconnection transformer. The

currents flow back through the solidly-grounded substation

transformer and contribute additional current to the fault. The

amount contributed would depend on the size and impedance

of the transformer.

Fig. 7 shows the sequence network to illustrate this

increased ground fault contribution from this transformer

configuration. When this Figure is compared to Fig. 6, it canbe seen that the zero-sequence current is divided between the

utility and DG connection. This increases the overall ground

fault contribution but reduces the ground fault current seen

at the substation.

Note: The DG contribution in Fig. 5 will be dependent on

the capability of the DG to supply fault current. In many

ground fault cases, the contribution due to the transformer

alone will be larger. The connection of the DG device (delta,

grounded-wye, or ungrounded-wye) is irrelevant for faults on

the utility side of the interconnection.


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S u b s t a t i o n

T r a n s f o r m e r

S e c o n d a r y

D G

* F a u l t c u r r e n t i s

a p p r o x i m a t e l y e q u a l t o t h e

c u r r e n t t h r o u g h t h e

s u b s t a t i o n n e u t r a l r e a c t a n c e .

S u b s t a t i o n

T r a n s f o r m e r

S e c o n d a r y

* * T h e g r o u n d e d - w y e

D G t r a n s f o r m e r

d e c r e a s e s t h e z e r o -

s e q u e n c e i m p e d a n c e

w h i c h i n c r e a s e s t h e

t o t a l f a u l t c u r r e n t

c o m p a r e d t o t h e

e x a m p l e w i t h o u t t h e

g r o u n d e d - w y e D G

t r a n s f o r m e r .

F a u l t *

F a u l t * *

( a )

( b ) Fig. 5. Grounded-wye/delta transformer for DG interconnection can feed ground

faults on 4-wire multi-grounded neutral systems, introducing additional fault

stresses and interfering with utility-side protective relaying (a) Without

Transformer Connection (b) With Transformer Connection

Fig. 6. Sequence Diagram for SLG Fault in system without any Ground

Wye/Delta Transformer Connection

Fig. 7. Sequence Diagram for SLG Fault with Grounded-Wye/DeltaTransformer Connected

If this connection is to be used on a typical North

American distribution system, some of the options for better

accommodating it are:

1. Ground the transformer through a neutralreactance of sufficient size to limit the fault

current contribution and the amount of

unbalanced load current the transformer would

have to absorb. (Discussed in next section.)

2. Increase ground trip pickup settings on feederbreakers and line reclosers.

Since this connection is unusual for distribution systems,

but has some clear advantages for interconnecting DG, the

remainder of this paper will be devoted to issues related to

implementing it with a neutral grounding reactor.

IV. GROUNDED-WYE (UTILITY)DELTA (DG)

INTERCONNECTION TRANSFORMER WITH A GROUNDING

REACTOR

As discussed in the previous section, a neutral reactor may

be added to this connection to make it more compatible with

typical distribution system design by limiting fault currents,unbalance currents, and harmonic currents. If the neutral

reactor is sized properly, the grounded-wye/delta transformer

connection can provide an effectively grounded DG interface

under all circumstances without excessive interference with

system operation and other utility customers on the system.

The interconnection diagram shown in Fig. 8 shows a typical

grounded-wye/delta installation with a neutral impedance.

Fig. 9 shows the sequence network for a SLG fault

condition to illustrate the effect of the added neutral reactor.

When compared to Fig. 7, the added reactor increases the


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effective zero-sequence impedance, decreasing the overall

ground fault contribution of the transformer. Not only does it

reduce the total ground fault current when compared to Fig. 7,

but when the zero-sequence current is divided between the

utility and the DG connection, more current will flow through

the utility connection allowing the ground fault protection to

work closer to its original ground fault settings.

With the addition of this neutral reactor, a neutral shift is

introduced. When sizing the neutral reactor, one design goal isto not expose the distribution system to line-to-ground

voltages during a ground fault during an islanding condition

that greatly exceed the voltages under the same conditions

with the utility source alone.

The selection of the grounding reactor size is a trade-off

that minimizes the risks associated with harmonics, ground

fault currents and overvoltages. However there is one caveat:

the addition of future DG with this transformer connection

may alter the optimal size of this neutral reactor, complicating

reactor sizing. The design must leave a certain amount of

margin for future DG addition.

Fig. 8. Grounded-Wye/Delta Transformer with Added Neutral Reactor

Z t r a n s f o r m e r




3 x Z N

D G S i d e

U t i l i t y S i d e

P o i n t o f C o m m o n

C o u p l i n g

P o s i t i v e S e q u e n c e

N e g a t i v e S e q u e n c e

Z e r o S e q u e n c e

Z p o s g e n


Z n e g g e n


Z z e r o g e n

A d d e d N e u t r a l

R e a c t o r

Fig. 9. Grounded-Wye/Delta Transformer Sequence Diagram for SLG Fault

with Added Neutral Reactor

A. Neutral Grounding Reactor Sizing

The proper sizing of a neutral grounding reactor on a

grounded-wye/delta interconnection transformer is a

compromise of three (3) conflicting goals. The impedance

must be:

1. High enough to limit the maximum fault currentcontribution to a value acceptable to utility to

which the proposed DG is to be connected, and

to prevent failure of the grounding bank itself

due to high fault currents flowing repeatedly in

its windings.

2. High enough to limit circulating currentssufficiently for continuous operation in

unbalanced conditions.

3. Low enough to maintain an effectivelygrounded system on the utility should the DG

become separated from the utility system while

the DG is operating.

In order to be low enough to maintain an effectivelygrounded system the following criteria must be met [2]:

3/ 10 XX and 1/ 10 XR

Therefore, to keep the system effectively grounded during

a possible islanding condition the reactor size needs to be

limited to the value calculated below [1]:

3/011 TGTN XXXX +

where

=NX neutral reactance

=0TX transformer zero-sequence reactance

=1TX transformer positive-sequence reactance

=1GX generator (DG) positive-sequence reactance

The above calculation only guarantees effective grounding

for the interconnection point. For faults at other locations on

the feeder the zero- and positive-sequence impedances

between the ground fault location and the DG need to be

added toXT0 andXT1. The resistance portion has been ignored

because of the small resistance values associated on

transformers and generators; however, they may need to beincluded with the addition of feeder [7].

Reference [4] states a typical ohmic value of a neutral

reactor to be within 1.0 to 1.5 times the transformer zero-

sequence reactance. This typical range of values is on a

system that limits temporary overvoltages during line-to-

ground faults to 122% and permits only a 5% reduction in

feeder ground protection sensitivity from a case with no DG

connected [4] . The authors have worked on cases where

values between 4 and 5 ohms have been used on 15-kV class

distribution systems for 5MW DG interconnects. This gives


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the reader some idea of the ranges of values necessary for

acceptable operation.

As stated previously, the addition of future DG with the

same transformer connection on the feeder may require the

neutral reactor size be altered in order to meet the above

design criteria. The technical solution may be straightforward,

but the commercial solution is often contentious.

V. CONCLUSIONS

Connecting DG with a grounded-wye/delta, or grounding

bank, transformer connection has advantages over other

common connections used for loads by maintaining an

effectively grounded system even when the DG becomes

islanded. However, this connection cannot exist on many

distribution systems without an appropriately-sized neutral

reactor to limit zero-sequence currents. The proper sizing of

the neutral grounding reactor is a compromise between

conflicting objectives and must be done with careful study.

The neutral reactor size may need to change to accommodate

future DG on the system in order to maintain the ground-fault

contribution and overvoltages to acceptable levelsThe implementation of this transformer (as with all

interface transformers) with the appropriate insulation level

will provide proper insulation coordination between utility

and load-side equipment.

VI. REFERENCES

[1] EPRI 1000419, Engineering Guide for Integration of DistributedGeneration and Storage into Power Distribution, Electric Power

Research Institute, Palo Alto, CA 2002.

[2] IEEE Guide for the Application of Neutral Grounding in ElectricalUtility SystemsPart I: Introduction, IEEE Standard C62.92.1-2000,

Sept. 2000.

[3] IEEE Guide for the Application of Neutral Grounding in ElectricalUtility Systems, Part IV Distribution, IEEE Standard C62.92.4-1991,

Jul. 1992.

[4] Nagpal, Plumptre, Fulton, and Martinich, Dispersed Generation-UtilityPerspective, IEEE Transactions on Industry Applications, May/June

2006, Volume 42 No. 3, Page(s):864 872.

[5] R.C. Dugan, On The Necessity Of Three-Phase Feeder Models For DgPlanning Analysis, IEEE Power Engineering Society Summer Meeting

Conference Proceedings, 2002, Volume 1, Page(s):438 441.

[6] R.C. Dugan, M.F. McGranaghan, S. Santoso, H.W. Beaty, ElectricalPower Systems Quality 2nd Edition. New York: McGraw-Hill, 2002, pp.

373-435.

[7] T.A. Short, Electric Power Distribution Handbook. New York: CRCPress, 2000, p. 651-748.

VII. BIOGRAPHIES

Robert F. Arritt (M96) is a Power Systems

Engineer for EPRI in Knoxville, TN. He holds a

BSEE degree from West Virginia Institute of

Technology, Montgomery, WV (2000) and an

MSEE degree from Worcester Polytechnic Institute

in Worcester, MA (2005).

His employment experience included Raytheon in

Sudbury, MA where he worked in the Power and

Electronic Systems Department. At Raytheon he was awarded the 2006

Raytheon Technical Honors Award for Peer and Leadership Recognition for

Outstanding Individual Technical Contribution and also received a 2005

Raytheon Authors Award for work on Phase-Shifted Transformers for

Harmonic Reduction.

Mr. Arritt has spent most of his career designing and modeling power

systems from the electronics level to ac generation. Recently, Mr. Arritt has

been actively involved in distributed generation impact studies.

Roger C. Dugan (M74SM81F00) is Sr.

Technical Executive for EPRI in Knoxville, TN. He

holds the BSEE degree from Ohio University, Athens,

OH (1972) and the MEEPE degree from Rensselaer

Polytechnic Institute, Troy, NY (1973).

From 1992-2004, he served as Sr. Consultant forElectrotek Concepts, Knoxville, TN. From 1973

1992 he held various positions in the Systems

Engineering department of Cooper Power Systems in

Canonsburg, PA and Franksville, WI. Roger has worked on many diverse

aspects of power engineering over his career because of his interests in applying

computer methods to power system simulation. The focus of his career has been

on utility distribution systems. He has been particularly active in developing

advanced methods for analysis of distribution systems with distributed

generation. He was elected an IEEE Fellow in 2000 for his contributions in

harmonics and transients analysis. He is coauthor of Electrical Power Systems

Quality published by McGraw-Hill, 2nd edition. He is currently Chair of the

IEEE PES Power Systems Analysis, Computing, and Economics Committee. He

was the 2005 recipient if the IEEE Excellence in Distribution Engineering

Award.

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ConexionTransformadoresEnGD