Erthing Transformer

8/17/2019 Erthing Transformer

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Abstract—The grounding transformer is a transformer

intended solely for establishing a neutral connection point on a

three-phase ungrounded power system. The transformer is

usually of the wye-delta or zig-zag transformer.

This paper first reviews the state of the art of the grounding

transformer to assist electric power engineers in the proper

understanding of the use and applications of these devices, and

then a zig-zag grounding transformer is modeled in

PSCAD/EMTDC simulator. After a 27.6 KV ungrounded three-

phase transmission system is constructed, different scenarios are

simulated and verified under different conditions with theconnection of the grounding transformer.

Index Terms—Fault Localization, Grounding Transformer,

Metering System, Modeling, Power System, Protection, Relaying,

Simulation, Ungrounded.

I. INTRODUCTION

he way to ground a power system is probably more

difficult to select than any other features of its design.

Historically, there has been a gradual trend in American

power system from ungrounded, to resistance grounded, to

solid or effective grounded [1]. The main reasons for this

trend can be readily traced. Most systems were initiallyoperated with their neutrals free, i.e., the neutrals were not

connected to ground. This was the natural thing to do as the

ground connection was not functional for the actual transfer

of power. People had a strong argument in this favor, since an

insulator failure on one of the phases could be tolerated for

some little time until the fault could be located and fixed. In

addition, most lines at that time were single circuit, and the

free-neutral feature permitted loads to be powered with fewer

interruptions than the neutral had been grounded. Another

important consideration was that relaying had not come into

general use, so that many prolonged outages were avoided by

the ungrounded operation.The principal virtue of an ungrounded-neutral system is its

ability, in some cases, to clear ground faults without

M. Shen, L. Ingratta, and G. Roberts are with the Energy Division at

Wardrop Engineering Inc., Mississauga, Ontario L4V 1V2, Canada. (e-mail:

[email protected])

Wardrop Engineering Inc. has been delivering solutions to power utilities

and industrial clients since 1955. Wardrop is internationally recognized as a

provider of engineering services in the specialized fields of power transmission,

distribution, and generation.

The vision at Wardrop is to create a company that stands for People,

Passion, Performance. Trusted Globally.

interruption. However, Limitations to ungrounded power

system began to develop with the growth of systems, both as

to mileage and voltage. This increased the currents when a

ground occurred, so that the increasing faults of transient

grounds (from lightning, or momentary contacts) were no

longer self-clearing. More recent transient-overvoltage

comparisons between isolated and grounded systems have

shown the former to give higher overvoltages, both during

faults and switching operations [1]. Therefore, it is necessary

to assume that an ungrounded-neutral system will result in

more equipment damages than some form of groundedsystem. Transformers must be designed on the basis of full

neutral displacement and in the higher voltage classes this will

result in a somewhat higher cost.

Most grounded systems employ some method of grounding

the system neutral at one or more points. These methods can

be divided into two general categories: solid grounding and

impedance grounding. Impedance grounding may be further

divided into several subcategories: reactance grounding,

resistance grounding, and ground-fault-neutralizer grounding.

Each method, as named, refers to the nature of the external

grounding circuit, from system neutral to ground rather than

to the degree of grounding [2].The best way to obtain the system neutral for grounding

purpose in three-phase systems is to use source transformers

or generators with wye-connected windings. The neutral is

then readily available. The alternative is to apply grounding

transformers when some system neutrals may not be

available, particularly in many old systems of 27.6 KV or less

and many existing 2400, 4800, and 6900 V systems. When

existing delta-connected systems are to be grounded,

grounding transformers may be used to obtain the neutral.

Grounding transformers may be the interconnected wye (zig-

zag), the wye-delta, or the T-connected type [2].

II. INTRODUCTION TO GROUNDING TRANSFORMERS

A grounding transformer is a transformer intended solely

for establishing a neutral ground connection on a three-phase

ungrounded system. The transformer is usually of the wye-

delta or zig-zag arrangement as shown in Figure 1. The

application of grounding transformers on delta-connected

ungrounded three-phase transmission/distribution systems is

well known. Ground-fault protection schemes that provide

selective and reasonably fast tripping are often incorporated

with these grounding transformers. Since grounding

Grounding Transformer Application,

Modeling, and SimulationM. Shen, Member, IEEE , L. Ingratta, and G. Roberts

T

©2008 IEEE.


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transformers are not encountered on a daily basis by most

electric power engineers, improper understanding and

applications of these devices and/or the associated ground-

fault protection systems sometimes occurs.

A

B

C

(a) Wye-delta grounding transformer

(b) Zig-zag grounding transformer

Figure 1. Wye-delta and zig-zag grounding transformer

The technical literature covering grounding transformers is

scattered. A number of technical publications [1]-[8] discuss

various aspects of the purpose, application, protection

philosophy, and specifications of different types of grounding

transformers. However, some of these materials are not

readily available. It appears that no single publication

discusses all aspects of the grounding transformers. This

paper first reviews the state of the art of the grounding

transformer to assist electric power engineers in the proper

understanding of the use and applications of these devices,

and then a zig-zag grounding transformer is modeled in

PSCAD/EMTDC simulator. After a 27.6 KV ungrounded

three-phase transmission system is constructed, different

scenarios are simulated and verified under different

conditions with the connection of the grounding transformer.

A. What is the Grounding Transformer?

One type of grounding transformer commonly used is a

three-phase zig-zag transformer with no secondary winding.

One application is to derive an earth reference point for an

ungrounded electrical system.

The internal connection of this transformer is illustrated in

Figure 2. Consider a three-phase Y (wye) transformer with an

earth connection on the neutral point. Cut each winding in the

middle so that the winding splits into two sections. Turn the

outer winding around and rejoin the outer winding to the next

phase in the sequence (i.e. outer A phase connects to inner B

phase, outer B phase connects to inner C phase, and outer C

phase connects to inner A phase).The impedance of the grounding transformer to three-

phase current is high so that when there is no fault or un-

balanced current on the systems, only a small magnetizing

current flows in the transformer windings. The transformer

impedance to ground current, however, is low so that it allows

high ground current to flow. The transformer divides the

ground current into three equal components; these currents

are in phase with each other and flow in the three windings of

the grounding transformer. The method of winding is seen

from Figure 2 to be such that when these three equal currents

flow, the current in one section of the winding of each leg is

in a direction opposite to that in the other section of thewinding on that leg. This tends to force the ground-fault

current to have equal division in the three lines and accounts

for the low impedance of the ground currents.

Figure 2. Winding connections of the zig-zag grounding transformer

B. Why the Grounding Transformers are Necessary?

Grounding transformers have been applied to ungrounded

three-phase power systems to 1) provide a source of ground-

fault current during line-to-ground faults, 2) limit the

magnitudes of transient overvoltages when restriking groundfaults occur and, 3) stabilize the neutral, and when desired,

permit the connection of phase-to-neutral loads [4].

Ungrounded three-phase systems are used mainly to

prevent an automatic shutdown when a ground fault on any

one of the three phases occurs. The majority of all faults are

of the single phase-to-ground variety. Therefore, continuity of

power is maintained when no automatic tripping occurs for

this common type of fault. However, ultimately, the fault must

be located and fixed. It goes without saying that it can be

annoying and time consuming to locate a ground fault by


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switching loads on and off to pinpoint and remove the single

phase-to-ground fault from the system. Consequently,

grounding transformers are commonly used to enable

automatic detection and, if desired, isolation of phase-to-

ground faults.

Many electric utilities have converted ungrounded delta

primary distribution systems to grounded wye systems to

provide for the automatic isolation of line-to-ground faults, to

help protect the system components, and to prevent orminimize possible injury to personnel. It is believed that the

use of grounding transformers on new systems will phase out

in the future, because generally, it is cheaper and simpler to

install a new grounded neutral wye system than a delta system

having an associated grounding transformer. However,

grounding transformers would normally be retrofitted to

existing delta systems, particularly; systems rated for 27.6 KV

or less. Most of the older systems in these voltage classes

were designed to be operated ungrounded.

C. Use of Grounding Transformers

The grounding transformer provides a source for zero-

sequence current, stabilizes the system neutral, and, if

properly sized, permits the addition of a neutral conductor to

overhead lines.

The preferred location for the grounding transformer is at

the source substation, connected either to the power

transformer leads or the station bus. If the grounding

transformer is to be used to supply a four-wire distribution

system, care must be taken to insure that switching cannot

cause the grounding transformer to be disconnected while the

power transformer continues to energize the lines. If the

grounding transformer were to be disconnected, a system

ground fault could cause 173% voltage to be applied to the

phase-to-neutral distribution transformers connected to theunfaulted phases. Also phase-to-neutral overvoltages are

possible due to load imbalances, even without a ground fault.

Small grounding transformers made from single-phase

distribution transformers have sometimes been used on three-

wire ungrounded distribution systems to derive a neutral for a

local four-wire system. Such applications must be carefully

engineered since the presence of the grounding transformer

on the distribution line will tend to degrade the sensitivity and

selectivity of residual ground relays. Application of small

grounding transformer on otherwise ungrounded systems

should be avoided since it is usually not possible to provide

ground-fault relaying that is fully selective and yet protectsthe grounding transformer from continuous overcurrent.

The calculations necessary to specify a grounding

transformer are discussed in [3].

D. Location of System Grounding Points

The selection of a system grounding point is influenced by

whether the transformer or generator windings are connected

‘wye’ or ‘delta’. ‘delta-wye’ or ‘wye-delta’ transformers

effectively block the flow of zero-sequence current between

systems. Although the wye connection is generally more

helpful to system grounding because of the availability of a

neutral connection, that fact alone should not be the sole

criteria for the location of the system ground point.

The system ground point should always be at the power

source. An old concept of grounding at the load or at other

points in the system because of the availability of a

convenient grounding point is not recommended because of

the problems caused by multiple ground paths and because of

the danger that the system could be left ungrounded andtherefore unsafe. The National Electrical Code recognizes this

danger and prohibits system grounding at any place except the

source and/or service equipment.

It is generally desirable to connect a grounding transformer

directly to the main bus of a power system shown in Figure 3,

without intervening circuit breakers or fuses, to prevent the

grounding transformer from being inadvertently taken out of

service by the operation of the intervening devices. In this

case, the transformer is considered part of the source power

transformer and is protected by the relaying applied for

transformer/bus protection.

Figure 3. General connection of grounding transformer to a

delta-connected or ungrounded power system

E. Rating of the Grounding Transformer

Since a grounding transformer is normally only required to

carry short-circuit ground current until the circuit breakers

clear the fault and de-energize the faulted circuit, it is

common to rate it on a short time such as 10 s. Under these

circumstances the physical size (and resulting cost) is

considerably reduced. If it is required to carry a continuous

percentage of unbalanced current, this will reduce the amount

of savings possibly.

The rating of a three-phase grounding transformer, in kVA,

is equal to the rated line-to-neutral voltage in kilovolts times

the rated neutral current that the transformer is designed to

carry under fault conditions for a specified time. Most

grounding transformers are designed to carry their rated

current for a limited time only, such as 10 s or 1 min.

Consequently, they are much smaller in size than an ordinary

three-phase continuously rated transformer with the same

rating.


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Rated voltage of a grounding transformer is the line-to-line

voltage for which the unit is designed.

In an application on a multigrounded neutral system, the

grounding transformer must have the capability to carry some

continuous neutral current. An estimate must be made of the

maximum expected load imbalance in order to specify this

rating. A grounding transformer constructed in accordance

with IEEE Std 32-1972 will have a continuous rating of 3%

for a 10 s rated unit. This value would correspond to a 200 Acontinuous rating for the 6600 A transformer specified above.

If higher values of continuous current are required, the size

and cost of the grounding transformer may increase. A 1 min

rated unit would have 7% of the continuous current rating.

F. Protection of the Grounding Transformers

When a grounding transformer is used on a system, the

protection philosophy should be as follows [5]:

• The system must be protected against faults in the

grounding transformer; however, any isolation of a

grounding transformer must not leave a system in a

totally ungrounded or in an inadequately grounded mode.

• Back-up protection should be provided for ground faults

that are not cleared by the primary protection device.

• Protection should be selective to prevent unnecessary

outages.

When a zigzag or grounded wye-delta transformer is used,

the effective grounding impedance is selected to provide

sufficient current for selective ground relaying. The available

ground fault current is generally on the order of 400 A.

The electrical protection scheme for the grounding

transformer is simple, consisting of overcurrent relays

connected to delta-connected CTs, as shown in Figure 4 (a),

or differential protection with backup ground relay, as shown

in figure 4 (b).

Each power transformer, bus, and feeder breaker would

have primary ground overcurrent relaying. This protection

could be sensitive instantaneous overcurrent relaying. Backup

protection would be provided by a time-overcurrent relay

connected to a CT in the neutral of the grounding transformer

as shown in figure 4 (C).

(a) Overcurrent protection

(b) Differential protection with backup ground relay

(c) Time-overcurrent protection

Figure 4. Ground-fault protection with zig-zag grounding transformer

A phase-to-ground fault should not be allowed to persist on

a grounding transformer with low or no neutral impedance

that permits a fault current magnitude greater than the

continuous current rating. Therefore, the selection of a CT

ratio associated with the grounding transformer depends more

on the pickup of the ground relay than the rating of the

grounding transformer. However, if a fault is allowed to

persist, then the CT ratio must be selected with the continuous

current in mind.

G. Fault Locating in Ungrounded or High-Resistance

Ground System

Common methods of localization are: 1) fault isolation by

network switching, and 2) circuit tracing using a signal

injector and a hand-held signal detector.Network switching is the simplest method. The system

operator deenergizes one feeder at a time until the fault

disappears. Then branch circuits are switched, eventually

loads are tested. This identifies the faulted network section.

This search process interrupts the continuity of service, which

is the advantage of these systems. In practice, the search is

postponed until there is a scheduled break in production.

However, often the search is frustrated by the disappearance

of the fault when all the manufacturing equipment is shut

down. And the search is manpower intensive and requires


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well-trained personnel that are familiar with the entire power

system network [9].

Circuit tracing with a superimposed signal is a preferred

method for locating a fault. The signal can be supplied in a

number of ways. For high-resistance grounded systems, a

common signal source is the modulation of the ground-fault

current through the grounding resistor. This may be

accomplished with a second resistor switched in parallel with

the grounding resistor or by shorting out a portion of thegrounding resistor. With either method, a pulsing circuit

operates a contactor, which switches in a lower resistance for

the grounding circuit. This increases the ground-fault current

momentarily, enough for detection by ammeters or by a

clamp-on detector [10].

For an ungrounded system, a pulsating electronic signal

injector (commonly referred to as a thumper circuit) is

attached to the faulted network, and hand-held detectors sense

the signal along the faulted circuit. The thumper circuit is an

electronic oscillator within the audio frequency range and is

coupled between the faulted phase and ground. The signal

travels along the fault path, and is detected by a receivercircuit. Such test equipment is portable and only needs to be

attached when looking for the fault.

The current practices for locating ground faults have

certain weaknesses, which have troubled many industrial

operations. These weaknesses stem from three conditions that

are frequently not considered by the localization methods.

They are: 1) intermittent fault conditions; 2) multiple faults on

the same phase; and 3) inverted ground faults [11]. A new

location technique which can uniquely identifies a fault

location by discerning the zero-sequence fault current is

proposed [11].

III. MODELING OF THE GROUNDING TRANSFORMER IN

PSCAD/EMTDC

As introduced in Section II, the internal connection of the

zig-zag grounding transformer is illustrated in Figure 2. In

PSCAD/EMTDC simulator, three single-phase 2-winding

transformers are used to model the zig-zag grounding

transformer. Oppositely connect the secondary winding of

each single-phase transformer to the primary winding of next

single-phase transformer in the sequence; a zig-zag grounding

transformer model is obtained (Shown in Figure 5).

# 1

# 2

# 1

# 2

# 1

# 2

Neutral Lead

Line Leads

Figure 5. The zig-zag grounding transformer model in PSCAD/EMTDC

A. Topology of Delta-connected Transmission System with

Grounding Transformer

In addition to the grounding transformer model introduced

in pervious section, a delta-connected transmission system is

also constructed in PSCAD (shown in Figure 6). The

grounding transformer is directly connected to the wye-delta

source power transformer. System simulations and analyses

will be taken place based on this system topology.

Depends on different system data, in this system, a 0.04Ohm resistor is applied to this model as the winding

impedance of this grounding transformer. Different balanced

and unbalanced loads are connected to the system.A

B

C

A

B

C27.6 [kV]

#2#1

115.0 [kV]

50.0 [MVA]A

B

C

RRL

RRL

RRL

Va

5.0 [MW]

Vb

1 0 0 0 [ oh m ]

5 0 0 [ oh m ]

1 0 0 [ oh m ]

Vc

# 1

# 2

0 . 0

4 [ oh m ]

# 1

# 2

# 1

# 2

Zig-Zag

GroundingTransformer

Balanced 3-phaseload Un-Balanced

3-phase loadAnd more ...

Figure 6. Delta-connected transmission system topology

IV. SYSTEM SIMULATIONS AND VERIFICATIONS

A. Normal Operation

As presented above, the grounding transformer creates a

neutral point for the 3-wire delta-connected transmission

system; therefore the original 3-phase ungrounded system is

converted to 3-phase grounded system, which now can supply

any unbalanced or single-phase loads. The unbalanced current

will go to ground and pass through the grounding transformerback to the main transformer.

The transmission system model is shown in Figure 7. The

main power transformer is a wye/delta (115kV/27.6kV)

transformer. Beside the main power transformer, a zig-zag

grounding transformer is connected to the system. Therefore,

a neutral point is obtained and connected to the ground.A

B

C

A

B

C27.6 [kV]

#2#1

115.0 [kV]

50.0 [MVA]A

B

C

RRL

RRL

RRL

Va

5.0 [MW]

Vb

1 0 0 0 [ oh m ]

5 0 0 [ oh m ]

1 0 0 [ oh m ]

Vc

# 1

# 2

0 . 0 4 [ oh m ]

# 1

# 2

# 1

# 2

I_ F a u l t

I a_

G r o u n d

I b_

G r o u n d

I c_

G r o u n d

Figure 7. 115/27.6 kV wye/delta transmission system with grounding

transformer


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Scenario 1. With Un-balanced loading

As shown in Figure 7, the system is powering some

balanced loads and some un-balanced loads. With

PSCAD/EMTDC simulator, an un-balanced current, I_Fault

(about 0.14kA, shown in Figure 8), is measured in the

grounding transformer.Main : Graphs

s 0.020 0.040 0.060 0.080 0.100 0.120 0.140 0.160

-0.20

-0.10

0.00

0.10

0.20

y [ K A ]

I_Fault

Figure 8. Un-balanced ground current

By measuring the currents in each winding of the

grounding transformer, 3 current signals, Ia_Ground,

Ib_Ground, and Ic_Ground are also measured and shown in

Figure 9.Main: Graphs

[s] 0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140 0.160

-0.20

-0.10

0.00

0.10

0.20

y ( K A )

I_Fault Ia_Ground

Main: Graphs

[s] 0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140 0.160

-0.20

-0.10

0.00

0.10

0.20

y [ k A ]

I_Fault Ib_Ground

Main : Graphs

[s] 0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140 0.160

-0.20

-0.10

0.00

0.10

0.20

y [ k A ]

I_Fault Ic_Ground

Figure 9. Un-balanced ground current vs. three currents in individual winding of

the grounding transformer

From Figure 9, we can see that the grounding transformer

creates a path for the un-balanced current, and also divides

the ground current to three in-phase, equal components. This

also verified one of the very important characteristics of the

grounding transformer, which we mentioned in Section II-A.

Scenario 2: With Balanced Loading

If the un-balanced load is disconnected from this system,

with the simulation, an almost zero kA current is measured in

grounding transformer (shown in Figure 10), which verifies

that the grounding transformer has very high impedance to

balanced three-phase currents.Main : Graphs

s 0.020 0.040 0.060 0.080 0.100 0.120 0.140

-0.20

-0.10

0.00

0.10

0.20

y [ K A ]

I_Fault

Figure 10. Ground current with balanced loading

B. Fault Analyses and Simulation Verifications

From the above simulation, it is clear that the grounding

transformer creates a neutral point for the transmission

system, and allows the un-balanced ground current pass by. In

addition, when the phase-to-ground fault is applied to the

system, the grounding transformer will also create a path for

the fault current. Mean while, the fault current will be sensed

by the CTs for the protection purpose. More important

characteristic, the grounding transformer has, is the grounding

transformer minimizes the overvltage on other un-faulted

phases, which will be simulated and analysed in this Section.

Scenario 1: Phase A to Ground Fault Applied

On this wye/delta transmission system, if a fault is applied

on the Phase A to ground during 0.08s to 0.12s shown in

Figure 11. Because the fault is applied on the Phase A, thevoltage on phase A will be dramatically decreased. However,

the voltage on Phase B and C are still maintained nearly at the

normal operating rate. Simulation verification is shown in

Figure 12 and Figure 13.A

B

C

A

B

C27.6 [kV]

#2#1

115.0 [kV]

50.0 [MVA]A

B

C

RRL

RRL

RRL

Va

5.0 [MW]

Vb

1 0 0 0 [ oh m ]

5 0 0 [ oh m ]

1 0 0 [ oh m ]

Vc

# 1

# 2

0 . 0 4 [ oh m ]

# 1

# 2

# 1

# 2

I_ F a u

l t

I a_

G r o u n

d

I b_

G r o u n

d

I c_

G r o u n

d

T i m e

d

F a u

l t

L o g

i c

F A U L T S

C B A

A - >

G

Figure 11. Fault applied between Phase A and ground


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Main: Graphs

[s] 0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140 0.160

-30

-20

-10

0

10

20

30

y ( K V )

Va Vb Vc

Figure 12. Phase voltages under Phase A to ground fault condition

Main: Graphs

[s] 0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140 0.160

-200

-150

-100

-50

0

50

100

150

200

y ( K A )

I_Fault Ia_Ground

Figure 13. Ground current under Phase A to ground fault condition

Scenario 2: Phase A and Phase B to Ground Fault

In some cases, some faults could be applied between two

phases and ground. In Figure 14, one scenario, phase A and

Phase B fault to ground is shown. With the grounding

transformer connected to the system, the Phase C voltage

should be maintained at the normal operation level. The

simulation result again verified this conclusion, which is

shown in Figure 15 and Figure 16.A

B

C

A

B

C27.6 [kV]

#2#1

115.0 [kV]

50.0 [MVA]A

B

C

RRL

RRL

RRL

Va

5.0 [MW]

Vb

1 0 0 0 [ oh m ]

5 0 0 [ oh m ]

1 0 0 [ oh m ]

Vc

# 1

# 2

0 . 0 4 [ oh m ]

# 1

# 2

# 1

# 2

I_ F a u

l t

I a_

G r o u n

d

I b_

G r o u n

d

I c_

G r o u n

d

T i m e

d

F a u

l t

L o g

i c

F A U L T S

C B A

A B

- > G

Figure 14. Fault applied between Phase A, Phase B and ground

Main: Graphs

[s] 0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140 0.160

-30

-20

-10

0

10

20

30

y ( K V )

Va Vb Vc

Figure 15. Phase voltages under Phase A and Phase B to ground condition

Main: Graphs

[s] 0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140 0.160

-200

-150

-100

-50

0

50

100

150

200

y ( K A )

I_Fault Ia_Ground

Figure 16. Ground current under Phase A and B to ground fault condition

V. REVENUE METERING SYSTEM INSTALLATION ON THREE-

PHASE UNGROUNDED SYSTEMS

Currently, in Ontario Electricity market, the utility

companies are conducting the wholesale revenue metering

system upgrade. According to the standard, the new metering

installations in the IESO-administered market shall conform

to Blondel’s Theorem. On this wye/delta with grounding

transformer system (shown in Figure 17), if the metering

system will be installed at point between the power

transformer and the grounding transformer, two elementscould be selected because the power source is a three-phase;

three-wire system before the grounding transformer.

However, if the metering installation point is selected after the

grounding transformer, a three-element system must be

installed because the power source now actually is a three-

phase; 4-wire system.

If a 3-element metering system is considered, it can be

concluded that there is no difference between installation

before grounding transformer point and installation after

grounding transformer point (shown in Figure 17). In

addition, because the distance between these two points is

only few meters physically. The line impedance between thesetwo points sure can be neglected. Therefore, no voltage

difference should be considered between these two points.

The metering instrument transformers, such as VTs, the

connections are shunt connection on the system. Usually they

are used as single purpose to supply the meters. Depends on

the VT’s secondary loads, but normally the loads are very

light. So the VT’s impedance to the system is very high.

Probably, only few mA current is drawn from the line on

which the VT is connected. Compare to the hundred and

thousand amperes fault current, this mA current definitely

could be neglected. The metering system will sure not affect

the relaying system at all unless the metering system itself

causes the faults on the system.

After all, the metering system itself could be treated as a

normal 3-phase load on the system, but just very light load.


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A

B

C

A

B

C27.6 [kV]

#2#1

115.0 [kV]

50.0 [MVA]A

B

C

RRL

RRL

RRL

Va

Vb

1 0 0 0 [ oh m ]

5 0 0 [ oh m ]

1 0 0 [ oh m ]

Vc

# 1

# 2

I a_

G r o u n d

I b_

G r o u n d

# 1

# 2

# 1

# 2

5.0 [MW]

I c_

G r o u n d

BeforeGroundingTransformerPoint

AfterGroundingTransformerPoint

I_ F a u l t

0 . 0 4 [ oh m ]

Figure 17. Potential revenue metering system installation points before or after

grounding transformer

The electric power systems, which are operated with no

intentional ground connection to the system conductors, are

generally described as ungrounded. In reality, these systemsare grounded through the system capacitance to ground

(shown in Figure 18). The line conductors have capacitances

between one another and to ground, as presented by the delta-

and the wye-connected sets of capacitances. In most systems,

this is extremely high impedance, and the resulting system

relationships to ground are weak and easily distorted.

If one conductor, for example phase A, becomes faulted to

the ground, the line A to ground voltage will be close to zero

voltage, and the line B and C to ground voltage will increase

to phase-to-phase voltage.A

B

C

A

B

C27.6 [kV]

#2#1

115.0 [kV]

50.0 [MVA]A

B

C

RRL

RRL

RRL

Figure 18. Three-phase ungrounded system

This overvoltage is 1.732 times the voltage normally on the

insulation. This sustained overvoltage or the transient

overvoltages on the ungrounded system may not immediately

cause failure of insulation, but may tend to reduce the life ofthe insulation.

For metering system installation on above ungrounded

system, the voltage classes of CTs, VTs, and other

instruments should be carefully considered.

VI. CONCLUSIONS

In this paper, first of all, the state of the art of the

grounding transformer is reviewed. Then, the electrical model

of the grounding transformer is built in PSCAD/EMTDC

simulator, and different scenarios are simulated and verified

under different conditions. Finally, the concern from the

revenue metering system installation on wye/delta

transmission systems in Ontario power network is addressed.

VII. REFERENCES

[1]

Electrical Transmission and Distribution Reference Book, ABB, 1997.

[2]

IEEE Recommended Practice for Grounding of Industrial and Commercial

Power System, IEEE Std 142-1991, June 1991.

[3]

IEEE Guide for the Application of Neutral Grounding in Electrical Utility

Systems, Part IV—Distribution, IEEE C62.92.4-1991, December 1992.

[4] Edson R. Detjen, and Kanu R. Shah, “Grounding Transformer

Applications and Associated Protection Schemes”, IEEE Transactions on

Industry applications, Vol. 28, N0.4, July / August 1992

[5]

L. J. Carpenter, “Connecting a Grounding Transformer to the System”,

General Electric Co., GER-1062.

[6]

Peter E. Sutherland, “Application of Transformer Ground Differential

Protection Relays”, IEEE Transactions on Industry applications, Vol. 36,

NO. 1, January/February 2000

[7]

http://en.wikipedia.org/wiki/Zigzag_transformer

[8]

http://ecmweb.com/mag/electric_basics_zigzag_transformers

[9] D. J. Love and N. Hashemi, “Considerations for ground fault protection in

medium-voltage industrial and cogeneration systems,” IEEE Trans. Ind.

Applicat., vol. 24, pp. 548–553, July/Aug. 1988.

[10]

D. H. Lubich, Sr, “High resistance grounding and fault finding on three

phase three wire (Delta) power systems,” IEEE Paper-7803–4090-6/97,

1997.

[11]

T. Baldwin, F. Renovich, L. Saunders, and D. Lubkeman, “Fault locating

in ungrounded and high-resistance grounded systems,” IEEE Trans. Ind.

Applicat., vol. 37, pp. 1152–1159, July/Aug. 2001.

VIII. BIOGRAPHIES

Mike Shen (Member’2006) was born in Jiangsu,

China, in 1972. He received the B.Sc. degree in

electrical engineering from Jiangsu University,

Jiangsu, China, in 1994, and the MASc degree in

electrical engineering from the University of

Waterloo, Waterloo, Ontario, Canada, in 2006. His

research interests are power system protection,

control and telecommunication, distributed

generation, SCADA system, metering system, power

quality, power electronics, electrical control,

electromechanical systems.

Mr. Shen has over 10 years of professional experiences in Electrical Engineering

from Electrical Automation, Mechatronics, AC/DC Motor Drives, Power

Electronics, Substation Automation, SCADA Systems, Power System Protection

and Control, Distributed Generation, Demand-side Management, Renewable

Energy, Electric System Modeling and Simulations. Currently, He is employed

at Wardrop Engineering Inc. as a design engineer.

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Erthing Transformer