Types Of Three-phase Transformer Connections & Application

TYPES OF THREE-PHASE TRANSFORMER CONNECTIONS & APPLICATION

 Learning Objectives  Introduction  Types of Three-phase Transformer Connections  General Considerations for Three-Phase

Transformer Applications  Advantages and Disadvantages  Recommendations

 To familiarize with the common variations of three-

phase transformer connections  To

familiarize

with

the

applicability

of

each

transformer connection for specific systems  To determine the advantages and disadvantages of

each transformer connection  To be able to properly select the appropriate

transformer connection for specific types of systems

Three-Phase Transformer Connections

Three-phase Transformer Connections The

three-phase

transformer

can

be

constructed by either: 1. Banking of three individual single-phase

transformers 2. Combined with one core - single tank

three-phase unit

The individual single-phase transformers can be banked by several configurations. Some of the common configurations are the following:  Delta-Wye  Wye-Delta  Delta-Delta  Wye-Wye  Open-Delta

The

single

unit

three-phase

transformer

windings are connected also by the same

configuration but its connections are made in one single core. Some other connections are the following:  Zig-Zag  Scott and T connection

Comparison between Three Single-phase and Single Tank Three-Phase Transformers

Types of Common Transformer Connections and their Characteristics

A. Delta-Wye Transformer Connection

A. Delta-Wye Transformer Connection

A. Delta-Wye Transformer Connection

Three-phase excitation current wave forms (solid) and the third harmonic components combined (dotted).

A. Delta-Wye Transformer Connection

•

Type of three-phase electric power transformer design with Delta-connected windings on its primary and Wye/Star connected windings on its secondary.

•

A neutral wire can be provided on wye output side.

•

Either a single three-phase transformer, or built from three independent single-phase units.

•

Equivalent term is Delta-Star transformer.

A. Delta-Wye Transformer Connection

•

•

Phase voltage = Line voltage / 1.732 Phase current = Line current

•

Used universally for connecting generators to transmission systems because of two very important reasons: 1. Generators are usually equipped with

sensitive ground fault relay protection.

2. Rotating machines can literally be

shaken apart by mechanical forces resulting from zero-sequence currents. The ∆-connected winding blocks zerosequence currents on the transmission system from the generator.

A. Open Delta - Open Delta Transformer Connection

A. Wye – Wye Transformer Connection

A. Wye – Wye Transformer Connection

•The most obvious way of transforming voltages and currents in a three-phase electrical system is to operate each phase as a separate singlephase system. This requires a four-wire system comprised of three phase wires plus a common neutral wire that is shared among the three phases. Each phase is transformed through a set of primary and secondary windings connected phase-to neutral.

The term ‘‘Y-Y connection’’ should be obvious from the fact that the vector diagrams of the primary and secondary windings both resemble the letter Y. Each phase of the primary and secondary circuits is 120 electrical degrees out of phase with the other two phases. This is represented by angles of 120° between the legs of the primary Y and the secondary Y in the vector diagram. Each primary winding is magnetically linked to one secondary winding through a common core leg. Sets of windings that are magnetically linked are drawn parallel to each other in the vector diagram. In the Y-Y connection, each primary and secondary winding is connected to a neutral point. The neutral point may or may not be brought out to an external physical connection and the neutral may or may not be grounded.

A. Wye – Delta Transformer Connection

A. Wye – Delta Transformer Connection

There are times when a grounded Y-Δ transformer is used for no other purpose than to provide a good ground source in an otherwise ungrounded system. Take, for example, a distribution system supplied by a Δconnected (i.e., ungrounded) power source. If it is required to connect phase-toground loads to this system a grounding bank is connected to the system

A. Delta – Delta Transformer Connection

•

Type of three-phase electric power transformer design with Delta-connected windings on its primary and Delta connected windings on its secondary.

•

No neutral wire provision.

•

Either a single three-phase transformer, or built from three independent single-phase units.

•

Phase voltage = Line voltage Phase current = Line current / 1.732

•

 Connection in three identical transformers are

symmetrical or balanced  Currents & voltages in each of the phase are the

same and their relationship to the line voltages and currents are identical  kVA rating of the bank = 1.732 x E x I  Associated power in balanced three-phase loads =

1.732 x E x I  kVA present in the transformers is equal to the kVA

delivered to the circuit

General Considerations for Transformer Connection Applications

General Considerations for Transformer Connection Applications • To

be

familiar

with

the

general

transformer characteristics is essential for the selection of the

appropriate

connection to meet a given service requirement.

The following are the transformer general characteristics: A. Ratio of kVA output to the kVA rating of

the bank B. Degree of Voltage Symmetry C. Voltage and Current Harmonics D. other operating peculiarities

A. Ratio of kVA output to the kVA rating of the bank  Connection in three identical transformers are

symmetrical or balanced  Currents & voltages in each of the phase are the

same and their relationship to the line voltages and currents are identical  kVA rating of the bank = 1.732 x E x I  Associated power in balanced three-phase loads =

1.732 x E x I  kVA present in the transformers is equal to the kVA

delivered to the circuit

 Thus, ratio of kVA load to the kVA present in the

bank is UNITY or equals to 1.0  This characteristic can be made as reference for

proper selection of transformer connections based on system kVA requirements

kVA output/kVA rating Ratio of various Transformer Connections

Transformer Connection Delta-Wye Wye-Delta Wye-Wye Delta-Delta

kVA output/kVA rating Ratio = 1.0 (100%)

< 1.0

   

Open-Delta

86.6%

T-type

86.6%

Zig-Zag

86.6%

B. Degree of Voltage Symmetry  Another basis is the symmetry with respect to the

lines and with respect to the neutral  Delta and Zig-zag:

 voltage and current symmetry with respect to

the three lines and lines to neutral  All other connections possess varying degrees of

dissymmetry; although three-phase load is balanced, introduce objectionable operating features such as:  Unbalanced regulation  Current distortion

 Open-Delta and T :  Dissymmetrical with respect to the three lines

and with respect to neutral  Wye-Wye connection:  Current dissymmetry between lines and neutral

 Wye-Delta or Delta-Wye connection:  Complete symmetry for all practical purposes is

maintained by the presence of the Delta

C. Voltage and Current Harmonics  Open-Delta and T :  introduces unbalanced regulation and third-

harmonic magnetizing currents in the circuit  Wye –Wye :  introduces third-harmonic voltage  Current

dissymmetry

between

lines

and

neutral  subjects parallel telephone circuits to serious interference  subjects the system itself to dangerous overvoltages under certain conditions

 Wye connection:  Thus,

Wye-Wye connection neutral is not recommended

with

isolated

 Balanced Three-phase Delta-Delta, Wye-Delta, &

Delta-Wye connection (equal and symmetrical windings and core):  Do not introduce third harmonics and their multiples into the line  Wave shapes of the magnetizing currents to

such banks are superior to the wave shapes of dissymmetrical banks

 Paralleled banks of Three-phase Delta-Wye and

Wye-Delta (same rating):  Fifth, seventh, seventeenth and nineteenth

harmonics = 180 degrees apart in phase  Transformer magnetization line harmonics are

minimized.

Symmetry & Harmonic Characteristics and Effects

DeltaWye

WyeDelta

Delta Delta

WyeWye

Voltage symmetry to the three lines











Voltage symmetry to the lines to neutral











Current symmetry to the lines











Current symmetry to the lines to neutral







Current Dissymmetry between lines & neutral

OpenD elta

T

ZigZag

 

Dissymmetrical with respect to three lines and neutral





Unbalanced regulation





Introduces third-harmonic magnetizing currents





Introduces third-harmonic voltage



Serious interference to parallel telephone circuits



Dangerous system overvoltages



D. Other operating peculiarities Transformers having equal characteristics (rating and symmetry) such as the Delta-Wye, Wye-Delta, and Delta-Delta, other factors must be considered in order to select the appropriate connection type.  Delta-Wye :  Neutrals can be derived either for loading or

grounding  Less expensive design in very high-voltage

(small line currents) systems since phase voltage = E / 1.732

 Balanced three-phase loads equally divided

among the phases regardless of unequal or different impedances of banked transformers  Unaffected by different transformer ratios  However, when banking transformers having

widely different kVA ratings leads to disproportionate sharing of loads among the three phases.

 Delta-Delta:  No neutrals available for loading or grounding  Less expensive design in very large current

(low-voltage) since phase current = I / 1.732  Balanced three-phase loads unequal division of

loads among the phases when banking with different transformer impedances  When

banking transformers having widely different kVA ratings leads to proportionate sharing of loads among the three phases.

Advantages & Disadvantages of Each Transformer Connection

A. Delta-Wye Transformer Connection advantages •

•

One of the major advantages of the ∆-Y connection is that it provides harmonic suppression. Recall that the magnetizing current must contain odd harmonics for the induced voltages to be sinusoidal. In the ∆-Y connection, however, the third harmonic currents, being equal in amplitude and in phase with each other, are able to circulate around the path formed by the ∆-connected winding.

A. Delta-Wye Transformer Connection advantages •

Another important advantage of the ∆-Y connection is that it provides ground current isolation between the primary and secondary circuits. Assuming that the neutral of the Yconnected secondary circuit is grounded, a load connected phase-to-neutral or a phase-to-ground fault produces two equal and opposite currents in two phases in the primary circuit without any neutral ground current in the primary circuit.

A. Wye - Wye Transformer Connection advantages

•The primary and secondary circuits are in phase; i.e., there are no phase angle displacements introduced by the Y-Y connection. This is an important advantage when transformers are used to interconnect systems of different voltages in a cascading manner. •Since the phase-to-neutral voltage is only 57.7% of the phase-to phase voltage, the windings of a Y-Y transformer require fewer turns to produce the same level of excitation in the core compared to windings connected across the phases.

A. Wye - Wye Transformer Connection advantages

•If the neutral end of a Y-connected winding is grounded, then there is an opportunity to use reduced levels of insulation at the neutral end of the winding. A winding that is connected across the phases requires full insulation throughout the winding. •A Y-Y transformer may be constructed as an autotransformer, with the possibility of great cost savings compared to the two-winding transformer construction.

A. Wye - Wye Transformer Connection disadvantages •

The presence of third (and other zero-sequence) harmonics at an ungrounded neutral can cause overvoltage conditions at light load. When constructing a Y-Y transformer using single-phase transformers connected in a bank, the measured line-to-neutral voltages are not 57.7% of the system phase-to-phase voltage at no-load but are about 68% and diminish very rapidly as the bank is loaded.

A. Wye - Wye Transformer Connection disadvantages

. Under certain circumstances, a Y-Y connected threephase transformer can produce severe tank overheating that can quickly destroy the transformer. This usually occurs with an open phase on the primary circuit and load on the secondary.

A. Wye - Wye Transformer Connection disadvantages

. Severe over-voltages due to Series resonance between the third harmonic magnetizing reactance of the transformer and line to-ground capacitance. . If a phase-to-ground fault occurs on the primary circuit with the primary neutral grounded, then the phase-toneutral voltage on the un-faulted phases increases to 173% of the normal voltage. This would almost certainly result in over-excitation of the core, with greatly increased magnetizing currents and core losses.

A. Wye - Wye Transformer Connection disadvantages

• If the neutrals of the primary and secondary are both brought out, then a phase-to-ground fault on the secondary circuit causes neutral fault current to flow in the primary circuit. Ground protection relaying in the neutral of the primary circuit may then operate for faults on the secondary circuit. •The obvious remedy for some of the disadvantages of the Y-Y transformer connection would be to simply solidly ground both the primary and secondary neutrals. In fact, this is standard practice for virtually all Y-Y transformers in systems designed by utility companies.

A. Wye - Wye Transformer Connection disadvantages

•Unfortunately, solidly grounding the neutrals alone does not solve the problem of tank overheating, ferroresonance, and operating primary ground protection during secondary faults.

A. Advantages of Delta-Delta or Ungrounded

Wye-Delta Connection:

•More economical transformer installation for smaller three-phase service with some single-phase loads is possible. •The load is isolated from ground faults on the utility side.

•DG would not typically feed utility-side ground faults except when resonance occurs. •Ungrounded interconnection can be provided for inverter-based systems requiring it.

A. Disadvantages of Delta-Delta or Ungrounded

Wye-Delta Connection:

•Utility-side SLG faults are difficult to detect. •Utility arresters are subjected to high steady-state over-voltages if islanded on an SLG fault. This is true for delta-wye connections as well. •These connections are highly susceptible to ferroresonance in cable-fed installations.

•There are more restrictions on switching for utility maintenance.

RECOMMENDATIONS

 Select the proper kVA output to kVA rating ratio

that is appropriate requirements.

for

your

system

kVA

 Evaluate if system loads can tolerate voltage and

current dissymmetry which can lead to unbalanced regulation, current distortion and interference in other systems (i.e. telephone circuits and other communication systems)  Evaluate if system loads are composed mostly of

non-linear loads which can produce undesired system harmonics.

 Check if there are presence of line-to-neutral loads

which necessitates neutral connection from the transformer.  Evaluate if system can tolerate voltage and current

harmonics which can easily damage sensitive electronic equipment, etc.  Evaluate

if system can not tolerate power interruption due to single-line-to-ground faults (SLG) due to critical opportunity loss or loss of lives such as in some areas of Healthcare facilities and hospitals

 Evaluate if neutral system ground is necessary for

easier ground fault detection in the system in order to avoid electrical fires due to arcing ground faults (i.e. hazardous areas, petro-chem facilities, wood manufacturing plants, etc.)  Last but not the least, don’t forget about the

bottomline:

PRACTICAL ECONOMICS

THE END!! Thank you for listening!!