Power Engineering: Transformer

Transformer parts:

Transformer part

Parts are shown in figure below:

Working principle:

A transformer works on the Faraday,^s law of electromagnetic induction. A transformer consists of a laminated magnetic core. It contains two magnetic core. When one of the winding is connected called primary is connected to AC voltage V₁ , an alternating flux of the same frequency as that of supply voltage is set up in the magnetic core inducing emf of self induction E₁ primary winding.

This alternating flux links with the other winding (secondary winding) inducing emf E₂ in this winding at the same frequency as that of the supply. This emf is known as emf of mutual induction. If N1 and N2 are number of turns of primary and secondary winding then the ratio is known as transformation ratio of transformer. If a load is connected across secondary winding, a current flows in both the windings. Thus electrical power is transferred magnetically from primary coil to secondary coil.

Buchholtz relay:

Buchholtz relay is used for protection of oiled filled transformers from incipient faults (minor faults ) below oil level. This relay is installed between the transformer tank and conservator.

The incipient faults in transformer tank below oil level actuate buchholtz relay so as to give an alarm. The arc due to fault causes decomposition of transformer oil. the product of decomposition contain more than 70% of hydrogen gas, which being light , rises upward and tries to go in the conservator. Buchholtz relay is fitted in the pipe leading to the conservator. The gas gets collected in the upper portion of the buchholtz relay; thereby the oil level in the buchholtz relay drops down. The float, floating in the oil level in the buchholtz relay tilts down the lowering oil level. While doing so the mercury switch, attached to the float is closed and the mercury switch closes the alarm circuit.

Thereby, the operators know that there is some incipient fault in the transformer. The transformer is disconnected as early as possible and the gas sample is tested. The testing of gas gives clue regarding the type of insulation failure. Buchholtz relay gives an alarm so that the transformer can be disconnected before the incipient fault grows into a serious one.

When a serious short circuit occur in the transformer, the pressure in the tank increases. The oil rushes towards the conservator. While doing so it passes through the buchholtz relay gets pressed by the rushing oil. Thereby they close another switch which in turn closes the trip circuit breaker. Buchholtz relay has following limitation. Only faults below oil levels are detected.

The relay is slow; minimum operating time is 0.1 second, average time 0.2 second. Such a slow relay is unsatisfactory.

However, it is an excellent relay to bring to notice incipient fault.

Buchholtz relays are not provided for transformers below 500kVA. (This is for economic considerations)

A separate buchholtz relay is provided with the tap changer to detect the incipient faults in tap changer. This does not respond to small arcing.

Filtration of Transformer Oil:

Removal of all suspended particles such as dust, rust, scales, colloidal carbon oxidation sludge, etc up to <1 unicorn, degassing and dehydration of transformer oil under vacuum, circulation of heated oil and filling of treated oil in transformer tank, de-acidification by evaporation and absorption method is a must for ensuring prolonged service life a transformer.

Transformer oil property

Oxidation stability		Sludge value less or equal to 1.2%, acidity 2.5mg of KOH per gram of oil
Dielectric strength		Should withstand 50KV rms for 01 min tested as IS:335
Saponification value expressed as mg of KOH per gram of oil acidity		Not more than one
Viscosity at 27⁰c		Should not be more than 27 centistock
Pour point		Max.=-10⁰c
Flash point		Min. = 145⁰c
Copper strip corrosion( at 100⁰c for 3hr)		There should not be any tarnish on copper
Volume resistivity when tested with 500V dc for one min at 90⁰c		Should not be less than 7.5X10¹² ohm cm.
S.K Value (chemical test to find extent of refining)		Should not be more than 4%
Furfurol value (chemical test to find out aromatic content)		8% max.
Dissipation factor or tan§ value		For new oil at room temp. 0.001or less;at 90⁰c it should not be more than 0.005
Interfacial tension		Not less than 0.04newton/m at 27⁰c
Max. Permissible water content (in ppm)		50
Note: It should be cross checked with manufacturer data sheet,

Standard: IS: 335-1993

Off load tap changer

It is a device fitted in the transformer, which is used to vary the voltage transformation ratio.

Here the voltage levels can be varied only after isolating the primary voltage of the transformer.

On load tap changer (OLTC)

The voltage levels can be varied without isolating the connected load to the transformer. To minimise the magnetisation losses and to reduce the nuisance tripping of the plant, the main transformer (the transformer that receives supply from the grid) should be provided with On Load Tap Changing facility at design stage. The downstream distribution transformers can be provided with off-circuit tap changer.

The On-load gear can be put in auto mode or manually depending on the requirement.

OLTC can be arranged for transformers of size 250 kVA onwards. However, the necessity of

OLTC below 1000 kVA can be considered after calculating the cost economics.

Parallel Operation of Transformers

The design of Power Control Centre (PCC) and Motor Control Centre (MCC) of any new plant should have the provision of operating two or more transformers in parallel. Additional

Switchgears and bus couplers should be provided at design stage.

Whenever two transformers are operating in parallel, both should be technically identical in

all aspects and more importantly should have the same impedance level. This will minimise the circulating current between transformers.

Where the load is fluctuating in nature, it is preferable to have more than one transformer

running in parallel, so that the load can be optimised by sharing the load between transformers.

Transformer vector group

Points to be consider while Selecting of Vector Group

Vector Groups are the IEC method of categorizing the primary and secondary winding configurations of 3-phase transformers. Windings can be connected as delta, star, or interconnected-star (zigzag). Winding polarity is also important, since reversing the connections across a set of windings affects the phase-shift between primary and secondary.

Vector groups identify the winding connections and polarities of the primary and secondary. From a vector group one can determine the phase-shift between primary and secondary.

When the hour hand is at 1O’Clockposition, the phase shift is -30 deg. Anti clock wise direction is negative. A connection designated by Dy 11 is delta-star transformer in which the lv line voltage phasor is at 11 O’ clock position that is a phase advance of +30deg. On the corresponding line voltage on hv side.

Transformer vector group depends upon:

1. Removing harmonics: Dy connection – y winding nullifies 3rd harmonics, preventing it to be reflected on delta side.

Parallel operations: All the transformers should have same vector group & polarity of the winding.
Earth fault Relay: A Dd transformer does not have neutral. to restrict the earth faults in such systems, we may use zig zag wound transformer to create a neutral along with the earth fault relay..
Type of Non Liner Load: systems having different types of harmonics & non linear Types of loads e.g. furnace heaters ,VFDS etc for that we may use Dyn11, Dyn21, Dyn31 configuration, wherein, 30 deg. shifts of voltages nullifies the 3rd harmonics to zero in the supply system.
Type of Transformer Application: Generally for Power export transformer i.e. generator side is connected in delta and load side is connected in star. For Power export import transformers i.e. in Transmission Purpose Transformer star star connection may be preferred by some since this avoids a grounding transformer on generator side and perhaps save on neutral insulation. Most of systems are running in this configuration. May be less harmful than operating delta system incorrectly. Yd or Dy connection is standard for all unit connected generators.

There are a number of factors associated with transformer connections and may be useful in designing a system, and the application of the factors therefore determines the best selection of transformers.

For example:

For selecting Star Connection:

A star connection presents a neutral. If the transformer also includes a delta winding, that neutral will be stable and can be grounded to become a reference for the system. A transformer with a star winding that does NOT include a delta does not present a stable neutral.

Star-star transformers are used if there is a requirement to avoid a 30deg phase shift, if there is a desire to construct the three-phase transformer bank from single-phase transformers, or if the transformer is going to be switched on a single-pole basis (ie, one phase at a time), perhaps using manual switches.

Star-star transformers are typically found in distribution applications, or in large sizes interconnecting high-voltage transmission systems. Some star-star transformers are equipped with a third winding connected in delta to stabilize the neutral.

For selecting Delta Connection:

A delta connection introduces a 30 electrical degree phase shift.
A delta connection ‘traps’ the flow of zero sequence currents.

For selecting Delta-Star Connection:

Delta-star transformers are the most common and most generally useful transformers.
Delta-delta transformers may be chosen if there is no need for a stable neutral, or if there is a requirement to avoid a 30 electrical degree phase shift. The most common application of a delta-delta transformer is as tan isolation transformer for a power converter.

For selecting Zig zag Connection:

The Zig Zag winding reduces voltage unbalance in systems where the load is not equally distributed between phases, and permits neutral current loading with inherently low zero-sequence impedance. It is therefore often used for earthing transformers.

Provision of a neutral earth point or points, where the neutral is referred to earth either directly or through impedance. Transformers are used to give the neutral point in the majority of systems. The star or interconnected star (Z) winding configurations give a neutral location.

If for various reasons, only delta windings are used at a particular voltage level on a particular system, a neutral point can still be provided by a purpose-made transformer called a ‘neutral earthing.

For selecting Distribution Transformer

The first criterion to consider in choosing a vector group for a distribution transformer for a facility is to know whether we want a delta-star or star-star. Utilities often prefer star-star transformers, but these require 4-wire input feeders and 4-wire output feeders (i.e. incoming and outgoing neutral conductors).

For distribution transformers within a facility, often delta-star are chosen because these transformers do not require 4-wire input; a 3-wire primary feeder circuit suffices to supply a 4-wire secondary circuit. That is because any zero sequence current required by the secondary to supply earth faults or unbalanced loads is supplied by the delta primary winding, and is not required from the upstream power source. The method of earthing on the secondary is independent of the primary for delta-star transformers.

The second criterion to consider is what phase-shift you want between primary and secondary. For example, Dy11 and Dy5 transformers are both delta-star. If we don’t care about the phase-shift, then either transformer will do the job. Phase-shift is important when we are paralleling sources. We want the phase-shifts of the sources to be identical.

If we are paralleling transformers, then you want them to have the same the same vector group. If you are replacing a transformer, use the same vector group for the new transformer, otherwise the existing VTs and CTs used for protection and metering will not work properly.

There is no technical difference between the one vector groups (i.e. Yd1) or another vector group (i.e. Yd11) in terms of performance. The only factor affecting the choice between one or the other is system phasing, ie whether parts of the network fed from the transformer need to operate in parallel with another source. It also matters if you have an auxiliary transformer connected to generator terminals. Vector matching at the auxiliary bus bar.

Application of Transformer according to Vector Group

1.) Dyn11, Dyn1, YNd1, YNd11

Common for distribution transformers.
Normally Dyn11 vector group using at distribution system. Because Generating Transformer are YNd1 for neutralizing the load angle between 11 and 1.
We can use Dyn1 at distribution system, when we are using Generator Transformer are YNd11.
In some industries 6 pulse electric drives are using due to this 5thharmonics will generate if we use Dyn1 it will be suppress the 5th harmonics.
Star point facilitates mixed loading of three phase and single phase consumer connections.
The delta winding carry third harmonics and stabilizes star point potential.
A delta-Star connection is used for step-up generating stations. If HV winding is star connected there will be saving in cost of insulation.
But delta connected HV winding is common in distribution network, for feeding motors and lighting loads from LV side.

2.) Star-Star (Yy0 or Yy6)

Mainly used for large system tie-up transformer.
Most economical connection in HV power system to interconnect between two delta systems and to provide neutral for grounding both of them.
Tertiary winding stabilizes the neutral conditions. In star connected transformers, load can be connected between line and neutral, only if
(a) the source side transformers is delta connected or
(b) the source side is star connected with neutral connected back to the source neutral.
In this transformers. Insulation cost is highly reduced. Neutral wire can permit mixed loading.
Triple harmonics are absent in the lines. These triple harmonic currents cannot flow, unless there is a neutral wire. This connection produces oscillating neutral.
Three phase shell type units have large triple harmonic phase voltage. However three phase core type transformers work satisfactorily.
A tertiary mesh connected winding may be required to stabilize the oscillating neutral due to third harmonics in three phase banks.

3.) Delta – Delta (Dd 0 or Dd 6)

This is an economical connection for large low voltage transformers.
Large unbalance of load can be met without difficulty.
Delta permits a circulating path for triple harmonics thus attenuates the same.
It is possible to operate with one transformer removed in open delta or” V” connection meeting 58 percent of the balanced load.
Three phase units cannot have this facility. Mixed single phase loading is not possible due to the absence of neutral.

4.) Star-Zig-zag or Delta-Zig-zag (Yz or Dz)

These connections are employed where delta connections are weak. Interconnection of phases in zigzag winding effects a reduction of third harmonic voltages and at the same time permits unbalanced loading.
This connection may be used with either delta connected or star connected winding either for step-up or step-down transformers. In either case, the zigzag winding produces the same angular displacement as a delta winding, and at the same time provides a neutral for earthing purposes.
The amount of copper required from a zigzag winding in 15% more than a corresponding star or delta winding. This is extensively used for earthing transformer.
Due to zig-zag connection (interconnection between phases), third harmonic voltages are reduced. It also allows unbalanced loading. The zigzag connection is employed for LV winding. For a given total voltage per phase, the zigzag side requires 15% more turns as compared to normal phase connection. In cases where delta connections are weak due to large number of turns and small cross sections, then zigzag star connection is preferred. It is also used in rectifiers.

5.) Zig-zag/ star (ZY1 or Zy11)

Zigzag connection is obtained by inter connection of phases.4-wire system is possible on both sides. Unbalanced loading is also possible. Oscillating neutral problem is absent in this connection.
This connection requires 15% more turns for the same voltage on the zigzag side and hence costs more. Hence a bank of three single phase transformers cost about 15% more than their 3-phase counterpart. Also, they occupy more space. But the spare capacity cost will be less and single phase units are easier to transport.
Unbalanced operation of the transformer with large zero sequence fundamental mmf content also does not affect its performance. Even with Yy type of poly phase connection without neutral connection the oscillating neutral does not occur with these cores. Finally, three phase cores themselves cost less than three single phase units due to compactness.

6.) Yd5

Mainly used for machine and main Transformer in large Power Station and Transmission Substation.
The Neutral point can be loaded with rated Current.

7.) Yz-5

For Distribution Transformer up to 250MVA for local distribution system.
The Neutral point can be loaded with rated Current.

Application of Transformer according according to Uses

Step up Transformer: – It should be Yd1 or Yd11.
Step down Transformer: – It should be Dy1 or Dy11.
Grounding purpose Transformer: – It should be Yz1 or Dz11.
Distribution Transformer: – We can consider vector group of Dzn0 which reduce the 75% of harmonics in secondary side.
Power Transformer: – Vector group is deepen on application for Example : Generating Transformer : Dyn1 , Furnace Transformer: Ynyn0.

Why 30°phase shift occur in star-delta transformer between primary and secondary?

30 deg phase shift between line voltage and phase voltage

The phase shift is a natural consequence of the delta connection. The currents entering or leaving the star winding of the transformer are in phase with the currents in the star windings. Therefore, the currents in the delta windings are also in phase with the currents in the star windings and obviously, the three currents are 120 electrical degrees apart.

But the currents entering or leaving the transformer on the delta side are formed at the point where two of the windings comprising the delta come together – each of those currents is the phasor sum of the currents in the adjacent windings.

When you add together two currents that are 120 electrical degrees apart, the sum is inevitably shifted by 30 degrees.

The Main reason for this phenomenon is that the phase voltage lags line current by 30degrees.consider a delta/star transformer. The phase voltages in three phases of both primary and secondary. you will find that in primary the phase voltage and line voltages are same, let it be VRY (take one phase). But, the corresponding secondary will have the phase voltage only in its phase winding as it is star connected. The line voltage of star connected secondary and delta connected primary won’t have any phase differences between them. So this can be summarized that “the phase shift is associated with the wave forms of the three phase windings.

This is the HV Side or the Switchyard side of the Generator Transformer is connected in Delta and the LV Side or the generator side of the GT is connected in Star, with the Star side neutral brought out.

The LV side voltage will “lag” the HV side voltage by 30 degrees. Thus, in a generating station we create a 30 degrees lagging voltage for transmission, with respect to the generator voltage.

As we have created a 30 degrees lagging connection in the generating station, it is advisable to create a 30 degrees leading connection in distribution so that the user voltage is “in phase” with the generated voltage. And, as the transmission side is Delta and the user might need three phase, four-wire in the LV side for his single phase loads, the distribution transformer is chosen as Dyn11.

There is magnetic coupling between HT and LT. When the load side (LT) suffers some dip the LT current try to go out of phase with HT current, so 30 degree phase shift in Dyn-11 keeps the two currents in phase when there is dip.

So the vector group at the generating station is important while selecting distribution Transformer.

Vector Group in Generating-Transmission-Distribution System

Generating TC is Yd1 transmitted power at 400KV, for 400KV to 220KV Yy is used and by using Yd between e.g. 220 and 66 kV, then Dy from 66 to 11 kV so that their phase shifts can be cancelled out. And for LV (400/230V) supplies at 50 Hz are usually 3 phase, earthed neutral, so a “Dyn” LV winding is needed. Here GT side -30lag (Yd1) can be nullify +30 by using distribution Transformer of Dy11.

A reason for using Yd between e.g. 220 and 66 kV, then Dy from 66 to 11 kV is that their phase shifts can cancel out and It is then also possible to parallel a 220/11 kV YY transformer, at 11 kV, with the 66/11 kV (a YY transformer often has a third, delta, winding to reduce harmonics).

If one went Dy11 – Dy11 from 220 to 11 kV, there would be a 60 degree shift, which is not possible in one transformer. The “standard” transformer groups in distribution avoid that kind of limitation, as a result of thought and experience leading to lowest cost over many years.

Generator TC is Yd1, can we use Distribution TC Dy5 instead of Dy11?

With regards to theory, there are no special advantages of Dyn11 over Dyn5.

In Isolation Application: -In isolated applications there is no advantage or disadvantage by using Dy5 or Dy11. If however we wish to interconnect the secondary sides of different Dny transformers, we must have compatible transformers, and that can be achieved if you have a Dyn11 among a group of Dyn5′s and vice versa.

In Parallel Connection: – Practically, the relative places of the phases remain same in Dyn11 compared to Dyn5.

If we use Yd1 Transformer on Generating Side and Distribution side Dy11 transformer than -30 lag of generating side (Yd1) is nullify by +30 Lead at Receiving side Dy11) so no phase difference respect to generating Side and if we are on the HV side of the Transformer, and if we denote the phases as R- Y-B from left to right, the same phases on the LV side will be R- Y -B, but from left to Right.

This will make the Transmission lines have same color (for identification) whether it is input to or output from the Transformer.

If we use Yd1 Transformer on Generating Side and Distribution side Dy5 transformer than -30 lag of generating side (Yd1) is more lag by -150 Lag at Receiving side (Dy5) so Total phase difference respect to generating Side is 180 deg (-30+-150=-180) and if we are on the HV side of the Transformer, and if we denote the phases as R- Y-B from left to right, the same phases on the LV side will be R- Y -B, but from Right to Left.

This will make the Transmission lines have No same color (for identification) whether it is input to or output from the Transformer. The difference in output between the Dyn11 and Dny5 and is therefore 180 degrees.

Transformer Protection–

The following protections are used to protect transformers up to 2000KVA, 11/0.415KV:
Over current & Short circuit (51): The relay operates when the primary current of the transformer exceeds the limit set in the relay due to overload with inverse time characteristics. Also there is provision for short circuit which trips the feeder instantly due to short circuit. The relay used is CDAG 51.

Earth Fault (51N): The relay operates instantly when the fault current exceeds the limit set in the relay. The relay used is CAG 14.
Transformer Differential (87T): The transformer feeder is protected from any faults that occur in the primary side of the transformer by using this relay. The relay used is DTH 31. This type of protection is used only in 11/6.6KV transformers.
WTI (49WTX/49WAX): The transformer is protected against increase in winding temperature. The relay is activated by WTI, which gives alarm and also trips the breaker, if it exceeds the limit. The relay used is VAA 34.
OTI (49TX/49AX): The transformer is protected against increase in oil temperature. The relay is activated by OTI, which gives alarm and also trips the breaker, if it exceeds the limit. The relay used is VAA 34.
Buchholz (63TX/63AX): The transformer is protected against internal faults by providing Buchholz inside. The relay used is VAA 34.
The transformer can be switched on if it trips on over current, WTI and OTI after some interval. If it trips on earth fault, the IR value of the transformer to be checked

Transformer Over Current protection

The over current protection required for transformers is consider for Protection of Transformer only. Such over current protection will not necessarily protect the primary or secondary conductors or equipment connected on the secondary side of the transformer.

When voltage is switched on to energize a transformer, the transformer core normally saturates.

This results in a large inrush current which is greatest during the first half cycle (approximately 0.01 second) and becomes progressively less severe over the next several cycles (approximately 1 second) until the transformer reaches its normal magnetizing current. To accommodate this inrush current, fuses are often selected which have time-current withstand values of at least 12 times transformer primary rated current for 0.1 second and 25 times for 0.01 second. Some small dry-type transformers may have substantially greater inrush currents.

To avoid using over sized conductors, over current devices should be selected at about 110 to 125 percent of the transformer full-load current rating. And when using such smaller over current protection, devices should be of the time-delay type (on the primary side) to compensate for inrush currents which reach 8 to 10 times the full-load primary current of the transformer for about 0.1 s when energized initially.
Protection of secondary conductors has to be provided completely separately from any primary-side protection.
A supervised location is a location where conditions of maintenance and supervision ensure that only qualified persons will monitor and service the transformer installation. Over current protection for a transformer on the primary side is typically a circuit breaker. In some instances where there is not a high voltage panel, there is a fused disconnect instead.
It is important to note that the over current device on the primary side must be sized based on the transformer KVA rating and not sized based on the secondary load to the transformer.

Over current Protection of Transformers >600V (NEC450.3A)

Transformer over current

1) Unsupervised Location of Transformer (Impedance <6%)

OverCurrent Protection at Primary Side (Primary Voltage >600V):

Rating of Pri. Fuse at Point A= 300% of Pri. Full Load Current or Next higher Standard size. or
Rating of Pri. Circuit Breaker at Point A= 600% of Pri. Full Load Current or Next higher Standard size.
OverCurrent Protection at Secondary Side (Secondary Voltage <=600V):
Rating of Sec. Fuse / Circuit Breaker at Point B= 125% of Sec. Full Load Current or Next higher Standard size.
OverCurrent Protection at Secondary Side (Secondary Voltage >600V):
Rating of Sec. Fuse at Point B= 250% of Sec. Full Load Current or Next higher Standard size. or
Rating of Sec. Circuit Breaker at Point B= 300% of Sec. Full Load Current.

Example: 500KVA, 11KV/415V 3Phase Transformer having Impedance of Transformer 5%

Full Load Current At Primary side = 750000/(1.732X11000) = 26.2A
Rating of Primary Fuse = 3X26.2A = 78.6A, So Standard Size of Fuse = 80A.
OR Rating of Primary Circuit Breaker = 6X26.2A = 157.2A, So standard size of CB = 200A.
Full Load Current at Secondary side = 500000/(1.732X415) = 695.6A.
Rating of Secondary of Fuse / Circuit Breaker = 1.25X695.6A = 869.5A, so standard size of Fuse = 1000A.