Transformer Types
1)Power Transformer
Power transformer used to refer to those transformers that used between generator &
distribution circuits.
Type 2 Winding transformer (HV/LV ratio is more than 2)
Autotransformer Winding ( 1 winding & HV/LV ratio is less than 2)
2)Distribution Transformer
Any transformer that takes voltage from a primary distribution circuit & step down it to a
secondary distribution circuit or consumer service circuit is called as distribution
transformer.
3) Instrumentation transformer.
1. Current Transformer
2. Voltage transformer
3. Capacitive voltage transformer.
What are the applications of Transformer in electrical power system.
A transformer is not only used to step & step down the voltage in the power system but
there are also other applications which are discussed below.
1.
2.
3.
4.
5.
6.
Step up & step down voltage without change in frequency.
Reducing the short circuit current.
convert 3ph 3 wire system to 3ph 4 wire system.
Voltage regulation – with OLTC & AVR
Reactive power control - with OLTC & AVR
Converting un grounded system in LV side in star delta transformer to grounded
system using earthing/ zig zag transformer.
7. isolation transformer.
Types of transformer based on application
There are 3 main applications of transformer-
1.
2.
3.
4.
As a generator transformer (Rating in MVA)
As a power transformer (Rating in MVA)
As a service or auxiliary transformer (Rating in KVA)
As a distribution transformer for power supply to household.
where the full load MVA rating of the generator transformer should be at least equal to
maximum MVA rating of the generator.
where the service or auxiliary transformer rating depends on the load list or total connected
load on its secondary side.
for the power transformer used in substation rating shall be as per the utility practices.
Transformer Equipment SLD (Example)
Transformer Standard ratings as per substation practices.
Transformer Vector Group
If we take Delta- Delta connect transformer then single line to ground fault can not be
detected because there is no any return path for ground fault due to this reason we do not
take delta connected transformer.
Why Delta -Star?
Reduce the unbalance on HV side.
SLG fault on LV side not reflected as SLG Fault at HV side & zero sequence current circulate
inside delta connected winding & third or multiple of third harmonics eliminated at HV side
due to delta connected.
Why vector group is important?
The vector group indicates the winding configurations & the phase angle displacement
between the primary & Secondary windings.
The three phase transformer consist of three nos. of primary winding, one for each phase &
three nos. of secondary windings wound on the same iron core.
The two most common configurations of winding connection are the delta & the star.
Winding connection designations:
Symbol for the high voltage winding comes first
First symbol: for High voltage always capital letters.
D= Delta, Y= Star, N= Neutral
First symbol: for High voltage always, capital letters.
d=delta, y=star, n=neutral
Third symbol :phase displacement expressed as the clock hour number (1,6, 11)
The vector for the high voltage winding is taken as the reference vector.
Displacement of the vectors of the other windings (LV ) From the reference ( HV) vector with
anticlockwise rotation is represented using clock hour figure .
Example: Dyn11: Transformer has a delta connected primary winding (D) a star connected
Secondary winding with Star neutral point is brought out & a phase shift of 30 deg leading
(11).
The phasor representing the HV winding is taken as reference & set at 12 o’clock.
Phase rotation is always anti-clockwise.
use the hour indicator as the indicating the phase displacement angle because there are 12
hours on a clock & a circle consists out of 360˚ each hour represents 30˚ thus 1=30˚,
2=60 ˚,3=90 ˚,6=180 ˚,9=270 ˚12=0 ˚ or 360 ˚.
Example: Note-Rotation will be anticlockwise.
•
•
•
•
•
Digit 12= 0 ˚ that the LV Phasor is in phase with the HV Phasor.
Digit 11= 30 ˚ that LV phasor is the HV phasor.
Digit 1= 30 ˚ that HV leads the LV winding.
Digit 5= 150 ˚ that HV leads the LV winding.
Digit 6= 180 ˚ that HV leads the LV winding.
Transformer vector group in generating power plant.
When we connect transformer with generator then we use the transformer with delta
winding on generator side & with star winding on transmission side.
As in a power plant, the most valuable equipment is the generator, so need maximum
number of protections.
Following is the reason we are providing delta connection on the generator side
1. Delta provide a circulating path for triple harmonics thus reduces the impact of
harmonics on generator.
Third harmonics voltage are eliminated since the third harmonics current is
circulated in the primary delta & blocks to flow in generator.
2. Delta connection traps the flow of zero sequence currents (commonly known as fault
current) so generator is protected from zero sequence fault currents, blocks zero
sequence between delta connected circuits.
Therefore, delta connection is required on generator side to prevent the zero-sequence
current entering into generator as rotating machines have very little withstand capacity for
zero sequence current enter in generator then generator then generator core may come
under saturation state, & we can not further use generator need heavy maintenance on
generator.
Why Neutral Grounding resistor?
The main goal of grounding through a resistance is to achieve the safety of equipment from
the line to ground fault current by limiting the amount of current flow by R.
NGR are used to protect Generators in special applications, in which generators are
connected to bus bars directly feeding single phase load. in generator the positive, negative
& zero sequence reactance are not equal. When the neutral is solidly grounded the line to
ground fault currents are usually higher than 3 phase short circuit current. This large fault
current will damage the laminated core which increases the cost of repair. Hence NGR need
to be introduced between the neutral of generator & the ground to limit this short circuit
current in thousands of Amps to thousands of Amps.
Transformer vector group in solar power plant.
In solar power plant, when we connect transformer with solar inverter, then we use the
transformer with delta winding on inverter side & with star winding on transmission side.
This is called as inverter duty transformer.
Now for power export import transformer i.e in power transmission purpose transformer
star-star connection preferred. in this transformer insulation cost is highly reduced & due to
cost economical purpose,& this is cheapest winding configuration transformer & provide
facility of graded insulation & so saving in transformer insulation cost & eventually saving in
overall cost of transformer.
What is Graded insulation?
In a transformer, one end is earthed, the insulation towards the earthed end could be
reduced or graded to reduce the cost.
This is worthwhile only for high voltage transformer standards specify graded insulation for
star/ interconnected for voltage above 66KV.
.In all others , uniform insulation is provided.
Large EHV/UHV Substation tie up transformer
In EHV Substation the YNa0d11 type transformer used widely, the meaning of YNa0d11 is
star connected HV winding with neutral brought out, auto transformer with 0 ˚
displacement .LV winding delta connected leading HV by 30 ˚.
with star connection configuration, the transformer insulation cost is highly reduced so we
are getting the advantage of transformer cost reduction with star winding & have delta
connected tertiary winding to get the advantage of delta connect winding.
Tertiary winding in a power transformer
Tertiary winding is normally provided in star connected auto transformer & star connected
transformer of rating 100MVA & above.
Purpose of additional inherent delta is as below
•
•
•
•
This winding is used as stabilizing winding.
The tertiary winding to take out by bushing from transformer to supply
tertiary auxiliary loads of the substations.
this tertiary winding is connected in delta & allows circulation of zero
sequence current in it. thus balancing zero sequence component of
unbalance load, this condition becomes very critical during short circuit fault
condition.
further, during impulse withstand voltage surges on HV winding transferred
surge voltages appear on tertiary winding terminals & are quite high & this
was made the tertiary winding as weak link in a power transformer & caused
very frequent failures.
to overcome this problem the rating of tertiary winding increased for high
current as well as voltage levels .say for 100 MVA 220/132 KV transformer
,voltage rating of Tertiary winding is now 33 KV ( Earlier it 11 KV ) & MVA
rating has been increased to 33 MVA (Earlier it was 16.66MVA ).
This improved rating of tertiary winding of a power transformer has
improved its performance & transformer failures have been reduced
considerably
Transformer vector group for LV distribution transformers
For LV distribution transformer –
Normally Dyn11 vector group using at distribution system.
Because star point facilitates mixed loading of three phase & single-phase consumer
connectors.
2nd reason is that in industries maximum loading is of non-linear type & they generates the
harmonics & these effect we do not want to reflect in primary grid side due to this reason
we use delta on primary side.
Type of non-linear load: for e.g. furnace heaters, VFDs & this non-linear load generates the
harmonics in system for that we use Dyn11.
so, what ever harmonics of LV voltage coming from industries, it will not be send in
upstream system due to delta winding in HV winding.
3rd reason is that whatever a single line to ground fault (SLG Fault) occurred at LV side of the
transformer, the reflected SLG fault current at HV side not having the zero sequence fault
contribution value.
In HV side its delta connection then zero sequence component will not be reflected at HV
side it only circulates within the winding & not coming outside of delta connected winding
thus HV side of transformer is unaffected by SLG fault ( only able to see positive & negative
sequence component are reflected when SLG fault occurred at LV ) & HV system protected
or unaffected by SLG fault.
Due to this above reason in the LV side star connected & HV side delta connected winding
configuration is preferred.