Supply authorities are under obligation to their customers to maintain the supply voltage between certain limits. Tap changers offer variable control to keep the supply voltage within these limits About 96% of all power transformers today above 10MVA incorporate on load tap changers
as a means of voltage regulation.
A tap changer is a device fitted to power transformers for regulating of the output voltage to required levels. This is normally achieved by changing the ratios of the transformers on the system by altering the number of turns in one winding of the appropriate transformer/s according to these two equationS ;
Turns ratio = secondary winding turns (Ns)/ primary winding turns (Np).
Secondary voltage = (supply voltage or primary voltage) / Turns ratio. The tap changer basic function is that it removes or connects some portion of the winding to the load side or source side. Reasons of using tap changer:- The main reason lays on voltage control and regulation of a transformer which can be explained in these states :-
1-To Supply a desired voltage to load.
2- To counter the voltage drops due to load variation.
3- To counter the input supply voltage changes on load.
4- To perform the mission of regulation of active and reactive power flows so that we can obtain a good profile system by tapping.
Location of taps:-
The tap changer is placed on high voltage side because of many factors :-
1) The HV winding generally wound over LV winding hence it
is easier to access the HV winding turns instead of LV winding.
2) Because of high voltage the current through the HV winding
is less compared to LV windings, hence there is less “wear” on the
tap changer contacts. Due this low current, in on load tap
changer the change over spark will be less. .3) the isolation & cooling methods is more accessable in this position.
Basic arrangements of tapped windings:-
Chosing the arrangement of taps depends on the system design& operating
Requirements & transformer system.
These arrangements are applicable to two winding transformes &autotransformers& phase-shifting transformers (PST). Where the tap winding are placed.
A)Linear arrangement is generally used on power transformers with moderate regulatig ranges up to a maximum of 20%. The tappd turns are added in series with the main winding and changes the transformer ratio. The rated position can be any one of the tap positions.
B) Reversing change over selector tap winding is added to or subtracted from the main winding so that the regulating range can be doubled or the number of taps reduced. During this operation, the tap winding is disconnected from the main winding but the main disadvantage of disconnecting main winding is the greatest copper losses occur.The rated position is normally the mid position or neutral position. C) The double reversing change-over selector avoids the disconnection of tap winding during the change-over operation. In phase-shifting transformers (PST) this apparatus is called the advance-retard switch (ARS).
D) Single a coarse change-over selector tap winding is connected either to the plus or minus tapping of the coarse winding. During coarse selctor operation, the tap winding is disconnected from the main winding.the copper losses in this method are lower compared to the two bove methods but it remains as aloss.The main disadvantage is the places higher demands on insulation materialand requires a larger number of windings. E) The multiple coarse change-over selector enables multiplication of the regulating range. It is mainly used for industrial process transformers (rectifier/furnace transformers).
Types of tap changers:-
There are two main types of tap changer
1)No Load Tap changer (NLTC) or De-energized Tap Changer (DETC)
No load tap changers are used in situations in which a transformer ratio does not require frequent changing, it is frequently employed in low power , low voltage transformers in which tap point is changed manually by hand and connected to anew tap terminal(tap point) to regulation requirements.In some systems , tap changing can be done using rotary or slider switch,it can mainly seen in solar power & wind projects, modulation of taps in this type costs less ,As it initially is separated the transformer from the system then regulating taps position in addition to cooling of NLTC is mainly done by natural air.
2)On Load TAP Changer(OLTC) or En-energized Tap Changer
Most of generating stations ,Substation system having Power Transformer with
On Load Tap Changer. Not only in Generating Station Transformer also in
Distribution system Transformer too.
This type is operated under load on conditions without moving transformer away of the system , which means they can change tapping position with transformer load current flowing. Some of the manufacturer’s requirements for this type of tap changer are: reliability, lowest cost, minimal maintenance, dielectric strength, electrical and mechanical life expectancy, overload and fault current capability.
This type may be generally classified into
a)mechanical tap changers
b)thyristor-assisted tap changers
c)solid state (thyristor) tap changer
these systems usually posses 33 taps ( one at centre which represents _rated tap_ and sixteen to increase and decrese the turn ratio) and allows for ±10% variation (each step providing 0.625% variation) ,this can be achieved by using automatic system, this operation costs more due to copper losses due to step urgent modifying of voltage ; current level while remaining loading. The common way for cooling this type is using oil…
On load tap changer structure:-
1. Selector Switches
These switches select the physical tap position on the transformer winding and
remain load current to not break.
are used in the circuit to increase the impedance of the selector circuit and limit
the amount of circulating current due to this voltage difference.
3. Vacuum Switch
This device performs the duty of a circuit breaker.
that makes and breaks current during the tap changing sequence.
4. Bypass Switch
This switch operates during the tap changing sequence but, at no time, does it make or. break load current, though it does make
before break each connection.
Operation of On load tap changer using reactor method :-
This method employs an auxiliary reactor to assist tap changing. The switches for the taps and that across the reactor(S) are connected as shown. The reactor has a center tapped winding on
a magnetic core. The two ends of the reactor are connected to the two bus bars to which tapping switches of odd/even numbered taps are connected. When only one tap is connected to the reactor the shorting switch S is closed minimizing the drop in the
reactor. The reactor can also be worked with both ends connected to two successive taps. In that case the switch ‘S’ must be kept open. The reactor limits the circulating current between the taps in such a situation. Thus a four step tapped winding can be
used for getting seven step voltage on the secondary according to the table of switching).
The advantage of this type of tap changer are:
1. Load need not be switched off.
2. More steps than taps are obtained.
3. Switches need not interrupt load current as a alternate path is
The major objection to this scheme seems to be that the reactor is in thecircuit always generating extra loss.
1-Tap-changer transformers are used in a power system to influence the voltage profile as well as the distribution of power flow
2-Tap changer is always placed in HV side.
3-basic arrangement of tapped winding for (2 -winding/Auto)transformer:-
-Reversing change over selector
-double reversing change-over selector
-single acoarse change over selector
-The multiple coarse change-over selector
4-The main types of tap changer is:
Off load tap changer/on load tap changer
5-OLTC is used for high voltage systems modification ; NLTC is used for Low voltage systems modification.
6-NLTC is mainly used in solar ; wind projects.
7-on load tap changing is controlled via automatic system at afew seconds.
8-off load tap changing is controlled manually by hands.