OLTC / DETC
for Power Transformers

OLTC / DETC
for Power Transformers

On-load and De-Energized Tap-Changers for Power Transformers

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On-load Tap-Changers (OLTC) and De-Energized Tap-Changers (DETC) are indispensable in regulating power transformers used in electrical energy networks and industrial applications. With over 50 years of experience, EMR is a leading player today in the tap changer market. With a comprehensive range of on-load tap changers for nearly all network applications, EMR is the trusted partner to Utilities, Consultants and Transformer Industry in and outside of India.

TYPE V

COMPACT & ECONOMICAL

Current rating up to 500 A & Voltage up to 132 kV (Y/D).

Special designs are avilable for 250 and 400 A on request.

Both star and delta applications are available.

Step voltage is 1500 Volts.

In-built selector switch model.

Bi-directional power flow.

Very compact dimensional arrangement.

Easy removal of diverter switch inserts, making it maintenance friendly.

Linear version is available up to 14 positions.

Up to 27 positions are available with the pre-selector arrangement.

No barrier board problems.

Electrical parameters

TYPE

Current Rating (A)

Voltage Variants

BIL kV/kVp

Max. Rated Step Voltage (Volts)

Max. Operating Position

Three Pole

Single Pole

Voltage Level (kV)

Vector

Linear

Pre-Selector

TYPE V

200, 350 & 500

350 & 700A*

40

Y, D

70/200

1500

14 & 18

27

76

140/350

123/76

230/550

Matching Products

Motor Drive Unit

TYPE M

MODERN, SUPERIOR & VERSATILE

Used in up to 765 kV Transformers

Three-pole design for neutral application at 350A, 500A & 600A rating for three-phase Y-connected windings.

Single-pole design at 350A, 500A, 600A, 800A, 1200A, 1500A and 1800A ratings for auto connected windings or single-phase transformers.

Available with ±9, ±11, ±13, ±15, ±17 steps.

Special design available with 700 A, 3 Phase & 2100 & 2400 A, Single Phase*

Available on both star and delta-insulation to ground, and tap selector size can be selected independently of one another.

Convenient for bell-type tank installations.

Additional device for potential tie-in of tap winding, during change-over operation of the change-over selector.

High-speed transition resistor-type diverter switch with arc extinction at first current zero.

Diverter switch uses sharp-action mechanics by energy accumulator mounted directly on diverter switch.

Minimum possible tap selector dimension because four avilable sizes ensure matched inpulse voltage withstandability.

Radial dimensions of the tap selector are reduced by special shaping of all parts on high potential. Distances between tap selector bars determined by actual voltage stress.

Optimised integration of the change-over selector into the fine selector contact circle.

Long life and reduced maintenance - rapid tap change operation. Low thermal stress on the transition resistors.

Special contacts for alternative oil application.

Simple tap selector design, effective contact cooling, high short-circuit withstandability.

Optimised integration of the change-over selector into the fine selector contact circle.

Long life and reduced maintenance - rapid tap change operation. Low thermal stress on the transition resistors.

Special contacts for alternative insulating liquids on request

Simple tap selector design, effective contact cooling, high short-circuit withstandability.

Electrical parameters

TYPE

Current Rating (A)

Voltage Variants

BIL kV/kVp

Max. Rated Step Voltage (Volts)

Max. Operating Position

Three Pole

Single Pole

Voltage Level (kV)

Vector

Linear

Pre-Selector

TYPE M

350, 500, 600 & 700*

350, 500, 600, 800, 1200, 1500, 1800, 2100* & 2400*

72.5

Y, D

140/350

3300

22

35

123

Y, D

230/550

170

Y

325/750

245

Y

395/950

300

Y

460/1050

Matching Products

Motor Drive Unit

TYPE D

HEAVY DUTY & TIME TESTED

Type ‘D’ Model is a true testimony of EMR’s time tested credentials for superior tap changer technology. This model is in successful service since 1975 catering to normal network applications and to demanding heavy duty furnace applications.

Current rating up to 500 A with 3-pole & up to 1500 A* with 1 pole.

Linear version applicable up to 18 position.

Special design is available for 300 A.

Voltage class up to 220 kV Class.

Both star and delta applications are available.

Maximum step voltage up to 2500 Volts.

Separate diverter and selector model.

Bi-directional power flow.

Up to 35 positions available for pre-selector arrangements.

Over 1500 in-tank installation in ultra heavy duty applications which involve 500 operations per day.

Electrical parameters

TYPE

Current Rating (A)

Voltage Variants

BIL kV/kVp

Max. Rated Step Voltage (Volts)

Max. Operating Position

Three Pole

Single Pole

Voltage Level (kV)

Vector

Linear

Pre-Selector

TYPE D

200, 400 & 500

200, 400, 600*, 800*, 1200* & 1500*

36

Y, D

75/200

2500

18

35

60

140/350

110

230/550

150

325/750

Matching Products

Motor Drive Unit

DETC

OC-R

R=Rotary

Up to 145kV

170 / 300kV

300kV

Electrical parameters

Current Rating (A)

Voltage Variants

BIL kV/kVp

Max. Rated Step Voltage (Volts)

Max. Operating Position

Single Pole

Voltage Level (kV)

Vector

Linear

Pre-Selector

200

11, 22 & 33

Y, D

80/250

825

9

17

OC-P

P = Pad mounted

300kV

Electrical parameters

Current Rating (A)

Voltage Variants

BIL kV/kVp

Max. Rated Step Voltage (Volts)

Max. Operating Position

Single Pole

Voltage Level (kV)

Vector

Linear

Pre-Selector

200

11, 22 & 33

Y, D

80/250

825

9

17

Power Transformers - frequently asked questions

What does an OLTC do in transmission-class power transformers?
An on-load tap changer in transmission power transformers enables continuous voltage correction while the transformer remains energized. This prevents grid voltage drift and ensures stable power flow across long-distance transmission networks.
Utilities specify OLTC for power transformers to manage fluctuating demand, renewable energy injection, and line drop compensation. This ensures voltage compliance with grid codes and reduces the risk of system-wide instability.
Vacuum interruption technology eliminates arcing in oil during tap transitions, reducing insulation degradation and contact erosion. This extends OLTC operational life and lowers outage frequency for transmission assets.
V-Type, M-Type, and D-Type OLTC designs are selected based on voltage class, load current, regulation range, step voltage, tap winding configuration, and short-circuit duty. OCTC configurations are chosen where voltage adjustment is required only during scheduled transformer shutdowns.
DETC is preferred when voltage tapping is infrequent and can be performed during planned outages. This reduces capital expenditure while still allowing seasonal or network-level voltage optimization.
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