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EHV Power Transformers

J S Sastry GM (EHV Transformers) Vijai Electricals

Topics to be covered
• • • • • • • • What is a Transformer Transformer principle Basic design concepts Construction features Materials used and improvements Types of failures and diagnostics Challenges to Transformer Designers Reactors

IF YOU HAVE NO DOUBTS YOU HAVE NOT UNDERSTOOD ANYTHING.
- a Spanish proverb.

What is a Transformer
• A transformer is a static equipment which transformers power from one circuit to another by stepping up or down the primary voltage without any change in the frequency. • The two circuits may or may not be connected but are magnetically coupled.

Features of Power Transformers
• Single Phase • Three phase
– Star or Delta connected Primary – Star or Delta connected Secondary – With or without Tertiary winding

• Provided with Off-circuit tap switch or On-load Tap Changer for voltage regulation

Transformers in Network

Transformer Operating Principle

Operating Principles Equivalent Circuit

Specification Parameters for Generator Transformer

Specification Parameters for Network Transformer

Design Principles - Core

Design Principles - Core
 Higher the number of steps in cross section, better is space utilization and smaller is the core diameter. 90 to 95 % utilization factor is optimal.

 Core area (A) is determined by the Flux Density (B) which in  turn depends on many factors - like loss capitalization and overall design economics. As the no load losses attract very high capitalization, attempts are continuously made to reduce them.

Design Principles - Core
• Improved manufacturing techniques like core building with 2-lamination packets, steplap joints, v-notched laminations, bolt-less cores are used. • Hi-β core steels like M0H, ZDKH, etc are available in which the specific core losses are lower than normal grades. • Amorphous steels drastically reduce no-load losses, but are now being used only in distribution transformers.

Design Principles Windings- L.V winding
 L.V Windings in Transformers are either
 Spiral wound for for low current ratings  Helical Wound with radial cooling ducts for higher ratings.

 The conductor used is paper insulated rectangular copper (PICC)  For higher currents, transposed conductors are used, to uniformly distribute the current across the cross section of the wire of coil.

Design Principles- L.V winding

Design Principles H.V Winding/1
HV winding is invariably uses PICC or CTC. Type of winding used is - Disc winding up to 132 kV - Layer winding or - Interleaved winding or - Rib shielded winding

Design Principles
Impulse Voltage Distribution
Cg Cs

α = K √ Cg/Cs

Design Principles
Impulse Voltage Distribution
V α= 0 α =5 α= 10 X

Windings - Interleaved winding

Windings - Rib Shield Winding

Design Principles Tertiary Winding/1
In Star-Star Connected Transformers and Autotransformers, Tertiary Winding is used- to stabilize phase to phase voltages in case of unbalanced load - Suppressing third harmonic currents in earthed neutral - reducing zero sequence reactance - for supplying auxiliary load or for connecting capacitors.

Design Principles Tertiary Winding/2
Tertiary is required to be designed for a power rating equal to one-third the rated power, it increases the cost of the transformer by 10- 12 percent. Tertiary winding is known to fail due to transferred surges and Short circuits Present practice is to do away with tertiary up to 100 MVA for 3 phase 3 limbed core transformers.

Design Principles Cooling
Most Common Cooling Methods Natural Circulation of Air and Oil Natural Oil Circulation and forced Air Circulation Forced Oil and Forced Air Circulation Forced Air Circulation and Directed Oil Flow through Windings Symbol ONAN

ONAF OFAF

ODAF

CORE

Level

16C

Ambient Temperature

Temp Rise

ON AN Temperature Diagram
AD- Oil in cooler AB- Oil in...
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