PROJECT SEMINOR

Problems encountered

in

NTPC

& proposed solutions.

PROBLEMS ENCOUNTERED IN SWITCHGEAR OF NTPC.

FAILURE OF THE SPRING MECHANISM IN THE CIRCUIT BREAKER.

FLASHOVER OCCURRENCE BETWEEN THE CONTACTS OF CIRCUIT BREAKER

THE PROJECT WILL COMPRISE WITH A STUDY DEALING THE CAUSES FOR THESE PROBLEMS & THE REMEDIES PROPOSED.

MAGNETIC ACTUATOR

CONCEPT

The basic concept of it is a simple-

PLUNGER, which is held magnetically

in both the open and close position by

permanent magnets.

WHY a Magnetic Actuator for VCB

Spring mechanism is not 100% suitable, this is clearly understood from its force travel characteristics.

Comparing the force travel characteristics of solenoid & spring mechanism, we will find that solenoid is suitable for closing operation spring mechanism for tripping operation

The plunger of magnetic actuator is basically driven by a solenoid.

The idea was 1st concieved in the year 2001.

Force Travel Diagram of a C.B.

DESIGN EVALUATION

1ST GENERATION ACTUATORS

Made from black round magnets.

shape was tubular DISADVANTAGES

1. Shape

2. Lesser Load / Kg value

2nd GENERATION ACTUATORS

Made of solid rectangular core with

rectangular magnets

More load / Kg value

Got two coils (closing & tripping)

Concept of split plunger is employed

DISADVANTAGES

1. Two coils

2. split plunger

3rd GENERATION ACTUATORS

Two coils in a single bobbin

solid plunger instead of split plunger

ADVANTAGES

1. More efficient

2. Maximum Load / Kg value

CONSTRUCTION

Moving part which facilitate the open / close movement is steel plunger. This plunger has a shaft which is connected to the breaker main shaft.

Movement of the plunger depends upon the coil energization polarities. Coil energization is done by discharging of ELECTROLYTIC CAPACITORS.

OPERATION OF ACTUATOR IS GOVERNED BY A MICRO CONTROLLER BASED ELECTRONIC RELAY

This relay takes the basic input from two proximity sensors each for open and close positions.

Operation of coil and magnets and the path the magnetic lines transverse, can be studied with the help of superposition theorem which is used in electrical circuit.

There are two sources in the model which contribute magnetic fields i,e coils & magnets.

To understand their effect on the plunger each of them is switched on independently, and then verify the cumulative effect of the sources to analyze the model with both sources contributing simultaneously.

FUNCTIONING

The functioning of the actuator i,e the closing & opening operations is done through an electrical control unit.

As the coils are to be fed with a constant voltage D.C pulse, it essentially consists capacitor & rectifier.

The controller requires auxiliary power supply for charging the capacitors.

The close / open command can be given by actuating contactors through push buttons.

Charging time for capacitors

2 to 3 sec’s for closing operation

0.3 sec’s for tripping operation

OPERATING MODES

MODEL1 : Magnets with coil excitation zero.

MODEL2 : Effect of coil excitation without magnets.

MODEL3 : Operation with coil & magnets simultaneously.

MODEL1: MAGNETS WITH COIL EXCITATION ZERO.

Plunger is in the close mode.

In this case excitation of trip coil is zero.

in this position the plunger is at the top i,e

(open position) together with the iron core

forms a path of low magnetic resistance

for the field of permanent magnets. In contrast the large gap at the bottom of the plunger

represents a high magnetic resistance.

EFFECT OF MAGNETS WITH COIL EXCITATION ZERO.

EFFECT OF COIL EXCITATION WITHOUT MAGNETS

OPERATION WITH COIL & MAGNETS SIMULTANIOUSLY

It can be seen that as the shaft permeability is high. Flux lines concentrate on the top of the plunger.

So if the plunger is to be pulled downwards MMF required will be more than which required to hold the plunger in close position.

MODEL 2EFFECT OF COIL EXCITATION WITHOUT MAGNETS

It can be observed that as the lower coil (trip coil) is energized it will produce flux proportional to H & force it through the magnetic path having higher permeability.

Flux lines directions are exactly apposite to that of magnets. So coil should produce sufficient field strength to pull the plunger against permanent magnetic force.

It can also be seen from the fig. that flux lines are also traversing through

the air path at the bottom.

As the reluctance of this path is more, major part of the MMF is lost to force the flux lines through air path.

MODEL 3 operation with coil & magnets simultaneously

During any operation coil has to overcome three basic components. latching force of the permanent magnets Air gap reluctance Residual flux in the core which align itself

along the last operational direction.To operate the actuator, coil is energized with

proper polarity to overcome these three.

The flux concentration in the lower part of the plunger has increased. Direction of flux lines are in downward direction it can be seen that the plunger

going to be pulled down.

This is

Tripping operation

Also the flux lines are concentrated in the air gap at the bottom of the plunger

Max losses because of air gap reluctance is in this portion. The

coil excitation should be enough to overcome this leakage flux.

Relation of MMF & Reluctance

flux linkage of the winding Inductance = -----------------------------------------------------

current

= Nø / I. Also L = N2.Pm

Ø = mmf x permeance = NI x Pm.

= NI / S since Pm = 1 / s.

S reluctanceThus ø directly controlled with ampere turns & reluctance of magnetic path.

Controlling of magnetic actuator is of two types.

1.Electronic controlling

2.Non Electronic control

Control Electronic Architecture.

Block diagram of Electronic Controller

Electronic Control Unit

The energizing of the coils of the actuator for closing & opening operations requires a continuous connection to some auxiliary voltage supply.

Under voltage and over voltage have no effect on switching times.

An electrolytic capacitor provides the surge power of upto 2600w required for energizing the opening &

closing coils in the actuator.

stores the electrical energy of less than 200J for a complete o-c-o operating cycle. After such an operating cycle, the capacitor recharges with in < 10 sec’s with a peak current of max 2A.

Power semiconductors (A combination of Mosfet Transistors and Thyristors)

Controls the current for switching the actuator coils.

The switching voltage induced by the inductivity of the coils on interruption of the current are reduced by parallel free wheeling diodes.

FPGA ( Field Programmable Gate Array ) controls the circuit breaker

Switching commands are only extended taking account of the switch position.

switch position is detected by two 1. inductive proximity sensors

2. the charging condition of the storage capacitor

inductive proximity sensors detect impermissible intermediate positions.

Eg: failure to reach a limit position & signals.

Applications of ECU

Energy consumption – 1 / 10th Unit consumes a power of – 2w On failure of aux supply voltage, storage capacitor

ensures that a breaking operation is possible for further 2 min’s. thus short voltage breakdowns are bridged without problems.

Under voltage release ( when the CCS tripping input 3 is connected to a commercial under voltage relay )

Over current release.

NON ELECTRONIC CONTROLLERIS SPECIALLY DESIGNED TO ACHIVE

Separate rectifier, capacitor & contactor units

For closing & tripping circuits

Use of ZENER

to ensure closing only when the closing capacitor is charged to 85% of its full voltage. Ready lamp will glow when charging is over.

Closing contactor supply taken from tripping condenser output & through trip coil. to ensure “trip circuit healthy” before closing.

Voltage doubling circuit can be made to cater variable input supply to controller.

Series parallel type coil can be used to cater variable input supply in case of direct battery operation.

Advantages Of Magnetic Actuator

Replaces the traditional spring wound mech Better reliability Simplicity in design Lengthening the life of circuit breaker Provision of manual trip facility Due to simplicity as well as the low number

of moving parts maintenance other than inspection is no longer required