SF6 properties

Ever Wondered Whats so great about SF 6 ? 

More on Circuit Breakers

Let us learn about Operating Duty of a CB

Various components of circuit breaker take different operating time 

More on Circuit Breakers

Some common problems encountered in CBs:

Make                                     Problem

ALIND................................. Chain

CGL interruptor.....................Tripping latch lever

Energoinvest  int....................Blocking lever and Coupling

Merlin German Inturrptor......C & D Rollers.


Now lets see some test reports of 25 KV single pole VCB:

Circuit Breaker Maintenance

Some ratings for knowledge:

Spring charge motor   DC 220 V

Coils  : Shunt Trip  110 V Dc

                Closing Coil   110 V Dc

There are three types of maintenance:
Corrective, Preventive and Predictive.

The governing IS for CBs:

• Routine test as per IEC 60065/IS 13118
• Testing alongwith panels to be tested as per IEC 60298/ IS 3427

Shown above is a 12 KV VCB, 1600 A, 25 kA

Interrupter Assembly:

• contacts
• arc interrupters
• arc chutes

Operating assembly

Operating rod

control panel

sealing system

Breaking Medium

• SF6
• Oil
• Vacuum and Air 

Auxiliary Equipment  :   Alarm Scheme, Annunciator etc\

What checks we should carry out in a traction substation regarding CB and interruptor?

Lets learn about Batteries

Batteries have evolved over recent past, some types:

 

Other types as follows:

 Shown above is 650 Ah VRLA battery.

Earthing

I.E. Rules 61  :   substation and equipments

I.E. Rules 67 : HV and EHV

Maxm Res. :
Large power stations   0.5 ohms
Major Substations      1.0 ohms

Small Substations    2.0 ohms
Others                       8 ohms


I.E. Rules  1956 Chapter I 

Low :          250 V
Medium:    650 V
HV:           11 KV
EHV:         33 KV

A note about slip test

It is conducted to calculate unsaturated Xd and Xq.
We feed subnormal voltage to armature, mechanically rotate  it, keep field open ( Vf) 
Here Nr is not equal to Ns.
but is almost equal to Ns.
Since mmf due to armature reaction is rotating at Ns, field poles gradually slip w.r.t. rotating armature mmf.
Thus R = 1/ u of path of mmf changes at twice slip freq. from maxm when axis of poles and axis of armature reaction  wave coincide and minm. when they are at right angles.

MCQs on Electrical Engineering

Q. The phase sequence of alternator wil reverse when 

• I(f) is reversed, keeping direction of rotation as same.
• I(f) remains same, direction of rotation is reversed.
• I(f) reversed, P x2 
• P x 2 ,  I(f)  same 

More about Transformers

The weakest link in the transformer is  Cellulose !

Condition of Cellulose depends upon Moisture, Temp and Oxygen.

Due top weak Cellulose, Partial Discharge  and creepage failures occur.

There is something called piper chart (H2O in paper Vs H2O in oil)

water in paper increases parabolically with water increase in oil.

                              Oil                            Paper
New T/F                  1                               1000
Old T/F                   1                                  500


 A typical 220 KVA T/F will have 56 T oil and 6 T paper.
 

Lets Learn about Transformers

 

Distance Relays

Reactance relays cant be made directional. A directional element cant be added to them since these relays operate under normal load condition and U.PF.

Reactance relays are used for ground faults because a ground fault relay should be independent of ground fault resisntance. ( which is variable).

Used for < 10 miles ( Short T.L.).

Mho relay or admittance relays ( for 50 miles +) is used for phase fault condition.

Arc resistance affects an impedance relay more than a reactance relay but less than Mho relay.

Bundled conductors

Advantages:-

• Reduced reactance.( L is reduced)
• Reduced voltage gradient. 
• Reduced corona losses.
• Reduced Radio intrerference.
• Reduced surge impedance.
• Oncreased capacity of transission.

Buildings Fire safety

IS 1646- 1982:

This IS covers following points: 

General Requirements

Power Equipment

Switchboards

Power Distribution and Motor Control Gear

Transformer Substation, Excluding Bell and Drawing Frame, Transformer and Transformers of Small Capacity

Earth Conections

Wiring Methods

Permitted Types of Wiring Methods

Lighting, Fittings and Accesories

Special Electrical Equipment and Appliances

Requirements For special Occupancies

General Godowns

Testing

Inspection

Symbols and connection diagrams of circuits

Insulation coordination

 

  

 

Why we use output choke in bridge rectifier?

Why we use capacitor bank in VFDs

Types of protection devices

Some notes on HVDC

Advantage of ground retuyrn:

• Earth resistance in case of dc for long lines is interdependent of length and this resistance is < that of metallic conductor if used.
• d.c line can be built in two stages.
• reliability increases in case of outage one monopolar out of the bipolar.

Disadvantages of  ground return:

• Electrolytic erosion of buried / immersed metallic structure.
• difficult design
• Ground currents can cause dangerous step and touch voltages.
• Ground current interferes with other services.

Z Bus Calculation