Current Transformer Applications and their connections

Current transformers are used for measurement of current in any circuit. It has same working principle of that transformer but only used for step down of current. 

Current Transformer are used in measurement circuits , it is advisable to use CT instead of connection ammeter in the circuit as currents above 20A ammeter size becomes so large which is not feasible that is why current transformers are used. 

Current transformer primary has only one turn. Current transformer primary is not really a turn but just a conductor. The primary winding of Current Transformer has very few turns, while the secondary winding have a many turns depends upon how much the current must be stepped down.

There is extra precaution required during connections of CT are that secondary of CT should never be kept open which otherwise leads to very high secondary voltages which ultimately leads to burning of CT or even explosion of CT. So such conditions should be avoided.

Now let’s discuss about the same by considering a Transformer, Let’s take a Transformer with one primary turn and many secondary turns. Now when current flows through primary this will leads to very voltage induced in secondary of the order of Kilovolts. This is leads to breakdown of insulation of CT.

Usually Secondary current of CT are available with 5A rating and Input current may be of any ampere value.

CT Connection for Transformers:-

It has been found that usually in Star/ Delta connected transformer at Star side CT connections are in Delta and at Delta Side of Transformer CT connections are done in Star. 

This is usually done for the following reason:-

The delta CT connection circulates the zero-phase-sequence components of the currents inside the delta and thereby keeps them out of the external connections to the relay. This is necessary because there are no zero-phase-sequence components of current on the delta side of the power transformer for a ground fault on the wye side; therefore, there is no possibility of the zero-phase-sequence currents simply circulating between the sets of CTs and, if the CTs on the wye side were not delta connected, the zero-phase-sequence components would flow in the operating coils and cause the relay to operate undesirably for external ground faults.

3-Phase 4 Wire connections in Energy Meter

You will see the 3- Phase 4-wire connections on an Energy meter in which neutrals of every CT secondary are get grounded.

These connections are most popular in electrical system for connections of energy meters where there are three phase energy meters needed to be installed.

Air conditioner Power Consumption

Whenever anyone going to Install an AC then first everyone planned power charges that they will have to pay after using AC.

Actually it is correct that one ton of air-conditioning is equal to 12,000 btu/hr. However the other question trying to convert a one ton A/C unit into kw/hr consumed is very dependant on the make and model of the air-conditioner, as well as its EER (energy Efficiency Rate). These range from 6-14 EER. Standard average one ton A/C unit consumes 1.335 KW/hr. 
#13650 btu= 4.0004205 kwh 
According to that for 1 Ton Ac 12000=3.51685 kwh 

And , for 2 Ton AC 24000 btu required , for 2 ton Ac , 
as per calculation 7.33706 kwh power required. 
But in practical is not there. 
As 2 Ton AC is taking max 14 to 15 Amp.Load. 
KW= VI 
= 15*220 
=3.3 kwh

So for 1 Ton AC Power Consumption stands at 1.7 KWH
For 1.5 Ton AC Power consumption stands at 2.5 KWH
Similarly for 2 Ton AC Power consumption will be 3.3 KWH.

66KV to 132KV Cables Tests requirement

Following tests as Shown in figure must be done for 66KV & 132KV Cables as per IS: 60840 and IS:7098 part 3.

You May visit at link for difference between XPLE and PVC Cables:-

http://electrialstandards.blogspot.in/2014/03/difference-between-xlpe-and-pvc-cables.html

Also find out advantages of XLPE cables over Oil filled Paper Insulated cables:-
http://electricalsystembasics.com/2014/04/xlpe-cables-advantages-oil-filled-paper-insulated-cables.html

As per IS Standards 7098 Part 3 :-
Cross-Linked Polyethylene Insulated Thermoplastic Sheathed Cables - Specification. Part -3 for working voltages from 66 Kv up to and including 220 kV.

is Copper as earth pit is better then GI earth pit?

It has been often found that most of think that material used for earthing will reduce earthing resistance.

It has been often misconception that copper earthing plate or rod will have lower earth pit resistance then GI earthing plate or rod.

Lets clear this fundamental regarding as often falling in this trap will lead to higher investment on earthing by using copper as earthing material.

If one considers a plate electrode, the approximate resistance to earth is:

Resistance formula for Pipe earthing

(R) = (100r/2πL) X loge (4L/d)

·          L= Length of Pipe/Rod in cm

• d=Diameter of Pipe/Rod in cm
• r = Resistivity of Soil Ohm-meter.

Resistance formula for Plate earthing

(R) = (r/A) X under root(π/A).

·         r = Resistivity of Soil Ohm-meter.

·         A=Area of Earthing Plate m3.

As can be seen from the above formulas, only the resistivity of the soil and the physical dimensions of the electrode play a major role in determining the electrode resistance to earth.

The material resistivity is not considered anywhere in the above formulas.

Hence, irrespective of the material of construction of the earth electrode, any material of given dimensions would offer the same resistance to earth.

Material will help in the sizing and number of the earthing conductor or the protective conductor. Because, the short time withstand current ratings do vary according to the material of construction of the conductor.

So will never be losing end even when you are spending more money on Copper earthing instead of GI earthing.

Difference between grounding and earthing

There is always misconception related to difference between grounding and Earthing.

Lets discuss the same:-

It’s generally Myth about ‘Grounding’ is that grounding done for current carrying parts to ground like transformer or generator neutral and ‘Earthing’ means connection of non-current carrying parts to ground, like metallic enclosures.

But as per IS 3043-1987  the terms ‘earthing’ and ‘grounding’ are synonymous. 

Possibly the various nomenclature is because of the usual conflicting using British language between your People in America & the British. 

As the British called it as being ‘earthing’, the People in America called it as being ‘grounding’. 

IEC & IS Standards refer as ‘earthing’, while IEEE & ANSI Standards refer as ‘grounding’.

To quote from IEC 60050:

“The terms ‘earth’ in addition to ‘ground’ have both been generally use to explain the most popular energy/signal reference interchangeably all over the world in the Electro-technical terminology. As the USA and other United States nations favor using the term ‘ground’, European nations such as the United kingdom and lots of other Eastern nations like the term‘earth’”.

The IEEE meaning of grounding is: “Ground (ground system) -

A performing connection, whether intentional or accidental, through which an electrical circuit or equipment is attached to the earth or some performing body of relatively large extent that serves  instead of the earth.”

To be used within Europe, when the generally recognized terms were changed as below, then your meaning continues to be same.

“Earth (earth system) - A performing connection, whether intentional or accidental, through which an electric circuit or devices are attached to the mass of earth or some performing body of relatively large extent that serves instead of the mass of Earth.”

Fault current direction

There are very misconceptions about the direction of fault current.

Usually it has been misinterpreted that as from source to load as everyone thinks that’s source will supply load current and voltage.

Fault current always flows backwards i.e. if fault occurred at Motor then first it's O/L get tripped not directly causing Transformer tripping. Although current is drawn from Source but fault current always from Fault end to backward, as during fault at load heavy current will flows at load end as during fault load resistance reduced to very low value . That is how Electrical system is designed for load fault current.

Electrical system should be designed in Such a way that protection equipment connected for protection of load much immediately isolate load from source.

e.g. if load rated current is 300A then load end MCCB/SFU must be of 400A with fault current rating depending upon Transformer rating and impedance. Now Load end MCCB is of 400A and protection equipment such be set at 300A or less then 300A so that if any fault occurred load must get disconnected as soon as current reached 300A level and Short circuit , Earth fault settings done accordingly.

If we are designing system for 160KW motor having rated current of 297A then we must select 315A as its main MCCB/SFU and O/L Relay setting done at 297A or less then 297A , Earth fault relay setting should be done at 10% and 400A MCCB/SFU will be used as main Source for whole electrical system of 160W motor.

Star Delta Starter Changing Direction of motor

Star Delta Starter connected motor direction can be changed by changing either connections at main input line connections and O/P connections at Main and Delta contactors.

Star-Delta Starter Connection changing circuit is shown above.