R.M.S Value and Average Value

R.M.S Value Or Effective Value

Definition:

The steady current which when flowing through a given resistor fora given time produces the same amount of heat as is produced by the alternating current when flowing through the same resistor for the same time is called R.M.S or effective value of the AC .

In electrical mainly for AC we are always take R.M.S value or Effective values for calculation.

Suppose we say the supply voltage in house 230volts, it means that the R.M.S value of the supply voltage is 230volts. Sometimes we can denote RMS value of voltage by E. As a whole ,we use V (or E) and I for RMS values of A.C voltages and currents respectively.

General formula is

                        R.M.S value=√ Area of squared wave/base

To find RMS value for AC is   =  I_m/√2           (it is only for symmetrical sine waves)

Average Value:

            The average value of  AC is defined as the D.C current which transfers across any circuit the same charge as is transferred by that AC during the same time..

Average value of AC=2/π*I_m

For rectangular wave form RMS value = average value

                        Average value= Area under the curve/base

Faults and Relays.

TYPES OF FAULTS AND RELAYS

In power system only two types of faults are present

1.     Over-current Fault

2.     Earth Fault

In these two main categories  there are many types of faults present

Example:

1.     Over-current Fault

·        Line to Line Fault

·        3 phase fault  etc.

2.     Earth Fault

·           Line to ground Fault

·           Double Line To Ground Fault etc.

So relate with equipment and environment, we use so many types of relays used to protect the power system elements.

TYPES OF RELAYS:

1.O/C & E/F RELAYS

These relays are used for general protection.It give protection over earth fault and over current fault.The main thing we have keep in mind that it is used in short transmission lines up to 33KV limit(may be used in 66KV).

2.Distance Relays

This Relay is used as over current relay in long transmission high voltage lines. It is accurate than ‘O/C & E/F RELAYS’ and we can figure out the exact position of fault.So it is much helpful to locate the fault in long tx.line.

3.Restricted earth fault relay.

This relays are used in equipment protection mainly for transformer.It is connected to the C.T in the earthed neutral link,we called a Neutral C.T.It is actually a earth fault relay that find any internal insulation failure or heavy earth fault related with equipment surrounding.

4. Differential relays

The definition of such a relay is “one that operates when the vector difference of two or more similar electrical quantities exceeds a predetermined amount.That means in a trans former we know it primary and secondary voltages so the ratio of Pr. And Sec. Always constant so if any abnormality occur the ratio will change and relay will operate.This is also for equipment protection.

5.Master Relay  

All the above relays are connected to master relay in a panel board and it will operate the breaker.

Protective scheme

Protective Scheme For Power System

          Hi friends  now we can see the protective action in a power system.Here i am going to explain you the very basic protective scheme.Don't think this is all,the power system contain very complex protective schemes because each and every devices are very costly and dangerous. Here i just illustrate only one P.T and C.T actually each phase has its own C.T and P.T . 

            The C.T is always series to the mains and its secondary should be short circuited(ie.always in a closed circuit).The P.T is always parallel with mains open circuited secondary.In most of the area of power system the C.T is the protecting device, ie we take the current reference . P.T is mainly for the metering of voltage.I think at the generating side only the frequency and voltage terms come into protective action.

            Both the C.T and P.T has many number of secondary  cores for various purposes. Mainly the C.T has dedicated cores for Metering,Protection,Backup Etc..The cores has some standards also like 5P10,.5s etc..according to the accuracy and other parameters.The C.T secondary measures in Ampere. it may be 1A to 5A normally. The P.T secondary measures in voltage, and it is normally 110v AC. we can discuss about C.T and P.T in the next post.........................THANK YOU

 

Name Plate Examples

SAMPLE NAME PLATE DETAILS 

33 KV Vacuum Circuit Breaker.

 33KV current Transformer

 L.A 11KV

 P.T 33 KV side

 11kv vacuum Circuit Breaker

 33/11 kv 5MVA transformer

Real And Reactive Power

KVA,KW,KVAR,KWh,Kvar.....

As an electrical engineer we seen a lot as KW,MW,KVA etc. These are some units related with power and energy. It varies with the nature of the power and energy. Before that we can just see what is reactive and real power.

REACTIVE POWER:

The book definition is ‘it is the power absorbed by the inductor or released by the capacitor’. We know in AC circuit only the inductance and capacitance are present. It may be a line capacitance or winding inductance.It is the imaginary power etc.

          This reactive power is simply circulating power due to the phase shift between the source and the loads. This additional power that appears to be flowing requires the equipment to be rated higher than that for a system operating at unity. Obviously these circulating currents while real..produce no useful work. It is the job of the Electrical Engineer to correct the load reactance as close as possible to unity to increase efficiency and reduce operational costs. That means simply match the capacitance and inductance.

          Start now,
Total power     KVA   =   Kilo Volt Amp(v*I)

Real power      KW    =    Kilo Watt(v*I*cosΦ)
Reactive power KVAR = Kilo Volt Amp Reactance  (v*I*sinΦ)

Total power  KVA=KW+jKVAR
Power * Time= Energy 
Eg:KW*Hour=KWh(unit of electrical energy)

Friends, the field of reactive power is a wide area and researches are now going on, so i am not yet expert. The interesing fact is Reactive power is needed to transport the real power!! So we minimize it what ever we can. We use capacitor banks and other FACTS(Flexible AC Transmission System) devices for minimize it.

Star and Delta Connections

STAR AND DELTA CONNECTIONS

We seen the 3 phase concept and now we can discuss the star and delta connection for 3 phase supply. Actually where is these connections exist? The answer is at

·       Transformers(Both power and instrumentation).

·       At motor winding.

·       At generator winding.

Actually where we handle the 3 phase supply directly, we can prefer the star or delta connection. Previous post i mentioned that only 3 phase loads are motors,so you may confuse about the transformers, actually transformer is not a load. It just change the voltage level.

Delta connection is also used to connect the windings of most three-phase motors because the phase windings are perfectly balanced and, therefore, do not require a neutral connection.

           Making a star connection has the advantage that two voltages

become available – a line voltage between any two phases, and a

phase voltage between line and neutral which is connected to the star point.

          In any star-connected system currents fl   ow along the lines ( IL), through the load and return by the neutral conductor connected to the star point. In a balanced  three-phase system all currents have the same value and when they are added up by phasor addition, we find the resultant current is zero.

3 Phase Concept

WHY WE USE 3 PHASE ?

In the electrical field we use only 3 phases exactly. Not more or less,Why? Because we minimum of 3 phases to produce a rotating magnetic field(RMF). We studied that more than one phase is enough to produce a torque, but imagine 2 equal magnitude phases come into action the phase angle will be 180⁰  and will oppose each other.

So we use 3 phases to produce a rotating magnetic field and can carry more power than a single phase. Remember one thing, the only 3 phase AC loads are motors. No other loads are directly consume or use the 3 phase AC supply. You can easily understand the concept with an IC engine with 3 cylinder.