MVAR for Mechanical Guys!!

Its been always a scary question for mechanical engineers working in power plants...

 "what is MVAR and how does it work? "

Well, the answer itself is scary too. Here I have tried my best to make you understand this in a simpler way.


First of all its VAR- Volt Ampere Reactive, The big M just stands for Mega! so you can call it KVAR as well when its value is in that range of course.

Here comes the wiki definition:

"In electric power transmission and distribution, volt-ampere reactive (var) is a unit used to measure reactive power in an AC electric power system. Reactive power exists in an AC circuit when the current and voltage are not in phase. The correct symbol is var and not Var, VAr, or VAR, but all three terms are widely used. The term var was proposed by the Romanian electrical engineer Constantin Budeanu and introduced in 1930 by the IEC in Stockholm, which has adopted it as the unit for reactive power."

Now for you I'm quickly mentioning the difference between resistance, capacitance and inductance as it is required for understanding VAR.

Resistor- Opposes the flow of current through it.

Inductor- Opposes the change in current through it. 

Capacitor- Opposes the change in voltage across its ends.


Now the time is to understand True, Reactive and Apparent power....

These inductors and capacitors dissipate zero power, yet the fact that they drop voltage and draw current gives the deceptive impression that they actually do dissipate power. This “phantom power” is called the reactive power.

True power                 P=I²R 

Reactive Power    Q= I²X        where x is the reactance of inductive and/or capacitive system

Apparent Power     S=I²Z          where z is the impedence of mixed system, Z²= R²+X²

Why inductors and capacitors don't consume power? Are you kidding me? no problem... here is the answer...

First of all we measure electrical power when it is converted to dissipating heat energy.

P=I²R

Now in case of inductor it is assumed to be a pure conductor so it won't get heat up. All it does is take electrical energy in one cycle to produce magnetic field and then in the next cycle it will give that consumed energy back. so in a complete cycle there won't be any net energy consumption or generation.
This is same with the capacitor as it stores electrostatic energy and does the same during a complete cycle.


Now that you know so much, let me remind you of phasor diagram, just have a quick look here

Now do you get it? The electrical guys use this rotating vector for vector manipulations of the electricity as in case of a cocktail of resistive, inductive and capacitive loads.

Hmm... now in the diagram above you can imagine that in a resistive load(First one) the voltage and current are in sync  but in case of capacitor(Second one) the current leads the voltage by certain angle and in case of inductor(Last one) it lags.
So if we want to add these current vectors in a mixed loading system,

Now You see, how easily they can be added in a vector way to have a resultant vector. Here the angle ⌽ will show weather the resulting vector is leading or lagging the voltage.

now that you know a lot about blah blah blah... lets get to the point....

This is how your Generator should look like

Since       E-jIX= V

where E is EMF, I is current flowing through stator windings and X is reactance of the system

Here is the phasor of normal generator connected to grid load

Here is how I draw the phasor.

Here V is the grid voltage( or GT terminal voltage) ... supposed to be ideally constant so taken on x-axis or as a reference.
The loading of generators are inductive mostly so the current I is lagging.
The EMF E leads the current by 90 degrees in pure inductive loading.

Now here The Apparent Power is

                                                                           P= VI

And The Reactive Power is

Now apply this to Generator in terms of MW and MVAR in Y and X axis respectively. while multiplying each term by V/X to convert it into power....

So you see how simple it was to come to Generator phasor diagram.

In the above diagram you can see horizontal component of the Apparent power is Mvar and the Vertical component is MW.

NOW!!!


Here comes the Generator capability curve....

Now try to digest this much for now!! 

I'll continue on the MVAR and Generator Capability Curve next time..... until then questions are welcome!!

Stalling in Fans

What is stall?

Separation of air flow from one or more fan blades. stalling is a major setbacks in axial fans.

Why does it occur?

In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases.This occurs when the critical angle of attack of the foil is exceeded. The critical angle of attack is typically about 15 degrees, but it may vary significantly depending on the fluid, foil, and Reynolds number.

Air is a mysterious substance. It is difficult to pin point the exact reason for stalling in a specific case of a Fan, but will occur in well designed air flow system only due to following reasons;

1. Turbulent flow

       

    Imagine a fan's blade that on one rotation cuts a part of air and pushes it forward. Now the gap created between fan blade back and non-pushed air can cause flow separation. But this occurs in a very high speed fans like Airplane propellers. 

    Now imagine ID fans(Induced Draft) where inlet turbulence is high then there is a good chance of flow separation at blades as shown in the picture.

2. Unexpected system Resistance

      

    Now when in equilibrium state, suppose an unexpected resistance occurs in air flow path, e.g. sudden closure of inlet duct damper or mechanical blockage. This causes the same effect as explained in a turbulent flow.

3. Pressure spikes

    

    In case of FD fan (Forced Draft) and ID fan (Induced Draft fan) working together balancing the furnace pressure in boiler. Suppose due to some furnace pressure deviations high enough to cause a temporary resistance in the flow will cause the same effect on the fan as explained in turbulent flow.

When or where it occurs in Power plants?

In case of  fans stalling occurs when unequal loading in a set fan occurs or change over form one  fan to another is in progress.

What is the effect?

Stalling may affect fan's loading, causing nonuniform load sharing.
Banging noise.