Sunday, 31 July 2016

Purpose of Interpole in DC Machine

For understanding the role of Interpoles, we need to understand the effect of armature reaction in the DC Machine. The effect of armature mmf on the main field flux is to distort the main field flux and to reduce the net main field flux. The figure below, shows the effect of armature mmf on the main field flux.

It is quite clear from the above figure that the flux at the location of Carbon Brush i.e. A, B and A are not zero and therefore an EMF will be induced in the coils undergoing commutation and will lead to the sparking. As we know that for better commutation, the coils short circuited by the brushes should have zero EMF induced in them. As the zero crossing of field flux is shifted due to armature reaction, the coils undergoing the commutation will have a net EMF induced in them. This induced EMF in the short circuited coil will delay the reversal of current in the short circuited coils and will result into poor commutation and sparking at the carbon brushes.

The question arises how to resolve this issue?

If we see the figure above, we observe that there is a net shift of zero crossing of net flux in the air gap by an angle Ɵ in the direction of rotation for Generator and opposite to the direction of rotation for Motor. So the cheap and easy solution shall be to shift the Carbon Brush at Zero Crossing of the air gap flux.

Thus carbon Brush need to be shifted by an angle Ɵ from Geometrical Neutral Axis (GNA) in the direction of rotation for Generator and opposite to the direction of rotation for Motor.

But this method of shifting the Carbon brush has a big disadvantage. What is that?
As the Armature Reaction depends on the current flowing through the armature winding which in turn depends on the load current. Therefore as the loading of the DC Machine varies the angle Ɵ will also vary and therefore we need to continuously shift the Carbon Brushes. So we need to find a smart way.

Again, looking back to the figure, if it could be possible to make the resultant or net air gap flux zero at GNA, then there would not have been any detrimental effect of armature reaction on commutation. Also, the existing flux at the GNA (at point C) is due to North Pole so we could use a South Pole (opposite of the pole which produced the imbalance at C) at C so that the net flux at C becomes Zero. Similarly at C’ we can use a North Pole to make net flux Zero there. Okay, this will work fine but how t change the magnitude field strength of this newly installed poles at C and C’? Hmmmm…..

We can use a winding on the newly installed poles at C and C’ and connect that winding in series with armature winding so that the strength of field due to newly installed poles at C and C’ varies proportionally will the loading of machine. Yes, this will work fine.

So we can conclude our solution as,

We will use Poles same as that of Main Poles ahead of GNA or Carbon Brush for Generator at the location of GNA or Carbon Brush and Poles same as Main Pole that of behind the GNA or carbon Brush for Motor at the location of GNA or Carbon Brush and will use winding on them and connect them in series with the armature winding as shown in figure below.

The Poles used in our smart solution is called the Interpole.

Interpoles are narrow poles placed at the GNA and fitted to the Yoke and also known as Commutating Poles or Compoles. For generator, the polarity of Interpoles must be same as that of main Pole ahead of it in the direction of rotation. For Motor, the polarity of Interpole must be same as that of Main Pole behind it.

So I expect that you understand the purpose of Interpoles as you only designed it. But there is one more interesting role of Interpole.

Interpole do not only nullify the effect of armature reaction but in addition, produces some extra mmf in the interpolar zone. This extra mmf in the interpolar zone induces rotational EMF in the short circuited coil undergoing commutation in such a direction to oppose the reactance voltage in the coil. Thus the resultant the resultant voltage in the short circuited coil becomes zero and the commutation is spark less.

Your comment and feedback is important to me. Thank you!

Saturday, 30 July 2016

Concept of Neutral Grounding

The concept of system grounding is extremely important, as it affects the susceptibility of the system to voltage transients, determines the types of loads the system can accommodate, and helps to determine the system protection requirements.

The system grounding arrangement is determined by the grounding of the power source. For commercial and industrial systems, the types of power sources generally fall into four broad categories:

Utility Service – The system grounding is usually determined by the secondary winding configuration of the upstream utility substation transformer.

Generator – The system grounding is determined by the stator winding configuration.

Transformer– The system grounding on the system fed by the transformer is determined by the transformer secondary winding configuration.

Neutral grounding is generally of three types:

  • Solid Grounding
  • Resistance Grounding
  • Reactance Grounding

Each of the grounding method serves a specific purpose and based on the suitability of our need, we use any one of the grounding method.

Solidly Grounded Systems:

The solidly grounded system is the most common system arrangement, and one of the most used. The most commonly used configuration is the solidly grounded star, because it support single-phase phase to neutral loads. In this type of grounding method, the star point is directly connected to the ground.

The figure below, shows the relationship between the phase and line voltage for Solidly Grounded System.

It can be seen from the above figure that the system voltage with respect to ground is fixed by the phase-to-neutral winding voltage. It means that the line-to-ground insulation level of equipment need only be as large as the phase-to-neutral voltage, which is 57.7% (100/1.732 = 57.7 %) of the phase-to-phase voltage. It also means that the system is less susceptible to phase-to-ground voltage transients. This is very important benefit of Solidly Ground System.

A common characteristic of solidly-grounded system is that a short circuit to ground will cause a large amount of short circuit current to flow. This condition is known as a ground fault. As can be seen from figure below the voltage on the faulted phase is depressed, and large current flows in the faulted phase since the phase and fault impedance are small.

The voltage and current on the other two phases are not affected. Thus a solidly grounded system supports a large ground fault current. Statistically, 90-95% of all system short-circuits are ground faults.

The occurrence of a ground fault on a solidly grounded system necessitates the removal of the fault as quickly as possible. This is the major disadvantage of the solidly-grounded system as compared to other types of system grounding.

A solidly-grounded system is very effective at reducing the possibility of line-to-ground voltage transients. However, to do this the system must be effectively grounded. One measure of the effectiveness of the system grounding is the ratio of the available ground-fault current to the available three-phase fault current. For effectively grounded systems this ratio is usually at least 60%.

To summarize,

The solidly grounded system is the most popular, is required where single-phase phase-to-neutral loads must be supplied, and has the most stable phase-to-ground voltage characteristics. However, the large ground fault current is a disadvantage and can be hindrance to system reliability.

Resistance Grounded Systems:

In Resistance Grounding method, the neutral point is connected to the ground by using a Resistor. The resistor is sized to allow 1-10 A to flow continuously if a ground fault occurs.

The resistor is sized to be less than or equal to the magnitude of the system charging capacitance to ground. If the resistor is thus sized, the high-resistance grounded system is usually not susceptible to the large transient overvoltages that an ungrounded system can experience.

If no ground fault current is present, the phasor diagram for the system is the same as for a solidly grounded system. However, if a ground fault occurs on one phase the system response is as shown in figure below. As can be seen from figure below, the ground fault current is limited by the grounding resistor.

If the approximation is made that ZA (impedance of winding) and ZF (Fault impedance) are very small compared to the ground resistor resistance value R, then the ground fault current is approximately equal to the phase-to-neutral voltage of the faulted phase divided by R. The faulted phase voltage to ground in that case would be zero and the unfaulted phase voltages to ground would be 173% of their values without a ground fault present.

The ground fault current is not large enough to force its removal by taking the system off-line. Therefore, the high resistance grounded system has the same operational advantage in this respect as the ungrounded system.

Reactance Grounding:

A Reactance Grounded system is one in which the neutral point is grounded through an impedance which is highly inductive. Reactance Grounding lies between the effective grounding and Resonant Grounding (will be discussed in next post). Reactance is provided to keep the fault current within safe limit. This method of grounding is used where the charging current is high like in capacitor bank, line reactors used for voltage control of transmission line etc.

Electrical Insulation Classes

Insulation Classes based on the temperature it can sustain is classified into the following classes:

The following are brief explanations of these insulation classes:

Class-Y Insulation: 

Class-Y insulations can withstands a temperature of up to 90°C and it is typically made of cotton, silk, or paper.

Class-A Insulation: 

Class-A insulations can withstands a temperature of up to 105°C. It is made of reinforced Class-Y materials with impregnated varnish or insulation oil.

Class-E Insulation: 

Class-E insulations can withstands a temperature of up to 120°C.

Class-B Insulation: 

Class-B insulations can withstands a temperature of up to 130°C. This has a form that inorganic material is hardened with adhesives. This is the first insulator using this structure.

Class-F Insulation: 

Class-F insulation can withstands a temperature of up to 155°C; for example, made of Class-B materials that are upgraded with adhesives, silicone, and alkyd-resin varnish of higher thermal endurance.

Class-H Insulation: 

Class-H insulations can withstands a temperature of up to 180°C. It is made of inorganic material glued with silicone resin or adhesives of equivalent performance.

Class-C Insulation: 

Class-C insulations withstand a temperature of up to 180°C or higher. It is typically made of 100% inorganic material.

As explained above, electrical insulation is classified with its maximum allowable temperature. By adopting an insulation technique of higher thermal endurance, the size of electrical machine can minimized.

Friday, 29 July 2016

Load Curve and Load Duration Curve

Load curve is the variation of load with time on a Power Station. As the load on a Power Station never remain constant rather it varies time to time, these variations in load is plotted on half hourly or hourly basis for the whole day. The curve thus obtained is known as Daily Load Curve.

Therefore, by having a look at the Load Curve, we can check the peak load on a Power Station and its variation. From the figure below, it is quite clear that the peak load (6 MW) on a particular Power Station is at 6 P.M.

The monthly load curve can be plotted using the daily load curve for a particular month. For this purpose the average load for different time for the whole month is calculated and the value thus obtained is plotted against time to get the Monthly Load Curve. Monthly Load Curve is used to fix the rate of energy.

In the same manner Yearly Load Curve can be obtained using the 12 monthly load curves. The Yearly Load Curve is used for calculation the Annual Load Factor.

Importance of Load Curve:

  • From the daily load curve we can have insight of load at different time for a day.
  • The area under the daily load curve gives the total units of electric energy generated.
         Units Generated / day = Area under the daily Load Curve in kW
  • The peak point on the daily load curve gives the highest demand on the Power Station for that day.
  • The average load per day on the Power Station can be calculated using the daily load curve.
  • Average load = Area under the daily Load Curve (kWh)/ 24 hrs.
  • Load curve helps in deciding the size and number of Generating Units.
  • Load Factor = Avg. Load / maximum Load =  Avg. Load x24 / 24xmaximum Load
                   = Area under daily Load Curve/Area of Rectangle having Daily Load Curve
  • Load curve helps in the preparing the operation schedule of the generating units.

Load Duration Curve:

Load Duration Curve is the plot of Load versus time duration for which that load was persisting. Load Duration Curve is obtained from the Daily Load Curve as shown in figure below.

From the above Load Duration Curve, it is clear that 20 MW of Load is persisting for a period of 8 hours, 15 MW of Load for 4 hours and so on.

It is also quite clear that, the area under the load duration curve is equal to the daily load curve and gives the number of units (kWh) generated for a given day. The load duration curve can be extended for any period of time i.e. it can be drawn for a month or for year too.

Thank you!

Thursday, 28 July 2016

Difference between Star and Delta Connection

As we know that Star & Delta Connection only possible in 3 phase system, so in our domestic system it is not possible, because generally all house hold electrical equipment are designed with single phase supply.

Difference between Star & Delta:

Star Connected System:

  • Star connection is used where we require Neutral terminal to obtain Phase voltage like above below.
  • In a star connected system VL=√3Vph, mean Phase voltage is root 3 times less than line voltage.
  • In a star Connected system IL=Iphase.
  • Star connected systems require less insulation level.
  • Star Connected system is used where low starting current is required.

Delta Connected System:

  • In a Delta Connected system Line Voltage is equal to Phase Voltage.
  • While phase current is √3 times less than Line current.
  • Insulation level is high because line voltage = Phase Voltage.
  • Delta Connected system is generally used where high starting Torque is required.