Thursday, 30 June 2016

What Happens if the Prime Mover of Synchronous Generator Fails?


If we take a Thermal Power Plant, then steam Turbine serves the purpose of prime mover and coupled with Synchronous Generator. Now if the steam supply to the steam turbine is suddenly cut off while the Synchronous Generator is connected to Grid, what will happen?



The answer is simple. If all the Generator protections are working fine then the Generator will be isolated from the Grid by Reverse Power Protection Relay. But here we will assume that no protection is working and then in investigate the behavior of Synchronous Generator.
Here are the various possibilities for a synchronous generator connected to grid when Prime mover is suddenly decoupled. The behavior of Synchronous Generator depends on whether the field winding is excited or not.

Case1: Field Excitation is available

In case the field excitation of Generator is available when the steam supply to the turbine is cut off, the Synchronous Generator will draw power from the Grid and will behave as a Synchronous Motor rotating in the same direction.

Case2: Field Excitation is failed

In this case we will consider two types of Synchronous Generator- Salient Pole and Cylindrical Pole.

Cylindrical rotor machine: It will work as Induction Motor running in same direction with speed less than synchronous speed.

Salient pole machine: It will work as Reluctance Motor running in same direction at synchronous speed.

One more situation is possible when the Prime mover is coupled but the Excitation alone fails, What will happen then?

In this case, the Machine runs as an Induction Generator whose rotor will rotate at a speed higher than synchronous speed.

Based on construction Induction Motor and Synchronous Machine have many similarities. Both have stators wired to have rotating magnetic field. The main difference comes in rotor construction. Induction Motor requires closed path for induced rotor currents created due to flux cutting. This flux cutting arises as the machine is not rotating at synchronous speed, as Slip is not zero.

In Synchronous Machine  also closed paths are available. In salient pole machine the damper bars act like squirrel cage rotor as salient poles are laminated so core does not provide proper closed paths. In case of cylindrical rotor machine there is a solid steel core which provides a closed current path. So when machine loses synchronism these will provide paths for circulating currents and create magnetic field.

Now consider a Synchronous Generator connected to Grid. When the excitation fails the power delivering capacity of machine suddenly reduces. So the mechanical power input is greater than electrical power output. This imbalance will accelerate the rotor and force it to run above synchronous speed. The circulating currents and then magnetic field are created.  The machine will start to generate power by induction and hence will work as Induction Generator.

Why don’t we then operate Synchronous Generator as Induction Generator?

We cannot operate Synchronous Generator as an Induction Generator. This operation is disastrous as the circulating current will heat the core and may damage the insulations. There is also a considerable change in the reactive power flow. A Synchronous Generator which was generating lagging power is suddenly changed to a machine absorbing lagging power i.e. generating leading power. These sudden changes will certainly cause disturbances in the network.

It should be noted that failure in field does not mean rotor magnetic field is absent. The machine draws reactive energy from Grid to create magnetic field. This field is created by the circulating rotor currents.

In industries, these situations are prevented by the Generator Protection system which isolates the generator from the grid immediately after it senses the fault.


Thank you!

Wednesday, 29 June 2016

Difference between Transducer and Sensor


Sensor :

As the name suggests, it is a device which reacts to a physical, chemical or biological condition. It senses a change in physical, chemical or biological condition. It can be considered as a detector.

A Sensor can sense in any form usually electronic i.e. due to some mechanical change, it can react in electrical form. Thus there is a conversion, similar to that of a transducer.
A classic example is a thermocouple or a pressure sensor which might detect pressure and convert it into electric current (3-15 psi to 4-20 mA)

Transducer:

The conversion of energy from one form to another is known as Transduction. A Transducer serves for this purpose.

A Transducer is more than a sensor. It consists of a sensor / actuator along with signal conditioning circuits.

A signal conditioning circuit, by the name is a circuit which conditions the signal so that it is strong enough for further processing. A system might contain many stages before the signal finally reaches its destination to derive meaningful information.



So one way to differentiate Sensor and Transducer is that the output from a sensor may or may not be meaningful i.e. most of the times it needs to be conditioned and converted into various other forms. The transducer output is always meaningful.



The output of a motor is meaningful. The output of a loudspeaker is meaningful. They are transducers. A sensor is nothing but just a primary element which senses any physical phenomenon or it gives an indication in any change of the physical phenomenon.

We can say that every transducer is also (or has) a sensor but every sensor need not be a transducer. Sometimes it is. Sometimes in a sensor, there is no conversion at all. For example, thermometer, where the temperature is sensed and is directly measured. In a transducer there is always a conversion i.e. transduction. For example, RTD, Thermocouple etc where the temperature is sensed and the measurement is made in terms of voltage.


Thus we can say that a SENSOR may or may not have a conversion and it only senses. A TRANSDUCER always involves a conversion and also has signal conditioning involved.

Thank you!

Why not Blocked Rotor Test by Keeping Induction Motor Rotor Winding Open?


As discussed in the previous post Blocked Rotor Test of Induction Motor, the rotor is prevented from rotating and then a small voltage is applied to the stator sufficient to pass the rated current through the stator winding and then leakage impedance of Induction Motor is calculated.

The question may arise that why don’t we keep the rotor winding of Induction Motor open to prevent rotor to rotate and then perform Blocked Rotor Test?



The answer is quite simple. If the rotor of the induction motor is kept open, then no current will flow in the rotor. Consequently the rotor copper loss would be zero. The rotor would not rotate even if it is not blocked. Hence the power measured would not be equal to the total copper loss and therefore the wattmeter reading cannot be used to determine the equivalent resistance referred to the stator.

In short, there is no point in doing the Block Rotor test on an induction motor with its rotor kept open.

If we try to pass rated current through the stator winding with the rotor open, then we will observe the stator voltage required is much greater than the rated stator voltage. Thus we are likely to damage the stator winding insulation and may burn the stator winding.


Thank you!

Blocked Rotor Test of Induction Motor


As the name of the test specifies, in Blocked Rotor Test, rotor of Induction Motor is blocked by external means so that it cannot rotate. The Blocked Rotor Test of Induction Motor is similar to the short circuit test of a transformer.

As in Transformer short circuit test, we apply a voltage which can circulate rated current in the winding, likewise in Blocked Rotor Test we apply a Voltage to the Stator winding of an Induction Motor by using VARIAC so that rated current flows through the stator winding. The voltage required to circulate the rated current through the Stator winding is around 10-15% of the rated voltage.

Since this voltage required to pass rated current through the stator winding under blocked rotor condition is only about 10 to 15 % of the rated stator voltage, the core losses during the Block Rotor test is negligible, mind that core loss is roughly directly proportional to the square of Voltage. Thus the wattmeter reading would effectively give the sum of stator and rotor copper loss. This wattmeter reading is then used determine the leakage impedance of Induction Motor as shown in figure below

.

Thus Blocked Rotor Test is performed on an Induction motor to calculate the leakage impedance of Induction Motor.

Form the figure above, as the rotor is blocked to rotate, mechanical loss will be negligible and therefore,

Total Power input Pin = Stator Ohmic Loss + Rotor Ohmic Loss

                              = W1 + W2

The input voltage V and Input phase current I are recorded. Here V = Line Voltage

But Pin = I2Reqs 

where Reqs = Equivalent Resistance of Induction Motor referred to Stator side.

Therefore,

Reqs = Pin/I2

Now, Blocked Rotor Impedance Zeq = Input Voltage / Input Current

                                                           = Vph/I
So,
Equivalent Blocked Rotor Reactance X =
                                                     
                                                                  

As, stator reactance X1 and rotor reactance per phase referred to stator X2 are normally assumed equal, hence

X1 = X2 =X/2

DC resistance of Stator winding is measured just after the Blocked Rotor test and the value obtained is multiplied by 1.1 to 1.3 to get the effective resistance of stator winding. Thus if R1 be the stator winding resistance and R2 the rotor winding resistance, then

R2 ≈ Reqs - R1 approximately.

In this way, the leakage impedance of Induction Motor is calculated using Blocked Rotor Test.

What is important for Blocked Rotor Test?

It should always be understand that, for large Motor of rating more than 20 kW if the Induction Motor characteristics is required near s=1 i.e. for getting an idea of starting torque, then Blocked Rotor Test shall be performed at line frequency as the rotor frequency sf = f =line frequency.

But if we need Induction Motor characteristics near synchronous speed the as the rotor frequency becomes sf therefore Blocked Rotor Test shall be performed at sf frequency not at line frequency.

Check this out for Electrical Machine. I will just to read this Book many times instead of reading many books one time. It is concept wise a great book and objectives & subjective questions at the end of the Book is very important and helpful for different kind of Exams.

Thank you!

Tuesday, 28 June 2016

Why Star Delta Starter Preferred in Induction Motor?


The main purpose of any starter is to reduce the requirement of high starting current. Normally the starting current of an induction motor is 6 to 7 times of the full load current. If one has an induction motor with a DOL starter, drawing a high current from the line, which is higher than the current for which this line is designed. This will cause a drop in the line voltage, all along the line, both for the consumers between the substation and this consumer, and those, who are in the line after this consumer. This is the reason for which a starter is to be used.

In a squirrel cage induction motor, the starter is used only to decrease the input voltage to the motor so as to decrease the starting current.



It is T.P.D.T switch used to first start the motor with the winding connected in star and then switch for delta connection in running position. TPDT stands for Triple Pole, Double Throw.

Why Star Delta Starter Preferred in Induction Motor?

If the winding of Induction Motor is connected in star, the voltage per phase supplied to each winding is reduced by 0.577.In general the voltage per phase in delta connection is Vs, the phase current in each stator winding is (Vs/Zs), where Zs represents the impedance per phase of the motor at standstill or start.

The line current or the input current to the motor is Ist (starting current) = (1.732*Vs)/Zs which is the current when it has to be started by DOL starter.

Now, if the stator winding is connected as star, the phase or line current drawn from supply at start (standstill)  = (Vs/Zs)/(1.732)

which is 0.333= (0.577*0.577) of the starting current, if DOL starter is used.

The voltage per phase in each stator winding is now Vs/1.732. So the starting current is reduced by 33.3%. Because of the reduction in starting current, starting torque reduces.

Therefore we can conclude that by using Star Delta starter, the starting current is reduced to approximately two-thirds. Since starting current is reduced, the voltage drops during the starting of motor in systems are reduced. 

Thank you!

Monday, 27 June 2016

Transformer Differential Protection


Differential Protection is based on the fact that any fault within electrical equipment will result into the difference in current entering it and the current leaving it. Thus by comparing the two currents either in magnitude or in phase or both we can determine a fault and issue a trip decision if the difference exceeds a predetermined set value.

In this post we will discuss about the Differential Protection of a Transformer. Consider an ideal transformer with the CT connections, as shown in figure below.



Suppose that current rating of primary winding = 100A

Current rating of secondary winding = 1000A.

Then if we use 100/5 and 1000/5 CT on the primary and secondary winding respectively, then under normal operating conditions the both CT currents will be 5 A in magnitudes.

By connections the primary and secondary CTs with due care to the dots (In actual CT polarity marking is by Terminal P1 and P2. Current is supposed to flow from P1 to P2), a circulating current can be set up as shown by dotted line.

No current will flow through the branch having overcurrent current relay in normal condition as

Differential current Id = 5-5 = 0 A

Now if an internal fault occurs within the Transformer like interturn short etc., then the normal mmf balance is upset i.e. under this condition, the CT secondary currents of primary and secondary side CTs will not match. The resulting differential current will flow through overcurrent relay. If the pickup setting of overcurrent relay is close to zero, it will immediately pick up and initiate the trip decision.

In practice, the transformer is not ideal. A differential current always flows through the overcurrent relay. Therefore overcurrent relay pick up is adjusted above the no load current value.

In Differential Protection of Transformer CT matching is an important thing to care care else differential protection won’t be efficient and reliable.

Let the transformer turns ratio given N1/N2 and the corresponding CT ratio be given by 1/n1 and as 1/n2.

Therefore,

Current in CT - 1 primary = I1

Current in CT - 1 secondary =I1/n1

Current in CT - 2 primary = N1I1/N2

Current in CT - 2 secondary = N1I1/N2n2

In normal operating condition of Transformer, the differential current through the Relay should be zero. Therefore,

Current in CT - 1 secondary = Current in CT - 2 secondary

I1/n1 = N1I1/N2n2

N2n2 = N1n1

Thus CT should be selected in such a way that their Turn Ratio satisfies

N2n2 = N1n1

In case, Transformer Tap is used then nominal Tap position should also be taken into account.

When dealing with three phase transformers, the transformer connections like Y-Y or delta – delta connection play a great role in determining CT secondary interconnections to establish circulating current scheme. This is because of the phase shifts typically of the order of +- 30° that result in the line currents when we move from primary to secondary side of the power transformer. If transformer winding are connected in Y configuration then use DELTA configuration for corresponding CT secondary interconnections and vice-versa. "


Thank you!

Saturday, 25 June 2016

Autoreclosing Philosophy in Distance Protection


What is Auto Reclosure?


Autoreclosing is a feature which is provided in the Line Circuit Breaker so that Single Pole of a Breaker may trip and close when command is given to do so.

Based on the time duration of fault existing in the Power System, faults can be classified into three categories as

  • Transient Fault
  • Semi-Transient Fault
  • Permanent Fault


Transient fault exists only for very short duration and these can the removed faster if the line is disconnected from the system momentarily so that arc extinguishes. After the arc is deionized, line can be reclosed to resume the service. Thus, in this way the interruption in the Power Supply is reduced and loss of revenue is also saved.

Philosophy of Auto Reclosure:


It has been found that 80% of the fault in Power System are Transient in nature, 12% are Semi-transient and remaining 8% are only permanenet fault. Semi-transient fault are those fault which take some finite time to clear by itself. For example, suppose a bord spanning the two lines sit then it will cause a fault which will clear by itself after the burning of cause of fault,after some time say 1sec. Thus we will expect, Autorecloser to take place for 1 sec i.e. Breaker shall close after a time delay of 1 sec. Here note that the time after which fault clears by itself is called DEAD TIME. Therefore, in our example DEAD TIME = 1 sec.

But for Permanent Fault, Autorecloser will not help as the cause of fault continuously exists so if we incorporate the Autorecloser the Breaker will again trip after the Autorecloser. So how many attempts will the Realy take to Autoreclose and after how much time it will take another consecutive attempt to Autoreclose?

“Here we come to another concept, called RECLAIM TIME. RECLAIM TIME is the time after which Relay will take another consecutive attempt to Autoreclose. This RECLAIM TIME is typically set at 25 sec. The number of attempt for Autorecloser is set in the Relay which is 4 for MiCOM P444 Distance Protection Relay. This means that Relay will take four Autorecloser shots and at the end of fourth shot, if still fault is existing, the Line will be taken out.” In figure below, a Numerical Relay is shown.



Thus we see that for permanent fault Autorecloser won’t help as we need to attend the fault and rectify it.

Autorecloser can be Single pole or Three pole. Here Pole means Breaker of any of the three phase i.e. either R, Y or B phase. Single Pole Autorecloser take place during Line to ground fault. It shall be noted that Autorecloser facility is provided only in Line Breaker and that to by Distance Protection Relay.

Single Autorecloser take place in the following conditions:

Zone-1 protection operated AND Carrier Channels are healthy AND Three Pole Tripping has not taken place.
OR

Zone-2 protection operated with Carrier Signal Received.

This seems surprising that only a single pole of Breaker trips during Zone-1 fault. But it’s true. The phase, say B phase, in which Line to ground fault has taken place will only trip and reclose after the DEAD TIME. If within the RECLAIM TIME, another fault take place then Three Phase trip will occur. During the DEAD TIME period, power is fed to the system via the two healthy phases.

In case of Three Pole Autorecloser, all the three phases are opened independently irrespective of type of fault be it Single L-G or L-L or L-L-L fault and reclosed after the DEAD TIME. During the DEAD TIME period, no power can be transmitted and therefore system is liable to operate unstably.


Hope you enjoyed the topic. Thank you!

What is Tunnel Diode?


A tunnel diode is a diode that exhibits a negative differential impedance region in its I-V characteristic due to quantum tunneling effects. That is, a \ -sloped region as shown by red colored region in the figure below.



It's a diode, a kind of semiconductor device usually with two terminals. Unlike a regular pn diode, Tunnel Diode conducts both ways. This is due to a peculiarity of the manufacturing process in which the impurity i.e. dopant concentration is intentionally high, that the semiconductor becomes a bit more like a conductor having small resistance.



Semiconductors are the only materials that exhibit this weird behavior. If we add impurities in a metal, it won't significantly alter its resistance.

Heavy doping in Tunnel Diode is intentional, because it alters the energy levels in the semiconductor. This allows such that the engineered gap between the p and n material types to be much larger than normal.



The above effect makes it very convenient for electrons to move and they are not allowed to jump from the lower energy level to the higher energy just like that; what happens here is quantum tunnelling, an effect where modern physics shows that electrons teleport through thin wall as shown in figure below.



Therefore in short,


Tunnel diode is a highly doped semiconductor device and is used mainly for low voltage high frequency switching applications. It works on the principle of Tunneling effect. It is also called as Esaki diode named after Leo Esaki, who in 1973 received the Nobel Prize in Physics for discovering the electron tunneling effect used in these diodes.

Thank you!

Concept of Infinite Bus


For understanding the concept of Infinite Bus, we will take some examples. Suppose we take an isolated synchronous generator of capacity 100 kW and supplying a load of, say 50 kW, at 50 Hz and rated voltage, say 400 V.

Now if we add a load of 20 kW what will happen?

The frequency (speed) and terminal voltage will reduce instantly say 49 Hz and 390 V, before you correct them. Instead if we add only 10 kW the result will be same, with frequency and voltage of 49.5 Hz and 395 V. Next time, if we keep reducing the applied load….

With say 1 kW addition frequency and voltage may be 49.95 Hz and 299.5 V
If we now add 0.1 kW (100 W) instead there may not be any measurable change in frequency and voltage. So we can say that for a load up to 100 W the 100 kW machine is an infinite bus.

This example is used to illustrate the concept of an infinite bus. All the numerical values for frequency and voltage used above are not calculated but assumed for illustration only.

In our day to day routine, in cities, if we switch on a 60 W bulb or a tube light you don’t notice anything electrically abnormal. But if you switch on an Air Conditioner (without a Voltage Regulator) you can clearly notice a sudden voltage dip which recovers. 

Theoretically an infinite bus is one where the frequency and voltage remain constant irrespective of the amount of load on it.

An infinite bus, to satisfy the above constraints, is represented by an equivalent generator having infinite moment of Inertia, M so that there will be no change in speed for any load addition and zero synchronous reactance, Xs so that there is no voltage drop for any load current and V = E, Induced generator emf.




In short infinite bus has number of generator connected to it and that means it has infinite active and reactive power capabilities. It can maintain its terminal voltage, frequency and any loading condition.

Friday, 24 June 2016

Deciding Factor for Selection of HVDC over HVAC Transmission


This is a famous question which most of the people ask to an Electrical Engineer that “Suppose we are setting up a power plant so shall we use HVDC or HVAC Transmission?” In this post I am going to answer this question with examples which will help to clear the concept. We have already seen the advantage of HVDC over HVAC in post HVDC - Advantage & Disadvantage.

The answer to this question lies in Economic Distance for HVDC Transmission. The cost per unit length of HVDC is much less when compared to AC Transmission for the same power capacity but the cost of terminal equipments used in HVDC like converters and Inverters are much higher which makes HVDC uneconomical. Now, as the length of line increases the cost of Transmission line for AC will also increase to a greater extent when compared with HVDC, therefore we can guess that there must be a specific distance beyond which the AC Transmission will be more costly.

This calls for a graph between the cost of AC Transmission Vs Line length and similarly cost of HVDC Transmission and Line length.



As clear from the graph, the vertical intercept of each line (line length = 0) is the cost of Terminal equipments like Transformer for HVAC and Converters & Inverters for HVDC. It is clear that for line length less than 800 km, HVDC is more costly.

It can be seen from the graph that there is a point corresponding to line length= 800 km where the cost of HVAC and HVDC are equal but beyond 800 km the cost of HVDC is less compared to HVAC. This 800 km is called Break even distance. Thus for transmission more than 800 km, it is cheaper and efficient to use HVDC.

Thus it can be seen that length of Transmission decides whether HVDC will be cheaper or HVAC.

Thank you!