By EEE, ADBU
System earthings and incidents on MV side that result in LV disturbances
By EEE, ADBU
Three abnormal situations leading to an uncontrolled release of SF6 gas
By EEE, ADBU
Line to Line Fault Definition, Fault Current Calculation
What is Meant by Line to Line Fault ?
When two conductors of a 3 phase system are short circuited line to line fault or unsymmetrical fault occurs.This fault is severe compared to symmetrical faults in power system.In order to safeguard power system network analysis of unsymmetrical faults (in this case line to line fault) must be done.
Fault Current Calculation in Line to Line Fault
Consider a 3-phase system with an earthed neutral.Assume a line-to-line fault between the blue (B) and yellow (Y) lines as shown in figure The conditions created by this fault lead to :
Again taking R-phase as the reference, we have,
June 27, 2017 at 12:27AM by EEE, ADBU
2 types of synchronisers required on a power system with two supplies
By EEE, ADBU
What is Bridge Rectifier? Operation, Circuit Diagram, Advantages, Disadvantages, Applications
June 24, 2017 at 10:24PM by EEE, ADBU
Why we use only 11KV / 22KV / 33KV / 66KV / 110KV / 230KV / 440KV. Why can’t we use other voltage like 54KV / 89KV etc?
Why electricity is being transmitted in multiple of 11?
So we can see all the voltages are made inevitably multiple of this value (1.1, which is the form factor of ac wave).Also it provides us the best economic construction of step up and step down transformers.
Right Answer:The answer given above is taken standard electrical book.But that is wrong the actual answer is when we want to maintain 10KV at receiving end, transmission companies transfer 10% extra than receiving end voltage.i.e., 11KV simillary 10% of 20/30/60/100KV gives us 11KV / 22KV / 33KV / 66KV / 110KV respectively.These questions sounds same :
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June 24, 2017 at 06:46PM by EEE, ADBU
Why functional specification of SCADA project is essential for its success
By EEE, ADBU
How to Draw Potier Triangle/ZPF characteristics ?
Regulation by Zero Power Factor ( ZPF) method of Alternator:
-As explained earlier OC and SC tests are conducted and OCC and SCC are drawn.
-ZPF test is conducted by connecting the alternator to ZPF load and exciting the alternator in such way that the alternator supplies the rated current at rated voltage running at rated speed.
-To plot ZPF characteristics only two points are required. One point is corresponding to the zero voltage and rated current that can be obtained from scc and the other at rated voltage and rated current under zpf load.
-This zero power factor curve appears like OCC but shifted by a factor IXL vertically and horizontally by armature reaction mmf as shown below in figure.
-By suitable tests plot OCC and SCC. Draw air gap line. Conduct ZPF test at full load for rated voltage and fix the point B.
-Draw the line BH with length equal to field current required to produce full load
current on short circuit..
-Draw HD parallel to the air gap line so as to cut the OCC. Draw DE perpendicular to HB or parallel to voltage axis.
-Now, DE represents voltage drop IXL and BE represents the field current required to overcome the effect of armature reaction.
-Triangle BDE is called Potier triangle and XL is the Potier reactance. Find E from V, IRa, IXL and .
-Use the expression E =√ (V cos Ø+ IRa)² + (V sin ) + IXL)² to compute E. Find field current corresponding to E. Draw FG with magnitude equal to BE at angle (90+ ) from field current axis, where is the phase angle of current from voltage vector E (internal phase angle).
-The resultant field current is given by OG. Mark this length on field current axis. From OCC find the corresponding E0. Find the regulation.
June 22, 2017 at 02:58AM by EEE, ADBU
Auto Transformer Principle of Operation,Working & Applications
An auto transformer is a electrical transformer having only single winding which acts as both primary and secondary,so in input and output connected to shared single winding.
1.Auto transformers usually smaller in size,because one winding is eliminated.
2.as size is small cost also low(so cheap in cost)
3.as the winding is same so leakage reactance will be less.
4.increased kVA rating.
∴ Saving of copper in auto transformer compared to two winding transformer,
∴ Wtw -Wa=kWtw
Why we need to go for Auto Transformer ?
We have some advantages of auto-transformer over normal two winding transformer.1.Auto transformers usually smaller in size,because one winding is eliminated.
2.as size is small cost also low(so cheap in cost)
3.as the winding is same so leakage reactance will be less.
4.increased kVA rating.
Construction, Principle of Operation Of Auto Transformer:
In Auto Transformer, one single winding is shared as primary winding as well as secondary winding.as in transformer copper wire wound on silicon steel.as shown in the figure input connected at fixed positions.but on other side we employ some tapping to get variable output voltages.Variable turns ratio at secondary can be obtained by changing the tappings of the winding.
Copper Savings in Auto Transformer:
If we compare conventional two winding transformer and auto transformer the amount of copper needed for auto transformer is less.weight of copper of any winding depends upon its length and cross - sectional area.and length depends on no.of turns,cross - sectional area varies with rated current.
copper in the section AC proportional to,
(N1-N2)I1
weight of copper in the section BC proportional to
(I1-I2)N2
Hence, total weight of copper in the winding of auto transformer proportional to,
In similar way it can be proved, the weight of copper in two winding transformer is proportional to,
N1I1+N2I2
=2N1I1(Since, in a transformer N1I1=N2I2)
Let's assume, Wa and Wtw are weight of copper in auto transformer and two winding transformer respectively,
∴ Wtw -Wa=kWtw
June 22, 2017 at 02:32AM by EEE, ADBU
9 Advantages of Three Phase System Over Single Phase System
Three Phase System Advantages
In modern power generation, transportation, distribution we use poly-phase system over single phase system.In poly-phase AC system it might be two , three or more individual circuits, operate at same frequency, their voltages and currents are out of phase from one another.But when it comes to practical scenario three phase system considered as poly-phase system.It is most reliable and efficient compared to all poly phase systems.Single phase system has some limitations and drawbacks so it's been replaced by three-phase system.Here we listed out major reasons why 3 phase/poly-phase system is better than single phase system.
Advantages of polyphase systems over single phase systems are
1. A polyphase/3 phase transmission line requires less conductor material than a single phase line for transmitting the same amount power at the same voltage.2. For a given frame size a polyphase/3 phase machine gives a higher output than a single-phase machine. For example output of a 3-phase motor is 1.5 times the output of single-phase motor of same size.
3. Polyphase/3 phase motors have a uniform torque where most of the single-phase motors have a pulsating torque.
4. Polyphase/3 phase induction motors are self-starting and are more efficient. On the other hand single-phase induction motors are not self-starting and are less efficient.
6. Per unit of output. the polyphase/3 phase machine is very much cheaper.
7. Power factor of a single-phase motor is lower than that of polyphase motor of the same rating.
8. Rotating field can be set up by passing polyphase current through stationary coils.
9. Parallel operation of polypahse alternators is simple as compared to that of single-phase alternators because of pulsating reaction in single-phase alternator.
It has been found that the above advantages are best realized in the case of three-phase systems.Consequently, the electric power is generated and transmitted in the form of three-phase system.
Final Points :
| Single Phase Supply | Three Phase supply |
| power delivered is pulsating | Power delivered is constant |
| Single Phase induction motors are not self starting as it does not have starting torque. | Three phase induction motors are self starting. |
| Parallel operation is not easy. | Parallel Operation is easy. |
| Efficiency of single phase motor is lesser. | High efficiency. |
| Single phase motors have pulsating torque. | Three phase motors have uniform torque. |
| Single phase motors have lower power factor. | Three phase motors have higher power factor. |
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June 22, 2017 at 01:57AM by EEE, ADBU
Practice for good grounding and bonding a home wiring system
By EEE, ADBU
Current Differential Relay Working || Types of Differential Relays
Current Differential Relay Working || Types of Differential Relays
What is differential relay?
In the over-current relays, a current is sensed but such relays are not very sensitive as these relays cannot distinguish between heavy loads and minor fault conditions. in such castes. differential relays can be need.
A differential relay in defined an the relay that operates when the phasor drill-me of two or more similar electrical quantities exceeds a predetermined value,
Thus a current differential relay operates on the result of comparison between the phase angle and magnitudes of the currents entering and leaving the system to be protected. Under normal conditions. the two currents are equal in phase and magnitude hence relay is inoperative. But under fault conditions. this condition no longer exist.
The relay is connected in such a manner that the difference between current entering and current leaving flows through the operating coil.If this differential current exceeds a preset value then the relay operates and opens the circuit breaker.
Almost any type of relay when connected in a certain way can be made to operate as a differential relay.
Types of differential relays:
1. Current differential relay.
2. Biased beam relay or percentage differential relay.
3. Voltage balance differential relay.
Current differential relay working:
Most of the differential relays are at current differential type. Consider an over current relay connected in the circuit so as to operate as the current differential relay. This it shown in the figure.
Two current transformers are used having same ratio are connected on the either side of the section to be protected.The secondaries of current transformers are connected in series, so they carry induced currents in the same direction. Let current I is flowing through the primary of current transformers towards the external fault. As the current transformers are identical, the secondaries of current transformers will carry equal currents. Due to the connection of relay, no current will flow through the operating coil for the relay, Hence relay will remain Inoperative. So relay cannot operate if there is an external fault.
The current flows through the fault from both sides. The two secondary currents through C.Ts are not equal. The current flowing through the relay coil is now i1+i2. This high current causes the relay to operate.
It should be noted that the fault current need not always flow to the fault from both sides. A flow on one side only or even some current flowing out of one side while a large current entering the other side can cause differential relay to operate. Thus the amount of current flowing through a relay coil depends upon the way the fault is being fed‘
Disadvantages of current differential relay:
1. The current transformers are connected through cables called pilot cables. The impedance of such pilot cables generally causes a slight difference between the currents at the ends of the section to be protected, A sensitive relay can operate to a very small difference in the two currents, though there is no fault existing.
2. The relay is likely to operate inaccurately with heavy through current flows.This is because the assumed identical current transformers may not have identical secondary currents due to the constructional errors and pilot cable impedance.
3. Under severe through fault conditions,. the current transformers may saturate and cause unequal secondary currents. The difference between the currents may approach the pick value to cause the inaccurate operation for the relay.
4. Under heavy current flows. pilot cable capacitance may cause inaccurate operation of the relay.
All these disadvantages are overcome in biased beam relay.
June 18, 2017 at 11:56PM by EEE, ADBU
What is Creeping in Energy Meters ?
Creeping in Energy Meters:
In some energy meters a slow but continuous rotation of the disc is obtained when the pressure coils are energized and there is no load current passing through the current coil i.e. current coil is not energized.The main reason for creeping is over-compensation the aluminium disk to over come the static friction of disk and another reason is over voltage across the shunt magnet.Due to this creeping consumers suffers from high tariff. This may be due to incorrect friction compensation to vibration, to stray magnetic field or to the fact that the supply voltage is in excess of the normal voltage. This unwanted effect is called as "creeping in energy meter".
Read Here : How Energy Meter Works?
How to reduce creeping error in energy meter?
To prevent such creeping of the meter two holes or slots are cut in the disc on opposite sides of the spindle. The disc tends to remain stationary when one of the holes comes under one of the poles of the shunt magnet. In some cases a small piece of iron wire is attached to the edge of the disc. The force of attraction of the brake magnet upon this iron wire is sufficient to prevent the creeping of the disc under no load condition.
June 18, 2017 at 04:49PM by EEE, ADBU
Lead Lag Compensation In Control System
LEAD LAG COMPENSATION IN CONTROL SYSTEM
In this post we are going to learn about types of compensations, why compensation of control system is required, lead and lag compensation in control system.
Series Compensation:
If we connect compensation circuit between plant and error detector it is called series compensation. The circuit for series compensation is as follows.
Parallel compensation:
If we connect compensation circuit as feedback type it is called parallel compensation. The circuit for parallel compensation is as follows.
Load compensation:
If we combine series and parallel feedback circuits it results in Load compensation. The circuit for load compensation is as follows.
Why compensation of control system is required?
1.We do compensation for a control system in order to achieve best performance.
2. To make an unstable system stable.
3. It increases steady state accuracy of the system.
4. Reduces overshoot of a system.
5. Generally compensation is done in the form of feed forward path gain adjustment.
6. When we apply compensation network to a control system it introduces poles and zeros in to the system which results in change of transfer function, this makes the performance of the system to alter.
Note: Increase in the steady state accuracy brings instability to the system.
By substituting
in above equation. we get transfer function.
here T and β are respectively the time constant and DC gain.
In the lag compensator pole is more dominating than the zero and because of this lag network introduces negative phase angle to the system when connected in series.
Substitute s = j𝛚 in the transfer function , also we have ɑ < 1 then we get transfer function as,
Where, θm is the maximum phase lead angle.β is generally chosen to be greater than 10.
Phase Lead Compensation:
A lead network has one pole and one zero, zero must be near to the origin and also the poles and zeros must lie on the negative real axis. By seeing the pole zero plot of lead compensation network you can get a clear idea, its figure is shown below.
Pole Zero Plot Of Lead Compensation Network :
So to add a dominating zero( zero near to origin) to a control system we use phase lead compensation network. The network is shown below.
Phase Lead Compensation Network:
From this circuit we have,
Now on equating above two I values we get,
Determining Transfer Function Of Lead Compensation Network:
Here transfer function is the ratio of output voltage to input voltage.
By applying Laplace transform on both sides of above equation we get,
we know,
By substituting this equation in the above equation we get transfer function,
Here T and ɑ are time constant and attenuation constant respectively.
This lead network when connected in series gives positive phase angle to the system which makes the zero to be close to origin .
Substitute s = j𝛚 in the transfer function , also we have ɑ < 1 then we get transfer function as,
In order to know at what frequency we get maximum phase lead we need to differentiate the above equation and equate to zero. Now we get,
Here θm is the maximum phase lead angle, corresponding magnitude of the transfer function at maximum θm is 1/a.
Advantages Of Phase Lead Compensation:
1. As the gain crossover frequency shifts to a higher value because of phase lead compensation the speed of the system increases.
2. Maximum overshoot of the system decreases because of phase lead compensation.
Disadvantages Of Phase Lead Compensation:
No improvement in steady state error.
Effects Of Phase Lead Compensation:
1. Increase in phase margin.
2. Faster Response.
3. Increase in velocity constant Kv.
4.The slope of the magnitude plot reduces at the gain crossover frequency so that relative stability improves and error decrease due to error is directly proportional to the slope.
Phase Lag Compensation:
A lag network has one zero and one pole, pole must be near to the origin and also the poles and zeros must lie on the negative real axis. By seeing the pole zero plot of lag compensation network you can get a clear idea, its figure is shown below.
Pole Zero Plot Of Lag Compensation Network :
So to add a dominating pole(pole near to origin)to a control system we use phase lag compensation network. The network is shown below.
From this circuit we have,
Determining Transfer Function Of Lag Compensation Network:
Here transfer function is the ratio of output voltage to input voltage.
By applying Laplace transform on both sides of above equation we get,
here T and β are respectively the time constant and DC gain.
Substitute s = j𝛚 in the transfer function , also we have ɑ < 1 then we get transfer function as,
In order to know at what frequency we get maximum phase lead we need to differentiate the above equation and equate to zero. Now we get,
Advantages Of Phase Lag Compensation:
1.Low and high frequencies can be attenuated.
2. Increase in steady state accuracy.
Disadvantages Of Phase Lag Compensation:
Decrease in speed of system.
Effects Of Phase Lag Compensation:
1. Increase in phase margin.
2. Decrease in bandwidth.
3. Increase in gain cross over frequency.
4.Response will be slower before due to decreasing bandwidth, the rise time and the settling time become larger.
Phase Lag Lead Compensation:
By cascading both lead and lag networks we can obtain the benefits of both lead network and lag network which makes the given system to work better. Given below is circuit diagram of lead lag network.
Circuit Diagram Of Lead Lag Network:
Determining Transfer Function Of lead lag compensation network:
Here transfer function is the ratio of output voltage to input voltage.
By applying Laplace transform on both sides of above equation we get,
By substituting,
in above equation we get,
Pole Zero Plot Of Lead Lag Network:
In the lag-lead compensation pole is more dominating than the zero and because of this lag-lead network may introduces positive phase angle to the system when connected in series.
Advantages Of Phase Lead Lag Compensation:
1. Improved accuracy.
2. Speed of the system increases because it shifts gain crossover frequency to a higher value.
In this way Lead Lag compensation networks are used in order to increase the performance of a system.
In this post we have learnt about lag lead compensators in control system.
To download this post on lag lead compensators in control system as Pdf click here.
June 14, 2017 at 11:05PM by EEE, ADBU
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