Sunday, 30 October 2016

Difference between Cylindrical and Salient Pole Rotor Synchronous Generator

We are quite familiar with Synchronous Generator or often called Alternator. There are mainly two types of Synchronous Generator from rotor construction point of view. The two main types of synchronous machine are Cylindrical Rotor and Salient Pole. Saliency simply means projection outward. Therefore from the literal meaning of Saliency one can guess that Salient Pole machines must have poles projecting outward. In general, the Cylindrical Rotor type machines are confined to 2 and 4 pole turbine generators, while salient pole types are built with 4 poles upwards and include most classes of duty. Both classes of machine are similar in Stator construction point of view. Each has a stator carrying a three-phase winding distributed over its inner periphery.

Within the stator bore is the rotor which is magnetised by a winding carrying DC current. The main difference between the Cylindrical Rotor and Salient Pole classes of machine lies in the rotor construction. The cylindrical rotor type has a uniformly cylindrical rotor that carries its excitation winding distributed over a number of slots around its periphery. This construction is unsuited to multi-polar machines but it is very sound mechanically. Hence it is particularly well adapted for the highest speed electrical machines and is universally employed for 2 pole units, and some 4 pole units.

The salient pole type has poles that are physically separate, each carrying a concentrated excitation winding. This type of construction is in many ways complementary to that of the cylindrical rotor and is employed in machines having 4 poles or more. Except in special cases its use is exclusive in machines having more than 6 poles. Two and four pole generators are most often used in applications where steam or gas turbines are used as the prime mover. This is because the steam turbine tends to be suited to high rotational speeds. Four pole steam turbine generators are most often found in nuclear power stations as the relative wetness of the steam makes the high rotational speed of a two-pole design unsuitable.

Most generators with gas turbine prime mover are four pole machines to obtain enhanced mechanical strength in the rotor since a gearbox is often used to couple the power turbine to the generator; the choice of synchronous speed of the generator is not subject to the same constraints as with steam turbines. Generators with diesel engine drivers are invariably of four or more pole design, to match the running speed of the driver without using a gearbox. Four-stroke diesel engines usually have a higher running speed than two stroke engines, so generators having four or six poles are most common. Two stroke diesel engines are often derivatives of marine designs with relatively large outputs and may have running speeds of the order of 125 rpm. This requires a generator with a large number of poles (48 for a 125 rpm, 50Hz generator) and consequently is of large diameter and short axial length. This is a contrast to turbine-driven machines that are of small diameter and long axial length.

Further it shall be noted that in Hydro Power plant, Salient Pole Generators are used as the speed of the prime mover (here water turbine is the prime mover) is less and therefore number of poles required will be more from P = 120f / N. To accommodate larger number of poles, Salient Pole construction is well suited.

It shall also be noted that Salient Pole machines cannot be used at higher speed because at higher speed the centrifugal forces will be large which may damage the Salient pole bolted on the Rotor core. This is why Turbo Generators uses Cylindrical Pole construction whereas Hydro generators use Salient Pole construction.

To summarize, the main differences between the Cylindrical Rotor and Salient Pole machines are as follows:

In salient pole type of rotor consist of large number of projected poles i.e. salient poles mounted on a magnetic wheel. Construction of a salient pole rotor is as shown in the figure at left. The projected poles are made up from laminations of steel. The rotor winding is provided on these poles and it is supported by pole shoes.

  • Salient pole rotors have large diameter and shorter axial length.
  • They are generally used in lower speed electrical machines, say 100 RPM to 1500 RPM.
  • As the rotor speed is lower, more number of poles is required to attain the required frequency. (N = 120f / P). Typically number of salient poles is between 4 to 60.
  • Flux distribution is relatively poor than non-salient pole rotor, hence the generated emf waveform is not as good as cylindrical rotor.
  • Salient pole rotors generally need damper windings to prevent rotor oscillations during operation.
  • Salient pole synchronous generators are mostly used in hydro power plants.

Non-salient pole or Cylindrical rotors are cylindrical in shape having parallel slots on it to place rotor windings. It is made up of solid steel. Sometimes, they are also called as drum rotor.

  • They are smaller in diameter but having longer axial length.
  • Cylindrical rotors are used in high speed electrical machines, usually 1500 RPM to 3000 RPM.
  • Windage loss as well as noise is less as compared to salient pole rotors.
  • Their construction is robust as compared to salient pole rotors.
  • Number of poles is usually 2 or 4.
  • Damper windings are not needed in non-salient pole rotors.
  • Flux distribution is sinusoidal and hence gives better emf.
  • Non-salient pole rotors are used in  nuclear, gas and thermal power plants.

Transformer Physical Protections

Transformer Physical Protection refers to the Protections which used the physical quantities to protect the Transformer. Apart from electrical protections which uses the electrical quantities to judge a fault and based upon the judgment, the electrical protections of Transformer isolates the Transformer. On contrary, physical protections continuously measure the physical quantities like oil / winding temperature, gas content in the Transformer Oil etc to judge a fault condition and isolate the Transformer from the fault. There are many physical protections provided in a Transformer, they are as follows:

2)    Transformer Oil Temperature Trip, OTI

3)    Buchholz Trip

4)    Pressure Relief Device, PRD

All the above physical protections have already been discussed in earlier posts except Pressure Relief Device. Therefore we will focus here in this post on Pressure Relief Device or PRD. PRD is basically a last line of defense and it shall not normally operate if all other protection viz. electrical protections are working properly but if in case they fail then PRD shall operate to isolate the Transformer.

When it is required to limit the pressure rise inside a tank, in order to prevent an excessive mechanical stress of the walls, it is necessary to use a safety valve set at a precise overpressure value. Here Pressure Relief Devive, PRD works as a safety valve. The tank of oil immersed Transformers is usually fit with this kind of protecting device; because in case of short-circuit due to an insulation failure, the dielectric arc between alive parts vaporizes the surrounding insulating fluid which generates a quick rise of the pressure inside the tank, with the risk of permanent deformations, or, failure of the tank walls with the consequent outflow of hot oil. Due to the rapidity of this event, it is necessary to mount on the Transformer an adequate protecting device which relieves quite suddenly the excess of pressure generated inside the tank by the failure mentioned above.

To understand the working principle of Pressure Relief Device, let us first understand the construction of simple Pressure Relief Device i.e. PRD. A PRD has a shutter which can open and provide path for the outflow of Transformer Oil. The shutter is spring loaded due to which it tightly covers the Transformer Oil Tank. Apart from theses, normally two Normally Open (NO) contacts are also provided with the PRD. Under Normal operating condition these contact remain open but when PRD operates these electrical contact changes its status from NO to NC which can be used to provide alarm in the Control Room. A typical PRD is shown in figure below.

The way of working of Pressure Relief Device i.e. PRD is quite simple. If the pressure inside the Transformer tank reach the operating pressure of the PRD or if the pressure inside the Transformer tank reaches the set point for the operation of PRD, the shutter lift slowly from its rest position disjoining from the main tightening gasket; in this condition the excess of pressure can’t be released in the environment, being the shutter still in contact with the external gasket .Now the internal overpressure operates on the whole internal surface of the shutter, which is bigger than the initial surface: consequently, the strength acting on the shutter is much higher than the spring-load on the other side; this brings to a quick and high lifting of the shutter followed by an equally fast reclosure as the excess of pressure is released in a very short time through the big opening created by the shutter lifting.This way PRD helps to protect the Transformer from over-pressure.

Saturday, 29 October 2016

Working Principle of Transistor

In this post I will focus on the working principle of the Bipolar Junction Transistor assuming that you are already aware of the construction details of the BJT or simply Transistors.

The basic operation of the transistor will be described using the pnp transistor. The operation of the npn transistor is exactly the same if the roles played by the electron and hole are interchanged. In the figure below, the pnp transistor has been drawn without the base-to-collector bias. This situation is similar to that of the forward-biased diode. The depletion region has been reduced in width due to the applied bias, resulting in a heavy flow of majority carriers from the p- to the n-type material.

As obvious from the figure above, Emitter to Base junction is forward biased, therefore majority carriers i.e. holes from the Emitter side to the Base side will start flowing and hence a current will set up from Emitter to Base.

Let us now remove the Emitter to Base bias of the pnp transistor as shown in figure below. Mind the similarities between this situation and that of the reverse-biased diode. Recall that the flow of majority carriers is zero, resulting in only a minority-carrier flow, in case of reversed biased diode.

Thus for the above situation only minority carriers i.e. holes flows from the n-side to the p-side. To summarize, we can say that one p-n junction of a transistor is reverse biased, while the other is forward biased under normal operating condition.

Now we will combine the two scenarios discussed above. In figure below both biasing potentials have been applied to a pnp transistor, with the resulting majority- and minority-carrier flow indicated. Note the widths of the depletion regions in the figure, indicating clearly which junction is forward-biased and which is reverse-biased.

As indicated in figure, a large number of majority carriers i.e. holes will diffuse across the forward-biased p-n junction into the n-type material. The question then is whether these carriers will contribute directly to the base current IB or pass directly into the p-type material.

Since the sandwiched n-type material is very thin and has a low conductivity, a very small number of these carriers i.e. holes will take this path of high resistance to the base terminal. The magnitude of the base current is typically on the order of microamperes as compared to milliamperes for the emitter and collector currents. The larger number of these majority carriers i.e. holes will diffuse across the reverse-biased junction into the p-type material connected to the collector terminal as shown in the figure. The reason for the relative ease with which the majority carriers can cross the reverse-biased junction is easily understood if we consider that for the reverse-biased diode the injected majority carriers will appear as minority carriers in the n-type material. In other words, there has been an injection of minority carriers into the n-type base region material. This is the reason, holes which arrived in n-type material will not go as base current IB rather will cross reversed biased np junction to share collector current IC.

Now after understanding this much we are able to get some mathematical relationship. Applying Kirchhoff’s current law to the transistor, we obtain      

IE = IB + IC

Thus we observe that emitter current is the sum of the collector and base currents.

The collector current in turn is made up of two components i.e. the majority and minority carriers as indicated in figure above. The minority current component is called the leakage current and is given the symbol ICO (IC current with emitter terminal Open). The collector current, therefore, is determined in total by equation,

IC = ICmajority + ICOminority

For general-purpose transistors, IC is measured in milliamperes, while ICO is measured in microamperes or nanoamperes. ICO, like Is for a reverse-biased diode, is temperature sensitive and must be examined carefully when applications of wide temperature ranges are considered. It can severely affect the stability of a system at high temperature if not considered properly. 

Analysis of Clamping Circuit

The clamping network is one that clamps a signal to a different DC level. The network contains a capacitor, a diode, and a resistive element, but it can also have an independent DC supply source to introduce an additional shift in voltage level. The magnitude of R and C to be used in the Clamper Circuit must be chosen such that the time constant Ƭ = RC is large enough to ensure that the voltage across the capacitor does not discharge significantly during the interval the diode is non-conducting. In our discussion, we will assume that for all practical purposes the capacitor will fully charge or discharge in five time constants.

The network shown in figure below will clamp the input signal to the zero level for ideal diodes. The resistor R is the load resistor or a parallel combination of the load resistor and a resistor designed to provide the desired level of R as determined by Ƭ = RC.

The above circuit can be can be well understood in two cases.

Case1: When 0< t < T/2, Diode Forward Biased

For the above time period, the diode will conduct and the circuit can be represented as shown below.

As clear from the above circuit, the diode shorts the Resistor R and hence the time constant Ƭ = RC is very small determined by the lead resistance. Therefore the Capacitor C gets quickly charged. As the output voltage is appearing across the short circuit, therefore Vo = 0.

Thus for 0 < t < T/2,

Vo = 0

Case1: When T/2 < t < T, Voltage changes to -V and Diode Reversed Biased

For this time period the diode does not conduct and acts like an open circuit. For this time period, the clamper circuit can be simply represented as shown below.

In this time period, the Capacitor will start discharging and as time constant of RC network is chosen high enough, the capacitor will not discharge completely till five time constant but before completely discharging the capacitor input voltage will again change its state from –V to + V assuming T < Ƭ.

Therefore during the above period,

Vo = -V –V = -2V

Thus for T / 2 < t < T,

Vo = -2V

Combining the two cases, we can get the waveform for Clamper Circuit as shown below.

From the above waveform we see that, total swing of the output is equal to the total swing of the input signal in a Clamper Circuit.

Some helpful tips for analyzing Clamper Circuit:

1)     Start the analysis of clamping networks by considering that part of the input signal that will forward bias the diode.

2)     During the period that the diode is in the forward biased state, assume that the capacitor will charge up instantaneously to a voltage level determined by the network.

3)  Assume that during the period when the diode is in the reversed biased state the capacitor will hold on to its established voltage level.

4)    Throughout the analysis maintain a continual awareness of the location and reference polarity for Vo to ensure that the proper levels for Vo are obtained.

5)    Keep in mind the general rule that the total swing of the total output must match the swing of the input signal.

Programmable Scheme Logic (PSL) in Numerical Relays

Programmable Scheme Logic or PSL is a kind of feature provided in Numerical Relays to implement the protection scheme of a particular type. This feature of Numerical Relays makes it easier to implement many protection schemes in a single Numerical Relay for example, in Distance Relay we can configure Distance protection, over voltage protection, Over Current Protection, Earth Fault Protection etc.

Now we will study about PSL. PSL is a logical block which is made from different but suitable DDB. Here DDB stand for Digital Data Bus. There are many DDBs offered in a Numerical Relay. Each DDB perform a unique function. Thus it is very important to have the knowledge of function of DDBs to implement a particular logic. Hope you got some idea of DDB but don’t worry I will go in detail with example to make it crystal clear.

Lets us begin with an example. Let us assume that we have an Alstom Relay P442 and we want to implement a protection feature called Local Breaker Back-up (LBB) Protection in the Relay. So we need to finalize our logic for the operation of LBB. The generalized logic for LBB protection is

Lock-out Relay Operated AND Current still existing

Under the above logic the LBB Relay shall initiate its timer and shall give the tripping command to isolate the fault after a fixed time delay say 200 ms. Assuming the above logic for LBB, we will design a logic using PSL in the Numerical Relay. But before designing the PSL, we need to give the input to the Relay, in our case there are two inputs, one will be the contact of Lock-out Relay and another Current Transformer (CT) input. So our first step will be to assign the inputs to the Relay and label a name to each input. In the second step, we need to configure the output of Relay and label a name to the Relay Output contact. Let,
Lock-out Relay contact Input is labeled as INPUT1. Mind that only digital inputs can be labeled. So we can not assign a name to CT input.

Likewise let the Relay output contact to trip Breakers to isolate fault be RL1.

Thus as per our logic when INPUT1 is high and over current still exists then RL1 shall get high after 200 ms to isolate the fault.

Carefully observe the figure above. We have used digital inputs and Over-current DDB I>1 (1st Stage Over-Current) in an AND gate to start the timer and if the input status do not change for 200 ms then RL1 will change its status from low to high but in the time window of 200 ms, if the input status changes then the timer will reset and RL1 will remain low.

Therefore, making a PSL in a Numerical Relay is just a logic building while having knowledge of function of each DDB to be used. This is just a brief of PSL to have some idea of PSL used in Numerical Relay. Hope you enjoyed this post.

Friday, 28 October 2016

Why Electric Field inside a Conductor is Zero?

In this post we will discuss, why electric field inside a conductor is zero. This is very basic but important concept to understand. So we will start will zero and will move further to explain this. Let us assume that a conductor is kept in an external uniform electric field E. The direction of electric field E is shown in the figure.

Before starting the discussion, there are two points to know.

1)      Negative charge move in the direction opposite to the direction of electric field.

2)      Positive charge move in the direction of electric field.

As we know that, a conductor has a lot of mobile or free electrons, therefore when keep the conductor in an external electric field, electrons will experience a force in the direction opposite to the direction of electric field E and will start accumulating at surface A of the conductor. As electrons are moving opposite to the direction of Electric Field E, positive charge will start building at the opposite face B of the conductor. This accumulation of charge on both surface of conductor A and B, will lead to development of Electric Field E’ inside the conductor and this developed electric field E’ will oppose the flow of further electron toward face A. As the accumulation of electrons increases on the face A, the strength of electric field E’ inside the conductor will also increase and will oppose the flow of electron more strongly. But as soon as the strength of developed electric field becomes equal to the strength of external electric field E, no net electric filed will be there inside the conductor to drive the electrons and hence further accumulation of electrons will stop.

Therefore at equilibrium,

Developed Field E’ = External Field E

Therefore, net force on electrons = 0 and hence no movement of electrons.

Thus we see that at equilibrium, the strength of electric field inside the conductor is zero.

Saturday, 22 October 2016

Events resulting into Magnetizing Inrush Currents – Study of Sympathetic Inrush

I would suggest to read Transformer Inrush Current before reading this article for better understanding. Any event on the power system that causes a significant increase in the magnetizing voltage of the transformer core results in magnetizing inrush current flowing into the transformer. The three most common events are as follows:

Energization of the Transformer. This is the typical event where magnetizing inrush currents are a concern. The excitation voltage on one winding is increased from 0 to full voltage. The transformer core typically saturates, with the amount of saturation determined by transformer design, system impedance, the remnant flux in the core, and the point on the voltage wave when the transformer is energized. The current needed to supply this flux may be as much as 40 times the full load rating of the transformer, with typical value for power transformers for 2 to 6 times the full load rating. Figure below shows the waveform during energization of a transformer.

Magnetizing Inrush Current during Fault Clearing. An external fault may significantly reduce the system voltage, and therefore reduce the excitation voltage of the transformer. When this fault is cleared, the excitation voltage returns to the normal system voltage level. The return of voltage may force a dc offset on the flux linkages, resulting in magnetizing inrush current. This magnetizing inrush current will be less than that of energization, as there is no remnant flux in the core. The current measured by the differential relay will be fairly linear due to the presence of load current, and may result in low levels of second harmonic current.

Sympathetic inrush. Energizing a transformer on the power system can cause sympathetic magnetizing inrush currents to flow in an already energized parallel transformer. Energizing the second transformer causes a voltage drop across the resistance of the source line feeding the transformers. This voltage drop may cause a saturation of the already energized transformer in the negative direction. This saturation causes magnetizing inrush current to supply the flux. The magnitude of the magnetizing inrush current is generally not as severe as the other cases.

While charging a Transformer in Power System, harmonic restraining of other connected Transformers must be taken care as it may otherwise lead to the tripping of other connected Transformers.

Saturday, 15 October 2016

Impulse Voltage Tests of Circuit Breaker and Standard Impulse Waves

This test is necessary for all indoor and outdoor breakers. The test is carried out as follows: Standard impulse wave of specified amplitude is applied five times in succession.

If flash-over or puncture of insulators does not occur, the circuit-breaker is considered to have passed the test. If puncture occurs or if on two or more applied test wave flash-over occurs, the circuit breaker is considered to have failed the test. During the test some waves should be applied with reversal of polarity.

The impulse voltage wave is generated in an Impulse Voltage Generator. During the test one terminal of the impulse generator is connected to the terminal of the circuit breaker pole. The other terminal is connected to the earth and the frame of the circuit breaker.

Standard lightning Impulse is a full impulse having a front time 1.2 μsec and time to half value of 50 μsec. It is described as 1.2/50 impulse as shown in figure below.

Standard switching impulse wave is characterized by prolonged wave-front and wave tail. The typical switching impulse wave has front fen of the order of 250 μs and half time of 2500 μs. The permissible deviation in the crest vale is of the order of 4 to 12%.

The switching impulse wave has been specified for high voltage circuit-breaker rated 420 kV and above.

What is Endurance Test of Circuit Breaker?

Endurance Test of Circuit Breaker is conducted to check the healthiness of its mechanical parts i.e. operating mechanism. In this test, Circuit Breaker is operated several times and checked for any damage of its mechanical parts / contacts. The breaker should be in a position to open and close satisfactorily. 

This test is also called Mechanical Test. In mechanical tests, the circuit breaker is opened and closed several times (1000). Some operations (about 50) are conducted by energizing the relays, remaining are by closing the trip circuit by other means. Mechanical tests on high voltage AC circuit breakers are conducted without current and voltage in the main circuit. Out of the 1000 operations, about 100 operations are made by connecting the main circuit (contacts) in series with trip circuit. 

No adjustment or replacement of parts is permitted during the mechanical tests. However, lubrication is permitted as per manufacturer’s instructions.

After the tests, the contacts, linkages and all the other parts should be in good condition and should not show any permanent deformation or distortion. The dimensions should be within original limits. During repeated operations of the circuit-breaker, the weaker parts in the assembly may fail. The circuit-breaker is then considered to have failed in the mechanical test. The tests are then to be repeated after improvement in the design and manufacture. Successful performance in Mechanical Endurance Tests proves the adequacy of design and also good quality of materials and manufacture.

Though 1000 close-open cycles are specified in the standards, the manufacturer may conduct 10,000 or more operations to ascertain the reliability and for getting design data.