Types Of Relays

Types Of Relays

Here is a detailed list of the different types of relays.

1. Latching Relay

Latching relays are also called impulse relays. They work in the bistable mode, and thus have two relaxing states. They are also called keep relays or stay relays because as soon as the current towards this relay is switched off, the relay continues the process that it was doing in the last state. This can be achieved only with a solenoid which is operating in a ratchet and cam mechanism.  It can also be done by an over-centre spring mechanism or a permanent magnet mechanism in which, when the coil is kept in the relaxed point, the over-centre spring holds the armature and the contacts in the right spot. This can also be done with the help of a remanent core.

In the ratchet and cam method, power consumption occurs only for a particular time. Hence it is more advantageous than the others.

2. Reed Relay

These types of relays have been given more importance in the contacts. In order to protect them from atmospheric protection they are safely kept inside a vacuum or inert gas.  Though these types of relays have a very low switching current and voltage ratings, they are famous for their switching speeds.

3. Polarized Relay

This type of relay has been given more importance on its sensitivity. These relays have been used since the invention of telephones. They played very important roles in early telephone exchanges and also in detecting telegraphic distortion. The sensitivity of these relays are very easy to adjust as the armature of the relay is placed between the poles of a permanent magnet.

4. Buchholz Relay

This relay is actually used as a safety device. They are used for knowing the amount of gas present in large oil-filled transformers. They are designed in such a way that they produce a warning if it senses either the slow production of gas or fast production of gas in the transformer oil.

5. Overload protection Relay

As the name implies, these relays are used to prevent the electric motors from damage by over current and short circuits. For this the heating element is kept in series with the motor. Thus when over heat occurs the bi-metallic strip connected to the motor heats up and in turn releases a spring to operate the contacts of the relay.

6. Mercury Wetted Relay

This relay is almost similar to the reed relay explained earlier. The only difference is that instead of inert gases, the contacts are wetted with mercury. This makes them more position sensitive and also expensive. They have to be vertically mounted for any operation. They have very low contact resistance and so can be used for timing applications. Due to these factors, this relay is not used frequently.

7. Machine Tool Relay

This is one of the most famous industrial relay. They are mainly used for the controlling of all kinds of machines. They have a number of contacts with easily replaceable coils. This enabkes them to be easily converted from NO contact to NC contact. Many types of these relays can easily be setup in a control panel. Though they are very useful in industrial applications, the invention of PLC has made them farther away from industries.

8. Contactor Relay

This is one of the most heavy load relay ever used. They are mainly used in switching electric motors. They have a wide range of current ratings from a few amps to hundreds. The contacts of these relays are usually made with alloys containing a small percentage of silver. This is done so as to avoid the hazardous effects of arcing. These type of relays are mainly categorized in the rough use areas. So, they produce loud noises while operated and hence cannot be used in places where noise is a problem.

9. Solid State relay

SSR relays, as its name implies are designed with the help of solid state components. As they do not have any moving objects in their design they are known for their high reliability.

10. Solid State Contactor Relay

These relays combine both the features of solid state relays and contactor relays. As a result they have a number of advantages. They have a very good heat sink and can be designed for the correct on-off cycles. They are mainly controlled with the help of PLC, micro-processors or microcontrollers.

Practical Tips about Three phase Transformer

Practical Tips about Three phase Transformer:-
In a Three Phase Transformer if one HG Fuse of one phase goes off then in the remaining two phases any one phase gets divided or gets added with the blown off phase...
Then also Three phase will be available.
but
i)it won't lag by 120 degree with subsequent phase
ii)if the voltage gets divided with other phase then the input voltage will be low only the half of the original voltage.
iii)if the voltages in the good phase gets added then there will be drastically high voltage and chances of damaging the fuses or other loads connected to it.

Megger Test

One of the best methods to test electrical insulation condition is a megger test. It is well known as insulation resistance meter test.  For finding defects and punctures in insulation of things like electric wires, antenna mounts, motor winding and many more. This method of testing is used widely all over the world.

Dedicated to excellence, Megger group has manufactured number of megger test equipments till now and they are still producing innovative test equipments to give better results. Even today, their expertly designed test kits are in demand across the world. It has a great application on small scale and even in large scale industries. Megger insulation test equipments are of great capability. They are manufactured as per universal standard specification norms.

Megger test procedure

The only compulsion for this test is a high voltage. For testing, a DV voltage of up to 1000 volts is applied to the insulation. Readings are noted as variations in insulation resistance with respect to time. Readings are taken and recorded with regular intervals on 1 minute and 10 minutes and many other interval times. Megger test is mainly used for purposes like finding the quantity of moisture in insulation or winding, current leakage over wet and bad surfaces of insulation, Winding decay or defects due to long term use beyond expiry and many more.

Megger test equipments

There are number of megger test equipments and instruments which suit different criterias. For every requirement, it has a fine solution. Some of them are listed below-

Some advanced insulation testers like MIT510/2 and MIT520/2 are good at tests with wide range of measurements. They provide duel operation mode. They use reliable electric line supply and even work on rechargeable batteries. They use different test voltages like 250, 500, 1000, 2500 and 5kv. It gives perfect analytical information. They give high quality performance with safety and comfort. Moreover there megger test devices are portable with different sizes. They are easy to carry on. Megger test has great applications in different markets such as Industrial Substations, mining, airports, Petrochemical, Railways, Wind/Renewable Energy with other electric utilities and many more.

Megger tests are available with expanded line of insulation testers. Their automated standardized tests include step voltage, Dielectric Discharge tests, absorption ratio and even PI. These testers accompany with different types of test clips to meet your need. You can use large test clips for larger test areas or compact test clips for with reserved access area. For testing at low voltages specially designed control circuit test clips are there. With advanced megger test equipment, up to five different tests can be run and programmed automatically. With compatible software you can monitor results on computer as these equipments are provided with USB connectors. It makes easy to run and retrieve information, reading and discrepancies with respect to result. It also allows the hardcopies of test prints.

In short, megger test is the best option available for insulation testing at affordable costs, for all purposes and in multiple industries with complete reliability.

Resistivity
S oil Ohm-cm (Range)
Surface soils, loam, etc. 100 - 5,000
Clay 200 - 10,000
Sand and gravel 5,000 - 100,000
Surface limestone 10,000 - 1,000,000
Shales 500 - 10,000
Sandstone 2,000 - 200,000
Granites, basalts, etc. 100,000
Decomposed gneisses 5,000 - 50,000
Slates, etc. 1,000 - 10,000

Effect of Added Salt in soil.
Percent by Weight of Moisture R esistivity, (Ohm-cm)
0.0 10,700
0.1 1,800
1.0 460
5.0 190
10.0 130
20.0 100
*For sandy loam; moisture content, 15% by weight; temperature 63º F (17º C)

Temperature
C        F    R esistivity (Ohm-cm)
20 68 7,200
10 50 9,900
0 32 (water) 13,800
0 32 (ice) 30,000
-5 23 79,000
-15 14 330,000
*For sandy loam; 15.2% moisture

Vector Groups

Vector Groups
Transformer nameplates carry a vector group reference such at Yy0, Yd1, Dyn11 etc.  This relatively simple nomenclature provides important information about the way in which three phase windings are connected and any phase displacement that occurs.
Winding ConnectionsHV windings are designated:   Y, D or Z (upper case)
LV windings are designated:    y, d or z (lower case)
Where:
Y or y indicates a star connection
D or d indicates a delta connection
Z or z indicates a zigzag connection
N or n indicates that the neutral point is brought out
Phase DisplacementThe digits ( 0, 1, 11 etc) relate to the phase displacement between the HV and LV windings using a clock face notation.  The phasor representing the HV winding is taken as reference and set at 12 o'clock.  It then follows that:
Digit 0 means that the LV phasor is in phase with the HV phasor
Digit 1 that it lags by 30 degrees
Digit 11 that it leads by 30 degrees
etc
All references are taken from phase-to-neutral and assume a counter-clockwise phase rotation.  The neutral point may be real (as in a star connection) or imaginary (as in a delta connection)
When transformers are operated in parallel it is important that any phase shift is the same through each.  Paralleling typically occurs when transformers are located at one site and connected to a common busbar (banked) or located at different sites with the secondary terminals connected via distribution or transmission circuits consisting of cables and overhead lines 
Basic Theory
An ac voltage applied to a coil will induce a voltage in a second coil where the two are linked by a magnetic path.  The phase relationship of the two voltages depends upon which way round the coils are connected.  The voltages will either be in-phase or displaced by 180 deg as below:

In phase		180deg displacement

When 3 coils are used in a 3 phase transformer winding a number of options exist.  The coil voltages can be in phase or displaced as above with the coils connected in star or delta and, in the case of a star winding, have the star point (neutral) brought out to an external terminal or not.
Example -  Dyn11We now know that this transformer has a delta connected primary winding (D) a star connected secondary (y) with the star point brought out (n) and a phase shift of 30 deg leading (11).  Connections and vector diagrams are as follows::

HV
LV

Other ConfigurationsBy connecting the ends of the windings in other ways a wide range of options becomes available as set out below.

Phase shift (deg)	Connections
0	Yy0	Dd0	Dz0
30 lag	Yd1	Dy1	Yz1
60 lag	Dd2	Dz2
120 lag	Dd4	Dz4
150 lag	Yd5	Dy5	Yz5
180 lag	Yy6	Dd6	Dz6
150 lead	Yd7	Dy7	Yz7
120 lead	Dd8	Dz8
60 lead	Dd10	Dz10
30 lead	Yd11	Dy11	Yz11

Tags :vector,groups,vector groups,transformer connections,dyn11,vector connection of transformers...

Microcontroller Interfacing Techniques

Micro controller interfacing Techniques are basically classified into 

For further reading continue this link....
Micro Controller Interfacing Techniques.pdf

Password for download is:sidhumms



Tags: Microcontrollers,interfacing micro controllers,ethernet interfacing of micro controllers,RS232 interfacing of micro controllers,serial interfacing of micro controllers,SPI interfacing of micro controllers,I2C interfacing of micro controllers.

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DIFFERENTIAL Relay

DIFFERENTIAL Relay
It is a relay that checks for current balance between the primary and the secondary side of a transformer.it is also used in other components of the power system like to protect cables.it is also known as a unit protection as it does not discriminate with any other protection schemes.the ct secondary currents that circulates in the coil of this relay from primary and secondary of the transformer cancels each other when the system is healthy.when the fault occurs in a system the balance is disturbed and the resultant current activate the relay and cause trip.
These relay are used in trnasformers between the primary and secondary of the transformer .
If there is any difference the Primary parameters and secondary parameters immediately the incoming breaker to the transformers is tripped and alarm is sent to panel.
For example,
The input power and the output power of the transformer should be constant.

If there any change between the primary and secondary currents or voltage or power factor there relay gets activated and trips the Breaker.

DIFFERENTIAL PROTECTION


Differential protection is a very reliable method of protecting generators, transformers, buses, and transmission lines from the effects of internal faults.

Figure: Differential Protection of a Generator
In a differential protection scheme in the above figure, currents on both sides of the equipment are compared. The figure shows the connection only for one phase, but a similar connection is usually used in each phase of the protected equipment. Under normal conditions, or for a fault outside of the protected zone, current I₁ is equal to current I₂ . Therefore the currents in the current transformers secondaries are also equal, i.e. i₁ = i₂ and no current flows through the current relay.
If a fault develops inside of the protected zone, currents I₁ and I₂ are no longer equal, therefore i₁ and i₂ are not equal and there is  a current flowing through the current relay.

Differential Protection of a Station Bus

The principle of the differential protection of a station bus is the same as for generators.

The sum of all currents entering and leaving the bus must be equal to zero under normal conditions or if the fault is outside of the protected zone. If there is a fault on the bus, there will be a net flow of current to the bus and the differential relay will operate.

Figure: Single Line Diagram of Bus Differential Protection

Percentage Differential Relays
The disadvantage of the current differential protection is that current transformers must be identical, otherwise there will be current flowing through the current relays for faults outside of the protected zone or even under normal conditions. Sensitivity to the differential current due to the current transformer errors is reduced by percentage differential relays.

Figure: Percentage Differential Relay

In percentage differential relays, the current from each current transformer flows through a restraint coil. The purpose of the restraint coil is to prevent undesired relay operation due to current transformer errors. The operating coil current | i₁ - i₂ |  required for tripping is a percentage of the average current through the restraint coils. It is given by

where k is the proportion of the operating coil current to the restraint coil current. For example if k = 0.1, the operating coil current must be more than 10% of the average restraint coil current in order for the relay to operate.

Differential Protection of Three Phase Transformers

Differential protection of three phase transformers must take into account the change in magnitude and phase angle of the transformed current.

Transformers Connected Y-Y or Delta-Delta
In these two connections, the primary and secondary currents are in phase, but their magnitudes are different. The difference in the current magnitude must be balanced out by the current transformer ratios.

Figure: Differential Protection for a Y-Y Connected Transformer
If the transformer ratio is
 
The secondary currents of the current transformers are

During normal operating conditions or when the fault is outside of the protection zone,
 

Therefore, the ratios of the current transformers on the two sides of the power transformer must be 

.

Sometimes standard current transformers with the ratios that satisfy the above equation are not available. In that case auxiliary transformers between one of the current transformers and the relay are used.

 

Transformers Connected Y-D or D -Y.

The primary and secondary currents have different magnitudes and they also have 30° phase shift. Both, the magnitude and the phase shift must be balanced by appropriate ratio and connection of the current transformers. The phase shift on a Y-D bank is corrected by connecting the C.T.’s on the D in Y, and on the Y side in D .
Refer to the following drawing. The full load current on the 66 kV side is

The full load current on the 230 kV side is

The secondary currents in the current transformers on the 66 kV side then are

The magnitude of the currents coming out of the differential relay should be the same
 
From that, the current in the D arms of the D connected C.T.’s should be
 
Ideally, the CTR on the 230 kV side of the transformer should be

The closest to that is the ratio
 
which is the ratio that will be used.. Using this ratio, the secondary current of the current transformers on the 230 kV side is

The current through the operating coil of the differential relay is then

The average current through the current restraint coil is
 
From that, the current through the operating coil as a percentage of the restraint current under normal full load conditions is

The percentage differential relays have settings for the allowable percentage difference. Examples of the percentage values are 15%, 30%, 40%, etc. Any of these relays could accommodate the 0.46% operating coil current without operating.

Connection of Differential Relays to a D -Y Connected Transformer.

Another problem that the differential relays used for transformer protection must overcome is the magnetizing inrush current.
The inrush current occurs when a transformer is being energized. Since during the energization of the transformer there is only current in and no current out, the inrush current appears to the differential relays as an internal fault. The inrush current has some characteristic properties. Its magnitude may be as high as sixteen times the full load current. It decays very slowly - from around ten cycles for small units to 1 minute for  large units. The harmonic content of the inrush current is different from normal load current and from fault currents. A typical waveform of inrush current has a large fundamental frequency component, a significant d.c. component, and 2nd and 3rd harmonic components. The 2nd harmonic component does not appear in the transformers under any other conditions except during energization. Desensitizing of the differential relay to the inrush current involves the use of the second harmonic component to restrain the relay from operating.

(a)
Sidharthan G
electricalmiracles.
(b)
Figure: Harmonic Restraint Circuit: (a) connection to current transformer   (b) tripping circuitss
Tags: relay,differential relay,transformer protection,generator protection.