Bouchloz Relay

• INTRODUCTION

Power Transformers are considered to  be  a  highly  reliable  type  of equipment, yet, in order to ensure the continuity  of  service  that  modern conditions  demand,  protective devices are required. The purpose of such devices is to disconnect faulty apparatus before large-scale damage is caused by a fault to the apparatus or  to  other  connected  apparatus. Such devices generally respond to a change  in  the  current  or  pressure arising from the faults and are used for  either  signaling  or  tripping  the circuits. 

Protective devices in the ideal case must be sensitive to all faults, simple in operation, robust for service and economically  feasible.  Considering liquid  immersed  transformers,  a near-ideal  'protective  device'  is available in the form of Gas and Oil relay  described  here.  The  relay operates on the well-known fact that almost every type of electric fault in a 'liquid  immersed  transformer'  gives rise to gas. This gas is collected in the body of the relay and is used in some way or other to cause the alarm or the tripping circuit to operate. 

The principle of the Gas and Oil relay was first successfully demonstrated and utilized by 'Buchholz' many years back.  In  a  series  of  experiments carried out extensively in Germany it was proved that the Relay is capable of  bringing  to  light  incipient  fault thereby preventing further spreading of the fault and extensive damage and thus saving expensive and protracted repairs. So successful is the principle of  this  Relay  that  despite  the continued search for better protective devices in other electrical fields the Gas-and-Oil Relay is still on its own in providing protection against a variety of faults.

• TYPES OF BOUCHLOZ RELAY

1.  Buchholz Relay with Mercury Switches
2.  Buchholz Relay with Magnetic Switches

• Bouchloz Relay with Mercury Switches

1. Uses mercury in switches which is toxic and also a carcinogen
   substance. Mercury is now prohibited in most parts of the world.
2. Relays with mercury switches are not accepted internationally by
   utilities and OEMs in most countries of the world.
3. Huge variation in gas volume and surge velocity readings from
   one relay to another.
4. Mercury  susceptible  to  oxidation's  resulting  in  no/false  signal
   upon prolonged use.
5. Switch activated by flow of mercury.
6. UN-branded locally made mercury switches prone to rejections in
   incoming, process and final inspection as well as transit.
7. May maloperate in one or more of the following conditions :a. External shocks to a transformer resulting in vibration.b.  Turbulence of oil due to starting of pump in forced cooled
   transformer.c.  Variation in angle of mounting of the Relay.d.  Earthquake of minor intensity.

• Buchholz Relay with Magnetic Switches

1. No use of mercury.
2. Consistent readings of gas volume and surge velocity.
3.  No affect of ageing
4. Switch activated by a magnet.
5. Branded  switches  imported  from  USA/Japan  are  free  from
   rejection in all stages.
6. Worldwide acceptance:
   a. Immune to such vibrations.
   b.  Highly stable and resistant and will not operate due to oil
   pump operation.
   c.  Immune  to  variations  of  angle  as  experienced  in
   transformer mounting.
   d.  Vibration proof to 6g accelerations.

• OPERATION

The  function  of  a  double  element relay will be described here. During normal operation of a transformer the Buchholz  relay  is  completely  filled with oil. Buoyancy and t he moment due t o counterweights keep the floats in their original top positions. In the event of some fault in the interior of the transformer tank, gas bubbles are produced  which  accumulate  in  the Buchholz  relay  on  the  way  to  the conservator. In consequence, the oil level  in  the  relay  enclosure  drops which  in  turn  lowers  the  upper bucket. 

This causes the magnetic switch to operate an alarm signal. The lower bucket does not change its position,  because  when  the  gas reaches the upper inside wall of the pipe  it  can  escape  into  the conservator. Hence, minor fault in the transformer tank will not trigger the lower  switching  assembly  and  will not trip the transformer. 

In case the liquid continues to drop due to loss of oil, the lower bucket also goes down. In consequence, the lower  switching  system  operates  if the level of oil goes below the bottom level  of  the  pipe  connected  to  the relay. 

Alternately in the event the liquid flow exceeds  a  specific  value  (which  is continuously adjustable, by means of a  flap)  the  lower  bucket  is  forced down,  thus  triggering  the  lower switching system to operate.

As the liquid flow rate decreases, or the level of the liquid rises, the bucket returns  to  its  original  position.  The single  element  relay  has  only  Trip element  and  it  responds  to  only  oil surges.  The  method  of  operation  is similar  to  that  described  for  double element relay. Single element relays are suitable for potential transformers and on load lap changers. 

The single element oil Surge relay has been  specifically  designed  for  use with on load tap change equipment and it will by-pass normal amounts of gas  which  are  generated  by  tap change  operations  and  will  only respond to oil surges and loss of oil. 

• APPLICATIONS

Double element relays can be used in detecting minor or major faults in a transformer.  The  alarm  element  will operate, after a specified volume of gas  has  collected  to  give  an  alarm indication.  Examples  of  incipient faults are :

1.  Broken-down core bolt insulation
2. Shorted lamination
3. Bad contacts
4. Overheating of part of winding's

The alarm element will also operate in the event of oil leakage, or if air gets into the oil system. 

The trip element will be operated by an  oil  surge  in  the  event  of  more serious faults such as :

1. Earth faults
2. Winding short circuits
3.  Puncture of bushings
4. Short circuit between phases

The trip element will also be operated if  a  rapid  loss  of  oil  occurs.  Single element relays can be used to detect either incipient or major faults in oil filled  potential  transformers, reactors,  capacitors  etc.  A  special single  element  relay  is  available  for the protection of on load tap-change equipment. 

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Thyristor

Thyristors, or silicon-controlled rectifiers (SCRs) have been the traditional workhorses for bulk power conversion and control in industry. The modern era of solid-state power electronics started due to the introduction of this device in the late 1950s.

Volt-Ampere Characteristics

The figure shows the thyristor symbol and its volt-ampere characteristics. Basically, it is a three-junction P-N-P-N device, where P-N-P-N and N-P-N component transistors are connected in regenerative feedback mode. The device blocks voltage in both the forward and reverse directions. When the anode is positive, the device can be triggered into conduction by a short positive gate current pulse; but once the device is conducting , the gate losses its control to turn off the device. A thyristor can also turn on by excessive anode voltage. Its rate of rise (dv/dt), by a rise in junction temperature ( TJ ), or by light shining on the junctions. 

The volt-ampere characteristics of the device indicate that at gate current IG = 0 , if forward voltage is applied on the device, there will be a leakage current due to blocking of the middle junction. If the voltage exceeds a critical limit (break over voltage), the device switches into conduction. With increasing magnitude of IG, the forward break over voltage is reduced. And eventually at IG3 , the device behaves like a diode with the entire forward blocking region removed. The device will turn on successfully if a minimum current, called a latching current, is maintained. During conduction, if the gate current is zero and the anode current falls below a critical limit, called the holding current, the device reverts to the forward blocking state. With reverse voltage, the end P-N junctions of the device become reverse-biased and the V-I curve becomes essentially similar to that of a diode rectifier. Modern thyristors are available with very large voltage (Several KV) and current (Several KA) ratings.

Switching Characteristics

Initially, when forward voltage is applied across a device, the off-state, or static (dv/dt), must  be limited so that it does not switch on spuriously. The (dv/dt) creates displacement current in the depletion layer capacitance of the middle junction, which induces emitter current in the component transistors and causes Switching action. When the device turns on, the anode current (di/dt) can be excessive, which can destroy the device by heavy current concentration. During conduction, the inner P-N regions remain heavily saturated with minority carries and the phenomena are similar to that of a diode. However, when the recovery current goes to zero, the middle junction still remains forward-biased. This junction eventually blocks with an additional delay when the minority carries die by the recommendation process. The forward voltage can then be applied successfully, but the reapplied (dv/dt) will be somewhat less than the static (dv/dt) because of the presence of minority carriers. For example, POWEREX SCR/diode module CM4208A2 (800 V , 25 A) has limiting (di/dt)=100 A/m

and off-state dv/dt =500 V/ parameters. A suitably-designed snubber circuit (discussed later) can limit di/dt and dv/dt within acceptable limits. In a converter circuit, a thyristor can be turned off (or commutated) by a segment of reverse AC line or load voltage (defined as line or load commutation, respectively), or by an inductance capacitance circuit-induced transient reverse voltage (defined as forced commutation).

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How to choose , use and calculations of the Flywheels !

 

 

 Load Equalization :

If the load fluctuates between wide limits in space of few seconds, then large peak demands of current will be taken from supply and produce

heavy voltage drops in the system. Large size of conductor is also required for this Process of smoothing out these fluctuating
loads is commonly referred to as load equalization and involves storage of energy during light load periods which can be given out during the peak load period, so that demand from supply is approximately constant. Tariff is also affected as it is based on M.D. (Maximum Demand) For example, in steel rolling mill, when the billet is in between the rolls it is a peak load period and when it comes out it is a light load period, when the motor has to supply only the friction and internal losses, as shown in figure

 

 Use Of Flywheels :

 

The method of Load Equalization most commonly employed is by means of a flywheel. During
peak load period, the flywheel decelerates and gives up its stored kinetic energy, thus reducing the
load demanded from the supply. During light load periods, energy is taken from supply to accelerate
flywheel, and replenish its stored energy ready for the next peak. Flywheel is mounted on the motor
shaft near the motor. The motor must have drooping speed characteristics, that is, there should be a
drop in speed as the load comes to enable flywheel to give up its stored energy. When the Ward -Leonard system is used with a flywheel, then it is called as Ward - Leonard Aligner control.

There are two choices left for selecting a flywheel to give up its maximum stored energy:

 

1.  Large drop in speed and small flywheel (But with this the quality of production will suffer, since a speed drop of 10 to 15% for maximum load is usually employed).
2.  Small drop in speed and large flywheel. (This is expensive and creates additional friction losses. Also design of shaft and bearing of motor is to be modified.) So compromise is made between the two and a proper flywheel is chosen.

 

Flywheel Calculations :

The behavior of flywheel may be determined as follows 

 Fly wheel Decelerating :- (or Load increasing)

Let :

TL : Load torque assumed constant during the time for which load is applied in (Kg.m)

Tf  : Torque supplied by freewheel in (Kg.m)

To : Torque required on no load to overcome friction internal losses in (Kg.m)

Tm : Torque supplied by the motor at any instant in (Kg.m)

ωo  : No load speed of the motor in (rad/sec)

ω : Speed of motor at any instant in (rad/sec)

s : Motor slip speed ( ωo-ω) in (rad/sec)

I : Moment of inertia of flywheel in (Kg.m2)

g : Acceleration due to gravity in (m/sec2)

t : Time in (sec)

When the flywheel decelerates, it gives up its stored energy. 

Tm = TL - Tf  ......... Equation (1)

Energy stored by flywheel when running at speed ‘ω’ is 1/2 Iω2/g.

If speed is reduced from ω0 to ω. 

The energy given up by flywheel is

= (I/2g)*[(ωo)² - (ω)²]   ......... Equation (2)

(ωo +ω)/2 : Mean speed . Asumming speed drop of not than 10% , this may be assumed equal to ω .
(ωo +ω)/2 = ω 
Also   (ωo - ω)= s 
From equation (2),energy given up = (I/g).ω.s
Power given up = (I/g).ω.(ds/dt)
But the torque = ( Power/ω )
Torque supplied by freewheel
 Tf = (I/g)*(ds/dt)
From equation (1), Tm = TL - (I/g)*(ds/dt)
For values of slip speed up to 10% of No-load speed,slip is proportional to torque or , 
S = K * Tm
Tm = TL - (I/g) * K *(dTm/dt)

This equation is similar to the equation for heating of the motor 

(TL - Tm) = (I/g) * K * (dTm/dt) 

g*(dt/IK) = dTm / (TL - Tm )

By integrating both sides 

ln (TL - Tm) = [(t*g)/(I*K)] + C1     ............ Equation (3)

At t = 0 , When load starts increasing from no load

 i.e. Tm = To

Hence, at t = 0       Tm = To 

C1 = - ln ( TL - To )

By substituting the value of C1 above, in equation (3) in ( TL - Tm ) = ( t*g)/(I*K) - ln ( TL - To )

ln [(TL - Tm)/(TL - To)] = - (t*g)/(I*K)

Tm = TL - (TL - To)* e^[( - t*g)/(I*K)]

If the load torque falls to zero between each rolling period, then Tm = TL - [1 - e^(- t*g)/(I*K)] , so  ( To = 0 )

Load Removed (Flywheel Accelerating)

Slip speed is decreasing and therefore ( ds/dt) is negative .

Tm = To + Tf  = To - [(I/g)*(ds/dt)]

To - Tm =(I/g)*K*(dTm/dt) 

(g*dt)/(I*K) = dTm / (To - Tm) 

After integrating both sides, 

ln (To - Tm) = [(t*g)/(I*K)] + C

At t = 0 , Tm = Tm'  motor torque at the instant, when load is removed .

C = - ln ( To - Tm' ) putting this value of C in the above equation 

ln (To - Tm) = [(t*g)/(I*K)] - ln(To - Tm')

ln [(To - Tm)/(To - Tm') = (- t*g)/(I*K)

To - Tm = (To - Tm')* e^[(-t*g)/(I*K)

Tm = To + (Tm' - To) * e^[(-t*g)/(I*K)]

Where Tm' = the motor torqur, at the instant the load is removed .

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Size and Rating of Motors !

The factors which govern the size and rating of motor for any particular service are its maximum temperature rise under given load conditions and the maximum torque required. It is found that a motor which is satisfactory from the point of view of maximum temperature rise usually satisfies the requirement of maximum torque as well. For class-A insulation, maximum permissible temperature rise is 40 °C whereas for class-B insulation, it is 50 °C. This temperature rise depends on whether the motor has to run continuously, intermittently or on variable load.

Different ratings for electrical motors are as under :

• Continuous Rating

It is based on the maximum load which a motor can deliver for an indefinite period without its temperature exceeding the specified limits and also possessing the ability to take 25 % overload for a period of time not exceeding two hours under the same conditions.

For example, if a motor is rated continuous 10 KW, it means that it is capable of giving an output of 10 KW continuously for an indefinite period of time and 12.5 KW for a period of two hours without its temperature exceeding the specified limits.

• Continuous Maximum Rating 

It is the load capacity as given above but without overload capacity. Hence, these motors are a little bit inferior to the continuous-rated motors.

• Intermittent Rating 

It is based on the output which a motor  can deliver for a specified period, say one hour or 1/2 hour or 1/4 hour without exceeding the temperature rise. 

This rating indicates the maximum load of the motor for the specified time followed by a no-load period during which the machine cools down to its original temperature.

Estimation Of Motor Rating

Since primary limitation for the operation of an electric motor is its temperature rise, hence motor rating is calculated on the basis of its average temperature rise. The average temperature rise depends on the average heating which itself is proportional to the square of the current and the time for which the load persists.

For  example, if a motor carries a load L1 for time t1 and L2  for time t2 and so on, then

In fact, heating is proportional to square of the current but since load can be expressed in terms of the current drawn, the proportionality can be taken for load instead of the current.

Generally, load on a motor is expressed by its load cycle. Usually, there are periods of no-load in the cycle. When motor runs on no-load, heat generated is small although heat dissipation continues at the same rate as long as the machine is running. Hence, there is a difference in the heating of a motor running at no-load and when at rest. It is commonly followed practice in America to consider the period at rest as one-third while calculating the size of motor. It results in giving a higher motor rating which is advantageous and safe.

Example : An electric motor operates at full-load of 100 KW for 10 minutes, at 3/4 full load for the next 10 minutes and at 1/2 load for next 20 minutes, no-load for the next 20 minutes and this cycle repeats continuously. Find the continuous rating of the suitable motor.

According to American practice, we will consider the period of rest as (20/3) minutes. In that case, the motors size is

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Changing the speed of induction motor 1

Protection System

The power system protection ?!

It is the process of making the generator,transformer and consumption of electrical energy as safe as possible from the effect of failures and events that make the power  system at risk .

Faults ?!
Any abnormal change of the voltage and current values

The main causes of faults ?!

1. Change of insulating material 
2. Due to humidity conditions 
3. Due to falls down of lines 
4. Due to open circuit 
5. Power swing 

 Classification of faults referring to fault period ?!

1. Permanent 
2. Transient 

Needs for protection ?!

1. Maintain acceptable operation 24 hr a day 
2. Protect the public 
3. Keep voltage and current and frequency within certain limits
4. Improve system stability 
5. Minimize damage for equipment

Components to be protected ?!

1. Generators 
2. Transformers
3. Buses 
4. Transmission lines
5. Loads or motors

Components of power system protection ?!

1. Current transformer  (C.T)
2. Voltage transformer (V.T)
3. Relay 
4. Circuit Breaker (C.B)  
5. Fuse
6.  Dc supply

Note :
Protection system not prevent fault to happen but to protect system when fault happen.

Classification of protection system ?!

1. Unit protection : That protect a define zone ,as ( Deferential relay , Restricted earth fault relay  )
2. Non unit protection : That protect all Transmission line (undefined zone) ,as (Fuse , Over current relay )

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Introduction to change of speed of Induction Motor

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