SAG in electrical power transmission.

What is  Sag.?

In electrical power transmission and mechanical design of overhead transmission line.

SAG.

A perfectly flexible wire of uniform cross-section, when string between the two supports at the same level, will form a catenary. However, if the sag is very small compared to the span, its shape approximation a parabola.

 The difference in level between the point of support and the lowest point on the conductor is known as sag 

The factors affecting the sag in an overhead line are given below.

1. Weight of the Conductor,

 This affect the sag directly. Heavier the conductor, greater will be the sag. In locations where ice formation takes place on the conductor, this will also cause increase in the sag.

2. Length Of the Span.

This also affect the sag. Sag is directly proportional to the square of the span length Hence other conditions, such as type of conductor, working tension, temperature etc. remaining the same a section with longer span will have much greater sag.

3. Working Tensile Strength.

The sag is inversely proportional to the working tensile strength of conductor if other conditions such as temperature, length of span remain the same. Working tensile strength of the conductor is determined by multiplying the ultimate stress and area of cross section and dividing by a factor of safety.

4. Temperature.

All metallic bodies expand with the rise in temperature and, therefore. The length of the conductor increases with the rise in temperature, and so does the sag.

Reference from .

Transmission and distribution of electrical power by-J.B.Gupta.

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Cooling Methods of a Transformer

Why it is needed to cool down the transformer?

No transformer is truly an 'ideal transformer' and hence each will incur some losses, most of which get converted into heat. If this heat is not dissipated properly, the excess temperature in transformer may cause serious problems like insulation failure. It is obvious that transformer needs a cooling system.

Transformers can be divided in two types as

1.               Dry type transformers.
2.               Oil immersed transformers.

Different cooling methods of transformers are

 For dry type transformers

• Air Natural (AN)

• Air Blast

For oil immersed Transformers

• Oil Natural Air Natural (ONAN)

• Oil Natural Air Forced (ONAF)

• Oil Forced Air Forced (OFAF)

• Oil Forced Water Forced (OFWF)

Cooling Methods for Dry Type Transformers

Air Natural or Self Air Cooled Transformer:  This method of transformer cooling is generally used in small transformers (upto 3 MVA). In this method the transformer is allowed to cool by natural air flow surrounding it.

Air Blast:   For transformers rated more than 3 MVA, cooling by natural air method is inadequate. In this method, air is forced on the core and windings with the help of fans or blowers. The air supply must be filtered to prevent the accumulation of dust particles in ventilation ducts. This method can be used for transformers upto 15 MVA.

Cooling Methods for Oil Immersed Transformers

Oil Natural Air Natural (ONAN):    This method is used for oil immersed transformers. In this method, the heat generated in the core and winding is transferred to the oil. According to the principle of convection, the heated oil flows in the upward direction and then in the radiator. The vacant place is filled up by cooled oil from the radiator. The heat from the oil will dissipate in the atmosphere due to the natural air flow around the transformer. In this way, the oil in transformer keeps circulating due to natural convection and dissipating heat in atmosphere due to natural conduction. This method can be used for transformers upto about 30 MVA.

Oil Natural Air Forced (ONAF):   The heat dissipation can be improved further by applying forced air on the dissipating surface. Forced air provides faster heat dissipation than natural air flow. In this method, fans are mounted near the radiator and may be provided with an automatic starting arrangement, which turns on when temperature increases beyond certain value. This transformer cooling method is generally used for large transformers upto about 60 MVA.

Oil Forced Air Forced (OFAF):   In this method, oil is circulated with the help of a pump. The oil circulation is forced through the heat exchangers. Then compressed air is forced to flow on the heat exchanger with the help of fans. The heat exchangers may be mounted separately from the transformer tank and connected through pipes at top and bottom as shown in the figure. This type of cooling is provided for higher rating transformers at substations or power stations.

Oil Forced Water Forced (OFWF):   This method is similar to OFAF method, but here forced water flow is used to dissipate hear from the heat exchangers. The oil is forced to flow through the heat exchanger with the help of a pump, where the heat is dissipated in the water which is also forced to flow. The heated water is taken away to cool in separate coolers. This type of cooling is used in very large transformers having rating of several hundred MVA.

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WHAT IS REAL POWER IN ELECTRIC ENGINEERING

What Is Real Power?

Real Power: (P)

Real Power is the actual power which is really transferred to the load such as transformer, induction motors, generators etc. and dissipated in the circuit.

Alternative words used for Real Power (Actual Power, True Power, Watt-full Power, Useful Power, Real Power, and Active Power) and denoted by (P) and measured in units of Watts (W) 

i.e. The unit of Active or Real power is Watt where 1W = 1V x 1 A

Real Power in DC Circuits:

In DC Circuits, power supply to the DC load is simply the product of Voltage across the load and Current flowing through it.

i.e., P = V I because in DC Circuits, there is no concept of phase angle between current and voltage.

In other words, there is no frequency (f) or Power factor in DC Circuits.

Real Power in AC Circuits:

But the situation in Sinusoidal or AC Circuits is more complex because of phase difference (θ) between Current and Voltage. 

Therefore average value of power (Real Power) is P = VI Cosθ is in fact supplied to the load.

In AC circuits, When circuit is pure resistive, then the same formula used for power as used in DC as P = VI

Real Power Formulas:

• P = V I  (In DC circuits)

• P = VI Cosθ    (in Single phase AC Circuits)

• P = √3 VL IL Cosθ   or       (in Three Phase AC Circuits)
• P = 3 VPh IPh Cosθ

• P = √ (S2 – Q2) or

• P =√ (VA2 – VAR2) or

Real or True power = √ (Apparent Power2– Reactive Power2)
 or
• kW = √ (kVA2 – kVAR2)

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What is RCCB and Why it is needed ?

Residual Current Circuit Breaker (RCCB)

 Much Needed Introduction....

A Residual Current Circuit Breakers is another different class of Circuit Breakers. A Residual Current Circuit Breaker (RCCB) is essentially a current sensing device used to protect a low voltage circuit in case of a fault. It contains a switch device that switches off whenever a fault occurs in the connected circuit.

Why needed  RCCB..?

Residual Current Circuit Breakers are aimed at protecting an individual from the risks of electrical shocks, electrocution and fires that are caused due to faulty wiring or earth faults.

RCCB is particularly useful in situations where there is a sudden earth fault occurring in the circuit.

e.g. A person accidentally comes in contact with an open live wire in the circuit.

In such situation, in absence of an RCCB in the circuit, an earth fault may occur and the person is at the risk of receiving an electrical shock.

However, if the same circuit is protected with RCCB, it will trip the circuit in fraction of a second thus preventing the person from receiving an electrical shock. Therefore, it is a good and safe practice to install RCCB in your electrical circuit.

Variants of RCCBs....

2 Pole RCCB: It is used in case of a single phase supply that involves only a live and neutral wire. It is as displayed in image below. It contains two ends where the live and neutral wires are connected. A Rotary switch is used to switch the RCCB back to ON or OFF positions. A test button helps to periodically test the RCCB functionality.

4 Pole RCCB: It is used in cases of a three phase supply connection involving three phase wires and a neutral. It is as displayed in image below. It consists of two ends where the three phases and neutral wire is connected. Besides this it is similar in construction and operation as 2 Pole RCCB.

RCCBs come in different ratings like: 30mA, 100mA, 300mA

How does it Protect?

As explained above, RCCB is meant for protection from earth faults and associated risk to human life such as electrical shocks.

The underlying fundamental principle behind operation of RCCB is that in ideal situations the current flowing in to the circuit through live (hot) wire should be same as the returning current from the neutral.

In case of an earth fault, the current finds a passage to earth through accidental means (such as accidental contact with an open wire etc.). As a result the returning current from neutral is reduced. This differential in the current is also known as “Residual Current”.

RCCB is designed such way that it continuously senses and compares for difference (residual current value) in current values between the live and neutral wires. Any small change in the current value on account of such event would trigger the RCCB to trip off the circuit.

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The History of 555 Timer IC – Story of Invention by Hans Camenzind

The 555 Timer IC is one of the renowned ICs, in the electronic circles. However, its history of invention is not known to many. This article takes you on a journey of 555 timer IC from the time of its creation to the present day time.

What is a 555 timer IC?

A 555 timer IC, is a multipurpose integrated circuit chip, that finds its application in timer, oscillation and pulse generation circuits. It is one of the prominent and popular inventions of the electronic world. A monolithic timing circuit, the 555 timer, is equally reliable and cheap like op-amps working in the same areas. It is capable of producing stabilized square waveform of 50% to 100% duty ratio.

The Birth of 555 Timer IC

Hans R. Camenzind, designed the first 555 timer IC in 1971, under an American company Signetics Corporation. It is this design work of his, that is most prominent in Hans’s distinguished career in the field of Integrated Circuit technology. In the summer of 1971, first design was reviewed, that used a constant current source and had 9 pins. After the review was passed, Hans thought of a new idea of replacing the constant current source by a direct resistance. This reduced the number of pins from 9 to 8, and enabled the chip to be fit in an 8 pin package instead of a 14 pin package. This new design was passed in the review in October 1971. The IC consists of 25 transistors, 2 diodes and 15 resistors. In order to define the timings, provision to attach R and C externally is provided.

In 1972, Signetics Corp. then released its first 555 timer IC in 8 pin DIP and 8 pin TO5 metal can packages, as SE/NE555 timer and was the only commercially available timer IC at that time. Its low cost and versatility, made it an instant hit in the market. It was later on manufactured by 12 other companies and became the best selling product.

Although, there is a belief that this IC got its name from the three 5k resistors in its internal circuit, Hans R Camenzind revealed in his book, “Designing Analogue Chips”, that it was Signetics manager, Art Fury's, love for the number “555”, that led to the naming of the circuit.

The basic working tutorial of timer 555 IC has been discussed in our article “555timer- A complete guide”.

Applications of 555 Timer IC:

Through the years, electronic hobbyists and engineers have explored various areas where this IC can be used. From temperature measurement to voltage regulators to various multivibrators, this IC has found its prominent place in thousands of applications. The implementation of 555 TIMER IC depends on its operating mode. It is this versatility of 555Timer IC, that makes it useful for many applications.

Basically, a 555 timer IC has three operating modes.

Bi stable mode: The Schmitt trigger

Mono stable mode: One shot mode

Astable mode: Free running mode

555 timer as a multivibrator:

Depending on which type of multivibrator, (astable/monostable or bistable multivibrator) is to be used, the operating mode of 555 timer is selected. For example, if we want to design a monostable multivibrator, we will wire the 555 timer in the monostable mode.

These multivibrators are used in various two state devices such as relaxation oscillators, timers and flip flops.

555 timer IC as a PWM generator:

Using the variable “control” input , a 555 timer IC can be used to create a pulse width modulation (PWM) generator. The duty cycle here, depends on the analogue input voltage.

This operation of 555 timer can also be seen in Switched mode power supply (SMPS)circuit. As these SMPS circuits work on pulse width modulation (PWM) , the 555 timer turns to be the most prominent choice for the designers, as it is cheap and easy to incorporate in a circuit design. Here in these circuits, two timer ICs are used; one operates in astable mode and the other in PWM mode.

Another area where 555timer IC is implemented is in small DC-DC converter circuits. The 555 Timer when operating in astable mode, can produce a continuous stream of pulses of specified frequency. The IC output is fed to the converter to produce desired output voltages. These converter circuits have wide industrial applications.

Other circuits which use 555 timer include that of temperature measurement, moisture measurement waveform generators, various timer circuits, etc.

In recent times, the CMOS version of the IC is most commonly used. Among them, the popular ones are the ICs manufactured by MOTOROLA like MC1455. It can be directly used as a substitute to the original NE555 IC. This IC is pocket-friendly and costs around 0.28 US$.

The bipolar and the CMOS versions of 555 Timer IC

Ever since the first IC was manufactured, over 12 independent companies have fabricated the same. The original design had some design flaws like unbalanced comparators, large operating circuits, and sensitivity to temperature.

Hence, Hans R Camenzind, redesigned the existing IC to diminish the design flaws. The design of this IC was better than its original design. The improved IC was then sold by ZSCTI555 but could not create a buzz like the original 555timer IC did. Hence the original design continued to be a hit in the market.

However, the classic bipolar 555 IC version like NE555 IC, uses bipolar transistors, which dissipate large amount of power and produce high current spikes. Hence, these ICs could not be used in low power applications. This paved a way to the design of a new and version of the same, the CMOS version.

CMOS stands for “complementary metal-oxide semiconductor” and uses a combination of both n-type MOSFET (NMOS) and p-type MOSFET (PMOS), in enhancement mode. All the PMOS transistors have input from either the voltage source or from other PMOS, whereas, all the NMOS transistors have an input connected to ground or to other NMOS transistor. This composition leads to reduction in the power dissipation and lower current spikes.

An example of the CMOS version of 555 timer is LMC555 produced by texas instruments.

The Derivatives of 555 Timer IC:

Now we know what all a single 555 timer IC can do. This means a 555 timer can be used as an oscillator and as a pulse generator in the same circuit. For this purpose, numerous pin compatible derivatives, for both bipolar and CMOS versions of 555 Timer IC were produced by many companies, over the years.

The ICs are available either in round metal can package, or the more commonly seen 8 pin DIP package.

Packages – 555 Timer IC

A 14 pin package variation of 555 timer IC, called the 556 IC, was manufactured which had two 555 timer ICs in one chip. Here, the two ICs share a common ground and supply pin. The other 12 pins are allocated to the inputs and outputs of individual 555 timers.

LM556, a dual timer IC manufactured by texas instruments is one such IC. These IC s are ideal for sequential timing application.

Other derivative in 16 bit DIP package, the 558 and 559 had four ICs, in which DIS and THR were connected internally. The 558 IC , is a quad IC and is edge triggered. This eliminates the need of using coupling capacitor for sequential timing applications.

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TYPES OF BUS BAR SYSTEM

TYPES OF BUS BAR SYSTEM

1 Single Busbar System

Single busbar system is as shown below in figure 

Single Busbar System

a. Merits

1. Low Cost

2. Simple to Operate

3. Simple Protection

b. Demerits

1. Fault of bus or any circuit breaker results in shut down of entire substation.

2. Difficult to do any maintenance.

3. Bus cannot be extended without completely deenergizing substations.

c. Remarks

1. Used for distribution substations up to 33kV.

2. Not used for large substations.

3. Sectionalizing increases flexibility.

2 Main & Transfer Bus bar System

Main & Transfer Bus is as shown below in figure 

a. Merits

1. Low initial & ultimate cost

2. Any breaker can be taken out of service for maintenance.

3. Potential devices may be used on the main bus.

b. Demerits

1. Requires one extra breaker coupler.

2. Switching is somewhat complex when maintaining a breaker.

3. Fault of bus or any circuit breaker results in shutdown of entire substation.

c. Remarks

1. Used for 110kV substations where cost of duplicate bus bar system is not justified. 

3 Double Bus bar Single Breaker system

Double Bus Bar with Double Breaker is as shown below in figure 

a. Merits

1. High flexibility

2. Half of the feeders connected to each bus

b. Demerits

1. Extra bus-coupler circuit breaker necessary.

2. Bus protection scheme may cause loss of substation when it operates.

3. High exposure to bus fault.

4. Line breaker failure takes all circuits connected to the bus out of service.

5. Bus couplers failure takes entire substation out of service.      

c. Remarks

Most widely used for 66kV, 132kv, 220kV and important 11kv, 6.6kV, 3.3kV

Substations.

4 Double Bus bar with Double breaker System

 Double Bus Bar with Double breaker system is as shown below in figure 

a. Merits

1. Each has two associated breakers

2. Has flexibility in permitting feeder circuits to be connected to any bus

3. Any breaker can be taken out of service for maintenance.

4. High reliability

b. Demerits

1. Most expensive

2. Would lose half of the circuits for breaker fault if circuits are not connected to both the buses.

c. Remarks

1. Not used for usual EHV substations due to high cost.

2. Used only for very important, high power, EHV substations.

5 Double Main Bus & Transfer Busbar System

Double main bus & transfer bus system is as shown below in figure

a. Merits

1. Most flexible in operation

2. Highly reliable

3. Breaker failure on bus side breaker removes only one ckt. From service

4. All switching done with breakers

5. Simple operation, no isolator switching required

6. Either main bus can be taken out of service at any time for maintenance.

7. Bus fault does not remove any feeder from the service

b. Demerits

1. High cost due to three buses

c. Remarks

1. Preferred by some utilities for 400kV and 220kV important substations.

6 ONE & HALF BREAKER SCHEME

a. Merits

1. Flexible operation for breaker maintenance.

2. Any breaker can be removed from maintenance without interruption of load.

3. Requires 1 1/2 breaker per feeder.

4. Each circuit fed by two breakers.

5. All switching by breaker.

6. Selective tripping.

b. Demerits

1. One and half breakers per circuit, hence higher cost

2. Any breaker can be removed from maintenance without interruption of load.

c. Remarks

1. Used for 400kV & 220kV substations.

2. Preferred.

7 RING OR MESH ARRANGEMENT

a.      Merits

Bus bars gave some operational flexibility.

b.      Demerits

1. If fault occurs during bus maintenance, ring gets separated into two sections.

2. Auto-reclosing and protection complex.

3. Requires VT’s on all circuits because there is no definite voltage reference point.

4. Breaker failure during fault on one circuit causes loss of additional circuit because of breaker failure.

These VT’s may be required in all cases for synchronizing live line or voltage indication

c.       Remarks

 Most widely used for very large power stations having large no. of incoming and outgoing lines and high power transfer.

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2.Electrical Instrumentation lab manual for diploma electrical engineering 3rd sem as per GTU

PRACTICAL 2

AIM: - TEST THE LOW RESISTANCE USING KELVIN BRIDGE.

APPARATUS:-

• Regulated dc supply-1no
• Standard resistance coil-1no
• Kelvin’s double bridge kit.
• Digital multimeter-1no,
• Patch codes.

CIRCUIT DIAGRAM:-

CLICK ON GIVEN LINK

THEORY:-

               Kelvin’s bridge is a modification of whetstone’s bridge and always used in measurement of low resistance. It uses two sets of ratio arms and the four terminals resistances for the low resistance consider the ckt. As shown in fig. The first set of ratio P

And Q. The second set of ratio arms are p and q is used to connected to galvanometer to a Pt dat an Approx. potential between points m and n to eliminate the effects of connecting Lead of resistance r between the known std. resistance‘s’ and unknown resistance R .The ratio P/Q is made equal to p/q. under balanced condition there is no current flowing Through galvanometer which means voltage drop between a and b, Eab equal to the Voltage drop between a and c, Eamd.

Now

Ead=p/(p+q)

Eab=I[R+S+ ((p+q)r)/(p+q+r)]

Eamd=I[R+ p/p+q [(p+q) r/p+q+r]] 

For zero deflection->

Eac=Ead

[P/P+Q]I[R+S+ {(p+q) r/p+q+r}] =I[R+pr/p+q+r] —______________________(3)

Now, if

P/Q=p/q

Then equation… (3) Becomes

R=P/Q=S-_________ (4)

Equation (4) is the usual working equation. For the Kelvin’s Double Bridge .It indicates

The resistance of connecting lead r. It has no effect on measurement provided that the two Sets of ratio arms have equal ratios. Equation (3) is useful however as it shows the error that is introduced in case the ratios are not exactly equal. It indicates that it is desirable to keep as small as possible in order to minimize the error in case there is a diff. between

The ratio P/Q and p/q.

R=P/QS

OBSERVATION TABLE:-

Sr no.	P                         (Ratio Arm Resistor)	Q  (Ratio Arm Resistor)	S Standard Resistor	R Measured Value	R Actual

PROCEDURE:-

1) The circuit configuration on the panel is studied.

2) Supply is switched on and increased up to 5v.

3) The unknown resistance is connected as shown.

4) The value of P,Q was selected such that a. P/Q=p/q

5) S was adjusted for proper balance and balance value of s was balanced.

6) The value of known resistance was calculated.

PRECAUTIONS:-

1) Check all the connections before turning ON the power supply.

2) Do not exceed the value of 5v.

3) Note the readings accurately.

CONCLUSION:-

FOR CIRCUIT DIAGRAM 

PLEASE CLICK ON THE BELOW LINK.....

DOWNLOAD FILE for practical 2

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