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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