Proximity Effect

Proximity Effect is a phenomenon which is observed in conductors carrying alternating current. When a conductor carries ac power, the constantly varying magnetic field induces eddy currents in the nearby conductors. In conductors where the current flows in the same direction, this results in increased current density in the nearby conductors due to the changes in the current distribution across the cross-section of the conductor. Thus the resistance of the conductor increases.

In the picture, the blue zone inducts the areas with high current density, the white zone indicates low current density caused by mutual induction.

When two conductors carrying current in the same direction are located close by, the current density on the sides of the conductor adjacent to each other will be lesser than the sides on the outside.

The reduces the net current carrying capacity of the conductor. This phenomenon is not observed when dc current flows through the conductor as there is no induction in dc.

Thus the AC resistance of a conductor may be many times the DC resistance. The AC resistance is directly proportional to the frequency of the power supply. The proximity effect is an important factor considered during the design of transformers, motors and multi-core cables.

Noise in Transformers

Transformer Noise is caused by a phenomenon called magnetostriction which occurs inside the transformers. Magnetostriction is a phenomenon by which a metallic objects experiences a distortion in its shape when it is placed inside a magnetic field. The objects can experience a change in the dimensions, expansion or contraction.

Inside a transformer, the core which is made in the form of laminated sheets also undergoes expansion and contraction due to the changing magnetic flux. This expansion and contraction occurs twice in an ac cycle. The fundamental frequency of the noise or vibration is double that of the frequency of the power supply. Thus a supply with a frequency of 50 Hz will cause noise or vibration whose fundamental frequency is 100 Hz.

In addition to the fundamental frequency, there are also harmonics whose frequencies are odd multiples of the fundamentals such as the 3rd harmonic, 5th harmonic, etc. A proper study of the noise and vibration spectrum is necessary to devise methods of reducing them.

Since, the core of the transformer is made of laminated steel sheets; these sheets experience unequal expansion and contraction when exposed to the magnetic flux. Hence, they rub against each other causing the distinct hum. The constant cyclic forces generated in the transformer core cause vibration which is carried to the different parts of the transformer body. In addition, they also cause noise. Thus when trying to reduce the hum of the transformer, both noise and vibration needs to be addressed. The noise of the transformer is measured in decibels (dB).

People can find the noise of a transformer disturbing and may oppose locating a transformer near their residence. In such circumstances, measures for reducing the impact of the sound may be explored.

Vibrations can be addressed by the fitment of supports or dampers. Noise can be reduced by mounting baffles and planning the location of the transformer.

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Substation, An introduction and types

Substations are installations in a Transmission and Distribution System which are involved in the connection of different sections of the transmission and distribution system, usually with the transfer of electrical power from one voltage level to another.

Substations play a vital role in integrating the generating, transmitting and distributing parts of an electrical system.

A substation is generally located in an open area. The substation contains of numerous equipments such as transformers, breakers, capacitors, measuring and protection devices and so on.

Based on their functions, Substations can be classified into four types

• Distribution Switching Substations
• Switching Substations
• Transmission Substation
• Customer Substation

Distribution Substations

Distribution Substations are substations which reduce the voltage from the Transmission level, say, 132 kV to a lower level such as 66kV or 33kV. The power at this reduced voltage is then supplied for domestic consumption via distribution transformers which are located in each street. These transformers then reduce the voltage to 440V and so on.

Customer Substations

These are substations that cater to only one big consumer, usually an industry. Industrial consumers though draw power directly usually at the secondary voltage level of the distribution i.e. 66kV or 33kV

Switching Substations

These substations are involved in switching. A power system may contain numerous feeders at the same voltage levels, the switching of these feeders for maintenance and the isolation of faulty feeders is vital to the reliable functioning of the system. The Switching Substations are used in merging two or three feeders into a single feeder. Switching substations contain all the components of a substation other than transformers. such as surge arrestors, current and voltage transformers, isolators,etc.

Transmission Substations

These are substations that are connect two transmission lines. The voltage levels in these substations are 66kV or higher. Usually these substations have transformers which connect two voltage levels. The substations also contain regulating equipment such as phase-shifting transformer, VAr compensators, reactors, etc. Elaborate arrangements are made for isolation of sections of the substations for maintenance in a manner that there is no interruption to the power supply.

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Solutions for Noise and Vibration in Transformers

Noise and Vibration in Transformers are undesirable aspects of Transformer Operation. Noise causes disturbance in localities where the transformer is located. Vibration in transformers can affect other components connected to it. Sometimes, excessive vibration can cause components inside the transformer to come loose.

Methods of Noise Attenuation:

Location of a transformer plays a crucial part in the level of noise. A transformer located in a corner of a building with walls on three sides will have its noise amplified.

If the transformer is to be placed in a solid mass such as concrete which cannot vibrate, a solid mounting is preferred.

If the transformer is to be mounted on other surfaces such as a structural frame, a column or a platform, flexible mounting pads would be ideal

The construction of a Wall around the Transformer can help contain the transformer noise within a small area.

Use of Double Walls: Double Walls or limp walls are arrangements which contain two glass plates between which is a viscous liquid. The viscous liquid helps in damping the noise as it passes through.

A cheaper alternative would be the construction of a screening wall made of wood or concrete which can reduce the transformer noise.

Reducing Vibrations in Transformers.

Vibration pads can be used to alleviate the issue of vibration.

All the components inside the transformer should be rigidly fixed by using spacers.

All external connections such as cables, etc should be attached by means of flexible couplings.

See Also : Noise in Transformers

Checking Lightning Arrestors

Lightning arrestors play a vital role in any substation by protecting equipment against lightning strikes and other surges. Lightning arrestors require very less maintenance and testing.

Lightning arrestors can deteriorate over a period of time due to factors such as dust, cracks, moisture ingress, degradation of the zinc oxide elements inside, etc. This can lead to failure of the lightning arrestor. When a lightning arrestor fails, it usually explodes causing a flashover and damage to the other equipment such as PTs, CTs, etc. Hence, it is imperative that the lightning arrestors in the system are kept in a healthy condition.

The usual tests carried out on Lightning arrestors are the Insulation Resistance Tests and the Hipot Test.

Harmonic Test (online test):
When the lightning arrestor is in line, a small leakage current flows through it. This current can be analysed for Harmonics. Online harmonics analysers for lightning arrestors are available. The leakage current is analysed for the presence of the 3rd Harmonic which usually indicates a failure in the near future. An arrestor thus identified can be isolated and sent for repair before any catastrophic failure can take place

The Insulation Resistance Test:
The tests are conducted with a High Voltage Meggar, usually 2500V. The value, usually in the order of megohms, is compared with the previous values and the test values of the manufacturers.

Hipot Test:
The Hipot test is conducted at about 175% of the rated voltage.

In addition to these tests, a visual inspection of the lightning arrestors for cracks, dust accumulation, broken fitments is also useful.

In the event of system overvoltages or adverse weather conditions such as thunderstorms, the lightning arrestors need to be tested more frequently

What are Surge Capacitors?

Surge capacitors are special capacitors connected to transmission lines. The function of these capacitors is to absorb the surge caused by waves which travel along the transmission lines.

The Surge Capacitor is always connected to the power supply. When the Surge appears it absorbs the surge, holds it on for sometimes and then releases it into the system.

Surge Capacitors have a very fast response time as they are continually in circuit.

However, the limitation of the Surge capacitors is that it cannot absorb a high current surge. High current surges can only be discharged by lightning arrestors.

Hence, the Surge capacitor is usually connected along with a lightning arrestor.

What are Bleeder Resistors in Capacitors

Capacitors are devices which store charge as an electrostatic field. When the supply connected to a capacitor is removed, the capacitor still retains the charge within itself.

Thus, when a capacitor is switched off, it still contains charge. Hence, an engineer working on a capacitor that has not been discharged can get an electric shock. It is, therefore, vital that all capacitors and other energy storage devices be discharged prior to service. Power capacitors usually have a resistor known as a bleeder resistor connected in parallel. The function of this resistor is to discharge the capacitor once the power supply has been removed.

These resistors are usually designed to reduce voltage across the capacitor to less than 50V (the permissible safe voltage for humans) within 5 minutes.Hence, service work in a capacitor should be started only after five minutes.

As a final precaution, the capacitors need to be discharged manually prior to starting the work.  (See article on Manual Discharge of capacitors)

What are the reasons for Bulging in Capacitors?

Bulging in capacitors is caused due to pressure inside the capacitor body. This pressure can be caused due to arcing between the capacitor plates and the resultant generation of gases.

Such capacitors need to be handled carefully. Always contact the capacitor manufacturers on handling or disposing the capacitor units.

Vents are provided in some capacitors as a defence against bulging. Vents need to be periodically checked for rupture which may indicate failure.

Transformer Classification based on the Cooling Medium

Dry - Type:

These transformers use air as the cooling medium. These transformers are generally used for indoor applications. This kind of cooling can be applied for transformers up to 20 MVA.

These Transformers can be further classified into

a)Natural Cooled:

These transformers can be Natural Cooled with Air (Air - Natural). The natural convection of the air removes the heat generated by the transformers. The symbol for this type of cooling is AA.

b)Forced Air Cooling:

This involves cooling the windings of the transformers with forced air, usually through a fan. The symbol for Forced Air cooling in Transformer Nameplates is AF.

Gases such as Nitrogen or Sulphur HexaFluoride can also be used for cooling transformers. These transformers are also considered to be amongst the "dry" type. These transformers need to be placed in sealed containers

Oil Cooled Transformers:

These are transformers which are cooled by means of oil. These transformers are fitted with fins through which the oils pass through as they transfer the heat to the atmospheric air. The oil in the transformers needs to be periodically sampled and checked for integrity. For more on transformer oil analysis Click here.

Like their air cooled counterparts, these transformers too can be further classified on the basis of the flow of oil.

a)Oil Natural Cooling:

In this type of cooling, the oil circulates through the transformer by way of convection. The heat collected by the oil is transferred to the surrounding air by means of cooling fins. The cooling class symbol for this kind of cooling is OA

b)Forced Air Cooled:

In this kind of transformer, cooling is achieved by means of oil driven by convection. As the oil circulates through the fins, air is forced from the outside by means of fans on the fins. This enhances the process of heat exchange and increases cooling of the oil. The designation for this kind of cooling is OA/FA. The fans in this kind of cooling are controlled by a mechanism which switches them on when the transformer oil temperature increases beyond a specific limit.

c)Forced Oil, Forced Air Cooling:

This involves forcing the oil inside the transformer through a special heat exchanger by means of a pump. Air is forced through the other side of the heat exchanger by means of fans. The designation for this type of cooling is FOA (Forced Oil Air).

d)Water Cooled Transformers:

Water can also be used in the cooling of transformers. In this method, the oil which passes through the heat exchanger is cooled by water. This method of cooling is designated as FOA

Comparison of Dry Type Transformers and Oil Filled Transformers

Dry Type transformers use air as the cooling medium. Oil Type transformers are considered a potential fire and safety hazard. Oil Type transformers require the development and maintenance of reliable fire safety and extinction procedures

Dry Type Transformers can be located closer to the load unlike oil transformers which require special location and civil construction for safety reasons. Locating the transformers near the loads may lead to savings in cable costs and reduced electrical losses.

Oil Type transformers may require periodic sampling of the oil and more exhaustive maintenance procedures.

However, though dry type transformers are advantageous, they are limited by size and voltage rating. Higher MVA ratings and voltage ratings may require the use of oil Transformers alone.

For outdoor applications, oil filled transformers are cheaper than dry types.

Ampacity - an introduction

Ampacity or 'ampere-carrying capacity' refers to the ability of a conductor such as wires, cables or busbars to carry current without getting damaged due to overheating.

The ampacity of a conductor should be optimimum with respect to the application. A lower ampacity would result in heating and damage to the insulation. An excessive ampacity selection would result in unnecessarily high costs.

The ampacity of any conductor depends on the following factors.

• The temperature rating of its insulation or the insulation class.
• The electrical properties of the conductor such as resistivity, etc.
• The heat dissipating capacity of the conductor; this depends on the shape of the conductor, the conductor location and ambient temperature, etc.