Trefoil Formation for Arranging Cables

Trefoil Formation refers to a method of arranging cables. The trefoil arrangement is primarily used in situations where the three phases are carried by individual cables rather than a single three phase cable.

In a three phase cable, since the individual conductors along with their insulation are placed near each other the net inductance is minimum as the magnetic field of the individual currents cancel each other out.

However, in single phase cables, when the cables are placed in a straight line the inductance is not cancelled. This can reduce the current carrying capacity of the cable by way of mutual inductance. It can also induce eddy currents in the cable sheath and metallic conduits which can cause heating. It is advisable to have conduits of non-ferrous metals.

Connecting the individual cables in the trefoil formation minimizes the magnetic field around the conductor and reduces the heating. There are special trefoil spacers which hold individual cables in place so that the magnetic fields cancel each other to the maximum.

XLPE Insulation for Cables

XLPE is the acronym for Cross linked Poly-Ethylene. It is a form of polyethylene which has crosslinks which join the individual polymer chains together. Polythene is a material that has numerous applications in the modern world. However, it has the disadvantage of having a low melting point. This downside is eliminated by "cross-linking" the polymer chains. The cross linkings increase the melting point.

XLPE has many qualities which make it extremely useful for cable insulation. It is flexible permitting smaller bending radius for the cables. It is light weight and water proof. It is also tough which minimizes the need for armouring.

XLPE cables are available for a wide range of voltage ranges from 600V to 154kV.

It is easier to handle and store compared to cables with paper insulation or lead insulation.

They are relatively maintenance free and have simple terminating and jointing procedures.

Some of the other features of XLPE are

1. It has a high softening temperature
2. It resists aging
3. It is light
4. It has resistance against stress cracking.

However, XLPE does have some disadvantages such as high cost, and the formation of water trees in the insulation due to ageing which result in partial discharge. Hence, recently, another polymer known as XLVLDPE(Cross linked Very Low Density PolyEthylene) is being used for cable insulation

ACSR Conductors in Power Transmission

ACSR is an acronym for Aluminium Conductor Steel Reinforced. These conductors are widely used in High Voltage Power lines.

Aluminium is a good conductor of electricity besides being cheap. However, its mechanical properties are not desirable. It is soft and cannot be hardened. It also has low tensile strength

This problem is resolved by providing a core of steel stranded cables inside an outer layer of aluminium stranded cables. The steel imparts excellent mechanical properties. Due to the skin effect, the bulk of the power is transmitted through the outer aluminium layer of the conductor which have better conductivity.

The amount of aluminium and steel strandings can be adjusted depending on the requirement for mechanical strength vis-a-vis electrical conductivity.

The conductors are sometimes impregnated with grease to protect against corrosion.

The strength of ACSR conductors is greater than that of copper conductors. The ACSR conductors also have a higher corona limit as they have a higer diameter.

Losses in a Transformer

The Efficiency of the transformer is the given by

(Power output/Power Input)*100

The Efficiency of the transformer is affected by the losses inside the transformer.

These losses can be categorized into three types

1. Copper losses,
2. Core Losses and
3. Stray losses

Copper Losses
These are losses caused by the heating of the conductor when current is passes through it. They are also known as I²R losses as the heat generated is proportional to the formula

H=I²R

Core Losses
Core losses are the losses which occur in the core of the transformer. There are two kinds of core losses, They are Hysteresis Loss and Eddy Current Losses

Hysteresis losses occur when the magnetic orientation of the molecules inside the core are reversed when the magnetic field changes. This reversal of orientation of the molecules results in the generation of heat.

Eddy current losses occur due to circulating currents in the form of eddies which are generated in the core. These eddy currents generate heat.

Stray losses
These are losses which occur due to the leakage of the magnetic flux of the transformer. This leakage can cause eddy currents in the fitments of the transformer such as the tank, channels, bolts, etc.

Types of DC Motors

DC Motors can be categorized into four types depending on the connection of the field and the armature windings:

DC Shunt Motors
In these motors, the field and the winding are connected in parallel. They are used in applications where there is minimal change in speed as the motor is loaded. The provide medium torque while starting.

DC Series Motors:
These motors are used for applications requiring high starting torque. Here, the field and the winding are connected in series. These motors can be used in applications requiring high starting torque such as in traction related applications. The load on these motors must never be reduced to zero as this may result in excessive speed.

Permanent Magnet Motors:
These are used in applications which require greater reliability. Here, the field is made up of permanent magnets. The efficiency of the motor is higher. Speed can be controlled by varying the voltage of the armature.

Compound wound motors:
These motors combine the features of shunt and series motors. They have on field winding connected in series to the armature and another field winding connected in parallel. They provide a heavy starting torque. This kind of motor can be used for loads which are not sensitive to speed variations.

Voltage dependent overcurrent protection

Overcurrent protection is a crucial component of the generator protection scheme. Overcurrent protection is used to protection the generator against overloading. It is also used to isolate the generator in the event of a short circuit fault.

However, there is one issue to be considered when designing a protection for a generator. In the event of a short circuit, the fault current is very high for a few milliseconds after a fault. This heavy current causes the generator voltage to drop. This drop in voltage causes the current to decay. Therefore, a high overcurrent setting may not operate in the event of a short-circuit.

To solve this problem, voltage dependent overcurrent relays bias the overcurrent setting with the measured voltage. That is, at normal voltage, the overcurrent relay operates if the current exceeds the setpoint. However, if there is a voltage drop, the overcurrent setting also progressive decreases according to the biasing. Thus, at lower voltages, the current required to operate the relay is very low.

A variation of the voltage-dependent relay is the voltage controlled. This relay has an undervoltage setting and a overcurrent setting. The overcurrent setting is set at a value less than the rated current of the generator. For the relay to operate, both the undervoltage and the overcurrent need to occur at the same time. This can occur only at the instant of a short circuit.

Power Quality Problems

Power quality is a vital attribute of any distribution and transmission system. It is the duty of the utility to deliver power to the consumer that is reliable and consistent at a predetermined voltage and frequency. However, the quality of power is affected by numerous issues such as sags, swells, over voltage, etc. The following are some of the problems that affect power quality.

Voltage Sags
They are also known as voltage dips. These are voltage drops which last for a short period, from 8milliseconds to 60 seconds. They are caused by momentary faults in the transmission lines which get cleared quickly. The drop in voltage may range from 10 to 90 percent.

Voltage Swells
These are momentary increases in the voltage exceeding 110% which last less than a minute. They can be caused by phase to ground faults.

Undervoltages
These are caused by a drop in the voltage to less than 90% of the voltage for more than 1 minute. They are also referred to as brownouts. Overloading of the distribution system can cause undervoltage.

Overvoltages
They are similar to voltage swells except that they can last for more than a minute. The voltage exceeds 110%. Overvoltages are caused by poor voltage regulation. They can also be caused by switching of capacitors or by sudden load reduction. Wrong setting of transformer taps can also lead to overvoltages.

Interruptions
Interruptions are defined as the drop of voltage to 10%. Interruptions are further classified into momentary interruptions lasting from 8 milliseconds to 3 seconds, temporary interruptions lasting from 3 seconds to 1 minute and sustained interruptions or long duration interruptions which last for more than a minute. Power interruptions can cause loss to production lines in industries. Some of the ways of mitigating the effects of interruptions are by installing a UPS, an emergency genset or by having power fed from two different feeders.

Unbalanced Voltages
When the voltages in all the three phases are not balanced, this situation occurs. This is caused by single phase overloading of one phase. Loads such as arc furnaces also lead to unbalanced loading. Unbalanced loading can be harmful for loads such as transformers and motors where the unbalanced currents which flow as a result of the unbalanced voltages can cause heating.

Voltage Fluctuation
These are caused by quick variations in the voltage between 90% to 105% of the rated value. Arc furnaces and welding transformers can cause voltage fluctuations. Voltage fluctuations can cause “flicker” in incandescent lamps

Transients - An Overview

A Transient can be defined as a momentary change in a parameter such as voltage, current or frequency that occurs when a system changes from one steady state operating condition to another. Transients occur during switching on and switching off loads, during starting motors, etc.

Transients can be classified into

Impulsive and
Oscillatory transients

Impulsive Transients
They are usually caused by lightning. These transients do not have any impact on the system frequency. They cause a very sharp change in either the voltage or the current. However, the change is in only one direction, the positive or the negative side i.e. they are unidirectional. They are characterized by a sharp rise followed by a decay. For instance a 1.5 x 60 microsecond, 2500V surge cause the voltage to rise to 2500V in a period of 1.5 microseconds. This will be followed by a decay to 50% of the voltage value in 60 microseconds.

The impulsive transient may appear differently in the waveform from different points in the system as it is a fast-changing phenomenon. This is because the transient can be modified by various components of the power system.

Oscillatory Transients
They differ from impulsive transients in that they are bi-directional, they occur on both the positive and negative sides of the waveform. Oscillatory transients can be classified into High, Medium, and Low Frequency Transients depending on the primary frequency of the transient.

High frequencies, with a primary frequency greater than 500 kHz, are caused by a reaction of the system to an impulsive transient. Impulsive transient can excite the natural frequency of the power system which can cause oscillations.

Medium Frequency Transients, with a primary frequency ranging from 5 to 500 kHz, are known as Medium frequency transients. They are generally caused by switching capacitor banks or charging large cables.

Low frequency transients have a primary frequency less than 5 kHz. They are usually caused by transformer energization and Ferro resonance.

Reclosers - An Introduction

Reclosers are switching equipment used in transmission lines. A recloser can sense an overcurrent condition by itself and isolate the system. It does not need an external relay to operate it. When the overcurrent condition is cleared, it closes automatically in a pre-determined sequence.

Reclosers are useful in distribution lines, where some of the overcurrent conditions are temporary in nature and clear quickly. They ensure that the system is normalized quickly.

After isolating a system, the recloser closes again after a preset time to see if the fault has been cleared. If the fault persists, the recloser trips again. It closes again for a few more times to see if the fault has cleared. Then it locks the system and permanently isolates it. The recloser then may need to be reset.

Like circuit breakers, reclosers consist of an interrupting mechanism with an insulating medium for arc extinction. They also contain coils for opening and closing, besides sensing transformers.

The tripping coils of reclosers are powered by the fault current which flows through the system. Unlike circuit breakers, where the closing coil is powered by an auxiliary supply, the reclosers are powered by a transformer which is located on the source side.

image courtesy: www.energobit.com

Classification of Relays

Relays can be classified on the basis of their function into five broad categories. They are Protective, Regulating, reclosing synchronism Check and Synchronizing, monitoring and Auxilliary.

Protection Relays
Protection relays are used in generators, transformers, feeders, transmission lines, etc. The primary function of these relays is to continually monitor a specific parameter such as current, voltage or power and to generate alarm/isolate the system or device in the situation of deviation from set limits for the parameter or a fault. For instance, an overcurrent relay may be programmed to operate when the current in a feeder exceeds a certain predetermined limit. These relays generally obtain their feedback from current or voltage transformers.

Regulating Relays
These relays are used to regulate a specific parameter such as the output voltage of a transformer. These relays operate a control equipment such as the tap changer of a transformer. These relays are not designed to respond to fault conditions.

Reclosing Relays,
These relays are used to put the system into operation. These relays are used to synchronize lines and feeders. These relays usually are used in connecting different components of an electrical distribution system such as generators, feeders, transformers, etc. They also come into play when restoring the system after a fault.

Monitoring relays
These relays are used to monitor conditions in a system such as the direction of power flow and generate alarms when there are deviations. Examples include the low forward power relay which generates an alarm when the power in a direction falls below the minimum set points. They are also used to monitor the continuity of systems such as pilot wires.

Auxilliary Relays
These relays are used generally for contact multiplication. The single contact available in a relay is used to trip a number of breakers. Besides, these relays are also to isolate the relay from other equipments such as breakers.

Skin Effect

Skin Effect refers to the tendency of alternating current(AC) to flow along the outer surface(skin) of the conductor rather than through the entire cross-section of the conductor.

Skin Effect is caused due to eddy currents form due to the magnetic fields created when current flows through the conductor. These eddy currents are strongest near the centre. The magnetic fields oppose the flow of the current. Hence, the current finds it easier to flow across the periphery of the conductor.

Conductors which carry AC such as busbars in substations are made hollow for this reason as current flows only along the surface. The Skin effect becomes more pronounced at higher frequencies. That is why radio antennae are made hollow.

Conductors in overhead lines and in cables are generally made of strands instead of one solid conductor.

Skin Effect does not occur when conducting DC.

What will be the impact of undervoltage and overvoltage on induction motors?

An overvoltage in an induction motor will cause the reactive component of the current inside to increase causing eddy current heating of the rotor and stress on the insulation.

In the case of an undervoltage, the low voltage causes the torque developed to reduce. This results in an increase of slip and a reduction in speed. The motor tries to reduce the slip by drawing more current. This overloads the motor and can cause overheating.

Power Transformers - Introduction

A Transformer can be defined as a static electrical device which transfers power from one circuit to another by means of electromagnetic induction. The transfer is accompanied without any change in the frequency.

The term power transformer is generally used to refer to transformers with a rating of 500kVA or greater.

Power transformers are used in distribution systems wherever there is a need to interface between different voltage levels i.e. to step up and step down voltages.

Power transformers are generally of the liquid-immersed type. However, Power transformers used for indoor applications may be air-cooled.

Based on the size ranges, transformers can be classified into three types.

Small Power Transformers : 500 to 7500kVA
Medium power transformers : 7500 to 100MVA
Large power transformers : 100MVA and beyond

The average life of a transformer is around 30 years.