Gang Operated Switches

          Gang operated Switches or G.O Switches as they are commonly known are switching devices used in overhead power lines.

          They are called Gang Operated as they are operated in a Gang, all three switches together, using a single mechanism.  They are also called air-break switches as they use air as the breaking medium or G.O.D.(Gang operated Disconnector) switch.

          These switches do not have any load-breaking capacity.  They can only be operated when the transformer is on no-load and only the small magnetizing current flows through them.  A substation must be disconnected from the load it is feed and only then can these switches located on the incoming lines of the substation be disconnected.

        
          These switches are used in lines with voltages about 5 kV.  They can also be motorized and operated remotely.  Gang operated switches can be mounted vertically or horizontally.

          A thin film of non-oxide grease is usually applied to the contacts of the Gang operated switches.  The Gang operated switches should be checked periodically for proper alignment and rigidity.

Non-directional Earth fault Protection using Residual Current Relay

The Residual Current based earth Fault relay works by measuring the vector sum of the three phase currents.  

Under healthy conditions, the vector sum of the three currents is zero. In the event of an earth fault, however, the fault current flows through the ground and hence, the vector sum of the currents is not equal to zero.  This is known as the residual current.  This current can be used to operate the earth fault relay.

The connection of the earth fault relay consists of three current transformers connected in parallel to each other.  This kind of earth fault protection is also known as unrestricted earth fault protection.

The residual current protection is usually set to operate at around 10% of the nominal current.  For fault currents lower than this value, as may be the case in high resistance grounded generators and transformers, the sensitive earth fault relay is used.  This is because, the three current transformers used in the residual current protection may not be exactly identical in response, even if they are from the same manufacturer.  Thus for very low setting, there is the risk of false operation of the relay due to errors in the current transformers.  

Since the sensitive earth fault relay uses one Core Balance Current Transformer instead of three individual current transformers, it can be set to lower values of earth fault current. 

Isolation Transformers

Isolation Transformers are transformers which serve to isolate two parts of a circuit. This may be required to ensure that a fault in one part does not affect the other parts of a circuit.

Strictly speaking, all transformers are isolation transformers. The primary and secondary sides are connected through induction and not through conduction. An exception is the autotransformer which cannot provide isolation as its windings are shared.         

Isolation transformers have a transformation ratio of 1:1 as no voltage transformation is involved.         
A grounded shield is usually placed between the primary winding and the secondary winding to prevent any risk of capacitive coupling between the windings. Capacitive couplings can compromise the isolation and provide a linkage between the primary and the secondary circuits. Isolation Transformers are widely used in electronic circuits and in medical instruments.

Construction of a Transformer

Transformers are indispensable equipments of the electrical system. The transformer alters the voltage level at the same frequency. Here is an interesting video about the Transformer Construction

Sensitive Earth Fault Protection

          The sensitive earth fault relay is a protective device that works by measuring the residual current across the three phases in a system. This is done using a Core Balance Current Transformer (CBCT).   In the ideal condition, the residual current will be zero as all the currents flow through the three wires and their magnetic fields cancel each other out. 

          In the event of a fault, the residual current over the three phases will not be equal to zero as the current from the faulted phase flows through the earth.

 

          The sensitive earth fault protection is usually used in alternators and transformers with high resistance grounding.  High resistance grounding restricts the earth fault current to less than 10A.  High resistance grounding enables electrical systems to continue running when one of the phases is faulted.  This prevents interruptions to the power supply.  This kind of earthing system provides time to identify and isolate the fault.

          Once an earth fault occurs in the high resistance grounding system, an alarm needs to be generated and the fault needs to be traced.  For this a reliable protection which detects earth faults even when the fault current is very low is necessary.  Undetected earth faults in this system are dangerous as a second earth fault in another phase may result in a short-circuit.  Conventional earth fault relays may not be accurate in detecting an earth fault at such low current values.

          The sensitive earth fault protection, as the name suggests, is a highly sensitive relay.  It can sense currents as low as 0.2% of the CT secondary current.

          The sensitive earth fault relay may be configured to either generate an alarm or a trip signal.

Bushings - An Overview

Bushings are insulating components which carry a conductor through a metallic component s for instance, bushings are found in Transformers as they carry the conductor from inside the transformer out to the terminals.  Bushings are usually made of ceramics.

          Bushings are also found in circuit breakers, alternators, motors and capacitors.  Some indoor substations use bushings to carry the conductors outside through the roof.  Electric locomotives also use bushings to support the overhead traction equipment.

          A bushing is usually a hollow ceramic tube through which the conductor passes.  Bushings also provide rigid support to the conductors inside.

          Bushings are used in both low voltage and high voltage applications.  In low voltage applications, the ceramic casing alone is sufficient to provide insulation to the conductor.  These bushings are known as non-condenser bushings.

           However, in high voltage application (>52 kV) the bushings are filled with insulating media such as oil, resin impregnated paper, oil impregnated paper, etc to provide greater dielectric strength.  Sulfur hexafluoride gas is also used in some high voltage bushings.   These bushings are also known as condenser bushings, as they form a capacitor between the live conductor and the equipment body which is at ground  potential

Bending Radius of Cables

The Bending radius of a cable refers to the sharpest possible bend a cable can take.  Cables are flexible conductors and they needed to be bent at numerous places as they carry power.

Cable manufacturers usually specify the minimum possible radius that a bend can take.  This is known as the bending radius.  If the cable is bent at a radius below the minimum specified radius, the cable can get damaged.

 As a rule of thumb, the bending radius is 10 to 15 times the diameter of the cable.  However, the bending radius value provided by the manufacturer should be consulted before laying any cable.

Coefficient of coupling in a transformer

In a transformer, the primary and the secondary windings are linked magnetically.  The magnetic flux created by the exciting current of the primary winding induces a voltage in the secondary.

In an ideal transformer, all of the flux generated in the primary winding will be linked to the secondary winding. 

In such a transformer, the efficiency will be maximum.  Modern Transformer cores use high silicon steel with high permeability and winding which are placed close to each other to achieve a high coefficient of coupling.

However, in actual transformers, some of the flux built in the primary winding is not linked to the secondary.  This flux which is not linked to the secondary is known as the leakage flux.  The leakage flux causes a drop in secondary voltage. 

Leakage inductance.

For the purpose of calculation, the leakage flux is assumed to be an inductance connected in series with the primary winding which causes a drop in the applied voltage in the primary.  This is known as the leakage inductance.

Turns Ratio and Voltage Ratio of Transformers, Are they the same?

Turns Ratio of a transformer is defined as the ratio of the number of turns in the Primary to the number of turns in the Secondary.

The voltage ratio is defined as the ratio of the Primary voltage of the transformer to the secondary voltage.

In transformers with small transformation ratios, the Turns ratio is usually equal to the Voltage ratio. However, for transformers with higher transformation ratio, the voltage ratio may be different from the Turns Ratio. This is known as Ratio Error.

Ratio error can be caused by a number of factors such as leakage inductance, copper losses within the transformer and inter winding capacitance.

The Ratio Error is detected by a ratio test.  A simple ratio test involves applying a fixed voltage on the primary and measuring the voltage induced in the secondary.

Video of the Week - The Induction Motor

The induction motor is at the very heart of industry.  This simple device efficiently converts electrical energy into rotating motion.  The principle of operation of the induction motor is described in this video.