df/dt Protection for Generators

The df/dt protection is used to identify abnormal changes in system frequency and take remedial actions in order to prevent generator overload and the resulting blackout.  The df/dt operates faster than ordinary under frequency relays as it is able to predict the under frequency much earlier.   

df/dt relays are also known as ROCOF relays (Rate Of Change of Frequency Relays) The df/dt is usually wired to a load shedding system which trips select breakers to isolate loads. 

The df/dt functions by measuring the rate of change of frequency.  When the frequency changes too fast, it is an indicator of a forthcoming under frequency.  The setting of the df/dt relay is in Frequency/Time in seconds. e.g. 0.3Hz/second or 0.4 Hz/.5 seconds.

Some manufacturers provide a more reliable setting involves specifying two frequency set points and the time taken for the frequency to cross the two limits.  For instance, a df/dt relays can be programmed to operate if the system frequency crosses 48.5 Hz and 48 Hz in 0.4 seconds

Factor of Earthing

The Factor of Earthing in a three phase system is defined as the ratio of the phase voltage (phase to earth voltage) on a healthy phase during the fault at the rated frequency to the phase voltage of the healthy phase during normal situation.  

When an earth fault occurs in one of the phases of a three phase system, the voltage vectors are distorted.  The neutral shifts in the direction of the fault phase.  This causes the voltage in the other two healthy phases to rise. 

The factor of earthing determines the rise of voltage in the healthy phases when one of the phases has a earth fault.

The factor of safety is 100% for an isolated neutral (floating neutral) system.  It is 80% for an effectively earthed system while it is 57.7% for a solidly earthed system.

The factor of safety is an important parameter while performing an insulation coordination in a system.  

Thermistors - an Overview

Thermistors are temperature sensors which have a sensing element usually made of polymers or ceramics.  Thermistors function by changing their resistance when the temperature increases. 

Thermistors find wide application in the industry, in automobiles and in electric appliances.  Their small size makes them ideal for use in electronic circuit boards and digital thermostats.

The principle of the Thermistor was first discovered by Michael Faraday in 1833.  However, the first practical thermistor was constructed by Samuel Ruben in the year 1930.

When the temperature of the Thermistor changes, the resistance of the Thermistor also changes.  The change can be either positive or negative.  Thus, we have PTC Thermistors (Positive Temperature Coefficient) and NTC Thermistors (Negative Temperature Coefficient).

NTC thermistors are used in temperature measurement while PTC thermistors are used in Electric current control. 

Thermistors are generally formed into a disc or bead and sealed in an enclosure made of plastic or gas.

Thermistors are highly accurate and have a quick response.  However, they have a limited range of measurement.  Another downside is that they do not have a linear response. 

Thermistors have high stability and are not affected by ageing.  This means that they need not be calibrated for long periods of time.    They are cheaper, rugged and are easy to produce. 

Pressure Transducers

Pressure is an important physical quantity to be measured in industrial systems.  

Transducers are one of the popular means of measuring Pressure.   Pressure Transducers work by converting the pressure signal into an analog electric signal usually a 4...20 mA signal. 

Pressure Transducers can be designed using many principles.  The most widely used of these are the capacitive and the Piezo-resistive transducer. 

Capacitive Pressure Transducers
The capacitive transducer consists of a diaphragm which works as one of the plates of a capacitor.  A fixed conductive surface acts as the other plate.  The permittivity of the space in between these plates varies as the diaphragm moves in response to the measured pressure. 

This change in capacitance is measured as the process pressure.  

The capacitive transducer is used to measure very low pressure values.  Very Accurate measurements are possible using the capacitive pressure Transducers. 

Piezo Resistive Pressure Transducers

Piezo Resistive Transducers work on the principle of the piezoresistive effect.  The piezo resistive effect refers to the change in the resistivity of a material in response to force or pressure.  The piezo resistive sensor is used widely in biomedical applications as well as in the automobile industry. 

Piezo Resistive Pressure Transducers

These sensors are low in cost and have high sensitivity.  They can be manufactured for a wide range of pressure measurement.

Piezo Resistive Pressure transducers consist of a diaphragm which is made of silicon.  The diaphragm bends due to the pressure of the system to be measured. 

Mounted on the diaphragm are four piezo-resistors which are usually arranged in the form of a Wheatstone bridge.  When the diaphragm bends due to the pressure, the piezoresistors are subject to either tensile or compressive stress.  This results in a change in resistance values which is measured through the Wheatstone bridge formation and is scaled as a pressure measurement

Inductive Proximity Sensors

Inductive Proximity sensors find wide application in the field of industrial instrumentation.  These sensors are extremely popular as they are reliable, robust and have a simple construction.  Inductive Proximity sensors are used to measure speed, detect motion and sense the position of objects.

The inductive proximity sensor consists of an oscillator, a coil and a detector.  The oscillator develops a high frequency signal which is fed to the coil. 

The high frequency signal develops a corresponding high frequency magnetic field at
the tip of the sensor.  When a metallic object comes in front of the sensor, eddy currents are induced in the object.  This acts as a load on the oscillator and the amplitude of the high frequency output drops.  This drop in the voltage is detected by the detector unit which causes the switching on or off of a transistor.  This results in a change of voltage level which is interpreted as a digital signal 0 or 1.

The inductive proximity principle can also be applied to speed sensors.  In speed measurement, the inductive proximity sensor is placed near the rim of a rotating object. The rotating object has a number of teeth along its rim.  When a tooth passes near the inductive proximity sensor, a pulse is produced. 

This sequence of pulses can be converted into an analog signal can be measured as the speed of the device.

Sympathetic Tripping

Sympathetic Tripping refers to the phenomenon in Electrical Systems when a protective device in a healthy section of the system operates for a fault in another section of the system.  Sympathetic tripping results in unnecessary loss of power for healthy equipment. 

There are many causes for sympathetic tripping.  The most common reason is undervoltage which occurs across the system when there is a heavy current due to a short-circuit or an earth fault. 

Another reason for sympathetic tripping can be the flow of capacitive currents in the healthy feeders when one of the feeders gets grounded. 

In Transformers and Generators the Differential relay sometimes operates for an overcurrent which is outside its zone.  This is due to the dc component of the earth fault current.

Preventing Sympathetic Tripping   

Sympathetic Tripping can be prevented by designing smaller feeders with the total loads equally balanced across the different feeders

Reducing the fault level can result in lesser currents in the event of faults.  The fault level can be reduced by the use of current limiting reactors which increase the impedance.

Extreme Inverse settings in IDMT relays can also help the relays discriminate between sympathetic overcurrents and genuine faults. 

Increasing fault clearing times in the faulty feeders reduces the duration of the undervoltage across the system. 

Modern Differential relays have an inbuilt dc filter which prevent sympathetic tripping due to dc components during earth faults.

Videos on Power Transformer Testing

Useful Videos on Power Transformer Testing by OMICRON