Study notes on Instrument Transformers & DVM's For Electrical Engineering Students

By Deepak Yadav|Updated : July 5th, 2023

Welcome to our comprehensive study notes on Instrument Transformers and Digital Voltmeters (DVMs) designed specifically for electrical engineering students. This resource aims to provide you with a solid foundation in understanding the principles, applications, and workings of instrument transformers and DVMs. Whether you are a novice student or an aspiring electrical engineer, these study notes will help you grasp the essential concepts and practical aspects of these vital electrical measurement devices.

In these study notes, we will delve into the world of instrument transformers, which are widely used to measure high voltages and currents safely. You will learn about current transformers (CTs) and potential transformers (PTs) and how they enable accurate measurements, protection, and control in power systems. Additionally, we will explore the fundamentals of digital voltmeters (DVMs), which are essential tools for measuring voltage in electrical circuits. Through these study notes, you will gain a comprehensive understanding of the working principles, types, and applications of DVMs, empowering you to make precise electrical measurements in your future engineering endeavours. Get ready to expand your knowledge and expertise in instrument transformers and DVMs with our comprehensive study notes.

Download Formulas for GATE Electrical Engineering - Signals and Systems

Table of Content
In this article, you will find the study notes on Instrument Transformers & Digital Volt-Meter which will cover the topics such as Instrument Transformers, Circuit Diagram of Instrument Transformers, Advantages of Instrument Transformer, Different Ratios of Instrument Transformers, Different Parameter in Instrument Transformers, Current Transformer (CT) & Potential Transformer, Analog to Digital Converter & Ramp Type Digital Voltmeter.

Instrument Transformers: Instrument transformer used for over current, under voltage earth fault and for relay protection.

It might appear that the extension of range could be conveniently done by the use of shunts for currents and multiplier for voltage measurement, as in DC. But this method is suitable only for small values of current and voltage due to the following reason

  • Difficult to achieve accuracy with a shunt on AC.
  • The capability of having a shunt of the large range is not possible.
  • The power consumed by multipliers becomes large as the voltage increases.
  • The measuring circuit is not isolated electrically from the power circuit.

Circuit Diagram of Instrument Transformer

byjusexamprep

Advantages of Instrument Transformer

  • Their reading does not depend upon circuit constants such as R, L & C as in the case of shunts and multipliers As in the case of shunts and multipliers.
  • Possible to standardize the instrument around their ratings this makes the replacement of the instrument transformer very easy.
  • The measuring circuit is isolated from the Power circuit.
  • Low power consumption in the metering circuit. Several instruments can be operated from a single-instrument transformer.

Ratios of Instrument Transformer

Transformation Ratio (K) It is the ratio of the magnitude of the primary phasor to the secondary phasor.

03-Instrument-transformers (1)

For the current transformer,

03-Instrument-transformers (2)

For potential transformers,

03-Instrument-transformers (3)

Nominal Ratio (NR): It is the ratio of the rated primary winding current (or voltage) to the rated secondary winding current (or voltage).

03-Instrument-transformers (4)

Where, CT = Current transformer

PT = Potential transformer

Turns Ratio (N)

For current transformer

03-Instrument-transformers (5)

For potential transformer

03-Instrument-transformers (6)

Ratio Correction Factor (RCF)

03-Instrument-transformers (7)

byjusexamprep

Different Parameters in Instrument Transformer

byjusexamprep

The burden of an Instrument Transformer

It is volt-ampere loading.

Secondary winding burden

03-Instrument-transformers (8)

Or Secondary winding burden

= (Secondary winding current)2 × (Impedance of secondary winding circuit)

Current Transformer (CT)

Transformation Ratio

03-Instrument-transformers (9)

03-Instrument-transformers (10)

Phase angle  degree 03-Instrument-transformers (11)

Where, 03-Instrument-transformers (13)

Potential Transformer (PT)

Actual Transformation Ratio (K)

03-Instrument-transformers (14)

03-Instrument-transformers (15)Analog to Digital Conversion

Voltmeter is an electrical measuring instrument which is used to measure the potential difference between two points. The voltage to be measured may be AC or DC. Two types of voltmeters are available for the purpose of voltage measurement i.e. analog and digital. Analog voltmeters generally contain a dial with a needle moving over it according to the measure and hence displaying the value of the same.

  • These are generally known as DVM that use digital formatting of the input fed to its input leads. Here, the result of measurement is shown in the form of discrete numbers for which they employ display devices for the decimal number system.
  • Digital voltmeter also attains an inherited greater speed of operation. Because the output obtained from these instruments comes to be a digital form so it becomes easier to use them directly as an input to many other devices like memory devices so that the result may be used further in future, this is called storage of data. 
  • Because of high accuracy, high-speed operation and greater reliability, they are frequently used in laboratories and industries for the purpose of experimentation and obtaining highly accurate results.
  • It can be accepted as a disadvantage of digital voltmeters that they always need some external power supply for their operation making them less portable and also bulkier but with the advancements made in the field of integrated circuits it has becomes possible to make such digital devices very compact, more efficient, low cost and having even greater accuracy.
  • These advancements have led to the verge that now some digital devices are there having less cost than their competent analogue ones with the same extent of accuracy. 
  • A/D converts an analog signal into the digital code which is proportional to the magnitude of the coming signal.

Vin ≈ k ×Digital output

Where k is step size or resolution.

Quantization error or Conversion error of an A/D

byjusexamprep

Conversion time,Tc: The time requires to convert an analog signal to the corresponding digital code.

byjusexamprep

On the basis of the A/D conversion method used digital voltmeters can be classified as:

  • Ramp-type digital voltmeter
  • Integrating type voltmeter
  • Potentiometric-type digital voltmeters
  • Successive approximation type digital voltmeter
  • Continuous balance type digital voltmeter

Nowadays digital voltmeters are also replaced by digital millimetres due to their multitasking feature i.e. it can be used for measuring current, voltage and resistance. But still, there are some fields where separated digital voltmeters are being used.

Ramp-type Digital Voltmeter

Its Operating principle is based on the measurement of the time it takes for a linear ramp voltage to rise from 0 V to the level of the input voltage, or to decrease from the level of the input voltage to zero. This time interval is measured with an electronic time-interval counter, and the count is displayed as a number of digits on electronic indicating tubes.

byjusexamprepThe working principle i.e., the Conversion from a voltage to a time interval is illustrated by the waveform 

byjusexamprep

  • At the start of the measurement cycle, a ramp voltage is initiated; this voltage can be positive-going or negative-going. The negative-going ramp is continuously compared with the unknown input voltage.

  • At the instant that the ramp voltage equals the unknown voltage, a coincidence circuit, or compactor, generates a pulse which opens a gate.

  • The ramp voltage continues to decrease with time until it finally reaches 0 V (or ground potential) and a second compactor generates an output pulse which closes the gate.

  • An oscillator generates clock pulses which are allowed to pass through the gate to a number of decade counting units (DCUs) which totalize the number of pulses passed through the gate.

  • The decimal number, displayed by the indicator tubes associated with the DCUs, is a measure of the magnitude of the input voltage.

     

  • The sample-rate Multi-vibrator determines the rate at which the measurement cycles are initiated.

  • The oscillation of this multivibrator can usually be adjusted by a front-panel control, marked rate, from a few cycles per second to as high as 1,000 or more.

  • The sample-rate circuit provides an initiating pulse for the ramp generator to start its next ramp voltage. At the same time, a reset pulse is generated which returns all the DCUs to their 0 state, removing the display momentarily from the indicator tubes.

byjusexamprep

byjusexamprep

If you are preparing for GATE and ESE, avail Online Classroom Program to get unlimited access to all the live structured courses and mock tests from the following link :

 

 Thank you,

 
Download BYJU'S Exam Prep, Best gate exam app for Preparation

Comments

write a comment

Follow us for latest updates