Natural lightning is very harmful and have drastic effects to objects exposed to external environment. Atmospheric lightening contains high current as well as high voltage impulses. The high voltage effects are tested with impulse voltages and high current effects are tested with impulse currents to make equipment safe of these damages.
Fig. 1.1 Lighting Impulse
If we look back into history, a little work has been done on impulse current generators design and evaluation and all the research conducted is based on use of analytical methods and mathematical tools and software base analysis.
Marx generators are very famous as far as hardware designs are concerned and were proposed in 1923. In the proposed design multiple capacitors were first charged in parallel through charging resistors by a high-voltage, direct-current DC source and then discharged in series through a test object by an instant spark-over of the spark gaps. 5
I.F.Gonus and his friends published a paper about analytical method for designing of impulse current generator. Mathematical derivations and expression, formulas, impulse current parameter for the waveform were calculated. Time constant, rise and decay times of wave form were calculated as well. Current generated characteristic were according to limitation of IEC-60060-1. 1
IEEE-PES wrote a document about surge (impulse) generator. In the paper types of impulses, parameters of impulse generator and tolerance of waveforms along with the consideration of test object were examined. 2
Pack S. and Jaufer S. in international conference presented their work in which complete evaluation of impulse current generator was made by numerical solution tool. A detailed thorough circuit of impulse generator was modeled by regression analysis and numerical simulation. By numerical method charging voltage and other circuit parameters were obtained and varied and resulting effects were calculated. 3
In 2nd Engineering conference on sustainable energy a paper was presented about impulse current generator simulation using ORCAD PSPICE software. By using ORCAD PSPICE impulse waveforms were generated while the characteristics parameters of waveforms were adjusted according to desired standards as given by IEC. Multiple simulations were performed to obtain required standarads. These waveforms can be used for testing of arrestors and to design protection schemes. 4
1.2 Problem Statement
Natural lightning occurs randomly and is very harmful and dangerous to the objects exposed in its path like aero planes, helicopters, buildings, antenna, electrical transmission & distribution networks. To minimize the problems caused by lightening over voltages and over-currents, different standards have been given by IEC to carry out impulse voltage and impulse current tests. Impulse current test system is used for testing of protection devices specifically breaking strength of circuit breakers and to test surge and lightening arrestors to avoid damages caused by natural lightening.
1.3 Aims & Objectives
Our objective was to design an impulse current generator producing a peak current of 100 kA. We aimed to have an Optimized design of impulse current generator at low voltage. These generators are designed at high voltages but we are using low AC voltages and the cost is reduced to a greater level. Design involved hardware assembly as well as software simulation-based designs were obtained. Testing of circuit breakers of different breaking strengths was carried out.
1.4 Motivations for thesis
Impulse current generators are having vast applications and different areas to serve in. They are used for the testing of breaking capacity of circuit breakers, testing of surge arrestors, lightening arrestors, aero plans and helicopters and wind turbine blades and military and plasma applications. Our main focus is to design impulse current generator to check breaking capacity of circuit breaker up to 100 kA. The content of thesis is mainly focused on the software simulation on PSIM and results, hardware-based design and the measurement of impulse. Testing of circuit breaker is carried out and mentioned.
1.5 Technique for Implementation of Project
Impulse of current is produced using the concept of charging and discharging of capacitor. Capacitors are connected in parallel and charged through DC voltage and then discharged across resistance. Rapid discharge produces peak current for fraction of seconds and the magnitude of impulse depends upon number of capacitors being discharged. Different current measurement techniques were examined including Rogowski-coil and current transformer. Finally, measurement of current is done using analog ammeter. Testing is done on circuit breaker connected in series with the resistor. Test Circuit is implemented on PSIM software and waveforms are obtained. Results are compared with hardware structure.
2.1 Impulse Current generation and characteristics
Lightning releases include both high voltage impulses and high current impulses which are damaging for transmission lines. Protective gear like surge diverters eject the lightning currents without destruction. They discharge the high currents and make them flow towards ground. Therefore, impulse current waves of high magnitude 100kA peak are generated and used in testing the lightening arrestors as well as in basic research on electric arc studies, non- linear resistors, and studies related to electric plasmas in high current discharges. Certain recognized standard tests performed via high impulses of currents are given below.
Table 2.1 Lightening impulse test standards
2.1.2 Impulse Current Waveform
The wave factors or parameters used in testing surge diverters are 4/10 and 8/20 µ s, the figures represent the nominal wave front and wave tail times with permissible tolerances of ±10%only respectively. Front time is the time required by the wave to reach 90% of its peak value. Tail time is the time required by the waveform to decay up-to 50 % of its peak value. Rectangular waves of lengthy period or duration can also be used for testing. The rectangular waves are normally having durations of 5 m s, with rise and fall times of 0.5 to 5.0 m s and this time is less than ±10% of their total time period. The time of the wave is said to be the total time of the wave during which the current is at least 10% of its peak value. The tolerance permitted on the peak value is +20% and -0%. The peak value can be more than the stated value but not less.
2.1.3 Circuit Description
To produce impulse currents of large value, a capacitor bank is have to be established where capacitors in parallel connection are charged to a particular value of voltage and then discharged through a series R-L circuit. To minimize the operative inductance and to prevent heat losses the capacitors are subdivided into minor units. And the arrangement of capacitors into a horse-shoe style diminishes or reduces the effective load inductance.
Fig. 2.1 Basic Circuit
2.1.4 Parts of Impulse Current Generator
The vital parts of an impulse current generator are as follows:
(i) A DC charging unit ,it is a voltage supply which will give a variable voltage to the capacitor bank,
(ii) Capacitors of high rating (0.5 to 5 µF) with very low self-inductance proficient of giving high short circuit currents.
(iii) An added air cored inductor of high current value,
(iv) Proper shunts and oscillo-graph for measurement drives
(v) A triggering unit and spark gap for the beginning or starting of the current generator.
Fig. 2.2 Impulse current generator model
Fig. 2.2 Impulse current generator model
Charge Storing System
In analog as well as digital electronic circuits, a capacitor is a elementary and fundamental element. it provides a memory element and It is used in filtering of signals. While resistors are called energy dissipating element, ideal capacitors store energy rather than wasting it. It is a passive element and stores energy in an electric field.
Fig. 3.1 Basic structure of the Capacitor and symbol
1.2 Modeling of Capacitor
The capacitor is typically made by two conducting plates separated by a dielectric which is insulator. When a voltage v is applied to the plates, a charge +q collects on one plate and a charge –q on the other plate.
Fig. 3.2 Basic structure of the Capacitor and symbol Capacitor Model
C is capacitance in Farads
Q is charge held on the plates in coulombs
V is potential difference across the plates in volts
If the plates have an area A and are parted by a space d, the electric field produced across the plates is as
And voltage through the capacitor plates is
The current entering in the capacitor is the rate of change of the charge across the capacitor plates.
The current-voltage correlation of a capacitor can be represented as
Capacitance denotes the capability of capacitor to store charge. units are in Farads (F). It is a function of the geometric features plate area (A) and capacitor – plate separation d , the permittivity (?) between the plates.
For DC, Capacitor acts as an open circuit . It does not like voltage cutoffs or else the current become infinity which is not possible physically.
1.2.2 Phase angle Between Current and Voltage
The voltage across the capacitor and Current going through a capacitor are 90 degrees out of phase. The current leads the voltage by 90 degrees.
The graph is shown below.
Fig. 3.3 Phase angle between current and voltages
1.2.3 Capacitive reactance
If we take the ratio of the peak voltage to the peak current we get capacitive reactance. That is
Xc has units of Ohms or Volts/Amperes. As the frequency reaches ? to 0 the quantity Xc goes to infinity and capacitor will act as an open circuit. As the frequency converts very large w approaches to infinity and the quantity Xc goes to zero and capacitor look like a short circuit.
1.3 Circuit Combination
Capacitors joined in parallel combine like resistors in series whereas capacitors in series combine like resistors in parallel connection.
1.3.1 Series circuit
Voltage divider rule is used for series connected capacitors. In series the capacitance is decrease and equivalent capacitance is less than the value of smallest capacitance in circuit.
Fig. 3.4 Series combination of two cap.
1.3.2 Parallel circuit
In the parallel arrangement of capacitors, the current division rule is applied. But capacitance is add up like resistance add up in series and equivalent capacitance is sum of all capacitance of capacitors.
Fig. 3.5 Parallel Arrangement of capacitor 14
1.4 Instantaneous power and energy
The instantaneous power supplied to a capacitor is given as.
P (t) = i(t)v(t)
The energy stored in a capacitor is the integral of the instantaneous power. The energy in the capacitor at time t is
E (t) =1/2*C*V(t)2
1.5 Real Capacitor
If the dielectric material between the capacitor plates has a finite resistivity instead of infinite resistivity in the case of ideal capacitor then there is a small amount of current flowing between the capacitor plates. Furthermore, there are lead resistance and plate effects.
Fig. 3.6 Circuit of non-ideal cap.
The resistance Rp is typically very big and it represents the resistance of the dielectric material. Resistance Rs is typically small and is the lead and plate resistance.
1.6 Equivalent Series resistance
Actually, we are focused on the Equivalent Series Resistance (ESR). ESR is a very important capacitor characteristic and must be considered in circuit design.
Usual values of ESR are in the m?-? range
Fig. 3.7 Non-ideal capacitor with series resistor
1.7 Uses and applications
• Smoothing the signals while converting them from AC to DC.
• Energy storage in form of electric field.
• Signal coupling and decoupling as a capacitor coupling that blocks DC and allow AC.
• Tuning, by connecting them to LC oscillator
• Timing circuits, due to the fixed charging and discharging time of capacitors.
• For electrical power factor correction
1.8 Working of a Capacitor
Current starts to flow through the circuit when DC power supply is given to the capacitor. There will be expanding potential difference (Vc) setup across two plates of capacitors while charging and the plates will be charged equally but inversely. The direction of flow of current from the battery passes from the positive plate to the negative through this capacitor but the stream is limited because of the insulating material that is partitioning the plates. Instead of this very little amount of current flows from positive to negative plate through this insulating material that is called dielectric. The stronger the dielectric the smaller will be the current and vice versa
Inside the dielectric of the capacitor an electric field is developed between positive and negative plates. After some time, positive charge from the battery will be aggregated on plate I (positive plate) while plate II (negative plate) will get the negative charge. After fixed interval of time, capacitor plates will be charged to its maximum capacity. This fixed interval is the charging time of this capacitor.
After charging time of capacitors is reached, the plates of the capacitor will be charged equivalent to the power supply voltage that is Vc = V. At the point the capacitor acts as open and current will stop flowing through the circuit and we can say that the charging phase of capacitor is done.
Fig. 3.8 Charging process
Capacitor become an open circuit for the flow of direct current and offer infinite resistance, R = ?, this concludes that after charging phase the current flow will be stopped completely. Discharging
After completion of charging phase of capacitor, capacitor switch is moved from the power supply and is then connected across a resistor. The voltage developed during the process of charging drops down to zero step by step. Charging and discharging rates are influenced by the resistances Rc and Rd separately.
Fig. 3.9 Dis-charging process
1.8.2 Discharging of a capacitor through inductor
If the fully charged capacitor of capacitance C is discharged through inductor having inductance L, capacitor voltage and current through the inductor will change gradually.
After specific time, inductor will be charged completely, and capacitor will have zero voltage. Maximum value of current exists through L. capacitor energy q02/2c is completely transformed into inductive energy Li02/2. As there is no energy loss the inductor will have potential difference equal and opposite to that of capacitor.
After that, inductor starts charging the capacitor in inverse way because of high potential difference. And now the inductive energy gets transformed to capacitive energy.
When capacitor charged to its maximum, the magnetic field established due to inductor depleted and the current flowing through the circuit become zero. Now, again capacitor drives current through the inductor by the process of discharging. As the energy loss is zero, the energy will continue to convert in the similar manner. There will be the state of oscillation of voltages/charge/energy between capacitor and inductor.
This oscillatory response is known a resonance. The frequency at which these oscillations occur is called resonant frequency of this circuit which can be calculated by value of L and C as shown in the following equation.
Fig. 3.10 Waveform for capacitor and Inductor
In the above graph the red sinusoidal curve shows the current through L and C assuming switch is close at t=0 while the green curve is for the voltages across L and C. the results are the continuous resonance.
The rate of charging and discharging of capacitor can be represented by time constant ?, that is the product of capacitance C and resistance R. smaller the values of R and C, time constant will be smaller indicating faster charging and discharging of capacitor and vice versa. Time constant is the time to increase the voltage of capacitor to about 63.2 % of its final value (voltage of the battery)..
Table 3.1 Time Constant
1.9 Capacitive AC Circuit
Capacitor and AC voltage supply together makes a purely capacitive AC circuit. AC supply is directly applied to the capacitor terminals. Charging and discharging of capacitor depends on the change in the value of supply voltage and frequency of AC supply determines the rate of charging and discharging. With the change in frequency the capacitance fluctuates in the AC circuit.
As power supply is AC the direction of flow of current reverses after half cycle. Charging current of capacitor is directly proportional to rate of change of the voltage across the capacitors plates. Through the capacitor plates the flow of current is actually zero. The feeling the current flows through the insulating medium that is separating the plates is because of the quick development of electrons on one plate and emptying out from the other.
Fig. 3.11 AC capacitive circuit
The circuit shown above, phase shift of 90-degrees exists between voltage and current that is voltage lags behind the current by 90-degrees. An oscillating sinusoidal voltage is produced by the AC control source.
At t=0, supply voltage has zero value causing maximum value of current through capacitor. The rate of change of this voltage of source increases and at its maximum peak that is at 900 as supply voltage neither increased or decreased for a instant of time the current through the capacitor become zero indicating full charging of capacitor.
Fig. 3.12 Capacitor charging and discharging
After that the supply voltage start decaying to zero, as the rate of change become decreasing so capacitor start discharging that is the current flows in opposite direction. At the point of 1800, the rate of change become maximum again hence zero current at this instant and the process continues.
Types of Capacitor
Depending on the dielectric (insulating material used between plates of capacitor) capacitor are of different kinds with different properties. Level of capacitance of capacitor can be changed by the dielectric being used. Usually, name to the capacitors is given on the basis of dielectric used. Some capacitors can tolerate voltage only in single direction, they are called polarized capacitor. While nonpolarized capacitors are those voltages can be applied across them in either polarity direction. In market different ranges of capacitors are available starting from small capacitors used in radio circuits or oscillators up to the large power metal-can capacitors that are used in smoothing circuits or high voltages applications. 7
1.9.1 Tantalum capacitor
Talinum capacitors are polarized ones that offers a very high level of capacitance but can not be used for under turn around one-sided condition or when they are subjected to voltages that are over their working voltage or high swell currents.
1.9.2 Silver Mica Capacitor
They are very much stable and have no or less losses. They give accurate results when procedure is not concerned of space requirements. They are usually used for RF applications and are of value approximated to 1000 u-f.
1.9.3 Polystyrene Film Capacitor
They are cheap and cost effective. They have frequency response up-to Kilo-Hertz.
1.9.4 Polyester Film Capacitor
They have a value of resistance equal to of 5% or 10%. These capacitors are normally available as leaded electronics components.
1.9.5 Metallized Polyester Film Capacitor
It is basically a type of polyester film capacitor having thin cathodes. This capacitor can be closed inside a medium small package.
1.9.6 Polycarbonate capacitor
The polycarbonate film has great stability over a wide range of temperature say – 55°C to +125°C and unwavering quality. Their dissipation factor is low
1.9.7 Polypropylene Capacitor
Here a polypropylene film is used for the dielectric. It is used for low frequencies, with 100 kHz or so on.
1.9.8 Glass capacitors
Glass is used as the dielectric and hence these capacitors have high cost. These capacitors have unusual states or performance trends regarding dramatically low loss, high RF current ability, no Piezo-electric noise.
It is also called super-capacitor or ultra-capacitor. They have a capacitance value of up to a few thousand Farads. They are used for giving memory hold-up supply and within automotive applications.
1.9.10 Dielectric Capacitor
These are multi-plate air-separated variable capacitors having an assembly of settled plates and mobile plates. These are also called rotor determine normal capacitance value. Maximum value of capacitance is obtained when two arrangements of plates are totally fit together. Trimmers are the preset compose non-polarized variable capacitors of small value equal to 500pF or less.
1.9.11 Film Capacitors
These capacitors include polyester, polystyrene, polypropylene, polycarbonate, metalized paper and Teflon. Value of capacitance is 5pF to 100uF. Long thin pieces of thin metal foil is coupled or joined with dielectric material and then twisted into a tight roll. Further they are fitted in paper or metal tubes. Thickness of dielectric film decreases the chances of punctures in it.
When dielectric is polystyrene, polycarbonate or Teflon, the capacitors are termed as “Plastic capacitors”. Here dielectric is a plastic not a paper. They work better when temperature is high. They are very reliable. They have a small tolerance.
1.9.12 Ceramic Capacitors/Disc Capacitors
It is cheap and cost effective. It is very dependable and reliable. Loss factor is low. Clay capacitors are non-polarized and have high value of capacitance and small size. They are used as de-coupling or by-pass capacitors. Ceramic capacitors have range of capacitance value from Pico-farads to microfarads, ?-F
Ceramic capacitors usually have a 3-digit code imprinted onto their body. The first two digits represent the capacitors value and the third digit shows the quantity of zero’s to be included. Letter codes are used to demonstrate their resistance value, for example, J = 5%, K = 10% or M = 20% and so on.
1.9.13 Electrolytic Capacitors
They are polarized capacitors with polarity markings. A thin layer of oxide acts as a dielectric. They have large value of capacitances and little size. They find application to remove or minimize ripple voltage or for coupling and decoupling application. They are damaged because of reverse voltage or polarity connected and over temperature.
1.9.14 Aluminum Electrolytic Capacitors
The range of capacitance value of aluminum electrolytic capacitor is from 1uF up to 47,000uF. They are composed of etched foil type or plain foil. DC current anodized the foil plates, this setup the plate material polarity. Etched foil type is smaller in size when compared to plain foil of equal value. When resistance go up to 20 % they cannot withstand high DC current as contrast with plain foil type aluminum capacitors. For part of coupling, by-pass circuit and DC blocking etched foil type capacitors are used and smoothing capacitors in power supplies are usually plain foil composes.
1.9.15 Tantalum Electrolytic Capacitors
Tantalum electrolytic capacitor is a type of polarized. These are available in both dry (solid) or wet (foil) type electrolyte whose rated Ac voltage is low. They have better dielectric properties, high stability, low leakage current and have smaller size when compared with the same valued aluminum capacitor. Mainly used for decoupling, blocking, filtering, by-passing and timin applications. They are polarized ones available in both wet (foil) a dry(solid) electrolytic types rated at substantially less Ac voltages. Tantalum capacitors are smaller than the equivalent aluminum capacitors with better dielectric properties having low leakage current and high capacitance stability. They are utilized as a part of blocking, by-passing, decoupling, filtering and timing applications. Typical range is between 47nF to 470uF.
Fig. 3.13 Capacitors Types
1.9.16 Motor capacitor
Motor capacitor can either be a run capacitor or start capacitor. This capacitor changes the current in the windings of single phase induction motor in order to build a rotational magnetic field.
184.108.40.206 Start capacitors
The value of capacitance and size are usually larger. They are used to increase motor starting torque by utilizing single phase motor startup phase. Start capacitor are disconnected as rotor achieve specific speed that was predetermined by means of centrifugal switch, this achieved speed is 75% of the maximum speed achieved for that motor type. The voltage ratings of start capacitors are 125 V, 165 V, 250 V, and 330 V. and have maximum of 70 uF capacitance value.
220.127.116.11 Run capacitors
Even when the centrifugal switch disengaged the start capacitor, these are left linked with the auxiliary coil as they are made for power factor correction or continuous duty for phase delay or power factor correction. at whatever point motor is controlled run capacitor remained powered. The range of capacitance value is 1.5 µF to 100 µF and voltage can be of 370 V or 440V top to top.
Rectification and DC Charging Unit
A rectifier is an electronic device that converts alternating current (AC) into direct current (DC) and the process is called rectification. An AC current or voltage is one which changes its direction while DC flows in only one direction.
High voltages are first step down using transformer and the resulting secondary voltages are rectified either by half wave or full wave rectifier. The output of rectifier is pulsating so a filter is used to remove these pulses. A filter may be a capacitor or choke or a combination of capacitor and resistor. A regulator is finally used to obtain a steady state DC output.
2.2 Rectifier circuits
Rectifier circuits are either single-phase or multi-phase. Three phase rectifiers are more common. For domestic equipment single-phase rectifiers and for industrial applications and transmission on HVDC three phase rectifiers are used. Rectifiers are of two types:
2.2.1 Single-phase rectifiers
18.104.22.168 Half-wave rectification
In this type of rectification one cycle of the wave is passed through forward biased diode and is rectified and other half is blocked due to the fact that diodes are reversed biased. It represents ramp function. Circuit has a single diode in a single-phase supply, or three diodes in a three-phase supply. Rectifiers have a unidirectional output current but it is not smooth but has pulses. There is a disadvantage that output has more ripples which are difficult to filter out the harmonics.
Fig. 4.1 Half wave rectifier
22.214.171.124 Full-wave rectifier
In a full-wave rectifier both positive and negative half cycles of the wave are rectified and a unidirectional output of same polarity is obtained. It can be approximated as an absolute value function. Full-wave rectification results in higher average peak value output and lesser ripples.
Full wave rectification is done either by center tap transformer using two diodes or bridge rectifier configuration using four diodes.
In case of bridge rectifier two diodes are forward biased for positive half cycle of the AC sinusoidal input and two are for negative half cycle and hence uni-directional current flows from load resistance RL for complete sinusoidal wave.
Fig. 4.2 Full wave rectifier
Fig. 4.3 Results of full wave rectification
For single-phase AC, if center tap transformer is used then two diodes back-to-back can form a full-wave rectifier.
Fig. 4.4 Rectification using centered taped transformer
2.2.2 Three-phase rectifiers
For most industrial loads and high-power applications, Other than domestic equipment, three rectifier circuits are used. Three-phase rectifiers are also may be half wave or full wave with center tap or bridge configuration.
Current Measurements Techniques
Current sensing is used to measure “how much” current is flowing in a circuit and to define when it is “very high” or a fault condition. It is vital to select the appropriate technology with a robust design to properly tolerate extreme conditions that can occur during a fault.
3.2 Measurement Methods
1. Resistive (direct)
a. Current sense resistors
2. Magnetic (indirect)
a. Current transformer
b. Rogowski coil
c. Hall effect device
Every method has benefits that make it an effective or suitable method for current measurement, but also has tradeoffs that can be critical to the end reliability of the application.
126.96.36.199 Current Sense Resistor
The resistor is a direct method of current measurement and it has advantage of linearity and simplicity . The current sense resistor is placed in line with the current that we have to measure and the resultant current flow origins a small amount of power to be changed into heat. This power conversion is what delivers the voltage signal. The current sense resistor is a economical solution with a steady temperature coefficient of resistance (TCR) of 100 ppm/°C or 0.01 %/°C.
188.8.131.52 Current Transformer
A current transformer provide isolation from lossless current measurement, line voltage, and a large signal voltage that can offer noise immunity. The method needs a changing current – such as an AC, transient current, or switched DC to provide a changing magnetic field that is magnetically attached into the secondary windings. The secondary measurement voltage can be calculated according to the turns ratio between the primary and secondary windings. This measurement method is reflected “lossless” since very slight resistive losses are there. A lesser amount of power is waste due to transformer losses from the burden resistor, core losses, and primary and secondary DC resistance
Fig. 5.1 ideal and practical model of CT
184.108.40.206 Rogowski Coil
A voltage is induced into a secondary coil that is proportional to the current flow through an isolated conductor. Different thing is that the Rogowski coil is an air core design as opposite to the current transformer that relies on a high-permeability core, such as a laminated steel, to magnetically couple to a secondary winding. The air core design has a lesser inductance to provide a earlier signal response and very linear signal voltage. It is considered as a lower-cost alternative to the current transformer. 17
Fig. 5.2 Rogowski coil with integrator circuit
220.127.116.11 Hall Effect
When a current-carrying conductor is placed in a magnetic field a difference in potential occurs perpendicular to the magnetic field and the direction of current flow. This potential is proportional to the magnitude of the current flow. The charges interact with the magnetic field causing the current distribution to change, which creates the hall voltage. 18
They can measure large currents with low power dissipation but some disadvantages such as non-linear temperature drift requiring compensation, limited bandwidth, low-range current detection requiring a large offset voltage that can lead to error, susceptibility to external magnetic fields, ESD sensitivity, and high cost make them less preferable
Fig. 5.3 Hall effect with presence and absence of magnetic field
3.3 Conversion of Galvanometer into ammeter
Galvanometer is used to detect the flow of current. It is a current sensor. To convert a Galvanometer into an Ammeter, a very low resistance known as “shunt” resistance is connected in parallel to Galvanometer. Value of shunt is adjusted in such a way that most of the current passes through the shunt and only a certain amount of current can pass through galvanometer which is enough to deflect its needle to full scale.
Fig. 5.4 Ammeter internal circuity
3.4 Using Oscilloscope
An oscilloscope cannot be used directly to measure electrical current. Instead an indirect measurement of electrical current is made via oscilloscope by attaching resistors of known value to the oscilloscope’s probes, measuring the voltage across the resistor, and then using Ohm’s Law to calculate the electrical current. Another easy way to measure current is to use a clamp- on current probe with an oscilloscope.
3.4.1 Initialize the Oscilloscope settings
• Plug in the oscilloscope.
• Press the AUTOSET or PRESET button.
• set the display to channel 1 AC and volts to the mid-range
• turn the magnification settings off
• ensure the trigger mode is auto and the trigger source is channel 1 and turn off the trigger hold-off
• set the intensity control to nominal
3.4.2 Calculate the Current
Attach a probe with the register to an electrical circuit. Resistor’s power rating should be equal or greater than the power output of the system.
Adjust the horizontal and vertical controls until you get a stable sine wave, and then take measurements.
Take a voltage measurement and then plug that value along with the value of the resistor into Ohm’s Law formula to calculate the current.
Testing Equipment (Circuit Breaker)
It is a switching device and can be operated automatically or manually. It is used for protection and controlling of electrical power system. An electric power system runs smoothly and effectively when there is no over current or over voltage. Short circuiting or sudden connection of the hot end wire to the ground wire would heat up the wires, causing arc and fire. The circuit breaker will avoid such situations which simply cut off or trip the faulty circuit or equipment by opening its contacts. 8
Fig. 6.1 Model
4.2 Types of circuit breakers
Circuit breakers are classified on the basis of their features such as voltage class, construction type, interrupting type, and structural features.
According to voltage level on which breakers can operate, they are categorized as:
1. High Voltage Circuit Breakers
2. Medium Voltage Circuit Breakers
3. Low Voltage Circuit Breakers
4.2.1 High Voltage Breaker
The definition of high voltage in power transmission is 72.5 kV or higher, according to a recent definition by the International Electro-technical Commission (IEC). High-voltage breakers are always solenoid-operated and current sensing protective relays are operated through current transformers.
Circuit breakers can be classified as live tank, where the enclosure that contains the breaking mechanism is at line or potential, or dead tank with the enclosure at earth potential or zero volts.
• Air Circuit Breaker
• SF6 Circuit Breaker
• Vacuum Circuit Breaker
• Oil Circuit Breaker
18.104.22.168 Air Circuit Breaker
This circuit breaker will operate in the air. These are used up to 15 KV. The quenching medium is an Arc at atmospheric pressure. The two types of air circuit breakers are
• Plain air circuit breaker
• Air blast Circuit Breaker
Air Blast breakers are used for system voltage of 245 KV, 420 KV. They are used for fast and frequent operations because of lesser arc energy and are used in Low as well as High Currents and voltage applications. They are small in size, have high speed and require less maintenance. They further consist of
• Axial blast breaker
• Axial blast with sliding moving contact.
22.214.171.124 SF6 Circuit Breaker
In the SF6 circuit breaker the current carrying contacts operate in Sulphur hexafluoride gas. It has an excellent insulating property and high electro-negativity. SF6 is 100 times more effective in arc quenching media than air circuit breaker. It is used for both medium and high voltage electrical power system from 33KV to 800KV. 13 They are of
• Single interrupter SF6 circuit breaker used up to 220
• Two interrupter SF6 circuit breaker used up to 400
• Four interrupter SF6 circuit breaker used up to 715V
126.96.36.199 Vacuum Circuit Breaker
Here vacuum is used to extinct the arc. It has dielectric recovery character, excellent interruption and can interrupt the high frequency current which results from arc instability, superimposed on the line frequency current. These are usually for voltages up to about 40,500 V with a longer life expectancy.
188.8.131.52 Carbon dioxide (CO2) high-voltage circuit breakers
It is a 75-kV high-voltage breaker. It uses carbon dioxide as the medium to extinguish the arc. They have working principle same as an SF6 breaker has and can also be produced as a disconnecting circuit breaker. By switching from SF6 to CO2 it is possible to reduce the CO2 emissions to a large extent.
184.108.40.206 Disconnecting circuit breaker (DCB)
It eliminates the need for separate dis-connectors while having a maintenance interval of 15 years thus reducing cost expenditures. Using DCB solution also reduces the space requirements within the substation and increases the reliability.
4.2.2 Medium-voltage circuit breakers
These breakers rated between 1 and 72 kV. They are enclosed into metal-enclosed switchgear line ups for indoor use, or sometimes installed outdoors in a substation. These are operated by current sensing protective relays operated through current transformers as input signal giving device. The characteristics of MV breakers are given by international standards such as IEC 62271. There is an advantage that medium-voltage circuit breakers do not rely on built-in thermal or magnetic overcurrent sensors.
4.2.3 Low-voltage circuit breakers
• Miniature circuit breaker (MCB)
• Low voltage power breakers:
• Molded Case Circuit Breaker (MCCB
220.127.116.11 Miniature Circuit breaker
Rated current is not more than 100 A for these breakers. Trip characteristics are not normally adjustable. Thermal or thermal-magnetic operations are there.
18.104.22.168 Low voltage power breakers
These circuit breakers are often installed in draw-out enclosures so that there is an option of removal and interchange without dismantling the switchgear. They can be mounted in low-voltage switchboards or switchgear cabinets. Large low-voltage molded case and power circuit breakers may have electric motor operators so they can open and close under remote control thus forming a part of an automatic transfer switch system for standby power.
22.214.171.124 Molded Case Circuit Breaker
They are rated current up to 2,500 A. Thermal or thermal-magnetic operation can be performed. Trip current may be adjustable in larger ratings.
126.96.36.199 Magnetic circuit breakers
Magnetic circuit breakers use a solenoid (electromagnet) in its structure which has increasing pulling force with increases in the current. As the current in the solenoid increases beyond the safe rating of the circuit breaker, the solenoid’s pull releases the latch holding breaker’s contact and contacts are open by spring action.
188.8.131.52 Thermal magnetic circuit breakers
They include both techniques with the electromagnet responding instantaneously to large surge or impulse of current (short circuits) and the bimetallic strip responding to less extreme but longer time over-current conditions. There is a a time response feature, that trips the circuit breaker sooner for larger over-currents with no intentional time delay but allows smaller overload currents to persist for a longer time and a time delayed operation is performed.
184.108.40.206 Magnetic-hydraulic circuit breakers
A magnetic-hydraulic circuit breaker uses a solenoid coil to provide operating force to open the contacts. Magnetic-hydraulic breakers include a hydraulic time delay feature using a viscous fluid. Short-circuit currents provide enough solenoid force to release the latch thus opening the contacts. Hydraulic energy may be supplied by a pump or stored in accumulators.
220.127.116.11 Common trip breakers
When there is more than one live conductor in a circuit, each live conductor must be protected by a breaker pole. To ensure that all live conductors are interrupted when any pole trips, a “common trip” breaker must be used. Two-pole common trip breakers are used with 120/240-volt systems where three-pole common trip breakers are typically used to supply three-phase electric power to large motors or further distribution boards.
There is another category which is related to the fact that which mechanism is used to actuate the circuit breaker, which specifies the mechanism of operation of the breaker, there are three further types:
1. Hydraulic Circuit Breakers
2. Pneumatic Circuit Breakers
3. Spring actuated Circuit Breakers
Another very important category is where to use the circuit breaker.
1. Outdoor Circuit Breakers
2. Indoor Circuit Breakers
4.3 Circuit breaker Specification on Data Sheet
4.3.1 Breaking capacity
Breaking capacity or interrupting rating is the current that a fuse, circuit breaker, is able to interrupt without being damaged or causing an electric arc or spark. This is the maximum current a circuit breaker can safely interrupt and not being destroyed. This short-circuit current rating is normally expressed in RMS symmetrical amperes.
4.3.2 Short Circuit Breaking Current of Circuit Breaker
Short circuit breaking capacity or short circuit breaking current of circuit breaker is defined as maximum current that can flow through the breaker from time of occurring short circuit to the time of clearing the short circuit without any permanent damage or harm in the CB. The value of short circuit breaking current is expressed in RMS.
4.3.3 Rated Short Circuit Making Capacity
Making capacity of a circuit breaker is the maximum current which the breaker can conduct at the time of closing. It is considered as the peak value of the first cycle when there is an imaginary and instant short circuit between the phases.
As it is expressed in maximum peak value, it is always more than rated short circuit breaking current of circuit breaker. Normally value of short circuit making current is 2.5 times more than short circuit breaking current.
4.3.4 Rated Voltage of Circuit Breaker
Rated voltage of circuit breaker depends upon its insulation system. For below 400 KV systems, the circuit breaker is designed to withstand 10% above the normal system voltage. For above or equal 400 KV system the insulation of circuit breaker should be capable of withstanding 5% above the normal system voltage. That means, rated voltage of circuit breaker corresponds to the highest system voltage.
A circuit breaker may have also to face lighting impulse and switching impulses during its life span. The insulation system of CB and contact gap of an open CB must withstand these impulse voltage waveform amplitudes of this disturbance is very high but extremely transient in nature. So a circuit breaker is designed to withstand this impulse peaky voltage for microsecond range only.
4.3.5 High Voltage Circuit Breakers specifications
Based on IEC 62271-100 (HV circuit breakers) and IEC 62271-1 (HV switchgear):
18.104.22.168 I r (Rated normal current)
It is the rated continuous / uninterrupted current that the circuit breaker can carry.
22.214.171.124 I k (Rated short-time withstand current)
This is the RMS value of the current which must be carried by the circuit breaker in the closed position during a specified short time under prescribed use and behavior. The specified time has a standard value of 1s, 0.5s, 2s and 3s. The rated short-time withstand current is equal to the rated short-circuit breaking current (I s-c).
126.96.36.199 I p (Rated peak withstand current)
The peak current associated with the rated short-time withstand current, and is defined according to the dc time constant. The rated peak withstand current is equal to the rated short-circuit making current (I m-c).
188.8.131.52 I s-c (Rated short-circuit breaking current)
The highest short circuit current that the circuit breaker shall be capable of breaking under the condition of use and behavior prescribed in IEC 62271-100.
184.108.40.206 I-mc (Rated short-circuit making current)
The short-circuit current that the circuit breaker can withstand as it is closing where the act of closing initiates the fault.
220.127.116.11 TRV (Transient recovery voltage)
This is the reference voltage which constitutes the limit of the prospective transient recovery voltage of circuit which the circuit breaker shall be capable of withstanding under short circuit.
4.3.6 Low Voltage Circuit Breaker specifications
Based on IEC 60947-2
18.104.22.168 In (Rated current)
It is the rated continuous / uninterrupted current that the circuit breaker can carry.
22.214.171.124 I-cm (Rated short-circuit making current)
The short-circuit current that the circuit breaker can withstand as it is closing where the act of closing initiates the fault.
126.96.36.199 I-cu (Rated ultimate short-circuit current)
This is the maximum symmetrical short-circuit current the circuit breaker can interrupt.
188.8.131.52 I c-s (Rated service short-circuit current)
It is the maximum current the breaker can interrupt multiple times and be returned to service without being damaged and is expressed as a % of I c