What do u mean by impulse voltage
Impulse voltage, also known as surge voltage or transient voltage, is a high-voltage electrical phenomenon characterized by:
Key Characteristics:
1. High amplitude (voltage peak)
2. Short duration (typically microseconds to milliseconds)
3. Rapid rise time (nanoseconds to microseconds)
4. Exponential or oscillatory decay
Types of Impulse Voltages:
1. Lightning impulse (e.g., lightning strike)
2. Switching impulse (e.g., power system switching)
3. Power frequency impulse (e.g., fault conditions)
4. Oscillatory impulse (e.g., resonant phenomena)
Parameters:
1. Peak voltage (Vp)
2. Front time (T1)
3. Tail time (T2)
4. Rise time (Tr)
5. Fall time (Tf)
6. Duration (Td)
Standard Impulse Voltage Waveforms:
Effects on Equipment:1. Insulation breakdown2. Electrical discharge3. Overvoltage stress4. Thermal stress5. Magnetic saturation
A high impulse voltage generator is a device used to generate high-voltage impulses for testing and research applications.
Types:1. Marx Generator 2. Cockcroft-Walton Generator 3. Tesla Transformer 4. Pulse Transformer 5. Solid-State Generator
Applications:1. High-voltage testing2. Insulation testing3. Electrical discharge studies4. Plasma research5. Particle acceleration6. Medical equipment testing (e.g., defibrillators)7. Aerospace and defense research
Key Components:1. High-voltage power supply 2. Impulse generator circuit3. Pulse shaping network 4. Voltage multiplier (e.g., Marx generator) 5. Output transformer 6. Load resistor or capacitor
Parameters:
1. Peak voltage (up to 1 MV)
2. Impulse duration (ns to μs) 3. Rise time (ns to μs) 4. Fall time (ns to μs) 5. Repetition rate (Hz to kHz) 6. Pulse shape (e.g., exponential, rectangular)
Effects on Equipment:1. Insulation breakdown2. Electrical discharge3. Overvoltage stress4. Thermal stress5. Magnetic saturation
Impulse Voltage Waveform
To understand the impulse voltage better let’s take a look at the impulse voltage waveform. In the below image, a single peak of high voltage Impulse waveform is shown
As you can see, the wave is getting to its maximum 100 percent peak within 2 uS. This is very fast, but the high voltage is losing its strength with a span of 40uS almost. Therefore, the pulse has a very short or fast rise time whereas a very slow or long fall time.
The duration of the pulse is called the wave tail which is defined by the difference between 3rd-time stamp ts3 and ts0.
Single Stage Impulse Generator
In order to understand the working of an Impulse generator, let stake a look at the circuit diagram of a single-stage impulse generator that is shown below
The above circuit consists of two capacitors and two resistances. The spark Gap (G) is an electrically isolated gap between two electrodes where electrical sparks happen. A high voltage power source is also shown in the above image. Any impulse generator circuit needs at least one large capacitor that is charged to an appropriate voltage level and then discharged by a load.
In the above circuit, the CS is the charging capacitor. This is a high voltage capacitor typically more than a 2kV rating (depends on the desired output voltage). The capacitor CB is the load capacitance that will discharge the charging capacitor. The resistor and RD and RE control the wave shape.
If the above image observed carefully, we can find that the G or spark gap has no electrical connection. Then how does the load capacitance get the high voltage? Here is the trick and by this one, the above circuit acts as an impulse generator. The capacitor is charged until the capacitor’s charged voltage is enough to cross the spark gap. An electrical impulse generated across the spark gap and high voltage gets transferred from the left electrode terminal to the right electrode terminal of the spark gap and thus making it a connected circuit.
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The response time of the circuit can be controlled by varying the distance between two electrodes or changing the capacitors fully charged voltage. The output impulse voltage calculation can be done by calculating the output voltage waveform with
Disadvantages of Single Stage Impulse Generator
The major disadvantage of a single-stage impulse generator circuit is the physical size. Depending on the high voltage rating, the components get bigger in size. Also, high impulse voltage generation requires a high DC voltage. Therefore, for a single-stage impulse voltage generator circuit, it gets quite difficult to get optimum efficiency even after using large DC power supplies.
Marx generator
Erwin Otto Marx provided a multistage impulse generator circuit in 1924. This circuit is specifically used to generate high impulse voltage from a low voltage power source. The circuit of multiplexed impulse generator or commonly called as Marx circuit can be seen in the below image.
The above circuit uses 4 capacitors (there can be n number of capacitors) that are charged by a high voltage source in parallel charging condition by the charge resistors R1 to R8.
During the discharge condition the spark gap which was an open circuit during the charging state, acts as a switch and connects a series path through the capacitor bank and generates a very high impulse voltage across the load. The discharge condition is shown in the above image by the purple line. The voltage of the first capacitor needs to be exceeded sufficiently to break down the spark gap and activate the Marx generator circuit.
When this occurs the first spark gap connects two capacitors (C1 and C2). Therefore the voltage across the first capacitor gets double by two voltages of C1 and C2. Subsequently, the third spark gap automatically breaks down because the voltage across the third spark gap is high enough and it starts to add the third capacitor C3 voltage into the stack and this goes on up to the last capacitor. Finally, when the last and final spark gap is reached, the voltage is large enough to break the last spark gap across the load which has a larger gap between the spark plugs.
The final output voltage across the final gap will be nVC (where n is the number of capacitors and VC is the capacitor charged voltage) but this is true in ideal circuits. In real scenarios, the output voltage of the Marx Impulse generator circuit will be much lower than the actual desired value.
However, this last spark point needs to have larger gaps because, without this, the capacitors don’t get into a fully charged condition. Sometimes, the discharge is done intentionally. There are several ways to discharge the capacitor bank in the Marx generator.
High DC Voltages:
Series Resistance Microammeter:
High d.c. voltages are usually measured by connecting a very high resistance (few hundreds of megaohms) in Series Resistance Microammeter as shown in Fig. 7.1.
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Only the current I flowing through the large calibrated resistance R is measured by the moving coil microammeter. The voltage of the source is given by
Measurement of High DC Voltages and Currents:
The voltage drop in the meter is negligible, as the impedance of the meter is only few ohms compared to few hundred mega-ohms of the series resistance R. A protective device like a paper gap, a neon glow tube, or a zener diode with a suitable series resistance is connected across the meter as a protection against high voltages in case the series resistance R fails or flashes over. The ohmic value of the series resistance R is chosen such that a current of one to ten microamperes is allowed for full-scale deflection.
The limitations in the Series Resistance Microammeter design are:
Series Resistance Microammeter are built for 500 kV d.c. with an accuracy better than 0.2%.
Resistance Potential Divider for DC Voltage:
A resistance potential divider with an electrostatic or high impedance voltmeter is shown in Fig. 7.2. The influence of temperature and voltage on the elements is eliminated in the voltage divider arrangement. The high voltage magnitude is given by [(R1 +R2)/R2] v2, where v2 is the d.c. voltage across the low voltage arm R2. With sudden changes in voltage, such as switching operations, flashover of the test objects, or source short circuits, flashover or damage may occur to the ground capacitances. To avoid these transient voltages, voltage controlling capacitors are connected across the elements.�
A Series Resistance Microammeter with a parallel capacitor connection for linearization of transient potential distribution is shown in Fig. 7.3. Potential dividers are made with 0.05% accuracy up to 100 kV, with 0.1% accuracy up to 300 kV, and with better than 0.5% accuracy for 500 kV.
Generating Voltmeters High voltage measuring devices employ generating principle when source loading is prohibited (as with Van de Graaff generators, etc.) or when direct connection to the high voltage source is to be avoided. A generating voltmeter is a variable capacitor electrostatic voltage generator which generates current proportional to the applied external voltage. The device is driven by an external synchronous or constant speed motor and does not absorb power or energy from the voltage measuring source.
Measurement of High AC Voltage
Series Impedance Voltmeter:
For power frequency a.c. measurements the Series Impedance Voltmeter may be a pure resistance or a reactance. Since resistances involve power losses, often a capacitor is preferred as a series reactance. Moreover, for high resistances, the variation of resistance with temperature is a problem, and the residual inductance of the resistance gives rise to an impedance different from its ohmic resistance.
Series capacitance voltmeter
The current Ic through the meter is:
where,
C= capacitance of the series capacitor, ω= angular frequency, and V= applied a.c. voltage.
Series capacitance voltmeters were used with cascade transformers for measuring rms values up to 1000 kV. The series capacitance was formed as a parallel plate capacitor between the high voltage terminal of the transformer and a ground plate suspended above it. A rectifier ammeter was used as an indicating instrument and was directly calibrated in high voltage rms value. The meter was usually a 0-100 μA moving coil meter and the over-all error was about 2%.
Capacitive Voltage Transformer (CVT)
Definition: The capacitive voltage transformer step-down the high voltage input signals and provide the low voltage signals which can easily measure through the measuring instrument. The Capacitive voltage transformer (CVT) is also called capacitive potential transformer
The capacitive potential divider is used in combination with the auxiliary transformer and the inductive element. The capacitive potential divider step-down the extra high voltage signals into a low voltage signal. The output voltage of the capacitive potential transformer is further step-down by the help of the auxiliary transformer.
Consider the circuit diagram of the capacitive potential transformer.
The capacitor or potential divider is placed across the line whose voltage is used to be measured or controlled. Let the C1 and C2 be the capacitor placed across the transmission lines. The output of the potential divider acts as an input to the auxiliary transformer.
The capacitor places near to the ground have high capacitances as compared to that placed near the transmission line. The high value of capacitances means the impedance of that part of the potential divider becomes low. Thus, low voltages pass to the auxiliary transformer. The auxiliary transformer further step-down the voltages.
The N1 and the N2 are the numbers of turns on the primary and the secondary winding of the transformer. The meter used for measuring the low value of voltage is resistive, and the potential divider is capacitive. Thereby, the phase shift occurs, and the output will be affected. To overcome this problem, the inductance is placed in series with the auxiliary transformer.
This inductance L consists the leakage flux of the auxiliary winding of the auxiliary transformer. The value of inductances is given as
The value of inductances is adjustable. The inductance compensates the voltage drops occurs in the transformer because of the reduction of the current from the potential divider. But, in actual practice, the compensation is not possible because of the inductance losses.
The voltage turn ratio of the transformer is expressed as
As the value of C1 is greater than the C2. Thus the value C1/(C1+C2) is small.
The low value of voltage is obtained.
Electrostatic voltmeter
An electrostatic voltmeter uses the attraction force between two charged surfaces to create a deflection of a pointer directly calibrated in volts. Since the attraction force is the same regardless of the polarity of the charged surfaces (as long as the charge is opposite), the electrostatic voltmeter can measure DC voltages of either polarity.
Typical construction is shown in the drawing. The pivoted sector NN is attracted to the fixed sector QQ. The small weight w counterbalances the moving sector and indicates the voltage by the pointer P. In newer instruments, the weight is replaced by a spring, thus allowing the meter to be used in horizontal and vertical positions; this form is shown in the photograph.
The fixed sector is insulated from the rest of the meter. The butterfly-shaped moving industry is made of thin aluminum foil. To minimize high electrical stress, the fixed and moving sectors are highly polished without sharp corners. An electrostatic voltmeter uses the attraction force between two charged surfaces to create a deflection of a pointer directly calibrated in volts.
series capacitance peak voltmeter (chubb-Fortscue method)
A simple, yet accurate, method for the measurement of peak values of ac power frequency voltage was proposed by Chubb and Fortescue in 1913.
Peak Reading AC Voltmeter Circuit : When a capacitor is connected to a sinusoidal voltage source, the charging current
where V is the rms value of the voltage and co is the angular frequency. If a half wave rectifier is used, the arithmetic mean of the rectifier current is proportional to the peak value of the a.c. voltage. The schematic diagram of the circuit arrangement is shown in Fig. 7.15. The d.c. meter reading is proportional to the peak value of the value Vm or
where I is the d.c. current read by the meter and C is the capacitance of the capacitor. This method is known as the Chubb-Frotscue method for peak voltage measurement.
The diode D1 is used to rectify the a.c. current in one half cycle while D2 bypasses in the other half cycle. This arrangement is suitable only of positive or negative half cycles and hence is valid only when both half, cycles are symmetrical and equal. This method is not suitable when the voltage waveform is not sinusoidal but contains more than one peak or maximum as shown in Fig. 7.16.
Spark gaps for measurement of High dc, ac and Impulse voltages –
Spark gap measurements
Spark gaps are commonly used for measuring high DC, AC, and impulse voltages due to their simplicity, reliability, and accuracy.
Here's an overview:
Principle: A spark gap consists of two electrodes separated by a gap. When the voltage across the gap exceeds the breakdown voltage, an electrical discharge (spark) occurs, creating a conductive path. The voltage at which the spark occurs is directly proportional to the gap length.
Types of Spark Gaps:
Applications:
1. High Voltage Measurement: DC, AC, and impulse voltages up to several hundred kilovolts.
2. Voltage Regulation: Monitoring voltage levels in power systems.
3. Surge Protection: Protecting equipment from voltage surges and transients.
4. Research and Development: Studying high-voltage phenomena and testing insulation materials.
Advantages:
1. Simple and Robust: Low maintenance and high reliability.
2. Accurate: ±1-2% accuracy achievable.
3. Wide Range: Measures voltages from a few kilovolts to several hundred kilovolts
4. Fast Response: Nanosecond response times for impulse voltage measurement.
Limitations:
1. Non-Linear Response: Spark gap voltage response can be non-linear.
2. Electromagnetic Interference (EMI): Spark gaps can generate EMI.
3. Limited Resolution: Gap length limitations affect measurement resolution.
Sphere Gap Measurement can be arranged either (i) vertically with lower sphere grounded, or (ii) horizontally with both spheres connected to the source voltage or one sphere grounded.
In horizontal configurations, it is generally arranged such that both spheres are symmetrically at high voltage above the ground. The two spheres used are identical in size and shape. The schematic arrangement is shown in Figs 7.19a and 7.19b. The voltage to be measured is applied between the two spheres and the distance or spacing S between them gives a measure of the sparkover voltage.
Factors influencing the spark over voltage of sphere gaps.
Module 4
Lightning phenomenon –Charge formation in the clouds
Lightning is a natural electrical phenomenon, which is consist from a lightning and thunder. A conditions for the formation of lightnings are storm clouds which arise when a sufficient amount of moisture, condensation particles and heat are present.
Lightnings in the storm clouds are formed by separating the charges. In the lower part of cloud is negative charge and in the upper part of cloud is positive charge. Due to vertical winds, which rise the water droplets which collide into the particle of ice in the cloud, the electrons charge separation occurs. This leads to excess electrons (negative charge) in the lower part of cloud.�
To the formation of negative charge also contribute freezing of water droplets when they are rising. The rising part of freezing droplets have negative charge, other water droplets are raised by the wind into the positive part of the cloud. The process of lifting the water droplets and freezing the other particles, leads to the distribution of negative charge into lower part of the cloud and positive charge in the upper part of the cloud.
Because of that in the lower and the upper part of the cloud appear sufficiently large electric field that generate the lightning.
Definition: Lightning stroke is the direct discharge of an electrical charge between the atmosphere and the object of earth. It is a sudden flow of electric charge between the electrical charge area of a cloud also called intra-cloud and another cloud called (CC lightning) or between the charged cloud and the ground (CG lightning).
The lightning strike is a process
lightning is not instantaneous phenomenon, is a process that takes place in steps. First is formed the so called "lider" which start to spread from the lower (negative lightning) or the upper (positive lightning) part of cloud to the ground. Lider spread where air ionization occurs more frequently, and therefore the lider have unusual shape.
Lider transmit weak violet light. When lider who is expanding from the cloud toward the ground looking for the way to the ground, the "streamer" which carries positive charge begin to rise up toward the cloud from the exposed parts of the ground. When lider and streamer meet, the conductive channel is established between the cloud and the ground. Through that channel electric charge from the cloud reach the ground. This is called lightning strike.
When this happen the electric current that flow through the channel rapidly increase to a few 100kA. In addition to that, the air in the channel become very hot, which is seen as a flash. Heated air also expand very rapidly, which causing sonic shock wave that we hear as thunder and could be nearby the lightning destructive.
Here's a detailed explanation of the lightning mechanism:
Step 1: Cloud Formation
Clouds form when warm, moist air rises into the atmosphere and cools, causing the water vapor to condense into droplets.
Step 2: Ice and Water Interactions
Within these clouds, water droplets and ice crystals interact, generating static electricity. The ice crystals tend to become positively charged, while the water droplets become negatively charged.
Step 3: Charge Separation
As the cloud grows, the positively charged ice crystals accumulate at the top of the cloud, while the negatively charged water droplets accumulate at the bottom. This separation of charges creates an electric field between the top and bottom of the cloud.
Step 4: Leader Formation
As the electric field strengthens, a channel of ionized air molecules, called a leader, begins to form between the cloud and the ground. The leader is a path of least resistance for the electrical discharge.
Step 5: Return Stroke
Once the leader reaches the ground, a massive surge of electricity, known as the return stroke, flows through the
Step 6: Thunder
As the return stroke travels through the air, it heats the air around it, creating a shockwave that produces the sound we know as thunder.
The entire process, from cloud formation to the return stroke, takes only a few seconds. Lightning can reach temperatures of up to 30,000°C, which is five times hotter than the surface of the sun.
In case a direct stroke occurs over the top of an unshielded transmission line, the current wave tries to divide into two branches and travel on either side of the line. Hence, the effective surge impedance of the line as seen by the wave is Z0/2 and taking the above example, the overvoltage caused may be only 10,000 (400/2) = 2000 kV. If this line were to be a 132 kV line with an eleven 10 inch disc insulator string, the flashover of the insulator string will take place, as the impulse flashover voltage of the string is about 950 kV for a 2 μs front impulse wave.
The incidence of lightning strikes on transmission lines and sub-stations is related to the degree of thunderstorm activity. It is based on the level of “Thunderstorm days” (TD) known as “Isokeraunic Level” defined as the number of days in a year when thunder is heard or recorded in a particular location. But this indication does not often distinguish between the ground strokes and the cloud-to-cloud strokes. If a measure of ground flashover density (Ng) is obtained, then the number of ground flashovers can be computed from the TD level. From the past records and the past experience, it is found that
It is reported that TD is between 5 and 15 in Britain, Europe and Pacific west of North America, and is in the range of 30 to 50 in Central and Eastern states of U.S.A. A much higher level is reported from South Africa and South America. No literature is available for the different regions in India, but a value of 30 to 50 may be taken for the coastal areas and for the central parts of India.
Indirect Stroke (also known as Indirect Lightning Strike):
Definition: An indirect stroke occurs when a lightning discharge strikes the ground or another object near a power system or structure, inducing over voltages and currents in the system.
Characteristics:
Types of Indirect Strokes:
Effects of Overvoltages:1. Equipment damage (transformers, switchgear, etc.)2. Power outages3. Fire risk4. Data loss and communication disruptions
Effects on Power Systems:
Factors Influencing Indirect Stroke Effects:
Protection Measures:1. Lightning arresters2. Surge protectors3. Grounding systems4. Shielding and screening5. Overvoltage protection devices (OPDs)6. Regular maintenance and testing
Power frequency Overvoltage –
Sudden load rejection,
Ferranti effect.
Control of over voltages due to switching.
Power frequency overvoltage is a type of electrical overvoltage that occurs in power systems and has the same frequency as the network. It can be caused by a number of factors, including:
Power frequency overvoltages can cause a number of issues, including:
Power frequency overvoltage protection devices are used to limit the impact of these overvoltages. They are often used in combination with circuit breakers, RCCB (Residual-Current Circuit Breaker), or RCBO (Residual Current Circuit Breaker with Overload protection).
Power Frequency Overvoltages in Power Systems:
The Power Frequency Overvoltages occur in large power systems and they are of much concern in EHV systems, i.e. systems of 400 kV and above. The main causes for power frequency and its harmonic overvoltages are
Power Frequency Overvoltages harmonics and voltages with frequencies nearer to the operating frequency are caused during tap changing operations, by magnetic or ferro-resonance phenomenon in large power transformers, and by resonating overvoltages due to series capacitors with shunt reactors or transformers.
The duration of these overvoltages may be from one to two cycles to a few seconds depending on the overvoltage protection employed.
1.Sudden Load Rejection:
Sudden load rejection on large power systems causes the speeding up of generator intervene to restore normal conditions. But initially both the frequency and voltage increase. The approximate voltage rise, neglecting losses, etc. may be taken as
where xs is the reactance of the generator (≈ the sum of the transient reactances of the generator and the transformer), xc is the capacitive reactance of the line at open end at increased frequency, E’ the voltage generated before the over-speeding and load rejection, f is the instantaneous increased frequency, and f0 is the normal frequency.
This increase in voltage may go to as high as 2.0 per unit (p.u.) value with 400 kV lines. The voltage at the sending end is affected by the line length, short circuit MVA at sending end bus, and reactive power generation of the line (due to line capacitive reactance and any shunt or series capacitors). Shunt reactors may reduce the voltage to 1.2 to 1.4 p.u.
2.Ferranti Effect
The Ferranti effect is an electrical engineering phenomenon where the voltage at the receiving end of a long transmission line is higher than the voltage at the sending end
What it is | An increase in voltage at the receiving end of a long AC transmission line when the load is very small or no load is connected |
Why it happens | The line's capacitance charges and discharges, causing a voltage drop across the line inductance |
When it happens | Occurs in medium and long transmission lines when the loads are lightly loaded or non-load |
What it can damage | Voltage-sensitive process controls, controllers, automated systems, and equipment |
Ferranti Effect
Definition: The effect in which the voltage at the receiving end of the transmission line is more than the sending voltage is known as the Ferranti effect. Such type of effect mainly occurs because of light load or open circuit at the receiving end.
Ferranti effect is due to the charging current of the line. When an alternating voltage is applied, the current that flows into the capacitor is called charging current. A charging current is also known as capacitive current. The charging current increases in the line when the receiving end voltage of the line is larger than the sending end.
Why Ferranti effect occurs?
Capacitance and inductance are the main parameters of the lines having a length 240km or above. On such transmission lines, the capacitance is not concentrated at some definite points. It is distributed uniformly along the whole length of the line.
When the voltage is applied at the sending end, the current drawn by the capacitance of the line is more than current associated with the load. Thus, at no load or light load, the voltage at the receiving end is quite large as compared to the constant voltage at the sending end.
Detail explanation of the Ferranti effect by considering a nominal pi (π) model:
Let us consider the long transmission line in which OE represents the receiving end voltage; OH represent the current through the capacitor at the receiving end. The phasor FE represents the voltage drop across the resistance R. The voltage drop across the X (inductance). The phasor OG represents the sending end voltage under a no-load condition.
It is seen from phasor diagram that OE > OG. In other words, the voltage at the receiving end is greater than the voltage at the sending end when the line is at no load.
For a nominal pi (π) model
Module-5
Non-Destructive Testing of Materials and Electrical Apparatus
Power frequency measurements- Schering bridge for audio frequency, transformer ratio arm
bridge. Partial discharge measurements- straight discharge detection, Balance detection.
High Voltage Testing of Electrical Apparatus-Testing of insulators, bushings, circuit breakers,
cables. Testing of transformers- Impulse test, Tests on surge arrestors.
Non-destructive testing refers to the use of testing techniques that do not alter any of the properties of the tested product. These properties could be its strength, integrity, appearance, corrosion resistance, conductivity, wear resistance, toughness and so on
Power frequency measurements- Schering bridge for audio frequency
Schering Bridge for Audio Frequency Range (50 Hz to 100 kHz):
The capacitance bridge used for dielectric measurements is shown in Fig. 9.16, where CARA and CBRB are the ratio arm capacitances. Substitution method is preferred in this range for greater accuracy. The specimen is connected across the standard variable capacitance CN. The bridge is balanced with and without the dielectric specimen CX. The specimen capacitance connected (CX) is the difference of the two readings of the standard capacitor CN.
The balance for the loss factor (resistive component balance) may be done in any one of the following ways:
Of these three, the third method is normally used, as CB can be made a variable air capacitor of high quality and the errors that arise within the resistances at high frequencies (skin effect, etc.) are avoided.
The Dissipation Factor in Schering Bridge tan δ for the third method is given as,
transformer ratio arm bridge
It is a common practice to use the four arm Wheatstone bridge network for a.c. measurements. In high frequency measurements, the arms with high values of resistances lead to difficulties due to their residual inductances, capacitances, and skin effect. Also, shielding and grounding becomes difficult in large arms. Hence, at high frequencies the transformer ratio arm bridge which eliminate at least two arms are preferred. These bridges are also useful for the measurement of low value of capacitances accurately.
The ratio arm bridges can be either voltage ratio type or current ratio type; the former being used for high frequency low voltage applications.
The schematic diagram of a ratio arm bridge (voltage ratio) is given in Fig. 9.18. Assuming ideal transformer conditions, for a null indication of the detector,
In practical transformers, the voltage ratio’slightly differs from the turns ratio due to the no load magnetizing current and is also affected by the load current. Therefore, the balance conditions- showb above involve errors. The errors are classified as the ratio and loading errors and are determined separately and compensated for in, the construction. A practical bridge constructed by General Radio Company (USA) has a useful range from a fraction of one pF to about 100 μF and covers a wide range of frequency from 100 Hz to 100 kHz, the accuracy being better than 0.5%.
For high voltage applications where sensitive measurements at fixed frequency (at 50 Hz) are required, the current comparator or the current ratio method (Fig. 9.19a) is used. This bridge has the advantage that full voltage is applied across the test capacitor but also has the drawback that a standard conductance has to be built for high voltages. It is difficult to construct a precision conductance suitable for high voltage operation.
This disadvantage is overcome by generating a low voltage signal Ef proportional to and in phase with the supply voltage E as shown in the modified Fig. 9.19b. At balance, there is no voltage across the current comparator winding. If the gain of the amplifier (A) is high, that is
Partial discharge (PD) measurements can be used to detect straight discharges, and are a key tool for assessing the condition of electrical equipment insulation:
What is partial discharge?
Partial discharge measurements- straight discharge detection