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Warm hints: The word in this article is about 3500 words and  reading time is about 15 minutes.   Based on the heat balance principle, the basic theory for the calculation of steady and transient temperature rise of transformer is discussed.   The calculation of transformer temperature rise is mainly used to make sure that the steady-state temperature rises produced by the transformer with rated load in long-term continuous operation will not exceed the limit specified in the standard or technical contract. In addition, the data of overload operation capacity of transformer running under various rated loads are also used to offer support to the measures taken to ensure safe overload operation of electrical systems.   However, from the author's current reading of recent info on transformer principles, design, and calculation theories available, it may be due to the limitation of space or different aspects of emphasis that they are always making an introduction to various practical formulas for transformer temperature rise calculation in a practical way. As to the involved thermal principle of calculating the transformer temperature rise is always a bit of an oversimplification I think, especially the thermal analyses demonstrating how heat energy be absorbed(or dissipate) and the temperature goes up(or goes down) during a heating process. In this article, therefore I would like to make some theoretical discussions to solve this problem and add some extra explanations needed.   Catalog   I. Brief Introduction II. Heating & Cooling Process 2.1 Heating process when the thermal power is constant 2.2 The cooling process in which the heated body is no   longer supplied with heat 2.3 Just as same as the situation of 2.1, but Τ≠0 when   t=0 III. Further Analysis of Some Formula 3.1 Mechanism of temperature-rising process of heated   body 3.2 Properties and applications of Τu in formula 2 3.3 Definition, function and derivation of the thermal   time constant Τ IV. Conclusion FAQ I. Brief Introduction   It is well known that heat is always transferred automatically from a high-temperature object to a low-temperature object. Heat can transfer (or move) in three ways: conduction, convection, and radiation, usually a combination of two or three of them may cause the transmission (also known as heat dissipation).   Before formally describing this section, I would like to quote two conclusions from laboratory research that are fundamental to thermal science:   1) the temperature rise of an object is directly proportional to the heat (heat) supplied by the outside world and inversely proportional to its own mass and Specific Heat Capacity.   2) the heat energy (calorific energy) that a heating object disperses into a cooler object(cooling medium) within a unit of time is directly proportional to the area of its radiating surface, the temperature difference, and the Heat Transfer Coefficient between the heating object and cooling medium.   Specific heat capacity: the ratio of the heat added to (or removed from) an object to the resulting temperature change per unit mass of material.   Heat transfer coefficient: when the temperature difference between the heating object and the cooling object is 1K, the heat from the unit dissipation surface area to the cooler object within a unit of time.   For the convenience of discussion, the heating object is referred to as the heated body in this paper, and the object which gets the heat released from the heated body is called the cooling body (or the cooling medium).   Take a dry transformer, for example, its winding and core are heating bodies, and the cooling body refers to the air around the transformer. For oil-immersed transformers, in addition to the winding and core known as heating bodies, the oil-filled in the transformer can also be called a heating one in relation to ambient air or cooling water. However, relative to the winding and core, oil is also called a cooling body(cooling medium).   Fig. 1 Transformers are found everywhere alternating current is used   II. Heating & Cooling Process   2.1 Heating process when the thermal power is constant Suppose that in the heating process (that is, the heat is still in a transition state, not reaches a stable state yet), the temperature rise by Τ relative to the cooling medium, the temperature rise increases dΤafter a tiny time unit dt.   The heat supplied by the outside during this dt period is a constant thermal power of Pdt(P. For windings in oil-immersed transformers, it refers to load loss; for oil, it refers to total loss). Let the heat supplied by the outside surroundings during this dt period be Pdt(P is the constant thermal power; for windings in oil-immersed transformer, P refers to load losses, and for oil, the total losses). Some of the Pdt is the heat absorbed when the temperature rise of the heated body increases by dΤ, and the other part is dispersed into the cooling medium.   In order to understand the nature of this physical phenomena in the heating process better, suppose the heated body is an isothermic one, therefore the density, specific heat capacity, and any other parameters being the same, including the heat dissipation capacity of each point on the surface.   According to the principle of thermal balance (it comes down to the law of energy conservation), the states of the heating process described above can be represented by the following equations:   Where   P——Constant value of thermal power c——Specific heat capacity of the heated body G——the mass of the heated body F——Heat dissipation surface area of the heated body k——Heat transfer coefficient  Τ——Temperature difference (or temperature rise) between the heated body and cooling body at a certain time   The first item on the right of Equation (1) indicates the heat energy absorbed by the heated body when it increases the temperature of dΤ; The second item indicates that the heat energy is dispersing to the cooling body while the heated body is storing heat.   When the temperature rise of the heating body does not go up, that is, when the temperature rise reaches a stable state, then there is dΤ=0 and Τ=Τu (Τu means the steady-state temperature rise).   At this point, from formula (1) there is:   According to the above analyses we can see: formula (1) is the mathematical expression of the heat balance principle under the transient state; formula (2) is the mathematical expression of the heat balance principle under steady-state.   From Formula (1) and Formula(2) we can get a first-order differential equation of Τ:   Where: From formula (4), the parameters on the right side of the equation are the physical parameters of the object itself, so Τ is a constant and has a dimension of time, so it is called the thermal time constant. In the physical sense, Τ is the ratio of the heat storage capacity of the heated body to the heat dissipation capacity per unit time of the heating system studied, which is an attribute of the heated body. Therefore formula (4) is considered to be the definition of Τ and the formula (4') is another expression for calculation.   To obtain the solution of formula (3) we let t=0 and Τ=0, then this is what we get:   Note that sometimes it is easier to express the formula (5) with temperature θ instead of temperature rise Τ, so it is reworded as follows: Where:   θ——The temperature of heated body at any given moment and there is θ-θа=Τ θа——The temperature of cooling body θu——The steady state temperature of the heated body that reaches a steady state Δθu——θu-θа=Τu   The rising curve in Fig. 1 shows the temperature rise of heated body changes with time t.   In order to visually see how the Τ changes with the temperature rise, Fig. 1  shows two curves with different Τ and same Τu. Fig. 2 Relation between temperature-rise(Τ)of heated body and time(t) As can be seen directly from formula (5), formula (5') and Fig. 1, the process of temperature rising of a heated body is characterized by the fact that it changes fast at the very start, then gradually slows down, and when the time t becomes equals to (4～6)Τ, it remains almost unchanged, at this point it can be assumed that Τ reaches Τu (theoretically t reaches ∞).   2.2 The cooling process in which the heated body is no longer supplied with heat   When the temperature rise of the heated reaches Τu and no heat will be emitted, the temperature rise begins to go down from Τu to zero, which we call the cooling process. At this point, its transient process equation can still be deduced by formula (1), in which you need only to let P be equal to zero.   Therefore the first-order differential equation of its temperature rise is as follows:   The solution is: All the symbols of parameter in the formula above are identical with those in formula (5), except that Τu is the initial value of temperature rise when t=0; when (4～6)Τ later, Τ≈0.   Why would we worry about Formula 6?    This is because the temperature rise of winding got at the end of the current transformer temperature rise test, is still the value of temperature rise calculated through measurements of the resistance value of the winding which will change with the temperature. The resistance value measurement is still carried out in the temperature rise test under a way of supply voltage been removed, thus the formula (6) should be used to calculate.     2.3 Just as same as the situation of 2.1, but Τ≠0 when t=0 When the heating body is supplied with constant heat power and Τ=Τ0 (≠0) when t=0 satisfied, the whole process of deduction of temperature rise calculation, with the exception of the situation of Τ=Τ0 when t = 0, is the same as 2.1, in other words, the formula of temperature rise can still be deduced from formula (5) as follows: Comparing this with the formula (5) we notice that there is a new second item occurs. Considering the physical meaning of the expression is not obvious enough, it is now rewritten (which will not change the result of the calculation) as: These two formulas above show a case that the transformer is suddenly asked to conduct a overload operation running beyond the steady load.   III. Further Analysis of Some Formula   3.1 Mechanism of temperature-rising process of heated body Formula (5) and Fig. 1 describe the rising process of heated body temperature from a mathematical point of view. This process is characterized by a rapid start and then a gradual slow down until it finally stops rising and reaches a stable temperature rise of Τu .   In this section, characteristics of heating and cooling mechanism in this process will be described from a physical point of view. So we divide the t into n small and equal time periods when t=Τu, that is Δt1=Δt2=……=Δtn=Δt (theoretically t=∞, but in practice, it is desirable to suppose that t=(4～6)T, considering the value required of Tu with a higher accuracy). Therefore, the derivative symbol "d" in this article is replaced with a increment sign "Δ".   Please note that the heat energy supplied by surroundings in each time period is equal to P·Δt (P is the constant thermal power).   Now let us take a look in the first time period Δt1. Since the temperature of heated body has already made be equal to the cooling body when t=0, that is Τ=0, according to the second item of formula (1), we can assume the heat energy emitted is also equal to 0 during the period of Δt1 until the end of present stage. Therefore, during the Δt1 period, the final increments of the temperature rise of the heated bodyΔΤ1 is determined by the total external heat energy (P·Δt). So the temperature rise at the end of first time period Δt1 is Τ1=ΔΤ.    According to the same analytical principle, we continue with the second time period Δt2. Since the initial temperature rise at the beginning of Δt2 is the that of the first time period Δt1, it can be included that Τ1=ΔΤ1. The heat emitted during Δt2 is no longer zero, but . At the end of the Δt2, the increment value of temperature rise of heated body ΔΤ2 is determined by , so there we have ΔΤ2<ΔΤ1 (That is, the increment of temperature rise in the second period is smaller than that in the first period).   Finally, at the end of the second time period Δt2, the temperature rise Τ2 is equal to ΔΤ1+ΔΤ2. The rest may be deduced by analogy, at the end of Δtn time period, the temperature rise is , also ΔΤn=0 has been illustrated at the same time.   According to the changes of temperature increment ΔΤn in each of time period above, there always are: ΔΤ1>ΔΤ2…>ΔΤn-1>ΔΤn (ΔΤn=0), therefore demonstrating the trend that all the heat absorbed by heated body in each time period gradually goes down from P·Δt absorbed in the time period of Δt1 to 0 absorbed in the time period of Δtn. The heat energy emitted in each of the corresponding time periods is gradually increased from 0 to (from the first time period Δt1 to the end of the NO.n time period Δtn). That also means the final temperature rise (steady temperature rise) is: After the temperature rise reaches the stable value of Τu, the heated body no longer absorbs external heat energy, which indicates that all the external heat energy has been dispersed to the cooling body.   3.2 Properties and applications of Τu in formula 2 Formula 2 is a theoretical formula derived from the heat balance principle for calculating the steady-state temperature rise of the heated body. It is difficult to calculate the temperature rise directly for complex heated bodies such as transformers.   Therefore, various manufacturers and scientific research institutions have respectively obtained many practical formulas, according to their own practical experience or scientific research results and the characteristics of the heated body structure and three different heat dissipation forms, including conduction, convection, and radiation.   Furthermore, a heated body such as a transformer having a complex structure does not always have the characteristics of the homogeneous isothermal body assumed in formula 2, and the heat-dissipation capability at each point on the surface is not equal, either.   Therefore, the steady-state temperature rise (Τu) calculated from formula 2 (including the relevant practical formula based on formula 2) on the whole indicates the average value of temperature rise at different points in the heat source.   The calculation of the maximum temperature rise (temperature) of an object at a "hot spot" of concern has so far could only be mainly estimated by experience and the use of temperature rise measurements in certain heaT tests (for example, confirming the difference between the maximum temperature rise and the average value or confirming the multiple values between them to estimate).   3.3 Definition, function and derivation of the thermal time constant Τ Formula 4 is the definition expression of thermal time constant. It is obtained in the derivation of formula 5 from formula 2 and formula 1. Therefore, it is possible to think that equation 4 is obtained under the admission that when t=∞ it has dΤ=0 and Τ=Τu, the latter of which is obtained under the boundary conditions of objective reality. Because the definition of "Τ" and some functions have been described in section 2.1, here i only do some additional analyses to formula 7 which shows applications of transformer temperature rise under a short-time overload operation. Fig. 3 What is a Transformer   The basic theory of transformer and working principle of transformer   Fig. 4 Large power transformers have their core and windings submerged in an oil bath to transfer heat and muffle noise, and also to displace moisture which would otherwise compromise the integrity of the winding insulation. Heat-dissipating "radiator" tubes on the outside of the transformer case provide a convective oil flow path to transfer heat from the transformer's core to ambient air.   First of all, according to the statistics of a large number of dry and oil-immersed power transformers with different capacities, which have been manufactured at home and abroad, the value of Τ is generally not less than 1h (for oil-immersed transformer, although the T of winding is quite low, about 5 min-20 mins, the T of oil is 1h～5h.    Noting that the oil actual temperature rise is generally not lower than the temperature difference between the winding and the oil and that the thermal time constant of the oil should therefore be considered to control the temperature rise of the winding during the overload operation, that is, the temperature difference between winding and ambient air or temperature of cooling water still plays a decisive role.    Although the Τu of the transformer will obviously exceed the temperature rise limit of rated-load operation, as long as the time t has been controlled within T, it can still make the actual temperature rise running in short-time overload not excess the temperature rise limit of short-time overload operation. These permissible limits, according to the standard of the load guide of the oil-immersed power transformer, is related to the operation type within nameplate capacity, namely, normal periodicity, long-term or short-term emergency, and generally higher than that of rated temperature rise.   It can be seen from this that the function of the thermal time constant T is relatively large, so it is necessary to pay close attention to its definition, various affecting factors, and derivation of its formula. However, I have seen some people added another boundary condition except dΤ=0 during the derivation of the expression of Τ: in extreme cases, the heat will not be transferred into the surrounding medium at all, which is also called "adiabatic condition". In this regard, I would like to put forward the following different views for discussion.   (1) In this article, the derivations of formula 4 and formula 5 have only used a "boundary condition" of dΤ=0 (Τ=Τu) when t=∞, so there is no need to add another adiabatic condition for derivation.   It is said that without heat dissipation, the time required to reach a stable temperature is called time constant (Τ), but that will only be true when the second item (dissipated heat) on the right of the equation meets a condition of , now that to reach a steady temperature Τ must be equal to Τu, which is at variance with objective reality of electric accessories including transformers.   Some might say that when people calculate the temperature of a transformer at a short-circuit current, don't they also use the formula obtained under the "adiabatic condition" to calculate the temperature of the transformer? Yes, but the case being considered is only the "short time" one, that is, the formula only applies when short-circuit durations never exceed 10s (actually 2s).    The time is very small compared with the thermal time constant of winding in oil-immersed power transformer (about 5min-20mins) and that of oil (about 1lh～5h). This is still true when compared with the thermal time constant of the windings of dry-type transformers (about or above 30min). In section 3.1 of this article, such a short duration makes it is possible to consider it as an adiabatic transient system. In a broad sense, if the heat energy emitted accounts for only a very small part of the heat supplied by the outside world during a same period, it can be roughly regarded as adiabatic process. At this point, it is precisely because of the recognition of  that it is in line with objective reality.   This proves that adiabatic conditions should not be used as the basis in the derivation of expression of Τ (formula 4) and that of transient temperature rise (formula 5).   (3) From the details of the derivation process in this article, I would like to say that the true expression of temperature rise of the heated body still has not been obtained yet in the end. Think about that, a task for you, my readers.   IV. Conclusion   (1) Starting from the principle of heat balance, this article expounds the model of the heating mechanism in the rising process of temperature rise of the heated body with concise mathematics and physical language.   (2) It is clearly pointed out in this paper that the steady temperature rise of the heated body can be calculated by using formula 2, and formula 5 and formula 7 are the formulas for calculating the temperature rise of the heated body during the transient process.   (3) It is pointed out that to obtain the expression of the transient temperature rise of the thermal time constant Τ and the heated body, the adiabatic condition should not be used for the process of derivation, which is not necessary either.   (4) Due to space constraints, this article has not covered the heat dissipation mechanism of heated body, but you readers can refer to other references about heat loss or transfer in Kynix and other sites.   FAQ   1. How hot is too hot for a power transformer? Transformers designed with high-temperature insulation systems can run safely at temps up to 200°F. But remember, a hot-running transformer is an angry transformer.    2. What is ambient temperature of transformer? The average ambient temperature for a transformer over a 24 hour period should not exceed 30 degrees Celsius. For instance, if the transformer ambient temperature was 40 deg. C for 12 hours, then the transformer must not exceed 20 deg.   3. Should doorbell transformer be hot? Transformers are always going to produce some heat. It's a part of the step-down process. It should only be warm to the touch, however.   4. What is maximum ambient temperature? In general, a safe range is between 60 and 75 degrees Fahrenheit or 15 and 25 degrees Celsius, although the cooler end of that range is better. Ambient temperatures above those ranges make it difficult for a computer's cooling system to keep it at a safe operating temperature.   5. What is the name of oil used in transformer? Mineral oil and Synthetic oil are the majorly used transformer oil. These are the petroleum products, like Naphthenic based transformer oil and Paraffinic based transformer oil. Naphthenic based transformer oils are known for their heat distribution, which is one of the main problems with transformer.   6. What will happen if the regulation of a transformer is poor? If the transformer supplies a very low lagging power factor, large secondary currents will flow resulting in poor voltage regulation due to greater voltage drops in the winding. ... Therefore positive regulation produces a voltage drop in the winding while a negative regulation produces a voltage rise in the winding.   7. What is high transformer temperature? Standard Ratings and Overload Capacity：Dry-type transformers are available in three standard temperature rises: 80C, 115C, or 150C. Liquid-filled transformers come in standard rises of 55C and 65C. These values are based on a maximum ambient temperature of 40C.   8. What is hot spot temperature in transformer? Modern transformers make use of thermally upgraded paper that has been chemically treated to improve the stability of cellulose structure. The rated hot spot temperature for this kind of paper is 110°C and it can be seen that an increase of 7°C will double the aging acceleration factor.   9. How much heat does a 75 kVA transformer give off? According to Cutler-Hammer, a 75-kVA, 150°F-rise, dry-type transformer has an efficiency of 97.2% at 1/4 load and 96.7% at full load. So, figure 3% loss at 75 kVA, which would represent 2,250 W.   10.What happens when transformer is overloaded? The weakening of the system will happen faster if the transformer is frequently overloaded. The net result of small, incremental increases in loading capacity over time is a weakened insulation system. Overloading causes overheating, and eventually thermal degradation that acts thrrough cracks in the insulation.   You May Also Like: Some suggestions about protecting transformers Learn Some Basic Knowledge about Capacitor Voltage Transformer  

Warm hints: The word in this article is about 2500 and  reading time is about 12 minutes. Summary In the power system, in addition to the traditional harmonic sources, such as electric arc furnace and frequency converter, the nonlinear loads such as new energy access and charging pile may produce a lot of harmonics. In order to prevent harmonic from further affecting the power grid, accurate monitoring and timely treatment of harmonic level in power grid is a necessary step. The correct measurement of harmonic content in power grid is the basis of monitoring and governance. This paper mainly introduces some basic  knowledge about capacitor voltage transformer including the capacitor voltage transformer symbol; testing; working principle; capacitive voltage transformer VS inductive voltage transformer and etc. Article core capacitor voltage transformer Abbreviation CVT English name capacitor voltage transformer Category Power Subject Power engineer compose Capacitive voltage divider and medium voltage transformer Field Energy         Catalogs I. What is Capacitor Voltage Transformer( CVT)3.1 Insulation Resistance Measurement1.1 The Composition of CVT3.2 Capacitance Measurement1.2 CTV Judgment of Common Anomalies3.3 Pressure Swing Ratio Test1.3 CVT Equivalent Circuit Model3.4 Polar MeasurementII.Terminal Sign of Capacitive Voltage TransformerIV.Working Principle of Capacitive Voltage Transformer （CVT)III.Capacitor Voltage Transformer TestingV.Capacitive Voltage Transformer VS Inductive Voltage TransformerⅥ. FAQ  Introduction I. What is Capacitor Voltage Transformer( CVT) 1.1 The composition of CVT 一、The capacitive voltage transformer is mainly composed of a capacitor voltage divider and a medium voltage transformer. The capacitor divider is made up of porcelain bushing and series capacitors installed in it. The porcelain bushing is filled with insulating oil that keeps 0.1MPa positive pressure, and steel bellows are used to balance different environments to maintain oil pressure. The capacitor divider can be used as a coupling capacitor to connect the carrier device. The medium voltage transformer is composed of a transformer, a compensating reactor, a lightning arrester and a damping device installed in a sealed tank, and the space on the top of the tank is filled with nitrogen. The primary windings are divided into main windings and fine tuning windings, and a low loss reactor is connected in series between one side and one winding. Due to the capacitance and the inherent nonlinear impedance of the capacitor voltage transformer sometimes cause Ferroresonance in capacitor voltage transformer, thus suppressing resonant damping device, damping device is composed of a resistor and reactor, connected across the two windings, normally the damping device has very high impedance, when iron magnetic resonance caused by overvoltage in medium voltage transformer affected before the reactor is saturated only resistive load, the oscillation energy will soon be reduced. 1.2 CTV Judgment of common anomalies (1)The secondary voltage fluctuation. The two connection is loose, the distributor is not grounded or the carrier coil is not connected. If the damper is a fast saturable reactor, it may be improper parameter matching. (2)The Secondary voltage is low. Its connection is bad and the electromagnetic unit failure or the capacitor unit C2 is damaged. (3)The secondary voltage is high.The capacitance unit C1 is damaged and the ground end of the partial voltage capacitor is ungrounded. (4)The oil level of the electromagnetic unit is too high. The next capacitance unit is leaking oil or electromagnetic unit into the water. (5)There is a different sound in the transportation. Bolt loosening of reactor or medium pressure rheostat in electromagnetic unit. 1.3 CVT equivalent circuit model Under the condition of steady state, the whole CVT equivalent circuit can be regarded as a linear system, which compensates the stray capacitance of reactor C. The influence of the primary stray capacitance C: of the intermediate transformer at the high frequency can not be ignored. CVT intermediate transformer core can be regarded as linear segments in the magnetization curve, ignoring the core magnetizing inductance, one or two intermediate transformer side leakage resistance reduction to compensation reactor.   II.Terminal sign of capacitive voltage transformer · A single phase transformer with a two - time winding   It represents a single-phase transformer with two times winding. A represents the primary winding terminal of capacitive voltage transformer, and N represents the primary winding grounding terminal of voltage transformer. A represents the two winding terminal terminal of a capacitive voltage transformer, and N represents the first winding grounding terminal of the voltage transformer. · Single phase transformer with two two times windings   It represents a single-phase transformer with two two times windings, A represents the primary winding terminals of capacitive voltage transformers, and N represents the primary winding grounding terminals of voltage transformers. 1A and 2A represent the two winding terminals of the capacitive voltage transformer, and 1n and 2n represent the primary winding grounding terminals of the voltage transformer. · A single phase transformer with two two windings with a tap A single phase transformer with two taps and two winding is represented. A represents the primary winding terminal of capacitive voltage transformer, and N represents the primary winding grounding terminal of voltage transformer. 1A1, 1A2, 2a and 2A2 respectively represent the two winding terminals of the capacitive voltage transformer, and 1n and 2n represent the primary winding grounding terminals of the voltage transformer. · A single phase transformer with a residual voltage winding and two two times windings   It represents a single-phase transformer with a residual voltage winding and two two winding. The A represents the primary winding terminal of capacitive voltage transformer, and N represents the primary winding grounding terminal of voltage transformer. 1A1, 1A2, 2a and 2A2 respectively represent the two winding terminals of the capacitive voltage transformer, and 1n and 2n represent the primary winding grounding terminals of the voltage transformer. Da and DN represent the residual voltage terminal.   Detail III. Capacitor Voltage Transformer Testing 3.1 Insulation resistance measurement The insulation resistance should be measured by the main capacitor, the partial voltage capacitor and the one or two winding insulation resistance of the intermediate transformer. 3.2 Capacitance Measurement The purpose of the test is to determine whether the capacitance of the voltage divider has a change, and the capacitor is insulated without water and dampness.   3.3 Pressure swing ratio test The test transformer exerts high voltage as far as possible. Due to the rise effect in the test, the high voltage voltage must be measured at the high voltage end. The voltage transformer used must be level 0.1 or above to ensure the accuracy of the test results. After the voltage is applied at the high voltage side, the voltage of the low voltage side is measured in turn on the two side and in the auxiliary side, and the voltage ratio is compared with the pressure ratio of the nameplate. 3.4 Polar measurement The purpose of polar measurement is to check the mark of the nameplate. Test method: using DC method, CAR instantaneous addition of 1.5V battery power "+", "-" with "N", respectively, with a multimeter or mA DC or mV meter, pay attention to the polarity put right, A1, A0 "+" X1, XD "-" pointer in the power supply to the deflection of the "+" direction; open to "-" deflection. The test of polarity and pressure variable ratio of windings is usually done in hand over and after overhaul.   IV. Working Principle of Capacitive Voltage Transformer （CVT) There is a video about CVT:   This vidoe explained How Capacitor Voltage Transformer CVT works.Capacitor potential transformer concept is explained.What is high voltage measurement using capacitor type voltage transformer is explained. How to measure high voltage? Educational tutorial on electrical engineering 126 by G K Agrawal. The basic part of the capacitive voltage transformer is the capacitor voltage divider, and it also includes the electromagnetic parts such as the intermediate transformer, the compensating reactor, the damper and so on. Its principle connection is shown in the following picture,the picture shows capacitor voltage transformer wiring diagram capacitance divider is composed of main capacitor C1 and voltage divider capacitor C2 series. Without considering the electromagnetic part, the voltage is divided by capacitance. The voltage on C2 is the following formula: K is the partial voltage ratio. When the two ends of the C2 are connected to the two load, due to C1, C2 The basic part of the capacitive voltage transformer is the capacitor voltage divider, and it also includes the electromagnetic parts such as the intermediate transformer, the compensating reactor, the damper and so on. Its principle connection is given below.   The capacitor voltage divider is composed of the main capacitor C1 and the partial voltage capacitor C2 in series, without considering the electromagnetic part, then the voltage is divided according to the capacitance inverse ratio, and the voltage on the C2 is:   In this formula,K is the ratio of partial pressure When the two ends of C2 are connected to two loads, the larger capacitance internal impedance is due to the existence of C1 and C2, which makes UC2 smaller than the capacitance partial voltage. The larger the load current is, the greater the error is. In order to reduce the capacitance internal impedance, a compensatory reactor L can be connected in series, and the UC2 is not related to the load as much as possible. In fact, because the capacitor has loss, the reactor also has resistance, so that the internal impedance can not be zero, so when the load changes, there will always be error. In order to further reduce the effect of load current, the measuring instrument is connected to the divider after the intermediate transformer TV is boosted. When the two side transformer short circuit occurs, the resistance in the circuit and the total reactance reactor L after compensation are very small, several times the short-circuit current may reach the rated current, will produce a very high voltage resonance in L and C2, in order to prevent overvoltage caused by the breakdown of insulation in capacitor C2 parallel at both ends of the discharge gap F1. Capacitor voltage transformer with capacitance and nonlinear inductance (e.g. TV magnetizing inductance etc.), when the transformer side suddenly close or receive two side and eliminate the impact of sudden short circuit, overvoltage in the transient process may cause nonlinear inductor saturation, which excite ferroresonance overvoltage, such as harmonic 1/3 resonant.  Because the resistance is very small, the resonance will last for a long time, which will cause damage to voltage transformers, instruments and relays, and may lead to incorrect operation of the protective devices. Therefore, the damping resistance RD or damper is often installed on the two side of the capacitive voltage transformer to consume the resonant energy as soon as possible to suppress the ferroresonance. For a common capacitive voltage transformer, a resonant damper is used. It is the capacitance and the inductor in parallel and then added to the damping resistance. In UHV power grid, a capacitive voltage transformer often uses a fast saturation type damper, which is composed of a fast saturation reactance and a damping resistor.     Analysis V. Capacitive Voltage Transformer vs Inductive Voltage Transformer Inductive Voltage Transformers (IVT), are used for voltage metering and protection in high voltage network systems. They transform the high voltage into low voltage adequate to be processed in measuring and protection instruments secondary equipment, such as relays and recorders). A Voltage Transformer (VT) isolates the measuring instruments from the high voltage of the monitored circuit. VTs are commonly used for metering and protection in the electrical power industry. It’s a standard transformer available in the market for step-up or step-down voltages. The advantage is it can be used for high load current and provides isolation.   However,as we mentioned in the above,capacitor voltage transformer is a specialized circuit whose purpose is to convert a high voltage AC signal to lower voltage, usually used with very high input voltages, and a large ratio between input and output voltage. It's usually only used in cases where you're trying to extract a very small amount of power from a high-power circuit, usually for monitoring the high-power circuit. Its advatage is economical but there is no galvanic isolation.   Ⅵ. FAQ 1. What is the function of capacitor voltage transformer?A capacitor voltage transformer (CVT), also known as capacitor-coupled voltage transformer (CCVT), is a transformer used in power systems to step down extra high voltage signals and provide a low voltage signal, for metering or operating a protective relay.   2. Why is CVT used?One of the advantages of a CVT is its ability to continuously change its gear ratio. This means that no matter what the engine speed it, it is always performing at its peak efficiency. CVTs often offer better fuel economy as a result, especially when driving in the city. ... This is because the transmission never shifts.   3. What do you understand CVT and CCVT?Capacitor Voltage Transformer (CVT) or Capacitor Coupled Voltage Transformer (CCVT) is a switchgear device used to convert high transmission class voltage into easily measurable values, which are used for metering, protection, and control of high voltage systems.   4. Why are capacitors used in transformers?At too high common mode frequencies, the inevitable capacitive coupling in the transformer will cause some of the common mode signal on the input to show up as signal on the output. The capacitor provides a more serious connection to ground for AC, while the resistor only a weak connection for DC to avoid ground loops.   5. Why is CVT hated?Because CVTs tend to lock an engine into a specific RPM, generally a high and noisy RPM, making the whole experience very hard on the ears. Also, CVTs are generally tuned for fuel economy rather than performance, and most of the magazines out there are wannabe racecar drivers.   6. What is the function of capacitor voltage transformer?A capacitor voltage transformer (CVT), also known as capacitor-coupled voltage transformer (CCVT), is a transformer used in power systems to step down extra high voltage signals and provide a low voltage signal, for metering or operating a protective relay.

  This article will be divided into three parts--individual transformer , parallel transformer and redundancy requirements for bulk power transformers.     Catalog   I. Individual Transformer II. Parallel Transformer III. Redundancy Requirements for Bulk Power Transformers FAQ   I. Individual Transformer   Just see the following picture, it includes the protection for banks where fuses are used on the primary. Overall differential protection may be applied by using CTs in the transformer primary bushings for larger or important banks. The common connection is shown with delta on the source (primary) side and wye-grounded on the secondary side. Other possible connections: delta–delta, wye–wye, or primary-wye– secondary-delta. Figure 1 – Transformer protection without primary-side circuit breaker   About the analysis of power circuit, you can see the article: Analysis of Switching Power Supply Principle   Note: Secondary circuits should have 51 and 51N relays. Therefore, transformer secondary breaker and relays may be omitted unless another source connects to the secondary bus. 51N relay can be omitted with 51G available.   For transformer banks with primary breakers, the protection is summarized in Figure 2.   Relay 51G provides backup protection for secondary bus and feeder faults and must be time-coordinated, with other ground relays protecting the various feeder circuits on the secondary bus. Similarly, phase relays 51 must be coordinated with the phase relays on the feeders. The common connection is shown with delta on the source (primary) side and wye-grounded on the secondary side. Other possible connections: delta–delta, wye–wye, primary-wye– secondary-delta, three-winding, or autotransformer.   Figure 2 – Transformer protection with primary-side circuit breaker   "52S may be omitted in some applications requiring 151G to coordinate with and trip the secondary circuit devices if used." II. Parallel Transformer   The protection for transformer banks where the secondaries are connected together by a bus tie breaker is summarized in the following picture(a,b,c).   The arrangement shown is typical for large- or critical-load substations, especially for industrial plants. The loads are supplied from separate buses that are connected together by a bus tiebreaker (52T) that may be operated either normally closed (NC) or normally open (NO).   Figure 3a – Single line diagram of transformer and secondary bus protection for a typical double-source supply with secondary tie and breaker   If you operated NO, the protection of the first picture and second is applicable. If operated with 52T NC, the protection of the first picture and second is applicable with the secondary side modified. Figure 3b – Secondary protection with high-side fuses With the bus tiebreaker closed, there is a possibility for the interchange of power between the two sources. Here, current flows from one source through its transformer, the secondary buses, and back through the other transformer to the second source. Generally, this is neither desirable nor permitted.   "To prevent this operation, directional time–overcurrent relays (67, 67N) are applied to each transformer."   Figure 3c – Secondary protection with high-side breaker The single-line connections are shown in Figure 3b and Figure 3c, with complete three-line connections in the following figure.   Note: They operate only for fault current that flows into the transformer and trip the secondary breaker (52–1 or 52–2). This is also important in removing a secondary fault source for faults in the transformer bank. The phase relays (67) can be set on a low of the minimum tap.   Load current certainly flows through the relay, but normally not in the operating direction. The low tap continuous rating must not be exceeded by increasing the maximum load current. The 67-time setting must coordinate with the protection on the transformer primary. When used, the ground relay can be set on minimum setting and time, because coordination is not necessary.   Figure 4 – Three-line connections for reverse-phase and partial differential backup protection The inverse-time–overcurrent relays (51, 51N) provide bus protection and backup protection for the feeder circuits. These relays trip both 52–1 (or 52–2) and 52T. This is a partial differential connection and these units must be time-coordinated with the protection on the several feeders that are connected to the bus.   "Only two-phase relays are required, but the third relay (shown optionally in Figure 4) provides additional redundancy. When a ground differential is used, as illustrated in Figure 3c, 67N and 51N are omitted."   Ground-fault backup is provided by 51G, 151G, and 251G inverse-time overcurrent relays (Figure 3abc). Relay 251G provides bus ground-fault protection and backup for the feeder circuit ground relays. It must be time- coordinated with these. It trips the bus tie 52T, as the fault could be either on the bus or on the associated feeders.   If the fault continues to exist with the bus tie open, relay 151G trips breaker 52–1 (or 52–2). Thus, 151G must coordinate with 251G. If the fault persists, it is between the secondary breaker, in the transformer winding, or in the grounding impedance.   Relay 51G set to coordinate with 151G is the last resort. It trips the high-side or primary breaker to remove the transformer from the service.   III. Redundancy Requirements for Bulk Power Transformers   When transformers are connected to bulk power systems, redundancy requirements for related protection need to be addressed. To provide the required redundancy, two separate differential schemes may be applied.   "Redundancy for transformer faults may also be obtained by a differential scheme and sudden pressure."   In such an application, the sudden pressure protection needs to be supplied with additional protection for faults on the transformer bushings and leads, as sudden pressure devices will not respond to faults in these areas. Redundant schemes for disconnecting the transformer from the system when a high-side breaker is not applied can be obtained by using various combinations of the methods. Take an example, two separate transfer trip systems may be applied although they are expensive. A cheaper alternative is to combine a transfer trip scheme and a faulty switch. It may be possible to delay closing the fault switch for a few cycles to allow time for the transfer trip scheme, provided it is operational, to de-energize the failed transformer before the closing of the fault switch. This would spare the power system from being subject to a solid fault when the fault switch closes, whenever the transfer trip scheme works properly.   When a high-side breaker is applied and it fails to operate, breaker failure protection is required to enable isolation of a faulted transformer. The breaker failure scheme may require the application of a fault switch, transfer trip scheme, or a second interrupting device if other local breakers are not available to isolate the transformer.   FAQ     1. What is the use of transformer? Transformers are employed for widely varying purposes; e.g., to reduce the voltage of conventional power circuits to operate low-voltage devices, such as doorbells and toy electric trains, and to raise the voltage from electric generators so that electric power can be transmitted over long distances.   2. What are the 3 types of transformers? There are three primary types of voltage transformers (VT): electromagnetic, capacitor, and optical.   3. What is the basic principle of transformer? A transformer consists of two electrically isolated coils and operates on Faraday's principal of “mutual induction”, in which an EMF is induced in the transformers secondary coil by the magnetic flux generated by the voltages and currents flowing in the primary coil winding.   4. Does a transformer convert AC to DC? A transformer is built to transfer the energy from one circuit into another circuit by way of magnetic coupling. ... An alternating current creates a magnetic flux in the core on its way through the first winding, inducing the voltage in the others. It can convert high and low voltages, it cannot convert AC to DC.   5. What are the main parts of transformer? There are three basic parts of a transformer: a. an iron core which serves as a magnetic conductor, b. a primary winding or coil of wire and. c. a secondary winding or coil of wire.   6. What are the classification of transformer? Depending upon the type of construction used, the transformers are classified into two categories viz.: (i) Core type, and (ii) Shell type. Depending upon the type of service, in the field of power system, they are classified as: (i) Power transformers, and (ii) Distribution transformers.   7. Can a transformer work on DC? As mentioned before, transformers do not allow DC input to flow through. This is known as DC isolation. This is because a change in current cannot be generated by DC; meaning that there is no changing magnetic field to induce a voltage across the secondary component.   8. How do you convert a transformer? This conversion is made by winding two separate conductors around a common iron core. Applying an alternating voltage to the primary conductor produces current which sets up a magnetic field around itself. This is known as mutual inductance.   9. What are two components of no load current in transformer? The no-load current of a transformer consists of two components: The Magnetization Current iM is the current required to produce the flux in the transformer core. The Core-loss Current ih+e is the current required to make up for hysteresis and eddy current losses.   10. Which type of transformer core is most efficient? SHELL CORE. The most popular and efficient transformer core is the SHELL CORE, as illustrated in figure (4). As shown, each layer of the core consists of E- and I-shaped sections of metal. These sections are butted together to form the laminations.   11. What is the power factor of transformer? The power factor of a distribution transformer is between (0.75 to 0.80) when secondary is connected to u.p.f loads.   12. Why do we need Transformers? Transformers help improve safety and efficiency of power systems by raising and lowering voltage levels as and when needed. They are used in a wide range of residential and industrial applications, primarily and perhaps most importantly in the distribution and regulation of power across long distances.   13. What is the difference between a step up transformer and a step down transformer? A transformer that increases the voltage from primary to secondary (more secondary winding turns than primary winding turns) is called a step-up transformer. Conversely, a transformer designed to do just the opposite is called a step-down transformer.   14. Are transformers dangerous? There is no established evidence that the exposure to magnetic fields from powerlines, substations, transformers or other electrical sources, regardless of the proximity, causes any health effects.   15. Why transformer rating is in kVA not in kW? Copper losses (I²R) depends on current which passing through transformer winding while Iron losses or core losses or Insulation losses depends on Voltage. ... That's why the transformer rating may be expressed in VA or kVA, not in W or kW.