UNIT 2 - Ideal and Practical Transformers and Tests PDF

UNIT 2 – IDEAL AND PRACTICAL TRANSFORMERS AND TESTS TOPIC OUTLINE Ideal Transformer EMF Equation Transformer Turn Ratio Practical Transformer Transformer Test  IDEAL Transformers are electrical devices consisting of two or more coils of wire used...

UNIT 2 – IDEAL AND PRACTICAL TRANSFORMERS AND TESTS TOPIC OUTLINE Ideal Transformer EMF Equation Transformer Turn Ratio Practical Transformer Transformer Test  IDEAL Transformers are electrical devices consisting of two or more coils of wire used to transfer electrical energy Transformer by means of a changing magnetic field. WHY AC VOLTAGE? EMF Equation of a Transformer The rate of change of flux with respect to time is derived mathematically. Let: ϕm be the maximum value of flux in Weber f be the supply of frequency in Hz N1 is the number of turns in the primary winding N2 is the number of turns in the secondary winding The EMF Equations of a Transformer: E= 4.44fϕmN E= 4.44fβAN Note: A is the of the core (m2 or cm2) β is the maximum flux density (Wb/m2)  NOTE 1 Weber = 1 x 108 Maxwells 1 Tesla (Wb/m2) = 1 x 104 Gauss (Mx/cm2) Derivation of Formula of EMF Equation of a Transform Using the Faraday’s Law Let E1 be the emf induced in the primary winding. Where Ψ = N1ϕ

Derivation of Formula of EMF Equation of a Transformer Since ϕ is due to AC supply ϕ = ϕm Sinωt So the induced emf lags flux by 90 degrees. Derivation of Formula of EMF Equation of a Transformer In getting the maximum value of EMF where w = 2πf Root mean square RMS value Derivation of Formula of EMF Equation of a Transformer Putting the value of E1 max in equation (6) we get
Putting the value of π = 3.14 in the equation (7) we will get the value of E1 as Derivation of Formula of EMF Equation of a Transforme Similarly Now, equating the equation (8) and (9) we get Note: Example Problem A Single-phase transformer has 480 turns on the primary winding and 90 turns on the secondary winding. The maximum value of the magnetic flux density is 1.1T when 2200 volts, 50Hz is applied to the transformer primary winding. Calculate a.) maximum flux of the core b.) cross sectional area Example Problems Transformer Turn Ratio (TTR) Transformer turn ratio K using winding number of turns K = N1/N2. Transformer turn ratio K using electromotive forces K = E1/E2 Transformer turn ratio K using winding number of turns K = I2/I1. Transformation Ratio of a Transformer  Ideal Transformer The transformer which is free from all types of losses is known as an ideal transformer It is an imaginary transformer that has no core loss, no ohmic resistance, and no leakage flux The ideal transformer has the following important characteristic: The resistance of their The core of the ideal The leakage flux of the The ideal transformer has primary and transformer has infinite transformer becomes 100 percent efficiency, i.e., secondary winding permeability. The infinite zero, i.e. the whole of the the transformer is free permeable means less flux induces in the core of from hysteresis and eddy becomes zero. magnetizing current the transformer links with current loss. requires for magnetizing their primary and their core. secondary winding. Equivalent Circuit of a Practical Transformer Characteristics of the Practical Transformer In practical transformers, which is different from ideal transformers. A practical transformer has, 1. Iron Losses 2. Winding resistances and, 3. Magnetic leakage, giving rise to leakage reactance. Characteristics of the Practical Transformer Transformer On Load Condition When the transformer is on the loaded condition, the secondary of the transformer is connected to load. The load can be resistive, inductive or capacitive. The current I2 flows through the secondary winding of the transformer. The magnitude of the secondary current depends on the terminal voltage V2 and the load impedance. The phase angle between the secondary current and voltage depends on the nature of the load. Transformer Loading Transformers can provide a voltage on their secondary winding but to transfer electrical power between their input and output they need to be loaded. The secondary current, IS which is determined by the characteristics of the load, creates a self- induced secondary magnetic field, ΦS in the transformer core which flows in the exact opposite direction to the main primary field, ΦP. These two magnetic fields oppose each other resulting in a combined magnetic field of less magnetic strength than the single field produced by the primary winding alone when the secondary circuit was open circuited. Transformer “No-Load” Condition When the transformer is operating at no load, the secondary winding is open-circuited, which means there is no load on the secondary side of the transformer and, therefore, current in the secondary will be zero. The no-load current consists of two components: a. Reactive or magnetizing component Im (It is in quadrature with the applied voltage V1. It produces flux in the core and does not consume any power). b. Active or power component Iw, also known as a working component (It is in phase with the applied voltage V1. It supplies the iron losses and a small amount of primary copper loss). FACTORS EDDY HYSTERESIS Induced circulating currents Magnetization and Cause (eddy currents) demagnetization of the core Mechanism Electrical energy lost as heat Energy lost due to the due to core resistance resistance of magnetic domains to realignment Dependency Depends on magnetic field Depends on material properties, frequency, conductivity, and flux density, and frequency core thickness Loss Type Resistive (ohmic loss) Magnetic (magnetic hysteresis) Material Reduction Laminated cores or materials Low-hysteresis materials like with high resistivity silicon steel Loss Manifestation Caused by changing magnetic Energy spent on realigning fields inducing currents magnetic domains Reduction Techniques Laminating cores, using high- Using materials with a narrow resistance materials hysteresis loop Transformer “No-Load” Condition The ammeter shown will indicate a small current flowing through the primary winding even though the secondary circuit is open circuited. This no-load primary current is made up of the following two components: 1. An in-phase current, (IE or Iw) which supplies the core losses (eddy current and hysteresis). 2. A small current, IM at 90o to the voltage which sets up the magnetic flux. Steps in Drawing Phasor Diagram 1. The function of the magnetizing component is to produce the magnetizing flux, and thus, it will be in phase with the flux. 2. Induced emf in the primary and the secondary winding lags the flux ϕ by 90 degrees. 3. The primary copper loss is neglected, and secondary current losses are zero as I2 = 0.
Therefore, the current I0 lags behind the voltage vector V1 by an angle ϕ0 called the no-load power factor angle and is shown in the phasor diagram above. 4. The applied voltage V1 is drawn equal and opposite to the induced emf E1 because the difference between the two, at no load, is negligible. 5. Active component Iw is drawn in phase with the applied voltage V1. 6. The phasor sum of magnetizing current Im and the working current Iw gives the no-load current I0. Transformer On-Load Condition Example Problem A transformer has 8 windings in its primary core and 5 in its secondary coil. If the primary voltage is 240 V, find the secondary voltage. Solution: V1 = 240 V, N1 = 8 turns, N2 = 5 turns Example Problem A voltage transformer has 1500 turns of wire on its primary coil and 500 turns of wire for its secondary coil. What will be the turns ratio (TR) of the transformer. Solution:  Transformer Test For confirming the specifications and performances of an electrical power transformer it has to go through a number of testing procedures. Some tests are done at a transformer manufacturer premises before delivering the transformer. Transformer manufacturers perform two main types of transformer testing – type test of transformer and routine test of transformer. Some transformer tests are also carried out at the consumer site before commissioning and also periodically in regular and emergency basis throughout its service life. Transformer Tests Types of Transformer Testing 1. Test done at factory a) Type tests b) Routine tests c) Special test 2. Test done at site a) Pre-commissioning tests b) Periodic/condition monitoring tests c) Emergency tests Routine Tests of Transformer Routine tests of transformer is mainly for confirming the operational performance of the individual unit in a production lot. Routine tests are carried out on every unit manufactured. Special Test Tests of Transformer Special tests of transformer is done as per customer requirement to obtain information useful to the user during operation or maintenance of the transformer. 8 Common Testing Procedures for Electrical Transformers 1. Turns Ratio Testing The transformer turns ratio test is used to make sure that the ratio between the windings of the primary and secondary coils follow the proper specifications. This test ensures that the transformer will provide the proper step-up or step down in voltage. The turns ratio is calculated by dividing the number of turns in the primary winding by the number of turns in the secondary coil. The ratio is calculated under no-load conditions, using a tool known as a turns ratio tester. Done correctly, the test can identify tap changer performance, shorted turns, open windings, incorrect winding connections and other faults inside transformers. If the transformer is in three phase, each phase is tested individually 8 Common Testing Procedures for Electrical Transformers 2. Insulation Resistance Testing Commonly known as the Megger test, insulation resistance testing measures the quality of insulation within the transformer. Testing is typically done with a megaohmmeter, a tool similar to a multi-meter but with a much higher capacity. Some variations in testing results in natural, depending on the moisture, cleanliness and the temperature of the insulation, but to pass, the insulation must demonstrate a higher resistance than prescribed international standards for the type of transformer. 8 Common Testing Procedures for Electrical Transformers 3. Power Factor Testing The power factor test determines the power loss of the transformer's insulation system by measuring the power angle between an applied AC voltage and the resultant current. For ideal insulation, the phase angle is 90 degrees, but in practice, no insulation is ideal The closer the phase angle is to 90 degrees, the better the insulation. This test can be repeated during the service life of the transformer and verified against the result obtained during manufacturing. 8 Common Testing Procedures for Electrical Transformers 4. Resistance Testing Resistance testing is conducted several hours after a transformer has stopped conducting current when it reaches the same temperature as its surroundings. The purpose of this test is to check for differences in resistance between windings and opens in the connections. This test ensures that each circuit is properly wired and that all connections are tight. 8 Common Testing Procedures for Electrical Transformers 5. Polarity Testing Polarity refers simply to the direction of current flow in a transformer, and testing is done to ensure that the windings are all connected the same way, and not in opposing ways that can cause a short circuit. Polarity is a vital concern if several transformers are to be paralleled or bank-connected. Polarity in a transformer is categorized as either additive or subtractive, and it is tested using a voltmeter. When voltage is applied between the primary bushings and the resultant voltage between the secondary bushings is greater, then it means that the transformer has additive polarity. 8 Common Testing Procedures for Electrical Transformers 6. Phase Relation Testing This test will detect if two or more transformers have been connected in a correct phase relationship. This test calculates the angular displacement and relative phase sequence of the transformers and can be conducted at the same time as ratio and polarity tests. The voltages of the phase of primary and secondary windings in each transformer can be recorded and comparisons made to get the phase relation between them.  8 Common Testing Procedures for Electrical Transformers 7. Oil Tests The oil that provides insulation and cooling properties for a transformer should be tested before the transformer is energized, and periodically as a part of a regular maintenance schedule. It is generally done with a portable testing unit which applies test voltage that increases in intensity until a breakdown point of the oil is detected. An oil sample test can detect several things on a transformer: a) Acid number b) Dielectric breakdown c) Power factor d) Moisture content e) Interfacial tension  8 Common Testing Procedures for Electrical Transformers 8. Visual Inspection Although this is the simplest of all tests, a visual inspection may reveal potential problems that can't be detected by other, more sophisticated forms of diagnostic testing. A standard procedure must be established to perform the visual test, identifying the elements to be viewed and criteria for pass/fail judgments. most standard visual inspections look for the presence of manufacturer's labels, signs of physical damage, the condition of welds, oil loss or leakage, integrity of wire connections, and the condition of valves and gauges (if present). Open and Short Circuit Test Open and short circuit tests are performed on a transformer to determine the: 1. Equivalent circuit of transformer 2. Voltage regulation of transformer 3. Efficiency of transformer Note: The power required for open circuit tests and short circuit tests on a transformer is equal to the power loss occurring in the transformer. Open Circuit Test  Open Circuit Test The connection diagram for open circuit test on transformer is shown in the figure. A voltmeter, wattmeter, and an ammeter are connected in LV side of the transformer as shown. The voltage at rated frequency is applied to that LV side with the help of a variac of variable ratio auto transformer. The HV side of the transformer is kept open. Now with the help of variac, applied voltage gets slowly increased until the voltmeter gives reading equal to the rated voltage of the LV side. After reaching rated LV side voltage, we record all the three instruments reading (Voltmeter, Ammeter and Wattmeter readings). Since voltmeter reading V1 can be considered equal to the secondary induced voltage of the transformer, wattmeter reading indicates the input power during the test. As the transformer is open circuited, there is no output, hence the input power here consists of core losses in transformer and copper loss in transformer during no load condition. Open Circuit Test Let us consider wattmeter reading Po Where, Rm is shunt branch resistance of transformer If, Zm is shunt branch impedance of transformer. Open Circuit Test Therefore, if shunt branch reactance of transformer is Xm, These values are referred to the LV side of the transformer due to the tests being conducted on the LV side of transformer. Therefore, it is seen that the open circuit test on transformer is used to determine core losses in transformer and parameters of the shunt branch of the equivalent circuit of the transformer. Example Problem An open circuit test was conducted to a certain transformer 2300/230 V. The readings during the test was listed below: Io = 2.54 A Note: The no load power factor is 0.12 Calculate the estimated core loss of the transformer. Solution Use 230 V for the open circuit test. Po = V1Iocosθ Po = 230 (2.54) (0.12) Po = 70.104 W This the core loss of the transformer. Open Circuit Test Open Circuit Test A voltmeter, wattmeter, and an ammeter are connected in HV side of the transformer as shown. A low voltage of around 5-10% is applied to that HV side with the help of a variac (i.e. a variable ratio auto transformer). We short-circuit the LV side of the transformer. Now with the help of variac applied voltage is slowly increased until the wattmeter, and an ammeter gives reading equal to the rated current of the HV side. After reaching the rated current of the HV side, we record all the three instrument readings (Voltmeter, Ammeter and Watt-meter readings). The ammeter reading gives the primary equivalent of full load current IL As the voltage applied for full load current in a short circuit test on the transformer is quite small compared to the rated primary voltage of the transformer, the core losses in the transformer can be taken as negligible here. Open Circuit Test Let’s say, voltmeter reading is Vsc. The watt-meter reading indicates the input power during the test. As we have short-circuited the transformer, there is no output; hence the input power here consists of copper losses in the transformer. Since the applied voltage Vsc is short circuit voltage in the transformer and hence it is quite small compared to the rated voltage, so, we can neglect the core loss due to the small applied voltage. Hence the wattmeter reading can be taken as equal to copper losses in the transformer. Let us consider wattmeter reading is Psc Open Circuit Test Where, Re is equivalent resistance of transformer. If, Ze is equivalent impedance of transformer. Therefore, if the equivalent reactance of transformer is Xe. These values are referred to the HV side of the transformer as the test is conducted on the HV side of the transformer. Hence the short-circuit test of a transformer is used to determine copper losses in the transformer at full load. It is also used to obtain the parameters to approximate the equivalent circuit of a transformer. Example Problem