1. The working principle of the transformer

 

    When a converter current U1 is applied to the primary winding, I0 flows in, and thus an alternating magnetic flux φ is excited in the core, and the magnetic flux φ is also entangled with the secondary winding. The potential ez is induced in the secondary winding due to the alternating action of the magnetic flux φ. According to the law of electromagnetic induction, the induced potential of the winding is proportional to the number of turns. Therefore, as long as the number of turns of the secondary winding is changed, the value of the potential ez can be changed. If the secondary winding is connected to the electrical device, the secondary winding has a voltage output, which is the working principle of the transformer.

     Assume that the number of turns of the primary and secondary windings is W1, W2 respectively. When the primary of the transformer is connected to a sinusoidal variable current power supply with a frequency of f and a voltage of V1, according to the principle of electromagnetic induction, the alternating magnetic flux φ in the core will be respectively The potential is induced in the primary and secondary windings. The primary winding induced potential is: e1 - W1 * dφ / dt where dφ / dt is the rate of change of the magnetic flux, and the negative sign indicates that the actual direction of the potential e1 is opposite to the positive direction of the potential when the magnetic flux increases. If the leakage impedance is not counted, according to the loop potential balance law:

           U1=- E1 Its value V1=E1=4.44 *f * W1*φm (1)

                The same reason can be found on the secondary side:

           U2= E2= 4.44* f* W2*φm

                (2) by the ratio of (1), (2)

          U1/U2 = E1/E2 = W1/W2 = K

        In the formula, K is the transformer's ratio, or the turns ratio. When designing the appropriate ratio, the primary voltage can be changed to the required secondary voltage.

        Power systems generally use three-phase power supply. Therefore, the most widely used one is the three-phase transformer. The operation of the three-phase transformer in the three-phase load balancing is basically the same as that of the single-phase transformer.

       2. Main parameters of the transformer:

             <1> rated voltage

            One of the functions of the transformer is to change the voltage, so the rated voltage is one of the important data. Rated voltage refers to the voltage applied between the line terminals of a multi-phase transformer or between the terminals of a single-phase transformer, or the voltage generated when no-load occurs, that is, when a rated voltage is applied to a certain winding at no-load, all other transformers The windings generate voltage at the same time.

             The rated voltage of the transformer should be consistent with the connected line voltage of this connection. China's power transmission and transformation line voltage level (kV) is 0.38, 3, 6, 10, 15 (20), 35, 63, 110, 220, 330, 500, 750. The voltage level of the transmission line is the voltage value of the line interruption. Therefore, the rated voltage on the side of the transformer connected to the line terminal is the same as the value listed above. The voltage at the beginning of the line (power supply) takes into account the voltage drop of the line. This level is high, and the voltage at the beginning of the voltage level below 35kV is 50% higher than the voltage level. And 35kV and above is 10% higher. Therefore, the rated voltage of the transformer is also increased accordingly. The voltage at the beginning of the line (kV) is 0.4, 3.15, 6.3, 10.5, 15.75, 38.5, 69, 121, 242, 363, 550. It can be seen that the transformer with a high voltage rated voltage equal to the starting voltage is A step-up transformer, a transformer equal to the line terminal voltage (voltage level), is a step-down transformer.

            The transformer product series is divided into high voltage voltage grades. Now the power transformer series is divided into 10kV and below series, 35kV series, 63kV series, 110kV series and 220kV series. The rated voltage is the line voltage and is expressed as a valid value.

           <2> Rated capacity:

           The main function of the transformer is to transfer electrical energy, so the rated capacity is its main parameter. The rated capacity is a conventional value for expressing power. It is the size of the transmitted electrical energy. It is expressed in kVA or MVA. When the rated voltage is applied to the transformer, it is used to determine the rated current that does not exceed the temperature rise limit under specified conditions.

          The rated capacity of the two-winding transformer is the rated capacity of the winding. (Because the efficiency of the transformer is very high, usually, the rated capacity of the secondary side is designed to be equal)

          Multi-winding transformers shall be specified for the rated capacity of each winding. The rated capacity is the rated capacity of the winding with a large amount; when the capacity of the transformer is changed by the cooling method, the rated capacity refers to the capacity with a large amount.

          China's current transformer rated capacity rating is increased by a multiple of ≈ 1.26, such as capacity of 100, 125, 160, 200 ... kVA, etc., only 30 kVA and 63000 kVA other than the capacity level is different from the priority number.

         Prior to 1967, the rated capacity rating of transformers was the R8 capacity series increased by R8=8≈1.33.

         For single-phase transformer SN = IIN^2 * U1N*(10)^-3 = U2N * I2N*10^-3(kVA)

              For three-phase transformer SN= *U1N *I1N * 10^-3=√3 *U2N *I2N *10^-3(kVA)

            The size of the transformer is also closely related to the voltage level. The voltage is low, the current is large when the capacity is large, and the loss is increased; the voltage is high, the insulation ratio is too large, and the transformer size is relatively increased, so the capacity of the low voltage must be small. The capacity of the high voltage must be large.

           <3> rated current

         The rated current of the transformer is divided by the rated capacity of the winding by the rated voltage of the winding and the corresponding coefficient (single phase is 1, three-phase), and the current obtained by the current through the winding end.

         Therefore, the rated current of the transformer is the rated current of each winding, which refers to the line current, and is also expressed by the effective value (note that the three-phase single-phase transformer is composed)

        <four> rated frequency

         The rated frequency refers to the operating frequency designed for the transformer. The frequency specified in China's standard is 50HZ.

        <5> No-load current and no-load loss

         The no-load current refers to when the rated voltage of the rated frequency is applied to one winding of the transformer (generally the primary winding), the other windings are open, and the current flowing through the terminal of the winding line is called the no-load current I. Its smaller active component Ioa is used to compensate for the loss of the core, and its larger amount of reactive Ior is used to excite to balance the magnetic pressure drop of the core.

         No-load current Io=

             Usually Io is expressed as a percentage of rated current: Io%=(Io/IN) *100= 0.1~3%

             The active component Ioa of the no-load current is the loss current, and the active power drawn is called the no-load loss Po, which is the active power drawn when the other windings are opened when the rated voltage of the rated frequency is applied to the terminals of one winding. Iron loss is also referred to as neglecting the resistance loss of the wire winding under no-load operation. Therefore, the no-load loss is mainly determined by the unit loss of the core material.

        <6> Impedance voltage and load loss

         A two-winding transformer is a short-circuit (usually secondary side) winding of one winding, and the voltage applied by the other winding is called the impedance voltage Uz. The multi-winding transformer has a Uz of any pair of winding combinations. Usually the impedance voltage is expressed as a percentage of the rated voltage

        Uz%=(Uz/UN)*100% (and should be converted to the reference temperature)

            One winding is shorted (usually two times). The active power drawn by the other winding through the rated current is called the load loss PR.

            Load loss = maximum resistance loss of a pair of windings + additional loss

          Additional losses include winding temperature loss, loop loss around the wire, structural losses and lead losses, where the resistive losses are also referred to as copper losses, and the load losses are also converted to the reference temperature.

          <7> Temperature rise and cooling method

          Temperature rise, transformer temperature rise, for air-cooled transformer is the difference between the temperature of the measured part and the temperature of the cooling air; for water-cooled transformer is the difference between the temperature of the measuring part and the water temperature at the inlet of the cooler (generally operated at altitude) 1000m and below)

          The oil-immersed transformer coil and top oil temperature rise limit is obtained because the insulation damage caused by Class A insulation at 98 °C is normal damage, and the annual average temperature to ensure the normal life of the transformer is 20 °C. The difference between the average temperatures is 13K, so the coil temperature rise limit is 98-20-13=65K.

          The maximum temperature of oil normal operation is 95 ° C, the maximum temperature is 40 ° C, so the top oil temperature rise limit is 95-40 = 55K.