A step up transformer is an electrical device that amplifies the voltage from the primary coil to the secondary coil. It is commonly used in power plants where voltage generation and transmission occurs.
The step up transformer has two main parts – the core and the winding. The core of a transformer is made of a material with a higher permeability than a vacuum. The reason for using high permeability substances is to limit field lines and reduce losses. Silicon steel or ferrite is used to prevent transformer from causing excessive eddy current and hysteresis loss. So the magnetic flux can flow easily through the core and increase the efficiency of the transformer.
The winding of the transformer is made of copper. Copper is extremely rigid and is ideal for carrying large amounts of current. These are covered with insulators to provide safety and durability for better performance. The winding is wound around the transformer core. Primary coils consist of fewer windings and thicker wires, and are specifically designed to carry low voltages and high currents. The secondary coil is the exact opposite phenomenon. This time the wire is thinner and has more turns. These wires are good carriers of high voltage and low current.
The primary winding has fewer turns than the secondary winding, Ns>Np (Ns= number of turns in the secondary coil, Np= number of turns in the primary coil). By the nature of the ideal transformer, the [Latex] \ frac {N_ {p}} {N_ {s}} = \ frac {V_ {p}} {V_ {s}} [/ Latex]. Therefore, the more turns of the secondary coil, the greater the induced voltage. But the power of the transformer should be fixed. Therefore, the step up transformer gradually increases the voltage and decreases the current to keep the power constant. Step up transformer is an indispensable part of power system. Transmission lines use step up transformers to transmit voltage over long distances.

The answer is no. A transformer is not used with direct current. Because the transformer itself is to use the alternating magnetic field to achieve the transmission of energy.And pulsating voltages with DC components do not work. Because the direct current will saturate the magnetic circuit of the transformer, lose the ability of “AC transformer”. If DC current cannot be transformer, the reason is because DC current does not change the magnetic field. What if you have an unsteady current? An unsteady current, if the Fourier transform is expanded, is a constant DC plus a number of AC’s, so in this combination, the DC part can’t change voltage, but the AC part can. But the premise is that the transformer core is large enough, otherwise DC will let the core magnetic saturation, so that the AC part of the change does not produce magnetic field changes. The coil with DC current only generates an automatic voltage induction potential opposite to the current at the moment of power-up and changes in magnetic field induction, and the magnetic field becomes stable after the DC is stabilized.

In addition, from the actual structure of the transformer, because the winding resistance of the transformer is generally relatively small, if the direct current is entered, its frequency is f=0, so it will produce a large amount of current. In a short time, a large amount of heat will be generated inside the voltage tester. If the time is a little longer, the heat generated by the current is enough to burn out the voltage tester.

So,to sum up,a transformer can not be used with direct current.

A transformer only works with which alternate current.Transformer is the use of alternating current mutual inductance phenomenon to change voltage, mutual inductance phenomenon only exists in AC circuit, DC no mutual inductance phenomenon.

We can also conclude from the working principle of the transformer that it is only suitable for alternating current. Transformer is the use of the principle of electromagnetic induction to change the AC voltage device, the main components are primary coil, secondary coil and core (magnetic core). In electrical equipment and wireless circuit, commonly used for lifting voltage, matching impedance, safety isolation, etc. In a generator, whether the coil moves through a magnetic field or the magnetic field moves through a stationary coil, an electric potential is induced in the coil. In these two cases, the value of the magnetic flux is unchanged, but the number of the magnetic flux of the chain intersecting the coil is changed, which is the principle of mutual induction. Transformer is a device that uses electromagnetic mutual inductance to transform voltage, current and impedance.

There is an alternating magnetic flux through a coil, the coil will produce induced electromotive force, in a transformer, AC current through the primary coil in the iron core to produce alternating magnetic flux, this alternating magnetic flux through the core through the secondary coil, so secondary induced electromotive force, secondary voltage. There is a condition that the flux must be alternating, that is, the primary voltage current must be alternating (induced electromotive force V=Ldi/dt, DC di/dt=0, so no EMF can be induced).

Alternating current, AC for short. Alternating current is also called “alternating current”, abbreviated as “AC”. Generally refers to a voltage or current that varies periodically from time to time in magnitude and direction. Its most basic form is sinusoidal current.

a novel system of electric distribution and transmission of power by means of alternate currents, affording peculiar advantages, particularly in the way of motors, which will at once establish the superior adaptability of these currents to the transmission of power, and will show that many results heretofore unattainable can be reached by their use; results which are very much desired in the practical operation of such systems, and which cannot be accomplished by means of continuous currents.

In our dynamo machines, it is well known, we generate alternate currents which we direct by means of a commutator, a complicated device and, it may be justly said, the source of most of the troubles experienced in the operation of the machines. Now, the currents so directed cannot be utilised in the motor, but they must — again by means of a similar unreliable device — be reconverted into their original state of alternate currents. The function of the commutator is entirely external, and in no way does it affect the external working of the machines. In reality, therefore, all machines are alternate current machines, the currents appearing as continuous only in the external circuit during their transit from generator to motor. In view simply of this fact, alternate currents would commend themselves as a more direct application of electrical energy, and the employment of continuous currents would only be justified if we had dynamos which would primarily generate, and motors which would be directly actuated by such currents.

But the operation of the commutator on a motor is twofold; firstly, it reverses the currents through the motor, and secondly, it effects, automatically, a progressive shifting of the poles of one of its magnetic constituents.

The transformer inrush current is the maximum instantaneous current drawn by the primary of the transformer when their secondary is open circuit.The characteristics of inrush current are as follows,
1, the inrush current contains a large number of high harmonic components (mainly second and third harmonic), mainly even harmonic, therefore, the change curve of excitation inrush current for the peak wave
2. The attenuation constant of excitation inrush current is related to the saturation degree of the iron core. The deeper the saturation, the smaller the reactance and the faster the attenuation. Therefore, at the beginning of the instant attenuation is very fast, and then gradually slow down, after 0.5 ~ 1s its value does not exceed (0.25 ~ 0.5)
3. In general, the larger the transformer capacity, the longer the duration of attenuation, but the general trend is that the attenuation speed of inrush current is often slower than that of short circuit current.
4. The value of excitation inrush current is very large, up to 6 ~ 8 times of rated current. When setting a circuit breaker to control a transformer, its speed break can be set according to the transformer excitation current.

The inrush current is related to the saturation degree of the core, and the remanence of the core and the phase Angle of the closing voltage can affect its magnitude.

The inrush current is not dangerous to the transformer because the inrush current exists for a short time. Of course, it is not good to charge the transformer repeatedly, because the multiple impact of large current will cause the mechanical force between the windings, which may gradually loosen its fixed objects. In addition, the excitation inrush current may cause the differential protection action of the transformer, so the transformer operation should be paid attention to.

The burden of the current transformer refers to the total impedance of the instrument, relay and connecting wire connected to the current transformer twice。

The impedance (current transformer) or admittance (voltage transformer) of the secondary circuit. Impedance or admittance within the transformer (the secondary winding itself) is not included. The load is usually expressed in terms of apparent power volt-ampere.

The burden of the current transformer is calculated as follows:
When calculating the burden of the CT, pay attention to the impedance conversion coefficient under different wiring modes and fault conditions.

The secondary burden of the CT can be expressed as impedance Z2(Ω) or capacity S(VA). The relationship between the two is: S=I2*I2*Z2

When the secondary current of the CT is 5A, S=25Z2

When the secondary current of the CT is 1A, S=Z2

The secondary burden rating (S) of the current transformer can be selected as 5, 10, 15, 20, 25, 30, 40, 50, 60, 80, 100VA.

Load calculation of current transformer for measurement.

Generally, the phase difference between impedances can be negative in engineering calculation. The calculation formula of secondary load Z2 is as follows:

Z2=Kcj.zkZcj+Klx.zkZlx+Zc

Where: Zcj——- Impedance of meter coil (Ω)

Zlx——- one-way impedance (Ω) of the connecting wire. Generally, the reactance can be ignored and only the resistance can be calculated.

Zc——- Contact resistance (Ω), generally 0.05~0.1(Ω).

Kcj.zk—- Impedance conversion factor of the meter

Klx.zk—- Impedance conversion factor of connecting wire

A、× This is a voltage transformer

B、√ This is a current transformer

The processes occurring inside the current transformer can be grouped into two:Magnetic flux is produced in a coil when ever there is a change in current flowing through the coil. Similarly change in magnetic flux linked with the coil induces EMF in the coil. The first process occurs in the windings of the current transformer. When the ac supply is given to the primary winding alternating flux is produced in the coil .The second process occurs in the secondary winding of the current transformer. The flux alternating flux produced in the current transformer links the coils in the secondary winding and hence emf is induced in the secondary winding. Whenever an ac supply is given to the primary coil, flux is produced in the coil. These flux links with the secondary winding thereby inducing emf in the secondary coil.

Current transformer is a measuring instrument that converts the high current of the primary side into the low current of the secondary side according to the principle of electromagnetic induction.

Current transformer consists of a closed iron core and winding. It has a small number of turns in the primary side winding and is connected to the line where the current needs to be measured. As a result, it often has all the line current flowing through it, and the secondary winding has a large number of turns. It is in series in the measuring instrument and protection circuit.

When the CT is working, the secondary loop is always in the closed state. Therefore, the impedance between the series coil of the measuring instrument and the protection circuit is very small, and the working state of the CT is close to short circuit.

The secondary rated load of the voltage transformer, similar to the transformer, has the load power that the secondary winding allows access, divided into rated capacity and large capacity. A voltage transformer is a voltage conversion device. It converts high voltage to low voltage so that the change of high voltage magnitude can be reflected with low voltage magnitude. Therefore, voltage measurement can be performed directly with ordinary electrical instruments through the voltage transformer.

1. Voltage transformer, also known as instrument transformer, is a voltage conversion device;

2. The capacity of the voltage transformer is very small, usually only tens to hundreds of volts;

3. The voltage on the primary side of the voltage transformer is the grid voltage, which is not affected by the secondary load, and its load is constant in most cases;

4. The secondary side loads are mainly instruments and relay coils, which have a large impedance and pass very little current. If the secondary load is increased indefinitely, the secondary voltage will decrease, resulting in increased measurement errors;

5. Use the voltage transformer to indirectly measure the voltage, which can accurately reflect the value of the high-voltage side and ensure the measurement accuracy;

6. No matter how high the primary voltage of the voltage transformer is, its secondary rated voltage is generally 100V, which makes the manufacturing of measuring instruments and relay voltage coils standardized. Moreover, it ensures the safety of instrument measurement and relay protection work, and also solves the difficulties of insulation and manufacturing process of high-voltage measurement;

7. Voltage transformers are often used in circuits such as power distribution instrument measurement and relay protection.

High voltage transformer is a transformer suitable for converting large voltage into small voltage or converting large current into small current in 1kv to 220kv power system for measurement, protection and use.

Generally divided into 3kv, 6kv, 10kv, 35kv, 66kv, 110kv, 220kv, and the 24kv system is being tested in Jiangsu. Among them, 10kv and 220kv are widely used. The secondary input is mostly 200V or 100V or 5A or 1A.