The purpose of the voltage transformers is to switch on the testing devices. Such devices are used, as a rule, in an alternating current circuit. With the help of them, you can carry out various voltage measurements. The measuring instruments in this case are completely isolated, and the current strength is regulated by a transformer. When it comes to testing the current strength, ammeters are used.
Content
- General definition
- Types of transformers and their components
- Testing process
- Single-band devices
- Multi-range modifications
- Oil models
- With gas-filled and dry dielectrics
General definition
Transformers are elements of an electric circuit that convert the magnitude of an alternating voltage. They can be:
- lowering, giving out less voltage at the output than at the input;
- enlighteners, performing counter-transformation;
- dividing, not changing the voltage value. They are used for galvanic isolation between sections of the electric network.
Up and down transformers are reversible: if we apply the nominal output voltage of the transformer to its secondary winding, we will receive the nominal input voltage at the primary one.
With currents in revolving there is a reverse picture. The primary winding is calculated for a current corresponding to the nominal power of the transformer. For the power, the cross-section of the magnetic wire and the diameter of the primary winding wire are selected.
The only secondary winding of the lowering transformer can be more only in the first place, just as much as its voltage is less. This ratio is called the transformation ratio. Therefore, the cross-section of the secondary winding wire at the lowering transformer is larger. In the lowering - everything is the other way around. The separator has all the same.
Speaking about what a voltage transformer is needed for, in electrical installations up to 1000 V measurement voltages are generated by connecting voltmeters directly to tires or other monitored sections of the network.
But in networks of 6 kV and above this is impossible, because:
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when measuring high voltage, it is required to reduce its value to the size accepted by a frame pointer device or an electronic digital converter. Resistive dividers will not perform the task with the required accuracy, and the use of a lowering transformer will make the device loud; - Isolation of the conductors for connecting the device must withstand the nominal voltage of the electrical installation. In addition, the phase-to-phase distances required by the PUE must be observed. It is impossible to do this.
Therefore, for measurements, the voltage value is reduced, and for this a voltage transformer is needed.
Types of transformers and their components
Today, high voltage instrument transformers are produced of various types. The most common subspecies can be considered single-phase modifications. They have quite high accessibility. In this case, the induction process is carried out quickly. Earthed transformers are usually produced three-phase. The parameter of the power supply voltage reaches 400 V. There are also cascade models. The primary winding of these devices is divided. Each section passes through itself only separately. Capacitive modifications are distinguished by the presence of a divider. Secondary wrap, with this there is one.
ADDITIONAL transformers are often connected to the meter. When you use them in the circuit, it is possible to determine the electrical resistance. Multimeters are combined measuring instruments. The frequency of vibrations in the network can be determined by using a frequency meter. If the voltage in it is sufficiently high, then without a transformer in this situation can not be avoided. Separately, it should also be mentioned about wattmeters and varmeters. They are intended for measuring the power of an electric current. Today, such basic types of measuring transformers are distinguished:
- Voltage devices.
- Permanent models.
- Variable current devices.
In addition, the division of transformers occurs according to the value of the transformation coefficient. In this case, a distinction is made between single-range and multi-range modifications. Depending on the type of installation, devices are divided into internal and external. Also, the transformer can be built-in. Additionally, there are on-board and even portable modifications. Some still share transformers by the type of dielectric. Today it is possible to distinguish oil, dry and gas-filled subspecies.
The principles of operation of voltage transformers of various types are quite similar, but there are still differences. If we consider voltage devices, then they include a magnetic drive and a secondary winding. At the top of the device there is a fastening ring. In turn, the current is in the middle of the device. Modifications of a permanent current presuppose the use of several magnetic drives and a core. In this case, the winding is used primary. The current in the device passes through the fastening ring.
Testing process
Tests of instrument transformers are carried out using a megaohmmeter. In this case, it is necessary to make measurements of the insulation characteristics. For this purpose, you should additionally use a voltmeter. The network connection is carried out through conductors. The resistance is checked for each phase separately. ADDITIONAL transformers can be tested according to the coefficient of electrical losses. In this case, the measurement of the tangent of the angle is carried out. With the help of an ammeter, it is possible to evaluate the wrap of the device.
Measuring transformers of alternating current, as a rule, are produced of the built-in type. They use only the primary one. They fit perfectly for installation on the support plane.
The maximum input voltage parameter can be 500 V. Pass-through modifications are used only as input systems. At the same time, they are not suitable for distribution work.
Additionally, it should be borne in mind that many models are manufactured with bushings. They are also called tire devices. Permanent transformers are distinguished by the presence of a core. As a rule, it is installed of an electromagnetic type. Devices such as conventional amplifiers work. In the process of increasing the voltage of the circuit, magnetization of the element occurs. Secondary wrap in this situation serves to strengthen the current.
Single-band devices
Single-range voltage transformers are usually manufactured with cores. They are installed in an alternating current circuit. In this case, the indicator of the power voltage should not exceed 300 V. If we consider the temperature characteristics, then the devices can be operated at a maximum of 40 degrees.
The operating voltage in the circuit must be maintained at a level of 200 V. The nominal frequency of the device does not exceed 50 Hz on average. By the class of accuracy, the models are quite different. The phase angular error in this case depends on the feasibility of the secondary sweep. The transformation ratio of the device is on average at the level of 50%. The primary current of the system can be accepted up to 3 A.
Multi-range modifications
Instrument transformers of this type are ideal for AC circuits. With this, the maximum voltage is maintained by them at a level of 500 V. The class of accuracy of the device depends on the type of core that is installed. With voltmeters, many modifications are able to work. Separately, note the high operating frequency range.
If you operate the transformer in a chain with alternating current, that is, this indicator is on average at the level of 55 Hz. Phase angular error in this case will be minimal. The parameter of the power supply voltage of the device generally does not exceed 300 V. To connect the measuring devices, terminals are used. Grounded modifications of this type of transformer are produced quite rarely.
Oil models
Instrument current transformers with oil dielectrics are very common today. They can be used in a circuit with alternating current. In this case, the parameter of the power voltage must not exceed 300 V. The class of accuracy of the device depends solely on the type of core. The minimum frequency is at the level of 3 Hz. At this maximum, the measuring devices are capable of being operated at 55 Hz.
The load parameter in the circuit, as a rule, does not exceed 5 A. Pliers are used for connecting with devices. Transformers can be installed on the outside plane. Models with a grounding system are produced quite often. Additionally, today there are tire modifications. They are used, as a rule, as input devices.
With gas-filled and dry dielectrics
Measuring devices of this type are capable of boasting a high frequency at the level of 60 Hz. In this case, the minimum is operated at 5 Hz. To turn on the appliance, you need a circuit with a constant current. The device is capable of withstanding a maximum of 6 A. It is prohibited to use the model at temperatures above 45 degrees. With all the measuring instruments, the devices interact in a way.
The limitation of the line voltage in the system comes from the core. In this case, there are two magnetic drives in the device, as a rule. In this case, the contacts are used with a protective tire. With this, sleeve modifications are quite rare. Separately, it should also be mentioned that there are many transformers of the earthed type. They use only the primary one. With this, dividing it into sections does not occur.
Instrument transformers with dry dielectrics are most often paired with multimeters. With this, it is also possible to connect voltmeters and ammeters to them. Due to the high rate of voltage, the accuracy of the results will be high. Wattmeters are quite rare for this type of model.
This is connected to a greater extent with a high rate of resistance inside the chain. Additionally, it should be borne in mind that today there are many built-in modifications. In this case, the parameter of their power voltage does not exceed 330 V. Bushing devices are used quite rarely. With this, the models have different connectors.
It is possible to connect a transformer for an alternating current circuit. The maximum load on the system can be in the region of 5 A. For the core to work properly, you should monitor the operating voltage parameter. As a rule, there are two magnetic wires in the device. Pass-through modifications of transformers are rarely produced.
