Capacitor voltage transformer
A voltage converter is in the area of electrical power engineering, a measurement converter for potential-free measuring AC voltage. The function of the voltage converter is a specific transformer the measured high -voltage to low voltage values proportional to implement. This lower voltage, usual voltages to 100 V can be measured in further consequence with measuring instruments and further processed. In the field of high voltage applications capacitive voltage transformers are used at rated voltages up to about 1 MV. The area of application of voltage transformers is including in the field of energy meters and power protection in the area of electrical power distribution in substations and power plants.
Construction
Voltage converters are generally constructed as transformers, consisting of a primary winding which is electrically connected to the voltage to be measured, and a secondary winding which is electrically isolated leads to the measuring instruments. For metrological applications have voltage transformer some constructive adjustments: So the phase shift of the voltage on upper to lower voltage may have only a small error, this is too important to capture the active power and reactive power correctly together with the current transformer. The ratio of voltage converters is usually chosen so that at the rated voltage on the primary winding, the secondary-side output voltage of 100 volts. For this, the secondary side is completed with a load whose value depends on the performance of the transducer, and extends the range of some 10 Ω up to a few 10 k.
The typical capacity of transformational voltage transformer for measurement applications from 10 to 50 VA, at its own power supply to the switchgear can be VA up to some 100. It is necessary to distinguish between the nominal load up to the converter complies with the accuracy class, and the thermal power limit up to which he can be charged without taking damage.
The accuracy is given in classes, with class 0.5 for example means a measurement tolerance of 0.5%.
Voltage transformer on the primary side are designed for either a connection between the outer conductors to be measured or for connection to earth. In the former case, the primary winding is completely isolated from earth and there are single-phase or three-phase version for three-phase alternating current. The connections are brought out high voltages. For voltage measurement at a single outer conductor to ground one end of the primary winding of the transformer is grounded, and has only a high-voltage terminal. The secondary winding is grounded at one end the two versions. Depending on the specific model, additional secondary-side windings are provided, which are used for example to monitor against ground faults. For this, these auxiliary windings of the transformer of each of the three phases are connected in series with three-phase current, so that the voltage at zero added accuracy.
Capacitive voltage transformers
In the range of high and very high voltages above 100 kV capacitive voltage transformers are used. The reason is that the insulation strength can be ensured only with great effort in the transformer at high voltages. Thus a capacitive voltage divider, as shown in the adjacent circuit, provided on the high voltage side. The drawn circuit on the high-voltage resistant capacitor C1 is housed in the interior of the insulating body and connected to the high-voltage line. This capacitor may also consist of a series connection of a plurality of individual capacitors, in order to increase the dielectric strength of the entire assembly. The coil L1 is used, the phase shift which is due to the capacitive voltage divider to compensate.
In addition to exclusive voltage and current transformers are also combined models, which summarize voltage and current transformers in a case and allow a compact system design.