Introduction to the Working Principle of High and Low Voltage Current Transformers

The working principles of high and low voltage current transformers are the same, they all adopt the principle of electromagnetic induction. The old code is called CT, and the new code is called TA. In standard electrical drawings, low voltage current transformers are generally represented by TA, and voltage transformers are generally represented by TV. Usually, CT and PT also refer to low voltage current transformer and voltage transformer. In fact, the principle is the same, large current becomes small current, the voltage level of high voltage knowledge is relatively high, and there are high requirements on insulation. The low voltage is the ordinary insulation level. The high-pressure ones have their own models, while the low-pressure ones have no definite model name.

A low voltage current transformer is composed of a closed iron core and a winding, while a voltage transformer is a transformer with an iron core. The secondary of the low voltage current transformer can be short-circuited, but not open-circuited. The voltage transformer secondary can be open but not short-circuited. The winding of the low voltage current transformer with many turns and thin wire diameter is connected to the measuring instrument, while the winding of the voltage transformer with few turns and thick wire diameter is connected to the measuring instrument. A low voltage current transformer with a few turns and a thick wire diameter is used as a primary winding and connected in series with the circuit to be measured, while a secondary remote fault indicator voltage transformer with a large number of turns and a thin wire diameter is used as a primary winding to be connected to the measured circuit. The measured circuits are connected in parallel.

Compared with the load on the secondary side, the primary internal impedance of the voltage transformer is so small that it can be ignored, and the voltage transformer can be considered as a voltage source; while the primary internal resistance of the low voltage current transformer is so large that it can be considered as a A current source with infinite internal resistance. The magnetic flux density of the voltage transformer is close to the saturation value when it works normally, and the magnetic flux density drops when it is faulty. greatly increased, sometimes well beyond saturation.

The classification of the two remote fault indicators:

There are many types and complex models of electric power secondary remote fault indicators, but they can be roughly divided into the following categories:

The following is a brief introduction to three different fault indicators according to the classification method of the installation site.

1. Overhead line fault indicator

The sensor and the display (indicating) part are integrated into one unit, and the indicator is mechanically fixed on a certain phase line of the overhead line (including bare wires and insulated wires). It should have a communication function, configure a communication module, and return information such as faults, line current, and low battery power to the data receiving device connected to it. It should be able to load and unload with electricity, and no false alarm should be given during the loading and unloading process.

2. Panel type fault indicator

It is mainly used in supporting equipment such as cable branch boxes and ring network cabinets. It is an indicator composed of a sensor and a display unit, usually the display unit is embedded on the operation panel of the ring network cabinet and switch cabinet. The detection principle is basically the same as that of the cable-type short-circuit and cable-type ground fault indicators. The difference is that the detector detects a fault and transmits it to the host through an optical fiber cable, and sends an alarm signal through the host. The sensor and the display unit communicate by means of optical fiber or wireless, and the primary and secondary should be reliably insulated.

3. Cable type ground fault indicator

The sensor and display (indicating) part are integrated in one unit, fixed on a certain phase cable line (busbar) by mechanical means, usually installed on the indicator on the power distribution equipment such as cable branch box, ring network cabinet, switch cabinet, etc. . In addition to the above requirements, a zero-sequence ground fault indicator also needs to be configured.