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The key problems related to current transformer

by:Transmart     2020-07-21
Why do the accuracy will be lower? The size of the current transformer secondary load impedance is very big harm to the accuracy of the voltage transformer. This is because, if the current transformer secondary load impedance increase many, more than the allowed by the secondary load impedance, the excitation regulator current values will increase greatly, and make the transformer core into the saturation, under such conditions, a very big part of the current will be used to produce current excitation controller and, in turn, makes the deviation of substantial increase of voltage transformer, its accuracy is as decreased. 2, to current transformer differential protection, the transformer core is good, should also increase transformer core cross section, why? In normal operation or differential protection system software category external short-circuit fault, on both sides of the differential protection current transformer current values and position in the same way, should not inject current differential relay protection cars, but in fact the two characteristics of current transformer is unlikely to be completely consistent, current excitation regulator is different, the secondary current is not easy, same car through the unbalance current relay. In order to reduce the unbalance current, it is important to improve the structure of the current transformer, make its not saturated state, or use special wear small ferrite core making transformer core, and increase the transformer iron core cross section. 3, the connection of the secondary winding of current transformer is what kind of method? Based on the relay protection device and the different provisions of the protection device, current transformer secondary winding generally has the following several kinds of connection methods: (1), (thoroughly Three phase) Star connection; 2, incomplete ( Two phase) Star connection; (3), delta connection; (4), three phase and wiring for zero sequence current; 5], two distance connection; [6], a phase with two current transformer series connection; Once, a phase with two current transformer in series connection. 4, what is the current transformer connection index? What's the connection index for? According to the current of automobile relay and ct (current transformer) secondary current ratio is current transformer connection index, namely the Kc = Ik/I2 Ik - type - - - - - - The current in the shed automobile relay; 12 - - - - - - - Man of current transformer secondary current; Connection index is a key of relay protection setting calculation, main parameters of various current maintenance posture value calculation, accurate measurement components must consider the connection index. 5, what is the battery voltage transformer? The protector what harm is there? According to the voltage transformer secondary side not by induction electric bus duct battery as the battery. Such as the 225 kv voltage transformer, variable than for 2200, a bus duct power even if not grounding device, its impedance ( Contains the bus duct capacitors and grounding resistance) In spite of the large, assuming the 1 m Ω, but seen from measured voltage transformer secondary impedance only 1000000 / ( 2200). 2 Ω, near fault, the battery charging current is very big, The battery charging current key decisions in the cable line resistors and two voltage transformer leakage reactance) , will cause the operation of voltage transformer secondary side small switch jump off or circuit breaker broken, make the operation of lose working voltage protector, will lead to misoperation or refusing action of protector. 6, the installation of the zero sequence current transformer ( 1) Cable joint and zero sequence current transformer brace using insulation on protection. ( 2) Produce single phase grounding, grounding device not only in the current through the, will also follow the skin through the cables. In order to avoid common failure zone of earthing device error operation, cable connector wire connection should be over the zero sequence current transformer grounding device. ( Wire connection back across the zero sequence current transformer grounding device) Common connection of voltage transformer, voltage transformer in the three-phase circuit in common wiring method has four kinds: the following figure 1, a single phase electric connection of voltage transformer, to asymmetrical three-phase circuit, secondary side can match the dashboard and auto relay, as shown in figure 1 ( 一) 。 Single-phase electric voltage transformer 2, 2, V/V wiring can be accurately measure two hue voltage, but can not measure the phase voltage, as shown in figure 1 ( b) 。 3, three single phase electric voltage transformer connected into Y0 / Y0 form, as shown in figure 1 ( c) 。 Can accurately measure the balance of supply and demand regulation phase voltage of the instrument panel and auto relay, and the rules of supply and demand equilibrium phase voltage insulation supervision and ammeter. 4, a three-phase voltage transformer connected into five core column Y0 / Y0 / Δ ( Open delta) , as shown in figure 1 ( d) In the figure. Connection into Y0 form secondary electromagnetic coil power supply system to the dashboard, automotive relays and insulation monitor current meter, etc. Auxiliary secondary electromagnetic coil connected into open delta, power supply system for insulation monitoring voltage relay. When three-phase system software all normal work, balanced three-phase voltage, on both ends of the open delta voltage is zero. When connected to a ground, openings on both sides of the triangle of zero sequence voltage, the insulation monitor voltage relay posture, signaling. In-depth analysis the following PT connect a wiring diagram of the V/V: left is appropriate wiring, working voltage equilibrium; Below is not correct connection, working voltage imbalance.
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