RU2105987C1 - Serviceability check technique for current transformer secondary circuits - Google Patents

Abstract

FIELD: electrical engineering. SUBSTANCE: serviceability of current transformer secondary circuits is recognized by equal angles between load current vector and voltage drop in secondary circuits of current transformer as well as between angles of respective phases pre-measured upon adjustment or in-service check by load current; open circuit is recognized by independent increase or decrease in angle. EFFECT: provision for on-load serviceability check of current transformer secondary circuits. 2 cl, 1 tbl

Description

 The invention relates to electrical engineering and can be used to test the health of the secondary circuits of current transformers.

Closest to the proposed method is a method of supplying current from an external source [1]
The disadvantage of this method is the need to decommission the electrical connection to check the health of the secondary current circuits. This requires organizational and technical measures to prepare the workplace for checking the disconnection of consumers at the time of work. In addition, when applying a direct current (measurement by a MMV bridge), the windings of current coils are not detected (with a small number of closed turns), and when applying alternating current from a constant source, it is not possible to identify unsatisfactory contacts of current circuits, since the current does not changes its value.

 The purpose of the invention is to conduct a functional check of the secondary circuits of current transformers without disconnecting the electrical connection.

 This goal is achieved in that the operability of the secondary circuits of current transformers is determined by the equality of the angles between the load current vectors and the voltage drop vector supplied to the phase-sensitive device [2] and the angles of the corresponding phases, previously measured after commissioning or operational testing by the load current, and violation of current circuits, for example, an increase in the contact resistance of the contacts, a short circuit in current coils, a short between cable conductors, a repeated short to ground, causes Depending on the type of damage, a decrease in the angle by a different amount.

 In FIG. 1 is a vector diagram of load currents, voltage drops in the secondary circuits of current transformers connected according to a full star circuit with a load in the form of a current relay 1РТ, 2РТ, 3РТ type RT-40/10, the current windings of which are connected in series.

The phases of the vectors I _A , I _B , I _C were recorded using clamp meters with respect to voltages U _AB , U _BC , U _CA , respectively.

Phases of the voltage drop vectors U $\genfrac{}{}{0ex}{}{\text{P}}{\text{AO}}$ , U $\genfrac{}{}{0ex}{}{\text{P}}{\text{IN}}$ , U $\genfrac{}{}{0ex}{}{\text{P}}{\text{With}}$ were removed using two conductors connecting the jacks for connecting the clamp meter to the device with the terminals of the panel of current circuits, respectively I _AO , I _BO , I _CO with respect to voltage U _AB , U _BC , U _CA.

At constant values of the active and inductive resistance of the secondary current circuits, the measured angles will also be constant. With an increase in transition resistance in contact joints (active resistance), the vector U $\genfrac{}{}{0ex}{}{\text{G}}{\text{AO}}$ , (U $\genfrac{}{}{0ex}{}{\text{G}}{\text{IN}}$ , U $\genfrac{}{}{0ex}{}{\text{G}}{\text{With}}$ ) will approach the vector I _A , I _B , I _C.

So, for example, the originally measured angle between the voltage U _AB and U $\genfrac{}{}{0ex}{}{\text{G}}{\text{AO1}}$ amounted to 50 ^o , while the angles for U $\genfrac{}{}{0ex}{}{\text{P}}{\text{IN}}$ and U $\genfrac{}{}{0ex}{}{\text{P}}{\text{With}}$ respectively 43 and 45 ^o . Considering that the phase "A" current relay in this case is installed closer to the terminal strip of the panel than the "B" and "C" relay, it could be concluded that the angle of incidence of the voltage of phase "A" (50 ^o ) is overestimated value due to poor contact in the current circuit, which was detected in the relay housing 1РТ. After eliminating this defect, the angle between the stresses U _AB and U $\genfrac{}{}{0ex}{}{\text{G}}{\text{AO}}$ amounted to 40 ^o .

Figure 2 shows the test circuit of the secondary circuits of the current transformer type TNL-10, H _TT 1500/5 with the current relay RT-40/10, RT = 40/20, included in the secondary winding of the cable (l = 25 m, Al) RT-85/1 ammeter "A" and terminal block of 9 series-connected terminals.

 The test results are listed in the phase angle measurement table.

The current in the primary circuit is 600 A, in the secondary 2A, φ _TK is the angle between voltage and current when using clamp meters. φ _{P is the} angle between voltage and voltage drop in the secondary circuit.

Δφ = φ _TC -φ _P
During testing at all stages, the current did not change its value (2A). The test results show that the angle between the secondary load current and the voltage drop changes with any changes in the secondary circuits of current transformers.

The tests carried out on a number of current devices made it possible to determine the phase of the voltage drop on their current coils. So, for example, the angle between the voltage drop and the passing current for a 3 phase disk electric meter φ 30 ^o relay PT-40/10 (serial connection of coils) 20 ^o relay PTV (5A) 60 ^o . As a result, it seems possible to identify a damaged element of the current circuits. When checking all current coils of active and reactive email. the high-voltage connection meter on one of the coils, the angle v was equal to 5 ^o .

After removing the counter cover, a loose contact was detected at the connection point of the current coil to the terminal. With the gradual twisting of the screw, the angle v also gradually approached 30 ^o . In this case, the current value did not change its value.

 In the process of operation, the contacts of aluminum cable conductors are of particular danger, the violation of which is associated with a temporary change in its (core) shape (metal fluidity). A complete violation of the current circuit causes a "fire of steel" of the current transformer followed by an electric accident. installation.

The magnitude of the EMF E ₂ can reach several kilovolts, which is a danger to the isolation of the current transformer and its secondary circuits, as well as to maintenance personnel. It should also be noted that the error of current transformers is affected not only by the magnitude, but also by cosφ _{2 of the} secondary load. If cosφ ₂ differs from 0.8 (with this value, permissible errors for accuracy classes are normalized).

With increasing cosφ _{2, the} angular error increases [3] Therefore, when checking errors, they can be controlled to some extent by varying cosφ _{2 by} adjusting the resistance of the secondary circuit, for example, by including a different number of cable cores in it, which will increase the accuracy of electronic metering. energies and actions of relay protection devices.

 The proposed method can be used at power plants and networks of all voltages.

 The technical and economic efficiency of the method is ensured by eliminating the need for disconnection and output to repair connections.

Claims (1)

 A method for checking the health of secondary circuits of current transformers, characterized in that they determine the angle between the current vector and the voltage drop vector in the secondary circuits of current transformers, and if this angle is equal to the angle of the corresponding phase previously determined after commissioning or operational testing by the load current, the health of the secondary circuits is established current transformers.

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