RU2015595C1 - Device for differential current protection of bus-bars - Google Patents

Abstract

FIELD: electric technique, relay protection of electric system elements. SUBSTANCE: device has intermediate current transformers equalizing currents of ports; protection current transformers operable with an error which can cause intolerable unbalance at external short-circuiting in comparison with an operate setting; auxiliary relay for locking differential protection operation. The device provides an enhanced sensitivity to inner damage currents due to locking at external short-circuiting heavy currents with the aid of the locking relay. The device can be used at any number of current transformers with different transformation rations. EFFECT: enhanced sensitivity, widened application range. 4 dwg

Description

 The invention relates to electrical engineering, and in particular to means of relay protection of elements of electrical systems, and is intended for use in power plants and substations to disable electrical damage to tires, especially with a voltage of 6-10 kV.

 A known scheme for differential protection of buses [1], containing current transformers (CT) in the phases of all bus connections, the secondary windings of which are connected in parallel and loaded with a current relay, included in the sum of the currents of the CT connections. The executive contact of the current relay commutes an intermediate relay, through the contacts of which the voltage is supplied to the trip coils of the circuit breakers of the supply connections.

 This scheme has inherent disadvantages: the need to install separate current transformers in the circuit of connections to shielded buses for differential protection devices; the impossibility of such a scheme in electrical installations equipped with current transformers with different transformation ratios; the impossibility of using existing current transformers if their error with external short circuits (CI) exceeds 10%.

 The known device for the differential protection of electrical installations [2] contains current transformers, to the secondary windings of which are connected the primary windings of intermediate current transformers (PTT) with two secondary windings, a differential-current organ included in the circuit formed by one system of secondary windings of the intermediate current transformer, and a semiconductor switch, the output of which is connected parallel to the differential-current organ, and is equipped with blocks for indirect control of the magnetic state of the main trans current shaper whose inputs are connected to the other secondary windings of the current transformers intermediate outputs connected to inputs of OR gate connected to the semiconductor switches.

 This device has inherent disadvantages: the need to install separate current transformers in the connection circuit to the shielded buses for the differential protection device; the inability to perform without changes in the primary circuits in existing electrical installations having a limited number of current transformers used for other existing protections; the complexity of the circuitry solution due to the use of saturation control units not manufactured by the industry, based on the use of differentiating circuits that are sensitive to switching noise, in which, in addition, non-selective operation of the device is possible: the use of units that do not have an industrial output in the circuit limits the scope of the circuit ; high voltage levels of the secondary PTT circuits, at which the input circuit of the reacting organ is effectively shunted by a semiconductor key (simulator), which reduces the insulation service life due to accelerated electrical aging, or requires the insulation of secondary PTT circuits of an increased class.

 The aim of the invention is to expand the field of application of increasing reliability and sensitivity.

 In FIG. 1 shows a block diagram of the main and current circuits of differential protection of tires; in FIG. 2 is a diagram of its operational circuits; in FIG. 3 and 4 are circuit diagrams of current differential protection current circuits for two-relay (or three-relay) and single-relay versions, respectively.

 The device for differential current protection contains included in the primary circuit of the supply connections 1-3 and outgoing from the protected bus connections 4-8 current transformers 9-16 relay protection, the secondary windings of which are connected to the primary windings of the intermediate current transformers 17-24. The differential winding relay 25 is connected to the secondary windings of all the PTTs, and the interlocking relay 26 is connected in series with the secondary windings of the PTTs of those connections, whose CTs do not pass with a 10% error by 10 percent error. The NO contact 27 of the differential relay is connected in series with the NC 28 relay 26 and an additional unit 29, including indicator and intermediate relays 30-34, winding 35. The closing contacts 36-38 of the intermediate relays are included in the circuit of the trip coils of switches 39-41 of the supply circuit Figure 1-3.

 The drawing shows the windings of the relay 42-49, current protection of the outgoing and supply connections, relay 50 monitoring the secondary circuits of CT.

 A device for differential protection of tires works as follows.

 In the operating mode, the currents of any of the supply connections 1-3, as well as the supplied connections 4-8 are converted by current transformers 9-16 connections, after which they pass through the primary windings of the intermediate current transformers (PTT) 17-24, as well as the windings of the current relays 42- 49 devices of relay protection of connections.

 The secondary currents of the CTs of all the connections are converted by the PTT so that the coefficients of transformation of the primary currents into the secondary circuits of the PTTs are the same for all the connections. The instantaneous values of the PTT currents are summed in their load branch - differential current electromechanical (type RT-40) or semiconductor (for example, based on optocouplers) or another relay.

 Since in the normal mode of operation of the buses, the algebraic sum of the instantaneous values of the currents of the supply and supply connections (conditionally positive directions of the currents to the buses) is equal to zero, the current of the differential relay is practically zero (due to transformation errors, there is an unbalance current lower than the operation threshold) . Contact 27 of this relay is open, the coils of the intermediate and indicator relays 30-35 are de-energized, contacts 36-38 of the relay 31-35 are also open, as a result of which the coils 39-41 of the disconnection of the power supply circuit breakers are de-energized.

 Connections to buses whose CTs with an external short circuit operate with an error exceeding 10% (for example, during reconstructions of primary network circuits, leading to an increase in short-circuit currents and preservation of previously installed CTs), or operate with an allowable error, however, requiring a large the settings are loaded on the secondary side of the PTT, except for the differential, blocking relay 26.

, где К_н - коэффициент надежности;
n_ТТ - коэффициент трансформации защитного ТТ;
n_ПТТ - коэффициент трансформации ПТТ;
К₁₀ - кратность первичного тока по отношению к номинальному значению при насыщении ТТ присоединения;
I_{1ТТ, ном} - номинальное значение первичного тока ТТ присоединения с дополнительным реле.As such connections in FIG. 1, 3, 4, connections 6, 7 are shown. The response current of the additional blocking relay 32 is selected from the condition:
I _av.dop ≅

where K _n - reliability coefficient;
n _TT is the transformation coefficient of the protective TT;
n _PTT - coefficient of transformation of PTT;
To ₁₀ - the magnitude of the primary current with respect to the nominal value at saturation of the CT connection;
I _{1ТТ, nom} - nominal value of the primary current of the CT connection with an additional relay.

 The above condition ensures that the blocking relay 26 is activated only when the current transformers of the connections are saturated, when the transformation error exceeds 10%, and the unbalance current is close to the differential relay trip setpoint, and, accordingly, the differential bus protection trips when external short-circuit occurs, causing the indicated CT error.

 The processes in the differential protection circuit for external short circuits differ depending on the short-circuit current level.

 Cases of external short circuits at currents that do not cause deep saturation of the protective current transformers, operating at the same time with an error not exceeding 10%, are accompanied by a fairly accurate short-circuit current transformation by both supply transformers and outgoing transformers. The current of the differential relay 1 in this case is less than the current of its operation and, therefore, does not cause bus tripping. A short circuit is disabled by the outgoing bus protection.

 In the event of external short circuits accompanied by currents causing an error in the protective current transformers, greater than 10%, the sum of the secondary currents of the PTT exceeds the response current of the differential relay, and the current of the arm working with the saturation of the CT exceeds the response current of the additional blocking relay. Since the operation of the auxiliary contacts of the auxiliary relay occurs faster than the operation of the make contact of the differential relay, the winding circuit of the index and intermediate relays 30-35 remains de-energized. Power connections remain on. Short circuit disconnection is carried out by the protections of the corresponding connections extending from the busbars.

When short circuits in the differential protection zone action tires _differential current I exceeds the triggering level relays 26 and due to lack of short-circuit current through the outgoing connections CT and CT saturation impossibility outgoing connections blocking relay is not activated and keeps the contact 28 in a closed state. The operation of the differential relay causes the closure of the contact 27 of the power circuit of the index 30, 32, 34 and intermediate 31, 33, 35 relays. The latter, through their contacts 36-38, supply voltage to the trip coils 39-41 of the circuit breakers of the supply connections. Damage is disconnected from power sources.

 The positive effect is to expand the field of application of differential busbar protection by using at substations with any short-circuit currents, both direct and alternating operational current, with current transformers having different transformation ratios.

 Such differential busbar protection can selectively work in substations with short-circuit currents greater than currents at which the current transformers of the outgoing connections operate with an error of 10%. In addition, the tire differential protection circuit does not require additional installation of individual transformers for its implementation.

Claims (1)

DEVICE FOR DIFFERENTIAL CURRENT PROTECTION OF TIRES, containing current transformers in the supply and outgoing from the busbars, to the secondary windings of which are connected the primary windings of the intermediate current transformers, to the secondary windings of which are connected a differential current relay with a make contact, a blocking relay, the outputs of which are designed for connecting to the circuit breakers of the connection switches, an operating current source, characterized in that, in order to expand the field I, reliability and sensitivity, it is further introduced a break contact of the locking relay, which is connected in series with the closing contact of a differential relay and the execution unit, the chain pins are connected to the terminals of a source of DC voltage, and outgoing connections for which the current transformer
K ₁₀ ˙I _1t.nom <I _{short-circuit max} ,
where K ₁₀ - the ratio of the primary current of the current transformer, at which the error is 10%;
I _1t.tnom - nominal value of the primary current of current transformers;
I _{KZ max} - the maximum value of the external current for the short circuit bus,
in series with the secondary windings of the intermediate current transformers connected in parallel with each other, a winding of the blocking relay is connected, the tripping current of which is selected from the condition
I _av.dop ≅

,
where K _n - reliability coefficient;
n _tt , n _ptt - transformation coefficients of the main and intermediate current transformers of the connection current, respectively.

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