RU2115206C1 - Current-limiting device - Google Patents

Abstract

FIELD: electrical engineering; alarm control systems. SUBSTANCE: device has resistor installation connected to transformer neutral, resistor installation monitoring unit connected in series with resistor installation built up of first and second stages. First stage functions to close ground switch which shorts out resistor installation and monitoring unit under phase-failure conditions; second stage functions to open transformer circuit breaker in response to short circuit in supply mains. EFFECT: improved design. 3 dwg

Description

 The invention relates to electrical engineering, namely to a circuit for protection against accidents, limiting overcurrents.

 It is known that in order to limit K3 currents to earth, which make up the vast majority of the total number of K3 in high-voltage networks, it is necessary to increase the equivalent channel resistance of the zero sequence circuit. The simplest way to limit the K3 currents to earth is to ground all or part of the transformer neutrals [1]. However, the operation of networks with isolated neutral requires isolation of the equipment by an order of magnitude higher than in networks with a grounded neutral [2]. Such networks are characterized by increased switching overvoltages and dangerous ferroresonant processes. The grounding regime of a part of the neutrals also has a number of drawbacks, the main of which is the likelihood of ungrounded sections of the network with unresolved K3 to the ground and the occurrence of dangerous switching and ferroresonant overvoltages.

 The development of energy systems, the increase in the number and capacity of transformers with dead-grounded neutral leads to a significant increase in K3 currents to earth, which necessitates the modernization or replacement of equipment with more powerful ones. At present, the most feasible in technical and economic terms and operational way to increase the reliability of equipment in emergency conditions is the use of grounding devices of transformer neutrals, providing inertia-free damping of transient electromagnetic and electromechanical processes in power systems when K3 occurs and is eliminated.

 Known AC power transmission with receiving and transmitting substations, the transformer neutrals of which are grounded through a current-limiting device containing a parallel-connected highly non-linear resistor, reactor and switching device [3]. This device cannot be implemented in practice, since powerful high-linear non-linear high-voltage resistors capable of limiting K3 currents to the ground to tens or even units of kA do not exist in the domestic and foreign electrical industry. In addition, relay protection and power transmission automation should be special, with tuning to the non-linear nature of transients during the occurrence and elimination of K3.

 A device for grounding the neutrals of transformers containing parallel connected arrester, oil current-limiting reactor TROM-35, high-resistance betel resistor installation [4]. This device is not widely used in power systems due to unacceptable cost, overall and operational performance.

 A device for grounding the neutrals of transformers through betel resistors [5]. However, it did not find industrial application in power systems due to the lack of methods for choosing the value of current-limiting resistance and calculating the resource and throughput of a resistor installation, taking into account reliability indicators that ensure its efficiency and performance.

 The closest solution to the claimed device is A.S. N 1700624 to protect the transformer connected to the transmission line through the separator by limiting the K3 current in the short circuit with betel resistors assembled into an installation of m-parallel branches with k-series resistors in each branch, and connected in series with a short circuit between high voltage terminals transformer and ground [6].

 The disadvantage of this device is that it is used only to limit the artificially switched short-circuit currents of single-phase K3 at substations without switches on the high voltage side.

 The objective of the invention is to create a reliable and efficient device to limit both artificial and random asymmetric K3 to the ground.

 The problem is solved in a device containing a short circuit and betel resistors assembled into an installation of m-parallel branches with k-series resistors in each branch, due to the fact that a control block for the operation of the betel resistor installation (BRU) is introduced into it, while the BRU one terminal is connected through a hardware clamp to the neutral terminal of the transformer, the other terminal is connected to the BRU through the control unit of the BRU containing a measuring current transformer, in the secondary circuit of which о includes the first stage of current protection with a control milliammeter and a disconnecting button shunting it, and the second stage of current protection, while the output of the first stage is connected to the short circuit circuit of the short circuit switch, shunting the series-connected BRU and the control unit, and the output of the second stage is connected to the transformer circuit breaker switching circuits or an autotransformer whose neutrals are grounded through the device.

 In FIG. 1 is a schematic diagram of a current limiting device; in FIG. 2 shows a diagram of an electrical system in which a current limiting device is used; in FIG. 3 - diagram of the operational circuits of the control unit of the BRU.

 The device according to FIG. 1 contains a BRU - 1, a unit for monitoring the operation of the installation 2. The BRU is connected to the neutral 4 of a transformer (T) or an autotransformer (AT) via one terminal through a hardware clamp 3. Another terminal of the switchgear is earthed via control unit 2. Between the neutral terminal and the ground, a short circuit 5 is connected in parallel with normally open contacts 5.

BRU -1 is assembled according to the method [6] from parallel-series-connected N = mxk betel resistors for given values: BRU active resistance - R _N , through-current through the installation - I _R , K3-t duration and the number of trips for the BRU service life - n

 The control unit - 2 contains a measuring current transformer 6, in the secondary circuit of which the first 7 and second 8 current protection stages of the switchgear are connected in series. The first stage 7 contains a sensitive current relay 9 and a milliammeter 10 connected in series with the normally closed shunt with button 11. The output signal of stage 7 acts to close the short circuit 5. The second stage 8 contains current relay 12. The output signal of stage 8 acts to turn off the 13 T switch and AT.

 The device 14 arranged in this way is switched on in neutral 4 of the T-15 of the powerful station 16 and (or) AT-17 of the node substation with switchgears of different voltage classes 18 and 19 (Fig. 2).

где
3I_ORH - устроенный ток нулевой последовательности, протекающий через БРУ, обусловленный несимметрией в системе; K_ОТС = 1,25 - коэффициент отстройки, учитывающий погрешность реле и необходимый запас; K_B - коэффициент возврата реле; K_TT - коэффициент трансформации трансформатора тока.Constantly switched in neutrals BRUs can be subjected to prolonged exposure to asymmetry currents in the normal mode of operation of the system. Experimental measurements of the zero sequence currents in the grounded neutrals of transformers with a power of 40 ... 400 MVA give average currents of 3. ..5 A. The maximum outliers for high-power T and AT reach 30-40 A, which is usually associated with the occurrence out of phase conditions and should be eliminated by appropriate automation. Betel resistors completing the switchgear switchgear are designed for continuous currents of 5-10 A. Thus, in normal mode, when protection of equipment from overcurrents is not needed, the switchgear can be temporarily (until the asymmetry is eliminated). For this purpose, in the control unit 2, a first stage 7 is provided, containing the first sensitive current relay 9, the operation setting of which is selected by the condition:

Where
3I _ORH - arranged zero-sequence current flowing through the switchgear due to asymmetry in the system; K _OTC = 1.25 - the offset coefficient, taking into account the error of the relay and the necessary margin; K _B - relay return coefficient; K _TT is the transformation ratio of the current transformer.

где
I_нд = 5. ..10 - длительно допустимый ток по ТУ на комплектующие резисторы;
m - количество параллельных колонок в БРУ.The sensitivity coefficient of the relay is defined as:

Where
I _nd = 5. ..10 - long-term permissible current according to the technical specifications for component resistors;
m is the number of parallel columns in the BRU.

If the sensitivity is insufficient, then the response current of the first stage should be selected by agreement with I _nd .

 The device 14 operates as follows.

 In the normal mode of operation of the system (without K3), if the current flowing through the switchgear 1 and the measuring current transformer 6 of control unit 2 exceeds the permissible limit, relay 9 activates, which closes the power circuit of time relay 21 with its contact 20 (Fig. 3). Time the relay 21 is detuned from the duration of a possible out-of-phase mode, for example, OAPV. Relay 21 with its first contact 22 closes the power circuit of the indicator relay 23, which, with its contact 24, turns on the signal on the control panel of the station: "Overload BRU". The second contact 25 supplies power to the inclusion circuit of the short-circuit switch 5, shunting BRU-1 at the time of elimination of the causes of the non-phase mode. The first stage also allows operational personnel to perform current visual monitoring of currents through the BRU using a calibrated milliammeter 10 by pressing button 11, and using button 26 to turn on, if necessary, a shunt short circuit 5.

The design mode for the resource and bandwidth of the switchgear is the emergency mode K3 in the electrical system with short-term (K3 shutdown time) exposure to the maximum possible overcurrents. For reliable operation during the service life of the switchgear, it is calculated on a given number of trips n with the maximum possible current I _R flowing through the unit at K3 at terminal T or AT or near busbars. With asymmetric K3 (Fig. 2) on lines 27, switchgear buses 16, 18, 19 or terminals T 28, or AT 29, 30, the zero sequence current occurring in neutrals 4 is limited by the active resistance of the switchgear. The operation threshold of relay 12 of the second stage 8 is determined by the zero-sequence current flowing through the BRU-1 and measuring transformer 6 in K3 mode with a maximum current of 3I _OR ≤I _R. If K3 does not turn off during the estimated time t, then when the relay 12 is activated, its contact 31 closes the power circuit of the time relay 32, which, with the time delay determined by the technical specifications on the switchgear, contacts 33 through the winding of the specified relay 34 and supplies power to the circuit breakers 13 station 16 or substation 18. Relay 34 contact 35 includes the alarm: "Unresolved K3. Transformer shutdown."

должна выбираться по требуемой эффективности токоограничения

при обеспечении допустимого повышения напряжения на изоляции нейтрали Т или АТ

С учетом указанных требований оптимальная величина сопротивления БРУ в нейтрали Т-15 станции 16 выбирается по условию:

эквивалентное сопротивление прямой последовательности схемы относительно местоположения К3; X_OC - эквивалентное сопротивление нулевой последовательности схемы без учета сопротивления трансформаторов X_т с R_N в их нейтралях.Resistance value

should be selected according to the required current limiting efficiency

while ensuring a permissible increase in voltage on the insulation of the neutral T or AT

Given these requirements, the optimal value of the BRU resistance in the neutral T-15 of station 16 is selected by the condition:

equivalent resistance of the direct sequence of the circuit relative to the location of K3; X _OC - equivalent resistance of the zero sequence of the circuit without taking into account the resistance of the transformers X _t with R _N in their neutrals.

эквивалентное сопротивление схемы по каналу прямой последовательности относительно местоположения К3; X_BC - сопротивление АТ, определенное по паспортному значению напряжения К3 между обмотками высшего и среднего напряжения; X₁₈ и X₁₉ - входные сопротивления прямой последовательности сетей соответственно среднего и высшего напряжения относительно шин 18 и 19; X $\genfrac{}{}{0ex}{}{\text{*}}{\text{T1}}$ = X_T1+ α•X₁₉₀, α = 1,0...0,8; а = 1,0...0,8;
X_T1 = K² • X_B + (K-1) • K • X_H, X_T2 = X_C - (K-1) • X_H, X_T3 = K • X_H,
сопротивления эквивалентной звезды АТ, определяемые по паспортным параметрам: X_B, X_C, X_H;

Использование токоограничивающего устройства согласно заявляемому изобретению позволяет отказаться от многоаппаратных токоограничивающих устройств (параллельно включенные реактор, резисторы, разрядник), что повышает в 5...10 раз его экономичность и облегчает условия эксплуатации. Выбор оптимальной величины сопротивления БРУ обеспечивает эффективное токоограничение без опасных перенапряжений на изоляции оборудования распредустройств, а введение автоматического контроля работы установки повышает надежность работы устройства.For ATs, the maximum voltage increase in the neutral occurs in the single-phase K3 mode at the medium voltage terminal 29 with the circuit breaker on the side of the medium voltage buses 18 open and the circuit breaker on the side of the buses - 19 higher voltage. Then the BRU resistance in the neutral of AT -17 substation -18 is selected by the condition:

equivalent circuit resistance along the direct sequence channel relative to the location of K3; X _BC - resistance AT, determined by the certified value of voltage K3 between the windings of high and medium voltage; X ₁₈ and X ₁₉ - input impedance of a direct sequence of networks of medium and high voltage, respectively, with respect to tires 18 and 19; X $\genfrac{}{}{0ex}{}{\text{*}}{\text{T1}}$ = X _T1 + α • X ₁₉₀ , α = 1.0 ... 0.8; a = 1.0 ... 0.8;
X _T1 = K ² • X _B + (K-1) • K • X _H , X _T2 = X _C - (K-1) • X _H , X _T3 = K • X _H ,
resistance of the equivalent star AT, determined by the passport parameters: X _B , X _C , X _H ;

The use of a current-limiting device according to the claimed invention makes it possible to abandon multi-device current-limiting devices (parallel connected reactor, resistors, arrester), which increases its efficiency by 5 ... 10 times and facilitates operating conditions. The choice of the optimal value of the BRU resistance provides effective current limitation without dangerous overvoltages on the insulation of switchgear equipment, and the introduction of automatic control of the installation increases the reliability of the device.

 Thus, the proposed device, which includes a range of developments from the methodology for choosing the resistance of a current-limiting installation to its assembly with industrially manufactured elements and ensuring its reliability and performance, can find wide practical application in power systems that require limiting K3 current levels that are dangerous for power equipment.

Claims (1)

 A current-limiting device containing a short circuit and betel resistors assembled into an installation of m parallel branches along K series resistors in each branch, characterized in that a control unit for the operation of the betel resistor installation (BRU) is introduced into it, while the BRU has one output through a hardware clamp connected to the neutral terminal of the transformer, the other terminal of the switchgear is grounded through the control unit of the switchgear, containing a measuring current transformer, in the secondary circuit of which and the second stage of current protection, while the output of the first stage is connected to the short-circuit switch on circuits, shunting the connected BRU and the control unit, and the second stage output is connected to the disconnect circuits of the transformer or autotransformer switches, the neutrals of which are grounded through the device.

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