RU2230415C1 - Device for checking neutral conductor of overhead lines for circuit contin uity - Google Patents

Abstract

FIELD: measurement technology; load protection against surges caused by open circuit in neutral wire. SUBSTANCE: proposed device designed for use on 0.4-kV overhead lines depends for its operation on using proportion between currents through neutral conductor and through grounded neutral of power transformer which is function of physical parameters of line and quantity of repetitive ground conductors on line. Device is characterized in that it has current sensors connected through computer unit, symmetry check-up and decision taking units to actuating mechanism. Device makes it possible to detect open circuit in neutral wire at any point of line without bringing any variations in line conditions and without impairing power characteristics. EFFECT: enhanced electric, fire, and explosion safety. 1 cl, 2 dwg

Description

The invention relates to measuring equipment, in particular to devices for monitoring the continuity of the neutral conductor, and can be used to protect consumers of electrical energy from overvoltages caused by a broken neutral wire and to ensure electrical, fire and explosion safety.

Ensuring the continuity of the neutral conductor requires constant monitoring of its condition and parameters and, in case of an accident, disconnecting the supply transformer. Currently, with respect to 0.4 kV networks, this is achieved by determining the phase-zero loop resistance. In addition, there are devices that use superimposed on the current network operating current and designed to control the grounding of a particular electrical installation. It should be noted that the measurement of the resistance of the phase-zero loop does not allow to determine exactly what caused the increase in the total resistance of the phase-zero loop: a zero break or an unacceptable increase in the resistance of phase wires. Devices that operate on operational current, allow you to control the continuity of only one part of the grounding circuit, and in addition, degrade the quality of electricity.

A device for automatically monitoring the health of the grounding circuit is known. It contains a separating transformer T2, from the secondary winding of which a three-phase rectifier VS is powered. The stabilization circuit includes resistors Rc connected in series with the rectifier diodes and the Zener diode V, which is connected to the common points of the diodes and neutrals of the secondary winding of the transformer T connected to the star [1, p. 58, Fig. 5.21].

The connection of the circuit is provided by connecting the primary winding of the isolating transformer T to the phase conductors of the monitored network and the common point of the secondary winding to the zeroed housing of the power receiver.

Zener diode V is a source of stabilized operating voltage U _op . Operational direct current in the circuit passes through the circuit: plus a power source - relay K - button S1 - power receiver body - grounding conductor - neutral power transformer T1 - phase wires of the network - primary winding of transformer T2 - minus the power source.

With proper Vanishing (nulling circuit resistance R _coupled ≈ 0), the current in the switch to the highest, and contacts anchor attracted relay. With an unacceptable increase in the resistance of the grounding circuit, relay K activates and switches contacts K1 and K2 in the signaling and protection circuits.

The disadvantages of this device are:

- the limited control zone, namely only the monitoring of the serviceability of attaching the installation grounding to the grounding line, because when the grounding line breaks behind the electrical installation, the device will not work;

- deterioration in the quality of electricity in this network due to the imposition of a constant operating current.

A device for monitoring the continuity of the grounding circuit of domestic installations is also known, which simultaneously measures the resistance of the neutral protective conductor and controls the continuity of the neutral grounding circuit of the supply transformer. The device contains the following elements [2, p. 60, Fig. 1]:

r ₀ - neutral grounding resistance; r _p is the resistance of the shunt; r _{w -} repeated grounding; IB - measuring unit; IOT - source of operational current, NZP - zero protective conductor; BK - neutral ground circuit control unit.

The device operates as follows. According chain OTH - r _w - WIP - r _n - OTH constant operating current flows, which is the voltage drop across r _w is measured and compared with the setpoint block IB. With the integrity of the zero protective conductor (NZP), the voltage drop by r _w exceeds the setting and the measuring unit signals the integrity of the NZP.

In the event of an open circuit breakdown, the voltage drop by r _w decreases almost to zero. The IB block gives a signal to turn off the supply transformer and at the same time gives an alarm.

The disadvantages of this device are:

- an increase in the resistance of the grounding line due to the inclusion of a shunt r _w in the cut-off of the wiring;

- deterioration in the quality of electricity in this network due to the imposition of a constant operating current.

- deterioration in the normal operation of the neutral grounding of the supply transformer due to the presence of a VD diode.

The basis of the invention is the technical task of providing automatic control of the continuity of the neutral conductor at any point in the network without compromising the quality of electricity.

This problem is solved in that in the device for monitoring the continuity of the neutral conductor in 0.4 kV overhead lines, containing measuring current sensors and an actuator, according to the invention, a computing unit, a symmetry checking unit and a decision making unit are introduced, while one of the measuring sensors is connected by its the input source of the input signal, which is the current in the neutral wire at the beginning of the line, and its output is connected to one of the inputs of the symmetry check unit and the input of the computing unit, the output of which It is connected to one of the inputs of the decision block, the other measuring sensor is connected by its input to the input signal source, which is the current in the neutral ground of the transformer, and its output is connected to the second input of the symmetry check block and one of the inputs of the decision block, the output of the symmetry check block connected to the third input of the decision block, the output of which is connected to the input of the actuator.

A feature of the claimed device is that its principle of operation is based on the measurement and use of the ratio between the currents in the neutral wire and in the neutral ground of the transformer. In the normal mode of operation of the network between the current in the neutral wire at the beginning of the line and the current in the neutral ground of the transformer there is a certain ratio, depending on the physical parameters of the network, which can be calculated for each specific network. Therefore, knowing the current in the neutral wire at the beginning of the line, it is possible to calculate the current in the neutral ground of the transformer, and the calculated value of this current must coincide with the measured value. When the neutral conductor breaks, the ratio between currents changes sharply. If the current value in the neutral ground of the transformer, calculated in the computing unit, differs from the measured value, the decision unit generates a trip signal supplied to the actuator. The symmetry check unit prevents false triggering of the device in case of a symmetrical load of consumers, when it is possible to violate the ratio between the currents in the neutral conductor at the beginning of the line and in the neutral ground of the transformer.

The essence of the proposed device is illustrated by drawings, where in FIG. 1 is a structural diagram of a device for monitoring the continuity of the zero conductor in 0.4 kV overhead lines, and FIG. 2 is a circuit diagram of the network section and the organization of measurements.

The device contains measuring current sensors 1 and 2, a symmetry check unit 3, which prevents false alarms of the device with a symmetrical load at the consumer, a computing unit 4, in which the value of the current in the neutral wire at the beginning of line I ₀₁ calculates the current value in the neutral ground of the supply transformer I _z.vych, which is then compared with the measured value of the same current I _s, the decision block 5, which on the basis of this comparison with a substantial divergence, which error value is determined from Eren, calculations and generates the selected reference response signal for interrupting the supply transformer, which is supplied to the actuator 6.

The device operates as follows. In normal operation of the network in the neutral wire at the beginning of the line current I ₀₁ flows; a grounded neutral transformer current I _s flows (FIG. 2). Using the current value I ₁ measured using the sensor 1, block 4 calculates the calculated current value I _З , which in block 5 compares with the measured value obtained using the sensor 2. If the calculated and measured values of I _З do not coincide, block 5 generates a signal to turn off the power transformer, which is fed to the actuator 6.

Below is a theoretical justification that there is a very definite relationship between the current in the neutral wire at the beginning of the line and the current in the neutral ground of the transformer. A 0.4 kV overhead line with a load concentrated at the end of the line is considered (Fig. 2). The following notation is used in the diagram: r ₀ — neutral grounding resistance; R _P - resistance to re-grounding the neutral wire; R is the resistance of the neutral wire section between the repeated ground electrodes; Z _N - load resistance.

Node voltage 0 to ground

Knowing the current I ₀₁ in the neutral wire at the beginning of the line, we determine the voltage in node 1 relative to the ground:

After which we determine the current in R _n , a re-earthing switch (PZ) connected to node 1:

The current in the neutral wire I ₀₂ between nodes 1 and 2 can be defined as the sum of the current in the neutral wire at the beginning of line I ₀₁ and the current I _З1 in the PZ connected to node 1:

Similarly, knowing the voltage U ₁ in node 1 and the current I ₀₂ between nodes 1 and 2, we find the voltage in node 2 relative to the ground:

U ₂ = U ₁ + I ₀₂ · R.

Current in the PZ connected in node 2:

The current in the neutral wire I ₀₃ between nodes 2 and 3:

I ₀₃ = I ₀₂ + I _z2 .

For the nth node of the network containing the nth number of PPs, you can write:

U _n = U _n-1 + I _0n · R,

Because the

I _s = I _P1 + I _s2 + ... + I _zn,

it follows that

I _s = kI ₀₁ .

Therefore, the current in the neutral grounding of the supply transformer and the current in the neutral wire at the beginning of the line are connected by a proportionality coefficient k, which depends on the physical parameters of the network (in particular, the neutral grounding resistance, the resistance of the neutral wire sections between the PP and the resistance of the PP itself), as well as the number of PZ on the line.

One embodiment of the device is described below. As current sensors 1,2 it is possible to use measuring current transformers with an appropriate measurement limit; as a computing unit 4, it is possible to use an operational amplifier with a fixed gain; as a block of checking symmetry 3, it is possible to use a logical element 2 AND-NOT, a high-level signal at the output of which appears in the absence of signals at its inputs (load symmetry). As a decision block 5, it is possible to use a comparator, which is triggered when the measured current I ₃ exceeds the calculated value. As the actuator 6, it is possible to use a magnetic starter of the corresponding magnitude.

Industrial applicability

The invention can be used to organize the control of the continuity of the zero conductor of overhead power transmission lines of 0.4 kV with a grounded neutral. The practice of operating such overhead lines has shown that the main causes of a zero wire break are:

- prolonged single-phase short circuit at the end of a long line, leading to the burning of the neutral wire in the places of contact connections;

- gradual electrical discharge erosion of the neutral wire during lashing with the phase in the places of the greatest sag.

In case of load asymmetry, a zero wire break leads to the appearance of significant overvoltages in consumers in the least loaded phases, which will lead to mass failure of consumer equipment and the occurrence of electrical, fire and explosive situations. The use of the invented device will avoid the negative consequences of a zero wire break by timely disconnecting the supply transformer from the line.

Sources of information

1. Life safety: lecture notes / Sidorov A.I., Bukhtoyarov V.F., Leukhina L.I. et al. - Chelyabinsk: ChSTU, 1997. - 4.VI. - 239 p.

2. Pyastolov A.A., Sidorov A.I., Kataeva N.K. Monitoring the continuity of the grounding circuit of municipal installations // Technique in agriculture. - 1988. - No. 5. - S.60-61.

Claims (1)

A device for monitoring the continuity of the zero conductor in 0.4 kV overhead lines, containing measuring current sensors and an actuator, characterized in that a computing unit, a symmetry checking unit, and a decision making unit are introduced, while one of the measuring sensors is connected to the source by its input the input signal, which is the current in the neutral wire at the beginning of the line, and its output is connected to one of the inputs of the symmetry check unit and the input of the computing unit, the output of which is connected to one of the inputs making decision, another measuring sensor is connected by its input to the input signal source, which is the current in the neutral ground of the transformer, and its output is connected to the second input of the symmetry check unit and one of the inputs of the decision unit, the output of the symmetry check unit is connected to the third input of the block making decisions, the output of which is connected to the input of the actuator.