CN106896275A - A kind of cable distributed capacitance Risk-warning control system - Google Patents

Abstract

The present invention relates to a kind of cable distributed capacitance Risk-warning control system, it is used to be monitored cable distributed capacitance, relay misoperation is prevented, the system includes the low frequency signal source being connected with dc bus positive pole and negative pole respectively and voltage balancing module and the small alternating current-direct current detection means being arranged at least one dc bus branch road.Compared with prior art, the present invention has the advantages that high security, high reliability, overall monitor, accurate measurement, easy to use, flexible economic.

Description

A cable distributed capacitance risk early warning control system

technical field

The invention relates to the field of cable distributed capacitance monitoring, in particular to a cable distributed capacitance risk early warning control system.

Background technique

When the DC system is grounded or the AC enters the DC, a loop will be formed through the distributed capacitance of the cable to generate capacitive current, which will cause some relays with low operating value and high sensitivity to malfunction. The main factors of malfunction are the action value (voltage and power) of the relay and the distributed capacitance value of the cable to the ground. "Eighteen Major Power Grid Anti-Accident Measures of the State Grid Corporation" makes corresponding requirements on the operating power and operating voltage of the relay from the design point of view, but when DC grounding or AC series connection occurs, if the distributed capacitance of the control cable exceeds the critical value, the outlet The relay will still malfunction.

Contents of the invention

The purpose of the present invention is to provide a high security, high reliability, comprehensive monitoring, accurate measurement, easy to use, flexible and economical cable distributed capacitance risk early warning control system in order to overcome the above-mentioned defects in the prior art.

The purpose of the present invention can be achieved through the following technical solutions:

A cable distributed capacitance risk early warning control system, which is used to monitor the distributed capacitance of cables and prevent relays from malfunctioning. A tiny AC and DC detection device on the branch.

The micro-AC/DC detection device includes a single-chip microcomputer and a DC detection circuit and an AC detection circuit respectively connected to the single-chip microcomputer. The DC detection circuit is provided with a DC current sensor and a first switch, and the AC detection circuit is provided with a AC current sensor and second switch.

The on-off of the first switch and the second switch are interlocked with each other.

The DC current sensor is a magnetic saturation current sensor, and the AC current sensor is a magnetic flux induction current sensor.

The system also includes a microcomputer insulation detector respectively connected to the positive pole and the negative pole of the DC bus.

The model of the microcomputer insulation tester is ATCWZJ5.

The low-frequency signal source is a portable insulation detector BXJY-A.

A protection device is provided on the small AC and DC detection device on each DC bus branch.

The range of the DC current sensor is ±30mA with an accuracy of 0.01mA, and the range of the AC current sensor is ±15mA with an accuracy of 0.01mA.

Compared with the prior art, the present invention has the following advantages:

1. High security and high reliability

The tiny AC/DC CT is small in size, which improves the space safety margin for the operation in the DC panel cabinet; the voltage equalization module participates in the balance control of the bus voltage, reducing the degree of voltage deviation. The use of high-reliability portable detectors as the source of low-frequency signals ensures the long-term reliable operation of the DC system.

2. Comprehensive monitoring and precise measurement

In addition to the functions of DC insulation monitoring, grounding line selection, and bus overvoltage/undervoltage monitoring, it also has the functions of AC intrusion into DC branch fault monitoring, bus voltage equalization control, and online measurement of distributed capacitance. Tiny AC and DC CTs can withstand high current impact and can measure accurately for a long time.

3. Easy to use, flexible and economical

The main components adopt distributed design, the interface is simple, easy to assemble and install, the system wiring is simple, convenient for maintenance and management; the expansion based on the original equipment is economical, and the operator can use it without retraining.

Description of drawings

Figure 1 is a diagram of a tripping circuit with a long cable.

Figure 2 is the equivalent circuit diagram of the DC system.

Figure 3 is the equivalent diagram of positive grounding.

Figure 4 is the equivalent diagram of the positive power supply grounding of the relay.

Figure 5 is a diagram of the online monitoring distributed capacitance system.

Figure 6 is a block diagram of AC and DC micro current detection.

Fig. 7 is a design diagram of a voltage equalization module, wherein, Fig. (7a) is a front view, Fig. (7b) is a left view, and Fig. (7c) is a top view.

Fig. 8 is a structure and control effect diagram of the voltage equalization module, wherein Fig. (8a) is a circuit diagram of the voltage equalization module, and Fig. (8b) is an output waveform diagram.

Figure 9 is a connection diagram of the DC insulation monitoring system.

Among them, 1. Low frequency signal source, 2. Voltage equalization module, 3. Micro AC and DC detection device, 4. Microcomputer insulation detector, 5. Protection device, 31. Single-chip microcomputer, 32. DC current sensor, 33. AC current sensor.

detailed description

The present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

Example:

One end of the hand jumper, three-jump relay and failure protection start-up optocoupler of the operation box is generally connected to the negative pole of the power supply, and the other end is connected to the control cable, as shown in Figure 1, and its equivalent circuit is shown in Figure 2. In Figure 1: T is the protection action contact; TBJ is the protection action relay; DL is the switch auxiliary contact; TQ is the tripping coil. In Figure 2, E is the equivalent power supply; R1 and R2 are the balance bridge resistances of the positive and negative poles respectively; R+ and R- are the insulation resistances of the positive and negative poles respectively; C ₊ and C- are the distributed capacitances of the positive and negative poles respectively; R is the resistance of the relay; C is the distributed capacitance of the positive pole of the relay to the ground. The long cable leading to the switch terminal box has a large distributed capacitance between its core wire and the shielding layer. When a point is grounded in the DC system, it may cause misselection of the portable grounding resistance tester based on the low-frequency signal injection method. It may even cause relays or optocouplers with small operating power to malfunction.

Analysis of Critical Value of Distributed Capacitance at One Point Grounding

(1) positive ground

The DC positive pole is grounded (point P in Figure 2 is grounded), and its equivalent circuit is shown in Figure 3. From the moment of grounding, the DC system charges the distributed capacitance of the cable through the relay.

According to the KVL theorem, the following relationship can be obtained:

Figure 3 shows the zero-state response under a simple DC excitation, and the joint formula (1) and formula (2) can be solved as follows:

u _R (t) = Ee ^-t/RC (3)

Assuming that the action time of the intermediate relay is t _d , the power P(t _d ) acting on the relay at this moment is:

According to the operating power P _R (5W or 7W) of the relay, the critical value of the distributed capacitance can be obtained, that is, P(t _d )=P _R :

If the DC negative pole is grounded (point N in Figure 2 is grounded), it means that the relay R and the distributed capacitance C are short-circuited, and the relay cannot operate and will refuse to operate.

(2) The positive power supply of the relay is grounded

The positive power supply of the relay is grounded (point Q is grounded in Figure 2), and its equivalent diagram is shown in Figure 4. The DC system charges the distributed capacitance of the cable through the relay, starting from the moment of grounding.

The initial and final values of the voltage across the relay are:

According to the KCL law:

Solve the voltage u _R across the relay as:

Wherein, τ=(R ₁ //R ₂ //R ₊ //R ₋ //R)·(C ₊ //C ₋ ).

Analysis of Critical Value of Distributed Capacitance When AC is Connected in Series

(1) Mechanism analysis

When the AC power enters the DC system from the bus bar, the distributed capacitance C of the cable will form a loop. The total loop impedance is:

Z＝R+1/(jωC) (10)

The voltage applied to the relay is:

u _R ＝u _s /[1+1/(jωRC)] (11)

When the AC power enters from the side of the relay near the cable, the circuit will be formed through the distributed capacitance C- of the cable, and the calculation process is the same as above. u _s is the voltage that enters the AC.

(2) Calculation of critical value

If the voltage u _R applied to the relay is higher than the action voltage of the relay during the change process, and the power is greater than the power starting value P _d , it will cause the relay to malfunction.

The power acting on the relay is:

P=u _R ² /R (12)

Calculate the critical value of the distributed capacitance according to P equal to the operating power Pd of the relay.

When the distributed capacitance is greater than the critical value of the distributed capacitance, the power is greater than the power startup value. Monitor the size of the distributed capacitance, and give an early warning when the distributed capacitance is close to the critical value.

Distributed capacitance online monitoring

When the distributed capacitance is large enough to meet the operating power of the relay, the relay will still malfunction when the DC single point is grounded, so it is necessary to detect the distributed capacitance of the branch circuit online.

Use the portable mobile device to set the frequency and voltage to generate low-frequency AC signals on the DC bus. If the AC CT detection device of a certain branch can detect the corresponding AC signal, the distributed capacitance of the branch can be measured and meet the corresponding standards. The technical solution for online measurement of distributed capacitance is shown in Figure 5.

Note 1 in Figure 5 provides a reasonable low-frequency AC signal; however, there is a lack of AC signal measurement CT, which needs to be added, as shown in Note 2 in Figure 5; in order to meet the requirements of stable and accurate measurement of distributed capacitance, a bus voltage equalization device needs to be designed, such as Figure 5 Note 3.

Design a sensor (miniature AC and DC current detection device) that can detect DC tiny current signals and AC tiny AC signals at the same time. The detection of tiny AC and DC currents is shown in Figure 6: the principle of DC tiny current detection is the principle of magnetic saturation. The oscillating circuit or modulated pulse detects the corresponding DC current in response to the magnetic field strength; the AC micro current detection principle is the magnetic flux induction principle, and the relevant AC current can be directly induced by the induction coil. The DC detection principle and the AC detection principle are mutually exclusive, and the use of the same coil switching reduces cost and volume.

DC current sensor startup: K1 is closed, K2 is disconnected, the DC current sensor works, and PWM or ADC output can be selected. The AC current sensor starts: K1 is disconnected, K2 is closed, and the AC current sensor works. The parameters of the micro AC and DC current detection sensor are as follows: DC, the minimum current detection accuracy is ± minimum current detection; AC, the minimum current detection accuracy is ± minimum current detection.

The trend analysis of the historical data of grounding resistance not only helps to analyze the insulation level and change rate of the secondary circuit, finds the early warning point of a sharp drop in insulation, and realizes the condition maintenance of the secondary cable; it also helps to monitor the CT zero drift online condition, evaluate the dehumidification effect of the heater, etc. When the trend of grounding resistance changes in reverse, it may be caused by CT zero drift. When the grounding resistance decreases rapidly, it may be due to external reasons such as moisture.

Bus voltage balance control

When the bus voltage offset occurs, the power electronic components are controlled by collecting the bus voltage, so that the equivalent resistance of the positive bus and the negative bus are equal, and the balance of the positive bus and the negative bus voltage is achieved. The design of the voltage equalization module is shown in Figure 7. Adjust the grounding resistance for the DC negative terminal to the ground pin, so that the absolute value of the voltage difference between the positive and negative busbars of the system and the ground does not exceed 10% of the rated voltage, that is, the negative pole of the DC system (busbar ) to ground voltage does not exceed 55% of the rated voltage. Among them, the voltage measurement is a fixed range, DC: 300V, AC: 250V.

When the voltage bus shift occurs, the voltage equalization module adopts the constant resistance mode in the DC electronic load, that is, the constant resistance mode is achieved by adjusting the current, and the effect is shown in Figure 8. In Figure (8a), when R4 is 1%, if the input voltage is 1V, then the voltage on the + of U1 (LM358) is 10mV, and the voltage on R1 is controlled to be 10mV, that is, the current flowing through R1 is I=10mV /0.01Ω=0.01V/0.01Ω=1A, so the resistance value RL of the equivalent electronic load=1V/1A=1Ω. From the output waveform in Figure (8b), it can be seen that the electronic load designed by the hardware is purely resistive.

The system consists of a microcomputer insulation monitor ATCWZJ5, a micro AC and DC CT, a voltage equalization module and a portable insulation detector BXJY-A (providing low-frequency signals). The system connection diagram is shown in Figure 9. On the basis of monitoring the grounding resistance of the DC system, the online monitoring function, trend analysis and risk warning of distributed capacitance are further added.

Claims (9)

1. a kind of cable distributed capacitance Risk-warning control system, is used to be monitored cable distributed capacitance, prevents relay Malfunction, it is characterised in that the system includes that the low frequency signal source (1) being connected with dc bus positive pole and negative pole respectively and voltage are equal Weighing apparatus module (2) and the small alternating current-direct current detection means (3) being arranged at least one dc bus branch road.

2. a kind of cable distributed capacitance Risk-warning control system according to claim 1, it is characterised in that described is micro- DC detecting loop and exchange that small alternating current-direct current detection means (3) is connected including single-chip microcomputer (31) and respectively with single-chip microcomputer (31) Measure loop, described DC detecting loop is provided with DC current sensor (32) and first switch (K1), described exchange Measure loop is provided with AC current sensor (33) and second switch (K2).

3. a kind of cable distributed capacitance Risk-warning control system according to claim 2, it is characterised in that described The break-make mutual linking of one switch (K1) and second switch (K2).

4. a kind of cable distributed capacitance Risk-warning control system according to claim 2, it is characterised in that described is straight Stream current sensor (32) is magnetic saturation current sensor, and described AC current sensor (33) is flux of magnetic induction current sense Device.

5. a kind of cable distributed capacitance Risk-warning control system according to claim 1, it is characterised in that the system is also Including the microcomputer insulation tester (4) being connected with dc bus positive pole and negative pole respectively.

6. a kind of cable distributed capacitance Risk-warning control system according to claim 5, it is characterised in that described is micro- Machine insulation tester (4) model ATCWZJ5.

7. a kind of cable distributed capacitance Risk-warning control system according to claim 1, it is characterised in that described is low Frequency source signal (1) is Portable insulation detector BXJY-A.

8. a kind of cable distributed capacitance Risk-warning control system according to claim 1, it is characterised in that every direct current Small alternating current-direct current detection means (3) on bus branch road is equipped with a protection device (5).

9. a kind of cable distributed capacitance Risk-warning control system according to claim 2, it is characterised in that described is straight The range of stream current sensor (32) is ± 30mA, and precision is 0.01mA, and the range of described AC current sensor (33) is ± 15mA, precision is 0.01mA.