Method and system for maintaining voltage of medium-voltage power supply system

Abstract

The invention relates to a voltage maintaining method and system for a medium-voltage power supply system, and belongs to the technical field of line protection detection. According to the scheme, when the power supply system breaks down, a voltage transformer and a current transformer are used for correspondingly detecting the voltage break variable and the current break variable in the power supply system and calculating the voltage amplitude and the current amplitude; if the current break variable of any phase is larger than a current set value, the voltage break variable of any line is larger than a line voltage set value, the voltage amplitude is smaller than a voltage amplitude set value, and the current amplitude is larger than a current amplitude set value, a current-limiting reactor is put into the power supply system; when the fault disappears, the current-limiting reactance is cut out; if the fault does not disappear, disconnecting the power supply system to isolate the fault, and ensuring the reliability of the power supply system. The fluctuation time of the bus voltage is effectively shortened, the fluctuation range of the bus voltage is reduced, and production equipment safety and continuous production of enterprises are guaranteed.

Description

Technical Field

[0001] The present invention relates to a method and system for maintaining the voltage of a medium-voltage power supply system, and belongs to the technical field of line protection detection. Background Art

[0002] When the power grid experiences phenomena such as large fluctuations in voltage or even short-term power outages for several seconds due to lightning strikes, short circuits, power supply failures, and other internal and external reasons, it is called "voltage sag". Most power grid faults are accompanied by "voltage sag" phenomena. The "voltage sag" phenomenon is extremely likely to damage motors and electrical equipment, affecting the safety of electricity use. In order to reduce the damage caused by "voltage sag" to the power equipment in the medium-voltage power supply system and ensure the safety of electricity use, generally, a protection device is set in the medium-voltage power supply system to achieve the protection purpose.

[0003] However, in the traditional medium-voltage power supply system protection scheme, when a fault occurs, the detection time of the protection device is about 60 ms, and the system voltage can only be restored after the protection reliably cuts off the fault. During this process, the duration of the voltage sag is at least more than 120 ms, which is extremely likely to cause sensitive loads in production enterprises, such as a large number of contactors, frequency converters, PLCs, and various digital devices, to trip or be damaged, resulting in the interruption of the process production, causing significant economic losses to the enterprise, and even affecting the safe and stable operation of the power grid. Summary of the Invention

[0004] The purpose of the present invention is to provide a method and system for maintaining the voltage of a medium-voltage power supply system, so as to solve the problem that the duration of the voltage sag is too long when a fault occurs in the traditional medium-voltage power supply system protection scheme, which is likely to cause damage to electronic devices.

[0005] To achieve the above purpose, the solution of the present invention includes: when a fault occurs in the power supply system, use a voltage transformer and a current transformer to detect the voltage mutation amount in the bus of the power supply system and the current mutation amount of each phase, and calculate the voltage amplitude and the current amplitude; if the current mutation amount of any phase is greater than the current set value, the line voltage mutation amount of any line is greater than the line voltage set value, and the voltage amplitude is less than the voltage amplitude set value and the current amplitude is greater than the current amplitude set value, put the current-limiting reactor into the power supply system; when the fault disappears, cut out the current-limiting reactor; if the fault does not disappear, disconnect the power supply system.

[0006] A method for maintaining the voltage of a medium-voltage power supply system according to the present invention includes the following steps:

[0007] 1) Detect the voltage mutation amount and the current mutation amount in the power supply system, and calculate the voltage amplitude and the current amplitude;

[0008] 2) If the sudden change in current of any phase is greater than the current setting value, the sudden change in any line voltage is greater than the line voltage setting value, and the amplitude of any line voltage is less than the voltage amplitude setting value and the amplitude of any phase current is greater than the current amplitude setting value, the current-limiting reactor is put into the power supply system;

[0009] 3) If the fault disappears, the current-limiting reactor is cut out; if the fault does not disappear, the power supply system is disconnected.

[0010] Further, in step 2), the meaning that the sudden change in current of any phase is greater than the current setting value is that the difference between the current value of any phase and the current value before the previous sampling period is greater than the sum of the difference between the current value before the previous sampling period and the current value before two sampling periods and the current setting value; the meaning that the sudden change in any line voltage is greater than the line voltage setting value is that the difference between the current voltage value between any two phases and the voltage value before the previous sampling period is greater than the sum of the difference between the voltage value before the previous sampling period and the voltage value before two sampling periods and the voltage setting value.

[0011] Further, in step 2), the current setting value is 0.2 times the rated phase current, and the voltage setting value is 0.2 times the rated line voltage.

[0012] Further, in step 1), the line voltage amplitude and the phase current amplitude are calculated using the three-point sampling value product algorithm.

[0013] Further, the power supply system includes a system bus and a power supply line connected to the system bus for supplying power to the next level. A current-limiting reactor is connected in series on the power supply line, and a fast switch for bypassing it is connected in parallel to the current-limiting reactor; a normal switch for cutting off the power supply line is also connected in series on the power supply line; the current-limiting reactor is correspondingly put into and cut out of the power supply system by disconnecting and closing the fast switch, and the power supply system is disconnected by disconnecting the normal switch.

[0014] Further, the sudden change in line voltage is collected by a voltage transformer arranged on the system bus, the sudden change in current of the corresponding phase is collected by a phase current transformer arranged separately on the power supply line for each phase, and the zero-sequence current value is collected by a zero-sequence current transformer on the power supply line.

[0015] Further, in step 3), the criterion for judging whether the fault has disappeared is as follows:

[0016]

[0017] In the formula, I ph,max represents the maximum phase current, I set,res represents the limiting current setting value, I ph,op represents the maximum phase current after the fast switch trips and limits the current, U line.min represents the minimum line voltage, U ε represents the rated line voltage.

[0018] A voltage holding system for a medium - voltage power supply system of the present invention includes a processor, and the processor is used to execute a computer program to implement the steps of the above - mentioned voltage holding method for the medium - voltage power supply system.

[0019] The beneficial effects of the present invention are as follows:

[0020] The present invention detects the voltage mutation and current mutation in the faulty power supply system and calculates the voltage amplitude and current amplitude; makes a judgment based on the voltage mutation, current mutation and the corresponding amplitudes to discriminate the severity of the fault and adopts corresponding countermeasures according to the severity of the fault. The present invention effectively shortens the fluctuation time of the bus voltage, reduces the fluctuation range of the bus voltage, ensures the continuous production of enterprises, solves the problem that in the traditional protection scheme of the medium - voltage power supply system, during the process of fault identification when a fault occurs, the voltage sag duration is too long, which easily causes damage to electronic devices in the faulty power system, and truly realizes the governance of "power supply flickering" of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a flowchart of a voltage holding method for a medium - voltage power supply system of the present invention;

[0022] Figure 2 is a circuit diagram of a voltage holding system for a medium - voltage power supply system of the present invention;

[0023] Figure 3 is a circuit diagram of the voltage holding system for the medium - voltage power supply system when the power supply system is normally powered;

[0024] Figure 4 is a circuit diagram of the voltage holding system for the medium - voltage power supply system when the power supply system has a fault. DETAILED DESCRIPTION OF THE INVENTION

[0025] To make the purpose, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be described in detail clearly and completely below with reference to the drawings and embodiments.

[0026] System embodiment:

[0027] As Figure 2 shown, a voltage holding system for a medium - voltage power supply system of the present invention is composed of a protection device 1, a normal switch 2, a fast switch 3 and a current - limiting reactor 4, and is connected to the system bus 5 of the power supply system where the holding system is located. The protection device 1 collects the current and voltage in the power supply system and controls the closing of the control switches of the normal switch 2 and the fast switch 3 to ensure the voltage stability in the medium - voltage power supply system.

[0028] As Figure 3As shown, when the power supply system is operating normally, the normal switch 2 and the fast switch 3 in the medium-voltage power supply system voltage maintenance system are both closed. At this time, the current-limiting reactor 4 is bypassed by the fast switch 3 and is in an off state; the medium-voltage power supply system voltage maintenance system is connected to the system bus 5, connects to the next-level bus through the normal switch 2 and the fast switch 3, and supplies power to the next level through the loop formed with the power supply line. The protection device 1 in the medium-voltage power supply system voltage maintenance system collects the operating states such as voltage, current, and switch position of this interval to determine the real-time operating state of the power supply system.

[0029] As Figure 1 shown, the medium-voltage power supply system voltage maintenance system performs the following steps:

[0030] 1) The protection device 1 in the medium-voltage power supply system detects whether the sudden changes in voltage and current in the power supply system are greater than the set values, and calculates the voltage amplitude and current amplitude.

[0031] The voltage amplitude and current amplitude are calculated using the three-point sampling value product algorithm.

[0032] The sudden change in line voltage is collected through the voltage transformer set on the system bus 5, the sudden change in current of the corresponding phase is collected through the phase current transformer set separately on the power supply line, and the zero-sequence current value is collected through the zero-sequence current transformer on the power supply line.

[0033] 2) If the sudden change in current of any phase is greater than the current set value, the sudden change in any line voltage is greater than the line voltage set value, and the amplitude of any line voltage is less than the voltage amplitude set value and the amplitude of any phase current is greater than the current amplitude set value, the current-limiting reactor is put into the power supply system.

[0034] The current is discriminated by phase, and the sudden change in current criterion formula is as follows:

[0035]

[0036] The voltage is discriminated, and the sudden change in voltage criterion formula is as follows:

[0037]

[0038] In the formula, t represents the current sampling point time, T represents the sampling period, i a represents the sampling value of phase A current, i b represents the sampling value of phase B current, i c represents the sampling value of phase C current, U ab represents the sampling value of line voltage AB, U bc represents the sampling value of line voltage BC, U ca represents the sampling value of line voltage CA, I ε represents the rated current value, U ε represents the rated line voltage value.

[0039] Magnitude discrimination, as an absolute criterion, calculates the magnitudes of line voltage and phase current through the three-point algorithm to determine whether there is a fault. Among them, the three sampling points are the current sampling point, the current sampling point - N, and the current sampling point - N*2, where N represents the number of forward sampling points (N≥1). To improve the accuracy of the criterion, the number of sampling point discriminations can be set.

[0040] 3) If the fault disappears, cut out the current-limiting reactor; if the fault does not disappear, disconnect the power supply system.

[0041] As Figure 4 shown, after the protection device 1 determines a fault, it issues a quick trip command to immediately trip the quick switch 3 in the medium-voltage power supply stabilization system, and the current-limiting reactor 4 is put into the circuit of the medium-voltage power supply stabilization system to maintain the voltage stability of the medium-voltage power supply system. The criterion for determining whether the fault has disappeared based on the voltage and current change characteristics in the system bus 5 is as follows:

[0042]

[0043] In the formula, I ph,max represents the maximum phase current, I set,res represents the set value of the limiting current, I ph,op represents the maximum phase current after the quick switch 3 trips and limits the current, U line.min represents the minimum line voltage, U ε represents the rated value of the line voltage. When the above three conditions are simultaneously satisfied, it is considered that the fault in the medium-voltage power supply system has disappeared; when any one of the conditions is not satisfied, it is considered that the fault has not disappeared.

[0044] Based on the sampling points, the present invention conducts sudden change discrimination and three-point algorithm magnitude verification, with a relatively fast discrimination time, improving the quick-acting performance of the protection. At the same time, it conducts verification from both relative value and absolute value aspects, improving the reliability of stabilizing the voltage.

[0045] Method embodiment: A method for maintaining the voltage of a medium-voltage power supply system in this embodiment has been introduced clearly enough in the system embodiment, and will not be elaborated here.

[0046] The present invention provides a method and system for maintaining the voltage of a medium-voltage power supply system. When a voltage sag occurs, it quickly identifies the fault through sudden change discrimination and three-point algorithm magnitude verification, and puts in a current-limiting reactor to stabilize the voltage of the power supply system. After putting in the current-limiting reactor, it determines whether the fault has disappeared according to the voltage and current change characteristics. If the fault disappears, the current-limiting reactor is withdrawn; if the fault does not disappear, the normal switch is tripped to isolate the fault, ensuring the reliability of the power supply system. The present invention can effectively shorten the fluctuation time of the bus voltage, reduce the fluctuation range of the bus voltage, ensure the continuous production of enterprises, and truly realize the governance of "power supply interruption" in the system.

[0047] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. The scope of patent protection of the present invention is subject to the claims. Any equivalent structural changes made by using the content of the specification and drawings of the present invention should, by the same token, be included within the protection scope of the present invention.

Claims

1. A method for maintaining the voltage of a medium-voltage power supply system, characterized in that, It includes the following steps: 1) Detect the voltage mutation and current mutation in the power supply system, and calculate the voltage amplitude and current amplitude; 2) When the current mutation of any phase is greater than the current set value, the line voltage mutation of any line is greater than the line voltage set value, and the line voltage amplitude of any line is less than the voltage amplitude set value and the current amplitude of any phase is greater than the current amplitude set value, the current-limiting reactor is put into the power supply system; 3) If the fault disappears, the current-limiting reactor is cut out; If the fault does not disappear, the power supply system is disconnected.

2. The method for maintaining the voltage of the medium-voltage power supply system according to claim 1, characterized in that, In step 2), the meaning that the current mutation of any phase is greater than the current set value is that the difference between the current value of any phase and the current value before the previous sampling period is greater than the sum of the difference between the current value before the previous sampling period and the current value before two sampling periods and the current set value; the meaning that the line voltage mutation of any line is greater than the line voltage set value is that the difference between the current voltage value between any two phases and the voltage value before the previous sampling period is greater than the sum of the difference between the voltage value before the previous sampling period and the voltage value before two sampling periods and the voltage set value.

3. The method for maintaining the voltage of the medium-voltage power supply system according to claim 1, wherein In step 2), the current set value is 0.2 times the rated phase current, and the voltage set value is 0.2 times the rated line voltage.

4. The method for maintaining the voltage of the medium-voltage power supply system according to claim 1, wherein In step 1), the line voltage amplitude and phase current amplitude are calculated using the three-point sampling value product algorithm.

5. The method for maintaining the voltage of the medium-voltage power supply system according to claim 1, characterized in that The power supply system includes a system bus and a power supply line connected to the system bus for supplying power to the next level. The current-limiting reactor is connected in series on the power supply line, and a fast switch for bypassing it is connected in parallel on the current-limiting reactor; a normal switch for cutting off the power supply line is also connected in series on the power supply line; the current-limiting reactor is correspondingly put into and cut out of the power supply system by disconnecting and closing the fast switch, and the power supply system is disconnected by disconnecting the normal switch.

6. The method for maintaining the voltage of the medium-voltage power supply system according to claim 5, characterized in that, The line voltage mutation is collected through a line voltage transformer arranged on the system bus, the current mutation of the corresponding phase is collected through a phase current transformer arranged in a split-phase manner on the power supply line, and the zero-sequence current value is collected through a zero-sequence current transformer arranged on the power supply line.

7. The method for maintaining the voltage of the medium-voltage power supply system according to claim 1, wherein In step 3), the criterion for judging whether the fault disappears is as follows: Wherein, I ph，max represents the maximum phase current, I set，res represents the setting value of the limiting current, I ph，op represents the maximum phase current after the fast switch trips and limits the current, U line.min represents the minimum line voltage, U ε represents the rated line voltage.

8. A medium-voltage power supply system voltage holding system, including a processor, characterized in that, The processor is used to execute a computer program to implement the steps of the medium-voltage power supply system voltage holding method as described in any one of claims 1 to 7.

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