Marine zero-sequence current protection method considering sensitivity and selectivity

Abstract

The invention relates to a marine zero-sequence current protection method giving consideration to sensitivity and selectivity, which comprises the following steps: detecting zero-sequence current recording at each part of a system, calculating to obtain zero-sequence current distribution of the system, if resistive current is detected, judging that a grounding fault exists, and according to the total zero-sequence current of each node of each system during the fault, judging that the grounding fault exists; judging a fault node; protection sensitivity setting action current is carried out according to the total zero sequence current related to the fault node, action time is set according to protection selectivity, and setting is carried out according to the characteristic that the action time is gradually increased step by step according to the connection relation with the fault point to form step delay. Zero-sequence current protection action current and action time are set according to a method of performing fault line identification and zero-sequence protection system setting through resistive zero-sequence current, and meanwhile, the requirements of protection sensitivity and selectivity are met.

Description

Technical Field

[0001] The invention relates to the field of power supply system protection, and in particular to a marine zero-sequence current protection method that takes both sensitivity and selectivity into consideration. Background Art

[0002] The ship distribution network is different from the land distribution network. Due to space limitations, the ship distribution network cannot use overhead lines, so it has to use power cables. The physical structure of power cables is more complex than overhead lines. As the scale of ship distribution networks continues to expand, the length and number of power cables increase, resulting in much greater capacitance to the ground than overhead lines. Therefore, when a ground fault occurs, the system's ground capacitance current value is more than ten times the value of the same overhead line ground fault, and the arc overvoltage during the fault is increased. Due to the significant ground capacitance current of the medium-voltage cable in the ship power supply system, the selectivity and sensitivity of the zero-sequence protection are mutually restricted, making the protection setting difficult. Summary of the invention

[0003] In view of the problem that the medium-voltage cable-to-ground capacitance current in the ship's power supply system is significant, which leads to the mutual restriction of zero-sequence protection selectivity and sensitivity, making protection setting difficult, a shipborne zero-sequence current protection method with both sensitivity and selectivity is proposed. The distributed capacitance of the medium-voltage cable to the ground is studied to solve the bottleneck problem of sensitivity and selectivity coordination of the zero-sequence overcurrent short-delay protection system caused by the excessive capacitance current of the system to the ground.

[0004] The technical solution of the present invention is: a marine zero-sequence current protection method taking into account both sensitivity and selectivity, which specifically comprises the following steps:

[0005] 1) Detect the zero-sequence current recordings at various locations in the system to calculate the zero-sequence current distribution of the system. Combined with the system grounding distributed capacitance measurement, if resistive current is detected, it is determined that there is a grounding fault. The faulty node is determined based on the total zero-sequence current of each node in each system at the time of the fault;

[0006] 2) According to the fault node, the total zero-sequence current sum related to the fault node is calculated as G1;

[0007] 3) According to the fault node, the maximum zero-sequence current in the line not directly connected to the fault node is calculated as G2;

[0008] 4) The setting method of the marine zero-sequence current protection method that takes into account both sensitivity and selectivity is as follows: the action current is set according to the protection sensitivity. If the protection sensitivity is not less than the coefficient K, the action current of the fault point is set to G1 / K; the action time is set according to the protection selectivity. The action time is set in a step-by-step manner according to the connection relationship with the fault point to form a step delay characteristic, that is, the closer to the fault point, the shorter the action time is set, and the farther away, the longer the action time is set. The action time is the shortest at the maximum zero-sequence current.

[0009] Further, the step 1) is a method for calculating the resistive current in the zero-sequence current distribution:

[0010] The zero-sequence voltage U0 and zero-sequence current I0 appear on the high-voltage side of the system, where the zero-sequence current is decomposed into the resistive current component I R and the capacitive current component I C , resistive current component I R In phase with the zero-sequence voltage U0, the capacitive current component I C The leading zero-sequence voltage U0 has a phase angle of 90°. By performing a vector dot product operation on the zero-sequence voltage U0 and the zero-sequence current I0, The phase difference between the zero-sequence voltage U0 and the zero-sequence current I0 is used as the reference to extract the resistive zero-sequence current component I R ,as follows:

[0011]

[0012] So we can get:

[0013]

[0014] Resistive zero-sequence current will only appear in the fault line and its upstream lines, and its value is determined by the neutral point grounding resistance of the grid-connected units; resistive zero-sequence current will not appear in non-fault lines.

[0015] Furthermore, the step 1) measures the system grounding distributed capacitance and performs a method for calculating the capacitive current in the zero-sequence current: the capacitance to ground is measured using the resistance-capacitance method, a direct current voltage DC is applied between the capacitance to ground and the grounding electrode, and a large resistor R is connected in series, and the grounding capacitance value is calculated by measuring the resistance voltage divider; a multimeter is used to measure the voltage on the resistor and the capacitor to obtain the voltage ratio, and then the capacitive reactance is calculated through Ohm's law that the series voltage divider is proportional to the impedance and the capacitive reactance, and the grounding capacitance value is calculated according to the capacitive reactance calculation formula Xc=1 / (2πfC), and the capacitance value is used to calculate the grounding capacitive current to analyze the arc overvoltage level of the power supply network. After the grounding capacitance value is calculated, the capacitive current is calculated through I=U / Xc.

[0016] The beneficial effects of the present invention are as follows: the present invention is a marine zero-sequence current protection method that takes into account both sensitivity and selectivity, and adjusts the zero-sequence current protection action current and action time according to the method of "identifying fault lines and adjusting the zero-sequence protection system through resistive zero-sequence current", while meeting the requirements of protection sensitivity and selectivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a zero-sequence voltage and zero-sequence circuit vector diagram when a single-phase grounding fault occurs in the method of the present invention;

[0018] Figure 2 A topological structure diagram of a power supply network example in the method of the present invention;

[0019] Figure 3 A schematic diagram of measuring distributed capacitance using the resistance-capacitance method in the method of the present invention;

[0020] Figure 4 The figure is a curve showing the relationship between the overvoltage multiple and the R / C current ratio during a single-phase grounding fault in the method of the present invention. DETAILED DESCRIPTION

[0021] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0022] The invention provides a marine zero-sequence current protection method that takes into account both sensitivity and selectivity. The zero-sequence current distribution of the system is calculated by recording the zero-sequence current at various locations of the system when a single-phase metal grounding fault occurs. Combined with the measurement of the system grounding distributed capacitance, the resistive current and the capacitive current are effectively distinguished. A zero-sequence protection strategy that takes into account both sensitivity and selectivity for distinguishing grounding faults using resistive current is proposed to prevent the bottleneck problem of sensitivity and selectivity coordination in the zero-sequence overcurrent short-delay protection system caused by excessive system-to-ground capacitance current.

[0023] The present invention proposes to use the resistive current component in the zero-sequence current to perform ground fault identification and protection setting. The specific steps are as follows:

[0024] 1. Detect the zero-sequence current recordings at various locations in the system to calculate the zero-sequence current distribution of the system. Combined with the system grounding distributed capacitance measurement, if resistive current is detected, it is determined that there is a grounding fault. The faulty node is determined based on the total zero-sequence current of each node in each system at the time of the fault.

[0025] 2. According to the fault node, the total zero-sequence current sum related to the fault node is calculated as G1;

[0026] 3. According to the fault node, calculate the maximum zero-sequence current of the line not directly connected to the fault node as G2;

[0027] 4. To meet the protection selectivity, the zero-sequence current setting value needs to be greater than G2. To meet the protection sensitivity, the zero-sequence current setting value should be less than G1. The setting method of the marine zero-sequence current protection method that takes into account both sensitivity and selectivity is to set the action current according to the protection sensitivity. If the protection sensitivity is not less than the coefficient K, the action current of the fault point is set to G1 / K. The action time is set according to the protection selectivity. The action time is set in a step-by-step manner according to the connection relationship with the fault point to form a step delay characteristic. That is, the closer to the fault point, the shorter the action time is set, and the farther away, the longer the action time is set. The action time is the shortest at the maximum zero-sequence current.

[0028] When a single-phase grounding fault occurs, zero-sequence voltage U0 and zero-sequence current I0 will appear on the high-voltage side of the entire system, where the zero-sequence current can be decomposed into resistive current components I R and the capacitive current component I C The vector relationship between the three is as follows: Figure 1 As shown, the resistive current component I R In phase with the zero-sequence voltage U0, the capacitive current component I C Leading zero-sequence voltage U0 has a phase angle of 90°. It is the phase difference between the zero-sequence voltage U0 and the zero-sequence current I0.

[0029] Therefore, the resistive zero-sequence current component I can be extracted by performing a vector dot product operation on the zero-sequence voltage U0 and the zero-sequence current I0, taking the zero-sequence voltage U0 as the reference quantity. R ,as follows:

[0030]

[0031] So we can get:

[0032]

[0033] Resistive zero-sequence current will only appear in the fault line and its upstream lines, and its value is determined by the neutral point grounding resistance of the grid-connected units; resistive zero-sequence current will not appear in non-fault lines.

[0034] Therefore, the resistive zero-sequence current can be used to identify the fault line and adjust the zero-sequence protection system to ensure that the protection can operate sensitively and selectively in the event of a ground fault.

[0035] like Figure 2 The power supply network topology diagram shown in the figure shows that the on-grid unit includes two high-power generating modules (TG), which are connected to the grid through the main jumper line to jointly supply power to the load and emergency load. In a single-phase ground short circuit test, the single-phase ground short circuit position is located at the jumper line between the power supply and the emergency load.

[0036] When a single-phase grounding fault occurs, the zero-sequence current in the system is mainly composed of the following two parts:

[0037] One part is the resistance current provided by the neutral point resistor of each generator; the other part is the ground capacitance current provided by the medium voltage connecting cables and all medium voltage equipment in the system. Here medium voltage refers to the primary side of the equipment.

[0038] The capacitance of medium voltage cables and medium voltage equipment to ground is measured by the resistance-capacitance method. A DC voltage is applied between the medium voltage cables and medium voltage equipment and the grounding electrode, and a large resistance R is connected in series, such as Figure 3 As shown, the grounding capacitance value of medium voltage cables and medium voltage equipment can be calculated by measuring the resistance voltage division. Use a multimeter to measure the voltage on the resistor and capacitor to get the voltage ratio, and then use Ohm's law to calculate the capacitive reactance, which is proportional to the series voltage division and the impedance and capacitive reactance. The grounding capacitance value can be deduced according to the capacitive reactance calculation formula Xc=1 / (2πfC). The capacitance value can be used to calculate the grounding capacitive current and analyze the arc overvoltage level of the power supply network. After calculating the grounding capacitance value, the capacitive current is calculated by I=U / Xc.

[0039] According to the length of medium voltage cables, the number of grid-connected power generation modules and the number of grid-connected medium voltage transformers, Figure 4 The relationship curve between the arc overvoltage level and the system resistive current / capacitive current ratio shown in the figure can be used to obtain the arc overvoltage level when a single-phase grounding fault occurs in the power supply system. This method can be used to determine whether the insulation withstand voltage level of the power supply system can withstand the arc overvoltage when a grounding fault occurs.

[0040] The two high-power generating modules in the front and rear power stations are connected in parallel to supply power to the entire ship. When a single-phase grounding fault occurs in the emergency jumper line, the zero-sequence current distribution of the entire system is summarized as follows:

[0041] ——When a single-phase grounding occurs, zero-sequence current will appear everywhere in the medium-voltage network of the whole system. The magnitude of zero-sequence current varies significantly at each location. The total grounding fault current at the fault point is about 27.5A.

[0042] ——The zero-sequence current flowing through the fault line (emergency jumper) is the largest, about 26A, which is the sum of the ground capacitance current of other non-fault lines in the whole system and the resistance current of the resistor cabinet;

[0043] ——The zero-sequence current flowing through the machine end of the high-power generation module is the sum of the resistance current of the resistor cabinet, the capacitance current of the medium-voltage cable to the ground at the machine end, and the capacitance current of the generator body to the ground, which is about 4.4A (G1);

[0044] ——The zero-sequence current value of many non-fault lines (such as emergency jumpers at other locations, load tie lines, main jumpers, and busbars) is large, and its value is equal to the cable-to-ground capacitance current of this line (and its downstream lines). The longer the cable, the greater the zero-sequence current. Figure 2The zero-sequence current at the middle main jumper is the largest, about 14.7A (G2).

[0045] According to the above zero-sequence current distribution characteristics:

[0046] ——In order to meet the protection selectivity, the zero-sequence protection should be set according to the capacitive current provided by the line when avoiding the single-phase grounding fault outside the zone. According to the capacitive current distribution of the load tie line and the non-fault emergency jumper line, the zero-sequence current setting value needs to be greater than 1.47A (G2);

[0047] ——In order to meet the protection sensitivity, the zero-sequence current setting value should be smaller than the resistive zero-sequence current IA (G1) at the machine end of the high-power generator module;

[0048] ——This leads to mutual constraints between the selectivity and sensitivity of zero-sequence protection, making protection setting difficult.

[0049] According to the method mentioned above about "fault line identification and zero-sequence protection system setting through resistive zero-sequence current", this typical power supply system and similar power supply systems can adopt the following setting method:

[0050] 1. According to the sensitivity of protection, the action current can be set according to the minimum operation mode, when a single-phase grounding fault occurs in the medium-voltage network when the ordinary power generation module is supplying power, and the protection can be set sensitively and reliably. Assuming that the resistive zero-sequence current provided by a single ordinary power generation module is IA, and the backup protection sensitivity is not less than 1.2 (K), the action current is set to IA / 1.2.

[0051] 2. Set the action time according to the protection selectivity. According to the load connection / emergency jumper, main jumper, bus tie, unit / shore power switch, the action time increases step by step to form a step delay characteristic for setting to meet the upper and lower protection selectivity.

[0052] The above-mentioned embodiments only express the specific implementation of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the invention patent. It should be pointed out that for ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be based on the attached claims.

Claims

1. A marine zero-sequence current protection method taking into account both sensitivity and selectivity, characterized in that: The specific steps include: 1) Detect the zero-sequence current recordings at various locations in the system to calculate the zero-sequence current distribution of the system. Combined with the system grounding distributed capacitance measurement, if resistive current is detected, it is determined that there is a grounding fault. The faulty node is determined based on the total zero-sequence current of each node in each system at the time of the fault; 2) According to the fault node, the total zero-sequence current sum related to the fault node is calculated as G1; 3) According to the fault node, the maximum zero-sequence current in the line not directly connected to the fault node is calculated as G2; 4) The setting method of the marine zero-sequence current protection method that takes into account both sensitivity and selectivity is as follows: the action current is set according to the protection sensitivity. If the protection sensitivity is not less than the coefficient K, the action current of the fault point is set to G1 / K; the action time is set according to the protection selectivity. The action time is set in a step-by-step manner according to the connection relationship with the fault point to form a step delay characteristic, that is, the closer to the fault point, the shorter the action time is set, and the farther away, the longer the action time is set. The action time is the shortest at the maximum zero-sequence current.

2. The marine zero-sequence current protection method taking into account both sensitivity and selectivity according to claim 1, characterized in that: The step 1) is a method for calculating resistive current in zero-sequence current distribution: The zero-sequence voltage U0 and zero-sequence current I0 appear on the high-voltage side of the system, where the zero-sequence current is decomposed into the resistive current component I R and the capacitive current component I C , resistive current component I R In phase with the zero-sequence voltage U0, the capacitive current component I C The leading zero-sequence voltage U0 has a phase angle of 90°. By performing a vector dot product operation on the zero-sequence voltage U0 and the zero-sequence current I0, The phase difference between the zero-sequence voltage U0 and the zero-sequence current I0 is used as the reference to extract the resistive zero-sequence current component I R ,as follows: So we can get: Only the fault line and its upstream lines will have resistive zero-sequence current, and its value is determined by the neutral point grounding resistance of the grid-connected units; In non-fault lines, no resistive zero-sequence current will appear.

3. The marine zero-sequence current protection method taking into account both sensitivity and selectivity according to claim 1, characterized in that: The step 1) measures the system grounding distributed capacitance and calculates the capacitive current in the zero-sequence current: The capacitance to ground is measured using the resistance-capacitance method. A DC voltage is applied between the capacitance to ground and the grounding electrode, and a large resistor R is connected in series. The grounding capacitance value is calculated by measuring the resistance voltage divider. A multimeter is used to measure the voltage on the resistor and capacitor to obtain the voltage ratio. The capacitive reactance is then calculated using Ohm's law, which states that the series voltage divider is proportional to the impedance and capacitive reactance. The grounding capacitance value is deduced using the capacitive reactance calculation formula Xc=1 / (2πfC). The capacitance value is used to calculate the grounding capacitive current, thereby analyzing the arc overvoltage level of the power supply network. After the grounding capacitance value is calculated, the capacitive current is calculated using I=U / Xc.

Images