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18650 rechargeable battery lithium 3.7v 3500mah
18650 rechargeable battery lithium 3.7v 3500mah

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LR1130 battery.Characteristics and models of neutral ground resistance, small power resistance multimeter auxiliary circuit diagram analysis

release time:2024-03-04 Hits:     Popular:AG11 battery

  

  The neutral point grounding resistance cabinet is mainly used to connect the neutral point of the main transformer and the grounding network in urban and rural distribution networks. Urban and rural distribution networks mainly refer to the three voltage levels of 10kV, 35kV, and 66kV. They are widely used in power systems. It is wide and occupies an important position. With the in-depth development of urban network transformation, the capacity of 10KV distribution network has increased rapidly and the network structure has become increasingly perfect. According to the needs of urban construction, overhead bare conductor lines are gradually being replaced by cables and insulated conductors. At the same time, due to overvoltage, Fire accidents in switch cabinets and expensive household appliances are also common. Therefore, how to effectively and economically limit the overvoltage problem of distribution network has become the focus of current power supply and consumption.

  The neutral point of the 10KV distribution network can usually be divided into ungrounded system, resistance grounding system and arc suppression coil grounding system. Since the choice of grounding method is a comprehensive issue involving factors such as the insulation level of lines and equipment, communication interference, relay protection composition, and the safety and reliability of the power supply network, the power supply system practices of my country's distribution network and large industrial and mining enterprises vary. Differently, in the past, most of them used the operation mode of ungrounded neutral point or grounding through arc suppression coil. In recent years, some urban power grids have vigorously promoted the operation mode of resistive grounding.

  In the mid-1980s, the 10kV distribution network in cities such as Guangzhou developed rapidly. A large number of cables were laid in the central area of the city. The single-phase grounding capacitor current increased rapidly, reaching more than 60A in 1987. Although arc suppression coils were installed, due to the large capacitor current and The operating mode changes frequently, and it is difficult to adjust the arc suppression coil. In addition, some cables with low insulation levels are used. In order to reduce the overvoltage level and reduce the possibility of phase-to-phase faults, the neutral point is grounded through low resistance.

  According to the research results of Ouzhuang substation in Guangzhou area, the neutral point is used to ground through low resistance. When Rn ≤ 10Ω, the single-phase grounding power frequency voltage can be reduced to about 1.4p.u in most cases. From the perspective of limiting arc ground overvoltage, when the arc is ignited and extinguished, the excess charge accumulated in the system can leak through Rn within half the power frequency cycle after the arc is extinguished, and the overvoltage amplitude can be significantly reduced. According to this requirement, the conditions that the low resistance value of the neutral point should meet are:

  Rn≤1/3ωC0

  When Rn=10Ω, the arc ground overvoltage can be reduced to less than 1.9p.u.

  If the resistance value of the neutral point is too low, the single-phase grounding current will be large, causing great interference to the communication line; if the resistance value is too high, the relay protection action will be unreliable. Generally speaking, when the current in the neutral point resistor is 100~200A, it will not cause interference to the communication line. Under this condition, the neutral point resistance value of the 10kV overhead line is 28.80~57.74Ω. For cable-based distribution networks, according to Japanese experience, when the current in the neutral point resistor is 400 to 800A, there is little interference with communication lines. Accordingly, the range of the neutral point resistance value of the 10kV cable distribution network It should be 7.2~14.4Ω.

  From the perspective of ensuring the reliability of the relay protection action, it should have a higher sensitivity when a single-phase ground fault occurs. There are two types of grounding relays: one is a grounding overcurrent relay, and the other is a grounding direction relay that operates according to the direction of the zero-sequence current.

  When using an overcurrent relay, there is no problem with the sensitivity of protection when a metallic grounding occurs. However, when grounding through a transition resistor, there are mainly a considerable number of single-phase ground faults in overhead lines. The resistance at the fault point is relatively large, and there are some problems with the sensitivity of the protection. For cable lines, the transition resistance when single-phase grounding is generally relatively small, which has little impact on the sensitivity of relay protection.

  Finally, starting from the requirement of limiting resonant overvoltage, when the cable line is particularly long, jωLe=1/jωC may occur, causing resonance. If the neutral point has an appropriate resistance, the abnormal voltage on the sound phase can be limited. .

  When the neutral point is grounded through low resistance, especially when a distribution network dominated by overhead lines is grounded in a single phase, the number of trips will be greatly increased. If the ring network power supply cannot be realized or the line is not equipped with reclosing, the number of power outages will increase. will increase and reduce the reliability of power supply. However, for cable-based power distribution, because of its extremely low failure rate, this problem is not prominent.

  Based on my country's specific conditions, it is recommended that distribution networks with cable-based capacitance currents above 150A can adopt low-resistance grounding methods, and the corresponding fault current levels are 400 to 1000A. For a 10kV system, the neutral point grounding resistance value can be RN=10~20Ω.

  (3) Implementation standards

  DL/T780-2001 "Neutral point grounding resistor for power distribution system"

  GB6450 "Dry-type power transformer"

  DL/T620-1997 "Overvoltage protection and insulation coordination of AC electrical devices"

  GB311.2-6 "High Voltage Test Technology"

  GB1208-1997 "Current Transformer"

  GB4208-93 "Enclosure protection level (IP code)"

  IEC289

  IEEE32-1972 Standard "Technology, Terminology and Testing of Neutral Point Grounding Devices"

  Other relevant national standards and power industry standards

  (4) Type

  ●Distribution network small resistance grounding resistance cabinet

  ●Resistance grounding resistance cabinet in distribution network

  ●High resistance grounding resistance cabinet for distribution network

  ●Generator neutral point resistance grounding resistance cabinet

  (5) Serialization of variety products

  ●Applicable voltage: 3.3kV~35kV;

  ●Allow flow: 5A~5000A;

  ●Nominal resistance: 0.1Ω~1500Ω;

  ●Allow flow time: 10s, 30s, 60s, 10min, continuous operation;

  ●Inlet and outlet lines: top in and bottom out, bottom in and bottom out, side in and side out, side in and bottom out;

  ●Installation location: indoors or outdoors;

  ●Allowable temperature rise: 760℃ when the flow time is 10s, 30s, and 60s; 610℃ when the flow time is 10min; 385℃ during continuous operation;

  ●Current transformer CT: optional

  Analysis of the auxiliary circuit diagram of a small-power resistance multimeter:

  In actual work, in order to analyze the circuit principle, when drawing the electrical schematic diagram based on the actual object, it is often necessary to measure the actual resistance value of the small resistance resistor, such as the constantan resistor (generally milliohm) used to detect the load current in high-end switching power supplies. level), high-power small-value resistors for over-current protection (some are below 0.1Ω), feedback resistors (generally a few tenths of ohms) connected in series with the current amplifier tube (E pole or S pole) in high-power amplifier circuits .

  The following is an auxiliary circuit diagram for accurately measuring small resistance resistors with an ordinary multimeter.

  Since the minimum range of electrical resistance of ordinary digital multimeters is 200Ω, due to accuracy limitations, it is often impossible to accurately measure the specific resistance values of these resistors, nor to judge their consistency, which is often difficult. To this end, the auxiliary circuit shown in Figure 1 was trial-produced and combined with the DC low voltage range of the multimeter (200mV, 2V, 20V) to achieve accurate measurement of small resistance resistors.

  Add a certain current to the resistor RX under test through a constant current source, and then use a multimeter to measure the voltage at both ends of Rx. The measured voltage value is divided by the constant current flowing through the resistor under test Rx, and the resistance of the resistor under test can be obtained. . Theoretically, the greater the current flowing through the resistor to be measured, the easier it is to accurately measure the resistance of the small-value resistor Rx. However, if the current is too large, it will cause serious heating of the constant current source and affect the stability of the current, resulting in the measured resistance. The value is inaccurate; second, the low-power resistor does not allow excessive current to flow. For this purpose, this circuit uses LM317 (U1), resistors R1, R2, and potentiometer Rp1 to form a simple 100mA constant current source.


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