Effective grounding system (also known as high current grounding system)
Small current grounding system (including ungrounded and grounded via arc suppression coil)
Resistance grounding system (including small resistance, medium resistance and high resistance)
High current grounding system
For 110kV and above systems. When the system is single-phase grounded, the other two relative ground voltages are basically unchanged, and the system overvoltage is low. It is beneficial to suppress the overvoltage of 110kV and above systems. However, the grounding current is very large, and it is difficult for the operating equipment to pass this current for a long time. The grounding voltage is very low, even zero, the system voltage is seriously unbalanced, many electrical equipment can not work normally, and the grounding point must be removed in time. The high-current grounding system requires the neutral point of some main transformers to be grounded to avoid excessive short-circuit current when single-phase is grounded. These main transformers must have a delta-connected winding to form a zero-sequence path that reduces the zero-sequence impedance. The zero sequence impedance of the main transformer is generally 1/3 of the positive sequence impedance, and the zero sequence impedance of the line is generally three times the positive sequence impedance.
As the main transformer of the 220kV hub substation , it must run in parallel. The 220kV side neutral point and the 110kV side neutral point of one main transformer must be directly grounded, and the other main transformer neutral points are grounded through the gap. The advantage is that the 110kV side zero-sequence impedance is stable, which is beneficial to the calculation and tuning of the zero-sequence setting of the 110kV system. The protection range of the zero-sequence overcurrent protection changes little, and it is easy to maintain its step characteristics; the stable zero-sequence power supply is not provided by the 220kV system. To maintain the directionality and stability of the zero sequence protection of the 220kV system. The main transformer 220kV side neutral point and the 110kV side neutral point are equipped with gap protection, and the protection action trips each side circuit breaker .
The main transformer as a 220kV load substation must be operated separately. At this time, the 220kV side neutral point of all main transformers must be grounded through the gap, and the 110kV side neutral point is all grounded. All main transformers cannot provide zero-sequence current in the 220kV system, and the 110kV-side zero-sequence impedance is stable. The main transformer 220kV side neutral point is equipped with gap protection, and the protection action trips each side circuit breaker.
As a 220kV substation with chain wiring, its 220kV side busbars run side by side and have two power supplies. Although the main transformer is operated in a separate column, it must have a main transformer 220kV side neutral point directly grounded, and the other main transformer 220kV side neutral point is grounded through the gap. The 110kV side neutral point must all be directly grounded. The main transformer 220kV side neutral point is equipped with gap protection, and the protection action trips each side circuit breaker.
All the main transformers of the 110kV substation currently operating are split and operate, and the power supply side busbar is a single power supply. Therefore, the 110kV side neutral point of the main transformer is grounded through the gap, and no gap protection is added.
The 0.4kV system is operated with a large current ground. For transformers with Y/Y0 wiring, the zero sequence impedance is large. Although the load to be connected is mostly single-phase load, since each load is small, it does not necessarily cause serious inconsistency of the three-phase load current (the neutral point current is less than 25% of the rated current), and the three-phase voltage will not be seriously caused. balance. However, when the line is short-circuited to the ground, the short-circuit current is small, and the circuit breaker (air switch) is often tripped or the fuse is blown, causing the accident to expand, and in many cases, a fire is formed. At this time, overcurrent protection should be installed at the neutral point of the transformer to trip the high voltage side circuit breaker. Obviously this is more complicated.
This disadvantage can be overcome by using a â–³/Y0 wiring transformer. However, it is difficult to make the tap changer of the oil-filled transformer, especially the on-load tap-changer. Although the transformer with â–³/Y0 wiring has the advantages of low zero-sequence impedance and small voltage difference when the three-phase current is unbalanced, the neutral point current must not exceed 60% of the rated current. For this reason, the 315kVA and below medium and small capacity transformers (especially oil-filled transformers) mostly use Y/Y0 wiring, while the transformers above 315kVA (especially dry-type transformers) use â–³/Y0 wiring.
At present, the power supply in large buildings is mostly three-phase five-wire system, which is one more ground wire than the original three-phase four-wire system. The ground and neutral wires are connected together at the low voltage screen (or transformer) (the small resistance grounding system is described separately) and are divided into two wires when they are taken out from the low voltage screen. Zero sequence can be connected to single-phase load, there will be working current, the ground wire has no working current, and its potential is always consistent with the earth. In order to ensure that the ground wire and the ground potential are consistent, it is necessary to repeat the grounding as required. The neutral line should not be grounded any more, and it should not be connected to the ground line to avoid the working current in the ground line. The metal enclosure of electrical equipment should be connected to the ground. When the insulation of the electrical equipment is damaged, a loop is formed with the ground wire. In severe cases, the fuse is blown or the air switch is tripped, and the potential of the casing is not raised, which poses no threat to personal safety.
Small current grounding system
When the neutral point of the system is not grounded, whether it is an overhead line or a cable , there are one or three relatively equal capacitors in normal operation. Since the capacitive reactance is substantially equal, the voltages of the three grounds are substantially equal, and the voltage to the ground of the neutral point is very low (not exceeding 2% of the rated voltage of the system). When one of the phases is grounded, the grounding relative to ground voltage is reduced (zero for metallic grounding), the ungrounded relative ground voltage is increased (the metallic voltage is the line voltage), and the grounding point current is the relative current for the metallic grounding. 3 times the ground capacitance current. If a voltage transformer that measures the voltage to ground is connected to the system, the output voltage is the rated voltage (the open triangle has a fixed connection). According to the level of this voltage, it can be judged whether the system has single-phase grounding. Since the ground point has only capacitive current, the system can operate for a long time. However, the stepping voltage of the grounding point will pose a great threat to the life safety of the surrounding personnel (within 20 meters). In addition, the voltage transformer that monitors the voltage to the ground is designed for 8 hours. Therefore, the grounding point should be found and removed from the grid as soon as possible. An arc is generated when the grounding point is not securely contacted. Since the arc current is not large, the arc cannot be maintained when the wire is away from the grounding point and will automatically go out. Intermittent arc current can cause overvoltage in the system. When the arc current is not large, the overvoltage will be limited to the range allowed by the system.
When the scale of the system increases, the single-phase grounded capacitor current also increases rapidly. When the grounding point generates an arc, the arc is not easy to extinguish. The system overvoltage caused by the intermittent arc current will exceed the allowable range of the system, resulting in insulation breakdown of the device. ACCIDENT.
In order to avoid the system can not allow overvoltage, and the arc of the grounding point is easy to extinguish, a reactor is added to the ground at the neutral point, so that the inductor current generated by it cancels the capacitance current of the grounding point, and the grounding current of the grounding point is lowered. The overvoltage amplitude is reduced to the extent that the system can tolerate and is conducive to arc extinguishing. This reactor is used as an arc suppression coil. A system in which the inductor current is greater than the capacitor current becomes an overcompensation system, a system in which the inductor current is smaller than the capacitor current becomes an undercompensation system, and a system in which the inductor current is equal to the capacitor current becomes a full compensation system. Without special measures, the full compensation system will resonate when the system is not single-phase grounded, and the system will not operate normally. The under-compensation system may be close to full compensation when the system cuts off a line, and is rarely used. The overcompensation system must operate the arc-suppression coil to exceed 10% of the system capacitance current and not exceed 10A during operation, otherwise operation is quite difficult.
The power supply for many systems is the delta wiring side of the transformer, and no neutral point can be drawn. At this point the system should be fitted with a grounding transformer that provides zero sequence current. There are two kinds of grounding transformers: one is to take out the neutral point of the transformer star-connected winding, the other side is connected to a triangle; the other is to use a zigzag wiring transformer (Z-shaped). When the grounding transformer only has an arc suppression coil, the capacity is not less than the capacity of the arc suppression coil.
There are usually multiple taps that generate different currents at phase voltages to correspond to different system conditions. Since we do not require the system to be single-phase grounded for long-term operation, the design of the arc suppression coil is generally operated at the maximum current tapping for 2 hours, or the upper oil temperature (oil-filled) winding temperature (dry) does not exceed the allowable value. Therefore, the arc suppression coil must be equipped with a thermometer for measuring the upper oil temperature (oil filling) or winding temperature (dry) with an alarm contact, and the unmanned station should have a remote transmission device.
When the system changes (increasing or decreasing the line length), the tapping of the arc suppression coil should be adjusted according to the regulations (overcompensation, under compensation). At present, the development and change speed of the power grid is relatively fast, so that the capacitance current to the ground of many small current grounding systems changes rapidly, and manual operation is frequent. As the grid expands further, the capacitor current also exceeds 100A, and the operating current of the arc suppression coil exceeds 10% of the system capacitor current and the target of no more than 10A cannot be achieved. So people developed automatic compensation arc suppression coils. Briefly introduce two kinds:
One-tuned (pre-set)
It consists of an arc suppression coil with on-load tap-changer, a single-phase PT, a resistor cabinet with a short-circuit switch and a controller. Before the operation, the short-circuit switch is in the opening position. After the control cabinet is energized, a special frequency voltage is injected into the arc-suppression coil. The single-phase PT and the arc-suppression coil are attached with CT to measure the neutral point voltage and the capacitance current component, and calculate the system capacitance. Current value. The controller automatically adjusts the arc suppression coil tap to the tap closest to the system capacitor current. Since the loop string has resistance at this time, resonance cannot occur. The controller continuously measures the system capacitance current. When the system capacitance current changes, the controller automatically adjusts the arc suppression coil tap to be close to the system capacitor current. When the system is single-phase grounded (the bus voltage of the busbar PT opening reaches 30V or more), the controller shorts the resistance switch, the current of the arc suppression coil can cancel the capacitance current of the grounding point, the grounding point current is small, and the arc can be quickly extinguished. After the system ground is restored, the controller opens the switch short circuit and the resistor limits the system resonance.
Two tracking
It consists of a transformer with a short-circuit coil, a thyristor and a harmonic elimination cabinet and a controller. When the short-circuit coil is open, the current of the primary coil is small, which is the no-load current of the transformer. When the short-circuit coil is short-circuited, the current of the primary coil is the rated current of the arc-suppression coil. Controlling the conduction angle of the two-way thyristor can adjust the short circuit degree of the short-circuit coil, thereby adjusting the operating current of the arc-suppression coil. After the whole device is running, the controller can change the conduction angle of the thyristor continuously to obtain the working current point of different arc coils, and then measure the neutral point (opening triangle voltage) voltage of the current point to calculate the system's Capacitor current and memory. The controller measures and changes the memory at any time as the system's capacitive current changes. When the single-phase grounding occurs in the system (the bus voltage of the busbar PT opening reaches 30V or more), the conduction angle of the thyristor is quickly turned on according to the memory, and the capacitance current of the grounding point is compensated to be close to zero. The arc can be extinguished quickly. After the system ground is restored, the thyristor is turned off.
Resistance grounding system (including small resistance, medium resistance and high resistance)
The urban power grid is dominated by cables. The characteristic is that the capacitor current is very large, often reaching more than 100A. The population density is extremely high, and the step voltage is easy to injure people when single-phase is grounded. So use a resistor grounding system. 10kV neutral point to ground to connect 5-10Ω resistor for small resistance system, connect tens-100Ω resistor for medium resistance system, connect several hundred-1000Ω resistor for high resistance system. When the single-phase grounding occurs through the resistance grounding system, the grounding is relatively lowered (zero when metallic grounding), the ungrounded relative ground voltage is increased (the metallic voltage is the line voltage), and the grounding point current is the capacitive current and the resistance-limited The sum of the resistive currents. After the protection is judged, the circuit breaker disconnects the grounding point from the system. As the neutral point access resistance increases from small to large, the overvoltage generated during single-phase grounding gradually increases. At present, the 10kV system in Beijing uses a small resistance grounding system with a neutral point resistance of 10Ω.
When the small resistance grounding system is running, the grounding resistance must not be taken out of operation, and the two grounding resistors must not be operated in parallel for a long time. At present, 10kV systems in Beijing use grounding to access neutral point resistance. There are two ways to connect to ground. One way is to access the bus. This method is more complicated in the switching operation. Before the 10kV bus is energized, the neutral point resistor is put into the main switch. The neutral point resistance protection action removes the 10kV main switch, and the 10kV main switch auxiliary contact cuts the neutral point resistance switch. Therefore, a pressure plate must be installed in the auxiliary contact circuit of the 10kV main switch, and an unmanned substation needs to be connected in series with a remote control contact. Another way is to access the main transformer 10kV lead, only add a separate CT and protection, no additional circuit breakers (separate grounding and resistance can be installed near the main transformer). In this method, when the three-volume transformer adopts such wiring, a grounding accident will occur, and the transformer will be cut off, so that the medium voltage side can be powered off (self-injection).
When the small resistance grounding system is single-phase grounded, the grounding current is mainly resistive. Therefore, judging the magnitude of the current can determine whether the protected component is grounded, and overcurrent protection is generally used. When the fixed value is small, it is easy to receive the interference of the capacitor current. For this reason, the capacitance current of the component in Beijing is required to be less than 30A at any time (except for the switching operation). The distribution transformer housing (oil-filled) or iron core and housing (dry) in a small-resistance grounding system must be separated from the neutral point if the grounding resistance is greater than 4Ω.
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