One, 35kV protection configuration
1, the necessity of grounding change:
Due to the characteristics of photovoltaic power stations, long underground cables are used for the Internet to increase the grid-to-ground capacitive current. When the system capacitance current reaches a certain level, the grounding current and its arc generated by the ground fault will not be extinguished on its own, causing system overvoltage, and thus endangering the insulation of other Internet cable lines, causing breakdown of the cable insulation. Due to the large capacitance of the cable to the ground, it is difficult to effectively extinguish the grounding arc by using the arc suppression coil compensation method. Most photovoltaic power stations use the neutral point to solve this problem through a small resistance grounding method.
2, ground change role:
1) Intermittent extinguishment and re-ignition of single-phase grounding arcs will produce arc grounding over-voltage, its amplitude can reach 4U (U is the peak value of normal phase voltage) or higher, and the duration is long, it will affect the electrical equipment. Insulation causes great harm, resulting in breakdown at weak insulation, resulting in significant losses;
2) As the continuous arc causes the air to be freed, the insulation of the surrounding air is destroyed, and short circuits between the phases are likely to occur;
3) Ferro-resonance overvoltage is generated, which easily burns out the voltage transformer and causes damage to the arrester and may even cause the arrester to explode.
According to the above, when a single-phase grounding occurs on the line, the zero-sequence protection action of the line trips the switch. Therefore, there is no direct correlation between the grounding zero sequence protection and the line zero sequence protection. The zero-sequence protection of the grounding transformer is back-up protection of the line zero-sequence protection. Only when the line switch refuses to act, it will trip the main low-voltage side switch and isolate the line fault.
Second, pay attention to the protection settings
If a step-over trip occurs, the zero-sequence protection of the feeder cabinet will cause the low-voltage side of the main transformer to move, because the line current superposition problem is first considered.
Assume feeder feeder zero-sequence protection setting is 60A 0.5S
Grounding zero sequence protection setting 80A 0.8S
When the line 1 is grounded with high resistance, the zero-sequence current reaches 50A, and the protection setting cannot be reached and the operation cannot be performed. After this, line 2 is again grounded through high resistance, and the zero-sequence current reaches 50A. It does not reach the protective action value and cannot operate. However, due to the superposition of the two line currents, the grounding zero-sequence current reaches 100A.
It can be seen that when two lines are grounded at the same time, the current flowing through the ground is no longer equal to the line current, and the current flowing through the ground is more than twice the line. Therefore, the ground zero-sequence current and the zero-sequence current of the line can not be set with the conventional coefficient (Kk=1.2~1.5). Otherwise, the fault current may not exceed the line protection setting, and the grounding current exceeds its protection setting. The value of the case, then the ground zero sequence protection will malfunction
Solution:
In order to ensure that the line zero-sequence current protection has enough sensitivity when the line is earth faulted, and the two high-resistance line currents are superimposed, the grounding zero-sequence protection does not malfunction, and the grounding zero-sequence current must be greater than the line zero-sequence current protection setting. 2 times the value. Therefore, the grounding zero-sequence protection value remains unchanged, maintains 80A 0.8S, and the feeder zero-sequence protection setting is modified to 20A, 0.3S, so that even if a high-resistance grounding occurs, the grounding zero sequence protection will not occur. Feeder malfunction.
Third, standard cable shield grounding point:
For the 35kV feeder in the station, the shielded cable is used, and both ends of the cable are grounded at the same time (this is an effective electromagnetic anti-jamming measure). The 35kV feeder zero sequence CT adopts a through-type CT, and the zero-sequence CT passes through the feeder and is installed in the switch cabinet cable outlet. At the same time, using the principle of electromagnetic induction, an unbalanced current is generated when a ground fault occurs, and a current is induced on the zero-sequence CT to enable the protection action.
If the cable shield grounding wire is incorrectly grounded, when the feeder cable shield is grounded at both ends, the current flowing through the cable shield induced current will induce current on the zero-sequence CT, affecting feeder zero-sequence protection. It will cause zero-sequence protection of the grounding transformer over-level action or non-faulty line action.
Why regulate it? Because the value of the three-phase current added during grounding is the zero-sequence current, and the zero-sequence current will form a path through the grounding line. The zero-sequence CT senses the magnetic field generated when the current flows, if the grounding line does not In the middle of the zero-sequence CT, there is no induction from the zero-sequence CT, so there is no current; in another case, some friends say that when my CT is placed over the cable head, then my ground wire is not Wear it on?
It is also not possible because, as mentioned above, when the non-triple earthed, there is a zero-sequence current flowing through, the stripped cable shield is not grounded, and the zero-sequence CT is directly placed on the stripped three-phase cable as it can sense the current vector and The zero-sequence current magnetic flux can accurately protect the zero-sequence protection. Never use the zero-sequence CT over the cable head. Be smart and ground the bottom line through the zero-sequence CT. Grounding from the outside, this will cause the zero-sequence CT induced current to be zero due to cancellation of the positive and negative directions. In addition, the grounding cable must be insulated from the cable head to the entire section through the zero-sequence CT to prevent the CT from touching the cabinet. Body grounded and lost zero sequence protection.
The correct approach is:
1. When the cable shield grounding wire is above the zero-sequence CT, the cable shield ground wire must pass through the zero-sequence CT from top to bottom. When passing through the zero-sequence CT, the grounding wire should be insulated to prevent contact with other conductors or metals and grounding occurs.
2. When the cable shielding grounding wire is under the zero-sequence CT, the cable shielding grounding cable cannot pass through the zero-sequence CT.
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