Why are some distribution lines only equipped with overcurrent protection and not quick break protec

1. The over-current protection action time limit is very short, which can meet the stability requirements of the system, and there is no need to install quick-break protection.

2. The line is too short, the quick-break protection area is too short or there is no protection area at all, and it is meaningless to install quick-break protection.

Overcurrent protection is a protection method that makes the protection device operate when the current exceeds a predetermined maximum value. When the current flowing through the protected element exceeds a certain pre-set value, the protection device is activated, and the selectivity of the action is ensured by a time limit, so that the circuit breaker trips or gives an alarm signal.

When the capacity of the distribution line is small, only overcurrent protection can be installed. The installation of speed break protection is mainly to protect the safety of the power supply circuit.

There is no time limitCurrent quick break protectionIt can't be the main protection, but it can be the backup protection.

In order to overcome this defect, the current quick break protection with a time limit is often used to protect the full length of the line The protection scope of the time limit quick break not only includes the full length of the line, but also penetrates into a part of the non-time limit protection of the adjacent line, and its action time limit is one step larger than the time limit protection of the adjacent line.

Difference between timeless current quick break protection:

When electrical equipment in the power grid fails, the current is short-circuited.

According to the basic operating principle of the relay, if the short-circuit current is set to the operating current of the relay by calculating the front land in advance, the faulty equipment can be protected. Shenji Xin overcurrent protection.

And current quick-break protection is based on this principle.

In order to ensure the selectivity of the action, according to the characteristics of the short-circuit current (the closer the fault point is to the power supply, the greater the short-circuit current), the over-current protection is with the action time limit, and the current quick-break protection does not have the action time limit, that is, when the short circuit occurs, it immediately acts and cuts off the fault, so it has no time limit characteristics, and is often used with the over-current protection.

The quick-break protection cannot protect the entire length of the line, and can only selectively protect a part of the line, and the remaining part is the dead zone of the quick-break protection. In order to avoid the above situation, the quick-break protection can also be made with a slight time limit, which is called the time-limited current quick-break protection. It works with timeless current quick break to eliminate the dead zone of current quick break protection.

Dear, hello, I am happy to answer for you, 10kV distribution line is only installed with overcurrent hidden answer, not installed with quick-break protection method, overcurrent that is, overcurrent protection. Three-stage current protection refers to a set of overcurrent protection mechanisms composed of current quick break protection (first stage), time-limited current quick break protection (second stage) and time-limited overcurrent protection (third stage). Section 1, the near area is short-circuited and trips in 0 seconds, which is generally protected to a distance from the exit of the bus side line; Section 2, with a time limit of about seconds to trip, generally protect the whole line, there may be a little extension; 3 sections, with a delay trip of n seconds, is generally set according to the maximum load current evaded, and the protection of the whole line includes a large part of the next stage, depending on the specific situation.

Faults in the power supply system such as lines and equipment will produce short-circuit currents. The short-circuit current is much larger than when the line is working normally, so this does not need to be explained too much. The current value is measured by the current transformer, and the duration of the current value, and the trip signal is output when the setting value is reached, which is the basic principle of overcurrent protection.

Therefore, the closer the matching current is to the power point, the greater the short-circuit current. Overcurrent section protection, also commonly known as quick-break protection. The current setting value of this protection is very large, and there is no setting time.

In other words, as soon as this current value is reached, the protection device must act immediately (the actual reaction speed is in the millisecond range)! However, in order to ensure the selectivity of the protection (the fault of the next line cannot make the protection of the higher level move), the quick break protection cannot protect the entire length of the line. So, don't look at its name as a section, the quick-break protection is not the main protection of the line!

When the first and end of the distribution line is short-circuited, the short-circuit current difference between the two is very small, if the quick-break protection is installed, the protection range is very small, or even there is no protection range, and the overcurrent protection can meet the system stability requirements as the main protection of the distribution line, so there is no need to install the quick-break protection.

The power line current quick-break protection is calculated and determined according to the setting of the three-phase maximum short-circuit current at the end of the line.

In fact, due to the influence of the natural environment meganucleus circle on the line and human factors, the contact resistance of the line is too large, especially when the short-circuit fault occurs near the end of the line, the short-circuit current is not the action value, and the current quick-break protection will not start.

The area on the line that cannot be protected by the current quick break is called the dead zone. When a short circuit accident occurs in the dead zone of current quick break protection, it is generally caused byOvercurrent protectionThe action trips, so the overcurrent protection is the backup protection of the current quick break protection family collapse.

Because the current quick-break protection can not protect the whole line, the over-current protection can protect the whole line, but can not meet the requirements of rapidity, then a set of time-limited current quick-break protection device can be added. For high-voltage power lines, the action time of time-limited current quick-break protection is generally seconds.

When the line is too long and the resistance is large, the short-circuit current at the end cannot reach the trip value, and it may not be tripped, so it cannot protect the full length of the line nucleus.

The current is quick to break that the current reaches this number of flashes and trips or extends for a short time, and it trips quickly. The tripping current of the overcurrent protection setting is smaller than the current quick break, and it will take a certain time to reach the tripping value, and it will trip if the current is still large.

Relay protection is a systematic project, a variety of protection to complete the cooperation, not only to avoid lightning and other instantaneous current, but also to be able to distinguish the location of the failure, a certain Fan Qin section excavation by a certain switch to trip; This requires setting different trip current values with different trip times to achieve this. The next level and this line are divided by a circuit breaker, and the line has a new setting after a circuit breaker.

The principle of current quick break protection

The basic principle of current quick break protection is to use the principle of electromagnetic induction, when the current in the circuit exceeds the rated value, the current quick break protection will produce a strong magnetic field, so that the current in the circuit is instantly disconnected. This is because when the current passes through the current quick break protection, a magnetic field will be generated inside the protector, and when the current exceeds the rated value, the magnetic field will become very strong, which will make the electromagnet inside the protector instantly attract and cut off the circuit.

In addition, the current quick break protection makes use of the principle of thermal effects. When the current in the circuit exceeds the rated value, the resistance inside the current quick break protection will generate heat, which will cause the thermal element inside the protector to change, thereby causing the trigger mechanism inside the protector to act and cut off the circuit.

1. Infinite current quick break protection is also the overcurrent protection section I, and the general protection range is 80% to 85% of the total length of the line. If its protection range is extended to the full length of the line, then if the protection range is extended to the next line due to the error of the TA itself, then the failure of the first end of the next line occurs, then the line and the next line overcurrent section I will be operated, and the protection will lose selectivity. So the full length cannot be protected.

2. The overcurrent section I is regarded as the main protection of the 10 thousand volt line, which requires the fault to be quickly cut off when the fault occurs (the quickness of the protection), so it cannot be delayed. Only the fallback protection is set for the delay.

These are three completely different mechanisms of action.

1: Overcurrent protection relies on the thermal components inside the circuit breaker, so overcurrent protection is also called thermal tripping.

Its action principle is basically the same as that of thermal relay and bimetal thermometer, the two conductive materials with a large difference in thermal expansion coefficient are bonded together, when the current is too large, the two materials will expand due to heating, the direct manifestation is that the length will become longer, the length of one side with a larger expansion coefficient increases more, and the other side is less, so that the whole composed of them is no longer flat, and will bend to the side with a smaller coefficient, and when bent to a certain extent, it will drive the mechanical structure to trigger the tripping mechanism, and then trip. This process is slower because it takes time for the metal to heat up and deform.

2: The current quick break protection relies on the principle of electromagnetic induction, also known as electromagnetic tripping.

The principle of action is relatively simple and crude! It is a coil in the main circuit of the circuit breaker, this coil is actually an electromagnet, when the working current in the circuit breaker is within the rated range, the magnetism generated by the electromagnet is not enough to attract the action of the tripping mechanism, but when there is a similar phase short circuit on the outlet side of the circuit breaker, a very large current will be generated instantly, and the current is large, the magnetism generated by the coil is strong, and then it is enough to attract the action of the tripping mechanism, and then trip!

3: The grounding fault actually depends on the principle of electromagnetic induction, and its action mechanism is also electromagnetic tripping, but it is completely different from the detection principle of quick-break protection, which detects the current in the main circuit, and the grounding fault detects the zero-sequence current, what is the zero-sequence current? For example, a 2P leakage protector, a fire and a zero, these two wires will pass through a current transformer together inside the circuit breaker, we all know that as long as there is current flowing through the wire, a magnetic field will be generated, we can measure the current value of the loop by detecting the strength of this magnetic field, but if you pass all the zero fire two wires of a circuit through a transformer, or pass all the three phases of the three-phase electricity through a transformer, the magnetic field of the zero live wire or the three-phase wire will cancel each other, At this time, the transformer cannot detect any magnetic field, so it will not induce current.

But if the live wire is grounded, then there will be a part of the current directly flowing into the earth, no longer through the zero line back, at this time their current is not the same, the magnetic field can not be offset, at this time the current transformer will detect the magnetic field, and then produce induced current, the induced current is very small, it will pass through an amplification circuit inside the circuit breaker (a small circuit board) to amplify, and then supply an electromagnet, this electromagnet will drive the trip mechanism action, and then leakage trip!

Under normal circumstances, the function of ordinary circuit breaker is also possessed by leakage circuit breaker, but there is one more function of zero sequence current detection, which is why the leakage circuit breaker has a large more piece than ordinary air opening, because there is a circuit board there, a separate electromagnet, and a transformer.

For high voltage, overcurrent protection is generally used for overload and short-circuit protection of lines or equipment, while current quick break is generally used for short-circuit protection. The over-current protection setting value is often small (generally only need to avoid the current caused by normal operation), and the operation has a certain delay; The current quick break protection generally has a large set value, and most of them are instantaneous actions.

Three-stage overcurrent protection includes:

1. Instantaneous current quick break protection (referred to as current quick break protection or current section) 2. Time-limited current quick break protection (current section).

3. Overcurrent protection (current section).

These three stages of protection constitute a complete set of protection.

They differ in the scope of protection:

1. Instantaneous current quick break protection: the protection range is less than the full length of the protected line, which is generally set to 85% of the total length of the protected line

2. Time-limited current quick break protection: the protection range is the full length of the protected line or 15% of the next circuit line 3. Overcurrent protection: the protection range is the full length of the protected line to the full length of the next circuit line.