Energy selective technology for low voltage circuit breakers
The first author, Ge Darin, male, born in 1941, graduated from Beijing Mining Institute in 1964, professor-level senior engineer.
1 The selectivity of the low-voltage protection device 1.1 defines the use of circuit breakers to protect, control and isolate electrical equipment and feedthroughs in power supply systems. In a radiation distribution system (see), the so-called selective trip means that the faulty load or feeder loop is disconnected from the grid, while the non-fault loop continues to supply power. If there is no selectivity, an electrical fault can cause multiple circuit breakers at the power grid to trip, causing large-scale power outages in the power grid, including power failures in the fault-free line, causing significant economic losses. The main faults in the power supply system are: overload, short circuit, inrush current, ground fault, voltage drop or transient current due to instantaneous power failure. In the event of a line failure, in order to ensure continuous power supply in the fault-free part, the cooperation between the protection devices must be considered.
1.2 Enhancing safety and reliability The purpose of distribution protection equipment is to protect against overload, short circuit, grounding and undervoltage. Sometimes a fault will directly or indirectly cause several different types of protection devices to operate at the same time, such as: large short-circuit current causes the under-voltage protection device to operate due to the line voltage drop; the zero-sequence current appears to cause the leakage relay to operate; TN and IT The short circuit protection device in the system is considered to be an overcurrent fault. As another example, a large short-circuit current will also cause the grounding protection device to act (a pseudo-zero sequence current is generated due to local saturation of the zero-sequence current transformer).
Therefore, for a grid, the selective research and evaluation of the protection system is based on the operational characteristics of the protection equipment. First of all, starting from the requirements of various types of faults on the protection device, the possibility of cooperation between the protection device and the (actually given) fault is studied, and the selectivity is improved under the premise of life and property safety to ensure continuous power supply.
Full selectivity is defined as when a distribution line fails, non-selective means that when the fault current exceeds a certain value, the fully selective trip condition is no longer guaranteed (see).
The selectivity cannot be guaranteed, the performance guarantee is selected for no reason, and the fault-free returning part also stops the circuit to maintain the power supply. In the power distribution system, it usually encounters two types of faults: short circuit and overload. When the overcurrent is 1.110 times the working current, it is called overload, higher than At 10 times, it is called short circuit, and it is necessary to use a circuit breaker with a momentary or short time release to cut off the short circuit or overcurrent fault. Selective studies should treat different categories of faults differently.
Overload area. The region starts with a long delay (LT) action value Ilt, and the trip curve Tb=/(/p) is an inverse time curve, usually at the bottom of the cable thermal stress curve. Use the most common method to plot the LT action current versus trip time (see ). For a certain overcurrent value, the time when the upper circuit breaker CB1 does not trip should be greater than the maximum breaking time of the circuit breaker CB2 (including the arc extinguishing time) to ensure the selectivity. Generally, the two-stage circuit breaker ratio should satisfy the /lt//lt2> short-circuit area. The selectivity is analyzed by comparing the action curves of the upper and lower circuit breakers. When the short circuit occurs, the selectivity between the two circuit breakers is based on the circuit breaker trip current setting value and/or different types of trip mechanisms. The buckle curves do not cross each other and overlap.
2 Short-circuit selective technology 2.1 Current-selective current selectivity is realized by different action values ​​of adjacent two-stage instantaneous release (ST). The terminal distribution system adopts fast circuit breaker and instantaneous trip (no delay). Protects short-circuit faults, but is generally locally selective. When a short-circuit fault occurs in different parts of the grid, the fault current value is different due to the line impedance, so selective protection can be achieved (see). Since CB1 and CB2 are installed at different locations on the line, the selectivity can be achieved by different setting values ​​of the instantaneous release. The ratio of instantaneous tripping current of the two circuit breakers /ins//ins2>1.5. Selectively, the current time curves of the two circuit breakers do not intersect and coincide. In the case of large fault current, if the horizontal part of the two curves cannot be guaranteed to cross or coincide, that is, the instantaneous trip value of the upper circuit breaker CB1/the right part is not at the upper part of the other curve, in order to achieve full selectivity, the following shall be adopted Measures: 1 Generally use a circuit breaker with a time delay release. This method is limited to terminal power distribution, the lower circuit breaker acts as a current limiting type circuit breaker; 2 uses a selective circuit breaker, meaning: the circuit breaker release has a Fixed or adjustable delay system; systems and circuit breakers shall be capable of withstanding the thermal and electrical stresses generated by fault currents during time-delayed trips.
The two circuit breakers are cascaded. The trip time of the upper circuit breaker is greater than that of the lower circuit breaker. The total breaking time of the circuit breaker with time selectivity should be greater than 20ms. When the fault occurs, the delay can reach several hundred ms (see). When the system (or circuit breaker) cannot withstand large short-circuit currents within the delay time, the upper circuit breaker CBi must have a high-set instantaneous release (HIN). The large fault current is cut off by the upper circuit breaker.
The current limiting circuit breaker has a high arc voltage and can quickly break the short-circuit current, thus limiting the short-circuit current. At the same time, the larger the short-circuit current value, the faster the current limiting action and the trip.
The lower stage uses a current limiting circuit breaker to ensure selectivity between the two stages. With this technology, rapid current limiting can quickly remove fault currents and limit thermal and electrical stresses in the system.
2.3 Selective tripping between data breakers at all levels of circuit breaker trips in a logic-selective radial grid (see). The working principle is very simple. When the detection current is greater than the setting value of a certain trip unit, the logic wait command is sent to the upper stage trip unit. If the circuit breaker of the upper stage does not receive the waiting command, the trip unit instantaneously operates.
With this type of system, faults can be quickly removed no matter which level fails, and logic selective technology can also be used for high voltage power grids.
Different selective methods can usually be used together to ensure the reliability of the grid supply. According to the manufacturer's matching table for selective research, energy selectivity is a new technology that greatly simplifies the study of low-voltage selective trips, and it is possible to achieve full-selective coordination between the lines at all levels at the lowest cost.
3 Energy-selective energy selectivity means that the current limit of the lower-level circuit breaker limits the energy generated by the short-circuit current and is insufficient for the upper-level circuit breaker trip device to operate. Energy-selective technology is a patented technology of Schneider Electric, which makes the CompactNS series circuit breakers have unparalleled advantages in other similar products: its fast, high current-limiting performance increases the power of the device, and the short-circuit current is greater. The stronger, the greater the breaking capacity; the various stresses generated by the system short circuit are limited, and the duration and amplitude of the fault current are reduced.
3. Selection of working curve Normally, the tb=/(/p) curve, that is, the current-time curve is used to study the selectivity. For the current limiting circuit breaker, when the fault current is >25In, the energy curve is needed to study. (The breaking time is less than 10ms at 50Hz) and the selectivity should be studied according to transient phenomena rather than periodic phenomena. In order to understand the energy selectivity, the following needs to be explained: the current waveform and the corresponding breaking energy when the circuit breaker is disconnected: b=I2dt ("general use").
The sensitivity of the energy trip corresponds to the current pulse, indicating that the above characteristic can be used with the /2t=/(/p) curve (see). Currently, IEC 60947-2 specifies the use of such a curve for circuit breakers. The breaking energy I2t value (ie Eb) ranges from 104107 (A2° s), the corresponding fault current is 1100 kA, Eb is commonly used in the order of 103, and the current is in the order of 102. Assuming that the first half of the breaking current is equivalent to the half-cycle of the sine wave and has the same initial slope as the expected short-circuit current, the breaking energy is a function of Ip, expressed by the following formula: or one step to obtain the breaking time tvb and the limit The peak current value is a few.
The parallel diagonal lines in the middle represent a constant breaking time at a given frequency. The t=20ms diagonal line corresponds to the most common breaking time when the Ip is greater than the instantaneous action value and the tb=/ and /2t=/(/p) of the electronic trip circuit breaker with the contact repulsion action value, then 2 breaks Energy is a function of the expected short-circuit current; 3 contact repulsion value / r (C point) 4 ultimate breaking capacity (point F) 5 breaking time tvb as a function of short-circuit current; 6 current-limiting peak as a function of short-circuit current; 710ms current The value is the current limit start value.
The pressure of the pressure trip system is caused by the energy generated by the arc and acts on the breaker breaking mechanism, and only acts when the fault current value is higher than the action value. Arc energy expands the gas in the mechanism and opens a piston to trip the circuit breaker (see 0). The role of the pressure trip system: the selective circuit breaker has a self-protection function, which improves the breaking and working reliability of the fast current limiting circuit breaker.
0 Pressure Trip System Working Principle The circuit breaker is correctly equipped with a pressure trip system to ensure selectivity between circuit breakers of different rated values ​​when overcurrent >20In. This energy selective tripping technique is used on CompactNS current limiting circuit breakers.
4 Advantages of energy selectivity The trip unit in the circuit breaker should have the following properties: reduce the electric power generated by the fault current, meet the system limit I and I2t value requirements; reduce the fault current interference, reduce the line voltage drop; 4.1 current limit Type circuit breakers with pressure trip system The pressure trip system cooperates with an electromagnetic or electronic trip unit to meet the above requirements. 1 indicates the "energy sensitivity" of this combination. The I2t energy is constant. The larger the short-circuit current is, the shorter the response time is. The energy passed by the current-limiting breaker follows the same curve in the figure, but it is slightly offset.
(1) Stress in electrical systems. Compared with the previous generation current limiting circuit breaker, the electrical stress and thermal stress in the system are limited. According to the figure, when Ip, the breaking time is 4ms; the peak current is 40kA; 2t=8X105 (A2s).
1 trip unit comprehensive curve (electromagnetic ten pressure or electronic ten pressure) (2) trip reliability. The pressure trip system is part of the short circuit protection breaking mechanism and is therefore related to the rated current of the circuit breaker. Both the electromagnetic or electronic trip unit are independent of the pressure trip system, thus enhancing the reliability of the work.
The duration of the voltage drop generated by the short circuit in the grid is related to the breaking time of the circuit breaker, that is, the arc voltage is opposite to the phase of the power supply voltage and ends when the current is interrupted. Therefore, the magnitude and duration of the voltage drop is related to the type of circuit breaker (and trip unit). 2 (a) is a non-current limiting circuit breaker, the voltage drop is obvious, will last 1015ms; the current limiting circuit breaker greatly reduces the amplitude and duration of the voltage drop due to the rapid generation of high arc voltage. The voltage drop in 2(b) lasts for about 5ms, 50% of the rated voltage, and its current causes the contact to repel. For larger short-circuit current values, the voltage drop is 30% of the rated voltage, but the duration is reduced to 34ms, the larger the short-circuit current, the smaller the voltage drop and duration. Therefore, it does not work for the undervoltage release coil.
(4) Selectivity. Strictly restricting the energy of the circuit breaker so that the upper circuit breaker cannot trip, thus achieving the purpose of continuous power supply.
4.2 The selective energy-selective technology of the CompactNS circuit breaker should correctly select the ratio of the rated value between the upper and lower circuit breakers and the corresponding trip unit. The CompactNS series circuit breaker can achieve the full selection of the limit breaking capacity (b) high limit. The relationship between the network voltage drop of the circuit breaker 2 and the type of the circuit breaker (1) is fully selective. 3 630A, 250A and 100A kinds of circuit breakers are selected respectively. After the three-level power supply system adopts different trip units, full selectivity can be achieved when the short-circuit current value is below 100kA. If CompactNS is selected, the full selection can reach 150kA. The selectivity rule is: the ratio of the rated values ​​of the adjacent two-stage circuit breakers is >2.5; the ratio of the rated values ​​of the trip units >1.6.4 is the partial selectivity between the two ( 3) CompactNS cascading technology. The French standard NFC15-100 allows the upper circuit breaker to help the lower circuit breaker to break the large short-circuit current value. The essence is that the upper circuit breaker plays a role in blocking the large short-circuit current, so that the breaking capacity of the lower-level circuit breaker can be less than the installation. The expected short-circuit current value, but also the short-circuit current can be broken.
4.3 Coordination with traditional protection equipment In the existing system, if a high current limiting performance CompactNS circuit breaker is used instead of the existing circuit breaker, the selectivity can be improved.
Increase the selectivity level and even achieve full selectivity; if installed in the upper level, the selectivity level is at least equal to the previous level, but the cascading performance is enhanced due to the high current limiting capability.
5 Conclusions Circuit breakers with high current limiting performance, the greater the fault current, the faster the breaking speed, and the ability to achieve full selective tripping between grid levels through simple rules. This patented technology greatly simplifies selectivity studies, greatly limiting the electrical and thermal stresses generated by short-circuit currents and line voltage drops. This is a new energy-based technology that is achieved by controlling the energy during the breaking process of the circuit breaker. Selectivity, this technology is a major contribution to improving the reliability of the grid's continuous power supply.
Interactive tables and chairs for children provide fun and safety for children of all ages and sizes. The desk features an adjustable tilt surface from 0 to 40 degrees, and an organized pull-out drawer for paper, coloring books, coloring utensils, and more. This set is ergonomically designed to provide comfortable sitting and correct body posture, which are important for good body posture and health, even from an early age. This table and chair table is recommended for ages 3-14. Initial tabletop setup takes only a short time, and it's easy to move after assembly.
Kids Ergonomic Chair,Single Piece Study Chair,Moveable Study Chair,Orthopedic Study Chair
Igrow Technology Co.,LTD , https://www.igrowdesks.com