Selection and Application of Breaker Segmentation Capability

In recent years, I have consulted and discussed with users of circuit breakers, and read some articles on the selection of circuit breakers in professional publications. The lack of publicity has caused users to have some biases in choosing low-voltage circuit breakers. Based on this, the author intends to discuss the selection and application of circuit breakers again, in order to attract others and remove the false and preserve the true.

一、Calculate the expected short-circuit current of the line to select the breaking capacity of the circuit breaker.

The calculation of the exact line expected short-circuit current is an extremely tedious task. Therefore, there are some simple calculation methods with small errors and acceptable engineering:

(1) For a transformer with a voltage level of 10/0.4kV, it can be considered that the short-circuit capacity of the high-voltage side is infinite (the short-circuit capacity of the 10kV side is generally 200~400MVA or even greater, so considering the infinite value, the error is less than 10%) .

(2) Article 2.1.2 of GB50054-95 "Code for Design of Low-Voltage Power Distribution" stipulates: "When the sum of the rated current of the motor connected to the short-circuit point exceeds 1% of the short-circuit current, the influence of the feedback current of the motor shall be included. ”, if the short-circuit current is 30kA, take 1% of it, it should be 300A, the total power of the motor is about 150kW, and the feedback current should be 6.5∑In when it is started and used at the same time.

(3) The impedance voltage Uk of the transformer means that the secondary side of the transformer is short-circuited (circuit). When the secondary side reaches its rated current, the primary side voltage is a percentage of its rated voltage. Therefore, when the primary voltage is at the rated voltage, the secondary current is its expected short-circuit current.

(4) The rated current of the secondary side of the transformer Ite=Ste/(1.732*Ue) where Ste is the capacity of the transformer (kVA), Ue is the rated voltage of the secondary side (no-load voltage), and at 10/0.4kV Ue= 0.4kV Therefore, the rated current of the secondary side of the transformer should be simply calculated as the transformer capacity × (1.44 ~ 1.50).

(5) According to the definition of Uk in (3), the short-circuit current of the secondary side (three-phase short-circuit) is the definition of Uk by I(3), and the short-circuit current of the secondary side (three-phase short-circuit) is I(3)=Ite /Uk, this value is AC RMS.

(6) Under the same transformer capacity, if there is a short circuit between the two phases, then I(2)=1.732I(3)/2=0.866I(3)

(7) The above calculations are all the current values when the transformer outlet is short-circuited, which is the most serious short-circuit accident. If the short-circuit point is some distance from the transformer, the line impedance needs to be considered, so the short-circuit current will be reduced.

For example, the SL7 series transformer (with three-core aluminum wire cable) has a capacity of 200kVA. When the transformer outlet is short-circuited, the three-phase short-circuit current I(3) is 7210A. When the distance between the short-circuit point and the transformer is 100m, the short-circuit current I(3) is reduced to 4740A; when the transformer capacity is 100kVA, the short-circuit current at the outlet end is 3616A. When the distance from the transformer is 100m, the short-circuit current is 2440A. When the distance is 100m, the short-circuit current is 65.74% and 67.47% of 0m, respectively. Therefore, when designing, the user should calculate the rated current of the installation (line) and the maximum short-circuit current that may occur there. And select the circuit breaker according to the following principles: the rated current of the circuit breaker In ≥ the rated current IL of the line; the rated short-circuit breaking capacity of the circuit breaker ≥ the expected short-circuit current of the line. Therefore, in the selection of circuit breakers, it is not necessary to put too much margin in order to avoid waste. The calculation of the exact line expected short-circuit current is an extremely tedious task. Therefore, there are some simple calculation methods with small errors and acceptable engineering:

(1) For a transformer with a voltage level of 10/0.4kV, it can be considered that the short-circuit capacity of the high-voltage side is infinite (the short-circuit capacity of the 10kV side is generally 200~400MVA or even greater, so considering the infinite value, the error is less than 10%) .

(2) Article 2.1.2 of GB50054-95 "Code for Design of Low-Voltage Power Distribution" stipulates: "When the sum of the rated current of the motor connected to the short-circuit point exceeds 1% of the short-circuit current, the influence of the feedback current of the motor shall be included. ”, if the short-circuit current is 30kA, take 1% of it, it should be 300A, the total power of the motor is about 150kW, and the feedback current should be 6.5∑In when it is started and used at the same time.

(3) The impedance voltage Uk of the transformer means that the secondary side of the transformer is short-circuited (circuit). When the secondary side reaches its rated current, the primary side voltage is a percentage of its rated voltage. Therefore, when the primary voltage is at the rated voltage, the secondary current is its expected short-circuit current.

(4) The rated current of the secondary side of the transformer Ite=Ste/(1.732*Ue) where Ste is the capacity of the transformer (kVA), Ue is the rated voltage of the secondary side (no-load voltage), and at 10/0.4kV Ue= 0.4kV Therefore, the rated current of the secondary side of the transformer should be simply calculated as the transformer capacity × (1.44 ~ 1.50).

(5) According to the definition of Uk in (3), the short-circuit current of the secondary side (three-phase short-circuit) is the definition of Uk by I(3), and the short-circuit current of the secondary side (three-phase short-circuit) is I(3)=Ite /Uk, this value is AC RMS.

(6) Under the same transformer capacity, if there is a short circuit between the two phases, then I(2)=1.732I(3)/2=0.866I(3)

(7) The above calculations are all the current values when the transformer outlet is short-circuited, which is the most serious short-circuit accident. If the short-circuit point is some distance from the transformer, the line impedance needs to be considered, so the short-circuit current will be reduced.

For example, the SL7 series transformer (with three-core aluminum wire cable) has a capacity of 200kVA. When the transformer outlet is short-circuited, the three-phase short-circuit current I(3) is 7210A. When the distance between the short-circuit point and the transformer is 100m, the short-circuit current I(3) is reduced to 4740A; when the transformer capacity is 100kVA, the short-circuit current at the outlet end is 3616A. When the distance from the transformer is 100m, the short-circuit current is 2440A. When the distance is 100m, the short-circuit current is 65.74% and 67.47% of 0m, respectively. Therefore, when designing, the user should calculate the rated current of the installation (line) and the maximum short-circuit current that may occur there. And select the circuit breaker according to the following principles: the rated current of the circuit breaker In ≥ the rated current IL of the line; the rated short-circuit breaking capacity of the circuit breaker ≥ the expected short-circuit current of the line. Therefore, in the selection of circuit breakers, it is not necessary to put too much margin in order to avoid waste.

二、 The ultimate short-circuit breaking capacity and operating short-circuit breaking capacity of the circuit breaker.

The IEC947-2 of the International Electrotechnical Commission and my country's equivalent use IEC's GB4048.2 "Low-voltage switchgear and control equipment - Low-voltage circuit breakers" standard, which defines the ultimate short-circuit breaking capacity and operating short-circuit breaking capacity of circuit breakers as follows:

The rated ultimate short-circuit breaking capacity (Icu) of the circuit breaker: according to the conditions specified in the specified test procedure, excluding the breaking capacity of the circuit breaker that continues to carry its rated current capacity;

The rated operating short-circuit breaking capacity (Ics) of the circuit breaker: according to the conditions specified in the specified test procedure, including the breaking capacity of the circuit breaker to continue to carry its rated current capacity.

The test procedure of the ultimate short-circuit breaking capacity Icu is o-t-co. The specific test is: adjust the current of the line to the expected short-circuit current value (such as 380V, 50kA), but the test button is not closed, and the tested circuit breaker is closed. position, press the test button, the circuit breaker passes 50kA short-circuit current, the circuit breaker opens immediately (OPEN abbreviated as O) and extinguishes the arc, the circuit breaker should be in good condition and can be closed again. t is the intermittent time (rest time), which is generally 3 minutes. At this time, the line is in a hot standby state, and the circuit breaker is connected again (CLOSE for short C) and then broken (O) (the connection test is to assess the circuit breaker. Electrical and thermal stability at peak currents and wear of moving and static contacts due to bouncing). This procedure is called CO. If the circuit breaker can completely break, extinguish the arc, and do not exceed the specified damage, it is deemed that its ultimate breaking capacity test is successful.

The test procedure for the operating short-circuit breaking capacity (Ics) of the circuit breaker is o-t-co-t-co, which is one more co than the test procedure for Icu. After the test, the circuit breaker can completely break and extinguish the arc without exceeding the specified damage, and it is determined that its rated short-circuit breaking capacity test has passed.

After the Icu and Ics short-circuit breaking tests, tests such as withstand voltage and protection characteristics recalibration should be carried out. Since the rated current must be carried after the operation of the short-circuit breaking, a re-test of the temperature rise needs to be added after the Ics short-circuit test. Icu and Ics have different short-circuit or actual assessment conditions, the latter is stricter and more difficult than the former, so IEC947-2 and GB14048.2 determine that Icu has four or three values, which are 25%, 50%, 75% and 100% Icu (for Class A circuit breakers, that is, molded case type) or 50%, 75%, 100% Icu (for Class B circuit breakers, that is, frame type). The Ics value determined by the manufacturer of the circuit breaker, all the Icu percentage values that meet the above standards are valid and qualified products.

Frame type circuit breakers, most (not all specifications) have three-stage protection functions of overload long-delay, short-circuit short-delay and short-circuit transient, which can achieve selective protection, so most main lines (including the outgoing line of the transformer) It is used as the main (protection) switch, and the molded case circuit breaker generally does not have the short-circuit short-time delay function (only overload long-time delay and short-circuit instantaneous two-stage protection), and cannot be used for selective protection. Can be used for branch circuits. Due to the different use (applicable) situations, IEC92 "Ship Electrical" recommends that the universal circuit breaker with three-stage protection should focus on its operating short-circuit breaking capacity value, while a large number of molded case circuit breakers used in branch lines ensure that it has sufficient The limit short-circuit capacity value.

Our understanding of this is that it is necessary to be careful when replacing the circuit breaker after the main line has cut off the fault current. The main line blackout will affect a large number of users. Therefore, two COs are required in the event of a short-circuit fault, and they are required to continue to carry the rated current for a period of time. The branch circuit has completed its mission after the breaking of the limit short-circuit current and the closing and opening again. It no longer carries the rated current and can be replaced with a new one (the impact of power failure is small). However, whether it is a frame type or molded case circuit breaker, there must be two important technical indicators of Icu and Ics. Only the Ics value is slightly different between the two types of circuit breakers. The minimum allowable Ics of the molded case type can be 25% Icu, and the minimum allowable Ics of the frame type is 50%. The circuit breaker with Ics=Icu is very few, even if the frame The formula is rarely Ics=100%Icu. (There is a foreign molded case circuit breaker using rotary double breaking (point) technology, which has excellent current limiting performance and a large margin of breaking capacity, which can achieve Ics=Icu, but the price is very high).

The Ics of my country's DW45 intelligent universal circuit breaker is 62.5% to 65% Icu. Internationally, ABB's F series and Schneider's M series are only about 70%. Model, Ics is probably between 50% to 75% Icu. When designers of some circuit breaker applications select circuit breakers based on their calculated expected short-circuit currents, they use the rated short-circuit breaking capacity of the circuit breaker to measure, and thus determine that a certain circuit breaker (the ultimate short-circuit capacity of this circuit breaker is greater than The expected short-circuit current of the line, and the operating short-circuit breaking capacity is lower than the calculated current) is unqualified. This is a misunderstanding.

For example, a transformer with a capacity of 1600kVA, the rated current of the secondary side is 2312A, the impedance voltage percentage Uk is 6%, the maximum expected short-circuit current should be 38.5kA, and the rated short-circuit breaking capacity of the circuit breaker for protection should be 40kA , If you choose 2500A of DW15-2500Y or 2500A of DW45-3200 as the main switch, it is competent. Since modern transformers and distribution cabinets are very close to each other, even installed together, so even if the branch circuit has a rated current of 100A, its expected short-circuit current is very large. Therefore, it is also required that the short-circuit breaking capacity of the molded case circuit breaker in the line should reach 380V and 40kA. Some articles concluded that a new type of molded case circuit breaker (frame current 160A, Icu380V, 50kA, Ics380V, 35kA) cannot be selected because its Ics is only 35kA, which is less than the expected line current of 38.5kA. This is a misunderstanding. This type of circuit breaker is used for branch circuits. Even though the short-circuit current through the branch circuit is 38.5kA, the Icu of this circuit breaker is 50kA, which is fully capable. Therefore, to judge whether the molded case circuit breaker is competent for a certain line protection switch, it is to see whether its Icu is greater than the expected short-circuit current of the line. And even if its Ics is smaller, it will not hinder its function. Because of various short-circuit accidents, such as two-phase short-circuit (the short-circuit current is three-half of the three-phase short-circuit value), or places far away from the power supply, such as 50m, 100m, even if it is a three-phase short-circuit, due to the impedance difference The reason is that when the three-phase is short-circuited, the accident current is about 50% to 60% of the three-phase maximum expected value.

三、The electrical clearance and creepage distance of the circuit breaker.

To determine the electrical clearance of electrical products, it must be based on the insulation coordination of the low-voltage system, and the insulation coordination is established when the instantaneous overvoltage is limited to the specified impulse withstand voltage, and the instantaneous overvoltage generated by the electrical appliances or equipment in the system must also be low. Impulse voltage specified in the power system. therefore:

(1) The rated insulation voltage of the electrical appliance should be ≥ the rated voltage of the power supply system

(2) The rated impulse withstand voltage of the electrical appliance should be ≥ the rated impulse withstand voltage of the power supply system

(3) The transient overvoltage generated by the electrical appliance should be less than or equal to the rated impulse withstand voltage of the power supply system.

Based on the above three principles, the rated impulse withstand voltage (priority value) Uimp of the electrical appliance has a great relationship with the maximum relative-to-ground voltage determined by the rated voltage of the power supply system and the installation category (overvoltage category) of the electrical appliance: The larger the relative ground voltage value, the higher the installation category [is divided into I (signal level), II (load level), III (distribution level), IV (power level)], and the higher the rated impulse voltage is. .

For example, when the relative ground voltage is 220V and the installation category is III, the Uimp is 4.0kV. If the installation category is IV, the Uimp is 6.0kV. The Uimp of electrical products (such as circuit breakers) is 6.0kV pollution degree 3 or 4, and the minimum electrical clearance is 5.5mm. The electrical clearance of DZ20 and CM1 series molded case circuit breakers are both 5.5mm (installation category III), which is only used for power level installation. For example, if the DZ20 series has a specification of 800 or higher, and the Uimp is 8.0kV, the electrical clearance is increased to ≥8mm. The actual clearance of the product, such as HSM1 series, when Inm (frame current) = 125.A, the clearance is 11mm, 160A is 16mm, 250A is 15mm, 400A is 18.75mm, 630 and 800A are both 300mm, are larger than 5.5mm.

Regarding the creepage distance, GB/T14048.1 "General Rules for Low-Voltage Switchgear and Control Equipment" stipulates: the minimum creepage distance and rated insulation voltage (or actual working voltage) of electrical appliances (products), the pollution level of the place where electrical appliances are used, and the product It is related to the nature of the insulating material used (insulation group). For example: the rated insulation voltage is 660(690)V, the pollution degree is 3, the insulation material group used in the product is IIIa (175≤cti<400, cti="" is="" the="" tracking="" index="" of="" insulation="" material="">, the minimum creepage distance is 10mm The creepage distances of the molded case circuit breakers mentioned above all greatly exceed the specified values.

To sum up, if the electrical clearance and creepage distance of electrical products meet the requirements of insulation coordination, the dielectric breakdown of the equipment will not be caused by external overvoltage or operating overvoltage of the circuit equipment itself. GB7251.1-1997 "Low-Voltage Complete Switchgear and Control Equipment Part 1: Type Test and Partial Type Test Complete Equipment" (equivalent to IEC439-1:1992), the requirements for insulation coordination are completely the same as those of GB/T14048.1 the same.

Some complete electrical appliance manufacturers propose that the copper bars for circuit breaker wiring should have a (air) distance between phases greater than 12mm, and some even propose that the electrical clearance of circuit breakers should be greater than 20mm. This requirement is unreasonable, it has exceeded the requirements of insulation coordination. For high current specifications, in order to avoid electrodynamic repulsion when short-circuit current occurs, or the conductor heats up when high current occurs, in order to increase the heat dissipation space, it is also possible to appropriately widen the space distance between phases. At this time, whether it reaches 12mm or 20mm, it can be solved by the complete set of electrical appliance manufacturers, or the electrical component factory can be asked to provide connecting terminals or connecting plates (pieces) with elbows. Generally, when the circuit breaker leaves the factory, arc shields are provided between the phases of the power supply terminal to prevent the short circuit between phases when the arc is ejected. In order to prevent ionized molecules from escaping when the short-circuit current is interrupted, the circuit breaker with zero flashover is also installed with this arc-shielding plate. If there is no arc shield, the bare copper bar can be wrapped with insulating tape, and the distance should not be less than 100mm.

四、 the application of four-pole circuit breaker

Regarding the application of four-pole circuit breakers, at present, there are no strict requirements for the use of national standards or regulations in China. Although the design specifications for regional four-pole electrical appliances (circuit breakers) have been issued The debate on pole electrical appliances is still going on, and the use in some areas has been swarming in recent years. Various circuit breaker manufacturers have also designed and manufactured various types of four-pole circuit breakers and put them on the market. The author agrees with an opinion that whether to use or not should depend on whether the reliability and safety of the power supply can be ensured, so in general:

(1) TN-C system. In the TN-C system, the N line and the protection line PE are combined into one (PEN line). Considering safety, it is not allowed to disconnect the PEN line at any time, so the four-pole circuit breaker is absolutely prohibited;

(2) TT system, TN-C-S system and TN-S system can use four-pole circuit breaker to ensure the safety of maintenance personnel during maintenance, but in TN-C-S and TN-S systems, the N pole of the circuit breaker can only be connected to N line, but not PEN or PE line;

(3) In the place where dual power switch is installed, since all neutral lines (N lines) in the system are connected, in order to ensure the maintenance safety of the switched power switch (circuit breaker), a four-pole circuit breaker must be used;

(4) For the single-phase main switch entering the residence, a two-pole circuit breaker with N pole should be selected (used as an isolator during maintenance);

(5) For the residual current protector (leakage circuit breaker) of the 380/220V system, the neutral wire must pass through the zero-sequence current transformer (iron core) of the protector to prevent the passage of no neutral wire and make the 220V load In case of malfunction due to leakage current, a four-pole or two-pole residual current protector with neutral wire should be used at this time.