When a short circuit occurs in the power supply network, a large short-circuit current may cause the electrical equipment to overheat or be damaged by the electric power, and the voltage in the network is greatly reduced, thereby destroying the normal operation of the electrical equipment in the network. To mitigate the consequences of a short circuit, it is necessary to calculate the short-circuit current to properly select electrical equipment, design relay protection, and select components that limit short-circuit current. Then, the point inrush current effective value Ic=1.5Id,=1.5*4.6=7.36KA, and the inrush current peak value ic=2.5Id=2.5*406=11.5KA. Other household electric appliance gree , https://www.greegroups.com
I. Calculation conditions
1. Assume that the system has an infinite capacity. After the user is shorted, the system bus voltage can be kept constant. That is, the calculated impedance is much larger than the system impedance.
Specific provisions: For the calculation of short-circuit current in 3~35KV-class power grid, it can be considered that the capacity of the system of 110KV and above is infinite. Just calculate the impedance of the network components of 35KV and below.
2. When calculating the short-circuit current in the high-voltage electrical appliance, only consider the reactance of the generator, transformer, and reactor, and ignore the resistance; for overhead lines and cables, only when the resistance is greater than 1/3 of the reactance Resistance, generally only count the reactance and ignore the resistance.
3. The short-circuit current calculation formula or calculation chart is calculated by three-phase short circuit. Because the short-circuit current of single-phase short-circuit or two-phase short-circuit is less than the three-phase short-circuit current. The electrical equipment capable of breaking the three-phase short-circuit current must be able to be disconnected. Single-phase short-circuit current or two-phase short-circuit current.
Simplified calculation method Even if some assumptions are set, it is very difficult to correctly calculate the short-circuit current, and it is not necessary for the average user. Some design manuals provide a simplified calculation chart. It saves the trouble of calculation. It is convenient to use. But what if you don't have a design manual at hand? Here is a calculation method for "mouth-style". Just remember that you can learn the short-circuit current calculation method if you remember 7 sentences.
Before you introduce simplified calculations, you must understand some basic concepts.
Main parameters
Sd three-phase short-circuit capacity (MVA) referred to as short-circuit capacity check switch breaking capacity
Id three-phase short-circuit current period component RMS (KA) referred to as short-circuit current check switch breaking current and thermal stability
IC three-phase short circuit first cycle full current RMS (KA) short name impulse current RMS check dynamic stability
Ic three-phase short circuit first cycle full current peak (KA) short name impulse current peak check dynamic stability
x reactance (Ω)
The system short circuit capacity Sd and the calculation point reactance x are the key.
2. Select a reference capacity (Sjz) and a reference voltage (Ujz) when calculating the value of the target. Convert each parameter in the short-circuit calculation to the ratio of the reference quantity of the parameter (relative to the reference amount), called The value of the standard (Electrician's home. This is the most special place for short-circuit current calculation, the purpose is to simplify the calculation).
(1) Baseline reference capacity Sjz = 100 MVA
The reference voltage UJZ is specified as 8. 230, 115, 37, 10.5, 6.3, 3.15, 0.4, 0.23 KV
With the above two items, the reference current of each level of voltage can be calculated, for example: UJZ (KV) 3710.56.30.4
Because S=1.73*U*I, IJZ (KA)1.565.59.16144
(2) The value of the standard value is calculated as the value of the standard value S* = S / SJZ. For example, when the short-circuit capacity of the 10KV bus is 200 MVA, its standard value capacity
S* = 200/100=2.
The voltage value is U*= U/UJZ ; The current value is I* = I/IJZ
3 Infinite large capacity system three-phase short-circuit current calculation formula Short-circuit current standard value: I * d = 1 / x * (reciprocal of the total reactance standard value).
Short circuit current RMS: Id= IJZ* I*d=IJZ/ x*(KA)
Inrush current RMS: IC = Id *√1 2 (KC-1)2 (KA) where KC impact coefficient, take 1.8
So IC = 1.52Id
Peak impulse current: ic =1.41* Id*KC=2.55 Id (KA)
When the secondary side of the transformer of 1000KVA and below is short-circuited, the impact coefficient KC is taken as 1.3.
At this time: the inrush current RMS IC = 1.09 * Id (KA)
Peak impulse current: ic =1.84 Id(KA)
With the above knowledge, the short-circuit current can be calculated. The formula is not much and simple. But the question is how to get the total reactance of the short-circuit point? For example: the reactance of the transformer of the regional substation, the reactance of the transmission line, the enterprise substation The reactance of the transformer, etc.
One method is to check the design manual, from which you can find the value of the reactance standard of common transformers, transmission lines and reactors. After obtaining the total reactance, calculate the short-circuit current by the above formula; there are some charts in the design manual, which can be directly Isolated short circuit current.
The following describes a "mouth-style" calculation method, as long as you remember 7 sentences, you can master the short-circuit current calculation method.
4. Simplified algorithm [1] System reactance calculation system reactance, one hundred megabytes is one. Capacity increase and decrease, reactance inverse ratio. Example of 100 system capacity: The reference capacity is 100 MVA. When the system capacity is 100MVA, the reactance of the system is XS*=100/100=1
When the system capacity is 200MVA, the reactance of the system is XS*=100/200=0.5
When the system capacity is infinite, the reactance of the system is XS*=100/∞=0
System capacity unit: MVA
System capacity should be provided by the local power supply department. When it is not available, the breaking capacity of the power supply outlet switch can be used as the system capacity. For example, it is known that the power supply outlet switch is W-VAC 12KV 2000A rated breaking current is 40KA. Then, the system capacity S=1.73*40*10000V=692MVA can be considered, and the reactance of the system is XS*=100/692=0.144.
[2] Calculation of transformer reactance
110KV, 10.5 in addition to transformer capacity; 35KV, 7 in addition to transformer capacity; 10KV{6KV}, 4.5 in addition to transformer capacity.
Example: The reactance of a 35KV 3200KVA transformer X*=7/3.2=2.1875
The reactance of a 10KV 1600KVA transformer X*=4.5/1.6=2.813
Transformer capacity unit: MVA
The coefficients 10.5, 7, and 4.5 here are actually the % of the transformer short-circuit reactance. Different voltage levels have different values.
[3] Calculated reactor reactance The rated reactance of the reactor is 10% off the rated capacity.
Example: There is a reactor U=6KV I=0.3KA rated reactance X=4%.
Rated capacity S=1.73*6*0.3=3.12 MVA. Reactor reactance X*={4/3.12}*0.9=1.15
Reactor capacity unit: MVA
[4] Calculation of overhead line and cable reactance overhead line: 6KV, equal to the number of kilometers; 10KV, take 1/3; 35KV, take 3%0
Cable: Multiply by 0.2 for the overhead line.
Example: 10KV 6KM overhead line. Overhead line reactance X*=6/3=2
10KV 0.2KM cable. Cable reactance X*={0.2/3}*0.2=0.013.
Simplified here, in fact, the reactance of overhead lines and cables is related to its cross section, and the larger the cross section, the smaller the reactance.
[5] Calculate the reactance of the short-circuit capacity and remove 100.
Example: It is known that the sum of the values ​​of the reactance standards of the components before the short-circuit point is X*∑=2, then the short-circuit capacity of the short-circuit point
Sd = 100/2 = 50 MVA.
Short circuit capacity unit: MVA
[6] Calculation of short-circuit current
6KV, 9.2 in addition to reactance; 10KV, 5.5 in addition to reactance; 35KV, 1.6 in addition to reactance; 110KV, 0.5 in addition to reactance.
0.4KV, 150 in addition to the reactance example: the sum of the reactance standards of each component before the short-circuit point is known as X*∑=2, and the short-circuit point voltage level is 6KV.
Then the short-circuit current of the short-circuit point Id=9.2/2=4.6KA.
Short circuit current unit: KA
[7] Calculation of short-circuit inrush current
When the secondary side of the transformer of 1000KVA and below is short-circuited: the effective value of the inrush current Ic=Id, the peak value of the inrush current is ic=1.8Id
When the secondary side of the transformer above 1000KVA is short-circuited: the effective value of the inrush current Ic=1.5Id, the peak value of the inrush current is ic=2.5Id
Example: Short-circuit current Id=4.6KA of known short-circuit point {1600KVA transformer secondary side},
It can be seen that the key to the calculation of the short-circuit current is to calculate the total reactance before the short-circuit point. However, it must include the system reactance.