1. Load calculation
According to the calculated load table, the load of two pumps running simultaneously is calculated as follows:
Since the water pumps are started one by one, the current at the start of a single unit:
(Used to select fuses, circuit breakers, adjust relay protection devices, and check motor self-starting conditions)
Pump cable selection:
Second, the selection of transformers
A single unit has a margin of 80% to meet the requirements. Substation with two main transformers, transformer selection:
Transformer optional: Capacity greater than 177.74 kVA. Because the mine already has SC-800/10/0.4 type transformers, it meets the requirements in terms of capacity.
Cable selection from the central distribution room to the central transformer room:
According to the economic current range, when the current of Yangzhuang cross-linked polyethylene insulating tape armored copper core cable is 19.48A,
Select the cross-sectional area of the conductor, when the cross-sectional area is for the three-shift current in East China, the current is between 29-41A and greater than 19.48A to meet the requirements of ampacity.
Due to the small distance between the central distribution room and the central substation, the voltage drop is negligible. Because there is no high-voltage electrical equipment underground, the cable from the diesel generator set on the well to the central distribution station underground is also YJV22-335, and the distance from the diesel generator set to the central distribution station underground is calculated to be about 1400 meters.
After checking the table: 10kV XLPE insulating tape armored power cable
3. Voltage loss calculation
Voltage loss:
4. Short circuit calculation
Short circuit protection: The physical process of three-phase short circuit in an infinite capacity power system, three-phase short-circuit current = periodic component + aperiodic component.
Namely:
As time tends to infinity, the aperiodic component tends to zero.
Namely:
Physical quantities related to short circuits:
Short-circuit steady-state current:
Unit value method: set the reference capacity, reference voltage - reference current - reference reactance, the standard unit value is the corresponding reactance value / reference reactance value Generally, it is only used for high-voltage circuit short circuit calculation.
Determine the reference value Set the reference capacity, reference voltage - reference current - reference reactance, take:
Reactance unit:
(1) The reactance unit of the power system, check the table to obtain the capacity of the circuit breaker:
(2) The unit value of the reactance of the overhead line, check the table:
(3) The unit value of the reactance of the power transformer is obtained:
In the case of K-1 in series:
(1) Total reactance unit:
(2) Three-phase current cycle component unit value:
(3) Other three-phase short-circuit currents are:
(4) The three-phase capacity is:
In the case of K-2 series and parallel:
(1) Total reactance unit:
(2) RMS value of three-phase current cycle component:
(3) Other three-phase short-circuit currents are:
(4) The three-phase capacity is:
5. Calculation of short circuit of low-voltage power grid
Often use ohm method: The impedance unit is often used ohm.
Usually the reactance includes: the impedance of all components, the impedance of the bus, the impedance of the primary coil of the current transformer, the contact resistance of each switch contact in the low-voltage line, and the smaller impedance can be ignored. When only one or two phases in the low-voltage line are equipped with a current transformer, the impedance of the primary coil of the current transformer is not included when calculating the three-phase short-circuit current, but its impedance should be included in the calculation of the circuit current of the calibration current transformer.
Example: A 10kV incoming line is connected to the bus bar through the transformer disconnector, and then multiple loops are separated, and the circuit has a circuit breaker and a current transformer with short circuit calculation.
(1) Calculate the resistance and reactance of each element in the short-circuit circuit:
1) The reactance of the power system S:
2) The electrical impedance check table of the power transformer T is obtained:
3) Resistance and reactance of bus WB1: check table according to the wire section spacing selection:
4) Resistance and reactance of bus WB2: check table according to the wire section spacing selection:
5) Resistance and reactance of bus WB3: check table according to the wire section spacing selection:
6) The resistance reactance table of the primary coil of the current transformer TA is obtained:
7) The resistance and reactance of the QF overcurrent coil of the low-voltage circuit breaker are obtained:
8) The contact resistance of each switch contact in the line, check the appendix table: the contact resistance of the disconnector QS is 0.03m?, the contact resistance of the knife switch QK is 0.4m?, and the contact resistance of the low-voltage circuit breaker QF is 0.6m?, so the total contact resistance is:
9) Resistance and reactance of VLV-1000-350 of low-voltage cable:
(2) Calculate the total resistance, reactance and total impedance of the short-circuit circuit:
(3) Other three-phase short-circuit currents are:
(4) The three-phase capacity is:
Calculation of two-phase and single-phase short-circuit currents in an infinite large-capacity power system:
(1) Calculation of two-phase short-circuit current
(2) Calculation of single-phase short-circuit current
Sixth, the selection of diesel generator sets
(1) Model capacity estimation of diesel generator set:
(1) Estimate the capacity of diesel generator sets according to 10%-20% of the transformer capacity, and take 15% to obtain:
(2) Calculated according to the capacity of starting a single or group motor:
7. Cable section selection
For high-voltage lines of 10kW and below and low-voltage power lines below 1kW, the conductor or cable section is usually selected according to the heating conditions, and then the voltage loss and mechanical strength are verified.
Low-voltage lighting line, due to the high voltage level requirements, first select according to the allowable voltage loss, and then verify the heating conditions and mechanical strength.
For long-distance high-current lines and high-voltage lines above 35kV, first select a section according to the economic current density, and then verify other conditions.
8. Disconnector selection
10kW circuit design product selection calculation:
(1) High-voltage disconnector: When overhauling high-voltage equipment, the power supply can be disconnected.
(2) Select according to normal conditions: voltage, current, frequency, mechanical load.
(3) Verification according to short-circuit stability: rated dynamic stability current, thermal stability current and continuous current.
(4) Overvoltage capacity: insulation level between ground and fracture, external insulation creepage ratio.
(5) Environmental conditions: ambient temperature, daily temperature difference, maximum wind speed, relative humidity, pollution, altitude, seismic intensity.
9. Selection of fuses
Calculation of the selection of fuses:
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