There is currently no unified calculation formula in the country for selecting the capacity of self-provided diesel generator sets, so the methods used by designers in actual work are also different: some simply determine it based on 10%-20% of the power transformer capacity; Some are added according to the capacity of fire-fighting equipment; some are selected according to the wishes of investors, which results in inaccuracy in selecting the capacity of self-provided generating units. If the capacity is selected too large, it will cause a waste of investment; if the capacity is selected too small, it will cause a waste of investment in the event of an accident. Cannot meet usage requirements. So, how to choose the capacity of self-provided generating units? After reviewing the information and referring to various experiences and practices of peers, the author summarized the following points:
(1) Plan or preliminary design stage
The capacity of the self-provided generator is calculated as 10%-20% of the total capacity of the power supply transformer.
(2) Construction drawing stage
(1) Select generator capacity according to calculated load
We know that the electrical load of a building can be divided into three categories: The first category is security load, which is a load that ensures the safety and reliable operation of people and equipment in the building, such as fire pumps, fire elevators, smoke prevention and exhaust equipment, emergency Lighting, communication equipment, important computers and related equipment, etc.; the second category is support load, that is, the basic equipment load to ensure the operation of the building, mainly work area lighting, some elevators, passage lighting; the third category is general load, that is Loads other than the above loads, such as air conditioners, water pumps and other general lighting and power equipment. When calculating the capacity of self-provided generating units, the first type of load must be taken into account, and the second type of load is determined according to the building function and power grid conditions. If the building function requirements are higher or the city power grid power supply is unstable, the second type of load should be considered. Class loads are taken into account, but if the first and second class loads are simply added to select the generator capacity, the selected capacity will be too large, because in the fire-fighting state, only the operation of the fire-fighting equipment needs to be ensured, and the second class load The load is not used; and when the power grid is outage in non-fire-fighting conditions, the fire-fighting equipment is not used. Therefore, the author believes that the larger of the two can be selected as the capacity of the generator set.
After the equipment capacity is calculated, the required coefficient Kx is selected according to the actual situation (generally 0.85-0.95), and the calculated capacity Pj=kxP∑ is calculated. The power of the self-provided generator set is calculated according to the following formula
Calculate P=kPj/η
In the formula: p - the power of the self-provided generator set kw;
Pj—the computing capacity of the load equipment kw;
P∑—Total load kw; η—The uneven coefficient of parallel operation of generators is generally 0.9, and 1 for a single unit;
k—Reliability coefficient, generally taken as 1.1.
(2) Calculate the generator capacity according to the starting needs of the largest single motor or group of motors
P=(P∑- Pm)/η∑+ Pm·K·C·cosψm(KW)
Pm—the capacity of the motor or group of motors with the largest starting capacity (kw); η∑—the calculation efficiency of the total load, generally 0.85;
cosψm—the starting power factor of the motor, generally 0.4;
K—starting multiple of the motor;
C—full voltage starting C=l.0, Y—△ starting C=0.67, autotransformer starting 50% tap
C=0.25, 65% tap C=0.42, 80% tap C=0.64.
(3) Calculate the generator capacity based on the allowable voltage drop of the busbar when starting the motor
P=Pn·K·C·Xd″(1/△E- 1)(kw)
Pn—the capacity of the motor or group starting motor unit that causes the largest bus voltage drop (kw);
K—starting current multiple of the motor;
Xd"—transient reactance of the generator, generally taken as 0.25;
△E—The allowable instantaneous voltage drop of the busbar is taken as 0.20 when there is an elevator and 0.25 when there is no elevator.
In actual work, the coefficient method can also be used to estimate the starting capability of the diesel generator set. Table 1 lists the required generator power under different starting modes as the minimum multiple of the power of the started motor for reference.
In recent years, variable frequency starting devices have become more and more widely used in civil buildings. Compared with other starting methods, variable frequency starting has a small starting current and a large starting torque. There is no impact current on the power grid and the busbar voltage drop caused is also very small. Therefore, The author believes that when the motor is started using variable frequency speed regulation, only the calculated load can be considered to calculate the capacity of the generator without considering the factors of motor starting.
Project example: Take a project designed by the author as an example. The project has a construction area of 10,000m2 and 12 floors. It is a second-class high-rise building. The security load is mainly fire-fighting load. Its capacity is 191kw. The largest motor is a sprinkler pump of 37kw. It adopts Auto-coupling step-down 80% tap step-down starting.
(1) Calculated according to calculation load
P=kPj/η=1.1x191/1kw=210.1 kw
(2) Calculation of the largest single motor starting requirements
P=(P∑- Pm)/η∑ +Pm·K·C·cosΨm
=(191—37)／0.85+37x6x0.64x0.4
=238.0 kw
(3) Calculation based on the allowable voltage drop of the generator bus when starting the motor
P=Pn·K·C·Xd″(1/△E- 1)
=37x6x0.64x0.25(1/0.20- 1)
=142.08 kw
Based on the above calculation, a generator set with ≥238.0 kw should be selected, so a 250kw generator set can be selected.