How Should Emergency Generator Units Be Selected?
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Generator sets are mainly used in important places. In case of power outages due to accidents or emergencies, emergency generator sets can quickly restore power and extend the power supply time for a period of time. This type of electrical load is called a first-level load. Communication equipment, instruments and computer systems, etc., which have strict requirements for power-off time, in addition to being equipped with generator sets, should also be powered by batteries or UPS.
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The work has two characteristics: First, it is used for emergency purposes, and the continuous working time is not long, generally only requiring a few hours of continuous operation (≤12 hours); Second, it serves as a backup. The emergency generator set is usually in a state of shutdown and waiting. Only when the main power supply fails and the power is cut off will the emergency generator set start operation to supply the emergency power load. When the main power supply returns to normal, switch to shut down immediately.
1. Determination of the capacity of diesel generator sets
The rated capacity of the emergency diesel generator set is the 12-hour rated capacity corrected by the environment (such as altitude, atmospheric pressure and air humidity, etc.). Its capacity should be able to meet the total calculated load of emergency power supply, and it should be verified according to the requirement that the generator capacity can meet the starting requirements of the largest single capacity motor in the first-level load. Emergency generators generally choose three-phase AC synchronous generators, with a rated output voltage of 400V.
2. Determination of the number of diesel generator sets
When there are multiple generator sets on standby, generally only one emergency diesel generator set is set up. Considering reliability, it is also possible to select two generator sets to operate in parallel for power supply. The number of generator sets supplied for emergency use generally should not exceed three. When multiple diesel generators are selected, complete sets of equipment with the same model and capacity, similar voltage regulation and speed regulation characteristics should be chosen as much as possible. The nature of the fuel used should be consistent to facilitate operation, maintenance and repair as well as the sharing of spare parts. When there are two urgently needed generator sets, the self-starting device should enable the two units to serve as backups for each other. That is, after a delay confirmation due to a power failure of the municipal power supply, a self-starting command should be issued. If the first unit fails to start automatically for three consecutive times, an alarm signal should be sent out and the second unit should be automatically started.
3. Selection of characteristics of diesel generator sets
Emergency generator sets should preferably be high-speed, turbocharged, low-fuel-consumption, highly reliable and of the same capacity diesel generator sets. High-speed turbocharged diesel engines have a large single-unit capacity and occupy less space. It is better to choose diesel engines equipped with electronic or hydraulic speed regulation devices as they have excellent speed regulation performance. The generator should be equipped with a brushless excitation synchronous motor, which operates reliably, has a low failure rate and is relatively convenient for maintenance and repair. The unit is installed on a common chassis equipped with shock absorbers. A silencer should be installed at the outlet of the exhaust pipe to reduce the impact of noise on the surrounding environment.
4. Control of diesel generator sets
The control of diesel generator sets should be equipped with rapid self-starting and automatic engagement devices. When the main power supply fails and the power is cut off, the emergency unit should be able to start up quickly and restore power supply. The allowable power outage time for first-level loads ranges from tens of seconds to several tens of seconds, which should be determined according to specific circumstances. When the main power supply for an important project is cut off, there should first be a confirmation period of 3 to 5 seconds to avoid the time of instantaneous voltage drop and the closing of the municipal power grid or the automatic commissioning of the backup power supply. Only then should the command to start the emergency unit be issued. It takes some time from the issuance of the instruction, the start of the unit, the acceleration to the point where it can carry the load. For general large and medium-sized diesel generator sets, a pre-lubrication and warm-up process is also required to ensure that the total loss system oil (engine oil) pressure, total loss system oil (engine oil) temperature, and cooling water temperature during emergency loading comply with the technical requirements of the product. The pre-lubrication and warm-up process of the unit can be carried out in advance according to different situations. For instance, emergency response units for important foreign affairs events in post and telecommunications, military communications, large hotels, large-scale public activities at night in public buildings, and important surgical operations in hospitals should be in a pre-lubricated and warm-up state at all times, so as to be able to start up quickly at any time and minimize the time of power outages due to faults.
After the unit is put into operation, in order to reduce the mechanical and current shock during sudden load increases, under the condition of meeting the power supply requirements, emergency loads are best increased in stages according to time intervals. According to national standards and national military standards, the first allowable loading amount of an automated diesel generator set after successful self-starting: for those with a rated power not exceeding 250kW, it shall not be less than 50% of the rated load. For those with a calibrated power greater than 250kW, the technical conditions of the product shall apply. When the requirements for instantaneous voltage drop and transition process are not strict, the sudden load increase or sudden unloading of the general unit should not exceed 70% of the unit's rated capacity.