Efficiency, parameters and environmental factors of generators

Abstract: Generator sets efficiency refers to the energy conversion efficiency in the process of converting the heat energy generated by diesel combustion into kinetic energy through the diesel engine, and then converting the kinetic energy into electrical energy through the electric bulb (such as the Stamford alternator). Generally speaking, diesel generator sets will generate a lot of friction during operation, which will waste energy and reduce the life of the generator, resulting in reduced generator efficiency; at the same time, due to the influence of harsh environment, the alternator efficiency will be further reduced.

I. Basic parameters of alternator

1. Standards met by alternators

Each country has its own relevant alternator standards. Stamford alternator can meet most of the general international standards and relevant standards of some countries.

(1) Land standards:

● IEC 34-1 International Electrotechnical Commission Standard;

● GB 755 General Technical Conditions for Rotating Electric Machines in China;

● BC 5000 British Standard;

● NEMA MG 1-22 North American Standard;

● C.S.A C22-2 Canadian Standard;

● UDE0530 German Standard.

(2) Marine standards:
● Lloyd's Register of Shipping;
● Det Norske Veritas;
● Bureau Veritas;
● Germanischer Lloyd;
● Registrar Italiano Navale.
(3) Other related requirements:
Such as military, communications, computer, data center and other related requirements.
2. Alternator capacity (power)
Different brushless alternator models (i.e. frame number, as shown in Figure 1) correspond to different powers. Users often choose the power of the alternator according to the total power of their loads. Users should fully understand the nature and size of their loads and consider special loads independently. Generally, Cummins will give it in the form of apparent power. Cummins generators are generally calculated based on the principle of a rated power factor of 0.8 when leaving the factory. Normally, we give the following three powers for each model of generator:
(1) Maximum continuous land power:
The appearance and structure of the land model are shown in Figure 2. It refers to the maximum power of the land generator in continuous operation. The technical data of all generators are based on this standard.
(2) Standby power:
This power is greater than the maximum power for continuous operation, so the temperature rise will increase and the performance will deteriorate. It allows long-term output, but this will reduce the service life of the motor.
(3) Continuous marine power:

Marine power is lower than land power, and the performance meets various standard requirements and is better than land power.

3. Voltage, frequency, speed, number of phases
In most cases, generator sets are used as backup or emergency power sources, or connected to the grid. The voltage frequency and phase number of the generator should depend on the power grid requirements of different countries and regions. For those without a power grid and using generators as the main power source, the voltage and frequency of the generator should be determined by its load condition, or the generator should be selected based on the load, or the voltage and frequency of the load should be selected based on the generator.
(1) Frequency: Generally, there are two types (power frequency): 50 or 60.
(2) Phase number: Generally, for commercial installations, it is 3 phases, and single-phase can be obtained from it.
(3) Voltage: The voltage is selected according to the specific requirements, and Stanford generators can provide and meet the voltage requirements of customers.
① Low voltage range: from 110 to 690V;
From 5 to 5, there are 12 leads to the terminal block, and customers can change the voltage according to their different connection methods and connect it to single-phase. For 5 or more, there are 6 leads to the terminal block, and the voltage can be changed according to the connection method.
② Medium voltage range: 2400-4160V;
③ High voltage range: 6000V~11KV.
(4) Speed
Usually divided into two types: 1500r/min and 1800r/min.
(5) The relationship between frequency, speed, and number of poles
F=（N×P） / 60
 
 
Among them: F is the frequency; N is the rotational speed RPM; P is the number of generator poles.
4. Power generation efficiency
The efficiency of a generator refers to the ratio of the output kilowatts (electrical energy) to the mechanical work input into the generator
That is, efficiency=(output power of generator output terminal)/mechanical power input to the generator
 
Therefore, the power input from the diesel engine to the generator shaft should be greater than or equal to the input power of the generator. This input power, excluding the connection efficiency between the diesel engine and the generator, should be the minimum output power of the diesel engine. The efficiency of a generator varies under different voltages and loads, as shown in the efficiency curve Figure 3 and performance gimbal Figure 4.

5. Temperature rise
Due to the copper loss, iron loss in the iron core, and other mechanical wear caused by the winding resistance of the generator, heat is generated during operation, causing an increase in the temperature of the generator winding and resulting in insulation aging or damage to the generator winding.
Cummins defined the relative rise in winding temperature of a generator as the temperature rise of the generator. The allowable temperature rise for various insulation levels based on an ambient temperature of 40 ℃ is listed in Table 1.

表1    发电机绝缘等级的最大允许温升表

Note: The maximum allowable temperature rise ambient temperature is 40 ℃; All Stanford generators use H-grade insulation materials.
The relationship between insulation level and temperature rise: Taking H-level insulation level as an example, when using H-level temperature rise, the winding life is 20000 hours or extended twice; When using B-level temperature rise, the lifespan of the winding is extended by 6 backups; When in standby use (H-level temperature rise+15 degrees), the lifespan of the winding is shortened by three times.

2、 Application environment
 
1. Relative humidity
(1) Humidity mainly refers to the content of moisture in the air, usually expressed as relative humidity.
(2) For areas with high humidity, we must carefully select the insulation and impregnation system of the generator to adapt to tropical climates.
(3) All Stanford generators can adapt to tropical climates.
2. Altitude
Due to the thinning of air at altitudes exceeding 1000 meters, and the fact that generators rely on fans for air cooling, the cooling effect of generators will deteriorate. Therefore, it is required to reduce the power of generators at altitudes exceeding 1000 meters to avoid overheating. But this does not mean that the generator power can increase when the altitude is below 1000 meters.
When exceeding 1000 meters, the actual output power of the generator should be multiplied by the correction factor in Table 2, and the power drop curve is shown in Figure 3.
Table 2 Correction Factors for Generators at High Altitude

3. Environmental temperature
According to internationally recognized technical requirements, the operating environment temperature for generators is generally defined as 40 ℃, and all designs and power are based on this environmental temperature.
(1) In fact, for a generator, the ambient temperature should be the inlet air temperature of the generator. Due to the generator working together with the diesel engine, the heating of the diesel engine can cause the temperature in the entire space to exceed 40 ℃. The ambient temperature of the generator during actual operation should not exceed 40 ℃, which is very important. If the actual operating environment temperature exceeds 40 ℃, the generator should operate at reduced power.
(2) For the ambient temperature, if it is below 40 ℃, the power of the generator can be higher than the rated power. When the ambient temperature exceeds 40 ℃, the power correction coefficient of the generator is listed in Table 5, and the power decrease curve is shown in Figure 6.
 

表3    高温环境下发电机修正系数表

 

 

4. Special climate and environment
Previously, we discussed the effects of humidity, height, and temperature on the generator, but these were all discussed independently. In practical application, the situation is much more complex, and other factors can also affect the normal operation of the generator.
(1) Factors affecting special environments
The following situations will have an impact on the generator:
① Corrosion gases with chemical properties that contain other gases in the air
② (At the seaside) There is salt water (fog)
③ Dust or sandstorms
④ Rainwater
(2) Solution measures
Therefore, we must comprehensively consider the impact of various complex climates on the generator to ensure its normal operation. Stanford Generator recommends the following three options to adapt to different climates:
① Anti condensation heater:
Used in environments with high humidity and prone to condensation in the air. When the generator is not running, turn on the heater to ensure that the body temperature of the generator is about 5K higher than the ambient temperature. Cut off the power supply to the heater during operation
② Protection level:
The standard IP22 is to prevent vertical rainwater from entering the generator, while Stanford provides IP23 to prevent vertical or 60 degree rainwater from entering the generator.
③ Air filter:
In some places, due to strong sandstorms or high levels of dust in the air.
 
Summary:
As is well known, synchronous generators have magnetic poles on their rotors, while asynchronous ones do not. This is equivalent to relying on magnetic poles to save excitation energy, so synchronous generators have high efficiency. The difference between asynchronous generators and synchronous generators is that when used as a generator, the rotor speed of the synchronous generator set operates synchronously with the rotating magnetic field; The speed of the asynchronous generator set is lower than the speed of the rotating magnetic field and decreases with the increase of load. For the environment, high temperature, humidity, and air pollutants inside the generator room are the most common factors causing generator failures. The accumulation of dust, dust, and other air pollutants can cause the performance of the insulation layer to deteriorate, and also increase the friction force of the rotor bearing part, resulting in heating. Moisture and air pollutants can easily form a leakage path to the ground inside the generator, causing generator failure. Excessive temperature in the computer room can make it difficult to dissipate the heat generated by the generator set during operation, resulting in a decrease in its output power and overheating of the unit. So the dust prevention, moisture prevention, ventilation and cooling of the computer room must be given sufficient attention.
Xiamen ZTA Power System Co.,Ltd
Power system supplier-Generator sets, Lighting Tower, Reefer Genset
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