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Diesel Generator

Overview

DG Set is a combination of a diesel engine and an alternator. Diesel engine is the prime mover, which drives an alternator to produce electrical energy. In the diesel engine, air is drawn into the cylinder and is compressed to a high ratio (14:1 to 25:1). During this compression, the air is heated to a temperature of 700–9000˚C. A metered quantity of diesel fuel is then injected into the cylinder, which ignites spontaneously because of the high temperature. Hence, the diesel engine is also known as compression ignition (CI) engine. DG set can be classified according to cycle type as: two stroke and four stroke. However, the bulk of IC engines use the four stroke cycle. Types of fuel or energy used in DG sets are Furnace oil and diesel.

Figure: Diesel Generator Set

Design & Operation

A diesel generating set should be considered as a system since its successful operation depends on the well-matched performance of the components, namely:

  • The diesel engine and its accessories.
  • The AC Generator.
  • The control systems and switchgear.
  • The foundation and power house civil works.
  • The connected load with its own components like heating, motor drives, lighting etc.

It is necessary to select the components with highest efficiency and operate them at their optimum efficiency levels to conserve energy in this system.

To make a decision on the type of engine, which is most suitable for a specific application, several factors need to be considered. The two most important factors are: power and speed of the engine. The power requirement is determined by the maximum load. The engine power rating should be 10-20 % more than the power demand by the end use. This prevents overloading the machine by absorbing extra load during starting of motors or switching of some types of lighting systems or when wear and tear on the equipment pushes up its power consumption.

An engine will operate over a range of speeds, with diesel engines typically running at lower speeds (1300 - 3000 RPM). Speed is measured at the output shaft and given in revolutions per minute (RPM). There will be an optimum speed at which fuel efficiency will be greatest. To determine the speed requirement of an engine, one has to again look at the requirement of the load. For some applications, the speed of the engine is not critical, but for other applications such as a generator, it is important to get a good speed match. If a good match can be obtained, direct coupling of engine and generator is possible; if not, then some form of gearing will be necessary - a gearbox or belt system, which will add to the cost and reduce the efficiency.

There are various other factors that have to be considered, when choosing a diesel engine for a given application. These include the following: cooling system, abnormal environmental conditions (dust, dirt, etc.), fuel quality, speed governing (fixed or variable speed), poor maintenance, control system, starting equipment, drive type, ambient temperature, altitude, humidity, etc. Suppliers or manufacturers literature will specify the required information when purchasing an engine. The efficiency of an engine depends on various factors, for example, load factor (percentage of full load), engine size, and engine type.

 

With the steady development of the Diesel Engine, the specific fuel consumption has come down from the value of 220 g/kWh in the 1970 to a value of around 160 g/kWh in present times. With the arrival of modern high efficiency turbochargers, it is possible to use an exhaust gas driven turbune generator to further increase the engine rated output. The net result would be lower fuel consumption per kWh and further increase in overall thermal efficiency. The diesel engine is able to burn the poorest quality fuel oils, unlike gas turbine, which is able to do so with only costly fuel treatment equipment.

Diesel Generator (DG) Set Selection and Installation Factors

  1. If a DG set is required for 100% standby, then the entire connected load in HP/kVA shold be added. After finding out the diversity factor (Demand/Connected Load), the correct capacity of a DG set can be found out.                                                                                                                                  Example: The connected Load = 650 kW, Diversity Factor = 0.54, Maximum Demand = 650 * 0.54 = 350 kW, % Loading = 70, Set Rating = 350/0.7 = 500 kW. At 0.8 Power Factor, the DG Set rating = 625 kVA.
  2. For and existing installation, record the current, voltage and power factor reading at the main bus-bar of the system at every half-an-hour interval for a period of 2 - 3 days and during this period the factory should be having its normal operations. The non-essential loads should be switched off to find the realistic current taken for running essential equipment. This will give a fair idea about the current taken from which the rating of the set can be calculated.                                                                                                                                                                                                kVA = √3 V I, kVA Rating = kVA/Load factor, where Load Factor is the ratio of average kVA to maximum kVA.
  3. For a new installation, an approximate method of estimating the capacity of a DG set is to add full load currents of all the proposed loads to be run in DG set. Then, applying a diversity factor depending on the industry, process involved and guidelines obtained from other similar units, correct capacity can be arrived at.

Unbalanced Load Effects

It is always recommended to have the load as much balanced as possible, since unbalanced loads can cause heating of the alternator, which may result in unbalanced output voltages. The maximum unbalanced load between phases should not exceed 10% of the capacity of the DG Sets.

Load Pattern

In many cases, the load will not be constant throughout the day. If there is substantial variation in load, then consideration should be given for parallel operation of DG Sets. In such a situation, additional DG Sets are to be switched on when load increases. The typical case may be an establishment demanding substantially different powers in first, second and third shifts. By  parallel operation, DG Sets can be run at Optimum Operating Points or near about, for Optimum fuel consumption and additionally, flexibility is built into the system. This scheme can also be applied where loads can be segregated as critical and non-critical loads to provide standby power to critical load in the captive power system.

Energy Performance Assessment of DG Sets

Routine energy efficiency assessment of DG sets involves following typical steps:

  1. Ensure reliability of all instruments used for trial.
  2. Collect technical literature, characteristics, and specifications of the plant.
  3. Conduct a 2 hour trial on the DG set, ensuring a steady load, wherein the following measurements are logged at 15 minutes intervals.
    • Fuel consumption (by dip level or by flow meter)
    • Amps, volts, PF, kW, kWh
    • Intake air temperature, Relative Humidity (RH)
    • Intake cooling water temperature
    • Cylinder-wise exhaust temperature (as an indication of engine loading)
    • Turbocharger RPM (as an indication of loading on engine)
    • Charge air pressure (as an indication of engine loading)
    • Cooling water temperature before and after charge air cooler (as an indication of cooler performance)
    • Stack gas temperature before and after turbocharger (as an indication of turbocharger performance)
  4. The fuel oil/diesel analysis is referred to from an oil company data.

Analysis - The trial data is to be analysed with respect to:

  • Average alternator loading.
  • Average engine loading.
  • Percentage loading on alternator.
  • Percentage loading on engine.
  • Specific power generation kWh/liter.
  • Comments on Turbocharger performance based on RPM and gas temperature difference.
  • Comments on charge air cooler performance.
  • Comments on load distribution among various cylinders (based on exhaust temperature, the temperature to be ± 5% of mean and high/low values indicate disturbed condition).
  • Comments on housekeeping issues like drip leakages, insulation, vibrations, etc

The following table format can be used for monitoring the performance of DG Set:

 

The following options will ensure that your diesel genset is operating at best efficiency and you can tap potential energy savings:

  • Ensure steady load conditions on the DG set, and provide cold, dust free air at intake (use of air washers for large sets, in case of dry, hot weather, can be considered).
  • Improve air filtration.
  • Ensure fuel oil storage, handling and preparation as per manufacturers’ guidelines/oil company data.
  • Consider fuel oil additives in case they benefit fuel oil properties for DG set usage.
  • Calibrate fuel injection pumps frequently.
  • Ensure compliance with maintenance checklist.
  • Ensure steady load conditions, avoiding fluctuations, imbalance in phases, harmonic loads.
  • In case of a base load operation, consider waste heat recovery system adoption for steam generation or refrigeration chiller unit incorporation. Even the Jacket Cooling Water is amenable for heat recovery, vapour absorption system adoption.
  • In terms of fuel cost economy, consider partial use of biomass gas for generation.  Ensure tar removal from the gas for improving availability of the engine in the long run.
  • Consider parallel operation among the DG sets for improved loading and fuel economy thereof.
  • Carryout regular field trials to monitor DG set performance, and maintenance planning as per requirements