GENERATOR / ALTERNATOR CONTROLS
4.1 Starting a Generator
A brief knowledge of the generator engine is essential before handling the associated electrical components relevant to the control and operation of the engine.
The prime mover for the main generator is termed as auxiliary engine, be it an IC, steam or gas turbine driven one.
Before starting a generator the logistic requirements for the prime mover should be fulfilled.
The prime mover requires basically rated Electric power supply, Fuel, Lubricating oil for all of its moving
parts, Jacket cooling fresh water and Air for starting the engine itself.
Electric Power Supply : The prime mover control circuit is powered by 24 V D.C. derived from Main / Emergency A.C. electric source stepped down and rectified at Battery Charging and 24 V Switch Board where 24 V from Main Emergency Battery is in parallel and floating.
Fuel : The fuel oil required for the prime mover ( engine ) should be available as per the standard quoted by the engine maker – quality, pressure and temperature which decides the viscosity of the fuel .
Lubricating Oil : The lub.oil specified by the manufacturer of the engine at specified grade, pressure and temperature should be available with pre lubrication pump arrangement and correct level in the crankcase is to be ensured.
Cooling Fresh Water : The engine cylinder ( jacket ) cooling fresh water inlet and outlet should be available as per the maker’s data standards – pressure and temperature.
Air : Starting air supply at specified pressure should be available.
Each of the above system has several sensors monitoring the status of different parameter values - pressure, temperature, level, RPM etc. - continuously and in case of abnormal values beyond the set values in the alarm monitoring system an alarm is annunciated and immediate attention and corrective measures are to be taken promptly.(For the normal parameter values of all process mediums as well as alarm and trip set point values Engine maker’s Maintenance Manual is to be referred and those data should be adhered to for safe and optimum performance of the engine.)
Alarms & Trip Features :
Various alarms normally found in generator prime mover system are mentioned below. But as the automation on board has much developed in recent days one can come across many more alarms.
Electric Power Supply System : D.C. 24 V Power Fail
Fuel Oil System : i) F.O. Pressure Low
ii) F.O. Temperature Low/High
iii) F.O. Filter Differential Pressure High
iv) F.O. Leakage Tank Level High
Cooling Fresh Water System : i) C.F.W. Inlet Temp. High/Low
ii) C.F.W. Outlet Temp. High
iii) C.F.W. Outlet Temp. High-High → Alarm &Engine Trip
iv) C.F.W. Inlet Press. Low
v) C.F.W. Expansion Tank Level Low
Lub. Oil System : i) L.O. Inlet Temp. High
ii) L.O. Filter Differential Press. High
iii) L.O. Pressure Low
iv) L.O. Pressure Low-Low → Alarm &Engine Trip
v) L.O. Sump Level High/Low
iv) Pre L.O. Pressure Low
Air System : i) Starting Air Press. Low
ii) Turbo Charging Air Abnormal
Others : i) No.1~n Cylinder Exhaust Gas Temp. High
ii) Cyl. Exh.Gas Mean Temp. High
iii) Turbo charger Inlet Temp. High
iv) Turbo charger Outlet Temp. High
v) Over speed → Alarm &Engine Trip
vi) Aux. Engine Start Fail
vii) Aux. Engine Emergency Stop
The alarms and trips of prime mover require periodic simulation tests to ensure positive functioning in case of actual alarm conditions.
Active and Passive Components of an alternator:
Any electric circuit has many elements like resistor, capacitor, inductor, battery etc. Circuit analysis is the process by which voltage or current is measured across the element. In a complete circuit there are two types of elements found, namely, active elements and passive elements. The active elements generate energy. Batteries, generators, operational amplifiers etc. are active elements. The passive elements cannot generate energy, but they drop energy. Resistor, capacitor, inductor etc. are passive elements because they take energy from circuit.
In a generator the generator itself is the only active element whereas all ensuing elements like ACB, AVR and its related components like reactors, capacitors etc are passive elements.
Two types of sources - i) independent and ii) dependent source.
An ideal independent source is an active element that provides a specified voltage or current that is completely independent of other circuit elements. In a complete circuit voltage or current source are most active elements which deliver power in the circuit.
An ideal dependent source is an active element in which the source quantity is controlled by another voltage or current.
Various Starting Positions for Starting the Aux. Engine :
Confirming all required logistics are available at correct parameter values the starting/stopping of the aux. engine can be performed in the following method.
1. Local :
a) From Local Control Panel ( LCP ) Manual Start / Stop :
There is an electrical panel incorporated with ‘START’ and ‘STOP’ push button switches and a
LOCAL / REMOTE change over switch along with speed processing electronic card located fitted near the aux. engine.
If required to start the engine locally by electric start turn the change over switch to ‘LOCAL’. Making sure all interlocks are clear push the ‘START’ push button. Air solenoid valve opens allowing starting air to crank the engine and when certain predetermined rotational speed is achieved fuel is injected and the engine runs up the rated speed. When fuel injection commences the ‘START’ push button can be released and now the air solenoid valve closes and cuts off air for starting.
To stop the engine press the ‘STOP’ push button switch. The stop solenoid in governor unit is energized , fuel fully cut off and engine stopped.
b) Emergency Start / Stop at Engine :
If the starting electric circuit has some problem the aux. engine can be started in ‘Emergency’ mode by the Starting lever near the governor unit of the engine. This has three positions. Vertical upright position of the handle is engine ‘STOP’ position. Vertical downright position is ‘START’ position. Slightly above the downright position is the ‘RUN’ position.
To start the engine, keep the c.o.s.at LCP to ‘LOCAL’, pull the starting lever fully down and the starting air will be let in and engine will start to crank. After a few seconds of the engine shaft revolutions move the lever to ‘RUN’ position and engine will now run at normal speed.
2. Remote :
a) From Main Switch Board –‘ MANUAL’ Start / Stop :
In the relevant Generator Panel of Main Switch Board ‘START’ and ‘STOP’ push button switches and a MANUAL / AUTO change over switch are incorporated. Select the c.o.s. position to ‘REMOTE’ at LCP of that engine; select the c.o.s. position to ‘MANUAL’ at MSBD. If ‘START’ push button switch is now pressed engine will start. By pressing ‘STOP’ push button switch engine can be stopped.
b) From Main Switch Board – ‘AUTO’ Start / Stop:
In LCP of that engine, select the c.o.s. position to ‘REMOTE’. Select the c.o.s. position to ‘AUTO’ in Generator Panel of MSBD. Also select the appropriate generator that is required to be started first automatically to ‘1st STBY' as per the circumstances determined by the ‘POWER MANAGEMENT SYSTEM’ (PMS) by the ‘STANDBY GENERATOR SELECTOR SWITCH’ in the synchronizing panel of MSBD.
Now the selected generator will be automatically started when deemed necessary ( increase in electrical load demand) by PMS, synchronized and load shared with already running generator/s and vice versa (decrease in electrical load demand) load removed from the generator that was on ‘1st STBY.’, started and loaded by PMS, disconnected from bus bar and stopped automatically .
In case the ‘1st STBY’ generator fails to start, automatically ‘2nd STBY.’ Generator will be started provided the relevant switches are already set in the appropriate positions.
Space Heaters :
Electrical equipments invariably use insulation materials (Class F insulation normally used in marine field which can withstand a maximum continuous operating temperature up to 155°C) to direct the flow of electric current in the specified path only. These insulating material characteristics have a definite relation with the temperature they are exposed to. The insulating property will be excellent within the rated temperature of that insulating material. But it will adversely deter with temperature above the rated value. It is said the insulation value of an insulator falls down to 50% when the rated temperature increases by 10°C beyond the rated value.
Hence hot spot temperature of electrical equipment should be known with the prevalent ambient temperature and the insulating material grade should selected carefully. Hot spot temperature is continuously measured by a special semiconductor device ‘Thermistor’ which has a negative temperature coefficient and incorporated in an electronic circuit.
Dust and moisture are the worst foes of electrical insulation. Dust can be controlled in enclosed electrical equipment by employing a ventilation system equipped with dust filter.
The problem of controlling moisture in electrical equipments in marine field led to innovative ways. Not only equipments but spaces in switch board, big size starter panel etc. also require moisture control so that the moisture does not condense and accumulate on electrical components. This accumulated moisture on insulation has the worst deteriorating effect and with this presence if the equipments are energized severe leakage and / or short circuit shall arise culminating in damage / loss of that electrical equipment.
Two types of space heater are used in marine field.
Big panels like Main Switch Board, Starter panels and electrical compartments where a number of electrical equipments are housed employ this type of space heater. It is similar to the shape of domestic immersion heater. Depending upon the area heater size and capacity will be decided. Normally the shape will be like a ‘U’ tube with either end having electrical connection terminal. The ambient air will be continuously heated to a specified temperature level so that the moisture is evaporated and not allowed to settle down on the equipment.
In big electrical machines these tubes will be installed at the bottom area (e.g.bottom quadrants of a circle) so that the heat produced will heat up the nearby air and the hot air rises above and a natural circulation of air occurs. The voltage rating will be that of the ship’s low voltage domestic supply (110 V or 220 V, 1Ø or 3 Ø).
In generators the space heating circuit arrangement is in such a way that the heating supply passes through a timer relay operated by the Normally Closed (NC) contact of the Air Circuit Breaker (ACB)in OPEN condition after the heater supply ON switch ensuring correct switching ON/OFF the heater.
As safety precaution before any sort of work is to be carried out in the electrical equipments with space heating switch OFF and isolating the supply and Tagging it OFF.
2. Voltage Injection type :
This type employs a heater transformer installed within the equipment panel unit supplied
With 1Ø main supply and stepped down to 10% of main supply voltage. When the machine is stopped, after a preset time delay by a timer relay, this supply is connected to two terminals of the electrical load. If it is a ‘STAR’ connected load the space heating current will flow through 2 winding coils only. The continuously generated heat by these two coils will warm up the third coil also which is placed adjacent to them along with the core on which the windings are wound.
If it is a ‘DELTA’ connected load the current will flow in all the three windings. The heat produced is sufficient to keep that equipment warm enough to drive away the moisture after evaporation without condensing and resting on the windings.
It is essential to design the main line contactor circuit to energize only when the heater circuit is isolated from the load while starting the equipment; or else low voltage heater and high voltage main supplies will be connected together and the space heating circuit fuses will blow and/or the space heating transformer may be burnt out.
4.2 Paralleling, Load Sharing & Changing Over Generators:
( For better understanding refer ‘ Marine Electrical Technology ’ manual by Elstan A. Fernandez –
Chapter 8 for Automatic Voltage Regulators & Chapter 10 for Paralleling of Alternators )
The alternator used on board is normally of Brushless Type which has either one of the following type of excitation for production of rated voltage.
The basic operating principle of an Automatic Voltage Regulator is the way in which the AVR controls the excitation of a generator.
The generator a.c. output of armature winding in the stator provides input for excitation of its
main field via the AVR which governs the level of excitation provided to the exciter field. The AVR responds to a voltage sensing signal derived from the main stator winding.
By controlling the low power of the exciter field, control of the high power requirement of the main field is achieved through the rectified output of the exciter armature. In this context it is better to have a fair idea about a brushless generator construction.
In a brushless generator the exciter and main generator are composed of a single machine with a small size exciter on free end and the other end with main generator. The stator of the exciter has D.C. field winding. The rotor of exciter carries 3 Ø A.C. armature winding and in succession an insulation disc on which rectifiers for the rectification of the exciter rotor A.C. output are mounted. This rectified D.C. voltage is connected to the main generator field winding which is again on the same rotor. The main generator A.C. armature winding is housed on the stator. This arrangement of having exciter armature, rectifiers on the insulation disc and main generator field on the same rotor entails elimination of slip rings & brush gear assembly for the connecting the D.C. supply to main generator field.
When the generator is started residual magnetism in exciter and main generator fields induce some voltages. Hence A.C. voltages appear in both armatures. The exciter output is rectified by the rectifiers and this D.C. strengthens the main field further thus increasing the main A.C. output. A tapping from this output is taken to the AVR which rectifies this A.C. and after certain comparison within its components gives D.C. output to exciter field. This closed link process continuously increases the main voltage till the rated value is achieved. Finally the AVR gives its output in such a level the main A.C. output value remains steady at the rated one.