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10/01/2013

Automatic Control System (Power Management System) of Electrical Power Plants

Automatic Control System for Ships Electrical Power Plants

The automatic control system is designed for controlling turbo- and diesel-driven electric generating plant, predominently onboard ships.
Automatic Control System (Power Management System) of Electrical Power Plants
The purpose of the automatic control equipment is to ensure that the power demand of a ship is generated in an economical manner and also to ensure that any faults occurring will not cause damage to the machinery.

During fully automatic operation the equipment carries out starting, synchronizing and load sharing on one or several diesel alternators as the power demand increases, as well as disconnecting and stopping the diesel alternator in the event of excessive power being available.

The starting of turbo alternator is usually done manually although synchronizing and load sharing is performed automatically. In the event of a fault on one set, a diesel alternator will automatically be started and connected to the main busbars. In the event of a serious fault, the set will immediately be disconnected and stopped, this involving the danger of black-out. At black-out a programmed number of sets will be started and the set which is first to reach frequency will be connected to the busbars. The other diesels are then synchronized in the usual manner. A special unit in the automatic is available for program-blocking of connection of large loads, thus allowing the automatic equipment to ensure that there is always sufficient power available before the load is connected.
DEIF Power Management System of Electrical Power Plant
DEIF Power Management System of Electrical Power Plant

The system is designed for unmanned engine rooms (UER). The only manual operations needed are: starting up the turbos, selection of the sequence of diesel alternators, blocking of a set whenever this may be required, resetting of start blocking caused by an alarm, choice of a program with a minimum number of units in operation.

The automatic control equipment is designed to meet the requirements made by classification societies on equipment in unmanned engine rooms. This implies that some of the functions of the automatic control equipments are monitored so that an alarm will be initiated if the system is not entirely in functional condition.

The automatic control equipment can be connected to the general alarm system on board, provided that this system is designed for breaking contact functions during alarm conditions.

Shutting Down. In order to avoid running the sets at excessively low loads, the set which is next in sequence will be shut down when the load has fallen to a value below a predetermined level.

The shutting down instruction can be initiated by the stop push button being depressed, by a critical or non-critical alarm being initiated, or for diesels by excessive alternator capacity being available. The ordinary shutting down program implies that the set is first unloaded.

When the load has fallen below about 10%, the circuit breaker is tripped and the engine is allowed to run for one minute for the temperature to equalize before the stop solenoid is energized for the period normally necessary for the engine to run down. Starting and stopping instructions interrupt each other, except when the stop instruction is caused by an alarm, the stop instruction then having priority.

Main Functions

The system is designed for the following functions:
  1. Prelubrication of stationary diesel alternator sets for adjustable periods and at adjustable intervals.
  2. Starting, synchronizing and load distribution among the sets when a starting order is received, in accordance with a predetermined program.
  3. Unloading, tripping of the circuit breaker and shutting down of a set on receipt of a stop instruction, in accordance with a predetermined program.
  4. Maintaining the frequency of the distribution network constant within ±0.1 Hz regardless of the load.
  5. Issuing starting instructions to diesel sets in accordance with a variable sequence, when the load on the sets which are then in service exceeds the predetermined level for an adjustable period of time.
  6. Issuing shutting-down instructions to diesel sets in accordance with a variable sequence, when the load on the sets then in service falls below a predetermined level for an adjustable period of time. The sequence of starting and stepping of sets may be selected by means of a switch in the control panel.
  7. Replacing a set on which a non-critical alarm is operative, by another unit.
  8. Stopping a device set when a critical alarm is initiated.
  9. Blocking the starting procedure for a set on which an alarm is operative, until the resetting push button or the alarm has been depressed.
  10. Starting and connecting to the distribution network a predetermined number of diesel alternator sets in the event of a black-out.
  11. Ensuring that sufficient alternator capacity is available before programmed loads are connected.
  12. Transmitting information concerning alarms to the general alarm system.
Synchronizing

As soon as the lower frequency alarm limit has been passed, the synchronizing unit will be switched in and will sense the difference in phase relationship and frequency between the alternator and busbars.

On the basis of these parameters, a control signal will be generated and will actuate the speed governor so that the generator will be rapidly brought into synchronism with the busbars. The voltage difference between the alternator and the busbars is detected by a breaker closing unit which will initiate closure of the circuit breaker as soon as the voltage difference tends towards zero at a sufficiently slow rate. If the difference in amplitude is excessively large, closure of the breaker is blocked.

The synchronizing process is supervised so that if it lasts for more than a predetermined period, a synchronizing alarm will be initiated causing the set to be stopped and further starting attempts to be blocked.

Starting Program for Diesel and Turbo Alternators

If the load on the sets in service should exceed a certain percentage of the rated output, a new set must be connected in. That level is normally 80 percent. However, in some systems the levels for starting a new set are detected on the basis of the sum. of all the sets then in service, e. g. the total power available.

This arrangement allows for better utilization of the sets. In order to avoid starting a new set as a result of the appropriate level being temporarily, e. g. when starting a new consumer with a large starting current demand, the starting instruction is delayed by an adjustable period.

On receipt of automatic starting order, the prelubrication pump will first be started and when its pressure switch delivers a signal to indicate that the oil system is pressurerized, the solenoid of the starting air valve will be energized. The supply of starting air is maintained until the ignition speed has been exceeded or for a predetermined maximum period, normally 5 s.

If the engine fails to start, the attempt is repeated, normally twice, at an interval of about 5 s. If all starting attempts have failed, the "starting failure" alarm will be initiated. This alarm will also be initiated if the engine has failed to attain normal speed after a predetermined period, normally 30 s.

When the ignition speed has been exceeded preexcitation is initiated by a contact function which persists until the voltage is acknowledged or an alarm is initiated. At this speed a time delay is also initiated for activating certain alarms, e. g. alarms for low voltage, low frequency and low lubricating oil pressure.

The turbo is always started manually since it is never used as a stand-by set and the taking into operation of a turbo is based on the fact that you know that the steam power supply will be sufficient for some time.

Furthermore, there are often certain visual checks to be done when starting up a turbo.

Supervision

Each set is protected against damage which could occur in the event of deviation from normal values. Such deviations are sensed by means of transmitters in the automatic equipment or on the set. If a fault is detected, an alarm will be initiated but this can be delayed for up to 60 s.

An alarm may initiate tripping, stopping and blocking of the starting procedure on the set concerned. Alarm indications are provided in the control panel, where resetting of the alarm can also take place. The alarms can be either non-critical (NCR) or critical (CR). A critical alarm will stop the engine automatically immediately, since the set is considered to be in danger of sustaining damage. Non-critical alarms cause shutting down in accordance with the ordinary shutting-down program and the starting procedure is blocked.

Black out Program

In the event of a black out, the automatic equipment will start a certain number of diesel alternators. The set which is the first to acknowledge the frequency receives the instruction to switch onto the busbars. If the first set does not succeed in switching onto the busbars, it will be blocked and the next is allowed to make the attempt, etc.

The reason for starting more than one diesel is primarily to prevent a fault on one set from delaying the clearing of the black out condition, and in addition, the load often increases rapidly when the black-out condition has been cleared, and it is then advantageous to have at least one additional alternator about to be synchronized onto the busbars.

The black-out condition is sensed by the automatic equipment by the fact that there is no voltage on the busbars and that all breakers are tripped. Both of these conditions must thus be satisfied.

Programming of Large Loads

If a relatively large load is switched onto the network, a load surge will occur and this may cause overloading and in the worst case, tripping of the circuit breakers due to overcurrent. The risk of not taking this faet into account is illustrated by the starting current of a directly starting induction motor being 5—10 times the rated current.

For preventing such damages automatic control system is provided with unit, designed to control switching in such large loads.

The request for switching in a certain load is made in the form of a closing contact function. The load to be switched in is exceeded, switching in of the load is blocked until conditions render this possible, i.e. until a new diesel has been synchronized onto the busbars.

If the power limit is not exceeded, clearance is given for immediate switching in of the load. When the load is switched in, the simulation is switched off, since the load will be metered by the ordinary equipment.