BBJ 20905 : GENERATOR SYSTEM MAINTENANCE
Embedded Files
TOPIC 9 (i) : PRINCIPLE OF CONTROL, METERING & PROTECTION
Automatic Voltage Generator (AVR)
AVR is the controller device to stabilize terminal voltage. It used to senses voltage decrease and increase and give command to exciter to increase and decrease the excitation current to rotor winding.
In Malaysia context, outgoing voltage is 220 Vac for single phase and 400Vac for three phase with +5% and -10% tolerance. (320Vac until 420Vac)
Output of the generator is rectified and controlled by the AVR and then fed to the field winding (rotor winding) thru carbon brushes and slip ring.
DC Generator Commutator Exciter
Alternator Rectifier Exciter and Stationary Exciter/Stationary Rectifier
Alternator Rectifier Exciter and Rectifiers (Brushless Exciter)
Static Exciter
Automatic Voltage Regulator Operation Principle
Step 1 : Diesel Generator output voltage abnormal
Step 2 : AC output of the generator is rectified and controlled by Auto Voltage Regulator (AVR).
Step 3 : The 'resultant' will fed to field winding through carbon brush and slip ring.
Step 4 : Voltage return to the stable and steady-state.
Excitation System Block Diagram
Engine Governor
The Governor purpose to control the fuel to the engine cylinders so as to control the speed of the unit, holding speed constant for all conditions of load imposed on the generator being driven by the engine.
To maintain frequency of the generator output, the engine speed must held constant.
Relationship between generator output frequency and the engine speed is express by :
Where ;
N = Speed (rpm)
F = Frequency (Hz)
P = Pole
Engine Governor's Standard
The applicable British Standard for the speed governing of Reciprocating Internal Combustion Engines is BS 5514: Part 4.
Analogy of Engine Governor
Governor Type
Governor consists of two technologies :
Mechanical - Hydraulic
Electronic
Fork Connection
Speed Setting Lever
Governor Arm
Stop Piece
Stop Screw
Control Arm
Curve Plate
Carrier Pin
Carrier Head
Angled Arm
Fly Ball
Governor Lever
Mechanical Hydraulic Governor
In a hydraulic governor, rotating centrifugal flyweights control a pilot valve, which in turn controls a pilot valve, which in turn controls the position of a power piston that actuates the fuel control linkage.
The Governor operates by three (3) mode of operation :
Steady Speed Mode
Load remove, Speed Increase Mode
Load Added, Speed Decreases Mode
Mechanical Hydraulic Governor
Steady Speed Mode
At steady speed, the flyweights and pilot valve are in a neutral position, with the flyweight force balanced by a speeder spring.
The pilot valve is positioned to shut off oil flow to or from the power piston so that trapped oil holds the power piston in a fixed position.
Load Remove, Speed Increase Mode
When load is removed and speed increases, the flyweights lean outward and displace the pilot valve to permit some trapped oil to escape.
The power piston moves in a direction that reduce fuel flow to the engine.
Load Added, Speed Decreases Mode
When load is added and speed decreases, the flyweights lean inward and displace the pilot valve to admit oil pressure to the end of the power piston.
The power piston moves the fuel control linkage to increase fuel flow.
Electronic Governor
Electric governor senses speed by sensing alternator frequency or the frequency of a magnetic pick-up.
Voltage spikes derived from the generated frequency are rectified and filtered to provide a DC voltage level proportional to the frequency.
An adjustable constant voltage source serves as a speed reference.
The frequency signal and reference signal are summed algebraically to produce an error signal that is applied to a solenoid to control the pilot valve of a hydraulic actuator.
Droop - Isochronous Relationship
Isochronous is Iso = same, Chronous = time
Isochronous governor is define as the governor that maintains the same speed in the mechanism control regardless of the load.
Droop = The percent change in speed as a unit goes from no load to rated load condition
𝐷𝑟𝑜𝑜𝑝 % = (𝑁𝑜 𝑙𝑜𝑎𝑑 𝑠𝑝𝑒𝑒𝑑 −𝐹𝑢𝑙𝑙 𝑙𝑜𝑎𝑑 𝑆𝑝𝑒𝑒𝑑)/ 𝐹𝑢𝑙𝑙 𝑙𝑜𝑎𝑑 𝑆𝑝𝑒𝑒𝑑 x 100
Isochronous Control Block Diagram
Droop Control Block Diagram
We can analyze droop control using a simple Linear Equation
P = S𝑝(𝑓𝑁𝐿 - 𝑓𝑠𝑦𝑠)
Where;
p = power output of generator
S𝑝 = Slope of curve, MW/Hz
𝑓𝑁𝐿 = Full load Frequency
𝑓𝑠𝑦𝑠 = Operating Frequency of the system
Metering
Diesel Generator (DG) display the all meter in a Generator Control Panel and Switchboard.
The meter display are :
Generator Control Panel
TOPIC 9 (ii) : PRINCIPLE OF CONTROL, METERING & PROTECTION
PROTECTION OF GENERATOR
Divided by sensory protection and electrical protection.
Protection hierarchy : ALARM & TRIP
Alarm : When reach to certain setting value, the ALARM will trigger.
Trip : When reach to certain setting value, the ALARM will trigger.
Overspeed Protection
Overspeed trip is the mechanism to cut the fuel supply to the cylinders in case the engine speed rises above the pre-set level.
It prevents the engine from getting damaged because of extremely high speeds.
using governor mechanism or centrifugal force concept.
General Damage due to overspeed protection malfunction
Winding Temperature
Three cost effective ways of detecting winding temperatures
Thermostats - Bi-metallic switches pre-set to a fixed temperature.
Thermistors - Temperature sensitive resistor with pre-set operating knee point.
RTD (Resistance Temperature Detector) - Resistive element with a linear resistance change, proportional to the temperature change.
RTDs are embedded within the stator winding assembly.
Winding Temperature Alarm & Trip Setting
Maximum ambient temperature of 40° C, the insulation class ratings also assume a service factor of 1.0 and an altitude of 3300 feet above sea level (beyond which, the thinner air has a reduced cooling capacity).
Bearing Temperature
Bearing temperature sensor are normally fitted to the bearing cartridge in Non Drive End (NDE) and Drive End (DE) side of alternator.
Function : to detect overheating of bearings, control signals should be set.
Bearing Temperature
Bearing Vibration
Vibration analysis plays a critical role in the condition-based maintenance of rotating equipment.
Help detect many serious problems at an early stage, allowing personnel to undertake remedial work in a timely manner and avoid deterioration in machine efficiency or, at worst, machine failure.
Over Current Relay & Earth Fault Relay
Over Current Relay
Function : Protects the power systems and its equipments such as generators against short circuits, ground faults and overloads.
Type of over current relay used in power system protection :
Standard Inverse (SI) or Normal Inverse
Very Inverse (VI)
Extremely Inverse (EI)
Long Time Standard Earth Fault
Earth Fault Relay
Ground Fault is a fault or contact occurs between the Live conductor to ground/neutral point.
In this fault the fault current directly flows to the ground. In this, The fault may be:
One phase to ground (L-G)
One or more phase to ground (LLG) or LLLG
Main reason of Earth fault is Insulation failure.
Under Frequency Relay & Reverse Power Relay
Under Frequency Relay
Used to shed automatically certain portion of load whenever the system frequently falls to such a low level.
When Pm < Pe, results in deceleration of machine and frequency decreases below nominal.
Pm = Mechanical Power input to Generator
Pe = Electrical Power output of Generator
Reverse Power Relay
Reverse Power Relay is directional protective relay that prevents power from flowing in the reverse direction.
Reverse power happen when generator output fails due to problem with prime mover. It mostly caused by:
Fuel supply problem in prime mover.
Engine problem.
Issues with speed controller.
Different frequency during synchronization.
Under Voltage Relay & Over Voltage Relay
Under Voltage Relay
Used to protect the generator from low voltage operation.
It sense the phase to phase voltage of the using instrument transformer (Potential transformer).
When the voltage drops below the rated voltage typically 85% (stage 2)-90% (stage 1) the under voltage protection will be activated.
Over Voltage Relay
Used to protect the synchronous Generator from high voltage.
Events causes Overvoltage:
Load rejection
Direct Lightning Strike
Back Flashover Lightning
Transformer Energize
Switching HT Switchgear
Fault Clearing
Turbine Overspeed
AVR Malfunction
THANK YOU!
Page updated
Google Sites
Report abuse