- Generator Protection Definition: Generator protection is the process of safeguarding generators from various electrical, mechanical, and thermal stresses.
- Types of Protection: Protective relays are used to detect both internal and external faults, ensuring comprehensive generator protection.
- Insulation Failure Protection: Longitudinal differential protection and inter-turn fault protection are crucial for preventing phase-to-phase and phase-to-earth faults.
- Rotor Fault Detection: Methods like potentiometer, AC injection, and DC injection are used to detect rotor earth faults, preventing severe mechanical damage.
- Backup Protection: Overcurrent relays and undervoltage relays provide essential backup protection for generators, ensuring faults are cleared if primary protections fail.
A generator needs protection because insulation stress, mechanical force and temperature rise can destroy the machine. Those forces act on every alternator in service. A machine that stays within rating still sees brief overloads. Protection must still clear a fault before that overload becomes a winding or rotor failure.
Relays and mechanical trips limit overload and abnormal running. Design and operation reduce the chance of a fault but they do not remove it. Generator protection devices detect the fault and trip or alarm so the machine is taken off the system.
Faults sit inside the machine or out on the network. Because generators are connected to electrical power systems, a system fault must be cleared from the generator before stator or rotor metal is damaged.
A generator sees more than one fault type, so more than one scheme is applied. Some schemes are unit (discriminative) and some are backup (non-discriminative). Settings must be coordinated so the scheme that should trip does trip, and the others stay stable.
Types of Generator Protection
The protection applied to the generator falls into two groups,
- Protective relays to detect faults occurring outside the generator.
- Protective relays to detect faults occurring inside the generator.
Besides the relays on the generator and its unit transformer there are lightning arrestors, overspeed guards, oil-flow devices and temperature devices for shaft bearings, the stator winding, the transformer winding and transformer oil. Some of those devices only raise an alarm.
The remaining schemes operate the generator master trip relay. A protective relay does not stop a fault from starting. It detects the fault and shortens its duration so the winding does not overheat to the point of permanent damage.
Surge capacitors, surge diverters or both are fitted so lightning and switching surges do not overstress the stator insulation. The schemes usually applied to the generator are summarised below.
Protection against Insulation Failure
The main stator scheme for a phase-to-phase or phase-to-earth fault is longitudinal differential protection of generator. The next stator scheme is inter-turn fault protection.
Older practice often omitted a dedicated turn-fault scheme. A breakdown between two points of the same phase in one slot, where a potential difference exists, often becomes an earth fault quickly. That earth fault is then seen by stator differential or by stator earth fault protection.
A high-voltage generator relative to its MVA rating packs a large number of conductors into each slot. On large high-voltage units a dedicated turn-fault scheme is now usual because that inter-turn voltage is higher.
Stator Earth Fault Protection
When the stator neutral is earthed through a resistor, a current transformer sits in the neutral-to-earth connection. An Inverse time relay is used across the CT secondary when the generator is connected directly to the busbar. When the generator feeds through a delta-star transformer, an instantaneous relay is used for the same current.
On a direct-connected machine the earth-fault relay must grade with other earth-fault relays on the system. That is why an inverse time relay is used. When the machine feeds a delta-star transformer the earth-fault loop is limited to the stator and the transformer primary, so grading with system earth-fault relays is not required. An instantaneous element is then usual.
Rotor Earth Fault Protection
A single rotor earth fault does not circulate current if the field is otherwise isolated. A second earth fault shorts part of the field winding, unbalances the air-gap magnetic field and can damage the bearings. Three methods are used to detect a rotor earth fault. The methods are
- Potentiometer method
- AC injection method
- DC injection method
Unbalanced Stator Loading Protection
Unbalanced load produces negative-sequence current in the stator. That current creates a field that rotates at twice synchronous speed relative to the rotor and induces double-frequency current in the rotor. That current is large and heats the rotor body and wedges.
An unbalance from a stator-winding fault is cleared by the generator differential. An unbalance from an external fault or unbalanced system load can last until backup relays time out. A negative-phase-sequence relay is then set to the machine I2^2 t withstand curve.
Protection against Stator Overheating
Overload heats the stator winding. A cooling failure or shorted stator laminations also heats the winding.
Overheating is detected by embedded temperature detectors at points in the stator winding. Those detectors are usually resistance elements that form one arm of a wheatstone bridge circuit. Smaller machines, often below about 30 MW in older practice, may have no embedded coil and instead use a thermal relay that measures stator current.
A current-only thermal relay sees overload heat. It does not see a cooling failure or shorted stator laminations. over current relays, negative-phase-sequence relays and coolant-flow devices still give some thermal overload protection.
Low Vacuum Protection
Low-vacuum protection is usually a regulator that compares condenser vacuum with atmospheric pressure. Older steam-set practice fitted it on machines above about 30 MW. The usual action is to unload the set through the secondary governor until vacuum recovers. If vacuum stays worse than about 21 inches of mercury, a figure that belongs to the turbine maker, the stop valves close and the main circuit breaker is tripped.
Protection against Lubrication Oil Failure
Some older descriptions skip a separate lube-oil trip because lube oil and governor oil often share a pump, so a pump failure already closes the stop valve. Many modern sets still fit a dedicated low-oil-pressure trip.
Protection against Loss of Boiler Firing
Two methods are used to detect loss of boiler firing. In the first, normally-open contacts on the fan motors may trip the generator if more than two motors fail. The second uses boiler pressure contacts that unload the generator if pressure falls below about 90% of the maker’s rated figure.
Protection against Prime Mover Failure
If the prime mover stops supplying mechanical energy, the generator keeps turning as a motor. It then draws electrical energy from the system instead of supplying it.
In a steam turbine the steam also cools the blades. Loss of steam then lets friction heat the blading and can distort it.
Loss of steam can damage the turbine and also impose a motoring load on the generator. A reverse-power relay trips the set when power flows into the machine.
Over Speed Protection
Mechanical overspeed devices are usual on both steam and hydro turbines. They act directly on the steam throttle or the main stop valve. A separate electrical overspeed relay is not usual on steam sets.
An electrical overspeed relay is common on hydro units because the governor is slower and the set is more likely to overspeed. When fitted, that relay is usually supplied from the permanent-magnet generator used for governor control.
Protection against Rotor Distortion
After shutdown the top and bottom of the turbine casing cool at different rates. That uneven temperature distorts the rotor. The rotor is therefore turned at low speed while it cools. Large modern rotors also have shaft-eccentricity detectors.
Protection against Difference in Expansion between Rotating and Stationary parts
During run-up the rotor heats at a different rate from the casing because the masses differ. The rotor then expands at a different rate from the casing, and that unequal expansion has to be controlled.
Larger machines therefore have independent steam supplies to selected casing joints. Axial-expansion measurement tells the operator where to feed that steam and also shows a dangerous expansion.
The shaft axial-expansion detector is similar to the rotor-distortion equipment, except that the detector magnets are fixed to the turbine casing.
Protection against Vibration
Vibration detectors are usually mounted on the bearing pedestals. The detector is a coil on springs between U-shaped permanent magnets. The coil voltage, which is proportional to vibration, is passed into integrating circuits and then to an indicating instrument.
Back up Protection of Generator
A large synchronous generator or alternator also needs backup. If the primary scheme fails to clear a fault, backup protection relays must still clear it. Overcurrent relays are the usual backup elements.
Synchronous reactance of a modern machine is often greater than 100 percent, so the sustained current into an external fault can sit below full-load current. A plain IDMT relay then has to sit near full load with a short time setting if it is to operate, and it will not grade with other overcurrent relays on the system.
That overcurrent relay would also be likely to trip for loss of field. The usual fix is an overcurrent relay supervised by an undervoltage relay. The undervoltage element controls the fault setting of the overcurrent element, as shown in the figure below.
