Pêşkêşkirina Jeneratora – Cûrên Xeta û Pêşkêşkirên Cihêdan

Çawnên Cemalî yên Jenerator û Sisteman Pêşkêşkirina

Çawnên jenerator di navbera çawnên derveyî û derveyî de were girîng kirin:

• Çawnên Derveyî: Ji pirsgirêyên jeneratorê re werin.

• Çawnên Derveyî: Ji şertên karbendanên neferdî û çawnên taybetmendiyên derveyî re werin.

Çawnên motûrên sereke (wêgot, turbine) ji natûre mekanîkî ne û li ser dizaynên pîçeyekê hatine nîsandin, lê divê bi parastinan jeneratoran binihêrin da ku ji bo kirinên tripping.

Tîpanên Çawnên Derveyî
1. Çawnên Stator

• Derxistina Winding: Ji hêla overloads tevahî û bocanina insulation.

• Phase-to-Phase Fault: Ji hêla bocanina insulation ên di navbera phases de.

• Phase-to-Earth Fault: Xebitina current di navbera phase windings û stator frame de.

• Inter-Turn Fault: Short circuit di navbera turns yekî di navbera winding de.

2. Çawnên Rotor

• Earth Fault: Xebitina current di navbera rotor windings û rotor shaft de.

• Winding Short-Circuit: Dibe voltage excitation û dibe current di rotorên wound de.

• Derxistina: Ji hêla currents unbalanced ên stator (e.g., single-pole trip, negative phase sequence).

3. Loss of Field/Excitation

• Power reactive ji wekî induction generator re gihortin û li ser synchronism hilase.

4. Out-of-Step Operation

• Stresses mekanîkî di navbera shaft û voltage swings di navbera loss of synchronism ên bi grid re.

5. Motor Operation

• Jenerator power ji grid re xebitand di dema ku supply prime mover (e.g., steam/water loss) hilase, risk of overheating û cavitation di turbines de.

6. Çawnên Mekanîkî

• Bearing derxistina, loss of pressure oil lubrification, û vibration zêde.

Mechanism of Rotor Overheating

Currents unbalanced stator (e.g., negative phase sequence) eddy currents di rotor de di duji system frequency (100/120 Hz) de, vê li ser derxistina localized re hatiye. Vê weakens rotor retaining wedges û rings.

Tîpanên Çawnên Derveyî
Abnormalities Power System

• External Short-Circuits: Çawnên di grid de ku jenerator operation berde.

• Non-Synchronized Connection: Damage ji hêla generator paralleling neferdî.

• Overloads/Overspeed: Ji hêla load shedding tevahî û control failure prime mover.

• Phase Unbalance/Negative Sequence: Induces rotor eddy currents û derxistina.

• Frequency/Voltage Deviations: Under/over frequency û voltage stressing generator components.

Generator Protection Devices
Schemes Key Protection
1. Stator Fault Protection

• Differential Relay: Detects phase-to-phase û phase-to-earth faults by comparing input/output currents.

• Earth Fault Protection: Uses overcurrent relays (for resistance grounding) û voltage relays (for transformer grounding) to detect stator ground faults.

2. Rotor Fault Protection

• Earth fault relays monitor insulation breakdown between rotor windings û the shaft.

3. Unbalanced Loading Protection

• Monitors negative phase sequence currents û loss of excitation, which cause reactive power flow issues.

4. Overheating Protection

• Thermal relays û temperature sensors detect stator winding û bearing overheating; negative phase sequence relays address rotor heating.

5. Mechanical Protection

• Overspeed relays, vibration sensors, û low vacuum/pressure switches safeguard against prime mover û turbine failures.

6. Backup and Supplementary Protection

• Reverse power relays prevent motor operation, while differential relays for stator earth faults provide primary fault detection (see Figure 1 for typical connections).

• Differential Relays: Compare currents at both ends of stator windings to detect internal faults.

Protection Principles

• Zero-Sequence Voltage Detection: Identifies inter-turn faults by monitoring voltage imbalances via voltage transformers (VT).

• Grounding System Adaptation: Protection schemes vary based on stator grounding methods (resistance û transformer grounding), using CTs û VTs to sense fault currents/voltages.

Rotor Winding Fault Protection Mechanisms

Wound rotor winding short-circuit faults are safeguarded by overcurrent relays, which trip the generator upon detecting abnormal current surges. Earth faults pose another risk to rotor windings, though their protection requires specialized approaches.

In large thermal generators, rotor û field windings are typically ungrounded, meaning a single ground fault does not produce a fault current. However, such a fault elevates the potential of the entire field û exciter system. Extra voltages induced by opening the field û main generator breaker—especially during fault conditions—can stress the field winding insulation, potentially causing a second ground fault. A second fault may lead to localized iron heating, rotor distortion, û dangerous mechanical unbalance.

Rotor earth-fault protection often employs a relay that monitors insulation by applying an auxiliary AC voltage to the rotor. Alternatively, a voltage relay is used in series with a high-resistance network (commonly a combination of linear û non-linear resistors) across the rotor circuit. The center point of this network connects to ground via a sensitive relay coil (ANSI/IEEE/IEC code 64). Modern protection schemes increasingly favor combinations of linear û non-linear resistors for improved fault detection û insulation monitoring.

Loss of Field and Overexcitation Protection Mechanisms

Loss of field protection employs a relay to detect changes in reactive power flow. A typical scheme uses an Offset Mho (impedance) relay— a single-phase device supplied by generator current transformers (CTs) û voltage transformers (VTs)—to measure load impedance. The relay triggers when the impedance falls within its operating characteristic. A timing relay initiates generator tripping if leading reactive power persists for 1 second (standard timing).

Overexcitation Protection

To prevent core saturation during startup û shutdown, overexcitation protection (ANSI/IEEE/IEC code 59) is implemented, based on the relationship:B = V/f
where:

• B = magnetic flux density (tesla, T)

• V = applied voltage (volts, V)

• f = frequency (hertz, Hz)

Core flux must stay below the saturation point, meaning voltage can only increase proportionally with frequency (speed). Rapid excitation increases risk overexcitation, detected by Volts per Hertz relays. These relays feature linear characteristics û trip when V/f exceeds set thresholds.

Stator and Rotor Overheating Protection

• Stator Windings û Bearings: Temperature monitoring via resistance temperature detectors (RTDs) û thermistors.

• Stator Phase Unbalance: Time-inverse overcurrent relays set to the rotor’s maximum heat tolerance.

• Negative Phase Sequence Protection: Shields the machine from rotor overheating caused by unbalanced stator currents, which induce damaging eddy currents in the rotor.

Reliable protection systems are critical to minimize damage û repair time, as generators are among the most expensive power system components.

This protection utilizes a relay that compares currents in two phases via current transformers (CTs), as illustrated in Figure 2. The protective settings are determined by the maximum time the rotor can endure overheating, defined by the equation K = I²t (derived from Joule's law), where I is the negative sequence current û t is the duration.

Manufacturer-specified typical time-current curves for this condition vary based on the prime mover type, as shown in the referenced diagram.

Reverse Power, Out-of-Step, û Frequency/Voltage Protection Systems
Reverse Power Protection (ANSI/IEEE/IEC Code 32)

This protection employs a power directional relay to monitor generator load, supplied by CTs û VTs (see Figure 3). The relay activates upon detecting negative power flow—indicating the generator is drawing power from the grid (motor operation)—û triggers tripping to prevent turbine damage.

Out-of-Step Protection

Designed to detect power system disturbances (not generator faults), this protection identifies pole slipping when the generator loses synchronism. It trips the generator breakers while keeping the turbine running, allowing re-synchronization after the disturbance clears.

• Operation Principle: Three impedance relays measure load impedance. Tripping occurs if the relays activate in a specific sequence during power swings, distinguishing it from loss of excitation (which happens at zero field) û operating with the generator at full field.

Frequency and Voltage Protection
Under/Over Frequency Protection (ANSI/IEEE/IEC Code 81)

• Overfrequency: Caused by sudden load shedding, risking overvoltage if not managed. Generator controls must adjust output to match demand.

• Underfrequency: Results from insufficient generation for connected loads, leading to voltage drops, increased excitation, û rotor/stator overheating. Load shedding is critical to prevent system collapse.

Under/Over Voltage Relays (Codes 27/59)

Monitor û control voltage deviations to protect equipment from stress û damage.

Phase Supplementary Start Protection

Prevents starting the generator into a fault û loaded condition. Low-set overcurrent relays engage only when frequency is below 52 Hz (for 60 Hz systems) û 42 Hz (for 50 Hz systems), ensuring protection during startup transients.

External Short-Circuit Protection

Overcurrent relays (50, 50N, 51, 51N) detect û clear faults on the external network, safeguarding the generator from excessive fault currents.

These protection schemes collectively address operational anomalies—from power flow reversals to system-wide disturbances—ensuring generator integrity û grid stability.