Jiangmen Hongli Energy Co.ltd , https://www.honglienergy.com
**Phase-in Operation of a Generator**
Phase-in operation refers to a condition where the generator absorbs reactive power from the system rather than supplying it. This occurs when the excitation current is reduced, leading to a decrease in the generator's internal voltage. As a result, the power factor angle becomes leading, and the stator current leads the terminal voltage. In this state, the generator supplies active power to the grid but draws reactive power from it.
Under normal operation, the generator provides both active and reactive power to the system, with the stator current lagging behind the terminal voltage. This is known as lagging operation. However, when the excitation current is gradually decreased, the generator shifts from supplying reactive power to absorbing it. The stator current then starts to lead the terminal voltage, marking the transition into phase-in operation.
During phase-in operation, the generator’s internal voltage (Eq) decreases, which affects the power angle relationship. For a constant active power output, the power angle must increase, reducing the static stability margin. The stability limit is influenced by factors such as the generator's short-circuit ratio, external reactance, and the performance of the automatic excitation regulator.
One key concern during phase-in operation is the increased magnetic flux leakage at the stator end, especially in large generators. This leakage can cause higher temperatures at the core ends, potentially affecting the generator’s thermal limits. Additionally, the terminal voltage of the generator may drop, which can impact the plant's auxiliary power supply. If the voltage drop exceeds 10%, it may disrupt the operation of critical equipment within the plant.
To ensure safe and stable operation, phase-in operation is typically determined through experimental testing. It involves evaluating how much reactive power can be absorbed while maintaining system stability, preventing overheating, and meeting voltage requirements.
**What factors limit phase-in operation?**
When the system has an excess of inductive reactive power, the grid voltage may rise, prompting the generator to reduce or even absorb reactive power. This allows the generator to transition from lagging to phase-in operation.
**What is phase-in operation of a generator?**
In normal conditions, the generator emits inductive reactive power due to inductive loads. When the generator enters phase-in operation, its terminal voltage drops, along with the plant power supply voltage. This mode is also referred to as under-excitation or loss of magnetism. The generator operates with reduced excitation, leading to a leading power factor.
In high-voltage transmission systems, the capacitive effect of long lines may exceed the inductive load demand. In such cases, the generator must emit capacitive reactive power, which requires reducing the excitation current. This results in phase-in operation, where the generator operates with a leading power factor.
Not all generators are suitable for phase-in operation, and special design considerations are required when ordering such units.
**What is the relationship between phase-in operation, under-excitation, and loss of magnetism?**
Generators operating below 500kV typically provide inductive reactive power. To do so, the excitation current must be increased, resulting in a lagging power factor. However, when the grid voltage is high and the transmission distance is long, the line’s capacitive effect may dominate. This necessitates the absorption of reactive power, requiring the generator to reduce excitation, entering an under-excited state. At this point, the power factor becomes leading, and the generator is in phase-in operation. If the excitation system fails, the generator loses its field current and enters a loss-of-magnetism state, requiring immediate shutdown.
**What is the power factor of a generator?**
The power factor is defined as the cosine of the phase difference between the generator’s voltage and current. It represents the ratio of active power (P) to apparent power (S), expressed as cosΦ = P/S. Active power is the real power delivered to the load, while reactive power supports the magnetic field needed for electromagnetic conversion.
The stator generates the induced electromotive force and delivers alternating current, while the rotor introduces direct current to establish the magnetic field. These two components operate independently except for mechanical coupling.
**What should you pay attention to when adjusting the power factor of a generator?**
It is generally recommended to adjust the power factor close to 1 for optimal efficiency. Key considerations include adhering to the power supplier’s regulations, ensuring that the rotor and stator currents remain within allowable limits, and avoiding unstable operation or oscillations. For single-unit generators, proper stabilization is essential before making adjustments.
Typical generator power factors range from 0.8 (lagging) to 1. Most generators are not designed to enter phase-in operation, and the power factor is often set according to grid scheduling requirements.
**Understanding active power, reactive power, and power factor:**
Power is divided into three types: active power (P), reactive power (Q), and apparent power (S). The power factor (cosΦ) is the ratio of active power to apparent power. In a right-angled triangle, active power and reactive power form the two legs, while apparent power is the hypotenuse.
Mathematically:
- Apparent Power S = √(P² + Q²)
- Active Power P = S × cosΦ
- Reactive Power Q = S × sinΦ
These values are crucial in determining the efficiency and stability of electrical systems.
**Main protection circuits of a transformer:**
Transformer protection includes differential protection and gas protection. Differential protection detects faults by comparing currents on both sides of the transformer. Gas protection monitors the decomposition of insulating oil, which can indicate internal faults.
**How does differential protection work?**
Differential protection uses Kirchhoff’s Current Law, where the incoming current equals the outgoing current under normal conditions. During a fault, the differential current increases, triggering a trip to isolate the faulty section.
**Primary frequency modulation in power systems:**
Primary frequency modulation is an automatic response to frequency deviations caused by load changes. It involves adjusting the power output of generating units to restore balance and limit frequency fluctuations. Secondary frequency modulation, on the other hand, is a manual or automated process used to restore the frequency to its nominal value after primary adjustments.
**Difference between primary and secondary frequency modulation:**
Primary frequency modulation is performed automatically by the governor, adjusting small load variations. Secondary frequency modulation is a larger-scale adjustment, often managed by control centers, to maintain the grid frequency at a stable level.
In summary, understanding these operational modes and protective mechanisms is essential for ensuring the reliability and efficiency of power systems.