I. The function of high-voltage isolation transformers
A high-voltage isolation transformer is a 1:1 voltage ratio transformer, meaning the voltage on both sides is the same. The transformer does not perform voltage transformation; it only establishes a magnetic connection in the circuit, not a direct electrical connection, thus providing isolation. The application and function of high-voltage isolation transformers vary depending on the situation.
1. Anti-interference effect: For example, after passing through a Y/Δ connected isolation transformer, it can block the transmission of some harmonics;
2. Impedance transformation function: Increases the system impedance, making it easier to coordinate with protection devices, etc.
3. Stabilizing system voltage: For example, reducing the impact on system voltage when starting up heavy-load equipment;
4. Function to prevent system grounding: When a single-phase grounding occurs on the load side of the isolation transformer, it will not cause a single-phase grounding of the entire system (the part above the isolation transformer);
5. Reduce short-circuit current: Limit the system's short-circuit current when a short-circuit fault occurs on the load side;
6. Other functions.
II. The function of intermediate frequency transformers
An intermediate frequency transformer (IF transformer) is generally used in intermediate frequency amplifier circuits or oscillator circuits. It is a type of transformer with a fixed resonant circuit, unique to superheterodyne receivers. However, the resonant circuit can be fine-tuned within a certain range to ensure an accurate resonant frequency is achieved after connection to the circuit. Fine-tuning is accomplished by changing the relative positions of the fine-tuning capacitor or magnetic core. In vacuum tubes, the primary and secondary windings of the IF transformer are typically tuned circuits, while those used in transistors come in two types: double-tuned and single-tuned.
(1) The intermediate frequency transformer has a frequency selection function. In the intermediate frequency amplifier circuit of the superheterodyne radio, the primary coils of intermediate frequency transformers T1 and T2 form a 465kHz resonant circuit with C1 and C2 respectively, which serves as the load of transistors VT1 and VT2. Therefore, only the 465kHz intermediate frequency signal is amplified, which plays a frequency selection role.
(2) The intermediate frequency transformer has a coupling effect. In the intermediate frequency amplifier circuit of the superheterodyne radio, the output signal of the first intermediate frequency amplifier (VT1) is coupled to the second intermediate frequency amplifier (VT2) through the intermediate frequency transformer T, and the output signal of the second intermediate frequency amplifier (VT2) is coupled to the detector stage through the intermediate frequency transformer T2.
III. The function of excitation transformer
1. Improve the ability of the power system to operate stably.
Regardless of the disturbances encountered, the power system's ability to operate stably can be improved by adjusting the excitation of the synchronous motor. When the power system is subjected to small or large disturbances, causing small or large speed variations in the synchronous motor, the static stability, dynamic stability, or transient stability will be adversely affected. In this case, excitation control will suppress or eliminate this effect, maintaining the synchronous stability of the synchronous motor.
2. Maintain the voltage level of the power system.
The relationship between the generator's internal electromotive force Eq, generator terminal voltage U, generator load current I, and generator reactance x can be expressed by the following formula: Eq = U + jIx Note: Eq, U, and I are vectors. When the grid load increases, that is, the generator current I increases. From the formula, it can be seen that if Eq remains constant, the generator terminal voltage U decreases. If an excitation regulator is installed, the excitation current (i.e., rotor current) can increase with the increase of load, that is, eq increases, thus maintaining the generator terminal voltage U at a certain level. Conversely, after the generator sheds load, the automatic excitation regulator can promptly reduce the excitation current to limit the terminal voltage from rising excessively. Automatically adjusting the generator's excitation can maintain a constant reactive power or power factor in the power supply system. Constant voltage is an important indicator of power supply quality.
3. The ability to increase generator power limits and power system transmission capacity.
4. Improve the operating status of the power system and synchronous generators:
● Improve the reliability of relay protection devices;
When a short-circuit fault occurs in the system, the short-circuit current is attenuated very slowly, or even not at all, by adjusting the excitation (forced excitation). This ensures that the short-circuit current causes the relay protection device to operate accurately and reliably within the set value and time.
● Balance the reactive power among generators during grid-connected operation to ensure a reasonable distribution of reactive power required by the system; When a short-circuit fault in the system is cleared, automatically adjust the excitation to accelerate the recovery of system voltage;
● By controlling the excitation, in addition to maintaining the constant voltage operation of the synchronous generator, the system can also be made to operate with constant reactive power or constant power factor, thereby improving the economic efficiency of power system operation.
5. The excitation regulator for synchronous motors should also meet the following requirements:
● It can automatically adjust the excitation as needed during different stages of the synchronous motor's "starting," "excitation activation," and "traction into synchronization" processes;
●When a synchronous motor or phase condenser is used to compensate for reactive power in the system, the excitation should be adjusted so that the motor has a good reactive power compensation effect and a certain phase-leading capability.
● Depending on the nature and severity of the mechanical load, the excitation regulator should have flexible operating modes to ensure energy efficiency in the power supply system.
