The reactive power compensation controller is a critical component in reactive power compensation systems, playing a central role in ensuring efficient and stable operation. Most manufacturers of such devices purchase controllers from third-party suppliers and assemble their own units, as designing and manufacturing high-performance controllers is a complex task. Only a few companies have the technical capability to develop advanced controllers, making them relatively rare in the market.
One of the key requirements for a controller is accurate measurement of reactive current. While voltage variation is typically small, the accuracy of voltage measurement is not as crucial—usually 1% is sufficient. In many cases, voltage measurement isn't even necessary for basic reactive power control, though it's often used for protection against overvoltage, undervoltage, or phase imbalance.
Current measurement, on the other hand, demands higher sensitivity. For low-end controllers using 8-bit microcontrollers, a sensitivity of at least 1% is acceptable. This means the controller can detect a 1% change in current, such as distinguishing between 100A and 105A in a 500A system. However, this is about sensitivity, not absolute accuracy. High-end controllers with DSP or 32-bit microcontrollers should aim for at least 0.1% sensitivity, which requires four significant digits in the measured value. While absolute accuracy is not always feasible, field calibration is recommended to ensure reliability.
The measurement of power factor also needs careful consideration. A sensitivity of 0.001 is ideal, but it’s more precise to focus on measuring the phase difference rather than the power factor itself. Reactive current is calculated using the formula Iq = I × sinφ, and sinφ should be directly derived from the phase angle. Using sinφ = √(1 - cos²φ) can lead to inaccuracies, especially when cosφ is close to 1. For example, if cosφ = 0.99, the corresponding phase angle is around 8.1°, and sinφ ≈ 0.14. Small changes in cosφ can result in large errors in sinφ, leading to incorrect reactive power calculations.
Phase difference measurement must cover the full range of -180° to +180°. Some controllers have an automatic CT polarity detection feature, but this limits the range to -90° to +90°, which may cause issues. For instance, during power generation, the controller might misinterpret the load, or when dealing with purely inductive or capacitive loads, it could mistakenly identify one as the other. This can lead to overcompensation and potential damage to the system.
In terms of display options, LED digital tubes are commonly used due to their low cost and reliability. Multi-position LED displays help reduce wiring complexity and simplify installation. While LCDs are popular for displaying Chinese characters and being more energy-efficient, they suffer from poor low-temperature performance, typically failing below -10°C. Therefore, they should only be used in environments where temperatures remain above that threshold.
Parameter setting is essential for most reactive power controllers. Since parameters like capacitor rating and CT ratio vary depending on the application, the controller must allow users to set and store these values. EEPROM devices like 24C02 or internal flash memory in microcontrollers are common solutions for non-volatile storage.
Protection functions are also crucial. Overvoltage and harmonic overload protection should be included to prevent capacitor damage. Detecting voltage harmonics helps identify the root cause of overloading, reducing the need for additional components like thermal relays, thus saving space and cost.
Capacitor switching strategies should be implemented step by step to avoid sudden current changes that could destabilize the system. For example, if a 40kVAR capacitor is needed, it should be switched in directly instead of incrementing by smaller steps. This approach minimizes switching frequency and improves efficiency.
Finally, the output circuit design affects the controller’s performance. Common outputs include AC contactors or solid-state relays. Electromagnetic relays require proper isolation to prevent electrical interference, while electronic relays (often based on thyristors) offer faster switching and lower power consumption. Bidirectional thyristors are another option, offering good reliability at a lower cost.
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