Abstract:In the power supply and distribution systems of the water treatment industry, load equipment such as aeration fans, lifting pumps, and sludge dewatering devices result in a significant amount of reactive power and a large number of harmonics from asynchronous motors. This leads to a decrease in the power factor of the system and poses considerable harm to the distribution system and the load. Consequently, the water treatment industry must enhance its focus on power quality. By utilizing active harmonic filters to treat and filter out harmonics, energy savings and cost reductions can be achieved.
Keywords:Water Treatment Industry Power Supply and Distribution Systems; Harmonics; Power Quality
1. Key Characteristics of Power Quality in the Water Treatment Industry:
The water treatment industry is experiencing stable growth, with factory construction increasing steadily year by year.
1.2 The 1.2 motors and pumps operate continuously with high power factors, resulting in stable reactive compensation.
The primary harmonic source is the variable frequency drive, with a significant harmonic content, necessitating the installation of active harmonic filter equipment for harmonic mitigation.
2. Main Equipment in the Water Treatment Industry
The main high-power equipment in wastewater treatment plants within the water treatment industry includes aeration fans, lift pumps, sludge dewatering equipment, and drying systems. Additionally, there are large-scale air conditioning systems, inverters, and ventilation equipment. The variable frequency drives and control components of these devices are typical nonlinear loads, which generate harmonics that flow into the power distribution system, polluting the grid. This not only poses potential impacts on reactive power compensation equipment but also affects the normal operation of various electrical devices, reducing system efficiency and increasing electricity costs.
2.1 Aeration Fan
The main function of the wastewater treatment aeration fan is to supply oxygen to the pond, primarily used in grit chambers. The aeration fan is connected to the pipeline, which then connects to the aeration disk at the bottom of the aeration pond. The aeration fan blows gas into the aeration pond, providing ample oxygen for microbial activity and promoting the progression of chemical reactions. When the aeration fan is switched on, the power factor is 0.8, and the total harmonic distortion rate is nearly 30%.
2.2 Pump Enhancement
The Lift Pump is an integrated pump product that combines a pump, motor, casing, and control system. Its primary function is to lift wastewater to a certain height and then allow it to flow by gravity. It can control the water volume, thereby regulating the wastewater concentration in the reaction pond. The power factor of the lift pump is 0.75, and the harmonic distortion rate is nearly 30% to 40%.
2.3 Sludge Dewatering
Methods of dewatering include natural drying, mechanical dewatering, and granulation. Natural drying and mechanical dewatering are suitable for wastewater sludge, while granulation is used for sludge that has been coagulated and settled. Dewatering equipment primarily generates 5th and 7th harmonics, with a power factor of 0.8 and a harmonic distortion rate of nearly 20%.
2.4 Frequency Converter
The primary function of a variable frequency inverter is to alter the frequency and amplitude of AC motor power supply, thereby changing the cycle of its motion field, achieving smooth control of the motor speed. Utilizing the soft start feature of the inverter ensures that the starting current begins from zero and does not exceed the rated current even at its peak, reducing the impact on the power grid and the demand for power supply capacity, thereby extending the lifespan of equipment and valves. The use of high-capacity variable frequency inverters requires the addition of an input reactor, effectively preventing disturbances and shocks to power components and preventing conflicts with active filters that could amplify harmonics. The power factor of the inverter is 0.9, with the total harmonic distortion rate ranging from nearly 30% to 50%.
3. Water Treatment Industry Electric Power Quality Case Studies and Solutions
3.1 Incident Phenomenon
A 2000kVA transformer at a Hunan wastewater treatment plant, along with two capacitor compensation cabinets on the transformer's low-voltage side, each with a compensation capacity of 1000kVar, features contactors for switching, all of which are automatically operated. During the debugging and operation of the wastewater treatment plant, the on-duty staff discovered that a set of capacitors and inductors in the low-voltage distribution room's compensation cabinet were burnt out. The remaining capacitors and inductors, which were still in operation, were found to have temperatures ranging from 80 to 100℃ upon on-site inspection. Additionally, the transformer's noise and temperature increased significantly.
3.2 Analysis of Capacitor Cabinet Damage Causes
- Quality issues with capacitors and inductors and other components themselves. The capacitors use poor black rubber filling internally, which is prone to expansion and bulging upon overheating; the external material is an iron casing, susceptible to corrosion in coastal cities or areas with high salt or fog; inductors use aluminum cores, with interfaces oxidizing and becoming explosive at high temperatures.
- During on-site installation, issues with the tightening of electrical connectors were encountered. If there is any loose connection at the interface, the connecting points may locally overheat.
- Parameter matching issues with capacitive reactance. Mismatched series reactance, such as using a 480V capacitor with a 7% reactance rate and a 525V reactor with a 14% reactance rate, can lead to resonance due to the mismatch between the capacitor and reactance.
- Environmental Factors. With changes in the on-site ambient temperature and humidity, the capacitance and inductance parameters vary, leading to a deviation in the series impedance rate. A series resonance occurs between capacitance and inductance, resulting in increased current and severe overheating.
- Paralleled resonance between harmonic voltage-activated capacitor cabinets and loads leads to bus voltage increase, resulting in capacitor damage or overvoltage protection or damage to load equipment.
3.3 Solution
The test point was at the transformer's main outgoing terminal. Based on the actual measured results of the electrical quality at the wastewater treatment plant, the transformer's main outgoing terminal contained 5th, 7th, 11th, and 13th harmonics, with a total harmonic current reaching 200A and a harmonic distortion rate of 27.6%. On-site inspection of capacitors, inductors, and other components revealed good condition and matched inductance rates. The cause analysis indicated that the excessive harmonic current led to fires in the lines and equipment. Ultimately, following our company's recommendation, a 300A active harmonic filter was installed on the transformer's outgoing side.
The harmonic distortion rate on-site is currently stable at below 10%, with the capacitor cabinet operating long-term stably.
APF active power filter employs a DSP+FPGA all-digital control method, parallelly connected in the system, compensating both harmonics and reactive power; capable of fully compensating harmonics from the 2nd to the 51st or compensating specific harmonics; equipped with comprehensive bridge arm overcurrent, DC overvoltage protection, and device overheating protection functions; features automatic operation detection, measurement supervision, and setpoint setting capabilities; equipped with intelligent heat dissipation and stepless speed control functions; offers dynamic expansion capabilities, supports plug-and-play, facilitating replacement.
5. Conclusion
The equipment in the water treatment industry, commonly using variable frequency drives, motors, and pumps, has led to a rapid increase in the types and quantities of nonlinear equipment loads, resulting in increasingly severe harmonic pollution. This poses significant harm to the power distribution systems and on-site equipment. However, the harmonic issues in the power supply and distribution systems of the water treatment industry have not received adequate attention. The direct and indirect economic losses caused by increased energy consumption, equipment failures, and shortened service life are quite substantial. Active harmonic filters play a crucial role in improving power quality, enhancing the safety and economic operation of the power grid, ensuring equipment performance, and reducing energy consumption.
