[Summary]To minimize harmonic emissions from wastewater treatment equipment, by identifying primary harmonic sources, detecting and calculating harmonic components, and employing active power filters for harmonic mitigation, we have significantly reduced the rate of three-phase current distortion in the power system, enhancing power quality. This has suppressed harmonic components, decreased electrical equipment failures caused by harmonics, and ensured the normal operation of the power system.
[Keywords]Harmonic Source; Harmonic Compensation; Active Power Filter; Current; Wastewater
1. Introduction
The issue of water pollution in our country is becoming increasingly prominent, severely hindering economic and social development. As the main facility for wastewater treatment, sewage treatment plants play a huge role in protecting the environment. With a vast scope, large capacity, and numerous equipment, wastewater treatment systems directly impact people's work and daily lives.
The primary electrical equipment in the wastewater treatment plant includes blowers and pumps, particularly aeration blowers and inlet lift pumps, which have high individual power ratings. Moreover, due to process requirements, many devices are controlled by variable-frequency drives (VFDs), such as aeration blowers often using VFDs for speed control to adjust the aeration volume and gas pressure in the biochemical pond, achieving optimal operational status for biochemical treatment. Inlet lift pumps commonly use VFDs for speed control to adjust the pump speed based on the liquid level in the pump tank for constant level control.
The wastewater treatment plants have high levels of automation control, requiring monitoring of flow rates, liquid levels, pressures, and water quality analysis in the process treatment procedures. It involves collecting operational status signals from key process and electrical equipment, setting up automatic control and automatic adjustment systems to meet the operational requirements of the equipment. This necessitates a large number of PLC controllers to be implemented.
Transformers, electric motors, inverters, PLC controllers, and other equipment in the wastewater treatment plant operate and generate a large amount of harmonic currents of various orders [1], making them the primary harmonic sources in the plant. These harmonics have varying degrees of impact and hazards on the plant's power distribution and electrical equipment, posing potential safety risks.
Comparison of Harmonic Compensation Devices
Currently, the primary method for harmonic control in wastewater treatment plants is to install harmonic compensation devices in the power system. These devices are further categorized into passive and active types.
A passive filter is a filtering device composed of a combination of filter capacitors, resistors, and inductors, which is connected in parallel with the source of interference. It not only serves as a filter but also provides appropriate reactive power compensation. The main drawback of a passive filter is that it is susceptible to the grid impedance and operating conditions, easily causing parallel resonance with the power system, leading to harmonic amplification and potentially overloading or damaging the filter. Additionally, a single filter can only compensate for harmonics at a fixed frequency, and the compensation effect is not always ideal.
An active filter is a new type of power electronic device that operates by first detecting harmonic currents in the compensation object. Subsequently, the compensation device generates a compensating current that is of equal magnitude but opposite polarity to the harmonic current, effectively canceling each other out and filtering out the harmonic components from the power grid. The advantages of this filter are that it can compensate for harmonics with varying frequencies and amplitudes, and its compensation characteristics are not affected by the grid impedance or operating conditions, making it widely applicable.
Application Examples of Active Power Filters
3.1 Project Overview
A wastewater treatment plant in a city in Shanxi Province has installed two SCBl0—800/10 800kVA 10/0.4kV dry-type transformers. The low-voltage system uses a single busbar segmented connection. Under normal circumstances, the bus coupler circuit breaker is open, and the two transformers operate in parallel.
Load factor for each transformer is approximately 70%.
The main high-power variable-frequency equipment in the factory includes two 90kW blowers and two 75kW water-lifting pumps, which are powered and controlled by the low-voltage variable-frequency cabinets within the substation. The variable-frequency inverters are installed in the variable-frequency control cabinets. The main motor loads and PLC cabinets in the entire factory are also powered by the low-voltage switchgear within the substation. Therefore, the main harmonic sources in the factory are concentrated within the low-voltage system of the substation.
3.2 Detection and Calculation of Harmonic Current
To determine the harmonic conditions within the low-voltage system, harmonic detection software was used to measure the harmonic voltages and currents within the system. The detection waveforms and data are shown in Figure 1.
It can be seen from the image:
(1) The measured voltage waveform distortion rate is approximately 1.7%, which generally conforms to the relevant national standards.
(2) The measured current distortion rate is approximately 7.9%, with a relatively significant current waveform distortion. The harmonic currents are primarily generated by the 3rd, 5th, and 7th harmonics.
Based on the capacity, load factor, and harmonic distortion rate of the transformer within the factory, the harmonic current of a section of busbar is calculated. The calculation formula for harmonic current is as follows:
Symbol meanings: S is transformer capacity, K is transformer load factor, U is rated voltage of transformer secondary side; IHR is harmonic current; THDi is total harmonic distortion rate of current, measured at 7.9%.
After calculation,
3.3 Harmonic Mitigation Solution
Due to the harmonic sources being primarily located within the low-voltage switchgear of the substation, we have considered centralized harmonic compensation. An active power filter has been installed in the low-voltage system, and based on the calculated results of the harmonic current, a filter with the maximum output compensation current of 75A has been selected.
Due to the simultaneous presence of harmonic compensation devices and reactive power compensation devices in the system, the compensation access point for the active power filter should be between the reactive power compensation device and the harmonic load, as close to the harmonic load as possible. Additionally, the compensation access point for the active power filter should be at the seven-tap of the current transformer, to ensure that the downstream of the current transformer does not include capacitive load current and the harmonic compensation current injected by the active power filter itself. The specific compensation scheme is shown in Figure 2.
3.4 Harmonic Mitigation Effect
After installing the active power filter, the special wave voltage and harmonic current within the low-voltage system were re-measured. The actual test results are shown in Figure 3.
Figure 2 shows that after applying the active filter, the system's three-phase voltage distortion rate has decreased from 1.7% to 1.5%, and the three-phase current distortion rate has dropped from 7.9% to 4.2%, with the primary 3rd, 5th, and 7th harmonic contents significantly reduced. The current waveform within the system has transformed into a smooth sine curve, demonstrating a notable harmonic mitigation effect. The harmonic mitigation device
The results are as follows:
Significantly reducing the three-phase current distortion rate in the power system, with an average reduction of about 50%, smooths out the current and voltage waveforms, enhancing the quality of electrical energy; suppresses the main harmonic components, reducing equipment failures caused by harmonics, ensuring the normal operation of the power system.
AcrelEMS-SW Smart Water Efficiency Management Platform
4.1 Platform Overview
AcrelEMS-SW's Smart Water Efficiency Management Platform offers an ecosystem from terminal perception and edge computing to energy efficiency management platforms. By installing protective, monitoring, analytical, and treatment devices at key nodes in the source, network, load, storage, and charging of wastewater treatment plants, it monitors the total and intensity of energy consumption in the plants. It focuses on the energy efficiency of major energy-consuming equipment, ensuring the safe and reliable operation of wastewater treatment plants, enhancing their energy efficiency, and providing scientific and refined solutions for energy efficiency management in wastewater treatment.
4.2 Platform Composition
The AcrelEMS Smart Water Utility Comprehensive Energy Management System consists of a substation comprehensive automation system, electric power monitoring, and energy management system. It covers water supply medium-voltage sub-distribution systems, electrical safety, emergency power supply, energy management, lighting control, and equipment maintenance operations, throughout the entire water utility energy flow. It assists operations and maintenance managers in real-time monitoring the operation status of the water supply distribution system through a single platform and an APP, and can be applied to the management needs of water utility support departments based on permissions.
4.3 Platform Topology Diagram
4.4 Platform Subsystem
Substation Integrated Automation System and Power Monitoring
Our water and power distribution system now includes relay protection and arc protection for 35kV and 10kV voltage levels, enabling remote measurement, signaling, control, and adjustment functions, and providing timely warnings for abnormal situations.
Monitor transformer, pump, and blower current, voltage, active/inactive power, power factor, load factor, temperature, three-phase balance, and abnormal alarm data.
4.4.2 Power Quality Monitoring and Management
A significant number of high-power motors and pumps in the water industry result in a large amount of harmonic distortion in the power distribution system. By monitoring the harmonic distortion, voltage fluctuations, flicker, and tolerance indicators of their power distribution system, the energy quality is analyzed, and corresponding energy quality treatment measures are configured to improve the quality of supplied electricity.
4.4.3 Electric Motor Management
Motor monitoring achieves protective, remote measurement, remote signaling, and remote control functions in water conservancy. Motor protectors can protect, monitor, and alarm against anomalies such as overload, short circuit, phase loss, and leakage. It efficiently and accurately reflects fault status, fault time, fault location, and related information, enabling health diagnosis and preventive maintenance for motors. Additionally, it supports integration with PLCs, soft starters, variable-frequency drives, etc., to achieve automatic or remote control of motors, monitor and control various process equipment, ensuring normal production.
Energy Consumption Management
Establishing a metering system for water utilities to display energy flow and losses, utilizing energy flow charts to assist in analyzing energy consumption patterns, and identifying areas of abnormal energy use.
Centralize all energy-related parameters on a dashboard, compare and analyze them from multiple dimensions, achieve energy consumption comparisons across various process stages, and assist leaders in managing the entire factory's energy consumption, energy costs, and standard coal emissions.
Energy consumption data is collected and statistically analyzed from wastewater treatment plants, waterworks, pump stations, etc., covering electricity, water, gas, and heat/cold consumption. Comparative analysis is conducted through year-on-year and month-on-month comparisons, calculating the total energy consumption and intensity, as well as the trend of standard coal equivalent calculation and CO2 emissions statistics.
Energy efficiency analysis is conducted based on a three-tiered measurement framework, aligning with the requirements of the energy management system. It allows for the assessment of energy efficiency levels across various workshops/functional departments, including year-on-year, month-on-month, and benchmark comparisons. By analyzing the output of wastewater treatment and energy consumption data collected by the system, a trend chart of specific energy consumption in wastewater is generated, along with year-on-year and month-on-month analyses. Additionally, the specific energy consumption of wastewater is benchmarked against industry/national/international advanced indicators, enabling the company to adjust production processes based on product specific energy consumption, thereby reducing energy consumption.4.4.5 Smart Lighting Control
The system provides lighting control management solutions for wastewater treatment plants, waterworks, pump stations, etc., supporting various control methods such as single control, area control, automatic control, sensor control, timed control, scene control, and dimming control. The modules can automatically identify sunrise and sunset times based on latitude and longitude to enable automatic control functions, maximizing the use of natural light for intelligent indoor and facility lighting, achieving the goals of safety, energy saving, comfort, and efficiency.
5. Hardware Selection List for Platform Deployment
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Application Scenario (0.4KV) |
Product |
Model |
Feature |
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0.4KV incoming/outgoing lines |
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APView500 |
Phase voltage and current + zero sequence voltage and current, voltage and current unbalance degree, active and reactive power, and electrical energy, event alarms and fault wave recording, harmonics (voltage/current 63rd harmonic, 63rd inter-harmonic, harmonic phase angle, harmonic content rate, harmonic power, harmonic distortion rate, K-factor), fluctuation/flicker, voltage transient rise, voltage transient drop, voltage transient, voltage interruption, 1024-point waveform sampling, triggering and timed recording, real-time waveform display and fault waveform viewing, PQDIF format file storage, 32G memory, 16D0+22D1, communication 2RS485+1RS232+1GPS, 3 Ethernet interfaces (+1 maintenance network port) + 1 USB interface supports USB drive data reading, supports 61850 protocol |
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APM520 |
This meter employs AC sampling technology to individually measure parameters such as current, voltage, power, power factor, and electrical energy in the power grid. It allows for setting the scale through the panel's membrane switch. It features an RS-485 communication interface, using Modbus protocol; it can also convert electrical signal into standard DC analog signals for output; or it can include switch input/output, relay alarm output, and other functionalities. |
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Power Quality |
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ARC |
Measure I, U, Hz, cosΦ, equipped with overvoltage protection, undercurrent lockout, and excessive grid harmonic protection functions; capable of controlling the switching of capacitors, RS485/Modbus protocol |
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ANSVC |
The ANSVC low-voltage reactive power compensation device is connected in parallel throughout the power supply system. It can compensate for changes in the power factor of the grid load by controlling the switching of electrical capacitors through a controller. The reactive power compensation device employs a modular assembly scheme, primarily consisting of capacitors, inductors, switching switches, and controllers. |
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ANAPF |
The ANAPF series active power filter collects harmonic currents from the current transformer system, quickly calculates and extracts the content of each harmonic current by the controller, generates harmonic current commands, produces a compensating current with the same magnitude but opposite direction to the harmonic current through power execution devices, and injects it into the power system, thereby canceling out the harmonic currents generated by nonlinear loads. |
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6 Summary
In wastewater treatment plant projects, harmonic compensation solutions should be scientifically and reasonably selected based on the type, magnitude, and distribution of the actual harmonic load. A single compensation scheme can be used, or a combination of multiple schemes. When both harmonic and reactive compensation are required in the power grid, a mixed dynamic harmonic elimination device can also be employed. Additionally, according to harmonic current calculations, an appropriate active power filter model should be chosen to achieve harmonic control in the wastewater treatment plant's power grid, ensuring power quality and the safe operation of electrical equipment.
Reference
- Sun Meijun. Industrial and Civil Distribution Design Manual [M], Beijing: China Power Press, 2005.
- Wang Zhaoan, Yang Jun, Liu Jinjun. Harmonic Suppression and Reactive Power Compensation. M: Beijing Machine Press, 1998.
