Inverter Helps Thermoelectric Enterprises Reduce Energy and Reduce Emissions
With the continuous advancement and refinement of frequency conversion technology, its remarkable energy-saving performance, superior starting characteristics, and comprehensive protection functions have been effectively applied in enterprises. This has enabled companies to save energy, reduce operational costs, and enhance overall system efficiency (ASD).
1. **Features of the Inverter**
Frequency control is considered one of the most advanced and promising technologies for speed regulation. The inverter is a key product used for adjusting the speed of three-phase asynchronous motors in industrialized countries. Its main advantages include:
- Excellent energy-saving effect, with potential savings up to 55%.
- A wide speed range, with a speed ratio of up to 20:1.
- Superior starting and braking performance, allowing for smooth starts, automatic acceleration and deceleration, and rapid braking.
- Comprehensive protection functions, including overvoltage, undervoltage, overload, overcurrent, power failure, short circuit, and stall protection, along with fault detection and display.
- Easy integration into computer systems, enabling remote control and efficient operation.
2. **Application of Frequency Control Technology in Cogeneration Systems**
2.1 **Central Heat Exchange Station and External Network Control**
The boiler system typically includes constant pressure water supply, hot water circulation, combustion control, and furnace pressure regulation. In cogeneration systems, superheated steam can be directly output or exchanged for steam and water before being exported. The thermal energy from steam or hot water is delivered to users through various heat exchange stations in the pipeline network.
2.2 **Control of External Network Circulation Pumps**
Maintaining stable pressure in the heating system is essential for its normal operation. The heating network is usually a closed-loop system, where theoretically, water consumption is minimal. However, practical issues such as leaks, blowdowns, and human errors inevitably affect pressure stability. Additionally, temperature fluctuations in the system can also cause pressure changes.
To ensure stable heat supply, the system must remain fully filled at the highest point, with pressure within safe limits. A constant pressure point is set on the return pipe, and the pressure is maintained at a stable value. This helps regulate the flow and head of the circulation pump, ensuring automatic adjustment of both parameters. It also helps prevent overheating and overpressure in the boiler system.
A constant-pressure water replenishment pump is used to adjust the system’s pressure. By controlling the pump’s speed, the water supply can be adjusted in real time, maintaining stable water volume. A pressure sensor converts the pressure signal from the constant pressure point into a (4–20) mA current signal, which is sent to a pressure regulator. The regulator compares this signal with a preset value and sends a frequency command to the inverter, which adjusts the pump motor speed accordingly.
Since water is incompressible, the pressure response is fast, making it easy to maintain the desired pressure level.
In the heating system, water acts as the medium for thermal energy transfer. The higher the water temperature, the more heat is carried per unit volume, and the greater the flow, the more heat is delivered. Using frequency conversion speed control allows for precise regulation of the circulation pump’s flow, ensuring the system operates safely and efficiently.
By following guidelines that recommend low water temperatures and small temperature differences, the system’s water temperature is adjusted based on ambient conditions. The return water temperature difference is used to regulate the circulation pump speed, indirectly controlling the system’s temperature. A temperature sensor converts the sampled temperature into a voltage or current signal, which is compared with a preset value by the regulator. The regulator then sends a frequency command to the inverter, which adjusts the pump motor speed accordingly.
2.3 **Control of Induced Draft Fan with High-Voltage Frequency Conversion**
High-voltage frequency converters are series-connected devices that use multiple single-phase three-level inverters connected in series to produce variable-voltage high-voltage AC. According to the basic principles of electrical machinery, the motor’s rotational speed depends on the operating frequency, as shown in the formula:
n = (60f)/P × (1 - s)
Where:
- n is the actual motor speed,
- f is the operating frequency,
- P is the number of motor pole pairs,
- s is the slip.
Adjusting the frequency allows for precise control of the motor’s speed. The slip is influenced by the load; as the load increases, the slip increases slightly, causing the motor speed to decrease.
The inverter consists of three main components: a transformer cabinet, a power cabinet, and a control cabinet. High-voltage electricity enters through the switchgear and is stepped down and phase-shifted via the power unit in the power cabinet. Each power unit is divided into three groups, corresponding to each phase, and their outputs are connected in series. The control unit communicates with each power unit via optical fibers, allowing for real-time adjustments based on user input.
On the input side, the phase-shifting transformer supplies each unit, and its secondary windings are divided into three groups. This multi-level phase-shifting rectification method improves the grid-side current waveform, resulting in a near-unity power factor. Additionally, the independent design of the secondary windings makes each power unit function similarly to a conventional low-voltage converter.
On the output side, the U and V terminals of each unit are connected in series using a star configuration to supply power to the motor. The PWM waveforms from each unit are reorganized into a staircase-like waveform, producing a clean sinusoidal output with low dv/dt. This reduces insulation damage to cables and motors, eliminates mechanical vibrations, and lowers stress on bearings and blades. The motor does not require derating, making it suitable for retrofitting existing equipment.
2.3.1 **Inverter Operation and Startup**
There are three main operating modes for the inverter:
- Local control: Directly operate the inverter using the touch screen on the control cabinet to start, accelerate, decelerate, reset, or stop.
- Remote control: Use a remote controller to send commands and setpoints through switches and analog inputs.
- DCS control: Integrate with a distributed control system for automated and centralized management.
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