Abstract : This paper mainly introduces the application of ZINVERT intelligent high-voltage variable frequency speed control system in the high-voltage variable frequency retrofit of the primary blower of the 440t/h circulating fluidized bed unit at Guangdong Lianzhou Yuelian Power Plant Co., Ltd. The paper elaborates on the system control method using automatic power frequency bypass, the achievements of the retrofit, and the system's operational reliability.
Keywords : High-voltage variable frequency drive; Circulating fluidized bed boiler; Primary air blower; Reliability; Energy saving; Automatic power frequency bypass
0 Introduction
Circulating fluidized bed boiler technology is a high-efficiency, low-pollution, and clean coal-fired technology that has developed rapidly in recent decades. Applying variable frequency speed control equipment to the blower of a circulating fluidized bed boiler can reduce the plant power consumption rate of the unit. Moreover, due to the excellent characteristics of variable frequency speed control, its application can lay the equipment foundation for the automatic control of the boiler unit and reduce the operating cost of the unit from the perspective of energy saving and operation management. The working condition of the primary blower of a circulating fluidized bed boiler is closely related to the fluidization state of the material in the furnace, combustion stability, temperature field distribution in the furnace, calcium-sulfur ratio of the material, and other important indicators for the normal operation of the circulating fluidized bed boiler [1]. The primary blower of a large-capacity circulating fluidized bed boiler plays a crucial role in the entire boiler flue gas system. Therefore, the high-voltage variable frequency drive that is matched with it must have extremely high operating stability and fault resistance to ensure the continuous operation of the primary blower to the maximum extent.
Long-term application has proven that the ZINVERT series intelligent high-voltage variable frequency speed control system is stable and reliable in operation, has strong fault tolerance, and offers comprehensive technical solutions. In March 2007, Guangdong Lianzhou Yuelian Power Plant Co., Ltd. selected the ZINVERT intelligent high-voltage variable frequency speed control system through bidding to retrofit the primary blowers of boilers #3 and #4 with high-voltage variable frequency technology. In July 2007, during the Class B overhaul of boiler #4 at Guangdong Lianzhou Yuelian Power Plant Co., Ltd., the high-voltage variable frequency drives for the primary blowers A and B of boiler #4 were successfully put into operation on the first attempt, thanks to the hard work of the power plant and Zhiguang Company's technical personnel. Since its commissioning, the equipment has operated stably and achieved significant energy-saving results.
1. User Operating Conditions Overview
Units #3 and #4 of the Guangdong Lianzhou Yuelian Power Plant Co., Ltd. are 135MW circulating fluidized bed (CFB) units. Units #3 and #4 were put into operation in March 2004 and May 2004, respectively. The boilers are HG-440/13.7-L.WM9 type ultra-high pressure intermediate reheat, single-drum natural circulation, CFB boilers designed and manufactured by Harbin Boiler Factory Co., Ltd., using CFB boiler technology from Alstom. Each boiler is equipped with two primary air blowers, A and B, which supply primary air to the boiler system through a main pipe. Each primary air blower has dampers at the inlet and outlet for adjusting the primary air volume. While damper adjustment is inexpensive and intuitive, it results in significant energy waste. With increasing environmental awareness and energy conservation and emission reduction requirements, this adjustment method is clearly outdated. The nameplate parameters of the primary air blowers for boilers #3 and #4 are as follows.
Primary air fan nameplate parameters: Model: L2N22606 Type: Single-pole double-support centrifugal type Rated speed: 1450 r/min Rated air volume: 136123 m3/h Rated air pressure: 26.0 kPa Primary air fan motor nameplate parameters: Model: YKK560-4 Rated power: 1250 kW Rated voltage: 6 kV Rated current: 136 A Rated speed: 1493 r/min
2. Technical Features of ZINVERT Series High Voltage Frequency Converters
The ZINVERT intelligent high-voltage variable frequency speed control system is a direct high-voltage output voltage source type frequency converter. It achieves high-voltage output by cascading multiple H-bridge power units, directly driving high-voltage motors without the need for voltage boosting or additional filtering devices. Its harmonic performance meets the most stringent requirements of IEC and national standards for power grid harmonics. The ZINVERT intelligent high-voltage variable frequency speed control system has the following main technical features:
1) Complete automatic bypass technology – unit automatic bypass and main circuit automatic bypass technology. In the event of a unit failure, the faulty unit automatically bypasses to ensure continuous system operation. The main circuit automatic bypass technology enables bidirectional free switching between power frequency and variable frequency operation modes with rapid transitions, without stopping the motor during the transition and without affecting production.
2) Strong resistance to voltage fluctuations – the system will not shut down when the grid voltage fluctuates within the range of 65% to 115% of the rated voltage.
3) Proprietary Core "STT" Technology – Based on years of research in motor operation and control, Zhiguang's technical staff developed the unique "STT" algorithm. This algorithm ensures that the motor can start the variable frequency speed control system directly and without impact at any speed within the speed range, without stopping the motor. This technology ensures that when external factors (such as instantaneous voltage drops, power outages, or external interference) cause the inverter to stop, it can be quickly and without impact restarted, regardless of the motor's condition.
4) High-voltage output sudden short circuit "SCP" protection technology – "SCP" protection technology can effectively protect the frequency converter when a two-phase sudden short circuit occurs in the output cable of the motor or frequency converter. ZINVERT intelligent high-voltage variable frequency speed control system possesses this patented technology.
5) Multi-circuit power supply technology for control power supply - The system has four control power supplies that serve as backups for each other and can automatically and seamlessly switch between them, ensuring that the system will not shut down due to control power supply failure to the greatest extent possible.
6) Intelligent acceleration and deceleration control - The system automatically identifies whether the acceleration and deceleration control speed is appropriate, and automatically adjusts the rate of change according to the working conditions. While achieving the control target as quickly as possible, it can also effectively prevent the system from stopping due to excessive acceleration or deceleration.
7) Unique capacitor charge and discharge test function - The system can perform charge and discharge tests on the unit filter capacitor to evaluate its service life and make it easy to understand the working status of the capacitor.
3. System Main Wiring and Control Method
The primary blowers (A and B) of boilers #3 and #4 at Lianzhou Power Plant are controlled by a ZINVERT-A6H1500/06Y intelligent high-voltage variable frequency speed control system. The system is equipped with a 6kV automatic bypass cabinet, enabling instantaneous automatic switching between power frequency and variable frequency operation. The main circuit of the system is shown in the diagram below.
Figure 1. Main circuit diagram of primary blower for boiler #4 at Lianzhou Power Plant
The control scheme takes the following into consideration in its design:
1) K1, K2, K3, and K4 disconnectors have no mechanical interlocking function; they are only manually disconnected during maintenance to create a clear disconnection point and ensure the safety of personnel. They are in the closed state under both power frequency and variable frequency operation conditions.
2) The power frequency bypass contactor J3 has an electrical interlocking function with the frequency converter incoming contactor J1 and the frequency converter outgoing contactor J2, and they cannot be closed at the same time.
3) When operating under variable frequency conditions, J1 and J2 are closed, and J3 is open.
4) When the power frequency bypass is in operation, J3 is closed and J2 and J1 are open.
When the primary blower is running at variable frequency, boiler operators remotely control the inverter from the central control room, performing operations such as setting the target frequency, starting, stopping, and emergency stopping, and monitoring the inverter's operating status. In variable frequency operation, the inlet and outlet dampers of the primary blower are fully open, and the air volume is adjusted using the inverter's operating frequency regulation method. During maintenance of the variable frequency speed control system, the primary blower operates at the mains frequency, and the air volume is adjusted using the original inlet and outlet damper regulation method. Operators can easily switch between automatic and manual operation of the system between mains frequency and variable frequency operation.
3.1 Switching from Power Frequency to Variable Frequency Operation Modes: After the primary blower has been running at power frequency and the variable frequency speed control system has been inspected and confirmed to meet operating conditions, the power frequency to variable frequency operation mode can be switched. The power frequency to variable frequency operation mode switch can be performed manually or automatically. Before switching, the damper of the primary blower to be switched is closed slightly, while the output of the other primary blower is increased. Most of the blower load is transferred to the primary blower that is not switching, thus minimizing the impact of the mode switch on production. The switching process is shown in Figure 2.
Figure 2. Flowchart of power frequency to frequency conversion
The primary air fan's operating state can only be switched after the system (variable frequency speed control system, boiler) meets the state switching conditions. In manual mode, follow the process shown in Figure 2; in automatic mode, press the "Industrial Frequency to Variable Frequency" button, and the system will automatically complete the switch.
3.2 Variable Frequency to Power Frequency Switching During Operation: When a shutdown fault occurs in the speed control system, the system automatically switches from variable frequency to power frequency. The switching process is shown in Figure 3.
Figure 3 Flowchart of frequency converter to power frequency conversion
The system determines that the damper is in position upon receiving the "damper position signal". The manual switching procedure is the same as the automatic switching procedure.
4. Testing and Results of the Renovation Project
On July 4, 2007, when the B-level overhaul of Boiler #4 was completed, under cold conditions, the power plant technicians and Zhiguang technicians conducted functional tests and cold-state energy-saving tests on the primary air fan control system after the frequency converter modification.
4.1 Comparison Test of Output of Primary Air Blower Variable Frequency Drive in Normal and Variable Frequency Operation for Boiler #4
Variable frequency drive (VFD) operation output test conditions: One induced draft fan, two primary air fans (VFD operation), one secondary air fan, and two high-pressure fluidizing fans are started. The high-pressure fluidizing air volume is 1000 m³/h, the furnace negative pressure is -50 Pa, and the primary air volume is controlled by adjusting the VFD operating frequency with the dampers fully open. Power frequency (PSV) operation output test conditions: One induced draft fan, two primary air fans (PSV operation), one secondary air fan, and two high-pressure fluidizing fans are started. The high-pressure fluidizing air volume is 1000 m³/h, the furnace negative pressure is -50 Pa, and the primary air volume is controlled by adjusting the damper opening. Comparative experimental data are shown in Table 1.
Table 1. Comparison of Primary Air Fan Operating Output
4.2 Test of the switching function between normal and variable frequency drives of the primary blower frequency converter for boiler #4
To verify the performance of the primary air fan control system after the frequency converter upgrade, especially the new control system's ability to withstand frequency converter failures, a primary air fan switching test between power frequency and frequency converter was conducted. During the power frequency to frequency converter switchover test, the transition was very rapid, and the furnace pressure and total primary air volume remained essentially unchanged; this test process will not be detailed here. The following is the experimental record of the primary air fan B switching from frequency converter operation to power frequency operation when the frequency converter of primary air fan B "failed". Test conditions: 2 induced draft fans operating; primary air fan A operating at frequency converter, primary air fan B operating at power frequency, total primary air volume 200,000 m³/h; 2 secondary air fans operating, secondary air volume 260,000 m³/h; 2 high-pressure fluidizing fans operating, high-pressure fluidizing air volume 1000 m³/h, furnace negative pressure -50 Pa. Thermal logic: When the primary air fan of boiler #4 experiences a "frequency converter major fault", the fan will automatically start at the power frequency when the inlet damper of primary air fan B is closed to 90% (the target for the inlet damper of primary air fan B is to be closed to 30%). Test data are shown in Table 2.
4.3 Experimental Conclusions
Experimental results show that the frequency conversion retrofit achieved the expected results, and the following experimental conclusions were drawn: 1) After the retrofit, the primary air fan not only met the operational requirements but also achieved significant energy-saving benefits. 2) After the retrofit, the primary air fan control system has excellent inverter fault tolerance. The conversion from power frequency to frequency conversion is rapid and has virtually no impact on boiler operation; the duration of the frequency conversion to power frequency process is short, and the lowest furnace negative pressure value is -600Pa, far lower than the boiler MFT value of -1500Pa. Data records are shown in Table 2.
Table 2 Boiler Operation Data Sheet During Variable Frequency to Fixed Frequency Conversion
5. Conclusion
The 135MW circulating fluidized bed boiler at Lianzhou Power Plant has maintained stable operation since the high-voltage frequency converter for its primary blower was put into service, demonstrating that high-voltage frequency conversion energy-saving retrofitting of auxiliary equipment for large circulating fluidized bed boilers is indeed feasible. Using high-performance high-voltage frequency converters coupled with efficient automatic bypass systems for high-voltage frequency conversion retrofitting of auxiliary equipment in large circulating fluidized bed boilers not only achieves excellent energy-saving results but also ensures the long-term safe operation of the unit.
References
[1] Zhu Aiqiang, Rui Xinhong (eds.). Circulating fluidized bed boiler equipment and systems. Beijing: China Electric Power Press, 2004.
[2] Training materials for ZINVERT series high voltage variable frequency speed control system, Guangzhou, Guangzhou Zhiguang Motor Co., Ltd., 2006.
About the author :
Liang Baohua (1970-) is a male from Yunfu City, Guangdong Province. He works in the Safety Production Technology Department of Guangdong Lianzhou Yuelian Power Plant Co., Ltd., and is engaged in the electrical technology management of the power plant.
Fu Jinjian (1984-) is a male from Mianyang City, Sichuan Province. He is a technical support engineer at Guangzhou Zhiguang Motor Co., Ltd., and is engaged in the research and application of high voltage frequency conversion technology.
