Abstract: High-frequency AC motor variable frequency speed control technology has become the first choice for energy-saving and power-reduction projects of thermal power plants because of its excellent speed control performance, significant power saving effect and wide applicability. In order to transform the condensate pumps of other ultra-supercritical units in future, and make the high-voltage motors and frequency converters after frequency conversion can be operated safely, stably, and efficiently, it is necessary to summarize some experiences in the practice of frequency conversion retrofitting in order to serve as a reference.
Keywords :Ultra-supercritical condensation pump Frequency converter Regulatory characteristics Energy saving
I. INTRODUCTION In this paper, the application of domestic multi-level high-voltage frequency converters in condensate pumps at Datang Ningde Power Plant is taken as an example. The operating conditions, basic principles, and precautions before and after the application of high-voltage inverters to condensate pumps are respectively described and passed. Contrast test of consumption compares the energy saving effect of variable frequency control of condensate pump and the traditional adjustment of valve adjustment, and further illustrates the application method of ultra-supercritical unit adopting high-voltage frequency converter to speed control and energy-saving transformation of condensate pump, which has the advantages of low investment and quick effect. Features.
Ultra-supercritical coal-fired generating units have the characteristics of low coal consumption, high technical content, good environmental protection performance, and resource conservation, and will certainly be the development direction of China's thermal power units in the future. The second phase of Datang Ningde Power Plant is a 2×660MW ultra-supercritical generating unit, which was put into commercial operation in December 2008 and June 2009 respectively. Since the start of production, the unit's various operating indicators are good. Each of the 2 units is equipped with 2 sets of multi-stage centrifugal condensate pumps with 100% capacity. The original designed operation mode of the condensate system is that two sets of 100% capacity fixed-speed condensation pumps are used for each other, that is, they are driven by a fixed-speed motor, and one is operated. One spare. The condensate flow is regulated by the deaerator upstream adjustment valve and is equipped with a bypass to ensure that the condensate pump operates safely under various conditions. Under normal operating mode, one condensate pump runs one standby and when the condensate pump fails, the other one is automatically put into operation. When the load changes, the water level of the deaerator is controlled by adjusting the door opening of the deaerator water, which results in a large throttling loss. In order to maintain the minimum flow of the condensate pump, the bypass valve is opened at the time of starting and stopping and low load, resulting in loss of energy. In addition, even if the unit is in full-load operation, the deaerator upstream adjustment door cannot be in the fully open position due to the design leaving a large margin. With the condensate pump running at a constant speed, the system has the following problems:
(1) The valve adjustment adjusts the throttling loss, the export pressure is high, the tube loss is serious, the system efficiency is low, and the energy is wasted.
(2) When the flow rate decreases and the position opening decreases, the pressure difference between the front and rear of the regulating valve increases and the work safety characteristics deteriorate, resulting in serious pressure loss and increased energy consumption.
(3) Long-term low valve opening of 10 to 40%, accelerating the wear of the valve body, leading to poor control characteristics of the valve, and causing greater vibration of the pipe near the condensate, which has a great impact on the safety of production.
(4) The high pressure of the pipe network threatens the sealing performance of the system equipment. In severe cases, it causes the valve to leak, it cannot be closed tightly, and the condensation pump outlet fine processor leaks.
(5) The equipment has a short service life, a large amount of routine maintenance, and high maintenance costs, resulting in great waste of various resources.
If a high-voltage frequency converter is used to control the frequency of the condensate pump motor, the variable load adjustment of the deaerator water flow is realized. The deaerator upper water conditioning door can always be fully open, and the bypass door is always closed, thereby avoiding the above-mentioned various power losses. In addition, the inverter can enable the motor to achieve a soft start, avoid the grid impact and mechanical shock caused by the direct start of the motor, greatly prolong the life of the motor, reduce the vibration of the pipeline, and improve the reliability of the system. In this way, it not only solves the control valve's poor control linearity, pure delay and other difficult to control shortcomings, but also improves the reliability of the system operation; more importantly, it reduces the pressure loss caused by the change in the valve orifice adjustment, Reduce the wear of the control valve, reduce the damage of the system to the sealing performance of the pipeline, extend the service life of the equipment, reduce the maintenance, improve the economical efficiency of the system, save energy, and provide a good way to reduce the power consumption rate of the plant .
The country ranks energy efficiency as the top priority. The main goal is to reduce energy consumption per unit of GDP by 20% by 2010. Among them, the consumption of standard coal per 10,000 yuan of GDP should be reduced to 0.98 tons; the energy consumption per unit of GDP must be reduced to 4.4% per year; the consumption of water resources per unit of industrial added value must be reduced by 30%; the discharge of major pollutants should be reduced. 10%. To this end, the state has formulated and implemented the "Mid-term and long-term energy-saving special plan" to determine the "Eleventh Five-Year" period during the energy reduction goals. The SASAC and the former State Environmental Protection Administration have also signed responsibility statements with the five major power generation group companies on energy conservation and emission reduction targets respectively. They have incorporated energy consumption into comprehensive corporate assessments and annual assessments, and implemented a “one-vote veto†system to implement energy conservation target accountability systems. Accountability.
Datang Ningde Power Plant, as a representative company of Datang Group, actively responded to the important policies of the national energy-saving emission reduction and building a resource-saving society. In 2009, we successively transformed the frequency conversion of the two unit condenser pump systems. Over the past year or so, the operation of the unit has shown that the safety and economic indicators of the condensate system after reformation have reached the expected goal, and the conversion of the condensate pump has been successful.
Second, the high-voltage frequency control technology 2.1 Pump load speed and energy-saving principle Frequency control in the water pump application and the fan is different, in many occasions, the load line characteristics change is the user water consumption (that is, the user artificially closed valve) caused of. In the process of speed regulation, the pump often requires constant pressure. At this time, the operating point of the pump changes as shown in Figure 1:
When the flow rate changes from Q1 to Q2, if the pump runs at a constant speed, the operating point will change from point A to point B, and the pressure will increase, threatening the safety of the pipe network; if the speed is adjusted, the pump operating point will change from point A to point C While maintaining the required flow, the pressure remains unchanged. The output power difference of the pump at points B and C is: PB-PC = (H3-H2) × Q2.
At both points A and C, despite the different pump speeds, the outlet pressure and the external pipe network pressure are still balanced due to the difference in the flow borne by the pumps in both cases. Due to the need of pressure balance, when the pumps are operated in parallel, the speed of the speed control pump can't be lower than N3, otherwise there will be no phenomenon of water going out at all. Not only is it not energy-efficient, but there is also a phenomenon of water pump idling.
If the water pump speed regulation is performed in a system with the same characteristics of the pipe network and there is no requirement for water pressure, the energy-saving benefit in this case is much more significant than the constant pressure water supply.
2.2 The principle of frequency conversion speed control Frequency control is to adjust the motor speed by changing the power frequency. For asynchronous motors, let f be the frequency of stator power supply, s is the slip ratio, p is the number of magnetic pole pairs, and n is the rotational speed. According to the basic principle of electrical machinery, the rotational speed of the motor satisfies the following relationship:
It can be seen from the above equation that the synchronous speed n0 of the motor (n0=60f/p) is proportional to the operating frequency f of the motor. Since the slip ratio s is generally small (0~0.05), the actual speed n of the motor is approximately equal to that of the motor. Synchronous speed n0, so change the power frequency f of the motor, can change the actual speed of the motor.
Frequency conversion speed regulation is to adjust the power frequency input to the AC motor so as to achieve the purpose of adjusting the output speed of the AC motor. That is, the frequency conversion speed control system receives AC 50Hz power directly from the power grid. After the frequency converter, the input AC power is converted into AC power with variable frequency amplitude and can be directly output to the AC motor to realize variable speed operation of the AC motor.
2.3 High-voltage frequency converter characteristics The frequency converter is the power converter in the motion control system. At present, China's high-voltage inverter presents three major trends:
(1) Power unit series multilevel technology is still the mainstream of the market;
(2) Towards high power;
(3) With the maturity of high-voltage frequency conversion technology, the demand for process control for frequency control will be greatly expanded.
High-voltage inverters do not have a mature and consistent main circuit topology like low-voltage inverters, but are limited to the contradiction between the voltage tolerance and high-voltage service conditions of power devices. Domestic inverter manufacturers use different power devices and different main circuit structures. In order to meet the requirements of various dragging equipment, there are also differences in various performance indicators and adaptation scopes.
High-voltage inverters can generally be divided into two major categories:
(1) AC-AC frequency converter (without DC link)
(2) AC-DC-AC frequency converter (with DC link)
Among them, AC-DC-AC frequency converters can also use large inductors to stabilize current ripple according to DC links. Current inverters are called current source inverters. DC-link inverters that use large capacitors to suppress voltage fluctuations are called voltage source inverters. . Figure 2 shows the block diagram of three high-voltage inverters.
No matter what kind of inverter, judging its advantages and disadvantages, we must first look at the effect of harmonics of its output AC voltage on the motor; secondly, we must look at the harmonic pollution of the power grid and the input power factor; again depends on its own energy loss (ie How is efficiency?
At present, many manufacturers in the market have introduced various high-voltage frequency converters with different principles and structures. It is very important to choose a product that is suitable for the transformation of thermal power plants fans and water pumps with variable frequency speed control. Practical experience shows that: the unit multi-level voltage source inverter is the first choice for frequency conversion and speed control transformation of thermal power plant fans and pumps. The topological structure of this kind of frequency changer was developed by the United States Robinson Company, so it is also called Robinson structure. Domestic manufacturers including Leader Huafu, Dongfang Hitachi, and Micro Energy Technology have adopted this kind of main circuit structure. Its structural features are:
(1) The direct in-line superposition of a single-phase bridge-type SPWM inverter with independent power supply is used in the output inversion part, and there is no problem of device equalization;
(2) Multi-phase multiple superposition rectification technology is adopted in the input rectification part, which can reduce the switching loss and increase the equivalent switching frequency, thereby reducing output harmonics, noise, and motor pulsation torque;
(3) The power unit modularization technology is adopted in the structure. Although the number of components is increased, the efficiency of the inverter can be as high as 96% due to the low IGBT driving power.
(4) The adoption of these technologies makes this type of inverter drive power, overall efficiency, and minimum harmonic pollution, making it a perfect harmonic-free inverter. The main advantages of series-connected multi-level voltage source inverters are:
1 Because the power unit is connected in series, the low-voltage IGBT with mature technology can be used to form the inverter unit, and the number of the unit can be changed to adapt to different output voltage requirements;
2 perfect input and output waveforms, can reduce the motor pulsating torque, making it suitable for any occasion and motor use, no special requirements for the length of the motor cable;
3 Because the multi-power unit has the same structure and parameters, it is easy to modularize the power unit and realize redundant design. In the event of failure of individual units, the bypass function can be used to make the system normal or derated.
4 The inverter can withstand -35 power supply voltage drop and 5 cycles of power loss, which can adapt to the voltage fluctuations caused by power supply switching between the power plant and power switching between backup and power supply pumps and other large loads.
2.4 Unit series multi-level high-voltage variable frequency speed control system structure Our company's transformation adopts Leadwell HARSVERT-A series high-voltage frequency converter. Its speed control system adopts multi-level serial structure control mode. The system structure is shown in Figure 3. As shown. The system consists of phase-shifting transformers, power units and bypass units. The 6kV series has 5 power units. Each 5 power units are connected in series to form a phase. Each power unit has the same structure and is interchangeable. The star connection method is used in series and the neutral point is floating. Each power unit is powered by the grid voltage via the secondary winding of the phase-shifting transformer. All power units receive instructions from the same central controller over the fiber to regulate the output voltage. The output voltage of the power unit is connected in series to obtain a variable frequency. Piezoelectric feed motor.
Third, the high frequency condensate pump frequency conversion plan implementation 3.1 The power system plan According to the condensate water system characteristic, comprehensively compares many kinds of power system solutions, we finally determine that our factory condensate water system frequency conversion transformation uses one to two manual bypass schemes. That is, a high-voltage frequency converter is used to switch the high-voltage frequency converter to the condensate pump to be operated by switching the high-voltage isolation switch. The high-voltage frequency converter can drive the A condensation pump motor to realize the variable frequency operation, and also can realize the variable frequency operation by switching the B condensation pump motor. Condensate pump motors on both sides have the power frequency bypass function, which can realize the variable frequency operation of any one motor, and the other one is in the standby power frequency. When the high voltage frequency converter fails, the system can interlock another power frequency motor to run.
Basic principle: It consists of six high-voltage isolation switches QS1~QS6 (see Figure 4). Among them QS1 and QS4, QS2 and QS5 have electrical interlock; QS3 and QS2, QS6 and QS5 install the mechanical interlocking device. If two power sources supply power at the same time, A condensate pump works in the variable frequency state, B condensate pump work in the power frequency state, QS3 and QS4, QS5 break the gate, QS1, QS2 and QS6 are in the closing state; B condensate pump works in the frequency conversion state When the condensate pumps work in the power frequency state, QS1 and QS2, QS6 are opened, QS3, QS4 and QS5 are in the closing state; if the frequency converter is inspected, QS3 and QS6 can be in the closing state, and other isolation switches are opened. The two loads can operate at the same time and frequency; when all the way to power overhaul, you can make variable frequency operation of any motor through split and disconnect switches.
When the frequency conversion running fault of the A condensation pump trips, the system interlock starts and the B condensation pump QF2 is switched to operate at the power frequency. When the frequency conversion running of the B condensate pump fails, the system interlocks and starts the A-condensation pump QF1 to switch the power frequency to run.
3.2 Control system scheme 3.2.1 Principle of reform The condensate pump frequency conversion reform must ensure the water level adjustment quality of the deaerator is not changed, and can ensure the normal operation of the unit when the working pump trips, low water pressure and other special working conditions occur under the premise of frequency conversion transformation . Renovation and utilization of existing equipment and systems, the original two water level adjustment doors fully open to reduce the throttling loss, when the high-voltage frequency converter trips, the standby condensate pump immediately starts in the power frequency mode, the condensate water hit the export of the mother tube, To ensure that the deaerator water level is stable when the inverter trips. The opening of the two adjustment doors is calculated from the current actual load, and quickly closed to the designated position within 10 seconds, minimizing the system disturbance, maintaining the deaerator water level within the normal range, and ensuring the unit operation.
3.2.2 Implementation of actual transformation The start and stop of the inverter is automatically completed by closing and disconnecting the 6kV switch of the condensate pump in the frequency conversion mode. That is to say, the operator presses the start of the condensate pump operation panel on the condensate pump operation panel. "and" stop " button to complete the 6kV switch closure, disconnection and start-stop control of the inverter. Because it is a frequency converter to control two condensate pumps, so at the same time only one pump in the frequency conversion mode, another Taiwan pump in the power frequency mode, in the logic of the condensate pump frequency conversion operating mode and frequency mode of operation, while in the original system were added a set of protection and a set of interlock, that is, the inverter heavy fault condensate pump trip protection, at the same time Standby pump interlock starts.
In normal operation, one condensate pump operates at a variable frequency, and the other condensing pump is used for standby. The operator can control the opening of the deaerator water level according to the actual situation. The frequency conversion operation is performed automatically. The inverter is adjusted by changing the output frequency. The speed of the condensate pump, so as to ensure that the water level of the deaerator is stable within the set value range of the operator, by controlling the amount of water from the condensate pump to the deaerator. When the water level fluctuates, through the cascade circuit formed by the three parameters of the condensate flow rate, the economizer outlet flow rate, and the deaerator water level in the DCS configuration, the speed command is output to the frequency converter to adjust the amount of water in the condensate pump. Stabilize the deaerator water level.
When the local equipment fails, for example, the inverter issues a “heavy fault alarm†or a sudden trip of the condensate pump, the high-pressure closing switch of the current condensate pump is disconnected and another high-frequency standby condensate pump is closed. Switch, standby pump frequency start. The frequency converter is automatically switched to the “manual†mode, and the two regulating doors are automatically switched to the “automatic†mode. When the industrial frequency pump is started, the deaerator upstream adjustment valve opening is still at a relatively large opening, and the condensation water volume will be Therefore, in order to prevent the deaerator water level from exceeding the specified value, the two regulating doors must be closed to the appropriate position in the shortest possible time. Therefore, the logic is designed to adjust the door for 10 seconds once the inverter is automatically cut manually due to the fault. Forced to close to the opening of the current load request within the time, put into the "automatic" mode of operation. This opening is also the ideal opening value for adjusting the door when the power frequency is normal. After adjusting the door to the position of the load calculation value and stabilizing, complete the undisturbed switching of the entire condensed water frequency conversion fault.
3.3 Cooling System Solution Since the inverter body has a certain amount of heat loss during operation, in order to ensure the inverter has a good operating environment, an independent cooling system needs to be provided for the inverter. According to the actual situation on the spot, the investment and operating costs of integrated cooling systems, equipment maintenance, and failure-free operation time, the actual installation location, total heat, operating costs, construction costs and other factors, the frequency conversion transformation using mandatory closed Cooling scheme.
In order to ensure the safe operation of the frequency conversion equipment and avoid the adverse effects of ambient temperature and dust on the equipment, an enclosed forced cooling system is independently added on the power cabinet side of the inverter. As an auxiliary device outside the frequency conversion power cabinet, this system can ensure that the frequency conversion power cabinet is always in the 25-40°C operating environment, greatly prolonging the filter replacement cycle and reducing the amount of on-site maintenance. There is no need to build a separate house for the frequency converter. The transformer cabinet is open-cooled. Forced cooling device and inverter power cabinet integrated design, attached to the top of the power cabinet. The refrigeration compressor unit is installed near the inverter cabinet.
The forced closed cooling system is shown in the figure below:
Through the actual operation, the forced airtight cooling device can meet the heat dissipation requirements during the operation of the high-voltage frequency converter. The equipment is easy to install and fast, and the heat exchange efficiency is high.
Fourth, the problem that should pay attention to when adopting the frequency conversion velocity modulation Choose the reliability to require high. The nature of the power plant determines that high-voltage frequency converters used in power plants need to have high reliability and ensure the safe production of power plants. The high-voltage frequency converters of American Robincon high-voltage frequency converter and Beijing Leader Huafu Co., Ltd. all use the entire power unit in series instead of directly connecting the power devices, which avoids the problem of voltage equalization caused by the direct series connection of the devices. Power unit bypass technology is adopted. When the power unit fails, the inverter can still continue to run with derating, which greatly improves the reliability of the system. When selecting a high-voltage frequency converter, this performance should be queried by the inverter manufacturer as a key reference indicator.
Inverter input harmonics affect the power system. If the inverter input current harmonics are large (such as the use of current source inverter, there is no filtering measures), the power system of the thermal power plant will have the following hazards: The relay protection device of the power supply system malfunctions, which may lead to large-scale power failure . Measurement instrumentation errors increase, affecting the measurement accuracy and control performance. Affects the normal operation of other power electronic devices, electronic computer systems and communications equipment. Loss of electrical equipment such as motors, transformers, and capacitors will increase. In severe cases, it will overheat or burn. Perfect Harmonic High-voltage Inverter The input current harmonic distortion is very small, which basically does not generate harmonic pollution to the power grid. The automation level of large and medium-sized thermal power plants is high. Most of them use automated instruments and computer control systems. The harmonics of the power system are very demanding. High voltage inverter voltages of US Robincon high-voltage inverters and Beijing Leader Huafu Co., Ltd. The current waveform is good and there is no such problem.
The effect of inverter output waveform on the motor. Since the application of frequency conversion in thermal power plants is a major part of the transformation of old equipment, the original ordinary motor is designed to operate directly in the power grid, and the voltage waveform of the power grid is basically a sine wave. If the inverter output waveform quality is not good, it will affect the motor. Inverter output harmonics can cause additional heating and torque ripple of the motor, noise increases, output dv/dt and common-mode voltage can affect the motor insulation. The quality of output waveforms of high-voltage inverters from US Robinson high-voltage inverters and Beijing Leader Huafu Co., Ltd. are all very high. It is not necessary to set up output filters, and the original ordinary asynchronous motors can be used.
Selection of high-voltage frequency converter into line knife, bypass knife gate, outlet switch integrated cabinet. When selecting the high-voltage frequency converter, it is possible to decide whether to use the line-in cutter, the bypass cutter, or the outlet-integrated cabinet according to the degree of importance of the auxiliary equipment. The condensate pump, when in normal operation, one operation and one standby, will not cause the unit to reduce load operation when the frequency converter is faulty, and no longer use the line-in cutter, bypass switch, outlet switchgear integration cabinet. Reduce cost and save investment.
The high-voltage frequency converter shall have reliable cooling device. When the cooling fan is stopped, the power unit will overheat and make the inverter stop running. Therefore, the high-voltage frequency converter shall have two independent cooling devices and the power supply of the cooling device. The system must have two separate power supplies from different low voltage sections.
High-voltage inverter should choose a good operating environment. The operation of the high-voltage frequency converter shall be selected in a clean, ventilated, and dry operating environment. The filters of the high-voltage frequency converter transformer cabinets and power cabinets shall be regularly cleaned. In particular, when the operating environment is poor, dust will block the filter, resulting in poor cooling and power unit over-temperature alarm, or even outage.
V. Energy-saving effect analysis 5.1 Equipment parameters 5.1.1 Condensate pump system parameters Condensation pump motor parameters Model: YKKL2300-4/1180-1
Rated power: 2300kW
Rated voltage: 6kV rated frequency: 50Hz
Rated current: 262.9A Rated speed: 1492r/min
Number of phases: 3 Wiring: 2Y
Cooling method: IC611
Motor inertia: J=133.4kg.m2
Production date April 2007 Manufacturer Xiangtan Motor Co., Ltd. 5.1.2 High voltage frequency converter Parameter model HARSVERT-A06/270
Technical solution Multi-stage module series, AC/DC, high-voltage rated input voltage / permissible variation range 6kV/±10%
System output voltage 0~6kV
Sensitivity to grid voltage fluctuation -35% to +15%
Phase-shifting transformer rated capacity 2900kVA
Inverter output voltage range 0~6kV
Inverter output current range 0~262.9A
Grid side converter type and components 30 pulse, diode three-phase full-bridge motor side inverter type and components IGBT inverter bridge series cooling method forced airtight cooling manufacturer Beijing Leide Huafu Electric Technology Co., Ltd. 5.2 power frequency / frequency conversion The state of electricity savings per hour under various loads is shown in the following table:
According to the data in the above table, it can be seen that the smaller the load of the unit is, the more energy-saving effect of the condensate pump motor becomes more significant after the frequency conversion is adopted. According to the average power saving of 750kWh per hour, and annual operation of 6,000 hours, the annual energy saving is 750×6000=4.5 million (kWh). Calculated according to the on-grid price of 0.4 yuan/kWh, the annual benefit is 0.4×450=180 (ten thousand yuan).
6. Conclusion Condensation pump has been operating stably for more than one year after the frequency conversion transformation and operation. This time, the 660MW ultra-supercritical unit condensate system high-voltage frequency conversion, the new frequency conversion equipment installed in the nearest location of the condensate pump, saving the cost of high-voltage cables and civil engineering; cooling systems are forced closed cooling structure design, air circulation, The dust is small, the environment is stable, and it is affected by the external environmental factors, which greatly reduces the work intensity of maintenance personnel. After the condensate system is put into operation, the test performance indicators are good: the noise and vibration of the two adjustment doors' shutoffs are significantly reduced. When the unit runs at 330MW, the condensate pump motor current is reduced from the original 170.5A to about 75.9A, and the energy saving rate Up to 55.5%, the average annual energy saving rate is 50.9%. The energy-saving effect is very obvious. After the frequency conversion of ultra-supercritical unit condensate pump, the tracking load and other parameters were changed to achieve continuous adjustment through adjusting the motor frequency, smooth and stable, large adjustment range, energy saving and consumption reduction effect is obvious, and the economy is higher. It is worth promoting and applying on a large scale throughout the country.