1 Introduction In the 50,000 t/year ABS resin unit of Daqing Petrochemical General Plant, all five ABS resin extrusion motors are DC motors. Although DC motors have good starting performance, strong overload capacity, and can smoothly and economically adjust speed over a wide range, the commutator of the DC motor must be frequently inspected and maintained, and the carbon brushes must be replaced regularly, resulting in a large workload for maintenance. Since the DC control system of the extrusion motor is a product of Japan's Daehaya Corporation, the price of spare parts is very expensive, leading to increased maintenance costs. To solve this problem, in the renovation of the 50,000 t/year ABS resin unit to a 100,000 t/year ABS resin unit, the newly added ABS resin extrusion motors adopt three-phase squirrel-cage induction motors and use variable frequency speed control. [B]2 System Requirements and Scheme Determination 2.1 Energy Saving Requirements[/B] Electrical energy saving is an important part of the national energy saving strategy. Therefore, this ABS unit expansion and renovation must select energy-saving equipment. In particular, the newly added ABS resin extrusion motor is 1,630 kW, and adopting an energy-saving scheme is a necessary requirement. 2.2 Reliability Requirements The ABS resin extruder is the core equipment for this expansion and renovation. Its abnormal operation will cause the entire extrusion granulation line to shut down. In order to ensure the normal production of ABS resin, the extrusion motor must be guaranteed to operate reliably. Therefore, the newly added ABS extrusion motor is considered to adopt a variable frequency speed control scheme, and the variable frequency speed control system is required to have high reliability. The following requirements are made for the variable frequency speed control system: 1) Long-term trouble-free operation. 2) Smooth and efficient speed adjustment over a wide range. 3) Speed ​​adjustment according to the supply of resin polymer. 4) Remote upper computer monitoring function. 2.3 Equipment Selection Compared with the variable frequency speed control schemes of various manufacturers in the domestic and foreign markets, ABB's medium-voltage frequency converter speed control scheme was selected due to its advanced technology, stable performance, and successful case at Daqing Petrochemical Company. The transformer, motor and frequency converter are all ABB products. [b]3 System Principle Overview 3.1 Frequency Converter Introduction[/b] The ACS1000 is a three-phase frequency converter used in squirrel-cage induction motors. Mature microprocessor control technology is used to monitor the electromagnetic state of the motor. These data, combined with direct torque control (DTC) technology, can achieve near-perfect sensorless motor control. With the addition of pulse encoder feedback, it can be applied to precise speed control or long-term operation in near-zero speed regions. The output voltage of the ACSl000 inverter is close to sinusoidal, which can be easily used with the standard induction motors currently in use without derating, making it an ideal choice for retrofit projects. (1) Fuse-free design The ACSl000 is a medium-voltage inverter without fuse protection. This patented design uses a new type of power semiconductor switching element IGCT as the circuit protection. The IGCT, placed between the DC circuit and the rectifier bridge, is different from the traditional fuse. It can directly and quickly isolate the inverter and rectifier sections within 25 microseconds, which is 1000 times faster than the fuse. The hardware and software protection features of the ACSl000 inverter can effectively protect the inverter from faults and damage caused by abnormal operation and equipment malfunction. (2) Control Equipment The ACS1000 frequency converter has advanced local control and remote control features. The control equipment is integrated inside the frequency converter cabinet and provides full digital and microprocessor technology based on process control, protection and monitoring functions, and provides backup for hardware protection circuits. The CDP 312 control panel is the basic user interface, through which users can monitor, modify parameters and control the operation of the ACS1000 frequency converter. 3.2 System Control Principle (1) System Composition The three-phase AC power supply supplies power to the rectifier bridge through a three-winding transformer (see Figure 1). In order to obtain 12-pulse rectification, there must be a 30° phase difference between the two secondary windings of the transformer. One secondary winding is star-connected and the other is delta-connected. In order to perform three-level switching operation, each arm of the three-phase inverter consists of two IGCTs: the output voltage of the IGCT switches between positive DC voltage, neutral point (NP) and negative DC voltage. Therefore, by using DTC technology, the output voltage and frequency can be continuously controlled between 0 and the maximum value. An LC filter is required at the inverter output to reduce harmonic content in the output voltage. After using this filter, the voltage waveform supplied to the motor is close to a sine wave (see Figure 2). Therefore, standard motors can be used directly at their rated capacity. This filter also eliminates the influence of dv/dt, thus completely eliminating voltage reflection in the motor cable and damage to the motor insulation. The charging resistor limits the DC circuit current when the inverter is powered on. When the DC voltage reaches 79% of its rated value, the IGCT conducts, and the charging resistor is bypassed. The main function of protecting the IGCT is to quickly shut it off in case of a fault to protect the rectifier bridge. The common-mode current of the inverter is limited by the common-mode reactor and attenuated by the common-mode suppression resistor. Due to its special structure, the common-mode reactor can provide comprehensive suppression of common-mode current flowing through the transformer secondary cable, DC circuit, output filter, and inverter internal grounding busbar. On the other hand, there is almost no restriction on the main DC current; the main current can flow freely. Transformer and motor parameters (as shown in Table 1). Two fuse-free rectifier bridges are connected in series, so the DC voltage is the superposition of the two rectifier bridges. Both rectifier bridges carry the full DC current. 3.3 Operation and monitoring of the frequency converter The ACS1000 frequency converter can be operated locally or remotely. The frequency converter uses DCS to process the switch signals in the cabinet and coordinate with various operation and status signals on site. It can also expand the control switch quantity according to the user's needs to enhance the system's flexibility. The frequency converter can be controlled by the host computer for speed regulation. It runs at high speed when the feed rate is large and at low speed when the feed rate is small. It can fully meet the process requirements. The host computer can monitor remotely, which is conducive to remote diagnosis and maintenance of the equipment. 3.4 Equipment operation status After more than 7 months of operation, the ACS1000 frequency converter has shown the following advantages: 1) Stable performance. There were no shutdowns due to frequency converter failures during operation, which provided a strong guarantee for the smooth production of the ABS unit. 2) Significant energy saving, greatly reducing power consumption. 3) It truly achieves soft start and soft stop, reducing the mechanical impact on the motor and extrusion equipment. 4) It has strong resistance to power grid fluctuations. It can still operate normally when the system voltage fluctuates. 4 Energy Saving Effect Analysis In practice, the energy saving effect is calculated based on 8160 hours (340 days) of operation per year. The actual operating data measured in the high-voltage feeder cabinet is shown in Table 2. The energy consumption of the extrusion machine in variable frequency operation is W[sub]1[/sub]=1.732UI[sub]1[/sub]COSφ =1.732×6×92×0.97=927KW. The variable frequency operation of the extrusion machine increases the power factor from 0.85 to 0.97. If the extrusion motor operates at the power frequency of 6 kV, the power factor can only reach 0.85. The extruder's operating current at the power frequency is I₂ = W₁ ÷ 1.732 ÷ U ÷ 0.85 = 105A. I₃ = I₂ - I₁ = 13A. The actual energy saving is W₃ = 1.732UI₃cosφ8 160h = 1.732 × 6 × 13 × 0.97 × 8160 = 1068960 kW•h. In conclusion, the variable frequency operation of the extruder not only meets the process requirements but also saves a significant amount of electrical energy.