Tangshan Huifeng Company's Gas Compressor Variable Frequency Energy-Saving Retrofit

Tangshan Huifeng Company Coal Gas Pressuring Machine Frequency Conversion Energy Saving Reconstruction

Guo Jijie, Tangshan Huifeng Coking and Gasification Plant

Hu Lingzhi, Shandong Xinfengguang Electronic Technology Development Co., Ltd.

Abstract : This article introduces the application of high-voltage frequency converters produced by Shandong Xinfengguang Electronic Technology Development Co., Ltd. in the gas compressor of Tangshan Huifeng Coking and Gasification Plant. Through the modification, the gas compressor achieved high-efficiency operation, realizing energy saving and consumption reduction.

Keywords: High-voltage frequency converter, gas compressor, energy saving and consumption reduction

1 Introduction

Tangshan Huifeng Coking and Gasification Plant is a subsidiary of Tangshan Shunli Industrial Group Co., Ltd. Huifeng Coking and Gasification Plant's main products include coke, sulfuric acid, coal gas, benzene, and other chemical products. The crude coal gas produced during the coking process undergoes tar removal, desulfurization, and benzene removal to become qualified refined coal gas. This refined coal gas is then transported via pipeline to a pressurization room, where it is further pressurized by a gas compressor before being supplied to users.

The company originally had three coke oven production lines, mainly supplying users: Tangshan City Gas Company and Guye Town Gas Company. To expand production, in 2010, Huifeng Coking Gasification Plant built a new coke oven production line to supply Heimao Carbon Black Company. However, the gas pipeline of the new production line was not connected to the original system, meaning that only a portion of the produced gas can currently be used, with most being released into the atmosphere, resulting in significant waste. Therefore, the company is constructing a new 50,000 cubic meter gas holder project. The aim is to store the refined gas produced during coking and then transmit it to the city gas network, increasing the company's revenue and reducing waste. The new gas station mainly consists of a 50,000 cubic meter gas holder, a pressurization station, a gas flow monitoring station, and pipelines. The 50,000 cubic meter gas holder serves as a buffer; when the coke oven gas production exceeds the user's demand, the excess gas is stored in the gas holder. When the user's gas demand exceeds the coke oven production, gas is drawn from the gas holder as a supplement. The pressurization station consists of 10 Roots blowers.

The gas consumption of urban residents (Guye and Tangshan) varies considerably. Cooking requires a large gas supply, while gas consumption is lower at night and in the early morning. If a constant-speed motor is used, the gas consumption cannot be adjusted, and low gas consumption can easily cause increased pressure in downstream gas pipelines, leading to a risk of leakage. The gas consumption supplied to Black Cat Carbon Black Company also fluctuates with production needs, requiring constant adjustment of its gas consumption as well.

2. On-site equipment introduction

The coking plant is equipped with 10 gas pressurizers, which are used in parallel. The gas pipelines supply gas to the outside in six directions. Under normal circumstances, seven are in use and three are on standby. The gas pressurizers are equipped with Roots blowers. Roots blowers have two main characteristics compared to centrifugal blowers: (1) the inlet and outlet doors must be fully opened when starting up, and (2) the gas entering the Roots blower through the inlet must be completely blown out. When the gas consumption is small, venting or backflow treatment measures are taken, resulting in waste. The gas pressurizers modified in this project are No. 1, No. 2, No. 3, No. 7, No. 8, and No. 10. The specific parameters of these six gas pressurizers are shown in Table 1.

Because the selected Roots blower is too large, it is often not necessary to operate the Roots blower at full load. As a result, the blower has to open the return valve or venting, which leads to low operating efficiency and serious waste. In order to improve the operating efficiency of the gas compressor, the plant leaders decided to upgrade the equipment with a high-voltage frequency converter.

Through a bidding process, the factory leadership selected six sets of JD-BP38 series (10kV) high-voltage frequency converters manufactured by Shandong Xinfengguang Electronic Technology Development Co., Ltd. as the applicable equipment for the renovation project, and the renovation was successful.

3. Technical Features of the JD-BP38 Series High Voltage Variable Frequency Speed ​​Control System

The Fengguang JD-BP38 series high-voltage frequency converter uses a high-speed DSP as its control core, employing speed-vector control technology and power unit series multi-level technology. As a high-voltage source frequency converter, its harmonic performance is far below the IEEE 519-1992 national harmonic standard. It boasts a high input power factor and excellent output waveform quality, eliminating the need for input harmonic filters, power factor compensation devices, and output filters. It also avoids problems caused by harmonics, such as additional motor heating, torque pulsation, noise, output dv/dt, and common-mode voltage, allowing the use of ordinary asynchronous motors. In 2007, the high-voltage frequency converter was recognized as a Chinese Famous Brand Product. Specifically, in addition to the performance characteristics of ordinary frequency converters, the Fengguang high-voltage frequency converter has the following outstanding features:

(1) Using a high-speed DSP as the central processing unit, the operation speed is faster and the control is more precise.

(2) Fly-start function. It can identify the speed of the motor and start the car directly without stopping the motor.

(3) Complete automatic switching technology between power frequency and variable frequency. Current high-voltage variable frequency speed control systems generally include a power frequency bypass switching cabinet. When the frequency converter fails, it allows the high-voltage motor to switch to power frequency operation. There are two types of bypass switching: manual and automatic. Manual bypass requires manual operation and is suitable for operating conditions without backup devices or for less critical situations. Automatic bypass can automatically switch to power frequency operation directly after a frequency converter failure. The automatic bypass switching cabinet provided by Xinfengguang Company can not only automatically switch from variable frequency to power frequency operation in the event of a frequency converter failure, but also instantly switch from power frequency to variable frequency operation after frequency converter maintenance is completed. The entire switching process will not affect the operation of the user's equipment.

(4) Restart function during rotation. If the high voltage is momentarily lost during operation and then restored within 3 seconds, the high voltage inverter will not stop. After the high voltage is restored, the inverter will automatically run at the frequency before the power outage.

(5) Automatic line voltage balancing technology (star point drift technology). When a unit in a phase of the frequency converter fails, in order to balance the line voltage, the traditional method is to reduce the voltage of the other two phases to the same voltage as the faulty phase. However, the automatic line voltage balancing technology adjusts the angle between the phases to ensure the maximum line voltage balance output under the premise that the phase voltage output is maximum and unequal.

(6) Unit DC voltage detection: Real-time display of the DC voltage of the detection system, thereby realizing optimized control of the output voltage, reducing harmonic content, ensuring the accuracy of the output voltage, improving the system control performance, and enabling operation and maintenance personnel to have a comprehensive grasp of the operating status of the power unit.

(7) The electrolytic capacitors in the unit have a lifespan that can be doubled due to the adoption of the company's patented technology.

(8) The heat dissipation structure is reasonably designed, and the unit is connected in series and then in parallel. The IGBT can withstand a lower voltage and has a wider overvoltage range (≥1.15Ue), resulting in higher equipment reliability.

(9) It has the function of sudden phase-to-phase short circuit protection. If a short circuit occurs at the output due to equipment failure or other reasons, and the frequency converter does not have the function of phase-to-phase short circuit protection, it will lead to a major accident. When such a problem occurs, the frequency converter can immediately block the output of the frequency converter, protect the equipment from damage, and avoid the occurrence of an accident.

(10) Current limiting function: When the inverter output current exceeds the set value, the inverter will automatically limit the current output to avoid overcurrent protection caused by the inverter during acceleration and deceleration or by sudden load changes, and minimize the number of shutdowns.

(11) Fault self-reset function: When the inverter causes overcurrent protection of the unit or the whole machine due to sudden load change, it can automatically reset and continue to operate.

4. Introduction to the Fengguang JD-BP38 Series High Voltage Variable Frequency Speed ​​Control System

4.1 Overall Structure

It adopts a direct high-voltage frequency converter circuit structure with multiple power unit connections, directly outputting to the high-voltage motor to provide drive voltage. Physically, it consists of three main parts: control cabinet, power cabinet, and transformer cabinet. Depending on the site process requirements, a bypass cabinet, host computer, and remote control box can also be selected. The structure is shown in Figure 1.

Figure 1. External appearance of high voltage frequency converter

4.2 Control Cabinet

The frequency converter control cabinet mainly consists of a main controller (CPU), UPS, PLC, human-machine interface, control power switch, switching power supply, relay, surge arrester, signal isolator, terminal block, cabinet door operation buttons, etc. The main components of the control cabinet are introduced below.

(1) Main control system

The main control system is the core of the frequency converter. It receives and processes control commands from the host controller and PLC, generates control signals for each phase and power unit, and simultaneously collects and processes fault information fed back from all faulty units. The Xinfengguang JD-BP38 series frequency converter adopts a high-performance main control system. The controller uses a 32-bit DSP with an operating speed of up to 150 MIPS, sufficient to complete some complex control algorithms. It also features 6 independent PWM outputs, 2 asynchronous serial communication ports, 16 channels of 12-bit AD input, and built-in 36K RAM and 256K Flash memory, capable of storing large-scale programs. The circuit board uses large-scale integrated circuits and surface-mount technology, ensuring extremely high system reliability.

(2) Uninterruptible power supply (UPS)

The UPS (Uninterruptible Power Supply) is installed at the bottom of the control cabinet. It is a purely online type. When the external 220VAC control power supply is normal, the UPS provides a stable 220V power supply to the control system. When the external power supply fails, the designed power redundancy system activates the corresponding control relays, switching to the 220V secondary winding of the transformer to continue providing control power. The UPS operates uninterruptedly, providing a stable power supply. Only when both the control power and high-voltage power fail simultaneously can the UPS continue to supply power to the system for 30 minutes using its own battery. Simultaneously, the inverter will issue an alarm signal, and the user should restore the control power supply as soon as possible.

(3) Built-in PLC

The frequency converter uses a built-in PLC to process internal switching signals, as well as field operation and status signals, enhancing the flexibility of its field applications. When the number of switching signals is insufficient, digital input/output (DIO) expansion modules can be used. As a mature industrial control component, the PLC provides flexible interfaces and reliability guarantees for the field application of frequency converters.

The standard interface of the control system for the high-voltage frequency converter is shown in Figure 2:

Figure 2 Standard interface diagram of the control system of high voltage frequency converter

4.3 Power Cabinet

The power cabinet is mainly used to install power units, realizing the series superposition of the units for three-phase output. The power unit is a high-voltage frequency converter component that uses power electronic devices for rectification, filtering, and inversion, and is also a major part of the main circuit of the high-voltage frequency converter. Each power unit is equivalent to an AC-DC-AC voltage-type single-phase low-voltage frequency converter. Each power unit consists of an H-bridge, outputting a set of SPWM waves. There are 8 units per phase, which are superimposed to output a set of 17-level sine waves. The sampling frequency of each power unit in the same phase is consistent, modulated by the same carrier wave, with the carrier waves differing by 1/N sampling periods.

When an overcurrent or overvoltage fault occurs in a power unit, the inverter immediately blocks the output of that unit. Through software control, the output current of the power unit can be bypassed by forming a current loop through the upper or lower bridge arm of the full-bridge inverter circuit.

The top of the power cabinet is equipped with a cooling fan, which is a German EBM fan. It is powered by 220V from the secondary detection winding of the phase-shifting transformer. The fan is started by the PLC through the circuit breaker. The fan starts when the frequency converter starts running at the starting frequency.

4.4 Transformer cabinet

The transformer cabinet mainly consists of components such as a phase-shifting transformer, a temperature controller, and a cooling fan.

The principle of a phase-shifting transformer: It transforms a high-voltage power supply into multiple sets of low-voltage circuits with mutual insulation on their secondary windings. Each secondary winding is connected in a delta configuration with an extended side, and there is a certain phase difference between them. A schematic diagram of a phase-shifting transformer is shown in Figure 3.

Figure 3 Schematic diagram of phase-shifting transformer

5. Variable Frequency Drive Retrofit Control Scheme

To ensure uninterrupted 10kV high-voltage power supply, a dual-circuit power supply is used, connected by a tie cabinet. If one circuit fails, the other circuit will continue to supply power stably.

To ensure the reliability of the gas pressurization system, gas pressurizers #1, #2, #3, #7, #8, and #10 are configured with a 4-in-operation, 2-in-standby configuration. If the frequency converter malfunctions and stops operating, the standby pressurizer immediately takes over, thus guaranteeing the gas supply to the pressurizers and improving the overall system's safety and stability.

The variable frequency system for gas compressors has the following characteristics: When the 220V AC control power supply to the inverter fails, since the inverter's control power supply and main power supply have no phase or synchronization requirements, the inverter can continue to operate using a UPS without stopping; when the speed setpoint signal is lost on site, the inverter provides an alarm and can continue to operate at the original speed, maintaining the unit's operating conditions unchanged; the inverter is equipped with a unit bypass function, which allows it to bypass the faulty unit and continue operating at reduced derating in the event of a partial fault, reducing losses caused by sudden shutdowns; if the inverter has more than three faulty units, it can automatically switch to mains frequency operation, ensuring that production is not affected.

6. Monitoring and Operation of the Gas Compressor Fan: When the user's gas consumption is relatively small, only a very low speed is required, and full-load operation is not necessary. Using a high-voltage frequency converter to adjust the speed of the Roots blower according to actual needs ensures gas supply safety while achieving energy conservation and consumption reduction.
The specific implementation process of high-voltage frequency converter monitoring for gas compressors is as follows: The frequency converter can be controlled locally or remotely. When remote control is selected, there are two options: DCS and remote control box. The frequency converter includes a built-in PLC for logic processing of switch signals within the cabinet, as well as coordination with various on-site operation and status signals (such as RS485). Furthermore, it can expand control switching quantities according to user needs, enhancing system flexibility.

On-site operators can remotely operate and monitor the inverter's operating status through the DCS and remote control box. The settings can be freely configured according to the working conditions, which can fully meet the process requirements.

7. On-site control status of the gas compressor

To address the problems identified on-site, system optimization and upgrades mainly need to solve two aspects: First, minimize discharge loss while meeting users' gas consumption requirements; second, minimize pump-pipe pressure differential, i.e., reduce pressure loss, while meeting gas pressure requirements. System optimization will address both kinetic and potential energy aspects simultaneously to reduce energy consumption and improve system efficiency as much as possible.

The system configuration diagram on site is shown in Figure 4.

Figure 4 Block diagram of the field system

The closed-loop control process of the system is as follows: Intelligent sensors monitor and process real-time data from each operating gas compressor, collecting and transmitting operating parameters such as compressor displacement Q_single, motor current I, compressor inlet and outlet pressures, compressor station outlet pressure P, and total displacement Q_total. These control parameters are then compared and optimized based on their desired values ​​and the compressor's characteristic curves. The gas supply pressure of the compressor station is the most critical parameter that the system needs to monitor and control. In this system, a high-reliability pressure sensor is installed on the outlet pipeline of the gas compressor station. The measured pressure signal is compared with the system's desired value, and the difference is sent to the DCS monitoring system for proportional and integral calculations. Finally, the output result is sent to the high-voltage frequency converter. The DCS monitoring system automatically adjusts the output frequency of the high-voltage frequency converter to control the speed of the gas compressor. As is known from the principle of the Roots blower, changes in blower speed cause corresponding changes in displacement; by changing the frequency, the desired displacement value is achieved. The system is designed as a closed-loop control system. It uses PID control to adjust the measured pressure signal of the system against the expected pressure value required for gas supply. The system automatically adjusts the output of the high-voltage frequency converter in a timely manner to ensure that gas is supplied to the outside at the expected pressure, thus realizing automatic control of gas supply.

8. Energy Saving Analysis

8.1 Energy-saving principle

Since its commissioning, the high-voltage frequency converter for the HSBC gas compressor has been operating well, basically achieving the expected transformation goals. Before the frequency conversion upgrade, the gas compressor supplied gas by opening the reflux valve or venting, which undoubtedly resulted in some energy waste. By adjusting the compressor speed to control constant pressure delivery in the pipeline, energy saving can be achieved, control performance can be improved, and operating noise can be reduced.

Based on the operating characteristics of the Roots blower, the load characteristic of the compressor is constant torque, and its shaft power is directly proportional to the first power of the rotational speed. Observations of the high-voltage frequency converter's operation on-site show that the operating frequency is generally between 25-45Hz, resulting in significant energy savings.

8.2 Improved system efficiency and reduced unit consumption

After the system was upgraded with a high-voltage frequency converter, the gas compressor met the system's gas supply requirements, improved system efficiency, and reduced system energy consumption. Analysis and calculations show that the average energy consumption per unit of gas supplied decreased by 0.015 kW∙h/ ^m³ after the upgrade. Currently, the average daily gas supply is 636,700 ^m³ /d. At a cost of 0.5 yuan per kW∙h, the annual electricity cost savings would be:

636700´365´0.015´0.5=1.742966 million yuan.

8.3 Adjust the gas supply to save gas.

Because the required gas supply fluctuates significantly and changes frequently throughout the day, implementing high-pressure variable frequency drive (VFD) upgrades at the station allows for flexible adjustment of the station's compressor operation based on external gas demand, minimizing gas waste while still meeting system pressure requirements.

9. Conclusion

After the gas compressor is retrofitted with frequency converters, in addition to significant energy savings, it also offers the following advantages: it enables soft starting of the motor, resulting in smoother startup; it effectively improves the on-site operating environment, which is highly welcomed by on-site operators; it reduces maintenance workload and costs, and extends the equipment's service life. With the increasing emphasis placed on energy conservation and emission reduction by the nation, enterprises are implementing various measures to reduce production costs, among which frequency converter technology plays a crucial role, achieving significant economic and social benefits and adapting to the national trend of building a resource-saving society.

About the author:

Guo Jijie, male, senior engineer, chief electrical engineer, works at Tangshan Huifeng Coking and Gasification Plant. Contact person: Guo Peibin

Hu Lingzhi, male, is a technical support engineer working at Shandong Xinfengguang Electronic Technology Development Co., Ltd.

Telephone: 0537-7237007 Address: Wenshang Economic Development Zone, Shandong Province, China Postcode: 272500