Theoretical Analysis of Four-Quadrant Frequency Converters and Their Application in Auxiliary Shaft Hoisting Machines

Theoretical Analysis of Four-Quadrant Frequency Converters and Their Application in Auxiliary Shaft Hoisting Machines

Abstract: This paper aims to explain the structural framework, theoretical analysis, and market scope of four-quadrant frequency converter technology. It focuses on the application practice of Guangzhou Zhiguang Electric's four-quadrant frequency converter in a secondary shaft hoist, illustrating the differences between four-quadrant frequency converters and traditional two-quadrant frequency converters used in high-power industrial systems. The topology of the four-quadrant frequency converter is briefly explained. Due to the semi-controlled bridge and circuit with freewheeling diodes, the rectified voltage Ud cannot be negative, and negative polarity on the DC side is not allowed. Therefore, the four-quadrant frequency converter uses insulated-gate bipolar transistors (IGBTs) to construct a rectifier module different from traditional frequency converters. This enables bidirectional energy flow when the main circuit load hoist operates in an inclined shaft. While feeding the energy generated by motor braking back to the grid, it also significantly improves the grid-side input power factor and reduces grid-side harmonics and dv/dt characteristics caused by diode rectification in traditional frequency converters. This paper combines the actual field application of the four-quadrant frequency converter to introduce its theoretical analysis and application characteristics!

Keywords: Four-quadrant coal mine hoist, filter, structural topology, Puzhiguang frequency converter

1. Application Site Introduction

Huating Coal Industry Group, established on April 24, 2002, is currently the largest state-owned modern coal enterprise in Gansu Province. Its mining area covers a total area of ​​134 square kilometers, with geological reserves of 3.37 billion tons. It is one of the key mining areas in the Huanglong Polar Region, one of the 13 large-scale coal bases planned by the state. The coal in the mining area is of excellent quality, characterized by high calorific value, high volatile matter content, high chemical activity, and low sulfur, low phosphorus, and low ash content – ​​the "three highs and three lows" characteristics. Huating Coal Industry Group has three holding subsidiaries, three wholly-owned subsidiaries, three branches, and secondary units such as Matigou Coal Mine and Shanzhai Coal Mine. The equipment used in this project is located at Shanzhai Coal Mine, situated in Cedi Town, Pingliang City, Gansu Province, northwest of Huating County. The average strike of the Shanzhai Coal Mine is 3.00 km, the average inclination is 3.431 km, the mining area is 10.2947 km², the proven geological reserves are 218.683 million tons, and the recoverable reserves are 153.078 million tons.

The Shanzhai Coal Mine underwent mine expansion and renovation in April 2003. The project was approved by the Gansu Provincial Development and Reform Commission with document number Ganfa Gai Nengyuan (2006) 889, designed by the Taiyuan Coal Industry Design Institute in January 2003, and constructed by Gansu Provincial Coal Industry Construction First Engineering Company. It was completed and accepted in June 2010. The main shaft has a slant length of 756m and an inclination angle of 16°. The 1350 conveyor roadway is equipped with one STJ1000/2×560S type conveyor belt, with a transport capacity of 800t/h, a belt width of 1000mm, a belt strength of 3150N/mm, a belt speed of 3.3m/s, and a motor power of 560KW. The load for this four-quadrant frequency converter application is its auxiliary shaft hoisting system. The auxiliary shaft is equipped with one GKT3×2.2-20 type single-drum winch, with a shaft inclination angle of 23°, a hoisting length of 565m, and a drum diameter of 3m. With a width of 2.2m, a lifting speed of 3.8m/s, and a static tension of 130KN, the wire rope has a diameter of 37mm. The motor power is 450KW, and the rated voltage is 6kV. It is mainly responsible for auxiliary hoisting tasks in the mine, such as transporting materials and lifting personnel. The underground auxiliary transportation adopts rail transportation, and the main auxiliary transportation line uses 30Kg/m mining steel rails with a total length of 2600m.

2. Theoretical Analysis of Four-Quadrant Frequency Converter Structure

2.1 Introduction to Four-Quadrant Frequency Inverters

In a four-quadrant frequency converter, the four quadrants refer to the four quadrants on the number axis of a mathematical model established by mechanical characteristic curves. Specifically, the first quadrant is the forward motoring state, the second quadrant is the regenerative braking state, the third quadrant is the reverse motoring state, and the fourth quadrant is the reverse braking state. The applications are mainly in industrial settings with mechanical potential load characteristics, such as coal mine hoists, oilfield pumping stations, centrifuges, and traction machines.

2.2 Main Circuit Topology of Four-Quadrant Frequency Converter

Based on the conventional unit series multilevel high voltage frequency converter (1), an improvement is made. Taking a 10kV 8-level series system as an example, the power supply side adopts a 48-stage phase-shifting transformer to supply the input voltage of the eight-stage unit body (4), and the unit output adopts a cascade mode. The output of the transformer secondary winding is staggered by a certain phase angle to effectively reduce the pollution of the grid side harmonics. The nth harmonic content is reduced to n=48k±1, where k is a natural number. The formula no longer contains low-order harmonics. Although high-order harmonics still exist, their amplitude is greatly reduced. An AC reactor is installed between the input side of the power unit body and the phase-shifting transformer. The rectifier bridge circuit of each power unit is composed of fully controlled IGBT devices (2). A voltage transformer PT (5) is added to the input side of the three-phase phase-shifting transformer. The secondary side voltage of the PT terminal is applied to the unit body through the intermediate isolation transformer as a voltage phase detection circuit (6), including the synchronous rectification, overcurrent comparison circuit, and phase-locked loop circuit connected to its primary side. The following figure is the main circuit structure diagram of the four-quadrant frequency converter:

Figure 1. Main circuit structure diagram of the four-quadrant frequency converter

2.3 Main Structure Topology of Fully Controlled Power Unit

The power unit's rectifier bridge uses fully controllable rectifier devices (IGBTs), with an energy storage stage in the middle, inverting to a two-phase output from the IGBT full inverter bridge. When the equipment operates in the first and third quadrants, energy flows forward; the input voltage is rectified by the rectifier bridge and inverted into an output AC voltage through the DC storage stage. In the second and fourth quadrants, the energy feedback control section on the rectifier side activates, controlling the phase and amplitude of the inverter voltage to ultimately invert DC into AC and feed the energy back to the grid. The energy fed back from the motor's energy consumption flows to the DC bus through the freewheeling diodes on the inverter side, achieving inversion through the unit input.

Figure 2. Main structure topology diagram of the fully controlled power unit

2.4 Implementation of Phase Detection Function

The phase detection stage, as shown in the figure, realizes the phase detection function of the primary voltage of the phase-shifting transformer. The primary voltage of the sampling phase-shifting transformer is connected to the primary of the isolation transformer via the secondary sampling PT. The secondary voltage of the isolation transformer is detected internally by the synchronous rectification circuit. The zero-crossing comparator circuit and the phase-locked loop circuit ensure that the phase of the primary voltage of the phase-shifting transformer and the secondary phase of the phase-shifting transformer are in phase with the grid input. The grid-connected inductor is connected in parallel with the positive phase of the unit input to realize the voltage feedback from the motor side to the grid side.

Figure 3 Phase principle diagram of the power detection unit

3. Process requirements for the auxiliary shaft hoist

3.1 Motor Nameplate Parameters

Motor nameplate parameters

Model YR560-12

Shanghai Electric Motor Factory Asynchronous Motor Manufacturer

Rotor asynchronous three-phase

Startup method: direct boot

Rated power (kW) 450

Rated voltage (kV) 6

Rated current (A) 55.7

Power factor 0.837

Rotational speed (r/min) 487

Table 1 Motor Nameplate Parameters

3.2 Operating conditions of the auxiliary shaft hoist

The hoist accelerates to a constant speed of 0.5 m/s at a depth of 565 meters, then accelerates to 0.8 m/s at 520 meters, reaches full speed of 3.4 m/s at 430 meters, decelerates to 0.5 m/s at 60 meters, and maintains a speed of 0.2 m/s below 20 meters before finally stopping. The reverse operation uses the same acceleration and deceleration method. The control circuit is controlled by Shanghai Electric Optoelectronics' electrical control system. During the upward pull, a maximum of six trolleys can be attached, with a load capacity of approximately 15 tons; during the downward pull, the maximum load is approximately 23 tons. The electric traction system cannot fully achieve four-quadrant operation; reliable braking force must be provided in the inclined shaft hoist to overcome the downward force caused by its own weight or inertia during descent or upward stops. The electrical control system screen is shown in the following figure:

Figure 4. Screenshot of the auxiliary shaft hoisting system

During the hoisting and lowering process, since the load may fall within any range below 25 tons of material, the frequency converter system must provide sufficient starting torque to the motor and shorten the acceleration time to below 20 seconds as much as possible, while the start-up holding time must be controlled to below 3 seconds. Before operation, the motor fan cooling system, brake pump, and hydraulic station lubrication pump must all be started to ensure basic operating braking conditions. During operation, the brake is released, and the speed of the winch is controlled by pushing the speed linkage. When stopping, the master switch is reset, the frequency converter must immediately stop output, and the brake linkage must be pushed to quickly stop the hoist. During the lowering process, the hoist decelerates, and potential energy begins to be converted into the kinetic energy of the motor. The motor operates in the fourth quadrant and is in reverse generation mode. Figure 5 below shows the motor and load diagram of the auxiliary shaft hoist on site.

Figure 5. Motor and load diagram of the auxiliary shaft hoist.

3.3 Introduction and Technical Requirements of the Electrical Control System

(1) Introduction to the electrical control system: The electrical control system of this mine hoist was provided by Shanghai Dianguang Explosion-proof Co., Ltd. It adopts advanced computer technology and has a high degree of intelligence. It has advanced automated control methods and perfect protection functions, which reduces the equipment failure rate and maintenance time.

(2) System functions and technical requirements:

1. Position control: including deceleration, stopping, overwinding, synchronization, and compensation for displacement changes caused by changes in steel length.

2. The speed, position, deceleration, and winding throughout the entire operation are monitored and protected by the main control PLC and the monitoring PLC.

3. Fault diagnosis function: It has fault diagnosis function for Siemens PLC, high voltage frequency converter and fieldbus.

4. Deceleration calibration: Independent deceleration calibration switches are installed at both ends of the wellbore for calibrating the speed at important locations.

5. It has a system designed to prevent human error during operation.

6. It has three or more safety loops, and at least one hardware safety loop protection must be set up.

7. The PLC operating system can control the hoist in various operating modes, including manual, semi-automatic, and maintenance modes.

8. It has the function of decelerating or accelerating according to a predetermined time or location, and can stop in an emergency when problems such as high brake oil temperature, overheating of the main motor, or serious failure of the frequency converter occur.

9. It has complete interlocking protection functions for switch cabinets, back-end systems, frequency converters, motors, switch cabinets, and PLC cabinets.

10. It features redundant design in both PLC hardware and software, and is equipped with multiple communication interfaces and various backup programs and solutions.

(3) Contents of the main safety circuit

A. Zero-position interlock of master controller handle

B. Working brake handle interlock limit switch

C. Speed ​​measurement disconnection protection

D. Undervoltage trip protection

E. Brake oil pressure high relay

F. Fault Protection

G. Constant speed overspeed

J. Overwind switch

K. Overwind reset switch

L. Shaft breakage protection

M, Spring Fatigue Switch

N. Slack Rope Protection

Figure 6 Manual operation control console of the electrical control system

4. Application of Four-Quadrant Variable Frequency Speed ​​Control System

4.1 Schematic diagram of the primary main circuit in the frequency converter field

Based on the actual on-site process and load conditions, a 630kVA four-quadrant high-voltage frequency converter provided by Guangzhou Zhiguang was selected. The rated voltage is 6kV, and the rated output current is 60A. The frequency converter operates in a manual one-to-one mode. GK is the high-voltage live indicator, R is the charging resistor, and J1 is the bypass contactor. After charging is complete, the charging resistor is bypassed. K1 is the disconnect switch. The schematic diagram of the primary main circuit of the frequency converter is shown in Figure 7.

Figure 7 Schematic diagram of the primary circuit of the frequency converter

4.2 Main circuit control method and power unit control

The phase detection power supply is taken from the secondary winding of the isolation transformer, and the primary power supply of the isolation transformer is taken from the PT signal. When the phase detection power supply is disconnected, the unit operates in a two-quadrant mode and can be used as a normal frequency converter. When the power terminal is plugged in, it operates in a four-quadrant mode. A grid-connected AC reactor is connected in series on the AC input side of the power unit to reduce the impact on the grid side. The control circuit is divided into a rectifier control drive board and an inverter control drive board, corresponding to control power supply 1 and control power supply 2, respectively. The unit control circuit is based on a programmable logic device. The main control circuit is based on a dual digital signal processor (DSP) and a very large-scale integrated circuit programmable logic controller (FPGA). Input and output currents are sampled through CTs and Hall sensors, and input and output voltage signals are sampled through PTs and voltage detection resistors. Communication between the power unit and the main controller is achieved through optical fiber. An optocoupler is used for intermediate transmission and AD conversion. All collected signals are transmitted to a touch screen or LCD panel for display as digital quantities via RS-485 and RS-232 communication.

Figure 8 shows the external structure of the unit. The fiber optic interface connects to the fiber optic communication of the main control circuit. Control power supplies 1 and 2 provide power to the rectifier and inverter circuits and the drive board. Inserting the phase detection terminal indicates operation in the four-quadrant frequency conversion state. The internal control circuit of the unit is mainly the drive board, which integrates functions such as internal temperature measurement, DC voltage detection circuit, IJBT drive circuit, and AC voltage detection. The internal protection devices of the unit include overvoltage protection, overcurrent protection, phase-locked loop protection, drive protection, and excessive voltage error protection. When a fault occurs inside the unit, the drive board will display the fault information to the user through status indicator lights for easy viewing of fault information.

4.3 Phase Detection Function Feedback

Because the four-quadrant frequency converter needs to achieve active inversion, it is essential to ensure that the primary input of the phase-shifting transformer is in positive phase sequence. Otherwise, the overcurrent protection of the equipment will burn out the internal circuit of the unit. Before applying high voltage, a phase sequence detector is required to check the wiring terminals on the secondary side of the PT in both the incoming line cabinet and the control cabinet to ensure that they are in positive sequence. The activation conditions for the phase detection relay are: no electrically controlled high-voltage button node, high voltage input, and no special major faults. The relay can only be activated when all conditions are met simultaneously to realize the unit feedback function. That is, the phase detection relay KA17 will activate, and after the feedback voltage, the DC voltage of the unit will rise from 780V to about 880V in about 10 seconds. UO4 and UO5 are PT voltage transformers that sample the voltage signal input from the 6KV high-voltage incoming line switch cabinet to the primary side of the transformer and then send it to the isolation transformer and controller through the secondary side for voltage detection and sampling.

The inverter's mainboard program already includes phase detection of the main circuit as an operating condition. If the program detects that the phase of the input voltage is not in the phase sequence where each of the three phases (A, B, and C) leads by 90°, the controller will consider the equipment faulty and prevent it from starting. It is crucial to ensure that the voltage fed back to the grid during operation in the second and fourth quadrants is in phase with the grid-side voltage.

Figure 9 Voltage and phase detection relay

4.4 Field Structure Composition of Four-Quadrant Frequency Converter

As shown in Figures 10 and 11, the inverter body consists of four main parts: the incoming line charging cabinet, the transformer cabinet, the inductor cabinet, and the power control cabinet.

(1) Incoming line charging cabinet: After the incoming 6KV power supply is introduced, a charging resistor is connected. The principle of the incoming line charging cabinet resistor bypass contactor is as follows: after the high voltage is applied (the live indicator GK flashes), the bypass resistor is closed after 3 seconds. After the charging process of the main body of the equipment is completed, it is bypassed. Its function is to complete the pre-charging of the DC bus circuit and avoid damage to the module inside caused by the strong current impact generated when the high voltage is sent in.

(2) Transformer cabinet: It mainly houses the phase-shifting transformer, which shifts the phase of the secondary output voltage and also filters harmonics. The output of a single power unit is a rectangular wave, and after phase shifting and superposition, the output is a voltage waveform close to a standard sine wave. The output uses phase-shifted multiplexed sinusoidal pulse width modulation (SPWM) technology and speed vector control to achieve the effect of low output harmonics.

(3) Inductor cabinet: Its function is to realize energy storage in the motor state and filtering in the feedback state. Its main significance is the same as the input reactor function of SVG products in dynamic reactive power compensation, and it is used as a grid-connected inductor. The feedback device is an external reactor structure. When wiring, it is necessary to ensure that the terminals of the reactor correspond to the terminals on the feedback device.

(4) Power control cabinet: It consists of main control core components such as controller and PLC. The transmission process is fiber optic transmission and encoding conversion. The controller internally processes various digital and analog signals sampled in through AD conversion into small current signals, and then converts them into voltage signals that the chip can recognize and transmits them to the main control board for processing. Various user control interfaces and communication functions are available for user use.

Figure 10 Composition of a four-quadrant frequency converter

4.6 Application Strategy for Deceleration of Auxiliary Shaft Hoist

The user's switchgear is an electrically held switchgear. After power is supplied, the inverter's start signal is point-controlled; a high-level start signal causes forward rotation, and removing the signal stops the inverter. A high-level stop signal causes reverse rotation, and removing the signal stops the inverter. During pull-up and release, acceleration is controlled by the inverter itself, assisted by the electrical control system. However, deceleration is controlled by the electrical control system. Therefore, the deceleration process must meet all user requirements, and coordination with the electrical control system's deceleration and the operator's emergency brake is crucial. Three deceleration schemes, in addition to normal shutdown, have been developed to meet the user's needs under different loads and at different times. Figure 13 below shows the background operation curve of the user's load. It is clear that the equipment starts and stops very frequently, operating continuously during short acceleration and deceleration periods. Suddenly decelerating from high speed to zero requires immediate restart within a short time, making the braking time critical. Therefore, three different deceleration modes, different from the normal deceleration scheme, have been specified to meet the on-site requirements.

Figure 13 On-site backend operation curve

(1) The first deceleration scheme is a zigzag deceleration, that is, the value set in the "DC braking frequency" in the parameter settings is the starting frequency of the second deceleration. The deceleration time from 50HZ to the set value is the value set in "deceleration time". The deceleration time of the second deceleration is the value set in "DC braking time", which is the deceleration time of this segment.

(2) Second deceleration scheme: It is a semi-manual deceleration scheme. When the "master handle" is in full position and there is a manual deceleration action, the second deceleration scheme will be executed. However, the target frequency of deceleration (the running speed after deceleration) is given by the "master handle". The deceleration time is set to the value set by "DC braking time", which is the time of the second deceleration scheme.

(3) The third deceleration scheme is for emergency braking. In case of an emergency during operation, the "brake handle" is pulled directly to lock the motor and the drum.

In summary, setting these three deceleration schemes in the frequency converter control system ensures that the high-voltage motor comes to a stop in the safest and fastest way during high-speed operation, reducing damage to the load motor reducer. For specific projects, the deceleration scheme and timing should be evaluated based on the specific project requirements. Currently, the on-site deceleration scheme fully meets the "instant start/stop" requirements of the Huating Coal Mine's on-site users for the four-quadrant frequency converter. Combined with routine inspection and maintenance of the frequency converter, the safe and reliable operation of the four-quadrant frequency converter equipment in the coal mine hoist is guaranteed!

5. Conclusion

The application of four-quadrant frequency converters significantly improves energy efficiency on-site. It reduces energy loss during braking and feeds energy back to the grid, achieving energy conservation and environmental protection. Compared to ordinary frequency converters, it is superior in function and offers more significant energy savings. Furthermore, it reduces harmonic pollution on the grid side, and the full-load power factor can approach 1. As the market demands increasingly higher levels of performance and energy efficiency from motors, four-quadrant frequency converters will be better positioned to serve various industrial sectors in the future!