Application of high voltage inverter in mine hoist

1 Introduction

Mine hoist is a large-scale lifting mechanical equipment. The motor drives the mechanical equipment to drive the wire rope to pull the container to run in the shaft or on the slope to complete the transportation task. Modern mine hoists have large lifting capacity, high speed and high safety. They have developed into fully automatic heavy mining machinery controlled by electronic computers.

The working process of the hoist is the same whether it is forward or reverse, and there are seven stages: starting, acceleration, medium-speed operation, stable operation, deceleration, low-speed operation, and braking. The time of each hoist operation is related to the system's operating speed, acceleration, and the depth of the inclined shaft or vertical shaft. The magnitude of each stage of acceleration is determined according to the process conditions, and the operation time is determined by the operator according to the on-site conditions.

At present, most of the mine hoists in large and medium-sized coal mines use the method of winding motor rotor series resistance for segmented step speed control. The basic principle is: when the winding asynchronous motor rotor is connected with additional resistance, the motor slip rate increases, and the motor runs at a lower speed. The larger the resistance connected, the lower the motor speed. The advantages of this method are simple equipment and convenient control; but the disadvantages are that the slip power is consumed in the form of heat on the resistor, and this method belongs to step speed regulation, and the mechanical characteristics are relatively soft. In actual application, the switching of the resistor is controlled by relay-AC contactor. Due to the frequent operation of the AC contactor during the speed regulation process, the equipment runs for a long time, and the main contact of the AC contactor is easy to melt, causing equipment failure. In addition, due to the step speed regulation, the speed control performance of the hoist is poor in the deceleration and creeping stages, which often causes inaccurate parking position. The frequent starting, speed regulation and braking of the hoist generates considerable power consumption on the resistor connected to the rotor external circuit, resulting in a large waste of energy. The current impact is large during the starting and speed regulation and shifting process; the vibration is large during medium and high speed operation, and the safety is poor. Therefore, for mining enterprises with higher requirements for safety and stability, this speed regulation method will soon be eliminated.

The rapid development of high-voltage variable frequency speed regulation technology, especially energy feedback technology, has created conditions for speed regulation and energy-saving transformation of mine hoists. Variable frequency speed regulation can achieve constant torque stepless speed regulation in the full range of 0-50 Hz, and the regenerative energy of the motor can be directly fed back to the power grid when lowering heavy objects. If vector control is used, static high-torque soft starting can also be achieved, and rapid lifting, lowering and instantaneous acceleration and deceleration can be achieved for shaft-type hoisting equipment.

Hengding Industrial International Development Co., Ltd. is a comprehensive coal enterprise integrating coal mining, washing and coking. It was listed on the main board of the United Exchange in September 2007 and is the first private energy enterprise in China to be listed on the major overseas capital market IPO. In recent years, Hengding Industrial Company has actively responded to the national energy-saving and consumption-reducing policies and has carried out frequency conversion transformation on the mine hoists of most of its coal mines. Hengding Industrial's several major coal mines in Pan County, Guizhou Province, have adopted a total of 7 FVA inverters from Beijing Lide Huafu Electric Technology Co., Ltd. to transform the mine hoist equipment. Practice has proved that the use of frequency conversion speed regulation has greatly improved the stability and safety of the hoist operation, reduced operating failures and downtime for maintenance, saved manpower and material resources, and improved the coal transportation capacity. The direct and indirect economic benefits are also considerable.

2 Project Overview

Panxian Lemin Hongxing Coal Mine is located in Lemin Town, Panxian County, Liupanshui City. The mine area is 1.7km2, the mine area has 44.3 million tons of recoverable coal seams, the designed recoverable reserves are 23.67 million tons, and the mine design output is 600,000 tons/year. This well is an inclined well, mainly used for underground material and material transportation. The total length of the inclined well is about 1km, the slope is 25o, and the vertical depth is about 420m. The well uses a single-drum single-rope winding winch. The on-site process conditions are shown in Figure 1.

The inverter directly uses 10kv high voltage electricity and outputs it to the motor after frequency conversion. The motor output shaft drives the drum to rotate after being decelerated by the speed reduction mechanism, thereby driving the wire rope wound on the drum to pull up or lower the trolley. The inverter provides two control modes: local (through the human-machine interface) operation and remote (through the operating console). The operating console controls the hydraulic system and the frequency setting and start-stop operation of the inverter through the push and pull of its two handles, thereby ultimately achieving precise adjustment of the winch's release, braking and trolley running speed.

The main equipment consists of the following parts:

(1) High-voltage variable frequency speed regulation system provided by Beijing Lide Huafu Company

The main parameters are as follows:

Model: harsvert-fva10/030, energy feedback type vector control high voltage inverter;

Rated voltage: 10kv;

Rated current: 30A.

(2) High-voltage motors produced by Jiamusi Electric Motor Factory

The main parameters are as follows:

Model:y4507-6;

Rated voltage: 10kv;

Rated current: 23.9a;

Motor pole number: 6 poles;

Rated speed: 989r/min.

(3) Reels, speed reduction mechanisms, hydraulic and electronic control systems manufactured by Guizhou Plateau Mining Machinery Co., Ltd.

The main parameters are as follows:

Model: jk-2.5×2.0p;

Roll diameter: 2.5m;

Maximum lifting speed: 3m/s;

Rope capacity of drum: 2km.

In addition to the above equipment, there are also depth indication system and lubrication system.

The lubrication station is used to lubricate the deceleration mechanism. The depth indication system is connected to the rolling shaft of the drum through a transmission rod, which is used to provide the relative position of the trolley with the ground when it is running underground, and transmit it to the operating table to facilitate the setting of acceleration and deceleration time points and protection logic. The hydraulic system consists of a hydraulic station and hydraulic gate valves. The hydraulic pump station drives a total of 4 groups of 16 hydraulic gate valves located in front, behind, left and right of the drum. Each gate valve is connected to a brake pad for emergency braking and release. The status signals of the frequency converter, the I/O points of the hydraulic system and all other auxiliary systems are sent to the plc on the operating table. The plc and the touch screen on the operating table use modbus communication to facilitate status monitoring and parameter setting. The control buttons and handles on the operating table are used for manual and automatic switching and start and stop operations of the winch operation.

3 High voltage variable frequency speed regulation technology

3.1 Four-quadrant inverter

For the frequency converter, the operating quadrant of the motor is defined as the operating quadrant of the frequency converter, and a plane rectangular coordinate system is established with the motor torque as the horizontal axis and the speed as the vertical axis, as shown in Figure 2.

When the output torque of the motor is the same as the direction of the motor, such as when the motor drives a fan, water pump, compressor, pulley, etc. in steady state or accelerated operation, or when the motor drives a hoist to lift heavy objects, the motor operates in motor mode, the inverter obtains active power from the power grid and outputs it to the motor, and the inverter operation mode is in the energy output state, that is, in the first and third quadrants. When the output torque of the motor is opposite to the direction of the motor, such as when the motor drives a fan and other shaft loads to decelerate and brake, or when the motor drives a winch to lower heavy objects, the motor operates in generator mode, the inverter will obtain active power from the motor and feed it back to the power grid, and the inverter operates in the second and fourth quadrants, in the energy feedback state.

Ordinary inverters use diode uncontrolled rectification or semi-controlled rectification on the grid side and can only work in the first and third quadrants, which is called a two-quadrant inverter. If fully controlled rectification is used on the grid side, the inverter can work in four quadrants. This inverter is called a four-quadrant inverter or energy feedback inverter.

When using energy-regenerative inverters on hoists, the inverter can drive the motor to output braking torque when parking or lowering heavy objects, which can not only achieve rapid braking (50 Hz parking only takes 10 seconds), but also convert the kinetic energy of the load into electrical energy and directly feed it back to the power grid.

3.2 Basic principles of energy feedback

The basic topology of the voltage source energy feedback inverter is shown in Figure 3.

In Figure 3, the grid-side inverter is called the active front end, which is responsible for controlling the energy exchange between the inverter and the grid; the reactor provides conditions for the energy flow between the grid and the inverter and limits the grid-side inverter current; the grid-side filter is used to absorb the harmonic current emitted by the grid-side inverter and prevent it from being injected into the grid.

Simplify the above structure, equate the grid-side inverter to a controllable voltage source, equate the grid and the filter to an ideal voltage source, and assume that the grid voltage angular frequency is ω, the effective value is e, and the phase is 0°; the grid-side inverter output effective voltage effective value is u, the phase is θ, and the inductance is l. Then:

It can be seen that the active and reactive exchange between the inverter and the grid is regulated by adjusting the amplitude and phase of the output voltage u of the grid-side inverter: when u lags e, θ》0°, p》0, the inverter absorbs active power from the grid; when u leads e, θ<0°, p<0, the inverter feeds back active power to the grid, that is, it is in the capacity feedback state; when ucosθe, the inverter sends inductive reactive power to the grid; when ucosθ=e, the grid-side power factor of the inverter is 1.

In summary, by precisely controlling the output voltage of the inverter on the grid side of the frequency converter, the size and direction of its active power and reactive power can be controlled to achieve energy feedback. In actual control, the power exchange between the frequency converter and the grid is often controlled by controlling the current on the inductor.

3.3 Implementation of energy feedback

At present, most of the inverters in China adopt the method of unit series multi-level. The topology with energy feedback is similar to the topology without energy feedback. The difference is that in each power unit, the original diode uncontrolled rectifier bridge is changed to an IGBT inverter bridge, as shown in Figure 4 and Figure 5. In addition, in order to filter out the high-order harmonic current of the switching frequency generated by the grid-side inverter and prevent it from being injected into the grid, a set of grid-side filters is added on the input side.

In order to control the DC bus voltage of each power unit, a separate controller is set in each power unit to execute the vector control algorithm of the grid-side inverter. When the inverter switches from the energy output state to the energy feedback state, the active power flows from the motor to the inverter, causing the DC bus voltage of each power unit to rise. After the controller in each power unit detects this rising voltage, it will reduce the active current set value flowing from the grid to the power unit to a negative number. At this time, the main controller will lock the amplitude, phase and frequency of the grid voltage in advance through the program, and then make the grid-side inverter of the power unit output PWM wave. The PWM waves generated by each power unit are superimposed to form a sinusoidal voltage and phase that are compatible with the grid, and then feed back active power to the grid.

4. Obvious energy saving effect

When operating at industrial frequency, the motor requires 5 to 7 times the starting current. When operating with a frequency converter, the motor can be started at zero speed and zero voltage (of course, the torque boost can be appropriately increased). Using vector control, the maximum dynamic torque at startup is 150% of the rated torque, and it can start with full load.

When the inverter is braking, the motor speed decreases with the frequency as the load decreases. This is because the power energy (potential energy) generated by braking is fed back to the grid through the grid-side inverter. In addition, when the motor is braking and stopping, the energy (potential energy) generated by the inertia of the motor can also be converted into renewable energy and fed back to the grid for reuse. Therefore, the use of inverters has obvious energy-saving effects.

5 Conclusion

Vector control technology realizes high torque, low current starting and precise adjustment of speed and torque, while energy feedback technology further optimizes the energy-saving effect of the inverter. Because the traditional variable frequency speed regulation energy saving only reduces the energy loss caused by valve or damper throttling under the condition of meeting the on-site process, while energy feedback also directly feeds back the regenerative energy of the motor in the generator state when it is running in the second and fourth quadrants to the power grid. Not only will it not consume the unusable or regenerated electric energy in the form of resistance heating like the rotor series resistance or energy consumption braking method, but this part of energy can also be utilized, making the energy-saving effect of the inverter more obvious.

Coal mining is also a high-energy-consuming industry. Modern large-scale electric machinery has replaced the traditional manual excavation method. The capacity of belt conveyors, hoists, gas exhaust fans, and main fans has gradually increased, and the application of high-voltage and high-power motors has been further promoted. Under the existing process conditions, achieving substantial energy conservation and consumption reduction will become a necessary condition for the long-term survival of enterprises.

The rapid development of high-voltage frequency conversion technology has provided a very favorable opportunity for energy conservation and consumption reduction in the coal mining industry. Relying on the academic advantages of Tsinghua University and with more than 10 years of technology and experience, Leader Huafu Company has vigorously promoted vector control energy feedback frequency converters, which will provide strong technical support and guarantee for energy conservation and consumption reduction in coal and other mining enterprises.