1 Introduction
In coal mine underground power distribution systems, a large number of inductive loads exist, such as three-phase asynchronous motors and transformers. These inductive loads consume a significant amount of reactive power, reducing the system's power factor and causing increased voltage drops and energy losses. Furthermore, some impulsive reactive loads can generate severe voltage fluctuations, deteriorating the power grid's quality and causing difficulties in starting or frequent burnouts of motors, particularly noticeable with high-power motors. Additionally, increased reactive current can lead to decreased insulation and aging of power lines and equipment, increasing the risk of leakage and short circuits. With the rapid development of modern mines, increasing mechanization, and the widespread use of high-power motors and electronic components, a large amount of reactive power inevitably circulates between various inductive loads and equipment and the surface power grid, generating various harmonics. This deteriorates power quality and causes serious energy waste, directly affecting the normal operation of the power grid and equipment.
The use of reactive power compensation has the following advantages:
(1) Reduce reactive power loss and reduce energy waste.
After compensation, the system power factor is improved, the current in the transformer and power supply line decreases, the reactive power loss is reduced, and the goal of energy saving and consumption reduction is achieved.
(2) Improve the power factor
The capacitive reactive current is used to offset the inductive reactive current generated by the load in real time, thereby improving the power factor of the underground power supply system.
(3) Controlling harmonics and purifying the underground power grid
Each compensation branch has the function of limiting inrush current and controlling harmonics, so as to achieve the purpose of safe operation of electrical components in the device and purification of the underground power grid.
(4) Improved the utilization rate of the power supply system
There is a large amount of reactive power circulating between underground electrical equipment and surface power supply. This reactive power inevitably occupies a lot of the power supply system capacity, causing a decrease in the load-carrying capacity of power supply lines, a decrease in the capacity utilization rate of underground transformers, and a decrease in the load-carrying capacity of control switches at all levels. After adding reactive power compensation, the apparent power of underground transformers is made close to the active power, which effectively improves the apparent power utilization rate. The power supply lines and control switches at all levels have a greatly improved load-bearing capacity due to the reduction of reactive current.
(5) Stabilize grid voltage
The large amount of reactive power generated by inductive loads underground inevitably leads to voltage fluctuations in the power grid. Large reactive power results in large voltage fluctuations, and rapid changes in reactive power cause even faster voltage fluctuations. Installing reactive power compensation systems will compensate for most of the reactive power locally, significantly reducing the reactive power in the power grid and decreasing the range of voltage fluctuations in both the grid and the secondary voltage of underground transformers.
(6) Reduce electrical accident rate and extend equipment service life.
The temperature rise of transformers, power supply lines, control and protection switches at all levels, and all main circuit connection points of the power supply system is directly proportional to the apparent current flowing through the system. A large apparent current inevitably leads to a rapid temperature rise. Once the temperature exceeds the insulation strength, it will inevitably cause various accidents such as aging, grounding, discharge, and arcing short circuits, and may even cause electric shock, shortening equipment lifespan, increasing maintenance workload, increasing maintenance expenditures, shortening equipment replacement cycles, and increasing equipment investment costs. After reactive power compensation, the apparent current of the system decreases by about 30%, reducing the actual current borne by all electrical equipment and decreasing the probability of various electrical accidents caused by high current. A lower accident rate inevitably increases equipment uptime, and reduced accident handling time inevitably increases normal production time. In short, the installation of compensation achieves the goals of reducing costs, saving investment, reducing accidents, and increasing production efficiency.
2 Static Var Generator Technology
The development of reactive power compensation technology has progressed from synchronous condensers to switched-on fixed capacitors to switched-on capacitors (SVC) to static var compensators (SVG). Static var compensators (SVG, also known as STATCOM) utilize fully controlled switching devices (such as IGBTs), resulting in a dynamic compensation effect unmatched by earlier reactive power compensation devices such as synchronous condensers and capacitors. With its lower harmonics, higher efficiency, and faster dynamic response, static var generators will become an important piece of equipment in power transmission systems.
One traditional method for compensating for harmonics and reactive power is to install passive filters, typically composed of series and parallel combinations of power capacitors, reactors, and resistors. This method can compensate for both harmonics and reactive power. Currently, commonly used reactive power regulation equipment in my country still consists of mechanical parallel reactors and switched capacitors. These static voltage regulation methods, due to their discontinuous regulation and slow response speed, are difficult to meet the needs of rapidly changing system operating modes. Another compensation device, the SVC (Sequential Voltage Regulator), has a faster response speed than capacitors, but it still exhibits impedance characteristics to the power grid. At low voltages, it cannot provide the necessary reactive power support for the system and has a weak ability to cope with grid anomalies. Because SVCs amplify certain characteristic harmonics in the system, filters are sometimes required, resulting in a large footprint. Too many SVC devices can easily cause system oscillations. The high-voltage dynamic reactive power compensation device (SVG) is a more effective voltage regulation method. Its reactive current output can remain constant over a wide voltage variation range, providing strong reactive power support even at low voltages, and can be continuously adjusted across the entire range from inductive to capacitive.
3. Xinfengguang Company's High Voltage Dynamic Compensation System (FGSVG)
The FGSVG series products adopt modern power electronics, automation, microelectronics and network communication technologies. They use advanced instantaneous reactive power theory and power decoupling algorithm with synchronous coordinate transformation to operate with the set reactive power properties and magnitude, power factor and grid voltage as control targets. They dynamically track changes in grid power quality to adjust reactive power output and can achieve curve setting operation to improve grid quality.
3.1 Features of FGSVG series products:
The FGSVG series products are designed to meet users' urgent needs for improving the power factor of power transmission and distribution networks, controlling harmonics, and compensating for negative sequence current. They feature the following characteristics:
(1) Modular design, easy to install, debug and set up.
(2) Fast dynamic response speed, with a response time ≤5ms.
(3) Provided that the compensation capacity is sufficient, the output current harmonics (THD) ≤ 3%.
(4) Multiple operating modes greatly meet user needs. The operating modes include: constant device reactive power mode, constant assessment point reactive power mode, constant assessment point power factor mode, constant assessment point voltage mode, and constant assessment point reactive power mode 2. The target value can be changed in real time.
(5) Track load changes in real time, dynamically and continuously smooth the reactive power compensation, improve the power factor of the system, control harmonics in real time, compensate negative sequence current, and improve the power supply quality of the power grid.
(6) Suppress voltage flicker, improve voltage quality, and stabilize system voltage.
(7) The FGSVG circuit parameters are carefully designed, resulting in low heat generation, high efficiency, and low operating cost.
(8) The equipment has a compact structure and occupies a small area.
(9) The main circuit adopts the H-bridge power unit series structure composed of IGBTs. Each group is composed of multiple identical power units. The whole machine output is a stepped wave composed of superimposed PWM waveforms, which is close to a sine wave. After being filtered by the output reactor, the sine wave is good.
(10) The FGSVG adopts a redundant design and a modular design to meet the requirements of high system reliability.
(11) The power circuit is modularly designed, easy to maintain and has good interchangeability.
(12) It has complete protection functions, including overvoltage, undervoltage, overcurrent, fiber optic communication failure, unit overheating, and uneven voltage protection. It can also record the waveform of the fault moment, which is convenient to determine the fault point. It is easy to maintain and has high operational reliability.
(13) The human-machine interface is user-friendly and provides interfaces such as RS485 and Ethernet for external communication, and adopts the standard MODBUS communication protocol. In addition to real-time digital and analog display, operation history event recording, historical curve record query, unit status monitoring, system information query, and historical fault query, it also has special functions such as system self-test after power-on, one-button start-up and shutdown, time-sharing control, oscilloscope (AD channel forced waveform recording), and voltage/current waveform recording at the moment of fault.
(14) The FGSVG design includes an interface for use with FC, realizing an effective combination of fixed compensation and dynamic compensation, providing users with a more economical and flexible solution.
(15) There is no transient impact, no inrush current, no arc reignition during switching, and it can be switched on again without discharge.
(16) When connecting to the system, there is no need to consider the phase sequence of the AC system, making the connection convenient.
(17) Can be installed in parallel, and the capacity can be easily expanded. Parallel operation uses fiber optic communication, which has a fast communication speed and can perfectly meet the requirements of real-time compensation.
3.2 FGSVG Principle
In AC circuits, there are three phases for voltage and current: when the load is purely resistive, the voltage and current are in phase; when the load is (or contains) inductive, the voltage phase leads the current phase; and when the load is (or contains) capacitive, the voltage phase lags the current phase.
Figure 1 Schematic diagram of principle
As shown in Figure 1, the basic principle is to connect the self-commutated bridge circuit in parallel to the power grid through a transformer or reactor. By appropriately adjusting the amplitude and phase of the AC output voltage of the bridge circuit, or by directly controlling its AC current, the circuit can absorb or generate reactive current that meets the requirements, thereby achieving the purpose of dynamic reactive compensation, as shown in Table 1.
Table 1 Schematic diagram of SVG operation mode
3.3 System Structure
The main circuit structure of the FGSVG system is shown in Figure 2:
Figure 2 System main circuit structure diagram
4 Coal Mine Application Cases
The power supply system of a certain coal mine adopts a 10kV dual-circuit power supply. The main power supply line comes from the 10kV outgoing line of the Xinyu 35kV substation, and the backup line comes from the 10kV Zhixin line of the self-owned power plant's 35kV direct supply station. Two KBSG-630/10 transformers are installed in the underground central substation, with a power supply voltage of 660V. Its power supply system is shown in Figure 3.
Figure 3 Power supply system of a coal mine
To compensate for reactive power in the power grid, the coal mine uses two sets of FGSVG-C3.0/10 capacitor banks manufactured by Xinfengguang Company. One set is installed in capacitor bank No. 1 to collect the CT current signals of the Yukuang line and the Zhixin line and perform reactive power compensation on the first section of the 10kV busbar. The other set is installed in capacitor bank No. 2 to collect the Yukuang line current signal and perform reactive power compensation on the second section of the 10kV busbar.
The SVG project has shown significant results after its successful commissioning. The data records before and after installation are shown in Figure 4.
Figure 4
Figure 4 shows the power factor of the jade mine line before the SVG was put into operation. As can be seen from Figure 4, the power factor was about 0.83 before the SVG was put into operation, which is not conducive to the stability of the power grid.
Figure 5
The left half of Figure 5 shows the power factor curve of the jade mine line after the SVG was put into operation. It can be seen that the power factor reached 1 after the reactive power compensation was put into operation. The right half shows the power factor curve before the SVG was put into operation. It can be seen that the power factor was only 0.84 after the SVG reactive power compensation device was removed. This shows that the SVG has a good effect on improving the power factor.
Figure 6
Figure 6 shows the reactive power value of the incoming switchgear of the 10kV2 section of the Yukuang line. As can be seen from Figure 6, the reactive power value of the incoming line can reach 450kvar and fluctuates greatly. At this time, the large reactive power value will increase the line loss and reduce the power supply capacity of the line.
Figure 7
The left half of Figure 7 shows the reactive power of the system when the SVG is not in operation, which can reach a maximum of 0.45 Mvar and fluctuates greatly. The right half shows the reactive power of the system after the SVG is in operation. It can be seen that the reactive power of the system after the reactive power compensation is put into operation is almost 0 and fluctuates less. This shows that the SVG has a significant effect on compensating the reactive power of the system and improving the power supply capacity of the line.
Figure 8
Figure 8 shows the power factor curves of the SVG system before and after commissioning on the 10kV1 section of the new line. The left half is before commissioning, which shows that the power factor is low and fluctuates greatly. The right half is the power factor after SVG commissioning, which shows that the power factor is 1. It can be seen that SVG has a significant effect on improving the power factor.
Figure 9
Figure 9 shows the reactive power values of the system before and after the commissioning of the SVG on the 10kV1 section of the straight-line. The left half shows the reactive power values of the system before the SVG was commissioned. It can be seen that the reactive power values of the system fluctuate greatly, and the maximum value reaches 1.25 Mvar. The right half shows the reactive power values of the system after the SVG was commissioned. It can be seen that the reactive power values are almost 0, small, and fluctuate little. This shows that the SVG has a significant effect on compensating for the reactive power of the system.
The above images and descriptions record the reactive power and power factor of the system before and after the commissioning of SVG in 10kV Section 1 (Zhixin Line) (Yuji Line under maintenance, not in operation, and not measured) and 10kV Section 2. As can be seen from the above, the connection of SVG to the power grid in Sections 1 and 2 has a significant effect on improving the system power factor, compensating for system reactive power, and improving the power supply quality of the lines, and is worth promoting.
5. Conclusion
Xinfengguang Company's high-voltage dynamic reactive power compensation device (SVG) is the latest technology in the field of high-voltage dynamic reactive power compensation. Compared with other reactive power compensation devices, it has the advantages of fast adjustment speed, wide operating range, continuous reactive power absorption, small harmonic current, low loss, and greatly reduced capacity and size of capacitors and reactors.
This device has significantly improved the power factor in coal mines. Its ability to operate stably for extended periods without inherent faults also demonstrates the strong adaptability of the SVG to coal mine power grids. Furthermore, it is evident that the SVG device plays a positive role in improving the characteristics and stability of coal mine power grids, making it worthy of widespread adoption in the power system.
