Reactive power compensation system based on magnetic valve controllable reactor

Abstract The natural power factor of electrified railways is low, and the existing parallel capacitor compensation method is difficult to make the system meet the standard requirements, which affects the economic benefits of the enterprise. When using a magnetic valve controllable reactor to regulate the reactive power of an electrified railway system, the main issues that need to be addressed include the measurement and rapid adjustment of the reactive power of nonlinear circuits to ensure that the power factor remains above 0.9. The magnetic valve-type controllable reactor, which uses DC current to control the magnetic saturation of the iron core to achieve smooth adjustment, is used as the compensation component, and the thyristor is used as the actuating component. It is controlled by the 80C196KC microcontroller to ensure rapid, accurate and reasonable compensation. sex. Experiments and prototype trial runs have shown that the dynamic reactive power compensation system can quickly compensate the system’s reactive power, keep the power factor at a high level, greatly improve the power supply quality, and increase the economic benefits of the power supply system. Keywords: electrified railway, magnetic valve type controllable reactor, reactive power, smooth regulation, power factor, economic benefit
1. Introduction With the continuous expansion of the scale of the power grid and the connection of various electrical equipment to the power grid, which consumes a large amount of reactive power, the problems of insufficient reactive power and large voltage fluctuations have become increasingly prominent. At this time, simply adjusting the generator excitation current cannot meet the requirements. Since the beginning of the 20th century, people have conducted a lot of research on reactive power compensation technology. In order to improve the load power factor, reactive power compensation such as synchronous condensers, shunt capacitors, shunt reactors, series capacitors, and modern static compensators have been gradually adopted. means. Control methods also include centralized control, decentralized control, and associated control. The control strategy has also shifted from classic control to intelligent control. ​

Electrified railways are important power users, and their reactive power problems have always been serious. The technical status of electrified railway electric locomotives and traction substation reactive power compensation devices is directly related to the economic benefits of transportation production. There are two methods to improve the power factor of electrified railways: one is to improve the power factor of the load (electric locomotive), which can be achieved by transforming the original electric locomotive or developing an electric locomotive with a high power factor; the other is to monitor and regulate the system without any power, so that the power factor always maintains a high value. The former method requires a large amount of funds and cannot be implemented in a short period of time. There are two types of reactive power compensation devices that are commonly used today: one is a switching capacitor bank, but when no electric locomotive passes through the power feeder, the parallel capacitor bank sends reactive power back to the system, and the power department implements reactive power compensation device “Reverse forward meter” (that is, the reactive power sent back to the power system by the user is accumulated with the absolute value of the reactive power taken), so that the power factor cannot reach the 0.9 standard; the switching capacitor bank also generates inrush current and Electromagnetic transients cause overvoltage. In actual operation, there have been system overvoltage accidents caused by using switches to switch capacitor banks; the second is to use thyristor controlled reactors (TCR), but they are expensive, occupy a large area, and have harmonic content. big. ​

The use of controllable reactors and parallel capacitor banks can meet the characteristics of electric locomotives with variable operating modes and rapid load changes. Moreover, the device can smoothly adjust reactive power, has low cost, high reliability, and small harmonics. It is an ideal choice for electrified railways. A better choice for system dynamic reactive power compensation. ​

The main work of this topic is: to study the control scheme of the reactive power compensation device to determine a more effective control method that is more beneficial to the power grid; to design the thyristor trigger circuit to determine the correct conduction angle; to conduct research based on the magnetic valve type Design of the control device of the controllable reactor; conduct experiments and debugging of the prototype.

2. Magnetic valve type controllable reactor and characteristics

Figure 1 is the schematic diagram of a magnetic valve type controllable reactor. As can be seen from the figure, the core magnetic circuit of the magnetic valve controllable reactor is composed of large-section sections and small-section sections connected in series. Two coils with N turns are symmetrically wound around two half iron core columns. The upper and lower windings of each half iron core column each have a tap with a tap ratio of δ = N2/N, and a thyristor K1 is connected between them. (K2). The upper and lower windings of different cores are cross-connected and then connected in parallel to the grid power supply.

Within the entire capacity adjustment range of the controllable reactor, the core of the large-section section is always in the unsaturated linear region of magnetism, and the magnetic resistance is negligible compared to the small-section section; the magnetic saturation of the small-section section can be designed to be close to the limit value. At this time, the harmonics generated by the controllable reactor are very small, about half of the harmonics generated by the thyristor-controlled reactor. At this time, the capacity has reached the limit value, so the overload capacity of the magnetic valve type controllable reactor is poor, but it is particularly suitable for voltage regulation and reactive power compensation in high-voltage distribution networks; if the problem of long-term overvoltage limitation is not considered, it can also be used Compensation for line charging power. ​

Within a power frequency cycle of the power supply, the thyristors are turned on in turn to play the role of full-wave rectification. Changing the firing angle of the thyristors K1 and K2 can change the size of the control loop current to change the saturation of the reactor, thereby smoothly and continuously adjusting the capacity of the controllable reactor [2].

3. Principle of dynamic reactive power compensation device

Figure 2 shows the wiring method of the electrified railway power supply system and dynamic reactive power compensator. The dynamic reactive power compensation system consists of a single-phase controllable reactor and a fixed capacitor. When the electric locomotive enters the jurisdiction of the traction substation, the fixed capacitor bank fully compensates the inductive reactive power of the locomotive, and the capacity of the controllable reactor is adjusted to the minimum (no-load); when the electric locomotive leaves the jurisdiction of the power grid, the capacitor moves to The system reverses reactive power. At this time, the capacity of the magnetic valve type controllable reactor is quickly adjusted to the maximum value to absorb the capacitive reactive power. During the change of the electric locomotive load, the controllable reactor quickly tracks and compensates for the remaining capacitance. reactive power, thereby ensuring a high power factor. At the same time, the capacitor bank also plays the role of 3rd, 5th and higher harmonic filters.

4. Reactive power detection and control loop In order to simplify the analysis and start from the actual situation, it is assumed that the voltage of the electrified railway system is a sine wave and the current is a non-sinusoidal wave, respectively expressed as:

In the formula, I1 is the effective value of the fundamental current, I is the effective value of the total current, and cosφ1 is the fundamental power factor. ​

Since the main function of the controllable reactor and capacitor bank in this system is to compensate for the fundamental reactive power, it can be expressed as:

In order to quickly adjust the capacity of the controllable reactor, a threshold can be set. When the absolute value of Q′ is greater than this value, we fully invest or fully exit the reactor (according to the sign of ΔQ). This threshold should be set according to the capacity of the capacitor bank in the dynamic compensation device to prevent the load from oscillating between the inductor and the capacitor. When the absolute value of ΔQ is less than this value, a corresponding trigger pulse is issued based on the relationship between ΔQ and the trigger angle. 5. Hardware structure of the control device The principle block diagram of the control device is shown in Figure 3. The functions of each part are introduced below. 5.1 Signal sampling part The voltage and current signals taken out from the primary side of the system are transformed and filtered and then sent to the microprocessor. 5.2 The sampling part uses the on-chip A/D of the CPU (80C196KC). Since its A/D is unipolar, a +2.5V voltage reference LM385 is used to raise the AC signal with a peak-to-peak value of 5V to a DC signal of 0-5V. 5.3 Signal output part The controllable saturation reactor changes the size of the control current by changing the trigger conduction angle of the thyristor, thereby changing the magnetic saturation of the iron core to smoothly adjust the capacity of the controllable reactor. In every power frequency cycle, a pulse must be sent to trigger the thyristor. At this time, the time base must be determined accurately. We take the voltage synchronization signal and use the zero-crossing comparator LM311 to turn the string signal into a square wave. Connect this square wave signal to the HSI of the CPU. 0 pin, using HSI. 0 interrupt, the thyristor can be triggered at any desired moment. In order to ensure the separation of strong current and weak current, the output signal is isolated by 4N25 photoelectric isolation and then connected to the thyristor. 5.4 External interface: Using a human-machine interface, the triggering angle of the thyristor can be manually adjusted through the keyboard, and values such as reactive power, power factor, current, and active power can also be displayed. When the keyboard is not in use, the firing angle of the thyristor can be automatically adjusted.

6. Harmonic suppression: Using the above-mentioned harmonic distribution and phase characteristics of controllable reactors, two sets of controllable reactors are connected in parallel. Through a certain control strategy, most of the harmonics generated by the two sets of reactors can be canceled out. Figure 4 is the wiring schematic diagram of a single-phase controllable reactor group. In the figure, L1 is the first unit of the controllable reactor group, its magnetic saturation under rated working condition is π, and its rated capacity accounts for one-third of the total capacity; L2 is the second unit of the controllable reactor group, its rated working The magnetic saturation in the state is 2π, and the rated capacity is two-thirds of the total capacity. ​

The control strategy for the controllable reactor is: adjust the controllable reactor L2 within the range of 0~1/3 rated capacity (total rated capacity of the two sets of reactors) to meet the capacity requirements; within the range of 1/3~1 rated capacity When the range changes, reactors L1 and L2 are coordinated and controlled so that most of the high-order harmonics generated by the two cancel each other out, that is, the harmonics generated by one unit’s reactor are bypassed (absorbed) by another unit’s reactor. ), that is, in in Figure 4. ​

Since the rated capacity of the reactor unit L2 accounts for 2/3 of the total capacity of the reactor group, according to the harmonic distribution characteristics of the controllable reactor, the maximum third harmonic current amplitude generated is (2/3) of the total rated fundamental current. ×7%, so the maximum third harmonic current generated by the reactor group within the capacity adjustment range of 0 to 1/3 is approximately 4.67% of the rated fundamental current. The corresponding magnetic saturation when the reactor unit L2 reaches 1/3 of the total rated capacity is β=π. Within the capacity adjustment range of 1/3 to 1, the magnetic saturation of reactor unit L1 changes between β1 = 0 and π, while that of unit L2 changes between β2 = π and 2π. It is not difficult to understand that within the above capacity adjustment range, the third harmonic generated by the reactor unit L2 is opposite to the third harmonic of the unit. If through control, β1 and β2 have the following relationship: The harmonic distribution of the reactor group can be calculated as shown in Figure 5. [2] The abscissa is the per unit value of the fundamental current, and the reference value is the rated fundamental current; the ordinate is the root mean square per unit value of each harmonic current, and the reference value is the rated fundamental current. Curve 1 shows the situation when a single reactor is running; Curve 2 shows the situation of a reactor group with harmonic self-compensation function. It can be seen from the figure that the harmonic content of the reactor group within the capacity adjustment range of 1/3 to 1 is not greater than 2.8% of the rated fundamental current.

The output current waveforms of the controllable reactor group with harmonic self-compensation function under different magnetic saturation β (unit L2) are shown in Figure 6.

7. Experimental analysis Two controllable reactors of 320 V and 1000 VA are used. The rated magnetic saturation of one group of controllable reactors is βn=2π; the other group of βn=π. The measured distribution of the third and fifth harmonics is shown in Figure 7. The third harmonic current distribution in the figure is consistent with the theory in Figure 5