CR3032 battery.Research on new rectifier power supply for DC electric arc furnace

Abstract: This paper analyzes the working principle and characteristics of a new rectifier power supply based on two technologies: freewheeling diode with neutral point and phase shift control, and adopts low flicker control method. Compared with AC electric arc furnaces, DC electric arc furnaces have much less interference and impact on the power grid and cause flicker to the power grid. But the whole

Abstract: This paper analyzes the working principle and characteristics of a new rectifier power supply based on two technologies: freewheeling diode with neutral point and phase shift control, and adopting low flicker control method.

Compared with AC electric arc furnaces, DC electric arc furnaces have much less interference and impact on the power grid and cause flicker to the power grid. However, the rectified output voltage Ud of the rectifier is a function of the thyristor firing angle α. In order to reduce the rectified output voltage, α must be increased. This will increase the phase shift factor φ between the AC voltage and the AC current, so a new reactive power loss is added. At the same time, the power factor is also reduced. In order to minimize the reactive power loss, the electric arc furnace transformer must have a tap switch. In addition, as a result of the use of the current control loop, arc fluctuations cause fluctuations in the firing angle α, thus causing changes in reactive power and causing voltage flicker. In order to limit flicker from exceeding the standard, the short-circuit capacity of the DC arc furnace is required to be Scc ≥ 50Sf. Furthermore, since there is a rectifier device in the main circuit of the DC arc furnace, characteristic harmonics related to the rectifier circuit will inevitably appear, so that the harmonics appearing in the system (except for lower harmonics such as the second and third harmonics) are still high.

In the case of large-scale ultra-high-power DC arc furnaces and weak power grids, the above phenomena will become more obvious, causing the voltage fluctuations and flickers of the system at the public power supply point of the power grid to often exceed GB/T12326-90 "Power Quality" To meet the specified values of "allowable voltage fluctuation and flicker", SVC (dynamic reactive power compensation device) and transformer tap switching switch are still required, which increases the one-time investment. In order to completely solve the interference of DC arc furnace on the power grid, especially the flicker problem, the advantages of DC arc furnace should be fully utilized. In the mid-1990s, the French company CEGELEC proposed a new rectifier power supply for DC arc furnaces that uses two technologies: freewheeling diode with neutral point and phase shift control, as well as a special control method. This article mainly conducts a detailed analysis of the rectified power supply and further conducts computer simulation research.

1 Characteristic analysis of new rectified power supply

Compared with the traditional rectified power supply, the characteristics of the new rectified power supply are: based on two technologies: freewheeling diode with neutral point and phase shift control, it simultaneously controls the arc current and the reactive power at the grid common contact point (pCC) power.

1.1 Freewheeling diode technology with neutral point

The wiring structure is shown in Figure 1. A freewheeling diode is connected to each half-bridge, and the neutral point is connected to the neutral point of the rectifier transformer. In this case, the conduction time of the thyristor changes (for a conventional three-phase fully controlled rectifier bridge, it is always 120°). When the firing angle α is between π/6 ~ 5π/6, the diode is partially Time conduction freewheeling, and with the increase of α, the conduction time of the thyristor decreases, while the conduction time of the diode increases. In addition, the rectifier has no inverter working state, that is, there is no conventional rectifier that returns the energy stored in the reactor to the grid when α is large, which greatly reduces the change in reactive power and is conducive to reducing flicker. The p and Q curves are shown in Figure 2. The rectifier can run at the origin (p=Q=0), which is very beneficial to the DC arc furnace where short-circuit operation often occurs.

1.2 Parallel phase shift control technology

Using parallel phase shift control means crossing the firing angles α1 and α2 (α1≠α2) between the half bridges. The wiring diagram is shown in Figure 3. Because the two secondary windings with neutral point star connection that supply power to these two sets of rectifier bridges are located on the same core of the rectifier transformer. In this way, the even harmonics and DC components that appear in the two secondary windings of the rectifier transformer controlled by the phase shift are completely canceled at the primary side of the rectifier transformer. In the case of parallel phase shift control, the DC output voltage is a function of a1 and a2. This apparent redundancy of parameters can not only control the DC output voltage, but also control the reactive power. However, in the case of single parallel phase shift control, it is impossible to reach the p=Q=0 point. Figure 4 shows the p and Q curves under this wiring method.

1.3 New rectified power supply

The new rectified power supply combines the two technologies of freewheeling diode with neutral point and phase shift control. It has the advantages of these two technologies at the same time, achieving high reliability and availability, and reducing harmonics and reactive power. The wiring structure of the new rectified power supply is shown in Figure 5, and the obtained p and Q curves are shown in Figure 6.

The new rectified power supply adopts a low flicker control method, which can theoretically achieve zero flicker. In Figure 7, ① and ② are the two extreme operating states of no shift and maximum shift control respectively. In principle, the rectified power supply can work at any point between ① and ②, and is determined by the following relationship:

Among them, the trigger angles a1 and a2 serve the purpose of balancing. Therefore, auxiliary conditions are given:

The firing angles α1 and α2 should satisfy the above equation (1) and control the reactive power and arc current at the same time. The variable p is the sum of the output of the current regulator and the corresponding feedforward amount; the variable q is the sum of the output of the reactive power regulator and the corresponding feedforward amount. The basic block diagram of the new rectifier power supply control is shown in Figure 8. The control system mainly consists of current regulator, reactive power regulator, feedforward link, power couple (p, q) link, etc. The control system is based on the given current of the system and the actual operating current of the DC arc furnace, the given (budgeted) reactive power of the rectifier power system to the power grid and the actual operating reactive power, and the two feedforwards of the control system. In the compensation link, these four factors are combined to decouple the relationship between p and q (1), and timely send out digital pulses to control the conduction of the thyristor, that is, adjust the firing angles α1 and α2 of the upper and lower arms of the thyristor to control The output of the rectifier device ensures that the output load current is always stable at the given current and the reactive power of the entire rectified power supply system is stabilized at the given (budgeted) reactive power. Assume that when the output load current increases (decreases) due to fluctuations in the power grid or changes in operating conditions in the furnace, the current negative feedback link and the feedforward compensation link in the link channel will collectively reduce (increase) the control signal, thereby controlling the trigger. The increase (decrease) of angles α1 and α2 causes the output voltage of the rectifier device to decrease (increase), and the load current controlled to output decreases (increases) until it stabilizes at the given current. The same goes for the reactive power negative feedback link of the new rectified power supply system and the feedforward compensation link in the link channel. By controlling the size of the trigger angles α1 and α2, the reactive power negative feedback link and the feedforward compensation link in the link channel are controlled.

The active power is stabilized at a given (budgeted) reactive power. The new rectified power supply system can adjust the arc current and reactive power independently by adjusting the firing angle α1 and α2.

The rectified power supply system also has a short-circuit blocking link. In the operation of DC electric arc furnace, material collapse is a normal working condition that frequently occurs during the melting period. This working condition is a DC side short circuit for the rectifier device. If only the constant current control loop is adjusted, overcurrent output in the deep control state will occur. If it lasts for a long time, the rectifier device may be damaged. Therefore, it is necessary to set up a short-circuit blocking ring. The control system judges the sampled input arc current and arc voltage signals, and selects the arc current and arc voltage based on the output characteristics and component characteristics of the rectifier device, as well as the state during short-circuit operation, combined with the conditions of the electrode position regulator and the furnace itself. And the set values of action delay and signal holding time, and adjust the set values according to the actual operating conditions (during the simulation process, set the arc voltage to 40V, the arc current to 150kA, and the delay to 20ms). When the arc current is greater than the set value and the arc voltage is less than the set value, the loop will judge that the operation is in a short circuit state and delay by one click. If the short-circuit state is eliminated within the delay setting value, the ring will not act, and the short-circuit identification signal will be released and the original state will be restored; otherwise, if the short-circuit state lasts longer than the delay setting value, the ring will send out a pulse blocking signal. , increase the trigger angle α1 and α2 to 150°, and at the same time give a signal to the electrode regulator to increase the level at full speed. After holding for one press time (this time is enough to make the electrode out of the short-circuit state), the trigger pulse automatically moves forward to the time of arc ignition. phase.