中文版
Current situation in the development of the reactive power low voltage capacitor compensation cabinet
- Categories:Industry News
- Author:
- Origin:
- Time of issue:2021-05-20
- Views:
(Summary description)I. The categories of reactive power capacitor compensation cabinets based on the capacitor connection mode: 1. The reactive power capacitor compensation cabinet type in which the three phases of the c
Current situation in the development of the reactive power low voltage capacitor compensation cabinet
(Summary description)I. The categories of reactive power capacitor compensation cabinets based on the capacitor connection mode:
1. The reactive power capacitor compensation cabinet type in which the three phases of the c
- Categories:Industry News
- Author:
- Origin:
- Time of issue:2021-05-20 14:51
- Views:
Information
I. The categories of reactive power capacitor compensation cabinets based on the capacitor connection mode:
1. The reactive power capacitor compensation cabinet type in which the three phases of the capacitor are switched on or switched off at the same time. In this kind of reactive power capacitor compensation cabinet, the three-phase power capacitor is used to calculate and control the input of the capacitor by detecting the current of a certain phase to achieve the purpose of compensation. Because the reactive currents which are provided by the capacitor to the three phases are equal, this kind of reactive power capacitor compensation cabinet is only suitable for the load condition where the currents of the three phases are basically balanced. In the case where the currents of the three phase of the load current are not balanced, it will not be possible that the three phases can be compensated in the proper manner. There may be too much compensation in one phase while there may be insufficient compensation in another phase. This kind of reactive power capacitor compensation cabinet has the largest number of users because its structure is simple and its price is low.
2. The reactive power capacitor compensation cabinet type in which the single-phase capacitor is used and the individual phases are switched separately. The single-phase power capacitor is used in this type of reactive power capacitor compensation cabinet to calculate and control the input quantity of the capacitors in each phase by detecting the currents in the three phases. It is equivalent to 3 sets of reactive power single-phase capacitor compensation cabinet. This kind of compensation device can make the reactive current in all the phases to receive a good compensation, but it does not has any effect in case that the active current is not balanced. In the case that the reactive power capacitor compensation cabinet is used in the case where the currents in the three phases are not balanced, its effect is better than The reactive power capacitor compensation cabinet type in which the three phases of the capacitor are switched on or switched off at the same time. Because the structure of this type of reactive power capacitor compensation cabinet is complex and its price is high, there are only a small amount in use.
3. The reactive power capacitor compensation cabinet type which is intended for adjusting the unbalanced currents. In this kind of reactive power capacitor compensation cabinet, the single-phase power capacitor is used. The input mode and the quantity of capacitors in each phase are calculated and controlled in a comprehensive manner by detecting the currents in the three phases, so as to carry out compensation and adjustment of the unbalanced currents. Being different from the reactive power capacitor compensation cabinet type in which the single-phase capacitor is used and the individual phases are switched separately, this kind of reactive power capacitor compensation cabinet makes use of the principle that the capacitors crossing between the phases can transfer the active current between the phases. By connecting different number of capacitors between among the phases as well as between each phase and the zero line, not only the reactive current in each phase can be well compensated, but also the unbalanced active current in the three phases can be adjusted to the balanced state. This kind of reactive power capacitor compensation cabinet has unparalleled effect when it is used in the load condition where the currents in the three phases are not balanced.
This kind of reactive power capacitor compensation cabinet has a complicated structure and has a high price, in addition it uses a new technology, so there is only a small amount in use. However because this type of reactive power capacitor compensation cabinet will surely replace the reactive power capacitor compensation cabinet type in which the single-phase capacitor is used and the individual phases are switched separately.
II. The categories of reactive power capacitor compensation cabinets based on the capacitor activation control mode:
1. The reactive power capacitor compensation cabinet type in which the capacitor activation is controlled by the AC contactor. Because the capacitor is a device whose voltage is not transient, a large inrush current will be formed when the capacitor is put into operation. When the inrush current is at its maximum, it may be 100 times as large as the rated current of the capacitor. The inrush current will have an adverse interference to the power grid, and reduce the service life of the capacitors at the same time. In order to reduce the inrush current, most of the reactive power capacitor compensation cabinet now use the special contactors for switching on or switching off the capacitors. This contactor has a set of auxiliary contacts in which the series current limiting resistor are in parallel with the main contact. In the process that the contactor is being closed, the auxiliary contact is first turned on so that the capacitor can be precharged through connecting itself to the circuit by means of the current limiting resistors. Then the main contact is connected and connects the capacitor, which has already been connected, to the circuit in a normal manner. In this way, the inrush current can be limited to be within 20 times of the rated current of the capacitor.
This kind of reactive power capacitor compensation cabinet has the advantages of low price and high reliability, and its applications are the most wide. Because the service life of the contact in the AC contactor is limited, it is not suitable for frequent switching on and switching off. So this kind of reactive power capacitor compensation cabinet is not suitable for the load conditions where frequent variations are present.
2, The reactive power capacitor compensation cabinet type in which the capacitor activation is controlled by the thyristors. This kind of reactive power capacitor compensation cabinet is the TSC sub category in SVC category. Because the thyristor tends to be damaged easily by the inrush current, the thyristor must be triggered at the time of zero crossing, that is, when the voltage at both ends of the thyristor is zero, the trigger signal is sent out. With the zero crossing trigger technology, it is possible to put the capacitor in operation with no inrush current. In addition, due to the fact that the times of the thyristor being triggered is unlimited, the quasi dynamic compensation (response time is in milliseconds) can be realized, so it is suitable for frequent switching on and switching off of the capacitors and is very suitable for the load conditions where frequent changes are present. The voltage drop in the thyristor when it is conducted is about 1V, and the loss is very large (using the reactive power capacitor compensation cabinet with a rated capacity of 100Kvar as an example, the rated current of each phase is about 145A, then the rated conduction loss of the thyristor is 145 × 1 × 3 = 435W). The cooling fins with a large area must be used and a fan also must be used. The thyristor is very sensitive to the change rate of voltage (dv/dt). It is easy to mislead and be damaged by the inrush current in case of abrupt changes in the voltage, such as the operating overvoltage and lightning strikes and so on, where the thyristor may be damaged because it has been conducted accidentally. Even in the occasion where a lightning arrester is installed, this situation is not improved. The reason for this is that a lightning arrester can only limit the peak value of voltage and can not reduce the change rate of voltage.
The structure of this kind of reactive power capacitor compensation cabinet is complex, and the price is high, the reliability is poor, the loss is large, except for the occasion where frequent changes in the load are present, it has little utilization value in other occasions.
3. The reactive power capacitor compensation cabinet type in which the capacitor activation is controlled by the compound switch. The compound switch technology is to use the thyristor in parallel with the relay contact. The capacitor is put into operation by the thyristor at the time the voltage crosses the zero and the capacitor is put out of operation when the current crosses the zero; the continuous current passes through the relay contact. Thus the conduction loss of the thyristor is avoided and the inrush current when the capacitor is put into operation is also avoided. However, in the compound switch technology, both the thyristor and the relay are used, as the result, the structure becomes quite complicated. In addition, the sensitivity of the thyristor to dv/dt is easy to be damaged.
4, The reactive power capacitor compensation cabinet type in which the capacitor activation is controlled by the synchronous switch. The synchronous switch technology is the technology developed in recent years. As the name implies, the contact of the mechanical switch can be closed or disconnected accurately at the time when needed. For the synchronous switch which controls the capacitor, it is necessary to close the voltage at both ends of the switch contact when the voltage is zero, so as to put the capacitor in operation with no inrush current as well as put it out of operation at the time when the current is zero. In this way, the switch contact can be disconnected with no arc.
Thyristor is refused in the synchronous switch technology, so it is still not suitable for frequent switching on and switching off. It can be predicted that the synchronous switch using the magnetic holding relay will replace the compound switch and the AC contactor.
III. The power capacitors used in the reactive power capacitor compensation cabinet
At present, the low voltage power capacitors used in the reactive power capacitor compensation cabinet are the metallized capacitors. The metallized capacitor is widely used because it is small in volume, its price is low and it is self healing.
The polar plate of the metallized capacitor is a vacuum evaporated aluminum film, the thickness of which is in the order of nanometer. Due to the aluminum film is very thin, in the event that the dielectric film has been partially broken down due to any weak point, the weak point itself and the nearby aluminum film will evaporate away. So there will be no short circuit fault and this is so called self healing effect.
The electrode extraction process of the metallized capacitor is to spray metallic conductive layer at both ends of the element after the coiling of the core element is completed. Then the lead wire is to be welded onto the conductive layer. Because the current of the polar plate should flow from the middle part of the element to both ends, and the aluminum film of the plate is very thin. and the resistance loss is large. Therefor it is hoped that, with the prerequisite that the resistance loss is to be reduced as much as possible, the core element will be coiled into a short and thick shape as much as possible. On the other hand, because the such a thin aluminum film polar plate does not have much mechanical strength, there is no strong connection between the end conductive layer and the polar plate of the core element. When the core element has any inhomogeneous deformation due to heating, certain part between the end conductive layer and the polar plate is easy to be peeled off and a local fault may occur. From this point of view, it is hoped that the core element will be rolled into a thin and long shape as far as possible.
The metallized power capacitor has two kinds of structures, i.e the rectangular structure and the cylindrical structure. The core elements in the capacitor with the rectangular structure is arranged side by side, which is suitable for occasion of ordinary applications. The core elements of the capacitor with the cylindrical structure are arranged in short series, which is suitable for occasions where serious harmonics are present.
The main problem in the operation of the metallized capacitor is the decrease in the capacitance. With the extension of the operation time, the capacitance of all the metallized capacitors will decrease due to the self healing process. However their extents of decrease are not the same. Some capacitors of a poor quality will also have the fault that their end conductive layer is separated from the polar plate, which shows that the capacitance is reduced to half of the rated value, one third of the rated value, or even even zero. For a certain brand, the larger the capacity of a single capacitor is, the longer the core element is and the thicker the diameter is. If the element length is longer, this will lead to the increase of the resistance loss. If the area of the conductive layer on the end face is larger and if the inner and the difference between the inner temperature and the outer temperature of the element is increased, it will be easier for the conducting layer to peel off the polar plate. Therefore, the reliability in using a single capacitor with a large capacity in parallel with a capacitor with a small capacity is not as high as the reliability when using the capacitors with a small capacity in parallel. There are fewer short circuit faults or explosion faults with the metallized capacitors.
IV. The controller used in the reactive power capacitor compensation cabinet
The earliest controller of the reactive power capacitor compensation cabinet performs the controlling on the basis of the power factor. This kinds of controllers are still in use because of their low prices. The biggest problem exists with performing the control on the basis of the power factor is the oscillation under a light load. For example, the capacity of the smallest capacitor in a reactive power capacitor compensation cabinet is 10 Kvar. the inductive reactive power of the load is 5Kvar and the power factor is 0.5 lag. At this time, when a capacitor is put in operation, the power factor will become 0.5 lead. The power factor will become 0.5 lag when the capacitor is put out of operation, so the process of oscillation will continue endlessly.
The new type controllers of the reactive power capacitor compensation cabinet perform the controlling on the basis of the reactive power. This requires that they must have the setting function, and the capacitor capacity in the reactive power capacitor compensation cabinet can be set, so that method for putting the capacitors in the capacitor into operation can be determined according to the reactive power of the load. As the result, the phenomenon of the oscillation under a light load can be eliminated.
With the continuous progress of the technology, there are more and more additional functions available on the controller of the reactive power capacitor compensation cabinet, such as the data storage, the data communication, the harmonic detection, the electricity detection and so on. The control elements used also range from the small-scale integrated circuit in the initial period to the 8-bit single-chip microcomputer, then to the 16-bit single-chip microcomputer, and then to the 16-bit DSP, until the most advanced 32-bit single-chip microcomputer. Now the price of a single 32-bit single-chip microcomputer has dropped to 30 yuan, so the cost of the hardware of the controller has not had much impact. Its performance has surpassed the 8 bits of single-chip microcomputer by more than 100 times. The main reason for the difficulty in popularizing it is that the technology development is too difficult.
V. Combining the reactive power capacitor compensation cabinet with other equipment
With the popularization of the reactive power capacitor compensation cabinet, the combination of the reactive power capacitor compensation cabinet with other equipment is an inevitable trend. For example, the combination of the reactive power capacitor compensation cabinet with a metering box, the combination of the reactive power capacitor compensation cabinet with a switch box, and so on. Through combining the reactive power capacitor compensation cabinet with other devices, the cost can be reduced, the space can be saved, the connection line may be reduced, andthe maintenance workload is reduced. There is no technical difficulty in the design and manufacture of the combined device. However, since no unified standards are available, the manufacturer can only organize the production according to the order.
Scan the QR code to read on your phone
Latest News
2726472126
Feidong Economic Development Zone LiuZi Road, Anhui Hefei，China
Copyright © 2020 Anhui Zonsen Electric Power Technology Co,Ltd. Powered by www.300.cn 皖ICP备18026616号