Abstract: Through the comparison of different intelligent low-voltage reactive power compensation devices currently in use. And a preliminary analysis of some of the problems in practical applications, and proposed the selection of reactive power compensation schemes and equipment selection recommendations.
In the distribution network, low-voltage reactive power compensation is of great significance to improve power quality and reduce line losses. Has been widely used. However, there are many manufacturers of reactive power compensation devices, and the user situation is very different. The actual operating effects of different compensation devices are different, which brings great difficulties to the management of reactive power compensation. Combined with the problems in practical applications, a simple analysis and summary of the three major switching switches, capacitors, controllers, and design schemes of the low-voltage reactive power compensation device are carried out, and some suggestions are put forward for everyone to discuss together.
1 Switching switch
The switching switch mainly includes three methods: contactor, bidirectional thyristor and compound switch.
Contactors are the longest used and most widely used method. Compared with other methods, its biggest advantage is that it is cheap, so it is the most widely used. However, when the contactor switches the capacitor, it is easy to generate a large inrush current, burn the contact of the contactor, and cause damage to the capacitor.
This method of contactor is suitable for reactive power compensation with relatively stable load and low frequency of power factor changes, and has good effects.
1.2 Non-contact switch composed of bidirectional thyristor
In this way, the capacitor bank is switched on and off by detecting the zero-crossing point of the voltage and controlling the on and off of the positive and negative thyristors in the upper and lower half waves. Since the input is at the zero-crossing point, the impact of closing inrush current is avoided, and the impact of overvoltage on the capacitor is avoided.
But a big problem with this method is heat dissipation. The thyristor is theoretically equivalent to a short circuit after it is turned on, and the rated current needs to be passed for a long time. In fact, the thyristor has a turn-on voltage drop, which is generally about 1.5 V. Take a compensation cabinet with a total compensation of 160 kvar as an example. The capacitors are divided into 8 groups. For every 20 kvar, the current flowing in phase A is 52.6 A, the power consumption of each thyristor is P=1.552.6=78.9 w, and the two thyristors are nearly 160 w. It is equivalent to lighting two 80W bulbs. The case temperature of the thyristor in normal operation does not exceed 80°C. If the thyristor is overheated for a long time, the performance of the thyristor may decrease or even burn out. Therefore, a radiator, fan and temperature control relay must be added. However, when the outdoor temperature is high in summer, the distribution room is not ventilated, and there are heat sources such as transformers, the ambient temperature of the device may reach above 50°C, and it is difficult to ensure that the thyristor temperature rise is less than 80°C even if the heat is dissipated. After adding the radiator, fan and temperature control circuit, the installation space in the compensation cabinet is larger, the assembly is very troublesome, and the reliability is also reduced. Moreover, harmonic currents are generated when the thyristor is switched on and off, which will be amplified after the system resonates, causing damage to the thyristor.
Therefore, for reactive power compensation cabinets similar to outdoor public transformers or special transformers, and power distribution rooms with poor heat dissipation, try not to install such non-contact switches composed of bidirectional thyristors.
1.3 Compound switch
This is a new type of capacitor switching switch that combines the advantages of contactors and bidirectional thyristors. Connect the contactor and the triac in parallel. Its working principle is to use the fast characteristics of the thyristor as a switching switch, and the contactor as a switch when the current continues to pass. When the capacitor bank needs to be put in, the thyristor is first put in when the voltage crosses zero to prevent the generation of closing inrush current, and then the contactor is put in to make it work in parallel with the thyristor; when the system is stable, the thyristor will be withdrawn. The contactor bears the operating current; when it needs to exit the capacitor bank, first turn on the thyristor to make it work in parallel with the AC contactor; then disconnect the AC contactor and exit the work, the capacitor is connected to the grid within a short time It is independently undertaken by the thyristor; finally, the trigger signal of the quality thyristor is cut off, so that the thyristor is naturally turned off when the current crosses zero.
The compensation device of non-contact switch and composite switch composed of bidirectional thyristor adopts zero-crossing switching, so the probability of capacitor damage of the compensation device using these two methods is small. For occasions with heavier harmonics, to prevent resonance, if you want to use a composite switch, you should consider filtering first.
In order to ensure the safety of the thyristor, the reverse peak voltage and on-state current of the VRSM selected by the thyristor are at least 2.5 times or even higher than the rated voltage and current of the system it bears. Of course, it is also subject to cost constraints. The price of non-contact switches and composite switches composed of bidirectional thyristors is usually 3-4.5 times that of contactors. If the parameter is selected again, the cost will increase sharply.
2.1 Structural characteristics of self-healing shunt capacitors
Most of the capacitors currently used are self-healing shunt capacitors. This type of capacitor is made by winding metalized polypropylene film. This material enables the capacitor to melt and evaporate the surrounding metal layer by the heat generated by the short circuit after the capacitor is short-circuited by breakdown, so that the insulation characteristics can be quickly restored and the capacitor can continue to be used. It has self-healing function.
In practical applications, structural capacitors with three-phase angular connection and integrated packaging are more common. The three-phase angular connection facilitates the automatic filtering of the third harmonic, and the integrated package facilitates installation. A small resistor is usually connected in parallel at both ends of this type of capacitor as a discharge loop for the capacitor. It is required that the voltage across the capacitor is less than 65 V after the power supply is disconnected. In addition, pressure insurance can be installed to prevent explosion.
But in fact, there are still more damages to the capacitors, even causing the accident to expand. The main reason lies in the overvoltage, overcurrent and temperature generated when the capacitor is switched. The cause of overvoltage and overcurrent is the switching time and frequent actions of the capacitor.
2.2 Capacitor service life
For reactive power compensation devices that use contactors as switching switches, it is not advisable to switch capacitors frequently. Otherwise, the overvoltage and inrush current generated during switching will cause harm to the capacitor and shorten the service life.
It is generally believed that a 10% increase in voltage will reduce the life of self-healing metallized shunt capacitors by half.
3 Intelligent reactive power controller
The function of the intelligent reactive power controller is to collect the relevant voltage and current, according to a certain control strategy, to achieve the control goal by switching the capacitor bank. The usual control targets are: power factor, voltage, reactive power or reactive current. The control target of most manufacturers' products can be set by a fixed value, or the power factor and voltage can be set at the same time.
The controller should try to meet the requirements of the recommended standards such as DL/T597—1996 ((Technical Conditions for Ordering Low-Voltage Reactive Power Compensation Controllers), JB/T9663 "Low-Voltage Reactive Power Automatic Compensation Controllers" and other recommended standards. In practical applications, The following functions should be focused on filtering according to the user's situation.
3.1 Overvoltage and overcurrent protection functions
The more the limit value of voltage and current can be set. The capacitor can be disconnected when the voltage or current exceeds the limit. This can prevent the capacitor from being impacted and prematurely damaged. If it has a harmonic over-standard protection function, the influence of resonance can be avoided and the capacitor will also be well protected. In addition, the setting of the switching time should be able to meet the time requirements for the complete discharge of the capacitor.
Of course, if the loss of component damage is less than the loss of insufficient reactive power compensation, it is another matter.
3.2 Light load automatic recognition
When the load is very light, it is possible to overcompensate when a set of capacitors are put in, and undercompensate when withdrawing, forming oscillation switching. The controller should be able to solve this problem easily. Users can also change the capacitor capacity according to the situation. Set the target power factor to solve.
3.3 Current phase determination function
Since the current wiring of the current transformer will affect the device's judgment of the power direction, in order to facilitate wiring and equipment maintenance, the controller should be able to automatically identify the current direction.
3.4 Strong electromagnetic compatibility
Because it is in a typical industrial environment, the intelligent reactive power controller should complete electromagnetic compatibility type test items such as electrostatic interference, fast transient pulse group anti-interference, surge, etc. The test level should be level 3. In the centralized bidding of reactive power compensation cabinets, this requirement should be clearly put forward for the controller.
For controllers directly exposed to strong electromagnetic interference environments such as large-capacity transformers, if there are problems such as display screens, unreasonable resetting, etc., there are reasons to suspect electromagnetic interference. Consider replacing the controller, adding shielding or putting a filter in the power circuit of the device.
4 Scheme design
In the design of low-voltage reactive power compensation scheme, the user's load situation should be carefully analyzed and studied, especially for users with special working conditions. The selection of equipment should follow the principles of reliable technology, good operating performance, and economical application. Two examples are given for illustration.
4.1 Reactive power compensation transformation in a textile factory
The transformer capacity is 315 kVA, and the compensation capacity is 8 groups of 25 kvar. The non-contact switch composed of bidirectional thyristor was originally adopted. The thyristor was found to be damaged after less than three months of use. Less than 2 months after the component was replaced, large-scale damage to the thyristor occurred again. The investigation found that the 5th harmonic current in the user load accounts for 12.6% of the total current. In severe cases, the capacitor will scream. The source of harmonics comes from the loom in the factory. It is suspected that harmonics have caused resonance to cause damage to the thyristor.
After the thyristor was sent to the manufacturer for inspection, it was also proved to be damaged by overcurrent. It is recommended to the user to install a filter device, but the user thinks that the one-time investment is too large and is not accepted, and would rather accept a monthly fine and component replacement. Later, the reactive power compensation cabinet was transformed again, and the non-contact switch composed of bidirectional thyristor was changed to a contactor. After the transformation, the contactor contacts were burnt out and the capacitors were damaged, but the frequency of damage was lower than that of the thyristor.
4.2 Electricity transformation for the relocation of a motor production plant
The original transformer capacity is 315 kVA, the compensation capacity is 15 kvar fixed compensation, 8 groups of 25 kvar cycle switching, using contactor switching. After the transformer capacity was changed to 500 kVA, the reactive power compensation remained unchanged, and the meter was changed to a three-phase three-wire electronic multifunctional electric energy meter. It turns out that the reactive power is sent backward, and the power factor is only 0.36. After investigation, it is found that there is only office load left in the factory and no other domestic electricity consumption of 15 kvar.
The fixed compensation capacity is too large, because the old meter is non-reversal, even if there is reactive power backward transmission, it cannot be found. The new table is very clear. It is normal after changing the fixed compensation capacity to 5 kvar.
In reactive power compensation, in order to complete the compensation economically and reliably, the scheme design plays an important role, but it is necessary to conduct a detailed analysis of the actual situation of the user and select the appropriate device for the specific working conditions. After the use method is determined, the device manufacturer can formulate a technical plan with clear parameters, and after a comprehensive comparison of similar products, it should be able to achieve a better compensation effect and achieve the largest comprehensive economic benefit.
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