The distributed power supply is often connected to the distribution network in situ. The relay protection in the traditional low-voltage distribution network system is designed on the basis of single-side power supply and radiation type structures. Generally, only simple protection can be used to meet the requirements. Grid relay protection requirements. With the large number of distributed power supply DGs in the power grid, the power flow will change, and the short circuit current of each line or bus will also change when a short-circuit fault occurs in the system. There are many new problems in relay protection. Traditional simple protection has been Can not meet the requirements.
Instantaneous current quick-break protection, definite time current quick-break protection and over-current protection. Instantaneous current quick-break protection can instantaneously cut off the fault, but it cannot protect the full length of the line. When the setting value is short-circuited according to the end of the protected line, the maximum three-phase short-circuit current flowing through the protection should not be set by the protection action. The definite time current quick-break protection can protect the whole length of the line, and the setting value is determined according to the line terminal fault sensitivity and the instantaneous current protection of the adjacent line. The overcurrent protection setting is generally set according to the maximum load current of the line and the overcurrent protection of the adjacent line, and the full length of the adjacent line can be protected.
Inverse time overcurrent protection
Inverse time overcurrent protection is generally intended to remove faults near the power supply as quickly as possible. This is a protection-time limit related to the size of the short-circuit current in the protected circuit. The greater the short-circuit current, the shorter the protection time limit; the protection time limit is related to the fault's distance, and the farther the action time is, the longer it will be.
Since most of the faults in the distribution network are transient faults, a three-phase automatic reclosing device should be configured in the non-full-cable lines to ensure that the power supply can be quickly restored after a fault.
Power system relay protection devices should meet the four basic requirements, namely: selectivity, quickness, agility and reliability. The following sections discuss the different effects of DG on the two relay protection configurations and the automatic reclosing, as well as some effects on the quadratic protection relay.
The main performance of DG three-phase current protection
1 Causes non-faulty line sensitivity to increase, protection against misoperation, so that protection will lose selectivity, and will expand the scope of the accident. Before DG access, when other lines fail, the short-circuit current is only provided by the system; after accessing DG, the DG and the system will provide a short-circuit current to the fault point, which will increase the fault current of the faulty feeder and increase the sensitivity, which may cause serious Protection misbehaving. The greater the access capacity of the DG, the greater the impact on the sensitivity of the protection.
(2) The sensitivity of the protection of the line is reduced, and protection is refused when it is severe; the sensitivity of the downstream protection is improved, and the protection is misoperation. When the downstream line of the DG access point fails, the short-circuit current is only provided by the system before the DG accesses. After accessing the DG, the DG and the system simultaneously provide the short-circuit current, but the DG upstream protection can only sense the short-circuit current provided by the system. Moreover, this fault current is smaller than before the access to DG, which results in a decrease in the protection sensitivity. In severe cases, this may cause protection refusal. On the other hand, the protection current of the downstream DG is larger than that before the access to the DG, resulting in increased sensitivity, and even in case of seriousness, it may cause protection misoperation. And the greater the capacity of the accessed DG, the greater the impact on the protection sensitivity.
When the DG capacity of the access system is fixed, the impact on the relay protection is small when it is incorporated into the end of the line. At the same time, a protection directional element should be added upstream of the DG to ensure that the protection will not be mistaken when the DG upstream or system side fails. . When the position of the access DG is fixed, the capacity of the access should be ensured, and the fixed value of the downstream protection of the DG and the adjacent line protection setting are increased to meet the requirement of selectivity; the sensitivity of the upstream protection of the DG is checked; When the DG comes out of operation, the protection has sufficient sensitivity and protection range.
DG counter-time limit
Impact of overcurrent protection
After the DG is connected, the magnitude and direction of the short circuit current will change, which will bring the following effects to the inverse time overcurrent protection:
Downstream of 1DG, the short-circuit current in the upstream decreases, which increases the time limit for protection action, which is not conducive to the rapid removal of the fault.
Downstream 2DG faults, the downstream short-circuit current increases, the protection operation time is shortened, and the protection quickness is improved.
Effect of DG on Automatic Reclosing
In the case of grid faults, transient faults account for the largest number, and the installation of automatic reclosing can improve the reliability of the supply and reduce the number of line outages. The incorporation of DG will cause hidden failures such as failure of reclosing due to automatic reclosing and non-synchronized closing. For example, after the line trips due to a fault, the formed islands maintain power and voltage near the rated value. The distributed power source may not jump off the line during reclosing action. The fault current at the fault point will be maintained by the distributed power supply. , resulting in failed reclosing, non-synchronous closing, etc., and even caused damage to the grid equipment.
Some other solutions
In view of the influence of DG on three-stage overcurrent protection and automatic reclosing, it is proposed in the literature that the use of a series reactor to limit the short-circuit current can eliminate the coordination problem between the distributed power supply and the protection to a certain extent and ensure the correct operation of the automatic reclosing. Or according to the concept of wide-area protection, using the current amplitude comparison to reduce the fault to a smaller area, and use the phase relationship between the current to find the fault line, this method has a very good value in the project.
Distributed generation based on new energy has brought both opportunities and challenges to the traditional power system. DG can make full use of clean energy, promote China's sustainable development, and make up for the shortcomings of traditional power grids in environmental protection and reliability. The traditional relay protection and automatic reclosing device based on the design of single-ended power supply system must keep pace with the times and make corresponding adjustments to maintain the safe and stable operation of the system. Otherwise, the incorporation of distributed power supply will inevitably lead to protection rejection. Problems such as moving or misbehavior may seriously cause a large-scale blackout and damage the interests of the national economy.
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