1. Including photovoltaic power distribution network protection scheme
1.1 Protection Scheme 1
Feeder line 2 is divided into two areas according to the location of the photovoltaic power supply connection: the upstream area of the photovoltaic power supply. Consists of lines AB and BC; Photovoltaic power supply downstream area. Consists of line CD and plus. Install circuit breaker 5 and protection device 5 in upstream area. The downstream area needs to be equipped with circuit breaker 3 and protection device 3 in order to remove the fault faster in case of failure. In order to reduce the damage of short circuit to photovoltaic power supply, inverse time overcurrent protection is introduced. The protection overcomes the short circuit the closer the point is to the power supply. Disadvantages of longer protection action time. Setting principle: Under the maximum operation mode, when the next level line outlet is short circuited. The action time limit of the upper level of protection is a time ladder higher than that of the next level of protection to ensure that the protection action time limit under other operation modes can meet the selectivity requirements. . When a short circuit occurs at the point, the protection 4 is set to instantaneous action, and the action time limit of the protection 3 is one time ladder higher than that of the protection 4. When the output power of the photovoltaic power supply becomes smaller or exits the operation, protection 3 and protection 4 can be reliably matched. Point short circuit. The action duration of protection 2 is one time step higher than that of protection 3.
When the photovoltaic power supply is connected. Protection 2 and protection 3 meet selectivity requirements. The inverse time limit protection of protection 1 is matched with the time limit of protection 2 in the same way. When the system and the photovoltaic power supply exist at the same time, the power direction element is installed at the protection 1, 2, and 5. Ensure the selectivity of action between each protection. A three-phase short circuit occurs in the positive direction near the protection installation site. As the bus voltage is reduced to zero. The directional element loses its discrimination basis. As a result, the protection refuses to move, and the directional current protection has a “dead zone”. Therefore, the directional comparison longitudinal protection is configured at protection 1 and 5 in the upstream area of the photovoltaic power supply, which can quickly cut out various faults within the protection range. Consider the failure of the upstream area of the photovoltaic power supply. The output power of the photovoltaic power supply is small or it is out of operation. As a result, the sensitivity of directional elements at 5 points of directional comparison longitudinal protection is insufficient. Configure weak feed protection at protection 5. Set the reclosing function at protection 1 and 5. Acceleration protection before reclosing is adopted. Use the conditions provided by reclosing to accelerate fault removal. If the failure of the upstream area of the photovoltaic power supply is instantaneous. Power is restored after reclosing action: If the fault is permanent, the fault is selectively removed by overcurrent protection 1 or 2. Protection l configuration general reclosing. The reclosing function of protection 5 is activated by protection 1 when protection 1 is judged to be a transient fault. At this time, the photovoltaic power supply exists, and the protection 5 reclossing needs to be checked during the same period. For feeder 1. The current protection is configured according to the traditional pre-acceleration or post-acceleration mode of reclosing. Protection action behavior: When any point in the upstream area of the photovoltaic power supply fails. The directional elements on both sides of the directional comparison longitudinal protection are judged to be positive. An area fault is considered to have occurred. Reliable operation Disconnect circuit breakers at protection l and protection 5. Then protect 1 circuit breaker quickly coincidence, if the fault is instantaneous, the fault disappears after coincidence. Then protect 5 circuit breakers overlap. Restore power supply to the entire feeder. If the fault is permanent, the fault is removed by the selective action of the inverse time overcurrent protection at protection 1 and protection 2. The output power of the photovoltaic power supply becomes smaller or exits the operation, resulting in a reduction in the sensitivity of the directional element at the protection 5, because the protection 5 is equipped with weak feed protection. Reliable operation on the weak power side is guaranteed.
When a failure occurs in the downstream area of the photovoltaic power supply, the acceleration mode before reclosing is taken as an example. First, protect the electrical flow rate at 3 places to protect instantaneous action. Then it overlaps. If it is a transient fault, the fault will disappear after coincidence: if it is a permanent fault, the fault is removed by the selective action of the inverse time overcurrent protection at the protection 4 or the protection 3. The downstream inverse time protection of the photovoltaic power supply is coordinated according to the maximum output of the photovoltaic power supply, when the output power of the photovoltaic power supply becomes smaller or exits the operation. The photovoltaic power supply operates selectively after a failure occurs downstream. It is not affected by the change of output power of photovoltaic power supply. When the adjacent feeder l is faulty. The upstream area of the photovoltaic power supply is equipped with a direction element. Protection 1 and protection 2 do not misoperate. Protect 5 actions may form islands. After the distribution network with photovoltaic power supply is separated from the main distribution network. Continued transmission to the independent distribution network, the formation of islands to the system, user equipment, maintenance personnel, etc., the imbalance between supply and demand in the operation process damages the power quality. The power supply reliability of the distribution network is reduced. Distribution networks need to have a strategy in advance to deal with the emergence of islands. This can be divided into islands. To maintain power balance and voltage frequency stability in the island. Isolated island is a new operation mode of photovoltaic grid connection.
1.2 Protection Scheme 2
The direction element is added to the current protection at both ends of the upstream line of the photovoltaic power supply, and the selectivity requirements are met by means of the method of communication at both ends. The upstream line of the photovoltaic power supply and the power supply side are equipped with two-stage directional current protection, and the opposite side is not equipped with protection. The current of section I of directional current protection is set, and k is set according to the principle of short-circuit action at the end of the line under the minimum operating mode of the system. Action time ￡I-0s, i.e., b=Krel^. Stare at i. (12) Where, ‰ is 0.9; k glaze is the minimum short circuit current of the short circuit at the end of the line. Direction current protection section II current setting, drop in accordance with the principle of avoiding load current setting. The action time limit takes the step type. The principle is the same as that of the conventional current section III setting. As shown in Figure 11. Protection section I and adjacent feeder protection section I constitute a communication unit. Protection for section I of adjacent feeders Do not act Enable protection for section I of local feeders, and vice versa. Section I of the local feeder is blocked. If the local feeder section I protects and does not receive the lock signal, it is considered an internal fault. Hops the local circuit breaker and sends the remote control information to the peer end. Trip the opposite circuit breaker as well. Isolate the fault. The direction element must be installed at the end of the last line upstream of the photovoltaic power supply. The two sides of the line are protected to form a communication unit. When the outlet protection section I is operated and the end power direction is positive. Local line fault was found. Jump both circuit breakers. The downstream protection of photovoltaic power supply is adjusted according to the increase of three-stage current protection. Protection action Behavior: After the point failure, the protection l power direction is positive. Current I section action; Protection 2 The power direction is negative, and the direction current protection does not work. Do not send a blocking signal to protection 1; Protect 1 trip, while sending remote control information to control circuit breaker 2 trip. Isolate the fault. Chu, point failure. Protection 1 and protection 2 direction current I segment operation, protection 3 power direction is negative. Directional current protection does not operate: Protection 2 sends a blocking signal to protection 1. Protection 1 does not act: Protection 3 does not send a blocking signal to protection 2. Protection 2 is judged to be an internal fault. Disconnect the circuit breakers on both sides of said C. Isolate the fault. Similarly, in the rear, when the point is faulty, the current section I in the direction of protection 2 acts, and the power direction of protection 4 is positive. Line CD is found to be faulty. Disconnect both circuit breakers. Isolate the fault. When multiple photovoltaic power supplies are connected. The configuration and setting analysis methods of protection are similar. The two protection schemes proposed in this paper can effectively solve the protection problems mentioned in the literature.
2. Analysis of numerical examples
The parameters of 10kV distribution network are 5. =600MV·A, Zhang = 10.5kV; The parameters of the overhead line are n= 0.27Q/km, go1_O.347q/km, and the load with rated capacity of 5MV-a and rated power factor of 0.85 is connected at each node. The capacity of the photovoltaic power generation system is 6, 12MV·A. The power distribution system is shown in Figure 12. This system is simulated by PSASP. Simulation scheme 1: Without the photovoltaic power supply, the maximum load current of each protection on feeder line 2 and the corresponding overcurrent protection set value. In the system minimum operation mode. Short-circuit current flowing through each protection when a two-phase short-circuit occurs at the end of each section of feeder 2. When the end of each section of the line fails in the minimum PV operation mode. The short circuit current flowing through each protection of feeder 2 is shown. The photovoltaic power output changes. When a fault occurs at the end of the line ∞, the protection 3 and 4 can reliably remove the fault. When the end of the line BC fails, the photovoltaic power output is O. Directional longitudinal protection can operate reliably, and then protect the circuit breaker at 1 place to coincide. If it’s a permanent failure. Protection 2 will reliably trip. When there is A permanent fault at the end of line A. Protection 1 can operate reliably. When the starting end of the adjacent feeder fails and the photovoltaic power output is maximum. The short circuit current flowing through the protection 1 and 2 is 1.26kA and 1.212kA respectively, because the direction element is installed at the protection 1 and 2 respectively, they will not misoperate. Simulation scheme 2: Take the impedance X of the system under the maximum operation mode. Blooood. = 0.091Q, take the impedance X under the minimum operating mode. One = 0.126Q; The end load impedance is 30+ i15.7n, and the capacity of the photovoltaic power supply is 6MV.A. According to formula (12), the setting value of section I of protection l and protection 2 is as follows:, b, = 5.44kA, k= 2.905kA. In the maximum operating mode of the system, when the three-phase short circuit occurs at the end of each line, the short circuit current flowing through feeder line 2 is as follows: Short circuit occurs at the end of line AB. The fault current is 7.002 kA. Short circuit at the end of line port C, fault current is 3.154 kA: short circuit at the end of line ∞, fault current is 1.215 kA: short circuit at the end of line DE, fault current is 0.834 kA. After the end of line AB is faulty, the direction current segment I of protection 1 acts, while the direction current segment I of protection 2 does not. No blocking signal is sent to protection 1. Then protect 1 trip. At the same time, send remote control information to control circuit breaker 2 trip and isolate the fault. Through the example analysis. After adopting the above protection scheme. For the distribution network with photovoltaic power supply. Both schemes can operate correctly.
3. Conclusions
In this paper, the two proposed protection schemes are verified by simulation using PSASP. The results show that the protection has good selectivity. For the time being. Qinghai Province is actively promoting the development of solar energy and the industry. The future distribution network will be a network containing a large number of photovoltaic power supplies, and the feeder protection scheme of the distribution network containing photovoltaic power supplies will be more and more widely used.
