Island protection method based on switching of fast action and slow action

Abstract

The invention provides a perfect scheme aiming at an existing traditional island protection problem, and aims to solve the problems of low voltage / frequency protection action speed when a power distribution network fails and voltage ride-through of a distributed power supply under a quick action setting scheme. According to the scheme, international experience is used for reference, a low-speed action setting scheme is selected, and voltage fault component detection is introduced for judging the island operation state. By detecting voltage fault components (such as zero sequence voltage and phase voltage break variable), switching to a rapid action fixed value group is realized, splitting of the distributed power supply is accelerated, and stable operation of the system is ensured. The invention provides a new islanding protection scheme, the scheme has the advantages of a low-speed action setting scheme and a high-speed action setting scheme at the same time, the requirement of islanding protection for the splitting speed and the requirement of voltage ride-through are met, the contradiction between the splitting speed and the voltage ride-through in traditional islanding protection is solved, operation is simple and easy to implement, and the islanding protection method is suitable for large-scale popularization and application. Rapid and accurate action of anti-islanding protection is facilitated, and the safe and stable operation capability of a power grid is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of anti-islanding protection of distribution networks, and in particular to an improved islanding protection scheme for solving the problems of existing protection schemes. Background Art

[0002] In order to ensure the safe and stable operation of the power grid, the photovoltaic grid-connected system must have both island protection and low voltage ride-through and high voltage ride-through capabilities. Islanding refers to the situation where the energy storage station continues to supply power to the surrounding area due to a fault on the main power grid side, becoming a self-sufficient power supply island that cannot be controlled by the main power grid. Islanding protection may bring the following hazards: 1) When the islanding effect occurs, the power grid cannot control the voltage and frequency in the island. The power quality in the island cannot be guaranteed. If the voltage and frequency of electricity exceed the allowable range, it may cause damage to the user's equipment. 2) Since the operation of the island is uncontrollable for the public power grid, the line is still energized when the island occurs, which will threaten the personal safety of maintenance personnel and reduce the safety of the power grid. 3) If the load capacity is greater than the total capacity of the VP system, the PV system will be overloaded and easily burned. 4) The reset and automatic reclosing of the circuit breaker will cause the main system and the island to lose step. 6) When the power grid is restored, there may be a large difference in phase between the grid voltage and the output voltage of the grid-connected inverter, resulting in a phase asynchrony on both sides of the switch, which will generate a large impact current at this moment, which may damage the photovoltaic power generation system and other equipment, causing a large closing impact current; it may cause the line to trip again. 7) Affect the automatic device, protection action and reclosing of the power grid; 8) If it is a single-phase photovoltaic power generation system, island operation will also cause unbalanced power supply of three phases; 9) If the load capacity is greater than the capacity of the photovoltaic power generation system, the photovoltaic power generation system will be overloaded and easily burned; 10) After being separated from the main power grid, the power in the unplanned island is unbalanced. If it runs for a long time, it will inevitably lead to serious deviations in voltage and frequency in the island system, causing serious damage to the energy storage system and load electrical equipment. Therefore, configuring anti-islanding protection to minimize the harm of the islanding effect and to disconnect the energy storage system from the grid side in the case of unplanned islanding has very important theoretical and engineering significance.

[0003] Low voltage ride-through means that when the grid voltage drops for a short time, the photovoltaic power station should remain connected to the grid for a period of time to maintain the stability of voltage and frequency. High voltage ride-through means that when the grid voltage of the photovoltaic power station suddenly rises due to power system accidents or disturbances, the photovoltaic power station can ensure continuous operation without disconnecting from the grid within a certain voltage surge range and time interval.

[0004] Voltage / frequency protection is a widely used anti-islanding protection at home and abroad. Its setting scheme can be divided into two categories: fast action and slow action. China currently adopts the fast action setting scheme. The voltage protection setting requirements given in GB / T 33593-2017 "Technical Requirements for Distributed Generation Grid Connection" refer to the formula in step S22. DER connected to the grid at 380V voltage level or connected to the user side at 10(6)kV voltage level will be disconnected from the grid within 0.2s when the frequency exceeds 49.5~50.2Hz.

[0005] However, the fast action setting scheme is not conducive to DER to achieve voltage ride-through when the distribution network fails. China's national standard GB / T19964-2012 "Technical Regulations for Photovoltaic Power Stations Connected to the Power System" and the State Grid Corporation's enterprise standard Q / GDW1617-2015 "Technical Regulations for Photovoltaic Power Stations Connected to the Grid" both require large and medium-sized photovoltaic power stations to have low voltage ride-through capabilities. The regulations clearly require that photovoltaic power stations connected to the public power grid through a voltage level of 10kV and above should have low voltage ride-through capabilities, that is: when the voltage at the photovoltaic power station's grid connection point drops to 0, the photovoltaic power station should be able to operate continuously for 0.15s without disconnecting from the grid; when the voltage at the photovoltaic power station's grid connection point drops to 20% of the rated voltage, it can ensure that it can operate continuously for 0.625s without disconnecting from the grid. Obviously, the traditional voltage protection setting scheme cannot meet this requirement. In addition, when the system experiences voltage / frequency disturbances, the duration may be as long as several minutes, and the fast action setting scheme may cause DER to disconnect from the grid on a large scale during the disturbance. With the increase in the proportion of DER access, the system stability problem caused by its large-scale disconnection from the grid is becoming increasingly prominent.

[0006] In order to meet the requirements of voltage ride-through, a slow action setting scheme is proposed. The more representative one is the scheme given by IEEE1547—2018 IEEE Standard for Interconnection and Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interfaces, in which the voltage protection action time limit is up to 1000s; the frequency protection action time limit is not less than 300s except for the instantaneous action when it is lower than 57Hz or higher than 62Hz. The use of a slow action setting scheme will obviously slow down the protection action speed during distribution network faults and island operation, and the main problem is that the waiting time for reclosing is prolonged during faults.

[0007] References

[0008] [1] China National Standardization Administration. Technical Specification for Grid-connected Relay Protection of Distributed Generation: GB / T 33982—2017

[0009] [2] China National Standardization Administration. Technical regulations for photovoltaic power station access to power system: GB / T 19964-2012

[0010] [S]. Beijing: China Standards Press, 2012.

[0011] [3] State Grid Corporation of China. Q / GDW 1617-2015 Technical Regulations for Photovoltaic Power Station Access to the Grid[S]. Beijing: State Grid Corporation of China.

[0012] Science and Technology Department of Power Grid Corporation, 2016.

[0013] [4] National Energy Administration. Photovoltaic grid-connected inverter technical specifications: NB / T 32004-2018[S]. Beijing: 2018.

[0014] [5]IEEE Standard for Interconnection and Interoperability ofDistributed Energy Resources withAssociated Electric Power SystemsInterfaces: IEEE Std 1547—2018 Summary of the invention

[0015] To solve the above problems, the present invention provides an island protection method based on switching between fast action and slow action. The method comprises the following steps:

[0016] S1: The control device detects the zero-sequence voltage / phase voltage mutation, voltage, and frequency information of the grid connection point in real time;

[0017] S2: Determine whether the zero-sequence voltage at the grid connection point reaches the protection threshold. If it is within the allowable range, execute S3; if it exceeds the allowable range, the island protection quickly operates to disconnect the DER according to the voltage and frequency changes;

[0018] S3: Compare the voltage amplitude with the nominal voltage value to determine whether the voltage ride-through condition is met. If the voltage amplitude is within the normal range, the PV power station continues to operate normally. If the voltage is continuously less than the voltage ride-through threshold, step S4 is executed to slowly decouple the DER.

[0019] S4: Entering the voltage ride-through operation state, after the ride-through time limit, the PV power station is slowly de-energized.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] The present invention proposes a perfect solution for the existing traditional island protection problem. By selecting a slow action setting scheme, the voltage ride-through requirement is met, and voltage fault component detection is introduced to judge the island operation state, and the voltage fault component is detected in real time, so as to achieve rapid switching to the fast action setting group and accelerate the decoupling of distributed power sources. This solution can ensure the stable operation of the system and solve the contradiction between the voltage ride-through requirement and the island fast action time limit in the traditional scheme. This invention has positive significance for the progress and development of distribution network anti-island protection technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a flow chart of an island protection method based on switching between fast action and slow action in the implementation of the present invention; DETAILED DESCRIPTION

[0023] The present invention provides an island protection method based on fast action and slow action switching, which can solve the problems of existing traditional island protection, ensure that the distribution network can operate stably when a fault occurs, and realize faster distributed power supply decoupling, thereby improving the safety and reliability of the distribution network. The scheme has important practical application value and promotion significance for the development of distribution network anti-island protection technology.

[0024] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below through specific implementations and in conjunction with the accompanying drawings.

[0025] Embodiment 1

[0026] like Figure 1 As shown, an island protection method based on switching between fast action and slow action provided by an embodiment of the present invention includes the following steps:

[0027] S1: The control device detects the zero-sequence voltage / phase voltage mutation, voltage, and frequency information of the grid connection point in real time;

[0028] S2: Determine whether the zero-sequence voltage at the grid connection point reaches the protection threshold. If it is within the allowable range (the component is normal), execute step S3; if it exceeds the allowable range, the island protection quickly operates to disconnect the DER according to the voltage and frequency changes;

[0029] S3: Compare the voltage amplitude with the nominal voltage value to determine whether the voltage ride-through condition is met. If the voltage amplitude is within the normal range, the PV power station continues to operate normally. If the voltage is continuously less than the voltage ride-through threshold, step S4 is executed to slowly decouple the DER.

[0030] S4: Entering the voltage ride-through operation state, after the maximum ride-through time limit, if the voltage reaches the normal value, normal operation will be restored, otherwise the PV power station will be slowly decoupled.

[0031] In one embodiment, the above step S2: determining whether the zero-sequence voltage U0 of the grid connection point reaches the protection threshold U 0PCC If it is within the allowable range, step S3 is executed; if it is beyond the allowable range, the island protection switches to the fast action setting scheme. According to the voltage and frequency changes, the island protection quickly acts to disconnect the DER, which specifically includes:

[0032] Step S21: The real-time detected frequency f PCC Compared with the normal frequency f0. If f PCC If f is within the normal range, then execute step S22; PCC If the frequency exceeds the allowable range (48Hz~50.5Hz) specified in the grid connection standards (references [1-5]) and lasts for more than 0.1s, the island protection will be activated immediately.

[0033] Step S22: The real-time detected voltage effective value U PCC With nominal voltage U N According to the voltage allowable range specified in the grid connection criteria (references [1-5]), the photovoltaic power station is disconnected within the corresponding setting time limit.

[0034] Step S3: Compare the grid connection point voltage amplitude U PCC With the nominal voltage value U N , determine whether the voltage ride-through condition is met. If the voltage amplitude U PCC Within the normal range, the PV power station continues to operate normally; if the voltage persists beyond the voltage crossing threshold, step S4 is executed to slowly decouple the DER;

[0035] Step S4: The system enters the voltage ride-through operation state, and the photovoltaic power station is slowly decoupled.

[0036] Furthermore, in step S21, the allowable range of frequency is 48 Hz to 50.5 Hz.

[0037] Furthermore, in step S22, the normal voltage value range is 0.85 to 1.1 per unit. N , the maximum opening time does not exceed 0.2s; if 50% U N ≤U<85%U N , the maximum opening time does not exceed 2s; if 85% U N ≤U<110%U N , the power grid operates continuously; if U>110%U N , the maximum opening time does not exceed 0.2s. N is the rated voltage of the grid-connected point, and the maximum trip time is the time from the occurrence of the abnormal state to the power supply stopping supplying power to the grid.

[0038] Furthermore, in step S3, the low voltage ride-through condition is:

[0039]

[0040] Among them, T d is the time value, U PCC is the instantaneous value of the grid connection point voltage.

[0041] Furthermore, in step S3, the high voltage ride-through condition is:

[0042]

[0043] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

[0044] The above embodiments are only used to illustrate the purpose of the present invention, rather than to limit its scope. It should be noted that various equivalent substitutions and improvements made without departing from the principles of the invention should be included in the protection scope of the present invention.

Claims

1. An island protection method based on switching between fast action and slow action, characterized in that: include: S1: The control device detects the zero-sequence voltage / phase voltage mutation, voltage, and frequency information of the grid connection point in real time; S2: Determine whether the zero-sequence voltage at the grid connection point reaches the protection threshold. If it is within the allowable range (the component is normal), execute step S3; if it exceeds the allowable range, the island protection quickly operates to disconnect the DER according to the voltage and frequency changes; S3: Compare the voltage amplitude with the nominal voltage value. If the voltage amplitude is within the normal range, the PV power station continues to operate normally. If the voltage is continuously less than or greater than the voltage crossing threshold, execute step S4 to slowly decouple the DER. S4: Entering the voltage ride-through operation state, the PV power station is slowly decoupled.

2. The island protection method based on switching between fast action and slow action according to claim 1 is characterized in that: In step S2, it is determined whether the zero-sequence voltage U0 of the grid connection point reaches the protection threshold U 0PCC , if it is within the allowed range, execute step S3; If it exceeds the allowable range, the island protection switches to the fast action setting scheme. According to the voltage and frequency changes, the island protection quickly acts to disconnect the DER, including: Step S21: The real-time detected frequency f PCC Compared with the normal frequency f0. PCC If f is within the normal range, then execute step S22; PCC If the frequency exceeds the allowable range (48Hz~50.5Hz) specified in the grid connection standards (references [1-5]) and lasts for more than 0.1s, the island protection will be activated immediately. Step S22: The real-time detected grid connection point voltage value U PCC With nominal voltage U N According to the voltage allowable range specified in the grid connection criteria (references [1-5]), the photovoltaic power station is disconnected within the corresponding setting time limit. Wherein, in step S21, the allowable range of frequency is 48 Hz to 50.5 Hz. In step S22, the normal voltage value range is 0.85 to 1.1 per unit. N , the maximum opening time does not exceed 0.2s; if 50% U N ≤U<85%U N , the maximum opening time does not exceed 2s; if 85% U N ≤U<110%U N , the power grid operates continuously; if U>110%U N , the maximum opening time does not exceed 0.2s. N is the rated voltage of the grid-connected point, and the maximum trip time is the time from the occurrence of the abnormal state to the power supply stopping supplying power to the grid.

3. The method according to claim 1, characterized in that In step S3, the grid connection point voltage amplitude U is compared. PCC With the nominal voltage value U N , determine whether the low voltage ride-through condition and high voltage ride-through condition are met. PCC Within the normal range, the PV power station continues to operate normally; if the voltage persists beyond the voltage crossing threshold, step S4 is executed to slowly decouple the DER. Among them, the low voltage ride-through conditions are: T d is the time value, U PCC is the instantaneous value of the grid connection point voltage. Among them, the high voltage ride-through condition is:

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