CN102590663B - Improved intermittent reactive power disturbance island detection method - Google Patents

Abstract

The invention discloses an improved intermittent reactive power disturbance island detection method. The method applies periodic reactive power disturbance to the inverter controller, and each reactive power disturbance cycle is divided into reactive power disturbance time and non-power disturbance time. Reactive power disturbance time. At the beginning of each reactive disturbance period, the voltage frequency detected by the frequency detection module is assigned to the reactive disturbance calculation module, and then the minimum reactive power disturbance required for this period is calculated by the reactive disturbance calculation module The amount of reactive power disturbance can ensure that the frequency of the common coupling point is out of the allowable range when the island occurs. The intermittent reactive power disturbance signal module generates the intermittent reactive power disturbance signal of this cycle according to the calculated reactive power disturbance. Periodic intermittent reactive power disturbances are applied to the inverter controller to achieve the purpose of detecting islands. The invention can timely and effectively detect the isolated island phenomenon, and can reduce the reactive power disturbance amount required for detecting the isolated island, and has little influence on the grid-connected power factor.

Description

An Improved Intermittent Reactive Power Disturbance Islanding Detection Method

technical field

The invention relates to an islanding detection technology in the application field of a distributed power generation system and a grid-connected inverter, in particular to an islanding detection method for intermittent reactive power disturbance without improvement.

Background technique

Due to the rapid consumption of non-renewable energy such as fossil energy and the increasingly serious environmental problems, new energy distributed power generation has gradually received extensive attention. In the new energy distributed power generation, the electric energy generated by wind energy, photovoltaic, fuel cell, etc. is converted into qualified electric energy by power converter and incorporated into the grid. As an interface between new energy sources and the grid, grid-connected inverters can transform electric energy and control power quality. Usually distributed generation systems include single-phase or three-phase grid-connected inverters, circuit breakers, local loads and public grids. The grid-connected inverter and the local load are connected to the public grid through a circuit breaker, and the connection point is the point of public coupling (PCC). When the external grid fails and is disconnected, the grid-connected inverter and the local load are disconnected from the grid to form a grid structure with a partial power generation device, which is called an island. There are many reasons for islanding, mainly including power grid failure, circuit breaker damage, and natural disasters. The occurrence of isolated islands is generally unpredictable, and may cause damage to distributed power generation devices and local loads due to voltage or frequency out of control, and even pose a certain threat to personal safety. Therefore, the distributed generation system must adopt an effective island detection method to detect the occurrence of islands.

Islanding detection methods are generally divided into three categories: (1) passive islanding detection methods; (2) active islanding detection methods; (3) external communication islanding detection methods. The effectiveness of an islanding detection method is usually measured by the non-detection zone (NDZ), which represents an area where a particular islanding detection method cannot detect an islanded state given the local load characteristics.

The passive islanding detection method judges the occurrence of islanding by detecting the system power parameters such as voltage amplitude, frequency, phase and harmonics of the common coupling point. Over/under voltage method (OVP/UVP), over/under frequency method (OFP/UFP) are the most commonly used passive island detection methods. The over/undervoltage method judges the occurrence of islanding by detecting whether the voltage amplitude of the common coupling point exceeds the allowable range of the grid voltage, while the over/under frequency method judges the occurrence of islanding by detecting whether the voltage frequency of the common coupling point exceeds the allowable range of the grid frequency. GB/T 19939-2005 (Technical Requirements for Grid-connected Photovoltaic Systems) stipulates that the grid frequency cannot exceed the range of 49.5Hz to 50.5Hz. The passive island detection method is simple to implement and does not affect the quality of grid-connected power. However, when the active and reactive power output by the inverter is close to the active and reactive power consumed by the load, the passive island detection method may fail and fall into the non-detection district.

The active islanding detection method is to introduce disturbances to the common coupling point through the grid-connected inverter, so that the power parameters (including voltage amplitude, frequency, etc.) of the system after the islanding occurs exceed the allowable range, so as to determine the occurrence of the islanding. Active island detection methods include frequency deviation method (AFD), sliding mode frequency deviation method (SMS), Sandia frequency deviation method, reactive power disturbance method (RPV), etc. The active island detection method reduces or even eliminates the non-detection area, and at the same time has a certain impact on the quality of grid-connected power. The traditional intermittent reactive power disturbance method is to introduce a constant amount of positive reactive power disturbance and negative reactive power disturbance in a disturbance period, which can eliminate the non-detection zone. However, the traditional reactive power disturbance method will introduce a large amount of disturbance to the reactive power output by the grid-connected inverter and reduce the output power factor of the inverter, which has relatively large defects.

The external communication detection method detects the occurrence of islanding by establishing a communication link between the power grid and the distributed generation device, but the high cost limits the application of the external communication detection method.

Contents of the invention

Aiming at the deficiencies of the prior art, the present invention proposes a method whose main purpose is to provide an improved intermittent reactive power disturbance islanding detection method. This method detects the voltage frequency of the common coupling point at the initial moment of the reactive power disturbance cycle, and calculates the minimum reactive power disturbance that needs to be introduced in this cycle, so as to successfully detect the occurrence of islanding.

An improved intermittent reactive power disturbance islanding detection system includes an islanding test main circuit, a frequency detection module, a reactive power disturbance control loop, an inverter controller, and a frequency determination module; the islanding test main circuit includes a PWM modulation based The single-phase/three-phase grid-connected inverter, RLC load, circuit breaker for connecting to the grid and the grid; the frequency detection module is used to detect the voltage frequency of the common coupling point every sampling period; the inverter controller is used to control the inverter The specified reactive power is output; the frequency determination module is used to determine whether islanding occurs; the intermittent reactive power disturbance control loop includes a reactive power disturbance calculation module and an intermittent reactive power disturbance signal module.

The specific detection steps of the method include:

Step 1, in each sampling period, the frequency detection module detects the voltage frequency of the common coupling point;

Step 2: At the initial moment of each reactive power disturbance period T, the reactive power disturbance amount calculation module calculates the output power of the inverter within the reactive power disturbance period of this period according to the frequency value f provided by the frequency detection module at this time Reactive power disturbance Q , when f ≤ 50Hz, the output reactive power disturbance in this cycle is:

When f >50Hz, the output reactive power disturbance is:

Where P is the output active power of the inverter, Q _f is the load quality factor, f _min is the minimum value allowed by the grid frequency, and f _max is the maximum value allowed by the grid frequency;

Step 3, the intermittent reactive power disturbance signal module generates the intermittent reactive power disturbance signal of this cycle according to the reactive power disturbance quantity Q , and each reactive power disturbance cycle T is divided into reactive power disturbance time _T1 and non-reactive power disturbance At time T ₂ , within the reactive power disturbance time T ₁ , the output reactive power disturbance is a fixed value Q , Q is the reactive power disturbance calculated by the reactive power disturbance calculation module based on the voltage frequency at the initial moment of this disturbance cycle, not reactive power The reactive power disturbance output within the power disturbance time _T2 is zero;

    Step 4, the inverter controller controls the inverter to generate intermittent reactive power according to the generated intermittent reactive power disturbance signal.

At the same time, the frequency value measured by the frequency detection module is judged by the frequency judgment module on the frequency value of the common coupling point voltage. If the frequency is greater than 50.5Hz or less than 49.5Hz, it is determined that the power grid is disconnected from the grid-connected inverter at this time. Islands occur.

Beneficial effects: the islanding detection method proposed by the present invention is simple and feasible. Compared with the traditional reactive power disturbance islanding detection method, only a reactive power disturbance control loop needs to be added in the software. Before outputting reactive power disturbance in each cycle, according to The frequency detection module obtains the common coupling point voltage frequency to calculate the reactive power disturbance of this cycle, and the obtained reactive power disturbance is the minimum value that can cause the common coupling point voltage and frequency to deviate from the allowable frequency range during islanding. Therefore, compared with the traditional intermittent bilateral reactive power disturbance method, only unilateral reactive power disturbance needs to be introduced in each cycle, and the amount of reactive power disturbance can be reduced. Finally, the grid-connected system can obtain a higher grid-connected power factor and effective islanding detection capability. The invention is suitable for new energy distributed power generation systems such as solar energy, wind power, and fuel cells, and can successfully detect the islanding phenomenon of grid-connected inverters The occurrence of the islanding detection time that meets the national standard requirements, and has little impact on the grid-connected power factor.

Description of drawings

Figure 1 Schematic diagram of the improved intermittent reactive disturbance island detection system;

Fig. 2 The timing diagram of reactive power output by the improved intermittent reactive power disturbance method;

Figure 3 is a flow chart of the implementation of the improved intermittent reactive disturbance islanding detection method.

Detailed ways

The technical solution of the present invention will be further described in detail below in conjunction with the accompanying drawings.

As shown in Figure 1, an improved intermittent reactive power disturbance islanding detection system includes an islanding test main circuit, a frequency detection module 5, a reactive power disturbance control loop, an inverter controller 8, and a frequency determination module 9 The island test main circuit includes a single-phase or three-phase grid-connected inverter 1 based on PWM modulation, an RLC load 2, a circuit breaker 3 for connecting to the grid, and a public grid 4; the frequency detection module 5 is used for each sampling period Detect the voltage frequency of the common coupling point; the inverter controller 8 is used to control the inverter to output the specified amount of reactive power; the frequency determination module 9 is used to determine whether islanding occurs; the intermittent reactive power disturbance control loop reactive power Disturbance calculation module 6 and intermittent reactive power disturbance signal module 7.

The specific detection steps of the method include:

Step 1, in each sampling period, the frequency detection module detects the voltage frequency of the common coupling point;

Step 2: At the initial moment (t ₀ ) of each reactive power disturbance period T, the reactive power disturbance amount calculation module 6 calculates The reactive power disturbance Q that the inverter should output, when f ≤ 50Hz, the reactive power disturbance output in this cycle is:

When f >50Hz, the output reactive power disturbance is:

Where P is the output active power of the inverter, Q _f is the load quality factor, f _min is the minimum value allowed by the grid frequency, and f _max is the maximum value allowed by the grid frequency;

Step 3, the intermittent reactive power disturbance signal module 7 generates the intermittent reactive power disturbance signal of this cycle according to the reactive disturbance quantity Q , and each reactive power disturbance cycle T is divided into reactive power disturbance time _T1 and non-reactive power disturbance Disturbance time T ₂ , within the reactive power disturbance time T ₁ , the output reactive power disturbance is a fixed value Q , Q is the reactive power disturbance calculation module 6 calculated according to the voltage frequency at the initial moment of this disturbance cycle, The output reactive power disturbance within the non-reactive power disturbance time _T2 is zero;

    Step 4, the inverter controller 8 controls the inverter to generate intermittent reactive power according to the generated intermittent reactive power disturbance signal.

At the same time, the frequency value measured by the frequency detection module is judged by the frequency judgment module 9 on the frequency value of the voltage at the common coupling point. If the frequency is greater than 50.5 Hz or less than 49.5 Hz and lasts for one power frequency cycle, it is determined that the power grid at this time Disconnected from the grid-connected inverter, an island occurs.

The timing sequence of the inventive method output reactive power is with reference to Fig. 2, and principle is as follows:

The output reactive power of the grid-connected inverter changes periodically, and each reactive power disturbance period T is divided into reactive power disturbance time T ₁ and non-reactive power disturbance time T ₂ , within the reactive power disturbance time T ₁ , the output The reactive power disturbance quantity is a fixed value Q , and the reactive power disturbance quantity output in the non-reactive power disturbance time _T2 is zero. As shown in Figure 2, the output reactive power of the inverter can be obtained The expression is:

，间歇时间。以上实施例仅为说明本发明的技术思想，不能以此限定本发明的保护范围，凡是根据本发明的设计步骤，对时间参数和无功参数所做的任何修改，均落入本发明保护范围之内。 Considering that the island detection time cannot be greater than 2s, set the disturbance period , reactive power disturbance time

, intermittent time . The above embodiments are only to illustrate the technical ideas of the present invention, and cannot limit the protection scope of the present invention with this. Any modification made to time parameters and reactive parameters according to the design steps of the present invention all falls within the protection scope of the present invention. within.

The working flow diagram of the inventive method is as shown in Figure 3, and the principle is as follows:

Each sampling cycle enters an interrupt program, the frequency detection module detects the frequency of the common coupling point voltage, and the frequency judgment module judges the island state. The specific judgment method is: the frequency detection module obtains the frequency of the public coupling point voltage. If the frequency is greater than 50.5Hz or less than 49.5Hz, it is considered that the power grid is abnormal. Islanding occurs, and the islanding protection module shuts down the inverter. If the grid frequency does not exceed the allowable range at this time, the inverter controller works normally. The counter in the flowchart is a power frequency cycle counter, the initial value is 0, and the count value is increased by 1 every 20ms. The cycle T of intermittent reactive power disturbance can be set to 2s, then when the counter counts to 100 cycles, it is reset to 0, and the output reactive power disturbance time _T1 is set to 200ms, that is, when the counter is 0 to 9, the output reactive power Disturbance, and when the counter is 10 to 99, no reactive power disturbance is output. The reactive disturbance amount of the intermittent reactive power disturbance method proposed by the present invention is related to the frequency of the PCC point before the reactive power disturbance, so when the counter is 0, the frequency of the PCC point at this time is obtained and assigned to f , and the value of f is used according to the value of f Different formulas can calculate the minimum reactive power disturbance Q required in this cycle. When the counter is 0 to 9, the output reactive power disturbance is Q , and when the counter is 10 to 99, the output reactive power disturbance is 0. When the counter counts to 100 cycles, reset to 0. Through the flow chart shown in FIG. 3 , the reactive power output sequence shown in FIG. 2 can be obtained, and the occurrence of islanding can be effectively detected.

Claims (1)

1. An improved intermittent reactive power disturbance island detection method, the improved intermittent reactive power disturbance island detection system includes an island test main circuit, a frequency detection module, a reactive power disturbance control loop, and an inverter controller And the frequency determination module; the main circuit of the island test includes a single-phase or three-phase grid-connected inverter based on PWM modulation, RLC load, a circuit breaker for connecting to the grid and the public grid; the frequency detection module is used to detect the public coupling every sampling period point voltage frequency; the inverter controller is used to control the inverter to output the specified amount of reactive power; the frequency determination module is used to determine whether islanding occurs; the reactive power disturbance control loop includes the reactive power disturbance calculation module and the intermittent The reactive power disturbance signal module is characterized in that the specific detection steps of the method include: Step 1, in each sampling period, the frequency detection module detects the voltage frequency of the common coupling point; Step 2: At the initial moment of each reactive power disturbance period T, the reactive power disturbance amount calculation module calculates that the inverter should The output reactive power disturbance Q, when f≤50Hz, the output reactive power disturbance in this cycle is: $\mathrm{<span\; class="notranslate">\; Q</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; P</span>}{\mathrm{<span\; class="notranslate">\; Q</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; f</span>}}{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; min</span>}}}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; min</span>}}}{\mathrm{<span\; class="notranslate">\; f</span>}}<span\; class="notranslate">\; )</span>$ When f>50Hz, the output reactive power disturbance is: $\mathrm{<span\; class="notranslate">\; Q</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; PQ</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; f</span>}}{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; max</span>}}}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; max</span>}}}{\mathrm{<span\; class="notranslate">\; f</span>}}<span\; class="notranslate">\; )</span>$ Where P is the active power output by the inverter, Q _f is the load quality factor, f _min is the minimum value allowed by the grid frequency, and f _max is the maximum value allowed by the grid frequency; Step 3, the intermittent reactive power disturbance signal module generates the intermittent reactive power disturbance signal of this cycle according to the reactive power disturbance quantity Q, and each reactive power disturbance period T is divided into reactive power disturbance time T ₁ and non-reactive power disturbance In the reactive power disturbance time T ₂ , within the reactive power disturbance time T ₁ , the output reactive power disturbance is a fixed value Q, and Q is the reactive power disturbance calculated by the reactive power disturbance calculation module based on the voltage frequency at the initial moment of the disturbance cycle. Work power disturbance, the output reactive power disturbance within the non-reactive power disturbance time _T2 is zero; Step 4, the inverter controller controls the inverter to emit intermittent reactive power according to the generated intermittent reactive power disturbance signal; the frequency detection module measures the frequency value, and the frequency determination module determines the frequency value of the common coupling point voltage Make a judgment, if the frequency is greater than 50.5Hz or less than 49.5Hz, and lasts for one power frequency cycle, it is determined that the grid is disconnected from the grid-connected inverter at this time, and islanding occurs.

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