CN109713711B - A coordinated reactive power control strategy for distributed photovoltaic inverters under voltage sag - Google Patents

Abstract

The invention discloses a reactive power coordination control strategy of a distributed photovoltaic inverter under a fault, which comprises the following steps: 1) calculating reactive power demand Q when network voltage drops _{Total demand} (ii) a 2) Calculating the fault apparent power of each distributed inverter when the voltage of the power grid drops; 3) calculating reactive capacity Q of each distributed inverter when grid voltage drops _imax (ii) a 4) According to the influence degree of the inverter at different access positions by the fault, providing a corresponding control strategy; 5) and the minimization of the active reduction value of the distributed photovoltaic inverter under the fault is considered, so that the recovery of the active power after the fault is eliminated is facilitated. The method considers the comprehensive influence of different access positions, illumination change and voltage drop degrees of the actual distributed photovoltaic inverters on the transient process and the reactive capacity of the distributed photovoltaic grid-connected inverter, fully utilizes the reactive capacity of the distributed photovoltaic inverters, realizes the minimum reduction value of the active power during the power grid fault period, is favorable for the active power recovery of the distributed photovoltaic after the voltage drop of the power grid is eliminated, and simultaneously avoids the cascading breakdown of the distributed photovoltaic inverters.

Description

A coordinated reactive power control strategy for distributed photovoltaic inverters under voltage sag

technical field

The invention belongs to the field of distributed photovoltaic power generation, and in particular relates to a reactive power coordination control strategy of a distributed photovoltaic inverter under voltage drop.

Background technique

In recent years, distributed photovoltaics have shown the characteristics of contiguous development and cluster grid connection. With the increasing penetration rate of distributed photovoltaic clusters, the randomness and volatility of distributed photovoltaic output will have a profound impact on the security and stability of the power grid. The grid will exacerbate the energy imbalance of the main grid, resulting in grid voltage collapse.

At present, research on distributed photovoltaics mostly focuses on the voltage and reactive power control of inverters. The German Association of Engineers proposed constant reactive power control, constant power factor control, adjustable power factor control and Q(U) control. On this basis, some literatures propose a Q(U) control strategy based on the combination of adjustable power factor control and Q(U) control, so that all inverters participate in grid voltage regulation and the total reactive power output is the smallest, but This strategy needs to obtain the reactive power output value of each inverter under different control strategies, which requires high reliability of the communication system. There are also literatures that introduce the voltage of the grid connection point into the adjustable power factor control, and automatically adjust the power factor curve according to the voltage amplitude of the grid connection point, which effectively reduces the unnecessary reactive power output of the inverter, but the inverter cannot output in this strategy. The inductive reactive power supports the grid. At the same time, some literatures propose to reduce the active power output by the inverter to suppress overvoltage.

Aiming at the control of photovoltaic inverters under grid faults, most of them focus on low-voltage ride-through (LVRT) of large photovoltaic power plants. Some literatures implement LVRT by coordinating control of the active current and reactive current of the inverter. There are also literatures that consider the capacity limitation of the inverter, and realize low voltage ride-through by preferentially providing reactive current and limiting active current. At the same time, there are also literatures discussing the reactive power output capability of the inverter, and on this basis, the reactive power control strategy is proposed, but the system analysis of the reactive power given during the fault is not done. There are also literatures that consider two fault conditions, symmetrical and asymmetrical. When the voltage drops asymmetrically, the positive and negative sequence currents are independently controlled by the positive and negative sequence rotating coordinate systems with completely symmetrical structure to realize the asymmetrical fault. Low voltage ride through. The above are all studies on the single-point centralized form of photovoltaic power supply, without considering the coordinated control between inverters under the condition of high photovoltaic penetration rate. Therefore, coordinated control is required for distributed photovoltaic inverters with multi-point access under fault conditions.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is to solve the problem that when the grid voltage fails in the current actual project, the distributed photovoltaic inverters cannot be coordinated and controlled to make full use of the reactive power and the unreasonable reduction of the active power during the fault period. Coordinated control strategy for reactive power of distributed photovoltaic inverters under voltage sag.

The present invention adopts the following technical solutions to solve the above-mentioned technical problems:

A voltage sag distributed photovoltaic inverter reactive power coordination control strategy, the method includes the following steps:

1) According to the operating state of distributed photovoltaics, detect the voltage change of the control point, and calculate the total reactive power required by each inverter under the voltage drop in combination with the requirements of the fault ride-through guideline in the "Technical Regulations for the Access of Photovoltaic Power Stations to the Power System". Power Q _{total demand} ;

2) Calculate the fault apparent power S _i of each photovoltaic inverter according to the solar radiation intensity and ambient temperature and the voltage drop of the common point of the inverter at different intervention positions, and then obtain the real-time reactive power of each distributed photovoltaic inverter. capacity Q _imax ;

3) Compare the reactive power required by each inverter and the total real-time reactive power capacity of each distributed photovoltaic inverter under the voltage drop, coordinate and control the reactive power output of each distributed photovoltaic inverter, and realize the entire distributed photovoltaic inverter during the fault period. The reduction value of the active power of the photovoltaic inverter is the smallest, so as to facilitate the recovery of the active power of the photovoltaic inverter after the fault is eliminated.

Further, a reactive power coordination control strategy of a distributed photovoltaic inverter under voltage drop of the present invention, step 1) The distributed photovoltaic is usually connected to the end of the distribution network, and it is necessary to consider line impedance, solar radiation intensity and ambient temperature. According to the requirements in the "Technical Regulations for the Access of Photovoltaic Power Stations to the Power System", the reactive power demand Q _{total demand} of the output response.

Further, in the reactive power coordination control strategy of a distributed photovoltaic inverter under voltage sag of the present invention, step 2) specifically includes the following steps:

201. Determine the fault apparent power and reactive power capacity of the photovoltaic inverter as follows:

According to the power relationship:

P ² +Q ² =S ² ;

For the reactive power output range of distributed inverter i in steady state operation:

In the formula, P _i and Q _i are the active power and reactive power output by each inverter respectively; S _N is the rated apparent power of the inverter.

202. Considering the solar radiation intensity and the influence of the ambient temperature, the active power output range of the photovoltaic inverter:

0≤P _i ≤P _max ;

203. In the fault state, in order to avoid the inverter current overcurrent, the inverter fault apparent power is introduced:

In the formula, U _N is the grid-connected voltage of the inverter under steady-state operation; U' is the grid-connected voltage of the inverter when the grid voltage drops; S _N is the rated _apparent power of the inverter; Fault apparent power.

204. When the grid voltage drops, the inverter can work at 1.1 times the apparent power, so the reactive power capacity of the distributed photovoltaic inverter is obtained:

205. In order to obtain a larger reactive power capacity, the active power of the inverter can be reduced by:

In the formula, P _cur is the reduction value of active power during the fault control period.

Further, in a layered coordination control strategy of distributed photovoltaic inverters under fault conditions of the present invention, step 3) specifically includes the following steps:

301. When the grid voltage drops, the distributed photovoltaic inverters connected at different locations detect that the voltage of the grid-connected point presents different drop degrees, and the reactive power demand Q _{total demand} and Q _imax of the output response calculated according to steps 1) and 204, Coordinate reactive power allocation by comparing the two;

302. When _{the total demand} of Q is ≤∑Q _imax ; it means that the reactive capacity of distributed photovoltaics can provide the required reactive power, and there is no need to reduce the active power to increase the reactive power capacity. At this time, consider the active power loss caused by the resistance on the line , by coordinating the reactive power of each distributed inverter to ensure the reduction of active power loss;

The connection position of each inverter is different, and the line impedance can be expressed as:

The active power loss caused by the line resistance of the output reactive power of each distributed photovoltaic inverter under the grid voltage drop can be expressed as:

In the formula, u _ipcc is the grid-connected common point voltage detected by the inverter when the grid voltage drops; Q _iref is the reactive power that the inverter needs to provide, which satisfies the constraints:

∑Q _iref = Q _{total demand} ;

Build the function using the Lagrangian function algorithm:

f=∑ΔP+λ(∑Q _iref -Q _{total demand} );

In order to find the minimum value of f, the following relationship should be satisfied:

Let Q _iref = k _i Q _{total demand} , then we can get:

303. When _{the total demand} of Q>∑Q _imax ; it means that the reactive power capacity of the distributed photovoltaic cannot provide the required reactive power, and the active power needs to be reduced to increase the reactive power capacity. active power to reduce the total active power reduction;

The sum of active power reduced by distributed inverters under grid voltage sag can be expressed as:

Among them, Q _iref needs to satisfy the following constraints:

∑Q _iref = Q _{total demand} ;

Using the Lagrangian function method, we get:

f=∑ΔP _i +λ(∑Q _iref -Q _{total demand} );

In order to find the minimum value of f, the following relationship needs to be achieved:

Then, the reactive power distributed by each distributed photovoltaic inverter is obtained:

In order to avoid the occurrence of the distributed reactive power, the reactive power allocated by a certain inverter is less than its reactive power capacity, the improved allocated power is:

Description of drawings

Fig. 1 is a flow chart of the method of the present invention.

Detailed ways

The technical solutions of the present invention will be further described in detail below with reference to the accompanying drawings. It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It should also be understood that terms such as those defined in the general dictionary should be understood to have meanings consistent with their meanings in the context of the prior art and, unless defined as herein, are not to be taken in an idealized or overly formal sense. explain.

As shown in FIG. 1 , the present invention proposes a reactive power coordination control strategy for distributed photovoltaic inverters under voltage sag, and the method includes the following steps:

Step 1), according to the operating state of distributed photovoltaics, detect the voltage change of the control point, and combine the requirements of the fault ride-through guideline in the "Technical Regulations for the Access of Photovoltaic Power Stations to the Power System" to calculate the total voltage required by each inverter under the voltage drop. Reactive power Q _{total demand} ;

Step 2), according to the detected voltage drop of the grid-connected common point, calculate the inverter fault apparent power and its reactive power capacity;

It specifically includes the following steps:

Determine the PV inverter fault apparent power and reactive power capacity as follows:

According to the power relationship:

P ² +Q ² =S ² ;

For the reactive power output range of distributed inverter i in steady state operation:

In the formula, P _i and Q _i are the active power and reactive power output by each inverter respectively; S _N is the rated apparent power of the inverter.

Considering the solar radiation intensity and the influence of the ambient temperature, the active power output range of the photovoltaic inverter:

0≤P _i ≤P _max ;

In the fault state, in order to avoid inverter current overcurrent, the inverter fault apparent power is introduced:

In the formula, U _N is the grid-connected voltage of the inverter under steady-state operation; U' is the grid-connected voltage of the inverter when the grid voltage drops; S _N is the rated _apparent power of the inverter; Fault apparent power.

When the grid voltage drops, the inverter can work at 1.1 times the apparent power, so the reactive power capacity of the distributed photovoltaic inverter is obtained:

In order to obtain a larger reactive power capacity, the active power of the inverter can be reduced by:

In the formula, P _cur is the reduction value of active power during the fault control period.

Step 3), coordinate and control the reactive power output of each distributed photovoltaic inverter according to the voltage drop degree of the common point detected by each distributed inverter, so as to realize the minimum reduction value of active power of the entire distributed photovoltaic inverter during the fault period, so as to achieve the minimum value of active power reduction of the entire distributed photovoltaic inverter. It is beneficial to the active power recovery of the distributed photovoltaic inverter after the fault is eliminated.

It includes the following steps:

When the grid voltage drops, the distributed photovoltaic inverters connected at different locations detect that the voltage at the grid-connected point presents different drop degrees, and the reactive power demand Q _{total demand} and Q _imax of the output response calculated according to steps 1) and 204 are compared by comparing The two coordinate the distribution of reactive power;

When _{the total demand} of Q≤∑Q _imax ; it means that the reactive capacity of distributed photovoltaics can provide the required reactive power, and it is not necessary to reduce the active power to increase the reactive power capacity. At this time, considering the active power loss caused by the resistance on the line, through Coordinate the reactive power of each distributed inverter to ensure the reduction of active power loss;

The connection position of each inverter is different, and the line impedance can be expressed as:

The active power loss caused by the line resistance of the output reactive power of each distributed photovoltaic inverter under the grid voltage drop can be expressed as:

In the formula, u _ipcc is the grid-connected common point voltage detected by the inverter when the grid voltage drops; Q _iref is the reactive power that the inverter needs to provide, which satisfies the constraints:

∑Q _iref = Q _{total demand} ;

Build the function using the Lagrangian function algorithm:

f=∑ΔP+λ(∑Q _iref -Q _{total demand} );

In order to find the minimum value of f, the following relationship should be satisfied:

Let Q _iref = k _i Q _{total demand} , then we can get:

When _{the total demand} of Q>∑Q _imax ; it means that the reactive capacity of distributed photovoltaics cannot provide the required reactive power, and the active power needs to be reduced to increase the reactive power capacity. At this time, it is considered to coordinate the distribution of the reactive power of each inverter to reduce the total active power reduction;

The sum of active power reduced by distributed inverters under grid voltage sag can be expressed as:

Among them, Q _iref needs to satisfy the following constraints:

∑Q _iref = Q _{total demand} ;

Using the Lagrangian function method, we get:

f=∑ΔP _i +λ(∑Q _iref -Q _{total demand} );

In order to find the minimum value of f, the following relationship needs to be achieved:

Then, the reactive power distributed by each distributed photovoltaic inverter is obtained:

In order to avoid the occurrence of the distributed reactive power, the reactive power allocated by a certain inverter is less than its reactive power capacity, the improved distributed power is:

Claims (2)

1. a voltage drop distributed photovoltaic inverter reactive power coordination control strategy is characterized by comprising the following steps:

1) detecting the voltage change of a control point according to the running state of the distributed photovoltaic, and calculating the total reactive power Q required by each inverter when the voltage drops and provided by combining the fault ride-through rule requirement in technical regulation of photovoltaic power station access power system _{Total demand} ；

2) Calculating the fault apparent power S of each photovoltaic inverter according to the solar radiation intensity, the ambient temperature and the voltage drop condition of the common point of the inverters at different intervention positions _i And further obtain the real-time reactive capacity Q of each distributed photovoltaic inverter _imax ；

3) Comparing the total reactive power required to be provided by each inverter when the voltage drops and the sum of the real-time reactive capacity of each distributed photovoltaic inverter, and coordinating and controlling the reactive output of each distributed photovoltaic inverter to realize the minimum reduction value of the active power of the whole distributed photovoltaic inverter during the fault period so as to be beneficial to the active power recovery of the photovoltaic inverter after the fault is eliminated;

the step 2) specifically comprises the following steps:

201. the method for determining the fault apparent power and the reactive capacity of the photovoltaic inverter comprises the following steps:

according to the power relationship:

P ² +Q ² ＝S ² ；

for the reactive output range of the distributed inverter i in steady state operation:

in the formula, P _i And Q _i Respectively outputting active power and reactive power for each inverter; s. the _N Rated apparent power for the inverter;

202. the active output range of the photovoltaic inverter under the influence of solar radiation intensity and ambient temperature is considered:

0≤P _i ≤P _max ；

203. in a fault state, in order to avoid overcurrent of the inverter current, introducing the fault apparent power of the inverter:

in the formula of U _N The inverter grid-connected point voltage under steady operation; u' is the voltage of the grid-connected point of the inverter when the voltage of the power grid drops; s. the _N Rated apparent power for the inverter; s. the _i Apparent power for the fault of the inverter;

204. when the voltage of the power grid drops, the inverter can work under 1.1 times of apparent power, so that the reactive capacity of the distributed photovoltaic inverter is obtained:

205. to obtain a larger reactive capacity, the active power of the inverter can be reduced:

in the formula, P _cur A reduced value for active power during fault control;

the step 3) specifically comprises the following steps:

301. when the voltage of the power grid drops, the distributed photovoltaic inverters connected at different positions detect that the voltage of the grid-connected point presents different dropping degrees, and the voltage drop inversions are calculated by combining the step 1) and the step 2)Total reactive power Q required by the converter _{Total demand} And Q _imax Comparing the two to carry out reactive power coordination distribution;

302. when Q is _{Total demand} ≤∑Q _imax (ii) a The reactive power of the distributed photovoltaic system can provide required reactive power, the active power does not need to be reduced to improve the reactive power, the active power loss caused by the resistance on the line is considered, and the active power loss is reduced by coordinating the reactive power of each distributed inverter;

the line impedance can be expressed as:

the active power loss caused by the line resistance of each distributed photovoltaic inverter output reactive power due to the voltage drop of the power grid can be expressed as follows:

in the formula u _ipcc Detecting the grid-connected common point voltage for the inverter under the voltage drop of the power grid; q _iref The reactive power required to be provided for the inverter meets the constraint condition:

∑Q _iref ＝Q _{total demand} ；

And (3) constructing a function by utilizing a Lagrange function algorithm:

f＝∑ΔP+λ(∑Q _iref -Q _{total demand} )；

To find the minimum value of f, the following relationship should be satisfied:

let Q _iref ＝k _i Q _{Total demand} The following can be obtained:

303. when Q is _{Total demand} >∑Q _imax (ii) a Meaning that the reactive capacity of the distributed photovoltaic may not provide the required reactive power, the active power needs to be reduced to increase the reactive capacity, at which point the reduction of the total active power by coordinated allocation of the reactive power of the inverters is considered;

the sum of the active power reduced by the distributed inverter under grid voltage sag can be expressed as:

wherein Q _iref The following constraints need to be satisfied:

∑Q _iref ＝Q _{total demand} ；

The method comprises the following steps of:

f＝∑ΔP _i +λ(∑Q _iref -Q _{total demand} )；

To find the minimum value of f, the following relationship needs to be achieved:

and further obtaining the reactive power distributed by each distributed photovoltaic inverter:

in order to avoid the distributed reactive power, the distributed reactive power of a certain inverter is smaller than the reactive capacity of the inverter, the distributed power is improved as follows:

2. the reactive power coordination control strategy for the voltage drop distributed photovoltaic inverters as claimed in claim 1, wherein the distributed photovoltaic in step 1) is usually connected to the end of a power distribution network, and the influence of line impedance, solar radiation intensity and ambient temperature is considered, and the total reactive power Q required by each inverter in voltage drop to be provided is calculated according to the requirements in technical provisions for photovoltaic power station access to power system _{Total demand} 。

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