Distributed photovoltaic power generation follows the principles of adapting to local conditions, clean and efficient, decentralized layout, and nearby utilization, fully utilizing local solar energy resources to replace and reduce fossil energy consumption. Distributed photovoltaic power generation generally utilizes the roof resources of existing buildings in enterprises, and operates mainly on the user side for self use, with excess electricity connected to the grid. A distributed photovoltaic system consists of photovoltaic modules and inverters. The inverter tracks the high-power point of the photovoltaic cell, controls the waveform and power of the grid connected current, inverts the electrical energy generated by the photovoltaic cell into a sine current and integrates it into the grid, so that the power transmitted to the grid is balanced with the high-power electrical energy generated by the photovoltaic array. The transmission energy of photovoltaic grid connected systems comes from photovoltaic cells, and the output voltage and current curves are determined by the characteristics of the cells to be nonlinear. The output power follows the changes due to the influence of light and temperature. Photovoltaic systems convert direct current into alternating current through power electronic converters and integrate it into the power grid.
1   The impact of distributed photovoltaic grid connection on power factor of the power grid
1.1 Power factor: Power factor is a coefficient used by power supply companies to measure the efficiency of users' electrical equipment. A low power factor can reduce the operational efficiency of the power grid. The calculation of power factor is obtained by the numerical values of active and reactive power of the user. Generally speaking, the higher the proportion of reactive power, the lower the power factor. Therefore, an important means to improve the power factor is to install reactive power compensation devices to reduce reactive power.
1.2 Power factor adjustment electricity fee: In order to improve the efficiency of electricity use, the former Ministry of Water Resources and Electric Power and the National Price Bureau issued the "Power Factor Adjustment Electricity Fee Method" (Water and Electricity Finance Document No. 215) in 1983. The method stipulates that power users with a capacity of 100 kVA or above must undergo power factor assessment. If they fail to meet the assessment standards, a power factor adjustment fee (i.e. power factor adjustment electricity fee) will be charged. Those who exceed the assessment standards will be rewarded proportionally. The assessment standard for user power factor is 0.85 or 0.90. If the power factor is much lower than the standard, it will not only cause a burden on the operation of the power grid, but also result in a huge amount of fines for power regulation. Due to the fact that user load and load nature may not be consistent at different times of the day, users generally install reactive power compensation devices (mostly capacitive devices) with automatic switching functions to automatically adjust the compensation intensity. The definition of reactive energy four quadrant measurement is specified in the "Multi functional Energy Meter Communication Protocol" (DL/T645-2007).
The forward and reverse directions of an electric energy meter are related to the reception (transmission) of electrical energy. Generally, the user's acceptance of the system's electrical energy is defined as positive; Internal power generation by users to supply power to the system is defined as reverse.
When the system delivers active and reactive power to the user, the energy meter operates in quadrant I and displays positive values for active and reactive power; This is the normal electricity consumption mode of the user, where both active and reactive energy come from the power grid;
(2) When the system delivers reactive power to the user and the user sends active power back to the system, the energy meter operates in quadrant II and displays negative active power (reverse active power) and positive reactive power; At this point, the user load cannot fully absorb the distributed photovoltaic power generation. In the case of surplus electricity being connected to the grid, active energy is sent back to the grid, while the reactive power required by the load still comes from the grid;
(3) When the user sends active and reactive power back to the system, the energy meter operates in quadrant III and displays negative values for active and reactive power; Some self generating users, like professional power plants, transmit both active and reactive power to the grid when there is no internal load;
(4) When the system delivers active power to the user and the user sends reactive power back to the system, the energy meter operates in quadrant IV and displays positive active power and negative reactive power; At this point, the user retrieves active power from the system, but the user's capacitor compensation is in an overcompensation state and sends reactive power back to the system.;
The calculation formula for user power factor as stipulated in the "Electricity Fee Method for Power Factor Adjustment" is:
                                  
In the formula, the directions of capacitive reactive power Qc and inductive reactive power QL are opposite, and the gateway meter calculates the power factor by adding the absolute values of forward and reverse reactive power. And active power only takes forward active energy, and reverse active energy does not participate in power factor calculation. When users transfer active power from distributed photovoltaics to the grid (with the energy meter operating in quadrant II or III), the power factor of the gateway meter will decrease.
The impact and consequences of power factor reduction in power supply grid
1) The equipment of the power system and electricity consuming enterprises cannot be fully utilized. Because equipment such as generators and transformers in the power system are not allowed to operate beyond their rated voltage and current for a long time under normal circumstances. So when both voltage and current have reached their rated values, a low power factor results in less active power output from the equipment. The lower the power factor of a device with the same capacity, the less active power it outputs.
2) Causing an increase in energy loss and a decrease in power supply quality in the power system. For transmission and distribution lines, the loss in the line is proportional to the square of the current. When transmitting the same amount of active power P=IUcos φ, the lower the power factor cos φ, the larger the current I=P/Ucos φ in the transmission line. The energy loss in the line increases proportionally to the square of the current.
3) When the power factor decreases and the line current increases, it will inevitably cause an increase in voltage drop in the line, which will lead to a decrease in voltage at the end of the line. To meet the voltage requirements of end users, the voltage at the beginning of the line needs to increase, which will reduce the power supply quality of the entire line.
4) The decrease in power factor increases the cost of electricity consumption. According to the requirements of the Ministry of Water Resources and Electric Power, the cos φ of the power factor feedback grid for industrial users cannot be lower than 0.90. For enterprise users who fail to meet the standard requirements, the power department will impose a certain proportion of electricity consumption rate fines on enterprise users in accordance with the "Measures for Adjusting Electricity Charges with Power Rates".


 

Distributed photovoltaic power generation follows the principles of adapting to local conditions, clean and efficient, decentralized layout, and nearby utilization, fully utilizing local solar energy resources to replace and reduce fossil energy consumption. Distributed photovoltaic power generation generally utilizes the roof resources of existing buildings in enterprises, and operates mainly on the user side for self use, with excess electricity connected to the grid. A distributed photovoltaic system consists of photovoltaic modules and inverters. The inverter tracks the high-power point of the photovoltaic cell, controls the waveform and power of the grid connected current, inverts the electrical energy generated by the photovoltaic cell into a sine current and integrates it into the grid, so that the power transmitted to the grid is balanced with the high-power electrical energy generated by the photovoltaic array. The transmission energy of photovoltaic grid connected systems comes from photovoltaic cells, and the output voltage and current curves are determined by the characteristics of the cells to be nonlinear. The output power follows the changes due to the influence of light and temperature. Photovoltaic systems convert direct current into alternating current through power electronic converters and integrate it into the power grid.
1   The impact of distributed photovoltaic grid connection on power factor of the power grid
1.1 Power factor: Power factor is a coefficient used by power supply companies to measure the efficiency of users' electrical equipment. A low power factor can reduce the operational efficiency of the power grid. The calculation of power factor is obtained by the numerical values of active and reactive power of the user. Generally speaking, the higher the proportion of reactive power, the lower the power factor. Therefore, an important means to improve the power factor is to install reactive power compensation devices to reduce reactive power.
1.2 Power factor adjustment electricity fee: In order to improve the efficiency of electricity use, the former Ministry of Water Resources and Electric Power and the National Price Bureau issued the "Power Factor Adjustment Electricity Fee Method" (Water and Electricity Finance Document No. 215) in 1983. The method stipulates that power users with a capacity of 100 kVA or above must undergo power factor assessment. If they fail to meet the assessment standards, a power factor adjustment fee (i.e. power factor adjustment electricity fee) will be charged. Those who exceed the assessment standards will be rewarded proportionally. The assessment standard for user power factor is 0.85 or 0.90. If the power factor is much lower than the standard, it will not only cause a burden on the operation of the power grid, but also result in a huge amount of fines for power regulation. Due to the fact that user load and load nature may not be consistent at different times of the day, users generally install reactive power compensation devices (mostly capacitive devices) with automatic switching functions to automatically adjust the compensation intensity. The definition of reactive energy four quadrant measurement is specified in the "Multi functional Energy Meter Communication Protocol" (DL/T645-2007).
The forward and reverse directions of an electric energy meter are related to the reception (transmission) of electrical energy. Generally, the user's acceptance of the system's electrical energy is defined as positive; Internal power generation by users to supply power to the system is defined as reverse.
When the system delivers active and reactive power to the user, the energy meter operates in quadrant I and displays positive values for active and reactive power; This is the normal electricity consumption mode of the user, where both active and reactive energy come from the power grid;
(2) When the system delivers reactive power to the user and the user sends active power back to the system, the energy meter operates in quadrant II and displays negative active power (reverse active power) and positive reactive power; At this point, the user load cannot fully absorb the distributed photovoltaic power generation. In the case of surplus electricity being connected to the grid, active energy is sent back to the grid, while the reactive power required by the load still comes from the grid;
(3) When the user sends active and reactive power back to the system, the energy meter operates in quadrant III and displays negative values for active and reactive power; Some self generating users, like professional power plants, transmit both active and reactive power to the grid when there is no internal load;
(4) When the system delivers active power to the user and the user sends reactive power back to the system, the energy meter operates in quadrant IV and displays positive active power and negative reactive power; At this point, the user retrieves active power from the system, but the user's capacitor compensation is in an overcompensation state and sends reactive power back to the system.;
The calculation formula for user power factor as stipulated in the "Electricity Fee Method for Power Factor Adjustment" is:
                                  
In the formula, the directions of capacitive reactive power Qc and inductive reactive power QL are opposite, and the gateway meter calculates the power factor by adding the absolute values of forward and reverse reactive power. And active power only takes forward active energy, and reverse active energy does not participate in power factor calculation. When users transfer active power from distributed photovoltaics to the grid (with the energy meter operating in quadrant II or III), the power factor of the gateway meter will decrease.
The impact and consequences of power factor reduction in power supply grid
1) The equipment of the power system and electricity consuming enterprises cannot be fully utilized. Because equipment such as generators and transformers in the power system are not allowed to operate beyond their rated voltage and current for a long time under normal circumstances. So when both voltage and current have reached their rated values, a low power factor results in less active power output from the equipment. The lower the power factor of a device with the same capacity, the less active power it outputs.
2) Causing an increase in energy loss and a decrease in power supply quality in the power system. For transmission and distribution lines, the loss in the line is proportional to the square of the current. When transmitting the same amount of active power P=IUcos φ, the lower the power factor cos φ, the larger the current I=P/Ucos φ in the transmission line. The energy loss in the line increases proportionally to the square of the current.
3) When the power factor decreases and the line current increases, it will inevitably cause an increase in voltage drop in the line, which will lead to a decrease in voltage at the end of the line. To meet the voltage requirements of end users, the voltage at the beginning of the line needs to increase, which will reduce the power supply quality of the entire line.
4) The decrease in power factor increases the cost of electricity consumption. According to the requirements of the Ministry of Water Resources and Electric Power, the cos φ of the power factor feedback grid for industrial users cannot be lower than 0.90. For enterprise users who fail to meet the standard requirements, the power department will impose a certain proportion of electricity consumption rate fines on enterprise users in accordance with the "Measures for Adjusting Electricity Charges with Power Rates".