Voltage regulation method after distributed energy access of power distribution network

Abstract

The invention discloses a voltage regulation method after distributed energy access of a power distribution network, which relates to the technical field of power distribution networks, and comprises the following steps: carrying out peak-valley normal period division according to a load characteristic curve, calculating the probability of each point on the load curve in a peak period or a valley period by adopting a semi-trapezoidal membership function, and completing the peak-valley normal period division; aggregating the plurality of DERs into a virtual power plant to participate in power market response time-of-use electricity price; when the time-of-use electricity price is not considered, the power distribution network carries out voltage regulation and control by directly adjusting energy storage charging and discharging and a distributed photovoltaic inverter power factor, and an energy storage and inverter coordinated voltage regulation model is constructed. According to the voltage regulation method after the distributed energy access of the power distribution network provided by the embodiment of the invention, various DERs accessed into the power distribution network are utilized to improve the capability of accepting distributed photovoltaics of the power distribution network, solve the problem that the terminal voltage of the power distribution network is out of limit due to the access of a large amount of distributed photovoltaics, and improve the operation safety of the power distribution network.

Description

Technical Field

[0001] The present application relates to the technical field of power distribution networks, and in particular to a voltage regulation method after distributed energy is connected to a power distribution network. Background Art

[0002] As energy shortage and environmental pollution become important factors restricting the sustainable development of society and economy, it has become a consensus of all mankind to develop renewable energy on a large scale, adjust the energy structure, and achieve clean and low-carbon development. At the same time, with the continuous expansion of the scale of the power grid, large-scale centralized power supply is increasingly difficult to meet the increasing requirements for power supply quality and safety and reliability. Distributed Energy Rescourse (DER) can be directly deployed on the load side to provide users with "plug and play" power supply services, realize distributed power supply, and alleviate the hidden dangers brought to the power system by large-scale centralized power supply. Therefore, distributed energy is developing rapidly around the world, and DER is being connected to the distribution network on a large scale.

[0003] The number of distributed photovoltaic grid-connected in medium and low voltage distribution networks is growing rapidly, and the capacity has increased significantly. In this context, the access of a high proportion of distributed photovoltaics has a huge impact on the power quality of the distribution network, which may cause voltage sag, flicker, voltage over-limit and harmonic resonance. Among these many risks, voltage over-limit is the most important factor affecting the absorption of distributed photovoltaics in the distribution network. Summary of the invention

[0004] The purpose of the embodiment of the present invention is to provide a voltage regulation method after the distributed energy is connected to the distribution network, utilize various types of DER connected to the distribution network, improve the ability of the distribution network to accept distributed photovoltaics, solve the problem of voltage exceeding the limit at the end of the distribution network caused by the large-scale connection of distributed photovoltaics, and improve the safety of the distribution network operation.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0006] The embodiment of the present invention provides a voltage regulation method after distributed energy is connected to a distribution network, comprising the following steps:

[0007] The peak, valley and normal periods are divided according to the load characteristic curve, and the probability of each point on the load curve being in the peak period or valley period is calculated using a semi-trapezoidal membership function to complete the division of the peak, valley and normal periods;

[0008] Aggregate multiple DERs into virtual power plants to participate in the electricity market and respond to time-of-use electricity prices;

[0009] When time-of-use electricity prices are not considered, the distribution network performs voltage regulation by directly adjusting the energy storage charging and discharging and the power factor of the distributed photovoltaic inverter, and constructs a coordinated voltage regulation model of energy storage and inverter.

[0010] In some embodiments, the minimum and maximum values ​​of the load curve are p n and p x , the load at a certain moment is p t The probability of being in the peak period and valley period is expressed as F(p t ) and G(p t )express:

[0011]

[0012] In some embodiments, the aggregating of multiple DERs into a virtual power plant to participate in the electricity market in response to time-of-use electricity prices also includes the following steps: when the virtual power plant operates at the most economical level, analyzing the voltage condition of the distribution network, and if the voltage of the distribution network still exceeds the limit at certain moments, adjusting the power factor of the distributed photovoltaic inverter to regulate the voltage, and establishing a distributed photovoltaic inverter voltage regulation model.

[0013] In some embodiments, the voltage regulation model objective function is to minimize the reactive power absorbed by the inverter, which is specifically expressed as:

[0014]

[0015] Where n NPV is the number of inverters involved in reactive voltage regulation, is the reactive power absorbed by the inverter at node i at time t.

[0016] In some embodiments, the energy storage and inverter coordinated voltage regulation model is specifically expressed as follows:

[0017]

[0018] In the formula, c ess and c pv are the regulation costs of energy storage and inverter respectively.

[0019] As energy shortage and environmental pollution become important factors restricting the sustainable development of society and economy, it has become a consensus of all mankind to develop renewable energy on a large scale, adjust the energy structure, and achieve clean and low-carbon development. At the same time, with the continuous expansion of the scale of the power grid, large-scale centralized power supply is increasingly difficult to meet the increasing requirements for power supply quality and safety and reliability. Distributed Energy Rescourse (DER) can be directly deployed on the load side to provide users with "plug and play" power supply services, realize distributed power supply, and alleviate the hidden dangers brought to the power system by large-scale centralized power supply. Therefore, distributed energy is developing rapidly around the world, and DER is being connected to the distribution network on a large scale.

[0020] Solar photovoltaic is the first choice for the development of renewable energy. By the end of 2020, my country's solar installed capacity has reached 250 million kilowatts. At present, the installed capacity of photovoltaic is still showing a rapid growth trend. According to the different capacities, photovoltaic power generation is divided into two forms of grid connection: one is to access the transmission network through medium and high voltage lines, and the other is to access the distribution network through medium and low voltage lines. According to the "Several Opinions of the State Council on Promoting the Healthy Development of the Photovoltaic Industry" and the power development plan, photovoltaic grid-connected power generation should follow the principle of decentralized development and local consumption. Photovoltaic decentralized grid connection and local consumption of electric energy have become a trend. At the same time, the state has widely promoted the construction of distributed small photovoltaic power stations and household photovoltaics through subsidies in rural areas. Driven by a series of favorable factors such as the maturity of photovoltaic power generation technology, the reduction of economic costs and the support of national policies, the number of distributed photovoltaic grid-connected in medium and low voltage distribution networks is growing rapidly, and the capacity has increased significantly. In this context, the access of a high proportion of distributed photovoltaics has a huge impact on the power quality of the distribution network, which may cause voltage sag, flicker, voltage over-limit and harmonic resonance. Among these many risks, voltage over-limit is the most important factor affecting the consumption of distributed photovoltaics in the distribution network.

[0021] In addition to uncontrollable distributed photovoltaics, the number of other DERs such as energy storage equipment, controllable loads, electric vehicles, micro gas turbines, and demand response in the distribution network is also increasing. These types of DER have certain control capabilities. The continuous access of these resources has also enabled the distribution network to gradually develop into an active distribution network (ADN) with certain coordination and control capabilities. A large number of DERs can also be aggregated to form virtual power plants (VPPs) that can participate in the electricity market and coordinate management. Based on active distribution networks and virtual power plants, DER can be better managed and regulated, giving the load side of the power grid great control potential.

[0022] In the context of large-scale access of DER to the distribution network, especially the access of a high proportion of distributed photovoltaics, the safe and stable operation of the distribution network will be challenged. By utilizing various types of DER connected to the distribution network, the ability of the distribution network to accept distributed photovoltaics can be improved, the problem of voltage exceeding the limit at the end of the distribution network caused by the large-scale access of distributed photovoltaics can be solved, and the safety of the distribution network operation can be improved.

[0023] There are two ways to adjust the voltage of energy storage. One is direct control, which controls the energy storage to charge during the period when the voltage of the distribution network is higher than the upper limit, and discharge during the period when the voltage is lower than the lower limit. The other is to consider the power market and use time-of-use electricity prices to guide the energy storage to charge and discharge. In the period when the voltage is higher than the upper limit, a lower electricity price is set to guide the energy storage to charge, and in the period when the voltage is lower than the lower limit, a higher electricity price is set to guide the energy storage to discharge. The distribution of the power flow of the distribution network is adjusted by charging and discharging the energy storage, reducing the active power reversed by the distribution network during the period of large photovoltaic power generation and the forward transmission power during the peak load period, thereby improving the voltage quality of the distribution network.

[0024] For transferable loads, it is impossible to control them directly and they can only be guided through time-of-use electricity prices. Similarly, lower electricity prices are set during periods of high photovoltaic power generation. Transferable loads transfer part of the load from peak electricity price periods to low electricity price periods, which is equivalent to increasing the load during low electricity price periods and increasing the absorption of photovoltaic power. It also achieves the reduction of active power backfeed and alleviates voltage over-limit.

[0025] The present invention provides a voltage regulation method after the distributed energy is connected to the distribution network. When the connection of distributed photovoltaics causes the voltage of the distribution network to exceed the limit, if a DER with regulation capability is connected to the distribution network, the DER in the distribution network can be used to adjust the voltage to ensure the safe operation of the distribution network and prevent the occurrence of abandoned light. Considering the situation of connecting distributed photovoltaics, energy storage and transferable loads to the distribution network, the photovoltaic inverter itself has a certain reactive power regulation capability, and the energy storage and transferable loads can change the distribution of the active power of the distribution network, and both can achieve the regulation of the distribution network voltage.

[0026] The present invention provides a voltage regulation method after the distributed energy is connected to the distribution network. In view of the voltage exceeding the limit caused by the high proportion of distributed photovoltaic connection, the DER in the distribution network is used to regulate the voltage and formulate different voltage regulation strategies. Considering the impact of time-of-use electricity prices, the peak, valley and normal periods are divided according to the net load of the distribution network, and the DER in the distribution network is aggregated into a virtual power plant to participate in the power market to respond to the time-of-use electricity price. The operation of DER in the virtual power plant is guided by the time-of-use electricity price. In this case, the economy of the virtual power plant is not contradictory to the security of the distribution network, that is, the virtual power plant can alleviate the voltage increase of the distribution network by operating at the optimal economy. If the voltage of the distribution network is still exceeding the limit at this time, the distributed photovoltaic inverter is used to regulate the voltage, and an inverter-based voltage regulation model is established, with the goal of minimizing the reactive power absorbed by the inverter. When time-of-use electricity prices are not considered, an economically optimal energy storage and inverter coordinated voltage regulation model is established to coordinate the energy storage charging and the reactive power absorption of the inverter. Compared with the voltage regulation strategies of reactive power first and active power later and active power first and reactive power later, the coordination of active and reactive power is more economical. Since the voltage regulation cost of the inverter is much lower than the voltage regulation cost of energy storage, the reactive power voltage regulation of the inverter is mainly used during voltage regulation. DETAILED DESCRIPTION

[0027] The technical solutions in some embodiments of the present disclosure are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present disclosure, not all of the embodiments. Based on the embodiments provided by the present disclosure, all other embodiments obtained by ordinary technicians in this field belong to the scope of protection of the present disclosure.

[0028] Unless the context requires otherwise, throughout the specification and claims, the term "including" is to be interpreted as an open, inclusive meaning, that is, "including, but not limited to". In the description of the specification, the terms "one embodiment", "some embodiments", "exemplary embodiments", "examples" or "some examples" and the like are intended to indicate that specific features, structures, materials or characteristics associated with the embodiment or example are included in at least one embodiment or example of the present disclosure. The schematic representations of the above terms do not necessarily refer to the same embodiment or example. In addition, specific features, structures, materials or characteristics may be included in any one or more embodiments or examples in any appropriate manner.

[0029] An embodiment of the present invention provides a voltage regulation method after distributed energy is connected to a distribution network, comprising steps: S100-S300.

[0030] S100, dividing the load characteristic curve into peak, valley and normal periods, and using a semi-trapezoidal membership function to calculate the probability of each point on the load curve being in a peak period or a valley period, thereby completing the division into peak, valley and normal periods.

[0031] The above division into peak, valley and normal periods is based on the load characteristic curve, where the highest point in the load curve must be the peak period, so the probability of being in the peak period is 1, and the probability of being in the valley period is 0; similarly, the lowest point in the load curve has a probability of being in the valley period of 1 and a probability of being in the peak period of 0.

[0032] Based on the above principle, the semi-trapezoidal membership function is used to calculate the probability that each point on the load curve is in the peak period or valley period, completing the division of peak, valley and normal periods.

[0033] The minimum and maximum values ​​of the load curve are p n and p x , the load at a certain moment is p t The probability of being in the peak period and valley period is expressed as F(p t ) and G(p t )express:

[0034]

[0035] S200 aggregates multiple DERs into a virtual power plant to participate in the electricity market and respond to time-of-use electricity prices.

[0036] There are many DERs in the distribution network but their capacity is small, so they cannot participate in the electricity market individually. Therefore, multiple DERs are aggregated into virtual power plants to participate in the electricity market and respond to time-of-use electricity prices.

[0037] The virtual power plant constructed above includes distributed photovoltaics, battery energy storage and transferable loads in the distribution network, and the virtual power plant is used to realize the comprehensive utilization of the three voltage regulation resources.

[0038] After a high proportion of distributed photovoltaics is connected, there is a serious flow reverse transmission during the period when the voltage of the distribution network exceeds the limit. At this time, the net load of the distribution network is negative and the absolute value is large. This period must be in the valley period in the division of peak, valley and normal periods, and the corresponding electricity price is the lowest. When the virtual power plant participates in the power market and operates with the best economic efficiency under the time-of-use electricity price, the battery storage is charged in the valley period, and part of the load of the transferable load in the peak period is transferred to the valley period. Both of these situations can reduce the reverse active power and alleviate the increase in the distribution network voltage. Therefore, while the virtual power plant operates with the best economic efficiency, it also plays a role in regulating the voltage. In this case, the security of the distribution network and the economy of the virtual power plant are not contradictory.

[0039] In some embodiments, step S200 further includes step: S210.

[0040] S210, when the virtual power plant operates at the most economical level, the voltage of the distribution network is analyzed. If the voltage of the distribution network still exceeds the limit at certain moments, the power factor of the distributed photovoltaic inverter is adjusted to adjust the voltage, and a distributed photovoltaic inverter voltage regulation model is established.

[0041] The objective function of the voltage regulation model is to minimize the reactive power absorbed by the inverter, which is specifically expressed as:

[0042]

[0043] Where n NPV is the number of inverters involved in reactive voltage regulation, is the reactive power absorbed by the inverter at node i at time t.

[0044] Constraints:

[0045] The power flow constraint is as follows:

[0046]

[0047] Where: P i and Q i is the active and reactive power injection amount of node i, V i and V j are the voltages at nodes i and j, G ij , B ij and δij are the conductance, susceptance and voltage phase angle between nodes i and j respectively.

[0048] The line transmission power constraint is as follows:

[0049] S l,t ≤S l,max (5)

[0050] Where: S l,t is the transmission power of line l at time t, S l,max The maximum power transmitted by the line.

[0051] The node voltage constraint is as follows:

[0052] V min ≤V i,t ≤V max (6)

[0053] Where: For medium voltage distribution network, V max =1.07, V min =0.93.

[0054] The inverter power constraint is as follows:

[0055]

[0056] When the electricity market or the division of peak, valley and normal periods is not taken into consideration, the operation of energy storage and transferable loads cannot take into account both the security of the distribution network and the economy of the virtual power plant. The voltage regulation strategy based on time-of-use electricity prices cannot be used. At this time, if time-of-use electricity prices are not taken into consideration, transferable loads will not be able to participate in the voltage regulation of the distribution network.

[0057] There are adjustment costs when the inverter and energy storage participate in voltage regulation. Inverter regulation is not limited by the number of adjustments and can be adjusted multiple times a day. The inverter reactive regulation cost is mainly affected by the investment cost and control cycle of the inverter, which is about 0.067 yuan / kvarh; the cost of energy storage regulation is limited by the life of the energy storage equipment. The number of charge and discharge cycles is limited and frequent charge and discharge will shorten the life of the energy storage equipment, which greatly increases the active voltage regulation cost. In addition, the active regulation cost is also affected by the energy storage investment cost and SOC value. The active regulation cost is about 0.6 to 1.0 yuan / kWh. It can be seen that the active regulation cost is much greater than the reactive regulation cost, which is about 9 to 15 times the reactive regulation cost.

[0058] S300, when time-of-use electricity prices are not considered, the distribution network performs voltage regulation by directly adjusting the energy storage charging and discharging and the power factor of the distributed photovoltaic inverter. The two methods are active voltage regulation and reactive voltage regulation of the distribution network. A coordinated voltage regulation model of energy storage and inverter is constructed. The specific expression is as follows:

[0059]

[0060] In the formula, c ess and c pv are the regulation costs of energy storage and inverter respectively.

[0061] The constraints include: power flow constraint as shown in equation (4), line transmission power constraint as shown in equation (5), node voltage constraint as shown in equation (6), inverter power constraint as shown in equation (7), and energy storage constraint as shown in the following equation:

[0062]

[0063] Where: is the SOC value of energy storage at time t, and are the upper and lower limits of SOC, S ess is the total capacity of energy storage, η is the charging and discharging efficiency of the energy storage system, is the charge and discharge power at time t, For charging status, In the discharge state, and are the maximum and minimum charging and discharging power of the energy storage system respectively.

[0064] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Other modifications or equivalent substitutions made to the technical solution of the present invention by ordinary technicians in the field should be included in the scope of the claims of the present invention as long as they do not depart from the spirit and scope of the technical solution of the present invention.

Claims

1. A voltage regulation method after the access of distributed energy to a distribution network, characterized in that: The following steps are involved: The peak, valley and normal periods are divided according to the load characteristic curve, and the probability of each point on the load curve being in the peak period or valley period is calculated using a semi-trapezoidal membership function to complete the division of the peak, valley and normal periods; Aggregate multiple DERs into virtual power plants to participate in the electricity market and respond to time-of-use electricity prices; When time-of-use electricity prices are not considered, the distribution network performs voltage regulation by directly adjusting the energy storage charging and discharging and the power factor of the distributed photovoltaic inverter, and constructs a coordinated voltage regulation model of energy storage and inverter.

2. The voltage regulation method after the distributed energy source is connected to the distribution network according to claim 1, characterized in that: The minimum and maximum values ​​of the load curve are p n and p x , the load at a certain moment is p t The probability of being in the peak period and valley period is expressed as F(p t ) and G(p t )express:

3. The voltage regulation method after the distributed energy source is connected to the distribution network according to claim 1, characterized in that: In the step of aggregating multiple DERs into a virtual power plant to participate in the power market in response to time-of-use electricity prices, the following steps are also included: When the virtual power plant operates at the most economical level, the voltage condition of the distribution network is analyzed. If the voltage of the distribution network still exceeds the limit at certain moments, the power factor of the distributed photovoltaic inverter is adjusted to regulate the voltage, and a distributed photovoltaic inverter voltage regulation model is established.

4. The voltage regulation method after the distributed energy source is connected to the distribution network as claimed in claim 3, characterized in that: The objective function of the voltage regulation model is to minimize the reactive power absorbed by the inverter, which is specifically expressed as: Where n NPV is the number of inverters involved in reactive voltage regulation, is the reactive power absorbed by the inverter at node i at time t.

5. The voltage regulation method after the distributed energy source is connected to the distribution network according to claim 1, characterized in that: The energy storage and inverter coordinated voltage regulation model is specifically expressed as follows: In the formula, c ess and c pv are the regulation costs of energy storage and inverter respectively.

Images