Operation control technology of source network load storage area based on flexible application of energy storage

This paper studies the operation control technology of source-network-load-storage area. Firstly, the flexible application mode of energy storage in the source-network-load-storage area is analyzed. Secondly, the definitions of absolute mileage and relative mileage are proposed, and the power coordination control model of energy storage units based on mileage balance is established to achieve the purpose of efficient utilization of energy storage resources. Finally, the operation control strategy of the source network load storage area is proposed, which verifies the effectiveness and practicability of the power coordination control model of the energy storage unit and the operation control strategy of the source network load storage area.


Introduction
With the maturity and commercialization of distributed new energy and energy storage technology, as well as the gradual deepening of power system reform, the construction and operation of the integration of source network load and storage has become a new system form of energy and power system.Under the bidirectional uncertainty of distributed new energy and power load, how to use flexible energy storage to improve the consumption level of distributed new energy and the safe and reliable power consumption level of users, and how to coordinate the complementary and optimal operation of distributed new energy, power grid, load and energy storage, as well as the different types of energy storage units in the energy storage system have become the core issues of increasing concern in the field of energy storage optimization.
A large number of research achievements have been made at home and abroad on the coordinated operation and control of source network load and storage.The Literature [1] aims at the complex diversity and random uncertainty caused by the access of new energy, a coordinated optimization control strategy of "source network load storage and charging" considering multiple operation scenarios was proposed, and a coordinated control architecture adapted to multiple operation scenarios was constructed.Reference [2] aiming at the problem that the regional frequency modulation control deviation and voltage fundamental frequency fluctuation of power system are greatly affected by repeated playing, a power system cooperative control method based on source network load storage optimization is proposed, and a power system cooperative control model is built to realize the horizontal complementarity of multiple sources and the vertical coordination of source network load storage.Literature [3] proposed a distributed state aware source network load storage cooperative scheduling mechanism under the power Internet of Things, defined the response strategy of each end of the source network load storage, and achieved the goal of reducing peak load and valley load and reducing scheduling loss.Literature [4] proposes a coordinated optimization strategy of network load storage system considering price response mechanism, and constructs a coordination optimization framework by introducing agent system and electricity price mechanism.Literature [5] has established the coordination and control architecture of the new energy system on the basis of fully considering the system functions such as monitoring, forecasting and dispatching management and the communication structure of the system, and has established the coordination and optimization model of the source, network, load and storage with the goal of maximizing the comprehensive benefits of the source side, network side and load side.However, the above achievements do not comprehensively consider the various operation control problems of the source network load storage system.
In terms of coordinated control of multiple types of energy storage, Han Xiaojuan et al. used empirical mode decomposition algorithm to suppress wind power fluctuations, adjusted the filter order k2 according to the SOC(State Of Charge) of batteries and supercapacitors, so as to achieve power distribution between hybrid energy storage, but did not have universality for other stabilization strategies [6].It is an effective method to adopt filtering control strategy for coordinated control of multiple energy storage systems [7][8][9][10].Shi Xiaohan et al. used a sliding average filter to separate the low-frequency components in the power command and then assigned them to the battery, and assigned the remaining components to the superconducting magnet energy storage [7].Duncan et al. used a low-pass filter to determine the power range required by super capacitors and energy storage batteries to suppress fluctuations, giving full play to the characteristics of the two types of energy storage [8].Tummuru et al. proposed a smooth control method based on low-pass filtering algorithm to achieve power distribution, which can effectively manage the energy stored [9].Ren Yongfeng et al. used a high pass filter to filter the mixed energy storage.The high frequency component was distributed to the supercapacitor, and the low frequency component was distributed to the vanadium flow battery [10].The above filter based energy storage coordinated control algorithm does not provide a theoretical basis for setting the initial value of the filter time constant and the adjustment step.Sun Yushu et al. proposed a multi type energy storage coordinated control strategy based on the model predictive control algorithm and Hilbert Huang transform [11].Hredzak et al. proposed a model predictive control system of energy storage battery supercapacitor multi power supply.The controller distributes fast current changes to supercapacitors, while the battery mainly responds to slow current changes.This method helps to extend battery life [12].However, the above research results need to further consider the energy storage life, so as to improve the energy storage efficiency and extend the overall life of the energy storage system.
Micro grid system control is generally divided into centralized control and decentralized control.In order to make the "plug and play" control of distributed generation in microgrid more flexible, the multi-agent based consistency control algorithm is widely used in the decentralized control strategy of microgrid.For the different subjects involved in the source, network and load coordination, based on the concept of system coordination multi-agent, the researchers introduced multi-agent and multi-layer price response mechanism in the literature to build a multi multi-level agent coordination optimization framework to solve the autonomous coordination problem of distributed micro grid distribution.For example, according to the source and load operation characteristics of direct grid connection or indirect grid connection, handle multi-level coordination problems between different levels and different entities.
The existing studies have fully analyzed the impact of energy storage on the power grid, but few have fully described and demonstrated the overall role of energy storage on the "source network load storage area".In addition, there is still a lack of research on mileage balance between internal components of energy storage.This paper refers to the different strategies adopted by energy storage to meet the multiple demands of "source network load storage area".The key innovation lies in the full mining of energy storage components and the equalization algorithm of component charging and discharging mileage.In this way, energy storage can improve its service life and shorten its maintenance cycle while meeting many demands of the power grid.
2. Flexible utilization mode of energy storage in load storage area of source network 2.1.Power and electricity balance 2.1.1.Energy storage to stabilize power fluctuation.During the operation of the distribution network or microgrid system, if the load changes suddenly and the system frequency changes, the power on the generation side will follow the regulation to maintain the power balance of the system.In order to maintain the stability of the system power during the regulation process, the energy storage system is added to the system, which can quickly provide the power shortage, thus stabilizing the dynamic process.The schematic diagram of energy storage to suppress power fluctuation is shown in Figure 1.The vertical axis in the figure represents the system power, the power below the power standard line Pdr represents the power that the energy storage system needs to absorb, and the power above the power standard line represents the power that the energy storage system needs to release.The whole dynamic process until the curve is stable is called the dynamic power shortage adjustment process caused by a sudden load change.Step load response diagram of a domestic energy storage power station.

Regulation response characteristics of energy storage system.
With the random fluctuation of load and distributed new energy power, the energy storage system should follow the fluctuation at a high speed to maintain the balance of active and reactive power of the system.Taking the step load regulation command received by a domestic energy storage power station as an example, as shown in Figure 2, reduce the power reference amplitude to the rated capacity of the energy storage power station.
It can be seen from Figure 2 that the energy storage network side converter can follow the change of AGC signal in real time and respond in time.The maximum response time of the converter on the side of the energy storage network to the step response can reach about 100ms.What really restricts the power response rate of the energy storage system is the response rate of the energy storage machine side (source side), such as the chemical side response rate of electrochemical energy storage, the flywheel motor response rate of flywheel energy storage, and the thermal dynamic system response rate of compressed air energy storage.

Frequency and voltage operation support
In recent years, the rapid development of distributed new energy has put forward higher requirements for the power grid's frequency and voltage regulation capabilities, bringing new difficulties and challenges [13,14].As a large number of power sources are distributed in the load concentrated area, the frequency and voltage operation support problem of the regional power grid, which is lack of sufficient frequency and voltage regulation power, is more prominent.

Power frequency modulation.
The output of renewable energy has randomness and volatility, and the two-way uncertainty of source load makes the problem of power system frequency stability more prominent.The fast power response capability of energy storage makes it have natural frequency modulation characteristics.Moreover, since energy storage can operate as power source or load in the power grid, it has the regulation capacity twice of its own capacity [15].
On the generation side, automatic generation control (AGC) auxiliary service is a kind of power auxiliary service mode with higher technical difficulty in paid auxiliary service.It is a service that the generator unit tracks the power dispatching instructions within its allowable output regulation range, and adjusts the power generation output in real time according to a certain regulation rate, as is shown in Figure 3, so as to meet the power system frequency and tie line power control requirements.As a fast frequency regulation resource, energy storage has greatly improved the auxiliary service capacity of AGC and the operation efficiency of traditional units through the planning and operation measures of energy storage on the generation side in many countries, which has a high economy and helps to improve the power grid's ability to absorb clean energy..

Dynamic voltage regulation.
When the load storage system of the source network operates synchronously with the large power grid, the control of the energy storage system on the voltage can be divided into two levels.The first level is to respond to the reactive power output command signal issued by the dispatching, and use PQ mode to control the output of reactive power; On the other hand, the system itself is equivalent to a load or power supply with a constant power factor according to the actual demand of the power grid.At this time, the PQ mode is used for energy storage to comprehensively adjust the active and reactive power output by the energy storage system to maintain a constant power factor.
When the energy storage system is configured in the isolated network system, it needs to make higher requirements on the response speed of the active and reactive power of the energy storage to meet the frequent fluctuation of the active and reactive load, so as to maintain the frequency and voltage stability of the isolated network system.When a small system is isolated, the energy storage system can dominate and maintain the voltage and frequency of the system in a short time, and can also jointly maintain the system voltage stability with other distributed resources.When there is only one voltage control point of the energy storage system, the converter of the energy storage system operates in VF mode, and makes reactive power response independently according to the system voltage; when multiple distributed resources jointly maintain voltage stability, energy storage can work in PQ mode or VdcQ mode to generate reference value of reactive power output with reactive voltage droop curve.

Operation standby and black start
Due to the unique bidirectional power transmission characteristics of the energy storage power station, the large capacity energy storage power station can be well used as the reserve capacity of the power grid to participate in the operation.The power grid is equipped with a certain energy storage capacity to generate electricity when peak load is required to meet the power demand and achieve the balance between power production and power consumption in the power system.At present, the state encourages the allocation of electric energy storage facilities in centralized new energy power generation bases and participation in peak shaving auxiliary services.Electric energy storage facilities above 10MW are subject to unified dispatching by power dispatching agencies.Energy storage facilities built in power plants can participate in peak shaving jointly with power plants or as independent entities [16].
In addition, according to the latest research and practice results, energy storage can also be used as the black start power supply of the power grid.In the case of major system failure or system wide power outage, restart the generator sets without self starting capability without the support of the power grid, gradually expand the scope of system recovery, and finally achieve the recovery of the entire system.Due to the complete power loss of the power grid, it is difficult for photovoltaic and wind turbine units to lock the phase of the voltage of the isolated grid system, which is difficult to be used as an independent self starting power source.It is necessary to cooperate with the synchronous unit type power generation units with fast energy storage, large capacity energy storage and supporting self synchronous control devices to conduct self synchronous control.

Coordinated control of energy storage power based on mileage equalization
When multiple energy storage units are included in the source network load storage system, after a period of energy storage operation, the utilization efficiency may be reduced and the maintenance cost may be increased due to different maintenance times of energy storage.In order to fill the defects of the traditional energy storage management mode on the imperfect life mileage management, improve the energy storage utilization efficiency, extend the overall life of the energy storage system, and improve the operation stability of the energy storage system, this paper proposes a power coordination control model of the energy storage unit based on mileage equalization.

Absolute mileage and relative mileage of energy storage
First, two kinds of mileage indicators of the energy storage unit are defined: the historical charging and discharging mileage of the energy storage unit, and the relative charging and discharging mileage.
The absolute charging and discharging mileage W of the energy storage unit refers to the total energy consumed or emitted during the charging and discharging process of the energy storage unit from its operation to the current time, which is a cumulative constant positive variable, as shown in Figure 4(a).
The relative charging and discharging mileage w of the energy storage unit refers to the quotient of the product of the total energy consumed or emitted by the energy storage unit during the charging and discharging process from its commissioning to the current time, the expected life of the energy storage unit and the maximum charging and discharging power.It is a variable between 0 and 1, representing the current actual life and capacity utilization of the energy storage unit, as shown in Figure 4

Coordinated control of energy storage power based on mileage equalization
Considering the situation of large-scale collaborative charging and discharging of energy storage systems with different capacities and models, the system power coordination control strategy is designed for the scenario where the energy storage system responds to fixed charging and discharging power commands.The overall idea of coordinated control of energy storage power based on mileage equalization is to unify the charge state and historical mileage of each energy storage unit as much as possible during charging and discharging.The strategy flow chart is shown in Figure 5.
It is assumed that n energy storage units of different types (with different maximum charging and discharging power, maximum capacity, expected life and initial state) are connected in parallel on the same bus.During the operation of the energy storage system, the reference value of output power of each energy storage unit is calculated in real time according to the SOC and relative charging and discharging mileage of each energy storage unit.The specific coordination strategy is designed as follows.
Assume that the total output (input) power required by the energy storage system is T 1 ~Tn , the expected life of each energy storage unit is, the maximum charging and discharging power is P 1 ~Pn , and the maximum capacity is C 1 ~Cn .The system needs to calculate the following variables of each energy storage unit in real time: According to the current charge C xi and the maximum capacity C i , the current state of charge of the ith energy storage unit is calculated as (1) Define and calculate the historical charging and discharging mileage of the ith energy storage unit as (2) Where , tnow represents the current time, Pci represents the value of charging power at a time in history, and Pdisi represents the value of discharging power at a time in history.Mileage means that the total energy consumed or emitted by the charging and discharging process of the energy storage unit from its operation to the present is a cumulative variable.The discrete approximate calculation method with fixed time interval can be used in practical calculation.The unit is MWh or MJ.
Define and calculate the relative charging and discharging mileage of the ith energy storage unit as The relative charge discharge mileage means the relative value of mileage consumption of the energy storage unit and capacity life of the energy storage unit, which obviously .The average value of the relative charging and discharging mileage of n energy storage units is , and the standard deviation is .
Assuming that the power coordination control command of the n energy storage units is , the power coordination logic is described below.
For the energy storage unit i, if it belongs to one of the following two cases, let : a) When discharging, , that is, the current charge of the energy storage unit is less than or equal to the minimum discharge capacity of the energy storage unit; during charging, , that is, the current charge of the energy storage unit is greater than or equal to the maximum charge of the energy storage unit.b) (According to the Laida criterion, the mileage deviates significantly upwards).Repeat the above process for n energy storage units.Suppose that the power command value of m energy storage units is not 0 after such processing, that is, n-m energy storage units are removed, and the remaining energy storage units are renumbered as 1~m, which is called distributable energy storage units.
Select the values of k, p and q as required (the initial value is k=1.7,p=0.8, q=1), and adopt the following coordination strategy to achieve the goal of balancing the relative mileage of SOC and energy storage unit.In case of charging mode, replace with .Among them, and are the allowable minimum and maximum state of charge of the jth energy storage unit (that is, the upper and lower limit requirements for the energy storage unit in normal operation). (4)
is the maximum relative mileage among m distributable energy storage units.K is the upper limit coefficient.If k>1, the smaller the k value, the more decentralized the distributed power of energy storage units at different mileage, and the faster the equalization speed.In addition, item represents the consideration of the current state of charge factor of the energy storage unit.
This method firstly eliminates the energy storage units with large relative mileage, and realizes the power coordination strategy according to the state of charge and historical mileage.The coordination strategy will be more inclined to allocate a larger power proportion to energy storage units with larger residual capacity and smaller relative mileage.The value of p and q will determine whether the current balance is more inclined to SOC balance or historical mileage balance.It not only allows the energy storage to maintain the longest power output with the most reasonable power command, but also gives consideration to the balance of the service life of the energy storage unit through the concept of mileage, thus maintaining the equality of the maintenance cycle of the energy storage unit.
In the power coordination control strategy of energy storage unit based on mileage equalization, the historical charging and discharging mileage and relative charging and discharging mileage of energy storage unit, to a certain extent, fill the lack of consideration of historical data in the process of energy storage operation optimization.The defined historical charging and discharging mileage of the energy storage unit is a consideration of the historical charging and discharging conditions of the energy storage unit, which is not only related to the operation time, but also related to the charging and discharging times and power; The defined relative charge discharge mileage is the relative value of the current design life of the energy storage unit and the maximum charge discharge power, which represents the current actual power life consumption of the energy storage unit itself.

Operation control strategy of source network load storage area
Based on the flexible application mode of energy storage in Section 2 and the coordinated control method of multi type energy storage power in Section 3, the operation control strategy of the source network load storage area is proposed as shown in Table 1.A simulation example is built using PSCAD/EMTDC based on the preliminary design scheme of a domestic source network load storage area multi energy complementary integration optimization demonstration project (hereinafter referred to as "the park").The system topology is shown in Figure 6.

4.2.2.
Example A: Make up the system power deficit in a short time.In the topology of the park, the energy storage system is connected to the 10kV bus of the gas turbine to make up for the power shortage of the gas turbine.As shown in Figure 7, the output command given by AGC to the gas turbine is 10.8MW.During the 2-3s period, the output of the gas turbine fluctuates for a short time.At this time, the energy storage detects the power shortage and puts it into operation for charging discharge cycle to maintain the overall output of the unit stable.This situation is also applicable to the supplement of low frequency power fluctuation deficiency caused by weather conditions (dark clouds, etc.) of photovoltaic units.
The response delay of the energy storage system is estimated to be about 50ms.

Example B: System spontaneous frequency regulation (primary frequency modulation).
Simulate the primary frequency modulation performance of the synchronizer [17], and formulate the primary frequency modulation control curve including the frequency dead zone for the energy storage system as shown in Figure 8.The charging and discharging power curve of the energy storage system under frequency fluctuation is shown in Figure 9.The DC bus voltage is collected through the information acquisition device.The centralized controller is planned to be located at the coordination control layer.The voltage outer loop control in the special controller is used to generate the reference current value iref of the current inner loop.Press a respectively through the transmission line × iref, b × iref, c × The iref (satisfying a+b+c=1) is distributed to three energy storage units as the reference current for current loop control.Finally, the DC side voltage is stabilized and the power coordination optimization is realized.The current loop controls the energy storage grid connected converter, which can effectively and indirectly control the output power of the energy storage system.The control block diagram is shown in Figure 10.
The energy storage converter operates in PQ mode (P=1.5MW), and the unit side adopts centralized control mode.The initial coefficients are a=0.5, b=0.3, c=0.2;At 2s, change the parameters to make a=0.3, b=0.6, c=0.1.The system response is shown in Figure 11.

Example D: Mileage of multiple energy storage components.
Taking the 600MW/2h centralized energy storage system in the load storage area of the source network as an example, using MATLAB, the power distribution mode of equal SOC proportion considering the historical charging and discharging mileage is adopted.Assuming that the centralized energy storage has five components, the initial SOC capacity ratio is 1.2:1.1:1:0.9:0.8, and the initial mileage is 5000:4000:3000:2000:1000 (unit: MW • min).
As is shown in Figure 12

Conclusions
This paper relies on the typical domestic source network load storage area, and considers a variety of flexible application modes of energy storage in the source network load storage area, establishes the energy storage unit power coordination control strategy based on energy storage mileage balance, which to some extent fills the lack of use of historical data in the traditional energy storage operation optimization process.Based on the power coordination control strategy of the energy storage unit, the operation control strategy of the load storage area of the source network is proposed.Based on a domestic source network load storage area, the simulation analysis of operation control strategy is carried out.The results show that the energy storage unit power coordination control model and the source network load storage area operation control strategy proposed in this paper are effective in short-term supplement of system power shortage and primary frequency modulation, and the power among multiple energy storage units is coordinated, which is helpful for the optimal utilization of the energy storage system.The next step we will continue to focus on energy storage to complete the "integration of source, grid, load and storage", focusing on the load demand.By optimizing and integrating the local power supply side, grid side, and load side resource elements, supported by energy storage and other advanced technology and institutional mechanism innovation, and with the goal of safety, green, and efficiency, we will innovate the power production and consumption mode, explore the development path for building a new generation of power system with a high degree of integration of source, grid, load Depth coordination of storage.

Figure 1 .
Figure 1.Energy storage to stabilize power fluctuation.

Figure 2 .
Figure 2. Step load response diagram of a domestic energy storage power station.

Figure 3 .
Figure 3. Dispatching AGC command power and energy storage system output power. (b).

Figure 4 .
Figure 4. Schematic diagram of absolute mileage and relative mileage.

Figure 5 .
Figure 5. Flow chart of power coordinated control of energy storage units based on mileage balance.

Figure 10 .
Figure 10.Storage module power distribution control system.

Figure 11 .
Figure 11.Power distribution curve of energy storage module, Unit: MW.
and Figure13, the minute power curve, SOC change of components are shown in the figure below.It can be seen that the equal SOC proportional distribution strategy charging and discharging mileage, compared with the simple equal SOC proportional distribution strategy, retains the advantages of improving the consistency of SOC, and at the same time, by considering the charging and discharging mileage of energy storage, through the secondary adjustment of power, avoids further widening the charging and discharging mileage gap between energy storage components, thus improving the consistency of life between components.

Figure 12 .
Figure 12.Minute power curve of components (considering equal SOC proportion distribution of historical charging and discharging mileage).

Figure 13 .
Figure 13.Minute level SOC curve of components (considering equal SOC proportion distribution of historical charging and discharging mileage).

Table 1 .
Operation control strategy of the area.System topology of a domestic source-network-load-storage area.