Hybrid energy storage control method based on low-pass charge-discharge coefficient constraint
Technical Field
The invention relates to the technical field of electric power, in particular to a hybrid energy storage control method based on low-pass charge-discharge coefficient constraint.
Background
Photovoltaic power generation is one of important components for realizing energy and electric power and sustainable development strategy in China. However, photovoltaic power generation has very strong volatility and randomness, so that great interference is caused to the stability of a new energy system, in order to more efficiently utilize renewable energy, the photovoltaic system is often used in cooperation with an energy storage device, and a light storage complementary system can improve the continuity, stability and reliability of the direct-current bus voltage of the system, so that the photovoltaic effect is maximized.
However, the common storage battery is often low in energy density and high power density, frequent charging and discharging of the storage battery are caused by frequent fluctuation of a direct current bus in a photovoltaic system, even the service life of the storage battery is seriously influenced by quick charging and quick discharging, and the system cost is increased; however, the super capacitor has small energy density and large power density, especially the super capacitor is developed rapidly at present, the number of times of the super capacitor can be charged and discharged circularly far exceeds that of the storage battery, and the super capacitor and the storage battery can form very good complementation, so that in the hybrid energy storage system, the super capacitor bears high-frequency power fluctuation, the storage battery bears low-frequency power fluctuation, the service life of the storage battery can be greatly prolonged, and the stability of the direct current bus is increased. For this reason, in the hybrid energy storage system, a hybrid energy storage control method is required.
Disclosure of Invention
The invention aims to provide a hybrid energy storage control method based on low-pass charge-discharge coefficient constraint, which can smooth the power fluctuation of a storage battery when the storage battery has different electric quantities, prolong the service life of the storage battery, better exert the electrical characteristics of the storage battery and be beneficial to maintaining the voltage stability of a direct-current bus.
A hybrid energy storage control method based on low-pass charge-discharge coefficient constraint is used for a hybrid energy storage system consisting of a super capacitor and a storage battery, a charge low-pass time coefficient lambda and a discharge low-pass time coefficient tau which can change according to the residual capacity SOC of the storage battery are respectively set, curve fitting is carried out on the change of the residual capacity SOC of the storage battery, the change curve and the change range of the charge low-pass time coefficient lambda and the discharge low-pass time coefficient tau are respectively obtained through conversion according to the fitted SOC curves, the charge-discharge power of the storage battery is smoothed according to the charge low-pass time coefficient lambda and the discharge low-pass time coefficient tau, and the power distribution of the storage battery and the super capacitor is shown as the following formula:
psc_ref=pdc-pbat_ref
Wherein p isdcIs the total power of the DC bus, pbat_refIs the reference power of the battery, psc_refS represents the laplacian operator for the reference power of the super capacitor.
The curve fitting of the residual battery SOC comprises primary fitting and multiple fitting, and the primary fitting curve is as follows:
the multi-fit curve is:
wherein, Vcur、Vmax、VminRespectively representing the current voltage, the maximum voltage and the minimum voltage of the storage battery; SOCcur、SOCmaxRespectively representing the current residual electric quantity and the maximum capacity of the storage battery; SOCn-1、SOCnDividing the current residual capacity SOC value of the storage battery into a previous moment and a current moment, wherein n is more than or equal to 1;
according to the primary fitting curve and the multiple fitting curves, respectively converting the variation curves of the charging low-pass time coefficient lambda and the discharging low-pass time coefficient tau according to the following formula:
wherein, taucur,τmax,τminRespectively representing a current discharge low-pass time coefficient, a maximum discharge low-pass time coefficient and a minimum discharge low-pass time coefficient; lambda [ alpha ]cur,λmax,λminRespectively the current charging low-pass time coefficient and the maximumA charge low-pass time coefficient and a minimum charge low-pass time coefficient.
The maximum discharge low-pass time coefficient taumaxAnd minimum discharge low-pass time coefficient tauminThe calculation method comprises the following steps:
Iout_max=k×SOCmax
when the battery is fully charged, there are:
SOC when the battery is lowshortageIn time, there are:
therein, SOCmaxFor the maximum capacity of the battery, k is a user-defined discharge multiple of the battery, typically taken as: k is more than 0 and less than or equal to 3, Iout_maxThe maximum current of the storage battery; SOCshortageFor a set quantity of electricity, V, during the power shortage of the accumulatorbat_shortageCorresponding SOC for the storage batteryshortageVoltage value of pdc_max、pbat_max、Vbat_maxThe maximum output power of the system, the maximum output power of the battery and the maximum voltage of the battery are respectively, C is the maximum value of SOC calibrated when the storage battery leaves a factory, and S represents a Laplace operator.
The maximum discharge low-pass time coefficient lambdamaxAnd a minimum discharge low-pass time coefficient lambdaminThe calculation method of (2) is as follows:
SOC when the battery is lowshortageAt first, there is a maximum charging current Iin_maxAnd a minimum charge low-pass time coefficient lambdamin:
Iin_max=k×SOCmax/10
When storing electricityWhen the battery capacity is sufficient, there is a minimum charging current Iin_minAnd maximum discharge low-pass time coefficient lambdamax:
Iin_min=k×SOCmax/200
Therein, SOCshortageFor setting the quantity of electricity, SOC, during the power shortage of the accumulatorenoughThe set electric quantity when the storage battery is fully charged generally means that more than 80% of the electric quantity is fully charged and less than 20% of the electric quantity is insufficient, namely 0 < SOCshortage<0.2SOCmax<SOCenough<0.8SOCmax;Vbat_shortage、VenoughThe voltages are corresponding to the short-circuit and full-electricity of the storage battery respectively; i isin_max、Iin_minMaximum and minimum charging currents, p, of the accumulatordc_maxIs the system maximum output power.
Compared with the prior art, the method of the invention can better take into account the electrical property change of the storage battery when the electric quantity allowance is different, can more smooth the power fluctuation of the storage battery in the hybrid energy storage system, and prolong the service life of the storage battery; the low-pass coefficient of the invention is more beneficial to maintaining the voltage stability of the direct current bus.
Drawings
FIG. 1 is a system block diagram of a hybrid energy storage system of the method of the present invention.
FIG. 2 is a primary V-SOC curve of the method of the present invention.
FIG. 3 is a multiple V-SOC curve of the method of the present invention.
FIG. 4 is a first fit V-time coefficient curve of the method of the present invention.
FIG. 5 is a multiple fit V-time coefficient curve of the method of the present invention.
Detailed Description
According to the hybrid energy storage control method, the charging low-pass time coefficient and the discharging low-pass time coefficient are designed, so that the electric performance of the storage battery in different capacities of SOC is fully utilized while the power of the storage battery is fully smoothed. As shown in fig. 1, the microgrid system of the method of the present invention includes a distributed power supply and a hybrid energy storage system, where the hybrid energy storage system includes a storage battery and a super capacitor; the distributed power source refers to photovoltaic power generation.
In the control method, the low-pass processing is carried out on the power of the direct current bus to be used as a power reference value of the storage battery, the difference value of the power of the direct current bus and the power of the storage battery is used as a power reference value of the super capacitor, namely:
psc_ref=pdc-pbat_ref (2)
in the formulae (1) and (2), pdc、pbat_ref、psc_refThe total power of the direct current bus, the reference power of the storage battery and the reference power of the super capacitor are respectively; tau and lambda are respectively a discharging low-pass time coefficient and a charging low-pass time coefficient; s represents the laplacian operator.
The discharge current of the storage battery is related to the capacity, the maximum discharge current of the storage battery can reach 4-5C according to experience, the safe discharge current of the storage battery is within 2C, and C refers to the nominal capacity of the storage battery. Maximum discharge current I of accumulatorout_maxThe calculation method of (2) is as follows:
Iout_max=k×SOCmax (3)
where k is the discharge rate of the battery, in order to protect the battery, it is usually: k is more than 0 and less than or equal to 3.
When the storage battery is fully charged, the discharge capacity is strongest, the discharge low-pass time coefficient is minimum, and the minimum discharge low-pass time coefficient tau is obtained at the momentminCalculated as follows (4):
in the formulae (3) and (4), pdc_max,pbat_max,Vbat_maxRespectively, the maximum output power of the system and the maximum output power of the batteryOutput power and battery maximum voltage, k is the user defined maximum discharge multiple, SOCmaxC is the maximum SOC value calibrated at the time of shipment of the storage battery, but the SOC is usually the maximum SOC value due to the aging of the storage batterymax≤C。
When the remaining battery capacity is insufficient, the discharge capacity is the weakest and the resistance is worse, so that the discharge low-pass time coefficient is the largest. Defining the SOC of the accumulator in a certain residual capacityshortageThe maximum charging low-pass time coefficient is belowmaxCalculated as follows (5):
v in formula (5)bat_shortageAfter the primary or multiple fitting is carried out on the storage battery, the storage battery corresponds to the SOCshortageVoltage value of (1), usually 0 < SOCshortage≤0.2SOCmax. Equation (5) illustrates the SOC when the remaining battery capacity ratio is setshortage=0.2SOCmaxAt a lower time, τmaxThe maximum value is kept constant.
When the electric quantity of the storage battery is insufficient, the storage battery can bear the maximum charging current and has the minimum charging low-pass time coefficient, so that the maximum charging current Iin_maxAnd a minimum charge low-pass time coefficient lambdaminCalculated according to the following equations (6) and (7), respectively:
Iin_max=k×SOCmax/10 (6)
the battery has minimal resistance to charging current when fully charged, and therefore the charging current should be minimal and smooth, with a maximum discharge low-pass time factor λmaxLet the current at this time:
Iin_min=k×SOCmax/200 (8)
at this time, the resistance of the storage battery to the charging current is the weakest, and the storage battery has the maximum discharging low-pass timeCoefficient lambdamax:
In the above equation (9), the remaining capacity SOC of the battery is definedenoughMaximum charging low-pass time coefficient lambda of above timemaxIs kept constant at maximum, Vbat_enoughIs fitted to the storage battery for the first time or multiple times and then corresponds to the SOCenoughVoltage value of (2), wherein 0.8SOCmax≤SOCenough≤SOCmax。
The primary fitting curve of the residual capacity SOC of the storage battery is calculated according to the formula (10):
wherein, Vcur,Vmax,VminRespectively representing the current voltage, the maximum voltage and the minimum voltage of the storage battery; SOCcur,SOCmaxRespectively the current capacity and the maximum capacity of the storage battery; the fitted curve of SOC obtained according to equation (10) is shown in FIG. 2, which provides a fast and simple SOC estimation method for unknown batteries, and the SOC is uniformly distributed according to the voltage.
Calculating a multi-fitting curve of the residual capacity SOC of the storage battery according to the formula (11):
wherein the SOCn、SOCn-1Respectively representing the battery residual quantity at the current moment and the battery residual quantity at the previous moment, wherein n is more than or equal to 1. As shown in fig. 3, which is the SOC fitting curve obtained according to equation (11), it can be seen that the curve is nonlinear, and the fitting method will approach the true SOC value more and more as the number of charging and discharging increases.
Respectively calculating the current discharging low-pass time coefficient tau according to the SOC curve of the storage battery obtained by the formula (11)curAnd a current charging low-pass time coefficient lambdacur:
τcur,τmax,τminRespectively representing a current discharge low-pass time coefficient, a maximum discharge low-pass time coefficient and a minimum discharge low-pass time coefficient; lambda [ alpha ]cur,λmax,λminRespectively a current charging low-pass time coefficient, a maximum charging low-pass time coefficient and a minimum charging low-pass time coefficient.
As shown in fig. 4, the values of the time constants according to equations (12) and (13) are changed with the change of the SOC, and the value of the low-pass time constant for time charging and discharging can be quickly determined according to the curve as long as the SOC is determined. FIG. 4 shows a time constant-V curve which is fitted for the first time, the time constant is linearly and uniformly distributed according to the voltage, different voltages correspond to different time constants, the time constant can be rapidly determined when an unknown storage battery is contacted for the first time, and a low-pass control strategy is determined. FIG. 5 shows a time constant-V curve which is fit for many times, the time constant is distributed in a curve, and low-pass coefficients can be reasonably used according to data recorded by the curve, so that the performance of the storage battery can be better exerted on the basis of smoothing the power of the storage battery.