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配电网的接线方式大多是辐射状链式接线,或采用环状接线,开环运行的状态。
以1条辐射状配电网馈线为研究对象,不考虑其与其他馈线间的联络关系,并进行适当简化后如图1所示。本文主要关注较长时间尺度的配电网馈线网损和电压偏差等稳态特征,因此,为便于分析,配电负荷采用侧重于综合负荷随接入电压呈一次关系的静态负荷模型,即恒电流模型,以体现负荷功率随电压的变化关系,且配电负荷沿线均匀分布,沿线电流连续变化。将一天分为高峰时段、低谷时段和平时时段,并假定高峰时段和低谷时段时间长度相同(均为$T$),高峰时段馈线始端电流为${I_1}$,末端电流为0,低谷时段馈线始端电流为${I_2}$,末端电流为0。则高峰时段沿线电流分布为:
图 1 负荷均匀分布的辐射状配电网馈线电流分布
Figure 1. Feeder current distribution of a radial distribution network with even load distribution
$$ {i_1}\left( x \right) = {I_1}\left( {1 - x} \right) $$ (1) 式中:
x ——距馈线始端的距离与馈线总长度之比,本文假定馈线总长度为1,则$x \in \left( {0,1} \right)$;
${i_1}\left( x \right)$ ——高峰时段沿线电流分布。
若馈线单位长度电阻为$R$,则高峰时段在$x$处的小馈线段$\Delta x$内线损功率为$i_1^2\left( x \right)R\Delta x$,可以得到高峰时段线损功率${P_1}$为:
$$ {P_1} = 3\int_0^1 {i_1^2\left( x \right)} R{\text{d}}x = I_1^2R $$ (2) 由式(2)可知,高峰时段线损功率取决于馈线始端电流和馈线总电阻(当馈线总长度为1时,单位长度电阻$R$即为馈线总电阻),等效为馈线始端电流在馈线总电阻上产生的损耗。
同样地,低谷时段沿线电流分布${i_2}\left( x \right)$和线损功率${P_2}$分别如式(3)、式(4)所示,式(4)中$R$仍代表馈线总长度为1时的馈线总电阻。
$$ {i_2}\left( x \right) = {I_2}\left( {1 - x} \right) $$ (3) $$ {P_2} = 3\int_0^1 {i_2^2\left( x \right)} R{\text{d}}x = I_2^2R $$ (4) -
在距馈线始端${x_1}$处接入储能系统(Battery Energy Storage System,BESS),储能系统也采用恒电流模型,分别在高峰时段、低谷时段以恒定电流进行削峰填谷。对于实际配网侧储能系统,在平时时段仍可能进行充电或放电,以缩短项目回收期或实现其他优化目标,本文仅考虑在高峰时段、低谷时段进行充放电的情况。配电网中常采用就地无功补偿,在线路中传输的无功较少,即可认为沿馈线负荷的功率因数较高,接入配电网的储能设备一般不参与AVC(自动电压控制,Automatic Voltage Control)调节,所配置的无功补偿装置通常用于补偿其自身的无功损耗,因此,可令储能系统运行于单位功率因数输出状态,或其输出功率因数与负荷功率因数基本相同。若储能系统在高峰时段提供电流为${I_{\text{B}}}$(${I_{\text{B}}}$>0),则高峰时段储能系统接入后,馈线电流分布如图2及式(5)所示。
图 2 储能系统接入后辐射状配电网馈线电流分布(放电)
Figure 2. Feeder current distribution of a radial distribution network when battery energy storage system is connected (discharge)
$$ {\tilde i_1}\left( x \right) = \left\{ {\begin{array}{*{20}{l}} {{I_1}\left( {1 - x} \right) - {I_{\text{B}}}}&{}&{0 < x \leqslant {x_1}} \\ {{I_1}\left( {1 - x} \right)}&{}&{{x_1} < x < 1} \end{array}} \right. $$ (5) 式中:
${\tilde i_1}\left( x \right)$——高峰时段储能系统接入后沿线电流分布;
${x_1}$ ——储能系统接入点距馈线始端的距离与馈线总长度之比。
从式(5)可以看到,$x \in \left( {0,{x_1}} \right)$时,令${\tilde i_1}\left( x \right)$=0,得到${x_0} = {{\left( {{I_1} - {I_{\text{B}}}} \right)} \mathord{\left/ {\vphantom {{\left( {{I_1} - {I_{\text{B}}}} \right)} {{I_1}}}} \right. } {{I_1}}}$,当储能系统注入电流${I_{\text{B}}}$不变时,若接入位置${x_1} < {x_0}$,则配电网馈线潮流方向不发生变化,由于配电网电压损耗主要由电压纵分量决定,沿线有功和无功需求均会增加电压降落纵分量,因此沿馈线潮流方向电压逐渐降低,馈线末端电流、电压达到极小值;若接入位置${x_1} > {x_0}$,则在$\left( {{x_0},{x_1}} \right)$区间内配电网馈线潮流方向将发生改变,沿馈线潮流方向电压也将出现波动,电流、电压极小值可能位于馈线段内。高峰时段在馈线${x_1}$处接入储能系统,且储能系统注入电流为${I_{\rm{B}}}$时的线损功率${\tilde P_1}$为:
$$ {\tilde P_1} = 3R\left[ {{I_1}{I_{\text{B}}}x_1^2 - {I_{\text{B}}}\left( {2{I_1} - {I_{\text{B}}}} \right){x_1} + \dfrac{1}{3}I_1^2} \right] $$ (6) 若储能系统注入电流${I_{\text{B}}}$不变,将线损功率${\tilde P_1}$对${x_1}$求导,令${{{{\text{d}}{{\tilde P}_1}} \mathord{\left/ {\vphantom {{{\text{d}}{{\tilde P}_1}} {{\text{d}}x}}} \right. } {{\text{d}}x}}_1}{\text{ = }}0$,得到:
$$ {x_1} = \dfrac{{2{I_1} - {I_{\text{B}}}}}{{2{I_1}}} $$ (7) 因$ {{{{\text{d}}^2}{{\tilde P}_1}} \mathord{\left/ {\vphantom {{{{\text{d}}^2}{{\tilde P}_1}} {{\text{d}}x_1^2}}} \right. } {{\text{d}}x_1^2}} = 6R{I_1}{I_{\text{B}}} > 0 $,则储能系统接入位置${x_1}$满足式(7)时,取得线损功率的极小值。此外,从式(7)中可知,${I_{\text{B}}}$越小,${x_1}$越接近馈线末端,馈线网损功率越小,随${I_{\text{B}}}$逐渐增大,取得线损功率极小值时${x_1}$将逐渐靠近馈线始端。
由式(6)可知,在高峰时段馈线始端电流${I_1}$、馈线单位长度电阻$R$一定时,线损功率${\tilde P_1}$与储能系统注入电流${I_{\rm{B}}}$、储能系统接入位置${x_1}$均有关,且${\tilde P_1}$随储能系统注入电流${I_{\rm{B}}}$、储能系统接入位置${x_1}$的变化均不是单调,因此,在储能系统注入电流和储能系统接入位置的可取值范围内,可能存在极值点或最值点,使得网损功率最小。
进而,令
$$ \left\{ {\begin{array}{*{20}{c}} {{{\partial {{\tilde P}_1}} \mathord{\left/ {\vphantom {{\partial {{\tilde P}_1}} {\partial {I_{\text{B}}}}}} \right. } {\partial {I_{\text{B}}}}} = 3x_1^2 - 3{x_1}\left( {2 - 2{I_{\text{B}}}} \right) = 0} \\ {{{\partial {{\tilde P}_1}} \mathord{\left/ {\vphantom {{\partial {{\tilde P}_1}} {\partial {x_1}}}} \right. } {\partial {x_1}}} = 6{I_{\text{B}}}{x_1} - 3{I_{\text{B}}}\left( {2 - {I_{\text{B}}}} \right) = 0} \end{array}} \right. $$ (8) 解得:${I_{\rm{B}}}{\text{ = }}{2 \mathord{\left/ {\vphantom {2 3}} \right. } 3}$ pu、${x_1}{\text{ = }}{2 \mathord{\left/ {\vphantom {2 3}} \right. } 3}$。
同时考虑到${{{\partial ^2}{{\tilde P}_1}} \mathord{\left/ {\vphantom {{{\partial ^2}{{\tilde P}_1}} {\partial I_{\text{B}}^2}}} \right. } {\partial I_{\text{B}}^2}} > 0$、${{{\partial ^2}{{\tilde P}_1}}/{\partial x_1^2}} > 0$,可知当${I_B}{\text{ = }}{2 \mathord{\left/ {\vphantom {2 3}} \right. } 3}$ pu、${x_1}{\text{ = }}{2 \mathord{\left/ {\vphantom {2 3}} \right. } 3}$时,网损功率${\tilde P_1}$取得极小值,通过与边界点的比较可知,其为最小值,$\min \left( {{{\tilde P}_1}} \right)$为0.11 pu。由式(8)解出的另一极值点为${I_{\rm{B}}}{\text{ = 0}}$、${x_1}{\text{ = 0}}$,此时网损功率${\tilde P_1}$为1 pu,不是全局最值点。可见,若${I_1}$=1 pu、$R$=1 pu,储能系统注入电流${I_{\rm{B}}}{\text{ = }}{2 \mathord{\left/ {\vphantom {2 3}} \right. } 3}$ pu,接入位置距馈线始端2/3处时网损功率取得极小值。
应该指出,单位长度电阻$R$取不同值时不影响网损功率取得最小值时的储能注入电流和接入位置,仅网损功率与$R$成正比关系。但由式(6)和式(8)可知,馈线始端电流${I_1}$的变化将影响网损功率取得最小值时的储能注入电流${I_{\rm{B}}}$和接入位置${x_1}$,且${I_1}$与${I_{\rm{B}}}$、${x_1}$均呈非线性关系,需根据不同的馈线始端电流${I_1}$进行计算。
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低谷时段,储能系统以电流${I_{\text{B}}}$(${I_{\text{B}}}$>0)进行充电,即储能注入电流为$ - {I_{\text{B}}}$,此时馈线电流分布如图3及式(9)所示。
图 3 储能系统接入后辐射状配电网馈线电流分布(充电)
Figure 3. Feeder current distribution of a radial distribution network when battery energy storage system is connected (charge)
$$ {\tilde i_2}\left( x \right) = \left\{ {\begin{array}{*{20}{l}} {{I_2}\left( {1 - x} \right){\text{ + }}{I_{\text{B}}}}&{}&{0 < x < {x_1}} \\ {{I_2}\left( {1 - x} \right)}&{}&{{x_1} < x < 1} \end{array}} \right. $$ (9) 从式(9)可以看到,$x \in \left( {0,{x_1}} \right)$时,令${\tilde i_2}\left( x \right)$=0,得到${x_0} = {{\left( {{I_2}{\text{ + }}{I_{\text{B}}}} \right)} \mathord{\left/ {\vphantom {{\left( {{I_2}{\text{ + }}{I_{\text{B}}}} \right)} {{I_2}}}} \right. } {{I_2}}} > 1$,与$x$的取值范围矛盾,可知在此种情况下,沿馈线潮流方向不发生变化,电流单调减小,沿馈线潮流方向电压也逐渐降低,馈线末端电流、电压达到极小值。低谷时段在馈线${x_1}$处接入储能系统,且储能系统注入电流为$- {I_{\rm{B}}}$时的线损功率${\tilde P_2}$为:
$$ {\tilde P_2} = 3R\left[ { - {I_2}{I_{\text{B}}}x_1^2 + {I_{\text{B}}}\left( {2{I_2} + {I_{\text{B}}}} \right){x_1} + \dfrac{1}{3}I_2^2} \right] $$ (10) 若储能系统注入电流$- {I_{\rm{B}}}$不变,将线损功率${\tilde P_2}$对${x_1}$求导,令${{{\text{d}}{{\tilde P}_2}} \mathord{\left/ {\vphantom {{{\text{d}}{{\tilde P}_2}} {{\text{d}}{x_1}}}} \right. } {{\text{d}}{x_1}}}{\text{ = }}0$,得到:
$$ {x_1} = \dfrac{{2{I_2}{\text{ + }}{I_{\text{B}}}}}{{2{I_2}}} $$ (11) 因式(7)得到的${x_1}$超出了其取值范围$\left( {0,1} \right)$,考虑到$ {{{{\text{d}}^2}{{\tilde P}_2}} \mathord{\left/ {\vphantom {{{{\text{d}}^2}{{\tilde P}_2}} {{\text{d}}x_1^2}}} \right. } {{\text{d}}x_1^2}} = 6R{I_2}{I_{\text{B}}} < 0 $,则储能系统接入并进行充电时,无论充电电流${I_{\text{B}}}$取何值,线损功率均随接入位置单调增大,接于馈线末端时网损功率最大,即充电电流${I_{\text{B}}}$越大、接入位置${x_1}$越大(越靠近馈线末端),馈线网损功率越大。
当储能系统接入位置为${x_1}$时,一天内的总网损(不考虑平时时段)为:
$$ \begin{array}{l}{\tilde{Q}}_{\text{L}}=\left({\tilde{P}}_{1}+{\tilde{P}}_{2}\right)T =3RT[{I}_{{\rm{B}}}\left({I}_{1}-{I}_{2}\right){x}_{1}^{2}-\\ 2{I}_{\text{B}}\left({I}_{1}-{I}_{2}-{I}_{\text{B}}\right){x}_{1}+\dfrac{1}{3}\left({I}_{1}^{2}+{I}_{2}^{2}\right)]\end{array} $$ (12) 若储能系统注入电流${I_{\text{B}}}$(充电时为$- {I_{\rm{B}}}$)不变,将总网损${\tilde Q_{\text{L}}}$对${x_1}$求导,令$ {{{\text{d}}{{\tilde Q}_{\text{L}}}} \mathord{\left/ {\vphantom {{{\text{d}}{{\tilde Q}_{\text{L}}}} {{\text{d}}{x_1}}}} \right. } {{\text{d}}{x_1}}}{\text{ = }}0 $,得到:
$$ {x_1} = 1 - \dfrac{{{I_{\text{B}}}}}{{{I_1} - {I_2}}} $$ (13) 储能系统未接入时,一天内的总网损(不考虑平时时段)为:
$$ {Q_{\text{L}}} = \left( {{P_1} + {P_2}} \right)T{\text{ = }}RT\left( {I_1^2 + I_2^2} \right) $$ (14) 则由于安装储能系统进行削峰填谷而增加的网损为:
$$ \begin{split} &\Delta Q = {{\tilde Q}_{\text{L}}} - {Q_{\text{L}}}= \\& 3RT\left[ {{I_{\rm{B}}}\left( {{I_1} - {I_2}} \right)x_1^2} \right.\left. { - 2{I_{\text{B}}}\left( {{I_1} - {I_2} - {I_{\text{B}}}} \right){x_1}} \right] \end{split} $$ (15) 将增加的网损$\Delta Q$对${x_1}$求导,令$\Delta Q'{\text{ = }}0$,解得接入位置与式(13)相同。若令$\Delta Q{\text{ = }}0$,则解为0或$2{x_1}$。考虑到${{{{{\text{d}}^2}\Delta Q} \mathord{\left/ {\vphantom {{{{\text{d}}^2}\Delta Q} {{\text{d}}t}}} \right. } {{\text{d}}t}}^2} > 0$,可知对于式(13),当${I_{\text{B}}} > \left( {{I_1} - {I_2}} \right)$时,无论储能系统接于馈线何处,均将导致网损增加,因此,从储能系统接入后降低网损的角度,储能系统充放电电流${I_{\text{B}}}$的范围是:
$$ {I_{\text{B}}} < \left( {{I_1} - {I_2}} \right) $$ (16) 以满足式(16)为前提,若充放电电流${I_{\text{B}}}$较小,则总网损取得最小值时储能接入位置${x_1}$更靠近馈线末端,注入电流${I_{\text{B}}}$越大,则总网损取得最小值时储能接入位置${x_1}$越靠近馈线始端,且在$\left( {0,2{x_1}} \right)$内接入储能系统进行削峰填谷时,均有助于减小网损。此外,将式(13)与式(5)进行比较可知,当按式(13)确定储能系统接入位置时,${x_1}$必然小于由式(5)确定的临界接入位置${x_0}$,不会造成馈线潮流方向发生变化。
当储能系统注入电流${I_{\rm{B}}}$和接入位置${x_1}$满足式(13)所示关系时,由式(15)计算的增加网损$\Delta Q$在储能系统接入位置在$\left( {0,2{x_1}} \right)$区间内时将为负值,即减小了一天内的总网损。
进而,令
$$ \left\{ {\begin{array}{*{20}{c}} {{{\partial \Delta Q} \mathord{\left/ {\vphantom {{\partial \Delta Q} {\partial {I_{\text{B}}}}}} \right. } {\partial {I_{\text{B}}}}} = 2.4x_1^2 - 6{x_1}\left( {0.8 - 2{I_{\text{B}}}} \right) = 0} \\ {{{\partial \Delta Q} \mathord{\left/ {\vphantom {{\partial \Delta Q} {\partial {x_1}}}} \right. } {\partial {x_1}}} = 4.8{I_{\text{B}}}{x_1} - 6{I_{\text{B}}}\left( {0.8 - {I_{\text{B}}}} \right) = 0} \end{array}} \right. $$ (17) 解得${I_{\rm{B}}}{\text{ = }}{4 \mathord{\left/ {\vphantom {4 {15}}} \right. } {15}}$ pu、${x_1}{\text{ = }}{2 \mathord{\left/ {\vphantom {2 3}} \right. } 3}$,同时考虑到${{{\partial ^2}{{\tilde P}_1}}/ {\partial I_{\text{B}}^2}} > 0$、${{{\partial ^2}{{\tilde P}_1}} /{\partial x_1^2}} > 0$,可知当${I_{\rm{B}}}{\text{ = }}{4 \mathord{\left/ {\vphantom {4 {15}}} \right. } {15}}$ pu、${x_1}{\text{ = }}{2 \mathord{\left/ {\vphantom {2 3}} \right. } 3}$时,增加的总网损$\Delta Q$取得最小值,$\min \left( {\Delta Q} \right)$为-0.284 4 pu。由式(17)解出的另一极值点为${I_{\rm{B}}}{\text{ = 0}}$、${x_1}{\text{ = 0}}$,此时$\Delta Q$为0,相当于未接入储能,不是全局最值点。可见,对于一定的高峰时段、低谷时段馈线始端电流,存在最优的储能注入电流${I_{\text{B}}}$和接入位置${x_1}$,使得增加的网损最小(负值),即减小的网损最多。
同样,单位长度电阻$R$取不同值时不影响总网损取得最小值时的储能注入电流和接入位置,仅总网损与$R$成正比关系。但由式(17)可知,馈线始端电流${I_1}$、${I_2}$的变化将影响总网损取得最小值时的储能充放电电流${I_{\rm{B}}}$和接入位置${x_1}$,且与${I_{\rm{B}}}$、${x_1}$均呈非线性关系,需根据不同的馈线始端电流${I_1}$、${I_2}$进行计算。
在实际配电网馈线中,馈线始端电流${I_1}$、${I_2}$随馈线负荷的变化而变动,当以削峰填谷为控制目标时,不可能在任何时刻都实现网损功率最小,此时,需在一定的负荷预测曲线下,通过不同季节典型日曲线或较长时间尺度生产模拟优化的方法求得储能系统最佳接入电流和接入位置,同时考虑储能的经济性,即全寿命周期成本和相关收益。
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本节仍针对不考虑与其他馈线联络关系的单条辐射状配电网馈线进行研究,如图1所示。配电负荷仍采用恒电流模型,且沿线均匀分布,同时,将一天分为高峰时段、低谷时段和平时时段,并假定高峰时段和低谷时段时间长度相同(均为$T$),高峰时段馈线始端电流为${I_1}$,末端电流为0,低谷时段馈线始端电流为${I_2}$,末端电流为0。
本节分析中,仍假定$x$为距馈线始端的距离与馈线总长度之比,馈线总长度为1,则$x \in \left( {0,1} \right)$。正常运行时,配电网馈线两端相角差较小,电压降落的横分量对电压损耗的贡献也较小,故认为电压降落的模即为电压损耗。若单位长度馈线的阻抗的模为$Z$,则在高峰时段,距馈线始端$x$处的小馈线段$\Delta x$内的电压损耗为${I_1}\left( {1 - x} \right)Z\Delta x$。因此,未接入储能系统时,高峰时段距馈线始端$x$处对于送端母线的电压损耗$\Delta {U_1}\left( x \right)$为:
$$ \Delta {U_1}\left( x \right) = \int_0^x {{I_1}\left( {1 - y} \right)Z{\text{d}}y} = {I_1}Z\left( {x - \dfrac{1}{2}{x^2}} \right) $$ (18) 低谷时段距馈线始端$x$处对于送端母线的电压损耗$\Delta {U_2}\left( x \right)$为:
$$ \Delta {U_2}\left( x \right) = {I_2}Z\left( {x - \dfrac{1}{2}{x^2}} \right) $$ (19) -
在距馈线始端${x_1}$处接入储能系统,储能系统也采用恒电流模型,分别在高峰时段、低谷时段以恒定电流进行削峰填谷。储能系统在高峰时段注入电流为${I_{\text{B}}}$,馈线电流分布如式(5)所示,则高峰时段接入储能后,距馈线始端$x$处的电压损耗$\Delta {\tilde U_1}\left( x \right)$为:
$$ \Delta {\tilde U_1}\left( x \right) = \left\{ {\begin{array}{*{20}{c}} {Z\left[ { - {I_{\text{B}}}x + {I_1}\left( {x - \dfrac{1}{2}{x^2}} \right)} \right],0 < x \leqslant {x_1}} \\ {Z\left[ { - {I_{\text{B}}}{x_1} + {I_1}\left( {x - \dfrac{1}{2}{x^2}} \right)} \right],{x_1} < x < 1} \end{array}} \right. $$ (20) 若储能系统注入电流${I_{\text{B}}}$不变,$x \in \left( {0,{x_1}} \right)$时,将电压损耗$\Delta {\tilde U_1}\left( x \right)$对x求导,令$ {{{\text{d}}\Delta {{\tilde U}_1}\left( x \right)} \mathord{\left/ {\vphantom {{{\text{d}}\Delta {{\tilde U}_1}\left( x \right)} {{\text{d}}x}}} \right. } {{\text{d}}x}}{\text{ = }}0 $,得到:
$$ {x_{\text{C}}} = 1 - \frac{{{I_{\text{B}}}}}{{{I_1}}} $$ (21) 考虑到${{{\text{d}}\Delta \tilde U_1^2\left( x \right)} \mathord{\left/ {\vphantom {{{\text{d}}\Delta \tilde U_1^2\left( x \right)} {{\text{d}}{x^2}}}} \right. } {{\text{d}}{x^2}}} < 0$,可知在$x = {x_{\text{C}}}$处,电压损耗达到极大值(不一定是馈线范围内的最大值)。需要说明的是,虽然式(21)中,电压损耗极大值点位置${x_{\text{C}}}$仅与储能注入电流${I_{\text{B}}}$、馈线始端电流${I_1}$有关,但储能接入位置${x_1}$将影响馈线范围是否会出现电压损耗极值点。若储能系统接入位置${x_1} < {x_{\text{C}}}$,即由式(21)确定的${x_{\text{C}}}$不在$\left( {0,{x_1}} \right)$范围内,则电压损耗无极大值点,沿馈线电压损耗单调增大。若${x_1} > {x_{\text{C}}}$,则电压损耗存在极大值点,极大值点位置至储能接入位置为电压损耗恢复段,且当储能接入位置${x_1} = 2\left( {{I_1} - {I_{\rm{B}}}} \right)/ {I_1} = 2{x_{\rm{C}}}$时,电压损耗恢复至0,但储能接入位置不能大于馈线全长,因此,若由式(21)确定的${x_{\text{C}}}$的2倍大于馈线全长时,电压损耗将不会恢复至0。
$x \in \left( {{x_1},1} \right)$时,将电压损耗$\Delta {\tilde U_1}\left( x \right)$对x求导,令$ {{{\text{d}}\Delta {{\tilde U}_1}\left( x \right)} \mathord{\left/ {\vphantom {{{\text{d}}\Delta {{\tilde U}_1}\left( x \right)} {{\text{d}}x}}} \right. } {{\text{d}}x}}{\text{ = }}0 $,得到$x = 1$,且为极大值,即从储能接入位置至馈线末端电压损耗单调增大,馈线末端$x = 1$处取得极大值,但不一定是馈线范围内的最大值,电压损耗最大值和最大值位置取决于储能接入位置和注入电流的大小。
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储能系统在低谷时段充电电流为${I_{\text{B}}}$,则储能注入电流为$ - {I_{\text{B}}}$,馈线电流分布如式(9)所示,则低谷时段接入储能后,距馈线始端x处的电压损耗$\Delta {\tilde U_2}\left( x \right)$为:
$$ \Delta {\tilde U_2}\left( x \right) = \left\{ {\begin{array}{*{20}{c}} {Z\left[ {{I_{\text{B}}}x + {I_2}\left( {x - \dfrac{1}{2}{x^2}} \right)} \right],0 < x \leqslant {x_1}} \\ {Z\left[ {{I_{\text{B}}}{x_1} + {I_2}\left( {x - \dfrac{1}{2}{x^2}} \right)} \right],{x_1} < x < 1} \end{array}} \right. $$ (22) 若储能系统注入电流$ - {I_{\text{B}}}$不变,$x \in \left( {0,{x_1}} \right)$时,将电压损耗$\Delta {\tilde U_2}\left( x \right)$对x求导,令${{{\text{d}}\Delta {{\tilde U}_2}\left( x \right)} \mathord{\left/ {\vphantom {{{\text{d}}\Delta {{\tilde U}_2}\left( x \right)} {{\text{d}}x}}} \right. } {{\text{d}}x}}{\text{ = }}0$,得到${x_{\text{C}}} = {{\left( {{I_2} + {I_{\text{B}}}} \right)} \mathord{\left/ {\vphantom {{\left( {{I_2} + {I_{\text{B}}}} \right)} {{I_2}}}} \right. } {{I_2}}}$,即电压损耗$\Delta {\tilde U_2}\left( x \right)$在$x = {x_{\text{C}}}$处取得极大值,但此时${x_{\text{C}}} > 1$,超出馈线范围,可知在$\left( {0,{x_1}} \right)$沿馈线电压损耗单调增大。
$x \in \left( {{x_1},1} \right)$时,将电压损耗$\Delta {\tilde U_2}\left( x \right)$对$x$求导,令${{{\text{d}}\Delta {{\tilde U}_2}\left( x \right)} \mathord{\left/ {\vphantom {{{\text{d}}\Delta {{\tilde U}_2}\left( x \right)} {{\text{d}}x}}} \right. } {{\text{d}}x}}{\text{ = }}0$,得到$x = 1$,且为极大值,即从馈线接入位置至馈线末端电压损耗单调增大,馈线末端$x = 1$处取得极大值,也是馈线范围内的最大值。
根据文献[1],采用馈线整体电压偏差$D$衡量馈线电压损耗程度。对于如图1所示的负荷均匀分布配电网馈线,高峰时段储能放电时,整体电压偏差为
$$ {\tilde D_1} = \dfrac{{\int_0^1 {\Delta {{\tilde U}_1}\left( x \right){\text{d}}x} }}{{{U_N}}} = \dfrac{{Z\left[ {\left( { - {I_{\text{B}}}{x_1} + \dfrac{1}{3}{I_1}} \right) + \dfrac{1}{2}{I_{\text{B}}}x_1^2} \right]}}{{{U_N}}} $$ (23) 式中:
${U_N}$——馈线电压标称值。
将${\tilde D_1}$对${x_1}$求导,令${{{\text{d}}{{\tilde D}_1}} \mathord{\left/ {\vphantom {{{\text{d}}{{\tilde D}_1}} {{\text{d}}{x_1}}}} \right. } {{\text{d}}{x_1}}}{\text{ = 0}}$,得到${x_1}{\text{ = }}1$,因此,储能放电时,总是布置在馈线末端才可使整体电压偏差取得极小值,同时也是最小值。
低谷时段储能充电时,整体电压偏差为:
$$ {\tilde D_2} = \dfrac{{\int_0^1 {\Delta {{\tilde U}_2}\left( x \right){\text{d}}x} }}{{{U_N}}} = \frac{{Z\left[ {\left( {{I_{\text{B}}}{x_1} + \dfrac{1}{3}{I_2}} \right) - \dfrac{1}{2}{I_{\text{B}}}x_1^2} \right]}}{{{U_N}}} $$ (24) 将${\tilde D_2}$对${x_1}$求导,令${{{\text{d}}{{\tilde D}_2}} \mathord{\left/ {\vphantom {{{\text{d}}{{\tilde D}_2}} {{\text{d}}{x_1}}}} \right. } {{\text{d}}{x_1}}}{\text{ = 0}}$,得到${x_1}{\text{ = }}1$,因此,储能充电时,总是布置在馈线末端才使整体电压偏差取得极大值,同时也是最大值。
忽略平时时段,则总体电压偏差$\tilde D$为:
$$ \tilde D = {\tilde D_1} + {\tilde D_2} = \dfrac{{Z\left( {{I_1} + {I_2}} \right)}}{{3{U_N}}} $$ (25) 可见,在本文所述的参与配电网削峰填谷场景下,储能系统的接入位置、充放电电流并不影响总体电压偏差值$\tilde D$,总体电压偏差主要由配电网馈线参数以及高峰时段、低谷时段馈线始端电流决定。而未接入储能时,高峰时段、低谷时段总体电压偏差$D$为:
$$ D = {D_1} + {D_2} = \dfrac{{Z\left( {{I_1} + {I_2}} \right)}}{{3{U_N}}} $$ (26) 因此,可知$\tilde D = D$,说明无论储能布置于何处,对总体电压损耗指标均无改善,而只能在一定程度上减小最大电压损耗,从而平滑电压损耗,即使得在高峰时段整体电压偏差适当减小,在低谷时段增大整体电压偏差适当增大。
Impact Analysis of Energy Storage Participating in Peak Shaving and Valley Filling for Distribution Network on Network Loss and Voltage Deviation
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摘要:
目的 针对配电网接入储能进行削峰填谷的应用场景开展研究,以明确储能接入对配电网侧网络损耗和电压质量的影响。 方法 分析了高峰时段储能放电时,网损功率随储能注入电流和接入位置的变化趋势,然后分析了低谷时段,储能系统接入并进行充电时,网损功率随储能注入电流和接入位置的变化趋势。对于储能接入配电网进行削峰填谷对配电网电压的影响,分别从高峰时段储能放电和低谷时段储能充电的角度,研究了储能不同注入电流和接入位置对配电网沿线电压分布的影响。最后,考虑配电网总电压偏差指标,研究了配电网接入储能后对总电压偏差指标的影响。 结果 通过仿真计算,分析了储能电流和接入位置取不同固定或连续值时储能参与配电网削峰填谷对网损功率和电压损耗的影响趋势,并与理论分析进行了对比分析。 结论 研究将对各类储能设施在配电网中的协调规划和优化运行提供有益参考,具有较好的工程参考价值。 Abstract:Introduction The application scenarios of peak shaving and valley filling by energy storage connected to the distribution network are studied to clarify the influence of energy storage access on network losses and voltage quality on the distribution network side. Method The paper analyzed the change trend of network loss power with the energy storage injection current and access position during peak hours of energy storage discharge, and then analyzed the change trend of network loss power with the energy storage injection current and access position when the energy storage system was connected and charged during valley hours. For the influence of energy storage connected to the distribution network for peak shaving and valley filling on the voltage of the distribution network, the influence of different energy storage injection currents and access positions on the voltage distribution along the distribution network was studied from the perspective of energy storage discharge during peak hours and energy storage charging during valley hours. Finally, considering the total voltage deviation index of the distribution network, the influence on the total voltage deviation index after the distribution network which was connected to energy storage was studied. Result Through simulation calculations, the influence trend of energy storage participating in peak shaving and valley filling for the distribution network on network loss power and voltage loss is analyzed when different fixed or continuous values of energy storage current and access position are taken, and it is compared with theoretical analysis. Conclusion The study will provide useful references for the coordinated planning and optimized operation of various energy storage facilities in the distribution network, and it has good engineering reference value. -
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