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目前并网光伏发电对电网电能质量的影响和解决方案 -----yl23455永利

2023-11-15

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1、并网光伏发电对电网电能质量的影响

光伏发电作为新兴能源发电的一种,光照、温度等外部条件随机性、波动性、间歇性的变化是光伏发电对电网产生影响的主要因素。其中DC-AC逆变器是并网光伏发电系统主要的器件之一,光伏逆变器质量的优劣在一定程度上决定光伏发电的电能质量能否达到并网要求。光伏发电并网运行时会产生谐波、电压波动和闪变、直流注入、孤岛效应等问题,使电网电能质量下降,对电网造成不利影响,严重时会于扰供用电系统及光伏发电设备自身的安全稳定运行。

 

(1)直流注入问题

并网光伏发电系统中另一亟待解决的关键问题:直流注入。直流注入影响了电网电能质量,同时也给电网中的其他设备带来不利影响。IEEEStd929-2000与IEEEStd547-2000明确规定,并网发电装置向电网注入的直流电流分量不能超过装置额定电流的0.5%。直流注入产生的主要原因有:1)电力电子器件自身分散性及驱动电路不一致不对称等;2)大功率控制器内部测量器件存在的零点漂移和非线性;3)各开关器件线路阻抗的不对称,寄生参数和寄生电磁场的影响等。

目前抑制直流注入的主要方法包括:1)检测补偿法;2)优化设计逆变器并网结构;3)电容隔直;4)虚拟电容法;5)装置隔离变压器等。

 

(2)孤岛效应的影响

孤岛效应是指由于人为因素或自然因素造成电网中断供电,但各个并网光伏发电系统没能及时检测出电网停电状态,从而光伏发电系统与其相连的负载仍独立运行的现象。随着并网光伏发电接入渗透率的不断扩大,孤岛效应发生的几率也逐渐增加。

孤岛效应的形成对整个配电网电能质量造成不利影响,主要包括:

1)当配电网切换至孤岛方式,仅仅依靠光伏发电系统供应电能,若该供电系统容量太小或未安装储能装置,均有可能造成用户负荷出现电压不稳定和闪变问题。

2)光伏发电系统出现孤岛效应之后,如果原供电模式为单相供电模式,有可能使配电网发生三相负荷不对称的问题,进而降低其余用户的用电整体质量。

3)在供电恢复过程中,由于电压相位之间不同步将会产生浪涌电流,有可能导致电网波形瞬间下跌。

4)在孤岛效应发生位置,其电压和频率波动性较大,降低了电能质量,且孤岛中的电压和频率不受电网控制,可能会造成系统电气设备损坏和重合闸故障等,同时可能会对电网维修人员造成个人安全隐患。

 

(3)针对孤岛效应产生的影响,主要有以下解决方法:

1)优化并网光伏发电系统孤岛检测方法,分析光伏发电对配电网故障电流大小、方向及分布的影响,提高故障情况下负荷切除速度和孤岛划分的选择技术。

2)提高孤岛检测技术的可靠性,配置快速有效的反孤岛保护功能,在异常情况下准确判断孤岛状态并迅速有效中断并网。

 

(4)针对光伏接入后的电能质量问题,提出抑制谐波的有效方法:

1)从谐波产生的源头入手,对谐波源进行改造,减少谐波注入。

2)装置有源或无源滤波器,以吸收某些特定次数的谐波电流。

3)装设附加的谐波补偿装置。

 

(5)电压波动和闪变

在传统配电网中,有功功率、无功功率随时间变化才会引起系统电压波动。而对于光伏发电而言,光伏发电系统有功功率的变化是引起接入点电压波动和闪变的主要因素。光伏发电系统核心部件光伏电池板的大功率点与辐照强度、天气、季节、温度等因素密切相关,这些自然因素的随机变化引起输出功率变化较大,致使负载功率在一定范围内变化频繁,从而引起并网用户负载端电压波动和闪变。

目前针对光伏电压波动和闪变问题的解决方法主要有:1)优化光伏并网逆变器控制策略,提高电压的稳定性;2)加大变电站母线短路容量;3)在光伏电站容量确定的情况下,提高其功率因数,以增加有功功率总量,从而降低无功功率变化量,满足电压波动的限值要求。

 

(6)谐波影响

光伏发电是通过光伏组件将太阳能转化为直流电,再经过并网型逆变器将直流电变换成交流电实现并网。在光伏发电系统中,逆变器是产生谐波的主要设备。并网逆变器内部电力电子元件的大量应用,提升了系统的信息化和智能化处理,但也增加了大量的非线性负载,造成波形失真,给系统带来大量谐波。逆变器开关切换速度的延迟,也会影响电网系统内部整体动态性能的输出,产生小范围的谐波。如果在天气(辐照度、温度)变化较大的情况下,谐波的波动范围也会随之变大。尽管单台并网逆变器输出电流谐波较小,但是多台并网逆变器并联后输出电流的谐波会产生叠加,从而形成输出电流谐波超标现象。此外,逆变器并联容易产生并联谐振,进而导致耦合谐振现象,造成特定次并网谐波电流扩大,产生并网电流谐波含量超标问题。

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