自发自用，余量上网”是指光伏发出来的电，优先给负载使用，负载用不完，再发到电网。初次接触光伏的朋友，会有很多疑问：光伏发出来的电，怎么能保证优先给负载使用，而不是光伏电送入电网，负载从电网取电？有什么依据能说明负载使用的电是光伏电，而不是电网送过来的电？光伏发电和市电频繁切换，会不会影响逆变器或者设备？ 

从电路原理上，电流都是从电压高的地方流向电压低的地方，在同一个时刻，电流的方向是唯一，就是说，电流在同一个点不能同时既流出又流入。我们以用户侧电表为节点，在同一时刻，电流只有一个方向，要么是光伏电流向电网，要么是电网的电流向负载。所以不存在同一时刻光伏电送入电网，负载从电网取电的情况。

光伏发电是一种电源，它可以输出电能，而且只能输出电能，而电网是一种特殊的电源，它既可以向负载提供电能，也可以作为负载接收电能，根据电流都是从电压高的地方流向电压低的地方这一原理，当光伏发电时，从负载上看，并网逆变器的电压始终比电网的电压要高一点，因此负载优先使用光伏发电，只有当光伏的功率小于负载功率后，并网点的电压才会下降，电网才会向负载供电。分布式光伏，自发自用，余量上网，一般要装两块电表，逆变器出来装一块电表，记录光伏发电，用户侧并网点装一块双向电表，用来记录光伏向电网送电以及用户向电网买电。

并网馈电系统中，市电主要用于构建电网的电压/频率/相位。并网逆变器不输出电压，而是跟踪电网相位和波形，只向电网输出电流，因为光伏逆变器的电压比电网电压要高，根据电路原理，电流是从电压高的地方流向电压低的地方，所以只要光伏能发电，就一定会先送到负载端。

从负载角度上，负载消耗电流，是从离自己最近的电流源获取电流。以屋顶系统为例，并网逆变器都是在市电变压器之后，当然是并网逆变器馈电，首先被利用。

从电能质量的角度上看，用户完全分辩不出自己用的电到底来自哪里。其实，也没有必要去区分到底是用哪一种电。如下图所示，并网点安装在变电房，并网点安装双向电表，电流有两个方向，可以计量光伏发出来的电，负载用了多少，上网送出多少。但厂房1到3，电流只有一个方向，就不能单独计量哪一个厂房光伏用了多少电，电网用了多少电。 

Tagraíonn "féin-úsáid, ceangal breise gréasáin" don chumhacht a ghineann an photóoltaic, a thugtar tosaíocht don lucht, agus ansin a tharchuirtear chuig an ngréasán cumhachta nuair nach n-úsáidtear an lucht suas. Beidh go leor ceisteanna ag cairde a thagann i dteagmháil le PV ar dtús: conas is féidir an leictreachas a ghineann PV a ráthú go n-úsáidfear an t-ualach ar dtús, in ionad iad a sheoladh chuig an ngréasán agus an t-ualach a fhaigheann leictreachas ón ngréasán? What is the basis for explaining that the electricity used by the load is photovoltaic electricity, rather than the electricity sent from the grid? An ndéanfaidh athrú minic idir ghiniúint cumhachta photóoltaic agus príomhchumhacht difear don inbhéirsteoir nó don trealamh?
1. Níl ach treo amháin reatha ann san am céanna
Ó phrionsabal an chiorcóid, sruthaíonn an sruth ón áit a bhfuil ardvoltas aige go dtí an áit a bhfuil ísealvoltas aige. Ag an am céanna, tá treo an sruth uathúil, is é sin nach féidir an sruth sruthú amach agus isteach ag an am céanna ag an bpointe céanna. We take the electricity meter on the user's side as the node. At the same time, the current has only one direction, either the photovoltaic current to the grid or the grid current to the load. Dá bhrí sin, níl aon staid ann go seolfar cumhacht photóvoltaach chuig an ngréasán cumhachta ag an am céanna agus go nglacfaidh an lucht cumhacht ón ngréasán cumhachta.
2. Why can photovoltaic power generation be used preferentially?
Photovoltaic power generation is a kind of power supply, which can output electric energy, and can only output electric energy. The power grid is a special power supply, which can not only provide electric energy to the load, but also receive electric energy as a load. According to the principle that the current flows from the place with high voltage to the place with low voltage, when photovoltaic power generation, from the perspective of load, the voltage of grid connected inverter is always a little higher than that of the power grid, Therefore, the load gives priority to photovoltaic power generation. Only when the photovoltaic power is less than the load power, the voltage of the grid point will drop and the grid will supply power to the load. Distributed PV, for self use and surplus grid connection, is generally equipped with two meters, one meter is installed outside the inverter to record photovoltaic power generation, and a two-way meter is installed at the user side and at the grid point to record photovoltaic power transmission to the grid and users' power purchase from the grid.
In the grid connected feed system, the mains power is mainly used to construct the voltage / frequency / phase of the power grid. The grid connected inverter does not output voltage, but tracks the grid phase and waveform, and only outputs current to the grid. Because the voltage of the PV inverter is higher than the grid voltage, according to the circuit principle, the current flows from the place with high voltage to the place with low voltage. Therefore, as long as the PV can generate power, it will be sent to the load end first.
Ó pheirspictíocht an luchta, tomhaltar an lucht reatha, a fhaightear ón fhoinse reatha is gaire. Taking the roof system as an example, the grid connected inverter is fed by the grid connected inverter after the mains transformer, which is used first.
3. An ndéanfaidh athrú minic idir an ghiniúint cumhachta fótáilteach agus an príomhchumhacht difear don inbhéirsteoir nó don trealamh?
As the light is unstable, the power of photovoltaic power generation is also unstable at large times. Therefore, sometimes the power used by the load may be photovoltaic power, sometimes it may be municipal power, sometimes it may be photovoltaic power and municipal power supplied at the same time. In fact, this is superficial. Theoretically, the electricity used by users is the electricity of the grid, because the inverter has a function to turn the electricity generated by components into the same electricity as the grid, with the same voltage, the same frequency and the same phase. Níl an próiseas athraithe seo ann i ndáiríre.
4. Cén chaoi a dhéanann úsáideoirí idirdhealú ar an leictreachas atá á n-úsáid acu, ar leictreachas photovoltaic ghréasáin nó ar leictreachas gréasáin?
Ó pheirspictíocht cáilíochta cumhachta, ní féidir le húsáideoirí a insint óna dtiocfaidh a leictreachas. I ndáiríre, ní gá idirdhealú a dhéanamh ar an gcineál leictreachais a úsáidtear. Mar a thaispeántar sa fhigiúr thíos, suiteálfar an pointe gréasáin chomhthreomhar san fho- shuíomh, agus suiteálfar an pointe gréasáin chomhthreomhar le méadar leictreachais dhá shlí. Tá dhá threoir ag an seoladh reatha, ar féidir an leictreachas a ghineann an photóoltaic a thomhas, cé mhéad ualach a úsáidtear agus cé mhéad leictreach Mar sin féin, i gcás gléasraí 1 go 3, níl treo amháin ag an reatha ach amháin, dá bhrí sin ní féidir tomhaltas cumhachta fótáilteach a thomhas ar leithligh ag an gléasra agus tomhaltas cumhachta an ghréasáin.
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