自發(fā)自用，余量上網(wǎng)”是指光伏發(fā)出來的電，優(yōu)先給負(fù)載使用，負(fù)載用不完，再發(fā)到電網(wǎng)。初次接觸光伏的朋友，會有很多疑問：光伏發(fā)出來的電，怎么能保證優(yōu)先給負(fù)載使用，而不是光伏電送入電網(wǎng)，負(fù)載從電網(wǎng)取電？有什么依據(jù)能說明負(fù)載使用的電是光伏電，而不是電網(wǎng)送過來的電？光伏發(fā)電和市電頻繁切換，會不會影響逆變器或者設(shè)備？

1.同一時刻電流只有一個方向
      從電路原理上，電流都是從電壓高的地方流向電壓低的地方，在同一個時刻，電流的方向是唯一，就是說，電流在同一個點不能同時既流出又流入。我們以用戶側(cè)電表為節(jié)點，在同一時刻，電流只有一個方向，要么是光伏電流向電網(wǎng)，要么是電網(wǎng)的電流向負(fù)載。所以不存在同一時刻光伏電送入電網(wǎng)，負(fù)載從電網(wǎng)取電的情況。
2.光伏發(fā)電為什么能優(yōu)先使用？
      光伏發(fā)電是一種電源，它可以輸出電能，而且只能輸出電能，而電網(wǎng)是一種特殊的電源，它既可以向負(fù)載提供電能，也可以作為負(fù)載接收電能，根據(jù)電流都是從電壓高的地方流向電壓低的地方這一原理，當(dāng)光伏發(fā)電時，從負(fù)載上看，并網(wǎng)逆變器的電壓始終比電網(wǎng)的電壓要高一點，因此負(fù)載優(yōu)先使用光伏發(fā)電，只有當(dāng)光伏的功率小于負(fù)載功率后，并網(wǎng)點的電壓才會下降，電網(wǎng)才會向負(fù)載供電。分布式光伏，自發(fā)自用，余量上網(wǎng)，一般要裝兩塊電表，逆變器出來裝一塊電表，記錄光伏發(fā)電，用戶側(cè)并網(wǎng)點裝一塊雙向電表，用來記錄光伏向電網(wǎng)送電以及用戶向電網(wǎng)買電。
      并網(wǎng)饋電系統(tǒng)中，市電主要用于構(gòu)建電網(wǎng)的電壓/頻率/相位。并網(wǎng)逆變器不輸出電壓，而是跟蹤電網(wǎng)相位和波形，只向電網(wǎng)輸出電流，因為光伏逆變器的電壓比電網(wǎng)電壓要高，根據(jù)電路原理，電流是從電壓高的地方流向電壓低的地方，所以只要光伏能發(fā)電，就一定會先送到負(fù)載端。
      從負(fù)載角度上，負(fù)載消耗電流，是從離自己最近的電流源獲取電流。以屋頂系統(tǒng)為例，并網(wǎng)逆變器都是在市電變壓器之后，當(dāng)然是并網(wǎng)逆變器饋電，首先被利用。
3.光伏發(fā)電和市電頻繁切換，會不會影響逆變器或者設(shè)備？
      由于光照不穩(wěn)定，時大時光，光伏發(fā)電的功率也不穩(wěn)定，因此負(fù)載用的電，有時可能是光伏電，有時可能是市電，有時可能是光伏電和市電同時供應(yīng)，其實這是表面上的。從理論上，用戶使用的電都是電網(wǎng)的電，因為逆變器有一個功能，能夠把組件發(fā)出來的電，變成和電網(wǎng)完全相同的電，具有相同的電壓，相同的頻率，相同的相位。這個切換過程實際上也是不存的。
4.用戶怎么區(qū)分正在使用的電，是太陽能光伏電還是電網(wǎng)電？
      從電能質(zhì)量的角度上看，用戶完全分辯不出自己用的電到底來自哪里。其實，也沒有必要去區(qū)分到底是用哪一種電。如下圖所示，并網(wǎng)點安裝在變電房，并網(wǎng)點安裝雙向電表，電流有兩個方向，可以計量光伏發(fā)出來的電，負(fù)載用了多少，上網(wǎng)送出多少。但廠房1到3，電流只有一個方向，就不能單獨計量哪一個廠房光伏用了多少電，電網(wǎng)用了多少電。

Spontaneous self-use, surplus Internet "refers to the photovoltaic electricity, priority to load use, load is not used up, and then sent to the grid.   Friends who are in touch with pv for the first time will have a lot of questions: how can we ensure that the electricity generated by PV is preferentially used by the load, rather than the photovoltaic power sent to the grid, and the load receives electricity from the grid?  What is the basis that the load is using photovoltaic power rather than electricity from the grid?  Will frequent switching between photovoltaic power generation and mains affect the inverter or equipment?  

1. The current has only one direction at a time  

In electrical principle, the current flows from a high voltage to a low voltage. At the same time, the direction of the current is unique. That is to say, the current cannot flow out and in at the same point at the same time.   We take the user-side electricity meter as the node, and at the same time, the current only has one direction, either the photovoltaic electricity flows to the grid, or the electricity flows to the load of the grid.  Therefore, there is no photovoltaic power into the grid at the same time, the load from the grid.  

2. Why can PHOTOVOLTAIC power generation be used preferentially?  

Photovoltaic power generation is a kind of power, it can output power, and can only output power, is a special kind of power supply and power grid, it can provide power to the load, also can be used as a load to receive power, according to the current from high voltage low voltage to this principle, when photovoltaic, look from the load,   The voltage of the grid-connected inverter is always higher than that of the grid, so the load preferentially uses photovoltaic power generation. Only when the photovoltaic power is less than the load power, the voltage of the grid-connected node will drop, and the grid will supply power to the load.  Distributed photovoltaic, spontaneous self-use, surplus Internet access, generally to install two meters, the inverter out to install a meter, record photovoltaic power generation, user side and dot installed a two-way meter, used to record photovoltaic power to the grid and users to buy electricity from the grid.  

In the grid-connected feeder system, the mains is mainly used to construct the voltage/frequency/phase of the grid.  The grid-connected inverter does not output voltage, but tracks the grid phase and waveform, only output current to the grid, because the voltage of the photovoltaic inverter is higher than the grid voltage, according to the circuit principle, the current flows from the high voltage to the low voltage, so as long as the photovoltaic power generation, it will be sent to the load end first.  

From the point of view of the load, the load consumes current and obtains current from the current source nearest to it.  Take the roof system as an example, the grid-connected inverter is after the mains transformer, of course, the grid-connected inverter feed, the first to be used.  

3. Will frequent switching between photovoltaic power generation and mains affect the inverter or equipment?  

Because the illumination is not stable, the power of photovoltaic power generation is not stable in large time, so the electricity used for load may be photovoltaic power, sometimes it may be municipal power, and sometimes it may be photovoltaic power and municipal power supply at the same time. In fact, this is superficial.   In theory, all the electricity the user uses is grid electricity, because the inverter has a function to convert the electricity from the components into exactly the same electricity as the grid, with the same voltage, the same frequency, and the same phase.  This switching process is also virtually nonexistent.  

4. How do users distinguish between the electricity being used, solar photovoltaic or grid?  

In terms of power quality, users can't tell where their electricity is coming from.  In fact, there is no need to distinguish which kind of electricity is used.  As shown in the figure below, the connection point is installed in the substation, and the connection point is installed with a bidirectional meter. The current has two directions, which can measure the electricity sent by the photovoltaic, how much the load is used and how much the Internet is sent.  However, from plant 1 to 3, the current is only in one direction, so we cannot measure separately how much electricity is used by the photovoltaic power plant and how much electricity is used by the grid.