光伏逆变器的原理和选型技巧


一,工作原理和特点:

工作原理:逆变器装置的核心是逆变器开关电路,简称为逆变器电路。该电路通过打开和关闭电源电子开关来完成逆变器的功能。

特性:

(1)要求高效率。

由于目前太阳能电池的价格高,为了最大限度地利用太阳能电池和提高系统效率,有必要提高逆变器的效率。

(2)要求高可靠性。

目前,光伏电站系统主要用于偏远地区,许多电站无人值守和维护。这要求变频器具有合理的电路结构,严格的元件屏蔽,并要求变频器具有各种保护功能,如:输入直流极性反接保护,交流输出短路保护,过热,过载保护等。

(3)输入电压要求具有宽的适应范围。

由于太阳能电池的端电压随负载和太阳光的强度而变化。特别是当电池老化时,其端电压变化很大。例如,对于12V电池,端子电压可以在10V和16V之间变化,这要求逆变器在大的DC输入电压范围内正常工作。

二,光伏逆变器分类

有许多方法可以对逆变器进行分类。例如,根据逆变器输出交流电压的相数,可以分为单相逆变器和三相逆变器。取决于逆变器使用的半导体器件的类型,分为晶体管逆变器,晶闸管逆变器和关断晶闸管逆变器。根据逆变电路的原理,可分为自激振荡型逆变器,步进波叠加型逆变器和脉宽调制型逆变器。根据并网系统或离网系统的应用,可分为并网逆变器和离网逆变器。为了便于光电用户选择逆变器,这里仅对逆变器进行分类。

1.集中式逆变器

集中式逆变器技术是将多个并联的PV串连接到同一集中式逆变器的DC输入端子。通常,三相IGBT功率模块用于高功率,低功率的场效应晶体管,并且同时使用DSP。转换控制器用于改善所产生的电能的质量,使其非常接近正弦电流,通常用于具有大型光伏电站(> 10kW)的系统中。最大的特点是系统功率大,成本低。然而,由于不同PV串的输出电压和电流不完全匹配(特别是当PV串由于混浊,阴影,污渍等而部分阻挡时),采用集中反转。改变模式将导致逆变器过程的效率降低和电力家庭的功率降低。同时,整个光伏系统的发电可靠性受到某个光伏单元的恶劣工作条件的影响。最新研究方向是利用空间矢量调制控制和新型逆变器拓扑连接的开发,以在部分负载条件下实现高效率。

2.串式逆变器

串式逆变器基于模块化概念。每个PV串(1-5kw)通过逆变器,在DC端具有最大功率峰值跟踪,并在AC端并联连接。市场上最流行的逆变器。

许多大型光伏电站使用串式逆变器。其优点是不受模块差异和串之间的阴影的影响,同时,光伏模块的最佳工作点与逆变器不匹配,从而增加了发电量。这些技术优势不仅降低了系统成本,还提高了系统可靠性。同时,在串之间引入“主 - 从”的概念,使得系统可以将几组光伏串连接在一起,并且在一串电能不能制造的情况下让一个或多个工作。单逆变器工作。从而产生更多电力。

3.微型逆变器

In the traditional PV system, the DC input end of each string inverter will be connected in series by about 10 photovoltaic panels. If one of the 10 series connected panels does not work well, the string will be affected. If the inverter's multiple inputs use the same MPPT, each input will also be affected, greatly reducing power generation efficiency. In practical applications, clouds, trees, chimneys, animals, dust, ice and snow and other occlusion factors can cause these factors, and the situation is very common. In the PV system of the micro-inverter, each panel is connected to a micro-inverter. When one of the panels does not work well, only this one will be affected. Other photovoltaic panels will operate at optimum operating conditions, making the overall system more efficient and generating more power. In practical applications, if a string inverter fails, it will cause several kilowatts of the panel to fail to function, and the impact of the micro inverter failure is quite small.

4. Power Optimizer

Adding a power optimizer (OptimizEr) to the solar power system can greatly improve the conversion efficiency and simplify the function of the inverter (Inverter). In order to realize a smart solar power system, the device power optimizer can ensure the best performance of each solar cell and monitor the battery consumption state at any time. The power optimizer is a device between the power generation system and the inverter. The main task is to replace the original optimal power point tracking function of the inverter. The power optimizer performs an extremely fast optimal power point tracking scan analogously by simplifying the line and a single solar cell corresponding to a power optimizer, so that each solar cell can achieve the best power point tracking. In addition, the battery status can be monitored anytime and anywhere by placing a communication chip, and the immediate return problem allows the relevant personnel to repair as quickly as possible.

Third, the function of the photovoltaic inverter

xxThe inverter not only has a direct AC conversion function, but also functions to maximize the performance of the solar cell and system fault protection. It can be summarized as automatic running and stopping function, maximum power tracking control function, anti-individual operation function (for grid-connected system), automatic voltage adjustment function (for grid-connected system), DC detection function (for grid-connected system), DC grounding detection Function (for grid-connected systems). Here is a brief introduction to the automatic operation and shutdown functions and maximum power tracking control functions.

(1) Automatic operation and shutdown function

After the morning sunrise, the solar radiation intensity gradually increases, and the output of the solar cell also increases. When the output power required for the operation of the inverter is reached, the inverter automatically starts to operate. After entering the operation, the inverter monitors the output of the solar cell module at all times. As long as the output power of the solar cell module is greater than the output power required for the operation of the inverter, the inverter continues to operate; until sunset stops, even in rainy days. The inverter can also operate. When the output of the solar cell module becomes smaller and the inverter output approaches 0, the inverter forms a standby state.

(2) Maximum power tracking control function

The output of the solar cell module varies with the intensity of the solar radiation and the temperature of the solar module itself (chip temperature). In addition, since the solar cell module has a characteristic that the voltage decreases as the current increases, there is an optimum operating point at which the maximum power can be obtained. The intensity of solar radiation is changing, and it is clear that the best working point is also changing. Relative to these changes, the operating point of the solar module is always at the maximum power point, and the system always obtains the maximum power output from the solar module. This control is the maximum power tracking control. The most important feature of inverters for solar power systems is the inclusion of maximum power point tracking (MPPT).

Fourth, the main technical indicators of photovoltaic inverters

1. Output voltage stability

xxIn a photovoltaic system, the energy emitted by the solar cell is first stored by the battery and then inverted by the inverter into 220V or 380V AC. However, the battery is affected by its own charge and discharge, and its output voltage varies widely. For example, a nominal 12V battery may have a voltage value ranging from 10.8 to 14.4V (beyond this range, the battery may be damaged) . For a qualified inverter, when the input voltage changes within this range, the steady-state output voltage change should not exceed the rated value of &Plusmn; 5%, and when the load is abrupt, its output voltage deviation should not Exceeding ±10% of rated value.

2. Waveform distortion of the output voltage

For sine wave inverters, the maximum allowable waveform distortion (or harmonic content) should be specified. Usually expressed as the total waveform distortion of the output voltage, its value should not exceed 5% (single-phase output allows 10%). Because the high-order harmonic current output from the inverter will generate additional losses such as eddy currents on the inductive load, if the waveform distortion of the inverter is too large, the load components will be seriously heated, which is not conducive to the safety of electrical equipment and seriously affects the system. Operating efficiency.

3. Rated output frequency

The deviation should be within &Plusmn; l%.

4. Load power factor

Characterizes the ability of an inverter to carry inductive or capacitive loads. The sine wave inverter has a load power factor of 0.7 to 0.9 and a nominal value of 0.9. In the case of a certain load power, if the power factor of the inverter is low, the capacity of the required inverter is increased, on the one hand, the cost is increased, and the apparent power of the AC circuit of the photovoltaic system is increased, and the circuit is increased. As the current increases, the losses increase and the system efficiency decreases.

5. Inverter efficiency

xx输出功率与输入功率之比表示为百分比。通常,PV逆变器的标称效率是纯电阻负载和80%负载的效率。由于光伏系统的高总成本,有必要使光伏逆变器的效率最大化,降低系统成本,并改善光伏系统的成本性能。目前,主流逆变器的标称效率在80%至95%之间,低功率逆变器的效率不低于85%。在光伏系统的实际设计过程中,不仅要选择高效逆变器,而且应采用合理的系统配置,使光伏系统负荷工作在最佳效率点附近。

6.额定输出电流(或额定输出容量)

表示变频器在指定负载功率因数范围内的额定输出电流。一些逆变器产品提供额定输出容量,其单位表示为VA或kVA。当输出功率因数为1(即纯电阻负载)时,变频器的额定容量是额定输出电压与额定输出电流的乘积。

7.保护措施

具有优异性能的逆变器还应具有完整的保护功能或措施,以应对实际使用中出现的各种异常情况,从而保护逆变器本身和系统的其他组件免受损坏。

(1)输入欠压保护:

当输入电压低于额定电压的85%时,应保护并显示变频器。

(2)输入过压保单持有人:

当输入电压高于额定电压的130%时,应保护并显示逆变器。

(3)过流保护:

逆变器的过流保护应确保负载短路或电流超过允许值时负载及时起作用,以保护其免受浪涌电流的影响。当工作电流超过额定值的150%时,应自动保护变频器。

(4)输出短路保单持有人

逆变器短路保护动作时间应不超过0.5秒。

(5)输入反接保护:

当输入端正,负极接反时,逆变器应有防护功能和显示。

(6)防雷保护:

逆变器应有防雷保护。

(7)过温保护等

另外,对无电压稳定措施的逆变器,逆变器还应有输出过电压防护措施,以使负载免受过电压的损害。

8.起动特性

表征逆变器带负载起动的能力和动态工作时的性能。逆变器应保证在额定负载下可靠起动。

9.噪声

电力电子设备中的变压器,滤波电感,电磁开关及风扇等部件均会产生噪声。逆变器正常运行时,其噪声应不超过80分贝,小型逆变器的噪声应不超过65分贝。

五,选型技巧

逆变器的选用,首先要考虑具有足够的额定容量,以满足最大负荷下设备对电功率的要求。对于以单一设备为负载的逆变器,其额定容量的选取较为简单。

当用电设备为纯阻性负载或功率因数大于0.9时,选取逆变器的额定容量为用电设备容量的1.11.15倍即可。同时逆变器还应具有抗容性和感性负载冲击的能力。

XX对于一般的感应负载,例如电动机,冰箱,空调,洗衣机,大功率泵等,在启动时,其瞬时功率可以是其额定功率的5至6倍。这时,逆变器将承受很大的力矩。浪涌。对于此类系统,逆变器的额定容量应留有足够的余量,以确保可靠地启动负载。高性能逆变器可在满负载下多次启动,而不会损坏功率器件。小型逆变器有时需要使用软启动或限流启动来保证自身安全。

六,安装注意事项和维护

1.安装前,首先应检查变频器在运输过程中是否损坏。

2.选择安装现场时,应确保周围区域内的任何其他电力电子设备不受干扰。

3.在进行电气连接之前,务必使用不透明材料覆盖或断开直流侧断路器。当暴露在阳光下时,光伏阵列会产生危险电压。

4.所有安装操作必须由专业技术人员执行。

5.光伏系统发电系统中使用的电缆必须牢固连接,具有良好的绝缘性和适当的规格。