102-104 Selection of inverter and controller
Author: Source: Datetime: 2017-01-16 11:53:28
4.3.2 Selection of inverter and controller
For the establishment of independent photovoltaic power generation system, in addition to capacity design, matching the appropriate inverter, controller and other hardware devices is essential.
PV inverter selection, the general design of the system to determine the DC voltage to select the inverter DC input voltage, according to the total load power and type to determine the inverter capacity and the number of phases, and then consider the impact of the instantaneous load Determine the inverter power surplus, usually the inverter's continuous power is greater than the load power, the inverter's maximum impact power is greater than the load start-up power. In addition, the inverter selection, but also consider the expansion of photovoltaic power generation system to leave some room.
Independent photovoltaic power generation systems, such as with AC load, you must be equipped with inverters; such as the DC load voltage and battery voltage is inconsistent, need to be equipped with DC converter.
Where the need to convert DC into the place where the inverter should be used, so the inverter itself is widely used. However, inverters with AC loads in stand-alone PV systems have some specific requirements, such as:
① in the operating range (meaning not only at full load, under the light load can also) high inverter efficiency;
② safe and reliable operation;
③ can be adapted to photovoltaic power generation system DC voltage of a wide range of changes;
④ resistance to transient high current impact, long continuous inverter can be used;
⑤ Inverter with inductive load, requiring high-order harmonic components of the AC output is small;
⑥ performance and better prices.
Inverter is the last level of independent photovoltaic system devices, investment in the system is also the first three key balance components,
Can directly affect the level of investment in the system, the use of performance and reliability. Therefore, for different application systems, matching the inverter is also designed to use an important work of the user.
A variety of inverters, a waveform of its output, the load is active, the output current phase number and the main loop topology, etc., as shown in Table 4-14.
|Output voltage waveform||Square wave inverter, sine wave inverter, step wave (quasi-sine wave) inverter|
|Output power going||Active inverter, passive inverter|
|Output the phase number of the AC submerged||Single-phase inverter, three-phase inverter, multi-phase inverter|
|Outputs the frequency of the AC power||Frequency inverter, medium frequency inverter, high-frequency inverter|
|Main Loop Topology||Push-pull inverter, half-bridge inverter, full-bridge inverter ~|
|Line principle||Self-oscillation type inverter, pulse width modulation inverter, resonant inverter|
|Input the nature of DC power supply||Voltage source inverters, current source inverters|
(1) Inverter efficiency
Inverter efficiency represents its own power loss. In general, the inverter efficiency can be required according to the following criteria:
Capacity of 100 ~ 1000kW inverter, the efficiency should be 96% to 98% or more;
Capacity of 10 ~ 100kW inverter, the efficiency should be 90% to 93% or more;
Capacity of 1 ~ 10kW inverter, the efficiency should be 85% to 90% or more;
Capacity of 0.1 ~ lkW of the inverter, the efficiency should be 80% ~ 85% or more;
It should be noted that the efficiency referred to here refers to the inverter in the full load conditions to achieve efficiency, good quality inverter at light load efficiency is also higher.
(2) Rated output voltage
Photovoltaic inverter in the provisions of the input DC voltage to allow fluctuations in the range, should be able to output constant voltage. For medium and small independent PV power plants, the transmission radius is less than 2km, the inverter output voltage is single-phase 220V and three-phase 380V, no longer boost to the user, the voltage fluctuation range is as follows:
① in the steady state operation, the voltage fluctuation range does not exceed the rated value of ± 5%;
② In the presence of impact load, the voltage fluctuation range does not exceed ± 10% of the rated value;
③ in normal operation, the inverter output of the three-phase voltage imbalance does not exceed 8%;
④ output voltage sine wave distortion requirements are generally less than 3%;
⑤ the output AC voltage frequency fluctuations should be less than 1%, GB / T 19064-2003 provides the output voltage frequency should be between 49 ~ 51Hz.
(3) Rated output power
Rated output power refers to the load power factor of 1, the inverter rated output voltage and rated output current of the product, in kV * A
(4) overload capacity
Overload capability is the time required for continuous operation of the inverter at rated output power. The standard stipulates as follows:
① the input voltage and output power rating, the inverter should be able to work continuously for more than 4h;
② input voltage and output power of 125% of the rated value should be able to work more than lmin;
③ input voltage and output power of 150% of the rated value should be able to work for more than 10s.
(5) Rated DC input voltage and range
The rated DC input voltage refers to the DC voltage value of the input inverter in the photovoltaic power generation system. Low-power inverter input voltage is generally 12V, 24V, 48V, medium and high power inverter usually have 110V, 220V, 500V and so on.
As the independent photovoltaic power generation system storage battery voltage is variable, which requires the inverter should be able to meet the input current and voltage can be changed within a certain range without affecting the output voltage changes, usually this value is 90% to 120% .
Inverter should have the following main protection functions to ensure safe and reliable operation of photovoltaic power generation system: ① overvoltage, undervoltage protection; ② over-current protection; ③ short-circuit protection; ④ reverse protection; ⑤ lightning protection.
(7） security requirements
① Resistor resistance Inverter DC input and the insulation resistance between the chassis, AC output and the insulation resistance between the case should be ≥ 50 MΩ
② Dielectric strength Inverter DC input and chassis should be able to withstand the frequency of 50Hz, sine wave AC voltage 500V, which lasted 1min of dielectric strength test, no breakdown or arc phenomenon. Inverter AC output and the chassis should be able to withstand the frequency of 50Hz, sine wave AC voltage 1500V, lasted 1min of dielectric strength test, no breakdown or arc phenomenon.
(8) Other requirements
① Use of environmental conditions The first PV system inverter with the normal conditions of use: the environment working temperature -20 ~ +5 O0C, relative humidity ≤ 93%, no condensation and elevation of altitude and so on. When the working environment exceeds the above range, consider reducing capacity usage or redesigning customization.
② electromagnetic interference and noise The switching circuit of the inverter is easy to produce electromagnetic interference, in the iron core transformer due to vibration and noise. In the design and manufacture of electromagnetic interference and noise must be controlled to meet the relevant standards and user requirements.
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