Hot spot effect
The hot spot effect refers to that under certain conditions, the shielded PV module in the series branch is used as a load to consume the energy generated by other illuminated battery components, and the blocked PV module will heat up at this time.
The blocked PV module will consume part of the energy or all energy generated by the light photovoltaic module, and reduce the output power. Seriously, PV modules will be permanently damaged or even burned.
Cause
1. The root cause of the hot spot effect is that some bad batteries are mixed, the electrode solder joint is poorly soldered, the battery is cracked, the battery features are damaged, and the battery is partially shaded.
2. The current and voltage of some batteries in the PV module change due to the local shadow. As a result, the local current and voltage of the battery assembly increase, resulting in local temperature rise on these battery components.
Protection Measures
A bypass diode is connected between the positive and negative poles of the PV module to increase the array reliability. Generally, the bypass diode is in the reverse bias voltage, which does not affect the normal operation of the component. The principle is that when a battery is blocked, other batteries trigger a large resistance. In this case, the diode is conductive, and the part of the total battery that exceeds the light current of the battery is distributed by the diode, thereby avoiding overheat damage of the battery. To prevent the energy generated by the light component from being consumed by the shadowed components.
PID effect
The potential evoked attenuation effect (PID, PotentialInducedDegradation) is that when the battery string is high voltage for a long time, leakage current exists between glass and packaging materials. A large number of charges are snapped on the surface of the battery sheet, which deteriorates the passivation effect of the battery surface. As a result, the component performance is lower than the design standard. If the PID is severe, the power of a PV module decreases by more than 50%, which affects the power output of the PV string. The PID phenomenon is most likely to occur in coastal areas with high temperature, high humidity, and high salt.
Cause
1. System design reasons: The lightning protection and grounding of the PV plant is implemented by grounding the frame edge of the PV array. As a result, a bias voltage is formed between a single component and the frame. The higher the bias voltage of the PV module, the more serious the PID is. If the P silicon component is grounded through the negative electrode of the inverter with a transformer, the positive bias voltage of the component frame relative to the battery sheet will be effectively prevented. However, the negative grounding of the inverter will increase the system construction cost.
2. The PV module causes are as follows: In a high-temperature and high-humidity environment, a leakage current is formed between the battery and the ground border, and a leakage current channel is formed between the packaging material, the backplane, the glass, and the frame. The change of vinyl acetate (EVA) is one of the ways to realize the anti-PID of the component. Under the condition of different EVA packaging film, the anti-PID performance of the component will be different. In addition, the glass in the PV module is mainly calcium sodium glass, and the effect of glass on the PID phenomenon of the PV module is still unclear.
3. The battery is faulty. The uniformity of the block resistor, the thickness of the anti-reflection layer and the refractive index have different effects on the PID performance.
Measures to Suppress PID Effect
1. Consideration from the component side
1) Use non-Na and Ca glass to improve the body resistance of the glass and block the formation of the leakage current path.
2) Packaging materials for non-ethylene-vinyl acetate copolymers.
Features: It is safe and reliable to suppress the PID effect from materials, but the cost of non-Na and Ca glass is high. In addition, the stability of new materials is unknown and cannot be popularized.
2. From the inverter side
The negative bias voltage is used to prevent leakage current caused by negative bias voltage.
Features: The solution is simple, cost-effective, and effective. However, the direct grounding of the negative electrode may cause security risks and threaten the normal operation and O&M security of the PV plant. After the negative electrode of the inverter is grounded, if the positive grounding of the PV module is faulty, the PV module is short-circuited. If the O M personnel contact the positive electrode, electric shocks may occur. Therefore, the negative grounding circuit must have the abnormal current monitoring and disconnection protection system, in this way, the PID effect can be suppressed and the operation safety of the plant equipment can be ensured.
