**Abstract:** Large and medium-sized photovoltaic power plants possess specific power supply characteristics and responses to frequency anomalies. These systems can contribute to voltage and frequency regulation in the power grid to a certain extent. They are also equipped with the ability to withstand voltage disturbances, which helps prevent disconnection during abnormal grid conditions and avoids power loss in the system.
**I. Technical Requirements for Grid Connection of Photovoltaic Power Stations**
Grid-connected photovoltaic power stations are categorized based on their connection voltage levels: small (0.4 kV), medium (35 kV), and large (66 kV or higher). Depending on whether they can send power back to the public grid through the connection point, they are classified as either reversible or irreversible.
The power quality requirements for these systems follow several national standards, including:
- GB/T 1459-1993: "Power Quality - Public Power Harmonics"
- GB/T 12325-2008: "Power Quality - Voltage Deviation"
- GB/T 12326-2008: "Power Quality - Voltage Fluctuation and Flicker"
- GB/T 15543-2008: "Power Quality - Voltage Unbalance"
Additionally, the DC component injected into the grid should not exceed 0.5% of the AC rated value. These systems are designed to support grid stability by responding to voltage fluctuations and maintaining connectivity during abnormal conditions.
**II. Essential Conditions for Connecting Photovoltaic Power Stations to the Grid**
1. Grid-connected circuit breakers must be installed at the inverter output points, ensuring operability, lockability, and clear disconnection points for the safety of maintenance personnel.
2. Lightning protection and grounding systems must comply with State Grid Corporation regulations, with regular testing of grounding resistance.
3. Protective functions must be in place to ensure equipment and personal safety, meeting the criteria of reliability, selectivity, flexibility, and speed.
4. Overcurrent, short-circuit protection, anti-islanding capabilities, and inverter protection must meet national grid requirements.
5. The design and configuration of the secondary DC system and battery discharge capacity must adhere to technical guidelines.
6. Power quality must meet specified limits for harmonics, voltage deviation, flicker, unbalance, and DC components.
7. Safety automatic devices must be fully configured as required.
8. A reliable independent communication path must be established between the photovoltaic plant and the dispatching agency.
9. Within six months of grid connection, a qualified test unit must conduct operational characteristic tests and submit the results to the grid company.
10. Required documentation, such as technical specifications, relay protection drawings, automation equipment details, and operating procedures, must be submitted to the dispatching authority.
**III. Characteristics of Grid-Connected PV Power Generation**
1. PV generation is inherently random, intermittent, and follows a daily cycle. It cannot be precisely forecasted, so it cannot be integrated into traditional power planning.
2. PV systems only generate electricity during daylight hours, with zero output at night. Output increases gradually after sunrise, peaking at midday.
3. When PV contributes to the grid, other energy sources must adjust their output to accommodate the fluctuating supply. Sudden drops in PV output due to weather require backup from conventional sources to maintain grid stability.
4. PV output varies rapidly depending on weather conditions, often dropping by up to 70% within minutes when clouds pass.
5. The reserve capacity needed for PV integration is typically maintained in a rotating standby mode, increasing with the scale of PV installations.
China’s solar power development follows trends of centralized large-scale projects connected via high-voltage lines and decentralized systems connected at low voltages. In regions like Northwest China, where solar resources are abundant but local demand is low, long-distance transmission is necessary for large-scale PV plants.
**IV. Impact of Grid-Connected PV on the Power Grid**
1. The variability and intermittency of solar resources can significantly affect grid stability.
2. PV systems connected via inverters may introduce harmonics and unbalanced currents, leading to voltage fluctuations and flicker.
3. As PV penetration increases, it can reduce the grid's peak load management capability.
4. Frequent solar fluctuations can cause voltage rises in transmission lines, potentially leading to stability issues over long distances.
5. Integration into distribution networks transforms the network from radial to multi-source, altering current flow and fault current characteristics.
6. PV system failures can interfere with relay protection and reclosing mechanisms, affecting overall grid reliability.
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