The power quality management of PV plants shares similarities with wind farms but has its own unique emphases. The core logic is: PV inverters are themselves harmonic sources and require reactive power support, while the grid has strict requirements for reactive power and harmonics at the point of common coupling.
I. Major Power Quality Issues in PV Power Plants
Reactive Power Issues (Core target for SVG mitigation):
Limited inverter reactive power capability: Although modern PV inverters have inherent reactive power regulation capability (typically able to adjust power factor within a range of ±0.8), this capability comes at the expense of active power output. Using inverters to provide reactive power means reducing power generation, directly impacting owner revenue.
Line and transformer reactive power consumption: Step-up transformers and collection lines are inductive and consume reactive power.
Grid dispatch requirements: According to national grid standards (e.g., China's "Technical Regulations for Connecting Photovoltaic Power Stations to the Power System"), PV power stations must possess dynamic reactive power compensation capability. They must be able to control voltage and power factor at the Point of Common Coupling (PCC) according to dispatch commands (typically required to be between 0.98 leading and 0.98 lagging). This is a mandatory requirement.
Harmonic Issues (Core target for APF mitigation):
Main harmonic source: PV inverters (DC-AC) are the primary harmonic sources. The harmonics they generate are primarily high-order harmonics, such as 13th, 17th, 19th, 23rd, 25th, etc., as well as some switching frequency harmonics (e.g., 1150Hz, 2350Hz).
Background harmonic amplification: The grid to which the PV plant is connected may have existing background harmonics. Inverters can interact with these, potentially causing resonance and amplifying specific harmonic orders.
Nighttime backfeeding issue: At night when the PV system is not generating power, station service power is drawn from the grid. During this time, station equipment like transformers and UPS systems can become harmonic sources, injecting harmonics into the grid.
Voltage Fluctuations and Flicker:
Changes in light intensity (e.g., passing clouds) can cause rapid fluctuations in PV output power, leading to voltage fluctuations and flicker at the PCC.
II. The Role and Configuration Strategy of SVG and APF in PV Power Plants
The primary principle for configuration is: First, meet the grid's mandatory reactive power requirements, then mitigate harmonics to protect the plant's own equipment and ensure safety.
(I) Configuration of Static Var Generator (SVG)
1. Role Positioning: Primary Dynamic Reactive Power Support and Voltage Regulation The core function of the SVG is to act as a powerful supplement and the main provider of reactive power, independent of the inverters. It can provide fast, smooth, and continuous reactive power adjustment without sacrificing active power generation, meeting grid dispatch requirements and stabilizing the PCC voltage.
2. Installation Location: PV Plant Point of Common Coupling (PCC)
The SVG must be centrally installed on the low-voltage side (35kV, 10kV, or 400V side) of the main step-up transformer.
Mitigation at this location allows for direct centralized adjustment of the PCC power factor, response to grid dispatch commands, and provides reactive power support for the entire PV plant.
3. Capacity Calculation (Critical Step): The SVG capacity must satisfy the maximum value of the following requirements:
a. Meet Grid Dispatch Requirements (Dominant factor): According to national standards, the SVG capacity should be 20% ~ 30% of the PV power plant's rated capacity.
Example: A 50MW PV plant typically requires an SVG with a capacity of ±10 Mvar to ±15 Mvar.
b. Compensate for Internal Reactive Power Deficit: Calculate the reactive power consumption of step-up transformers and collection lines, and consider the reactive power demand of station service loads at night.
c. Voltage Support during System Faults: Ensure the SVG can provide sufficient reactive power to support voltage during grid faults, achieving Low Voltage Ride-Through (LVRT).
Conclusion: SVG capacity is usually dictated by grid codes and is a "standard feature" for PV power plants.
(II) Configuration of Active Power Filter (APF)
1. Role Positioning: Mitigating High-Order Harmonics and Suppressing Resonance The core function of the APF is to precisely filter the high-order harmonics generated by PV inverters, preventing them from being injected into the public grid beyond limits, and to suppress potential harmonic resonance, protecting plant equipment like transformers and capacitors.
2. Installation Location: Primarily Centralized, supplemented by Localized Mitigation
Option A (Most Common): Centralized Installation at the PCC
Location: Installed alongside the SVG on the low-voltage side of the main step-up transformer.
Advantages: Convenient installation, centralized management, effectively mitigates the total harmonic current injected into the grid by the entire plant, ensuring PCC harmonic content complies with national standards (e.g., GB/T 14549).
Applicability: Suitable for the vast majority of utility-scale PV plants.
Option B (Specific Cases): Distributed Installation at Inverter Step-Up Transformer LV Side
Location: Install smaller-capacity APFs on the low-voltage side of the combiner box transformers for each or multiple unit power generation modules (e.g., 2MW units) in large plants.
Advantages: More thorough mitigation, prevents harmonic currents from flowing in collection lines causing losses, and can more effectively suppress local resonance.
Disadvantages: Higher cost, more maintenance points.
Applicability: Suitable for very large-scale plants, plants with exceptionally long lines, or plants with particularly severe harmonic issues.
3. Capacity Calculation:
Measurement Method (Recommended): Perform power quality measurements at the PCC or the LV side of combiner box transformers to obtain real harmonic current data. This is the most accurate method.
Estimation Method: APF capacity I_APF ≥ PV Plant Rated Current × Current THDi × Simultaneity Factor
The THDi at the inverter output is usually controlled below 1.5%~3% (including internal filters), but the superposition of multiple units and background harmonics must be considered.
Simultaneity Factor: Considering that not all inverters operate at full load simultaneously and harmonic phases partially cancel each other, a factor of 0.6 ~ 0.8 can be used.
Recommendation: Always conduct field measurements, as harmonic characteristics are closely related to inverter models and grid impedance.
III. Configuration Scheme and Typical Architecture
A standard power quality mitigation architecture for a PV power plant is conceptually structured as follows:
PV Arrays: Multiple groups of PV arrays connect to inverters.
Inverter & Step-Up Unit: Group string/central inverters (the harmonic sources) feed into combiner box transformers. Optionally, Distributed APFs can be installed here for targeted resonance suppression and harmonic control.
Main Step-Up Transformer: Steps up the voltage for grid connection.
Point of Common Coupling (PCC) Mitigation Layer: Located at the PCC on the LV side of the main transformer. This layer houses:
A large Centralized SVG for bulk dynamic reactive power support and voltage stability, responding to grid commands.
A Centralized APF (Primary choice) for filtering total harmonics.
Passive capacitor/reactor banks for basic reactive power compensation.
Recommended Scheme:
SVG: Centrally installed on the main LV bus, capacity sized at 20%~30% of the plant's total capacity.
APF: Prioritize a centralized mitigation scheme, installed on the same bus as the SVG.
Coordinated Control: The SVG and APF should be integrated into the PV plant's SCADA system to receive grid dispatch commands and enable automated operation. SVG and APF can operate independently or be integrated into a unified device (Hybrid-APF).
IV. Special Product Selection Requirements
The PV plant environment is special, demanding high equipment specifications:
Protection Rating: Outdoor installation requires at least IP54 and corrosion resistance class C4 or higher to withstand high temperatures, humidity, wind, sand, and saline-alkali conditions.
Voltage Level: Must directly match the plant's voltage level (e.g., 0.4kV, 10kV, 35kV).
Response Speed: Must be extremely fast (<5ms) to respond to power fluctuations caused by passing clouds.
Wide Voltage Adaptation Capability: The grid voltage fluctuation range can be large; equipment must operate normally within a wide voltage range.
Heat Dissipation Capability: Heat dissipation design is crucial in high-temperature environments.
Summary
Configuring SVG and APF for a PV power plant is a critical techno-economic decision:
SVG is mandatory: Its capacity is determined by national mandatory standards. It is the core equipment for meeting grid reactive power dispatch and voltage support requirements—it's the "pass" for grid connection.
APF is highly recommended: Its necessity depends on field-measured harmonic data and the grid background. It is "insurance" to ensure efficient plant operation, avoid penalties for harmonic exceedances, and prevent equipment damage.
Return on Investment (ROI): This investment is not just a compliance cost but a necessary expenditure to protect power generation revenue (avoiding inverter curtailment due to reactive power dispatch), enhance plant operational reliability, and extend equipment lifespan.
During the project's early stages, it is highly recommended to conduct a detailed power quality assessment and simulation and commission professional agencies for field measurements. This will help develop the most economical, efficient, and reliable mitigation plan, ensuring the maximization of the plant's life-cycle benefits.
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