The choice of grid connection scheme for photovoltaic power plants usually depends on the scale of the power plant, the access conditions of the local power grid, and economic considerations.
The two grid connected voltage levels of 35kV and 10kV each have their own advantages and disadvantages. Below, we will compare these two schemes from the technical, economic, and operational perspectives, and provide specific examples.
Technical Comparison
35kV scheme
Advantage
Transmission distance: suitable for long-distance transmission, reducing line losses.
Equipment capacity: Supports larger capacity photovoltaic power plants, suitable for large-scale projects.
Voltage stability: High voltage transmission has less impact on the power grid, which is beneficial for voltage stability.
Shortcoming
Cost: The construction and maintenance costs are relatively high, including substation equipment, cables, etc.
Construction complexity: requires more complex engineering design and construction, occupying more land resources.
Safety requirements: High voltage equipment operation requires professional personnel and has high safety requirements.
Parameter examples
Photovoltaic power station capacity: 10 MW to 50 MW.
Step up substation: with a capacity of 10 MVA to 50 MVA, and a high voltage side voltage level of 35 kV.
High voltage switchgear: rated voltage 35 kV, rated current 630 A to 1250 A.
Cable cross-section: The cross-section of high-voltage cables is usually between 150 mm ² and 400 mm ².
Line length: suitable for transmission distances of more than 10 kilometers.
10kV scheme
Advantage
Cost: The construction and maintenance costs are relatively low.
Easy construction: The equipment has a small volume, occupies less land, and has a shorter construction period.
Flexibility: Suitable for small and medium-sized photovoltaic power plants, with flexible access to the power grid.
Shortcoming
Transmission distance: suitable for short distance transmission, where line loss increases beyond a certain distance.
Capacity limitation: Suitable for small-scale photovoltaic power plants, may not be sufficient for large capacity projects.
Power grid impact: It has a significant impact on voltage fluctuations in local power grids.
Parameter examples
Photovoltaic power station capacity: 1 MW to 10 MW.
Step up substation: with a capacity of 1 MVA to 10 MVA, and a high voltage side voltage level of 10 kV.
High voltage switchgear: rated voltage 10 kV, rated current 630 A to 1250 A.
Cable cross-section: The cross-section of high-voltage cables is usually 70 mm ² to 150 mm ².
Line length: suitable for transmission distances within 5 kilometers.
Economic comparison
Cost analysis
35kV scheme: The overall investment is relatively high, but the cost per watt is low, making it suitable for large-scale projects.
10kV scheme: The initial investment is relatively low, but with the expansion of the power station scale, the unit cost may increase.
Payback period
35kV scheme: Due to the large investment and long payback period, but with good long-term returns.
10kV scheme: The payback period is relatively short, suitable for quickly recovering funds.
Operational level
Mocha ITOM
35kV scheme: The operation and maintenance requirements are high, requiring a professional team to conduct regular inspections and maintenance.
10kV scheme: relatively simple operation and maintenance, with lower maintenance costs.
Fault handling
35kV scheme: The scope of the fault impact is relatively large, and more complex coordination work is required when dealing with the fault.
10kV scheme: The fault has a relatively small impact range and is relatively easy to handle.
Actual case
Example of 35kV scheme
Assuming a large-scale photovoltaic power station project is located in a remote area with a total installed capacity of 30 MW, it needs to transmit electricity to a substation located 30 kilometers away.
Photovoltaic power station capacity: 30 MW.
Step up substation: Capacity of 30 MVA, high voltage side voltage level of 35 kV.
High voltage switchgear: rated voltage 35 kV, rated current 1250 A.
Cable cross-section: High voltage cable cross-section of 400 mm ².
Line length: 30 kilometers.
Example of 10kV scheme
Assuming a medium-sized photovoltaic power station project is located in the outskirts of a city with a total installed capacity of 5 MW, it needs to transmit electricity to a substation located 3 kilometers away.
Photovoltaic power station capacity: 5 MW.
Step up substation: capacity 5 MVA, high voltage side voltage level 10 kV.
High voltage switchgear: rated voltage 10 kV, rated current 630 A.
Cable cross-section: High voltage cable cross-section of 150 mm ².
Line length: 3 kilometers.
The steps and specific considerations for selecting a suitable step-up substation:
1. Determine the scale and capacity of the power station
The scale of the power station determines the capacity demand of the booster substation and is the basis for selecting a booster substation.
Steps
Estimate the total installed capacity of a photovoltaic power station: Calculate the total installed capacity based on the number of photovoltaic modules and the rated power of each individual module.
Determine maximum output power: Considering factors such as sunlight conditions and conversion efficiency, calculate the maximum output power of the photovoltaic power station.
Consider future expansion: Reserve a certain amount of capacity margin to meet possible expansion needs in the future.
2. Understand the requirements for grid access
The requirements for grid access determine the voltage level and other technical indicators of the step-up substation.
Steps
Consult the local power grid company to obtain technical requirements and regulations for grid access.
Determine the voltage level for connection: Determine the voltage level for connection to the power grid according to the requirements of the power grid company (such as 10kV, 35kV, etc.).
Understand the location of the grid connection point: Determine the distance between the photovoltaic power station and the grid connection point.
3. Consider geographical location and environmental factors
Geographical location and environmental conditions affect the design and installation of step-up substations.
Steps
Assess site conditions: Investigate the topography, climate conditions, etc. of the location of the photovoltaic power station.
Consider transportation and installation: Ensure that the substation equipment can be transported smoothly to the site, and take into account the difficulties during the installation process.
Lightning protection and protection: Design lightning protection grounding systems and protective measures based on local meteorological conditions.
4. Evaluate cost-effectiveness
Cost benefit analysis is one of the key factors determining the final plan.
Steps
Calculate initial investment: including costs for purchasing substation equipment, civil engineering, cable laying, etc.
Evaluate operation and maintenance costs: Consider the long-term operation and maintenance costs, including regular inspections, repairs, spare parts, etc.
Calculate economic benefits: Taking into account factors such as power generation revenue and government subsidies, calculate the investment payback period and rate of return.
5. Choose the appropriate type of substation
Based on the above analysis results, select the most suitable type of step-up substation.
Type and Characteristics
Dry type transformer: suitable for indoor installation, does not require oil immersion, and is easy to maintain.
Oil immersed transformer: suitable for outdoor installation, with good heat dissipation performance and large capacity.
Modular substation: Integrated with transformers, switchgear, protective devices, etc., it is easy to install and occupies a small area.
Prefabricated substation: factory prefabricated, on-site assembly, short installation cycle.
Example
Example 1: Medium sized photovoltaic power station (with a capacity of 10MW)
Photovoltaic power station capacity: 10 MW.
Capacity of the step-up substation: 10 MVA.
Voltage level: High voltage side 10 kV, low voltage side 0.69 kV.
Transformer type: dry-type transformer.
High voltage switchgear: rated voltage 10 kV, rated current 630 A, vacuum circuit breaker.
Low voltage distribution cabinet: rated voltage 0.4 kV, rated current 400 A.
Cable cross-section: High voltage cable 150 mm ², low voltage cable 70 mm ².
Example 2: Large scale photovoltaic power station (capacity 50MW)
Photovoltaic power station capacity: 50 MW.
Capacity of the step-up substation: 50 MVA.
Voltage level: High voltage side 35 kV, low voltage side 0.69 kV.
Transformer type: Oil immersed transformer.
High voltage switchgear: rated voltage 35 kV, rated current 1250 A, vacuum circuit breaker.
Low voltage distribution cabinet: rated voltage 0.4 kV, rated current 630 A.
Cable cross-section: High voltage cable 400 mm ², low voltage cable 150 mm ².
