• Zhejiang Zhilu Transmission and Distribution Equipment Co., Ltd

Design of Main Wiring Schemes for 10kV/0.4kV Photovoltaic Grid - Connection Cabinets

Abstract

1. Introduction

• Voltage level conversion: Efficiently stepping down 10kV power from the grid or PV inverter to 0.4kV for distribution;

• Power protection: Implementing reliable overcurrent, short - circuit, and overvoltage protection;

• Grid - code compliance: Adhering to international standards (e.g., IEC 61936, IEEE 1547) for grid connection.

2. Design Principles and Technical Requirements

2.1 Basic Design Principles

1. Safety: Minimize electrical hazards through proper insulation, grounding, and protective device integration.

2. Reliability: Ensure continuous power supply with redundant components and fault - isolation capabilities.

3. Flexibility: Design for easy maintenance, capacity expansion, and adaptation to changing grid conditions.

4. Economy: Optimize costs by balancing component selection, wiring complexity, and installation requirements.

2.2 Technical Requirements

• Voltage and Current Ratings:

• 10kV side: Rated voltage 10kV, rated current determined by PV system capacity (e.g., 630A for 10MW systems).

• 0.4kV side: Rated voltage 0.4kV, rated current matching LV load demands (e.g., 2000A for large - scale distribution).

• Short - Circuit Capacity: Select components (e.g., circuit breakers, switches) with sufficient short - circuit breaking capacity (e.g., 25kA for 10kV, 50kA for 0.4kV).

• Protection Coordination: Ensure that protective devices (fuses, relays) on both voltage levels operate selectively during faults.

3. Typical Main Wiring Schemes

3.1 Single - Busbar Scheme

• Configuration:

• 10kV side: A single busbar connects incoming feeders from the grid or PV inverters, with circuit breakers or load break switches for protection.

• 0.4kV side: The busbar distributes power to LV loads via circuit breakers and distribution panels.

• Advantages: Simple structure, low cost, easy installation.

• Disadvantages: Limited reliability; a busbar fault disrupts the entire system.

• Application: Small - scale PV projects (e.g., <1MW) or temporary power supply systems.

3.2 Single - Busbar Sectionalized Scheme

• Configuration:

• A bus - sectionalizing circuit breaker divides the 10kV/0.4kV busbar into two sections. Each section can operate independently or be connected through the sectionalizer.

• Advantages: Improved reliability; one section can continue operation during maintenance or faults in the other.

• Disadvantages: Higher cost due to additional switching equipment.

• Application: Medium - scale PV plants (1MW–10MW) requiring enhanced reliability.

3.3 Double - Busbar Scheme

• Configuration:

• Two separate busbars on the 10kV/0.4kV sides enable parallel operation or load transfer. Bus - coupler breakers connect the busbars, allowing flexible power distribution.

• Advantages: High reliability, flexibility for maintenance and capacity expansion.

• Disadvantages: Complex wiring, high investment cost, and increased maintenance requirements.

• Application: Large - scale PV power plants (>10MW) or critical grid - connection points.

3.4 Ring Main Unit (RMU) - Based Scheme

• Configuration:

• 10kV side: RMUs with load break switches, fuses, and circuit breakers form a ring - type network, providing backup power in case of faults.

• 0.4kV side: Standard single - busbar or sectionalized layout for LV distribution.

• Advantages: Compact design, quick fault isolation, suitable for urban PV projects.

• Disadvantages: Limited short - circuit handling capacity compared to busbar - based schemes.

• Application: Distributed PV systems in residential or commercial areas.

4. Component Selection and Integration in Wiring Schemes

4.1 10kV - Side Components

• Circuit Breakers: Vacuum or SF6 circuit breakers for high - voltage protection, rated for 10kV and appropriate current levels.

• Isolating Switches: Manual or motor - operated switches for safety during maintenance.

• Current/ Voltage Transformers: Used for metering, protection, and monitoring.

4.2 0.4kV - Side Components

• Low - Voltage Circuit Breakers: Molded - case circuit breakers (MCCBs) for overcurrent and short - circuit protection.

• Distribution Panels: Modular panels with busbars, contactors, and terminal blocks for LV power distribution.

• Power Factor Correction Devices: Capacitor banks to improve power factor on the 0.4kV side.

5. Protection and Control System Design

5.1 Protection Functions

• Overcurrent Protection: Set current thresholds to trip circuit breakers during overloads or short - circuits.

• Differential Protection: Compare current differences between incoming and outgoing feeders to detect internal faults.

• Earth Fault Protection: Monitor zero - sequence current on the 10kV side to identify ground faults.

5.2 Control and Monitoring

• Relay Protection Systems: Digital or electromechanical relays for protection logic implementation.

• SCADA Integration: Connect the grid - connection cabinet to a supervisory control and data acquisition (SCADA) system for remote monitoring and control.

6. Case Studies and Application Examples

6.1 Case 1: 5MW Distributed PV Project

• Wiring Scheme: 10kV single - busbar sectionalized layout with RMUs, 0.4kV single - busbar for LV distribution.

• Rationale: Balances cost and reliability for a medium - scale project, allowing partial operation during faults.

6.2 Case 2: 20MW Utility - Scale PV Plant

• Wiring Scheme: 10kV double - busbar configuration, 0.4kV single - busbar sectionalized for LV distribution.

• Rationale: Ensures high reliability and flexibility for large - scale power injection into the grid.

7. Conclusion