Anti-Backflow Control Strategies for Grid-Connected PV Systems: Principles, Schemes, and Applications

PV Anti-Backflow Control is a critical technology ensuring the safe and stable

operation of grid-connected photovoltaic (PV) systems. Its core objective is to prevent

reverse power flow from PV generation into the grid, thereby safeguarding both the grid infrastructure and PV equipment. The mainstream control

schemes are as follows:

1. Power Cutoff Scheme (Fast Trip Scheme / Switch-Based Scheme):
* Principle: An integrated anti-islanding/anti-backflow device is installed at the grid connection point to continuously monitor system status (PV/storage output, current direction). Upon

detecting islanding operation or reverse current flow (i.e., PV current

flowing towards the grid), the device immediately trips the grid

interconnection breaker or designated PV switch/inverter. If backflow

persists, it progressively disconnects additional units. When load

increases cause grid current to flow towards the load, the device

determines insufficient PV output and automatically reconnects some units.
* Characteristics: Low cost, simple retrofit, fast response. However,

it incurs PV curtailment losses, potentially increasing user

electricity costs, and represents a "hard" protection method.

2. Power Regulation Scheme (Soft Regulation Scheme / Meter & Inverter-Based Scheme):
* Principle: An anti-backflow meter (paired with CTs) or an anti-islanding/anti-backflow device is installed on the main grid bus, continuously

monitoring bus power, current magnitude, and direction. Upon detecting

reverse current flow (backflow) towards the grid:
* Single-Inverter System: The anti-backflow meter sends the backflow power signal via communication

(e.g., RS485) to the corresponding inverter, which rapidly reduces output power until the reverse current approaches zero.
* Multi-Inverter System: Multiple inverters connect via communication

interfaces to a data concentrator (often integrated into an anti-backflow box). The concentrator aggregates signals from the anti-backflow meter/CTs, calculates total backflow power, and coordinates

multiple inverters to reduce output synchronously via communication

commands.
* Characteristics: Relatively convenient installation, enabling "soft"

backflow prevention without PV curtailment. The core device is the anti-backflow meter (e.g., ADL400N), featuring high accuracy, fast response,

bidirectional metering, and communication interfaces (supporting

interaction with inverters or EMS). Applicable to both single-inverter (single/three-phase) and multi-inverter PV systems.

3. Energy Management System (EMS) Scheme:
* Principle: A monitoring device (e.g., anti-islanding/anti-backflow device or anti-backflow meter) is installed at the grid connection point. It collects

real-time PV export power and current direction signals and uploads them to

the EMS. Based on preset strategies and real-time data analysis, the EMS sends power regulation commands to PV

inverters (or storage PCS) to dynamically control their output and

prevent backflow.
* Characteristics: Achieves soft backflow prevention and maximizes PV

utilization. The system offers powerful functionality, including data

storage, reporting, HMI, and advanced management features. However,

system configuration and costs are relatively higher.

Universal Core Mechanism of Anti-Backflow Control:
All schemes rely on real-time monitoring of current direction and magnitude (typically via CTs).

When current direction reverses (flowing towards the grid):

• In the Cutoff Scheme, the controller issues a trip command.

• In Regulation and EMS Schemes, the controller (integrated within the

• anti-backflow meter, data concentrator, inverter, or EMS) issues commands to

• reduce inverter/PCS output power until reverse current is eliminated.
  For systems with energy storage, the PCS can dynamically adjust

• charge/discharge power to match load demand and avoid backflow.

Importance of Anti-Backflow Control:

• Ensures Grid Stability: Prevents uncontrolled reverse injection of excess PV power from the user side, maintaining grid security and stable

• operation.

• Protects Equipment Safety: Avoids overheating, damage, or potential

• safety hazards to user-side equipment (e.g., inverters, transformers) caused by reverse current.

• Prevents Energy & Economic Loss: Reduces energy waste and economic losses from feeding power back to the grid at low (or zero) compensation rates.

• Supports Diverse Applications: The technology is equally applicable to

• energy storage systems, microgrids, EV charging piles, enabling precise

• energy flow management.

Scheme Selection Recommendations:
Choosing a PV anti-backflow control scheme requires comprehensive consideration of:

1. System Scale & Capacity: Small residential systems may suit single-inverter meter schemes or simple cutoff schemes; large

commercial/industrial systems are better suited for multi-inverter coordinated schemes or EMS schemes.

1. Application Requirements: Clarify tolerance for PV curtailment, required automation level, and data management functionality.

2. Cost-Benefit Analysis: Balance equipment procurement, installation, and long-term O&M costs against expected benefits (e.g., value of reduced

curtailment losses, enhanced safety).

1. Technical Feasibility: Ensure the scheme reliably meets specific system

backflow prevention performance targets and grid interconnection standards.