Anke Electrical Co., Ltd.

Summary:As global warming intensifies, environmental protection issues brought about by economic development have become increasingly prominent. China has accordingly set the "peak carbon emissions, carbon neutrality" goals, and the development of clean energy has become an essential approach. In recent years, the capacity of individual wind turbines has continuously increased, and their installation locations have expanded from land to sea. A new protection scheme has been proposed for large-scale offshore wind turbine units, integrating arc fault sensors and current measurement into the protection of offshore wind turbines. This has introduced single and dual criterion solutions, enhancing the operational safety of offshore wind turbine units. This article mainly presents several suggestions on the application of arc fault protection in wind turbines through research and analysis of wind power generation technology.

Keywords:Arc light, protection, wind turbine, application

1. Wind Power Generation System Types

1.1 Classification Based on Engine Capacity

Wind power generation systems, when categorized by the capacity of the units, typically fall into four types, each with its own capacity range. First, the Small Unit, which generally has a capacity of 0.1 to 1 kW. Second, the Medium Unit, with a capacity usually ranging from 1 to 1000 kW. Third, the Large Unit, featuring a capacity typically between 1 to 10 MW. Fourth, the Giant Unit, with a capacity generally exceeding 10 MW.

1.2?Division based on the operational characteristics and control methods of the generator

If categorized based on the generator's operational characteristics or control methods, there are generally two types. The first is a Variable Speed Constant Frequency Wind Power Generation System. The wind turbines in this system are typically adjustable in speed, allowing for the reasonable utilization of wind energy to enhance the system's power generation efficiency. The second is a Constant Speed Constant Frequency Wind Power Generation System. This type is less commonly used in today's era. While it is convenient to use, this system generally has lower wind energy utilization and lower power generation rates, with certain shortcomings.

1.3 Operation Method Classification

Based on the operation methods of wind power generation, it can generally be categorized into two types. First, there is the grid-connected wind power generation system. This system is similar to the general power generation model, usually requiring the use of a large power grid to enhance the utilization of wind energy. Its advantage lies in its lower costs and is relatively common during this period. Second, there is the off-grid wind power generation system. This type of system typically operates independently and has a smaller capacity. It is commonly found along rural and urban roads.

1. Arc Flash Protection Principle

2.1 The Generation of Arc Light

Arc light is the ultimate form of gas discharge. When electrons around a gas atom are ionized into plasma, they become free electrons and, under the influence of external factors or a high electric field, radiate electromagnetic waves. These electromagnetic waves emitted outward from the discharge channel are known as arc light. According to relevant literature, the energy of arc light is mainly concentrated in the ultraviolet light band from 300 to 400nm and the visible light band from 500 to 600nm, with the central wavelengths approximately at 330nm and 530nm, respectively. Moreover, there is a tendency for the energy of arc light in the ultraviolet light band to be greater than its energy in the visible light band.

2.2 Arc Sensor

Arc fault sensors are photo-sensitive components. When a short circuit produces an arc, the light intensity suddenly increases, causing the electronic components within the arc fault sensor to convert the light signal into an electrical signal. This signal is then transmitted through a dedicated fiber optic to the arc collection unit or arc protection unit. The photoelectric conversion is completed within the arc collection unit or arc protection unit, thereby providing a basis for subsequent actions. Arc fault sensors should possess strong anti-interference capabilities, accurate optical measurement precision, and high reliability.

2.3 Rapid Protection

The severity of an arc fault depends on the magnitude of the arc current and the duration of the disconnection time. The energy produced by the arc, I2t, increases exponentially with the disconnection time T, as shown in Figure 1. From Figure 1, it can be observed that when the arc burning duration exceeds 100ms, the released energy begins to急剧 increase. Subsequently, various fault effects can cause severe damage to the cables and copper busbars of switchgear. Beyond 200ms, the energy reaches the level of cabinet combustion, hence the faster the protective action, the better. To ensure that the equipment is not structurally damaged, it is crucial to minimize the disconnection time. According to relevant standards, the pure arc fault trip time should be less than 10ms, and the action time with current dual criteria should be less than 20ms.

Figure 1: Diagram illustrating the dangers of arc light

Arc Flash Protection in Wind Power Applications

Arc light protection devices for wind power generation are installed inside the tower foundation control cabinet, primarily consisting of the main control unit and arc light sensors. The main control unit manages and controls the entire arc light protection system, processing and judging individual signals or two types of signals by detecting arc light and current increment signals. The arc light sensors are installed inside the medium-voltage switchgear and the tower foundation inverter. When the tripping conditions are met, it issues a tripping command to isolate the fault. The arc light sensors first transmit the detected arc light signals to the arc light collection unit, which then feeds back the signals to the main control unit via optical fibers.

3.1 Overall Structural Design

Arc fault sensors are individually installed on the high-voltage and low-voltage sides of the wind turbine generator housing, and within the inverter, referred to as single criteria. If both arc fault sensors and current transformers for measuring current are installed within the inverter simultaneously, the arc fault control unit will simultaneously judge the signals from the arc fault sensors and the incremental signals of the current. If the incremental current of any phase or the arc fault sensor signal is triggered, the arc fault control unit will send a disconnection command to the medium-voltage switchgear, causing the wind turbine generator to quickly disconnect from the grid. This method is called dual criteria.

3.2 Current Monitoring

The current transformer is utilized to measure the incoming or outgoing current of the variable frequency drive. The main function of the current transformer is to convert high currents in AC circuits into a proportionally smaller current (the standard in China is 5A) for measurement and relay protection purposes; it also isolates the measuring, protective, and control devices from high voltages. During use, the primary coil should be connected in series with the load line of the current to be measured, while the secondary coil is connected to form a closed circuit with a resistor. The working principle is similar to that of a transformer, but the specific type selection needs to differentiate between measurement and protection applications. The protective equipment should be selected based on the characteristics of the measured line, with the final selection primarily considering the current ratio and capacity. The current detection device for wind turbines is placed at the incoming cable of the medium-voltage switchgear cabinet, where three current transformers are installed to measure the currents of phases A, B, and C, respectively. The current transformers transmit the measured current values to the arc control unit.

3.3 Master Control Unit Functional Design

The main control unit of the arc suppression device, in addition to performing logical processing of signals from the arc sensor and current transformer, should also meet the following functions: 1) Self-check capabilities, including checks for hardware failures, software failures, and abnormal conditions of the arc sensor. Upon detecting its own fault, the device emits an alarm signal and simultaneously locks the entire protection system; 2) Indicators for operational and abnormal device states; 3) Display of information such as time, set values, configurations, collected data, and actions, along with event and operation record query functions; 4) Communication capabilities supporting RS485, Ethernet, and compliance with MODBUS protocols or IEC60870-5-103, 104 communication protocols.

Control of 4 Wind Turbine Units

4.1 Fixed Pitch Control Technology

The pitch control technology requires the blades on the hub of wind power generation equipment to be securely fixed. In the event of blade stall, it can control the maximum power to achieve quality control of wind power generation, which is the working principle of fixed-pitch wind turbines. This technology originates from Danish wind turbine generator technology and utilizes the blade airfoil stall theory. It enables the airflow attack angle to reach a specific value at the rated wind speed, causing vortices to appear on the blade surface and thereby reducing efficiency to limit power output. The fixed-pitch control technology has been adopted by many advanced manufacturers worldwide, producing numerous large-scale wind turbine generator sets, with an industry-wide usage rate of up to 70%.

4.2 Variable Pitch Control Technology

The variable pitch control technology of variable pitch wind turbines, achieved through the change in the longitudinal axis of the blades [6-8]. In this field, Vestas' wind turbines can be studied. The adjustment phase of this technology is divided into three stages: (1) The first stage is the startup operation. If the variable pitch wind turbine is in operation, the calculation of the fixed blade pitch angle will gradually expand, at which point the pitch angle needs to be adjusted. If the speed has reached 0.5 times the rated speed, a full adjustment of the pitch angle is required to position it appropriately, allowing the turbine to maintain a controllable speed and complete the grid connection and power generation function. (2) Entering the second stage, where the wind speed is below the rated speed, the power output is entirely dependent on the aerodynamic performance of the blades. Adjust the generator slip rate based on the wind speed to operate as close as possible to the optimal tip speed ratio for optimized power output. (3) Entering the third stage, if the power output can maintain consistency with the rated power, the system will gradually begin stable operation. At this point, the output power needs to be adjusted; if the output power exceeds the rated power, the pitch angle of the blades needs to be adjusted. Generally, if the wind turbine generator capacity exceeds 750kW, variable pitch control technology can be used; if it is less than 750kW, fixed pitch stall regulation technology can be used.

Ankorri ARB5 Series Arc Protection: Principles and System Composition

Arc protection devices initiate protection through dual criteria of arc + current surge or single arc criteria, with an operation time of

Arc flash single criterion action time ≤ 7ms; arc flash + current dual criterion time ≤ 20ms.

    The ARB5 series arc protection devices consist of the ARB5-M main control unit, ARB5-E expansion unit, ARB5-S arc probe, arc fiber optic cables, and tail fibers, among other accessories.

7. Conclusion

Arc light products are just beginning to be applied in wind turbines, capable of timely warning and tripping circuit breakers for arc and overcurrent phenomena in wind turbines, effectively preventing severe accidents such as fire outbreaks in wind turbines. Future integration with fire suppression systems like aerosol products can enhance the protection further.

Reference:

1. Wang Hai, Research on the Application of Arc Flash Protection in Wind Turbine Generators
2. Liu Lijiong, Chai Weijun, Zheng Xiaocui. Arc Protection Method for Low-Voltage Switchgear Based on Electronic Sensing Technology [J]. Automation and Instrumentation, 2020, 35(12): 98-102.

[3] Ankerui Enterprise Microgrid Design and Application Handbook, 2020.06 Edition.

[4] Ankerui User Substation Power Distribution and Monitoring Solution, 2021.10