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There are many flexible AC transmission system components designed to enhance the performance, reliability and efficiency of electrical power systems. Below are some of the most common types of these devices.
Static Synchronous Compensator (STATCOM)
Commercially available integrated systems which utilise a thyristor SCR to generate or absorb reactive power in response to visible system conditions. They operate by adjusting the output of the voltage amplitude in a fraction of a second to maintain nominal voltage in less than ideal operating conditions without affecting steady-state voltage levels. This correction capability offers significant voltage stability benefits during stress periods, such as peak loading, contingencies and transient disturbances associated with large switched loads or generation outages.
Static Synchronous Series Compensator (SSSC)
SSSC employs a thyristor-controlled voltage source to inject a voltage out of phase with the line current, thereby controlling the power flow in a desired manner. This allows for the adjustment of power distribution between parallel routes under varying conditions, optimizing usage and preventing overloads on critical lines. It also provides series reactive compensation to mitigate transient stability issues such as oscillations among generators and loads. The SSSC's fast, adjustable output effectively manages these dynamics to enhance reliability.
Thyristor-Controlled Reactor (TCR)
A TCR is a reactor whose magnetic field strength and hence reactive power output are adjustable via thyristors overriding the natural saturation timing. By delaying the thyristor conduction phase angles, the TCR partially saturates the reactor, reducing its current offsetting inductance and lowering reactive power output. TCRs are employed to absorb excess reactive power, which could otherwise lead to voltage surges or limit load capacity and to maintain targeted voltage levels across key network buses during load fluctuations.
Thyristor-Controlled Switch (TCS)
TCS uses a thyristor to short-circuit an inductor or capacitor, allowing pre-energized to current through the switch and dumping it safely. Timing the TCS effectively bypasses the device during nonpeaking load times and only activates when needed to clear peak currents, lowering voltages seen by load. This Smart Grid switching lowers over current risk and extends system life, intelligently conditioning feeds to balanced loads, manage phase equalisation across feeders and mitigate the impact of intermittent generation like rooftop PV.
Phase-Controlled Rectifiers (PCR)
Phase Control Thyristor SCRs can function as rectifiers in circuits converting AC to DC while controlling the DC output voltage level or 'bucking' DC cable power by phase shifting AC. PCRs extract maximum power from AC sources like generators under variable loads by adjusting thyristor firing angles to control DC voltage linearly over a wide load range. PCRs provide steady DC used to power sensitive electronics and equipment vital for system monitoring, communications and control.
Phase-control thyristors are widely used in several industries to control power in different applications.
Heating systems
In the heating industry, thyristors are extensively used in electrothermal devices such as induction heating systems, where they control the power supplied to metalworking applications like melting and forging through precise temperature regulation. In resistance heating, thyristors manage current flow to heating elements in furnaces, kilns and other industrial heaters, achieving energy-efficient heating in a safe and user-friendly system. They feature prominently in large heaters and heaters for hazardous environments, replacing mechanical contactors in major energy consumers like steel, aluminium and glass.
DC Motor Speed Control
Utilising phase-controlled thyristors in motor drives permits precise speed management of critical tasks in industries ranging from manufacturing to transportation. It enables regulated power delivery to DC motors, facilitating a broad range of operational speeds while maintaining torque. This is particularly important for applications like conveyor belts or fans where demands fluctuate. Controlled speed also benefits electric vehicles and rail systems, improving travel efficiency and energy conservation. Here it allows regenerative braking by reversing the current flow.
Uninterruptible Power Supplies (UPS)
Phase control SCRs optimise AC-DC conversion and battery charging in uninterruptible power supplies. By managing the rectifier circuit, they ensure efficient power backup transitioning during outages and full system protection. Their controlled power release safeguards sensitive equipment from surges, facilitating consistent conditioned power. Advanced multi-phase UPS systems use phase control for load balancing across powered phases, improving reliability in data centres and telecommunications infrastructure systems where continuous power must be maintained despite variable grid fluctuations or loss.
Motor control
Phase-control thyristors precisely vary AC motor voltage and frequency for fine-speed control in industrial applications like pumps, compressors and fans. This capability is vital where required torque must be maintained across speed ranges. Phase control also facilitates softer motor starts and smoother variable speed operation. In fan applications, it improves airflow control in HVAC systems. Moreover, it plays a role in electric vehicle motor drives, enabling efficient power utilisation for varying speeds and torque needs in traction systems.
Power control
A thyristor can switch currents at the required angles as controlled resistance on an alternating cycle, thus taking any value from zero to full resistance. The resistance can be controlled so that air is drawn through a heating element at any required rate. The faster the air is drawn, the higher the resistance; hence, more heat is transferred. Phase control thyristors are used in power control applications, including induction heating, electric furnaces, motor control and variable-speed fans. Induction heating uses a thyristor to control the coil current, an electric furnace that controls the heating element and a motor to control the speed of as fan.
Here are some features buyers should consider when purchasing phase control thyristors.
Voltage rating
Phase control thyristors have a wide voltage rating, typically from 200 V to 3000 V. This wide rating allows them to be used in many applications, from low-voltage industrial motor control to high-voltage power systems. The voltage rating is important as it determines the maximum voltage the thyristor can withstand without breaking down.
Current rating
Like voltage rating, thyristors' current ratings depend on their usage. In industrial applications, common ratings are 40 A to 200 A. High-power situations, like HVDC devices, need 1000 A or more. Current ratings are essential as they define the maximum continuous current the thyristor can handle.
Heat sink
Phase control thyristors control power at high voltages, producing heat. Without proper cooling, their internal components may degrade, leading to failure. In industries, this means heat sinks, usually radiators or fans, are added to dissipate heat and keep functioning. In electric furnaces, ineffective cooling leads to elevated temperatures, damaging components. Therefore, adequate heat management is essential for long-term reliability and performance.
Power rating
Phase-control thyristors have a typical power rating of around 1000 volts, 50 hertz and 40 amperes. This means a thyristor can control about 40 kw of power. The thyristor must be mounted on a heat sink that allows the thyristor to operate at around 100°c for dependable operation.
Reverse Voltage
Phase-control thyristors typically have a reverse voltage of about 1 V to 3 V. Some thyristors are built with extra protection against reverse bias, such as RK diodes. It may be required on some anti-parallel thyristors to extend their lives.
Firing circuit
A few thyristors need electrical firing, which can be achieved with power-electronic components like pulse transformers and optical isolators. Low-power thyristors can also be fired using a low-power opto-isolator.
When buying these products, here are some things to keep in mind.
Application needs
To select the appropriate phase-control thyristor for a customer, one needs to understand the application requirements. For example, the customer should be able to choose a photoconductive SCR to get an electronic interface or a standard SCR for an electrothermal device. In addition to the application, factors such as voltage rating, current rating and heat dissipation requirements should also be considered.
Reputation
The reputation of a manufacturer can greatly affect the quality of products. Therefore, customers ought to purchase from a well-known brand to get quality, durable products. They should also consider the manufacturer’s customer service and warranty policy.
Operating environment
The operating environment for these devices will affect how buyers use them. For example, if the products are to be used in wet or high-temperature conditions, ensure to get one with a moisture cover. Also, in choosing these products, consider the installation space, as some of them can be too bulky.
Electrical environment
Get the thyristor that matches electrical system standards. Select one with the required current and voltage ratings. Also, get one that can function at the required frequency. Buyers should also ensure to get products that match their control requirements. For example, if the system requires precise control, a high-frequency thyristor opt for a fast switching thyristor.
Heat management
Buyers also need to consider thermal management. They should ensure that the phase control thyristor has suitable heat sinks and a cooling system. This will help them effectively manage heat and prevent the devices from failing.
Documentation and support
To effectively use these products, customers should ensure they have all technical support information. Therefore, they should buy from manufacturers that can provide operational manuals, installation guides and application notes. Customers should also ensure they buy from manufacturers that can provide customer service in case they have further-related queries.
Yes, there are some designed to be used in environments with high temperatures, humidity and dust. They have protective covers that ensure the devices are not affected by such conditions.
Yes, there are integrated circuits that combine a thyristor with other power devices such as transistors. These devices can perform multiple functions such as AC to DC conversion and motor control.
Manufacturers make phase control thyristors primarily from silicon. However, they also use silicon carbide to make devices that operate at high voltages and temperatures.
These devices would require regular maintenance checks for the cooling system and the heat sinks. They also need frequent inspections to ensure there are no electrical connections among the devices.
These devices can last up to 20 years operating in good conditions. The lifespan can be affected by constant thermal cycling and exposure to high voltages.
