A high-voltage transformer is a device that converts high-voltage AC power to low-voltage AC power or vice versa. High-voltage transformers are mainly used for testing electrical equipment and components under high voltage conditions in laboratories or factories. They can also be used for power transmission and distribution, as well as for special applications such as HVDC, FACTS, and SVC.
High-voltage transformers are designed to withstand high voltages and currents without damaging their insulation or core. They are usually oil-immersed, single-phase, core-type transformers with bakelite sheets separating the primary and secondary windings. The cooling system, the bushings, and the voltage regulation of these transformers are also important factors to consider.
This article will explain the basic principles, types, and applications of high-voltage transformers, as well as their testing methods and standards.
What is a High Voltage Transformer?
A high-voltage transformer is defined as a transformer that operates at a voltage level higher than 35 kV. According to IEEE Standard C57.12.01-2015, high-voltage transformers can be classified into three categories based on their insulation level:
- Distribution transformers: rated up to 69 kV
- Power transformers: rated from 69 kV to 230 kV
- Extra-high-voltage (EHV) transformers: rated above 230 kV
High-voltage transformers have different functions and specifications depending on their applications. Some common applications of high-voltage transformers are:
- Testing electrical equipment and components under high voltage stress
- Transmitting and distributing electric power over long distances
- Converting AC power to DC power or vice versa for HVDC systems
- Controlling reactive power and voltage stability for FACTS and SVC systems
- Generating high frequency or impulse voltages for special purposes
Types of High Voltage Transformers
There are different types of high-voltage transformers based on their construction, winding configuration, and output characteristics. Some of the most common types are:
Testing transformers are used to generate high voltages for testing the insulation strength and performance of electrical equipment and components, such as cables, insulators, switchgear, motors, etc. Testing transformers can be either single-unit or cascaded, depending on the required output voltage.
A single-unit testing transformer consists of a single transformer with a primary winding connected to a low-voltage source and a secondary winding connected to a high-voltage load. The output voltage can be varied by changing the input voltage or by using a tap changer on the primary or secondary winding.
A cascaded testing transformer consists of two or more transformers connected in series on the secondary side. The primary windings are connected to separate low-voltage sources or to a common source with different phase angles. The output voltage is the sum of the secondary voltages of each transformer.
The advantages of cascaded testing transformers are:
- The higher output voltage can be achieved with smaller and cheaper units
- Lower insulation stress on each unit due to lower individual voltages
- Higher efficiency and lower losses due to lower currents
The disadvantages of cascaded testing transformers are:
- More complex wiring and synchronization of the primary sources
- Higher risk of partial discharge and corona due to higher total voltage
- Higher capacitance and inductance of the secondary circuit due to longer wires
The rating of testing transformers depends on the type and duration of the test. For example, for cable testing, the transformer should have a high kVA rating to supply sufficient current during breakdown. For insulator testing, the transformer should have a low kVA rating but a high resistance in series to limit the current during breakdown.
The table below shows some typical ratings of testing transformers for various purposes.
|Routine test for electric motors & switchgear||Small||2 to 3 kV|
|Insulation testing||10 to 20 kVA||50 kV|
|Routine test of cable||50 kVA||10 to 30 kV|
|EHV transformer & insulator testing||20 to 50 kVA||100 – 200 kV|
|String insulator testing||0.5 to 1 kVA per kV||500 to 2000 kV|
|High voltage cable testing||100 to 500 kVA||100 to 500 kV|
Power transformers are used to step up or step down the voltage level for power transmission and distribution. Power transformers can have multiple windings with different voltages and currents on each side. They can also have different winding connections, such as delta, wyestar, or zigzag. depending on the phase and voltage requirements.
Power transformers have different ratings based on their voltage, current, power, frequency, and insulation levels. They are also classified by their cooling methods, such as oil-immersed, air-cooled, or water-cooled.
Power transformers are essential for efficient and reliable power transmission and distribution. They can reduce power losses, improve voltage regulation, and increase system stability and reliability.
HVDC transformers are used to convert AC power to DC power or vice versa for high-voltage direct current (HVDC) systems. HVDC systems are used for long-distance power transmission, interconnection of different grids, and integration of renewable energy sources.
HVDC transformers can have different configurations, such as single-phase, three-phase, or twelve-phase. They can also have different winding connections, such as star-star, star-delta, or delta-delta. The choice of configuration and connection depends on the type and topology of the HVDC system.
HVDC transformers have special features and requirements compared to conventional AC transformers. They have to withstand high DC voltages and currents, harmonics, polarity reversal, and transient overvoltages. They also have to provide adequate insulation and cooling.
FACTS transformers are used to control reactive power and voltage stability for flexible AC transmission systems (FACTS). FACTS are devices that can improve the performance and efficiency of AC power systems by using power electronics.
FACTS transformers can have different functions and types, such as shunt reactors, series reactors, phase-shifting transformers, static VAR compensators (SVC), static synchronous compensators (STATCOM), thyristor-controlled series capacitors (TCSC), or unified power flow controllers (UPFC).
FACTS transformers have to meet specific technical and operational challenges, such as high switching frequency, harmonics, thermal stress, and dynamic loading. They also have to provide accurate voltage regulation and fast response.
Special transformers are used for specific purposes or applications that require special features or characteristics. Some examples of special transformers are:
- Impulse transformers: used to generate high voltage pulses for testing or measurement purposes
- High-frequency transformers: used to operate at frequencies higher than 50/60 Hz for applications such as induction heating, radio transmission, or switching power supplies
- Instrument transformers: used to measure voltage or current in high voltage or high current circuits by stepping down the values to a safe and standard level for metering or protection devices
- Isolation transformers: used to isolate two circuits from each other for safety or noise reduction purposes
- Auto-transformers: used to step up or step down the voltage by using a single winding with a tap point
- Arc-furnace transformers: used to supply power to electric arc furnaces for melting metals
- Traction transformers: used to supply power to electric locomotives or trains
Applications of High Voltage Transformers
High-voltage transformers have various applications in different fields and industries. Some of the most common applications are:
- Testing electrical equipment and components under high-voltage stress in laboratories or factories
- Transmitting and distributing electric power over long distances with reduced losses and improved efficiency
- Converting AC power to DC power or vice versa for HVDC systems that can interconnect different grids, integrate renewable energy sources, or transmit power underwater or underground
- Controlling reactive power and voltage stability for FACTS systems that can improve the performance and reliability of AC power systems
- Generating high frequency or impulse voltages for special purposes such as induction heating, radio transmission, or lightning simulation
Testing Methods and Standards for High Voltage Transformers
High-voltage transformers have to undergo various tests before installation and operation to ensure their quality, safety, and performance. The tests can be classified into two categories: type tests and routine tests.
Type tests are performed on a sample of transformers to verify their design and construction according to the specifications and standards. Type tests include:
- Short-circuit withstand test: to verify the mechanical and thermal strength of the transformer under short-circuit conditions
- Lightning impulse test: to verify the insulation strength of the transformer under lightning surges
- Switching impulse test: to verify the insulation strength of the transformer under switching surges
- Temperature rise test: to verify the cooling system and thermal performance of the transformer under rated load
- Sound level test: to verify the noise level of the transformer under no-load and load conditions
Routine tests are performed on every transformer before delivery to check their functionality and quality. Routine tests include:
- Winding resistance test: to measure the resistance of each winding and calculate the copper losses
- Ratio test: to measure the voltage ratio of each winding pair
- Polarity test: to verify the polarity or phase relation of single-phase or three-phase transformers
- Dielectric test: to verify the insulation strength of the transformer under high voltage stress
- Temperature rise test: to verify the thermal performance of the transformer under rated load and ambient conditions.
Some of the routine tests are also performed at the site before commissioning the transformer, such as winding resistance, ratio, polarity, insulation resistance, and dielectric tests. These tests are done to assess the condition of the transformer after installation and transportation and to compare the results with the factory test reports.
In addition to routine tests, some special tests may be performed on request by the customer or as per specifications. These tests are done to obtain additional information about the transformer that may be useful for operation or maintenance purposes. Some examples of special tests are:
- Impulse test: to verify the insulation strength of the transformer under lightning or switching impulses
- Sound level test: to measure the noise level of the transformer under no-load and load conditions
- Short-circuit test: to verify the mechanical and thermal strength of the transformer under short-circuit conditions
- Partial discharge test: to measure the amount and location of partial discharges in the transformer insulation
- Dissolved gas analysis: to analyze the gases dissolved in the transformer oil and detect any faults or defects
Special tests are performed according to IEEE C57.12.90 and IEEE C57.12.91 standards for liquid-immersed and dry-type transformers, respectively.
High-voltage transformers are devices that convert high-voltage AC power to low-voltage AC power or vice versa. They have various types, such as testing, power, HVDC, FACTS, and special transformers, depending on their construction, winding configuration, and output characteristics. They have various applications, such as testing electrical equipment and components, transmitting and distributing electric power, converting AC power to DC power or vice versa, controlling reactive power and voltage stability, and generating high frequency or impulse voltages. They have to undergo various tests, such as type tests, routine tests, and special tests, to ensure their quality, safety, and performance.