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The impedance of a transformer is a crucial parameter in its design and operation. It affects the transformer's performance, efficiency, and safety. During the design, manufacturing, and operation of a transformer, the factors that influence impedance need to be carefully considered, and its value should be reasonably controlled. Transformer impedance reflects the degree to which the transformer resists the flow of current.
1/Definition:
Transformer impedance refers to the internal electrical resistance of the transformer, which consists mainly of the resistance of the windings, the resistance of the insulating material, and the core losses in the magnetic circuit. It includes three components: resistance, inductive reactance, and capacitive reactance.
2/Representation:
Transformer impedance is usually expressed in percentage or ohms to describe the electrical characteristics of the transformer and its ability to match the load.
3/Role of Impedance Voltage in Operation:
Impedance voltage is a crucial economic and technical parameter that affects transformer efficiency and operation. The short-circuit impedance is equivalent to the transformer's internal resistance. The short-circuit impedance plays a decisive role in the amount of short-circuit current that occurs when the transformer’s low-voltage side experiences a fault. When transformers operate in parallel, the transformer with higher short-circuit impedance will carry a larger share of the load under equal conditions. Generally, as transformer capacity increases, short-circuit impedance decreases.
4/For Power Plant Main Transformers (Step-Up Transformers):
From an operational perspective, it is better for the impedance voltage to be smaller, as this results in higher efficiency. For transformers of the same capacity, a smaller impedance voltage leads to higher operational efficiency, less voltage drop, and smaller voltage fluctuation, making voltage quality easier to control and maintain. From the standpoint of limiting system short-circuit currents, a larger impedance voltage is preferable to prevent excessively large short-circuit capacity, which could make it difficult to select electrical equipment (such as circuit breakers, isolators, cables) or cause damage during operation due to excessive short-circuit currents. Therefore, impedance voltage should be selected based on system and equipment operation requirements, ideally as small as possible.
5/For Auxiliary High-Voltage Transformers in Power Plants:
The impedance value directly affects the short-circuit current and voltage level of the high-voltage auxiliary system. The minimum allowed impedance value for auxiliary transformers such as startup or standby transformers is limited by the selection of electrical equipment (such as circuit breakers, isolators, cables) within the plant, specifically the rated breaking current and dynamic stability current. The maximum allowable impedance is determined by the voltage requirements of the largest motor that starts under load. Therefore, impedance values should be optimized based on these factors.
The formula for calculating transformer impedance is:
Z = R + jωL + 1/(jωC)
Where:
In this formula, LLL, which represents the inductance, is typically the largest component of impedance in transformers. In practice, for everyday selection, comparing common data values is often sufficient, and detailed calculations are rarely required. Larger impedance values result in smaller short-circuit currents, which offer certain benefits, but also increase voltage drop, which sometimes cannot be adjusted via taps. If the impedance is too small, the short-circuit current will be large, leading to issues with equipment sizing, and requiring an increased short-circuit capacity, which is economically inefficient. Therefore, the impedance value must be chosen based on a comprehensive analysis. Standard product specifications can usually be relied upon, but for specific values, manufacturers can be consulted.
The magnitude of transformer impedance is influenced by multiple factors, including:
1.Design and Manufacturing:
The materials, layout, and geometric structure of the windings, as well as the material and structure of the core, all affect the impedance.
2.Rated Power and Voltage Ratio:
Generally, the higher the rated power of a transformer, the lower its impedance; conversely, the higher the voltage ratio, the higher the impedance.
3.Load Characteristics:
Factors such as the size, type, power factor, and harmonic content of the load can influence transformer impedance.
4.Short-Circuit Impedance:
Short-circuit impedance refers to the impedance when the secondary side of the transformer is short-circuited, and the primary winding is subjected to an applied voltage. The magnitude of the short-circuit impedance indicates the voltage drop across the transformer under rated load conditions. Generally, for small transformers, the short-circuit impedance Uk(U_k(%)Uk( ranges from 4% to 10.5%, while for larger transformers, it ranges from 12.5% to 17.5%. The value is typically expressed as a percentage on the nameplate.
The short-circuit impedance plays a vital role in transformer performance and reliability. A higher short-circuit impedance means that the transformer can withstand higher fault currents, reducing the damage from short-circuit currents and improving grid reliability. The effects of short-circuit impedance include:
1.Impact on Short-Circuit Current:
For transformers with the same rated capacity, the transformer with a higher short-circuit impedance will have a smaller short-circuit current, reducing the forces on the windings. On the other hand, the transformer with a lower impedance will experience higher short-circuit currents, leading to higher mechanical stresses on the windings.
2.Impact on Voltage Drop:
Short-circuit impedance also affects the voltage drop when the transformer operates under rated load. Transformers with higher short-circuit impedance will have a higher voltage drop.
3.Impact on Parallel Operation:
In parallel operation, if one transformer with higher short-circuit impedance is fully loaded, the transformer with lower impedance will become overloaded, while the transformer with higher impedance will operate underloaded.
1)Determining Voltage Drop:
Transformer impedance governs the voltage drop when the load changes. As the load increases, the voltage drop increases. A higher impedance leads to a smaller voltage drop. Proper control of transformer impedance ensures that load equipment operates normally and avoids excessive voltage fluctuations.
2)Influencing Short-Circuit Current:
Transformer impedance also determines the magnitude of short-circuit current. Lower impedance leads to larger short-circuit currents, while higher impedance limits short-circuit current. Correctly choosing transformer impedance protects both the transformer and connected equipment from excessive short-circuit currents.
3)Energy Consumption and Efficiency:
A lower impedance means higher energy loss in the transformer because more power is dissipated in the load. Higher impedance reduces energy loss and improves transformer efficiency.
Transformer impedance can be measured using methods such as the bridge method (single-arm or double-arm), high-capacity DC power source method, and constant voltage and current source method. The selection of impedance for factory use transformers is a complex process requiring the consideration of various factors. During selection, the load characteristics, system short-circuit current, voltage stability, economics, and efficiency must be carefully evaluated, referencing relevant standards and optimizing the design.
Key points to consider:
(1)Load Characteristics: The size, type, power factor, and harmonic content of the load will affect impedance selection. For larger loads or when harmonics are present, transformers with higher impedance may be required to limit short-circuit currents and reduce harmonic effects.
(2)System Short-Circuit Current: The impedance value directly influences the short-circuit current in the high-voltage auxiliary system. A higher impedance may be needed to limit short-circuit currents and protect electrical equipment such as circuit breakers and isolators.
(3)Voltage Stability: Transformers with lower impedance tend to offer more stable output voltages, but their short-circuit current tolerance may be lower. Therefore, impedance should be selected to balance voltage stability with the ability to withstand short-circuit currents.
(4)Economics and Efficiency: While transformers with higher impedance may limit short-circuit currents, they might increase energy consumption and manufacturing costs. Economic and efficiency considerations must also be factored into impedance selection.
(5)Equipment and Busbar Requirements: The impedance value of factory-use transformers is influenced by the selection of electrical equipment like circuit breakers, isolators, and their rated breaking currents. The maximum impedance value is also determined by the voltage requirements of the largest motor starting on the bus.
(6)Standards: Refer to standards like "Technical Parameters and Requirements for Oil-Immersed Transformers" and "Technical Parameters and Requirements for Dry-Type Transformers" to understand recommended impedance values and selection ranges. Generally, the impedance for factory-use transformers falls within 2% to 8%, but the final selection should be based on actual conditions.
