lv hv transformer | 11kv to 415v transformer diagram

The LV HV (Low Voltage to High Voltage) transformer is a critical component of electrical power systems, responsible for stepping up voltage levels to facilitate efficient long-distance transmission. These transformers are ubiquitous in power generation plants, substations, and industrial facilities, playing a vital role in delivering electricity to end-users. Given their importance, ensuring their reliable and efficient operation is paramount. This necessitates rigorous testing and adherence to industry standards, particularly ANSI C57.15, the standard covering voltage regulators, which provides a framework for assessing transformer performance and identifying potential issues before they lead to costly failures.

This article delves into the intricacies of LV HV transformers, focusing on the critical testing procedures mandated by ANSI C57.15, and referencing relevant aspects often discussed in ABB transformer technical data sheets and catalogs. We will also touch upon related topics like typical LV transformer impedance charts, LV transformer substations, and the common 11kV to 415V transformer diagram. Understanding these aspects is crucial for engineers and technicians involved in the design, installation, and maintenance of power systems.

The Importance of ANSI C57.15 Compliance

ANSI C57.15 sets the benchmark for testing voltage regulators, which, while not all LV HV transformers are, the testing principles and methodologies are highly relevant and often applied, even if in modified form, to these transformers. The standard outlines a series of tests designed to verify the transformer's electrical, mechanical, and thermal performance characteristics. Compliance with ANSI C57.15 ensures that the transformer meets the specified requirements and is capable of operating reliably under various operating conditions.

Key Tests Defined by ANSI C57.15 (and Applied to LV HV Transformers):

While not every LV HV transformer is a voltage regulator, the following tests are routinely adapted and applied to ensure their performance:

1. Resistance Measurements of All Windings:

This test is fundamental in determining the integrity of the transformer windings. Resistance measurements are taken on all windings (primary, secondary, and tertiary if applicable) to detect any open circuits, shorted turns, or loose connections. These issues can lead to overheating, reduced efficiency, and premature failure. Accurate resistance measurements are crucial for calculating I²R losses (copper losses) and verifying the winding material and dimensions.

The process involves injecting a DC current into the winding and measuring the voltage drop across it. Ohm's Law (R = V/I) is then used to calculate the resistance. The measured resistance values are compared to the manufacturer's specifications and any significant deviations indicate a potential problem. Ambient temperature must be carefully recorded and accounted for, as resistance varies with temperature. Temperature correction factors are applied to normalize the resistance values to a standard temperature (usually 20°C or 75°C).

2. Ratio Tests on All Taps:

The turns ratio of a transformer is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding. This ratio determines the voltage transformation ratio of the transformer. The ratio test verifies that the actual turns ratio matches the nameplate value and that the transformer is properly wired.

Ratio tests are performed by applying a voltage to one winding (typically the primary) and measuring the voltage on the other winding (typically the secondary). The ratio of the applied voltage to the measured voltage is the turns ratio. This test is performed on all tap positions of the transformer to ensure that the tap changer mechanism is functioning correctly and that the voltage ratio is accurate across the entire range of tap positions. Significant deviations from the expected ratio can indicate shorted turns, open circuits, or incorrect wiring.

3. Polarity and Phase Relationship Tests:

These tests determine the polarity and phase relationship between the primary and secondary windings. Correct polarity is essential for proper parallel operation of transformers and for preventing circulating currents. Phase relationships are critical for ensuring correct connections in three-phase systems.

Polarity tests can be performed using a DC voltmeter. A DC voltage is applied to the primary winding, and the polarity of the voltage induced in the secondary winding is observed. The polarity is marked on the transformer terminals to ensure correct connections during installation. Phase relationship tests involve comparing the phase angles of the voltages on the primary and secondary windings using a phase meter or oscilloscope.

4. No-Load Loss (Core Loss) and Excitation Current Measurements:

No-load loss, also known as core loss or iron loss, is the power dissipated in the transformer core when the primary winding is energized with rated voltage and frequency, but the secondary winding is open-circuited. This loss is due to hysteresis and eddy currents in the core material. Excitation current is the current drawn by the primary winding under no-load conditions.

These tests are performed by applying rated voltage and frequency to the primary winding and measuring the input power and current. The input power is the no-load loss, and the input current is the excitation current. These measurements are compared to the manufacturer's specifications to assess the condition of the core. Excessive no-load loss can indicate core insulation degradation or core lamination damage.

5. Load Loss (Copper Loss) and Impedance Measurements: