Complete Guide to 138kv High Voltage Transformers

What Are 138kv Transformers?

A 138kv transformer is equipment designed to convert 138,000 volt high voltage electricity from transmission lines down to lower distribution level voltages, or vice versa. It consists of two separate winding wire coils, a laminated steel core, and an enclosure or tank for insulation and cooling.

Purpose and Function

The purpose of a 138kv transformer is to either step down extra high voltage power from transmission lines to subtransmission voltages of 69kv or 34kv for lighter commercial loads, or to step up subtransmission voltages to 138kv for more efficient long distance transmission.

This is possible thanks to the two sets of winding coils. Voltages induced across the primary windings get transferred by magnetic coupling of the shared core to the secondary windings at the transformed voltage. This allows easy conversion between high and low voltage levels.

Classes and Configurations

138kv transformers come in single or three phase configurations. Single phase units with a double winding core handle up to 25mVA and can be mounted on pads or poles. Three phase transformers with three connected cores handle anywhere from 30mVA to over 100mVA and require a protected concrete slab.

They also come in classes like ONAN, ONAF, ONFA etc, indicating different cooling set ups. ONAN is the most common, referring to mineral oil immersed construction with natural air cooling.

Why Are 138kv Transformers Important?

These voltage transforming units provide crucial links between bulk transmission networks and lower voltage distribution that powers communities. Without them, it would be impossible to deliver electricity efficiently, safely and economically across vast distances to end users.

Enable Long Distance Transmission

By stepping up voltages to 138kv or higher, line losses and voltage drops are greatly reduced, allowing economic transmission of large blocks of power over hundreds of miles. The higher the voltage, the more power that can be sent efficiently over infrastructure like high voltage lines and towers.

Reduce Voltage to Safer Levels

While high voltages like 138kv minimize losses for transmitting wholesale power, that level of voltage poses safety hazards for typical homes, buildings and local grids. Transforming voltage down to 34kv or 13kv at substations enables much safer distribution from local poles and lines.

Support Grid Reliability

The ability to shift voltages up and down facilitates flexible transmission and distribution, helping balance loads across infrastructure. This greatly improves overall grid reliability and stability, preventing blackouts or shortages when demand spikes in certain areas.

How Do 138kv Transformers Work?

Faraday’s law of electromagnetic induction and the properties of tightly coupled windings enable efficient and reliable voltage transformations. Let’s break down the key operating principles.

Alternating Magnetic Fields

The laminated steel core guides an alternating magnetic field produced by high voltage flowing through the primary winding. This changing field induces, or transforms, a corresponding voltage across the secondary windings physically encircling the same core at a ratio determined by turns ratio between the two windings.

Turns Ratio & Voltages

A 138kv transformer with primary windings of say 266 turns and secondary 13,800 turns would reflect a 138:34 kv turns ratio. This transforms 138kv down to 34kv while also transforming much higher 34kA potential full load amps to lower 266A at the 138kv transmission side.

Cooling Systems

As transformers heat up internally, cooling oil and external radiators or fans activate at set temps to maintain safe operations. Ambient vs load cooling classes differentiate transformers, larger ones with vast surface areas can rely more on passive air cooling, smaller ones need proportionally more active cooling.

Typical Uses of 138kv Transformers

These high capacity transformers see extensive use throughout electrical utilities for transmission networks as well as industrial plants and large facilities with substations.

Electrical Substations

The most common application is stepped down voltage transformation from incoming 138kv lines at distribution substations before re-routing out locally at safer 69/34/13kv levels to commercial zones and population centers via subtransmission lines and poles.

Power Plants

At generation stations, the transformers operate in reverse, stepping up the 15-25kv output of turbogenerators to 138kv or higher for injection to broader state level transmission grids via switchyards for transporting substantial electric power efficiently over long distances.

Large Scale Industrial Sites

Some heavy industry sites like mines, oil refineries, or manufacturing plants require reliable on-site substations with 138kv transformers to meet in-house loads from reasonably close utilities instead of operating expensive captive power plants.

Critical Operations and Maintenance

Like all electrical infrastructure, proper installation, controls, and regular upkeep help maximize longevity while minimizing risks for such major investments.

Commissioning Tests

Once installed on a pad or pedestal, technicians complete a battery of vital checks before energizing, including wiring polarity, grounding resistance, visual inspection, SF6 pressure/density, oil sample DGA etc to ensure safety, losses within spec and baseline data.

Real-Time Monitoring

Microprocessor relays and sensors continually measure voltage, load, temp, gases etc, issuing alarms for abnormal fluctuations indicating problems like overheating, arcing faults or insulation issues needing immediate investigation.

Preventative Maintenance

While robustly built, scheduled maintenance programs aimed at cleaning, testing sensitivity of protection relays, sealing cracks, filtering insulating oil, minimizing oxygen/moisture intrusion etc help minimize age related deterioration and keep units running reliably.

We hope this detailed overview gives you a solid understanding of what 138kv transformers are, why they’re so important for transmission infrastructure, and how they are operated and cared for. Contact us with any other questions!

FAQs

What are the most common types of 138kV transformers?

The most widely used types are mineral oil-immersed 138kV transformers. ONAN transformers are standard, relying on natural oil circulation and air. ONAF and ONFA use directional oil flow and air or oil & air forced cooling systems for higher capacity ratings.

What are the main components of a 138kV transformer?

They consist of two separate high and low voltage winding coils (often copper or aluminum) wrapped around a laminated steel core for magnetism, enclosed inside a welded steel tank filled with insulating mineral oil plus bushings, tap changers, pumps, radiators, gauges, relays etc.

How often should maintenance be performed?

ANSI standards recommend monthly to quarterly inspections for small issues. Comprehensive annual servicing checking power factor, dissolved gases, contact resistance and cleaning cells is ideal. Larger utilities practice targeted 5-10 yearly planned outage upkeep.

What are warning signs of transformer problems?

Frequent overheating, oil leaks, increased vibration and noise, reliability issues like unexplained trips or odd voltages and currents signify emerging troubles. Rapid action can prevent major and costly breakdowns.