A COMPREHENSIVE GUIDE TO TRANSFORMER TURNS RATIO TEST SET CALIBRATION

INTRODUCTION

A transformer turns ratio test confirms ratio accuracy of a transformer. The test ensures the correct ratio of both primary turns and secondary turns. It can also help to identify issues related to tap-changer performance, shorted turns, incorrect winding connections, and open windings. Due to the many benefits a transformer turns ratio test can reap, it is crucial and also within your best interest to have your transformer tested frequently.

There are a multitude of reasons a Transformer Turns Ratio Test Set might need re-calibration. While using in service, the windings of the test set can become damaged or deteriorated, resulting in spikes, surges, contamination, faults or other unintended consequences. These side effects of a lack of calibration can have a costly or even damaging effect on those using the test set.

In this white paper, we will cover the basics of transformer turns ratio test sets, the benefits and frequency of calibration, a brief overview of how to calibrate a test set and finally, how to choose a calibration lab for your Transform Turns Ratio Test Set.


WHY RATIO MEASUREMENTS OF TRANSFORMERS ARE IMPORTANT

Ratio measurements of a transformer are of extreme importance for several reasons, such as:

Transformer turns ratio tests should be completed on a frequent basis to ensure the accuracy of a transformer. During testing if ratio results differ from the expected target values, the likeliness of a defect or error is high. Untargeted results could indicate a defect in winding or a tap-changer.

STYLE VARITIES OF TRANSFORMER TURNS RATIO TEST SETS

Before testing a transformer, it’s important to observe which transformer test set is necessary. Types of transformer turns ratio test sets vary in style and test connections; however, all test sets consist of at least two high leads and two low leads. Different styles of transformer turns ratio tests sets include:

SINGLE-PHASE MODELS

Single-phase models are designed to measure the turns ratio and exciting current windings in power transformers. These models are primarily used for testing single-phase power with low ratios; however they can be used to test three-phased transformers. If used to measure three-phase transformers each phase has to be tested separately.

THREE-PHASE MODELS

Three-phase models are designed to measure the turns ratio of power, instrument and distribution transformers in a manufacturing setting. Unlike the singe-phase model, the three-phase model can test high ratios.

BENEFITS OF CALIBRATION

The proper functionality of a transformer turns ratio test set is of utter importance. When working with electricity and high voltages, precise equipment is necessary to ensure correct operation and safe handling. In order to assure the accuracy of a turns ratio test set, calibration should be performed on a frequent basis.

Calibrating a turns ratio test set can reap many benefits. Test sets that have been calibrated regularly are known to obtain the following:

ACCURACY

Calibration of a turns ratio test set minimizes any ratio measurement uncertainties. When calibration is performed, uncertainties within the ratio measurement are processed to determine an acceptable margin of error.

RELIABILITY

Turns ratio test sets that are calibrated frequently are guaranteed to be more reliable than those that are not. This is because measurements of a test set have been calibrated to be the most accurate.

SUSTAINABILITY

Any instrument that has been calibrated is said to have a longer life-span than those that have not. This is because instruments that are calibrated are well-maintained and properly used.

HOW FREQUENTLY SHOULD A TURNS RATIO TEST SET BE CALIBRATED

A turns ratio test set should be calibrated regular to ensure it is accurate and reliable. Many test sets will have a manufacturer recommendation on how frequent the instrument should be calibrated. Although following this schedule could be beneficial, it is important to note that turns ratio test sets should also be calibrated after the following:

Notably, making a personal calibration schedule for a turns ratio test set is highly recommended. Every test set is used differently; for example, some more frequently than others. For this reason, a test set should be scheduled to be calibrated annually, quarterly or monthly depending on its usage.

A transformer turns ratio test is considered a low voltage test used to determine the ratio of a high voltage winding. The turns ratio test is performed on every tap of a winding with either a single-phase or three-phase model. During testing, the test sets are used to apply voltage to the H marked leads and measure the X marked lead. Ratio is then measured by dividing the induced voltage reading by the applied voltage reading.

**CAUTION** DO NOT ATTEMPT TO PERFORM THIS TEST IF YOU ARE NOT A CERTIFIED TECHNICIAN IN THE FIELD. TO FIND A CERTIFIED CALIBRATION LAB, CLICK HERE.

STEP-BY-STEP PROCEDURE
STEP ONE

Disconnect and isolate the transformer from any incoming and outgoing cables. Be sure to apply working ground to the incoming and outside cables prior to disconnecting from the transformer. Notably, disconnected cables should have a significant amount of clearance from the switch gear terminals.

STEP TWO

Connect the test set to the H designated lead – ensure that the connector is marked with H. Also, be sure to check that the connector’s index notch lines up precisely.

STEP THREE

Same as step two but with the X designated lead. Ensure that the connector is marked with X and check the connector’s index notch is lined up properly.

STEP FOUR

When using a three-phase model, connect H1, H2 and H3 designated test leads to the corresponding terminal. Connect to the H0 test lead if present.

STEP FIVE

Same step as four but with the X designated test leads. Connect X1, X2 and X3 to the corresponding terminals.

STEP SIX

After everything is connected, perform turns ratio measurements for every tap. It’s important to ensure that all measured ratios are with the calculated ratios.

Although this step-by-step guide is beneficial for understanding how a turns ratio test is performed, it is in your best interest to seek a professional to perform these tests on a transformer. Correct voltages are crucial for safe handling of any equipment dealing with electricity. By choosing a professional to complete this test procedure, safety can be guaranteed.

CHOOSING A CALIBRATION LAB

Finding the right calibration service provider for you transformer turns ratio test set can often be a stressful task. Oftentimes, individuals do not know if they should go with the equipment’s manufacturer or a calibration lab. When weighing out your options, it’s important to keep in mind what will be best for you. Nine times out of ten, you will find that choosing a calibration lab to perform your services will be more beneficial than choosing the equipment’s manufacturer.

When browsing for the right calibration lab, it’s important to ask yourself these four key questions to ensure it is a great match:

IS THE CALIBRATION LAB ACCREDITED? IF SO, IS TURNS RATIO TEST SETS INCLUDED IN THEIR SCOPE?

Accredited calibration labs are nationally recognized as being competent and complaint with ISO 9001 or ISO/IEC 17025 standards. This means they are highly knowledgeable in performing calibrations for anything under their scope of accreditation. If your turns ratio test set is included within a labs scope, then choosing this lab guarantees equipment accuracy.

IS THE TURNAROUND TIME ACCEPTABLE?

It’s important to choose a lab that will have a quick turnaround time. This will ensure minimum downtime for your company. When downtime is extended over a large course of time, your company loses money and in other cases, customers.

WHAT IS THE TRACEABILITY?

An accredited calibration lab is required to have all measurements traceable through NIST. Traceability is very beneficial because its guarantees the equipment used to calibrate your turns ratio test set is working within its intended specifications.

CAN THE LAB CALIBRATE MY OTHER KEY EQUIPMENT?

When looking for a calibration lab, it is important to find one that is capable of calibrating all your key equipment. This keeps all your equipment’s calibration records in one place. This is beneficial because recalibration schedules for each instrument will be easier to track, especially if the calibration lab offers an asset management system.