Technical library

Standard Battery Testing Requirements Summary

Maintenance professionals tasked with maintaining electric motor reliability are often concerned with testing their critical motors at voltages well above nameplate levels. It can certainly feel counterintuitive to test a 460V motor to 2,000V or a 4,160V motor to 9,320V. After all, the motor is only rated for this lower voltage, right? Some may even have had experiences with motor failure following a hipot or surge test at these elevated voltages. These events can create even more hesitation in using high voltage testing as an in-service evaluation technique.

Megger’s article “Transformer Winding Resistance Measurement: Field Challenges” was presented at the 2020 NETA PowerTest Conference in Chicago, Il. This article takes an in-depth look at the lesser known facts associated with the DC winding resistance (WR) measurements, diving deeper into topics such as selection of the correct test current, and the importance of compliance voltage during the test. Phenomena as core saturation, current stabilization, the influence of winding inductance on readings, and the effect of temperature, are also explained.Article Brief:DC winding resistance measurements (WRM) performed on distribution and power transformers are used to diagnose internal issues such as shorted turns, burnt or open windings, poor connections, and problems with On-load tap changers (OLTC) and De-energized tap changers (DETC). Industry acceptance criteria recommends that winding resistance between phases should be within 2% of each other. Additionally, comparison may also be made with original factory measured data, where differences within 5% is considered satisfactory. Results should be temperature corrected when measuring against factory readings or historical measurements. If readings obtained are outside of industry standards, complementary tests are recommended to assess winding condition. Although performing a WRM is relatively straight forward, there exist several challenges that one can encounter while testing in the field. The following section discusses these field challenges and solutions to support winding resistance testing.

Transformer Turns Ratio Testing: Lesser known facts that are affecting your ResultsOverviewTransformer Turns Ratio (TTR) Measurement is a diagnostic field test performed on a transformer to assess winding and core condition. The traditional method of performing this test has been limited to applying an AC voltage across each phase of the HV winding and measuring the AC voltage induced on the corresponding LV winding phase. The measured ratios are compared with calculated ratios from the nameplate voltages and as per international standards the calculated % error needs to be within ±0.5%. A number of factors such as core permeability, mutual and leakage flux, excitation losses, and winding configuration can influence the % error obtained. Through field testing it has been found that accurate results are obtained by energizing all the three phases simultaneously from the LV winding side and measuring the voltage induced on the HV winding.

This article describes the application of DFR and HV DFR as a preventive method to evaluate the condition and prioritize maintenance activities on HV and EHV bushings and relates some case studies. 

The  Institute  of  Electrical  and  Electronic  Engineers  (IEEE)  Standards  Association recently approved the publication of IEEE Std. C57.161–2018, Guide for Dielectric Frequency Response Test. The work was developed within the IEEE Transformers Committee and the Dielectrics Subcommittee. The industry needed a document to better understand the field application of a non-intrusive and non-destructive method to assess the condition of oil-paper insulation in power and distribution transformers.

The Narrow Band Dielectric Frequen-cy Response (NBDFR) test method consists of a series of power factor measurements ranging from 1 to 1000 hertz. This aggregate of the CHL (pri-mary to secondary) measurements constitutes the dielectric response of the test specimen. The subsequent evaluation consists of a geometric analysis of a plot where the mea-sured power factors and correspond-ing frequencies are graphed. As an insulation system ages, the response migrates towards the high frequency end of the plot. When the frequency corresponding to the lowest magni-tude within the response (the trough) is used as a reference point, the move-ment of the response may be quanti-fied; enabling the transformer may be classified in terms of condition.

Power utility companies operate nationally important critical infrastructure and thus face considerable regulatory compliance issues. Testing power system elements like relays and current transformers under these rules is a time consuming and costly operational expenditure. In a bid to streamline its testing operation, Georgia Power deployed new relay test sets from Megger. For their test engineers, the results speak for themselves.

Power swing which is principally caused by an oscillation in active and reactive power of transmission line, consequent to an enormous disruption in power system, which if not blocked, could cause wrong operation to the distance relay which may lead to tripping the healthy part of the transmission line. In the absence of power swing function it may result in severe damage to the machines or cascading tripping in the grid resulting in blackouts. To prevent such scenarios, intelligent electronic devices (IEDs) have power swing block (PSB) detection and trip logics incorporated with distance schemes. 

Transformers are critical assets of power system network and their adequate protection is a matter of vital importance. Modern protection relays are usually defined as intelligent electronic device (IED) s, and they provide huge flexibility and enhanced protection features to assure correct operation of the equipment. Transformer protection IEDs are equipped with key features such as differential and restricted earth fault (REF) protection along with several other functionalities as per the guidelines of IEEE C37.91.