Substation Testing and Commissioning: Power

Transformer Through Fault Test

Morteza Talebi, Power Grid EngineeringYigitcan Undug, Electric Power SystemsMarch, 2018, College Station, TX, USA

OUTLINE

• Motivation• Advantages of Transformer Through Fault Test (TFT)• Theory of Transformer TFT

– Calculation procedure– Testing procedure

• Real World Example• Conclusion• References

Advantages of Transformer TFT

This test will provide proof for:• CTs polarity and ratios• Power transformer winding connection and Phase

displacement• Protective relay wiring correctness• Protective relay setting• Potential circuit secondary

• Lots of valuable information - KNOWNS

Theory of Transformer TFT3 phase balance source will be in series with transformer

Since it is a balanced system, only positive sequence network is applied:

Theory of Transformer TFT-Calculation ProcedureCalculating the p.u. value of source output voltage and current

𝑉𝑉𝐺𝐺𝑝𝑝.𝑢𝑢 =𝑉𝑉𝐺𝐺𝑠𝑠𝑠𝑠𝑢𝑢𝑠𝑠𝑠𝑠𝑠𝑠𝑉𝑉𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵

𝐼𝐼𝐺𝐺𝑝𝑝.𝑢𝑢. =𝑉𝑉𝐺𝐺𝑝𝑝.𝑢𝑢.

𝑍𝑍𝑥𝑥𝑥𝑥𝑥𝑥𝑥𝑥Calculating transformer low and high side Base currents:

𝐼𝐼𝐻𝐻𝐵𝐵𝐵𝐵𝑠𝑠𝑠𝑠 = 𝑀𝑀𝑀𝑀𝑀𝑀3𝑀𝑀𝐻𝐻𝑥𝑥𝑥𝑥𝑥𝑥𝑠𝑠

, 𝐼𝐼𝐿𝐿𝐵𝐵𝐵𝐵𝑠𝑠𝑠𝑠= 𝑀𝑀𝑀𝑀𝑀𝑀3𝑀𝑀𝐿𝐿𝑥𝑥𝑥𝑥𝑥𝑥𝑠𝑠

Calculating current flow through the transformer primary and secondary side

𝐼𝐼𝐿𝐿𝑥𝑥𝑥𝑥𝑥𝑥𝑠𝑠 = 𝐼𝐼𝐺𝐺𝑝𝑝.𝑢𝑢. × 𝐼𝐼𝐿𝐿𝐵𝐵𝐵𝐵𝑠𝑠𝑠𝑠𝐼𝐼𝐻𝐻𝑥𝑥𝑥𝑥𝑥𝑥𝑠𝑠 = 𝐼𝐼𝐺𝐺𝑝𝑝.𝑢𝑢. × 𝐼𝐼𝐻𝐻𝐵𝐵𝐵𝐵𝑠𝑠𝑠𝑠

Theory of Transformer TFT-Testing Procedure• Safety

– Prior to performing the through-fault test, it is important that all safety procedures regarding the switching, locking, and tagging be observed and formalized to avoid protection engineers and technicians from being exposed to substation equipment that has not been properly de-energized.

• Protection zone– Location of connecting the test source in one side and shorting and grounding

of bushing on the other side of protection zone based on the relaying functional diagram and substation layout has to be identified.

• Identification– Primary conductors and phasing need to be visually proven and identified to

have a correct phasing prior to the test.

• Temporary generator phase rotation

• Known current magnitude and a phase angle

Simple Example

• Standard 3 phase Delta-Wye Xfmr• A-B connection • ABC Rotation• Differential relay zone is from CT1 to CT2

Simple Example-cont.

Phase angle relationship between testsource voltage and current in primary sideof Transformer

Phase angle relationship between currentin CT1 and CT2 seen by protectiverelay

Real World Example

Modified single line diagram of a 50 MW solar project

Transformer information3 𝑃𝑃HASE ⁄24 ⁄32 40 𝑀𝑀𝑉𝑉𝑀𝑀⁄115 34.5 𝐾𝐾𝑉𝑉 𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷 −𝑊𝑊𝑊𝑊𝐷𝐷

%𝑍𝑍 = 7.33 at 24 MVA

Real World Example-cont.

Real World Example-cont.

As it can be seen the expected value of A and C phase are opposite of the measured value!

Phase A and C were rolled!

Conclusion• TFT applies 3 phase balance source test which will be

in series with transformer positive impedance• Circuit test to check the correctness of whole circuit• Will give enough time to relay technicians and

engineers to troubleshoot• Cost of doing the test!

• Cost of not doing the test…

References