750kV Transformer On-Site PD and Induced Withstand Test: Case Study and Recommendations

I. Introduction

The Guanting–Lanzhou East 750kV transmission and substation demonstration project in China was officially put into operation on September 26, 2005. This project includes two substations—Lanzhou East and Guanting (each equipped with four 750kV transformers, three of which form a three-phase transformer bank in operation, with one on standby)—and one transmission line. The 750kV transformers used in the project were independently developed and manufactured in China. During the on-site commissioning tests, excessive partial discharge (PD) was detected in the Phase A main transformer at Lanzhou East Substation. A total of 12 PD tests were conducted before and after commissioning. This paper analyzes the reference standards, procedures, data, and issues related to the PD tests of this transformer, and offers practical engineering recommendations to support future on-site testing of 750kV and 1000kV transformers.

II. Basic Transformer Parameters

The main transformer at Lanzhou East Substation was manufactured by Xi’an XD Transformer Co., Ltd. Key parameters are as follows:

• Model: ODFPS-500000/750

• Rated Voltage: HV 750kV, MV (with ±2.5% tap changer) kV, LV 63kV

• Rated Capacity: 500/500/150 MVA

• Maximum Operating Voltage: 800/363/72.5 kV

• Cooling Method: Forced oil circulation with air cooling (OFAF)

• Oil Weight: 84 tons; Total Weight: 298 tons

• HV Winding Insulation Level: Full-wave impulse 1950kV, chopped-wave impulse 2100kV, short-time induced withstand voltage 1550kV,  power frequency withstand voltage 860kV

III. Test Procedure and Standards

(A) Test Procedure

According to GB1094.3-2003, the partial discharge test procedure for transformers consists of five time periods—A, B, C, D, and E—with specified applied voltages for each. The pre-stress voltage during the C period is defined as 1.7 per unit (pu), where 1 pu = Um/√3 (Um being the maximum system voltage). This value is slightly lower than Um specified in GB1094.3-1985. For the Lanzhou East transformer, Um = 800kV, so the pre-stress voltage should be 785kV.

(B) Withstand Voltage Requirements

• The short-time induced withstand voltage for the Lanzhou East  transformer is 860kV. According to the State Grid Corporation of China's  "Commissioning Test Standards for 750kV UHV Electrical   Equipment," the on-site test voltage should be 85% of the factory  test value, i.e., 731kV, which is less than the required pre-stress  voltage of 1.7 pu (785kV).

• To resolve the conflict between pre-stress voltage and  ommissioning withstand voltage, relevant standards state that if the pre-stress voltage exceeds 85% of the factory withstand voltage, the actual pre-stress voltage should be agreed upon by the user and  manufacturer. The "Technical Specification for 750kV Main  Transformers" explicitly specifies that the on-site PD test  pre-stress voltage equals 85% of the factory withstand voltage. As a result, the pre-stress voltage for the on-site PD test of the Lanzhou East transformer was set at 731kV. The PD measurement and withstand test were  combined, with the withstand test phase serving as the pre-stress stage of  the PD test.

(C) Acceptance Criteria for Partial Discharge

Under a test voltage of 1.5 pu, the transformer's partial discharge level must be less than 500 pC.

IV. Test Process

From August 9, 2005, to April 26, 2006, a total of 12 PD tests were conducted on the Phase A main transformer at Lanzhou East Substation. Key test information is summarized below:

Overall, the PD level of the MV winding of Phase A main transformer before commissioning ranged between 600 and 910 pC, exceeding the 500 pC acceptance criterion. However, after retesting on April 26, 2006, following commissioning, the PD level dropped to 280 pC, meeting the requirement.

V. Test Analysis

(A) Partial Discharge Inception Voltage (PDIV) and Extinction Voltage (PDEV)

• Definition Issues: GB7354-2003 and  DL417-1991 provide imprecise definitions of PDIV and PDEV. For instance,  the "specified value" in the definition is not clearly  defined—though 500pC is commonly assumed, this leads to significant  inconsistencies in practical application. Additionally, background noise  during on-site tests often reaches tens to hundreds of picocoulombs, making it difficult to identify a clear onset of discharge.

• Case Observations: In the 12 PD  tests conducted on the Lanzhou East Phase A transformer, the PD level  increased gradually with voltage, without a distinct jump (maximum step change ~200pC), making it impossible to determine a clear PDIV. In some  tests, measurable PD was already present at low voltages, making it  difficult to assess whether PDIV had decreased. Moreover, the latest   national standard GB1094.3-2003 does not mention PDIV or PDEV, leading to  inconsistent interpretation and determination among practitioners.

(B) Discharge Localization

• Limitations of Common Methods: The widely used ultrasonic PD localization method detects the time difference of ultrasonic waves generated by discharges arriving at sensors on the  tank wall. However, this method faces challenges such as immature  technology, requirement for sufficiently large discharge energy (within sensor sensitivity range), and inaccurate localization due to multiple      reflections and refractions of ultrasonic waves from inner windings.

• Case Results: During  pre-commissioning tests, PD localization equipment provided only a rough  estimate of the discharge location. The control room monitoring system failed to detect PD variations with voltage, limiting the usefulness of   the results. Later-installed online monitoring systems also failed to  detect relevant changes during the April 26, 2006 test. Therefore, ultrasonic localization results should be treated with caution when PD levels are low.

(C) Severity of Discharge

Although the standard specifies a 500pC limit at 1.5 pu, in practice, there is no significant difference between 500pC and 700pC—they belong to the same order of magnitude. Moreover, when PD is below 1000pC, there is typically no visible discharge trace inside the transformer, and on-site oil drainage inspections rarely reveal abnormalities. Returning a 750kV transformer (large and heavy) to the factory for repair carries high risks.

VI. Recommendations

(A) Increase Insulation Level

The induced withstand voltage of the Lanzhou East transformer is relatively low. Considering the short history and limited experience in domestic 750kV transformer manufacturing, along with the necessity of on-site PD tests, it is recommended that future 750kV main transformers have an induced withstand voltage of no less than 900kV.

(B) Relax On-Site Commissioning PD Test Criteria

Overseas, PD tests are strictly performed only at the factory, not repeated on-site. In China, however, on-site PD testing is a mandatory commissioning item. It is recommended to relax the acceptance criterion for on-site PD tests of 750kV transformers to less than 1000pC, for the following reasons:

• Transformers with PD levels between 500–1000pC often show  reduced PD upon retesting after a period of storage or operation (e.g., Lanzhou East Phase A transformer).

• When PD is below 1000pC, no visible discharge traces are  typically found, on-site inspections rarely detect issues, and factory  return poses high risks.

• On-site PD tests for 750kV and 1000kV transformers are  effectively "quasi-withstand tests":

• Small voltage margin: For the Lanzhou East transformer, the PD  test voltage at 1.5 pu (693kV, ±3% measurement uncertainty: 672–714kV) is  very close to the commissioning withstand voltage of 731kV, leaving only  a 2.4% margin. Even if future 750kV transformers have an induced   withstand voltage raised to 900kV, the commissioning test at 765kV still  leaves limited margin. Similarly, for 1000kV transformers, the PD test  voltage (1.4 pu = 889kV) is very close to the 935kV withstand level.

• Long duration: While the standard withstand duration is only  about 56 seconds (at 108Hz test frequency), the full PD test applies 1.5  pu for up to 65 minutes. Repeated testing may cause cumulative insulation  damage, affecting transformer lifespan.

• There are few cases where repeated on-site tests reduce  excessive PD to acceptable levels; instead, PD levels may increase (e.g., Lanzhou East Phase A transformer: 700pC on August 10, 2005, increased to  910pC by September 23).

(C) Redefine PD Inception and Extinction Voltages

Existing standards lack clear definitions for PDIV and PDEV, which can mislead test interpretation (as seen in the Lanzhou East case). It is recommended to redefine these terms with explicit numerical criteria and include guidance for cases where PDIV and PDEV are not clearly observable.

(D) Strengthen Research on Practical On-Site Techniques

• Collect Real Transformer PD Patterns: Most typical PD patterns in literature are from laboratory  simulations, which differ from real transformer behavior. Illustrative  diagrams are insufficient for guiding field work. It is essential to  collect and analyze real-world PD patterns and compile them into reference  manuals for qualitative analysis and localization.

• Advance Anti-Interference Research: External interference is a major challenge in on-site PD testing. Current  measurement systems cannot distinguish between genuine discharges and   interference, relying heavily on operator experience. More research is  needed on interference sources and suppression methods.

(E) Require Certification for Test Personnel

PD measurement is the most technically demanding and unpredictable of routine on-site high-voltage tests. However, misjudgments are common. Personnel should undergo systematic training in fundamental principles, equipment wiring, component matching, interference elimination, and PD localization, and must obtain certification before being allowed to conduct tests.

(F) Regular Calibration of Test Instruments

GB7354-2003 clearly states that PD measuring instruments must be calibrated at least twice a year or after major repairs. In practice, this is often not strictly followed, with some instruments used for years without calibration—errors as high as tens of times have been recorded. Strict enforcement of calibration per national standards is recommended to ensure measurement accuracy.

(G) Use Online Monitoring When Necessary

Online monitoring technology has significantly improved. For 750kV transformers with PD levels exceeding limits but not critically high, enhanced online monitoring is a reasonable approach. In addition to PD, parameters such as temperature, core and clamp grounding current, and oil chromatography should be monitored to comprehensively assess transformer health.

VII. Conclusion and Outlook

• Conclusion: Existing standards  provide inadequate definitions for PD inception and extinction voltages,limiting their usefulness in guiding on-site tests. The insulation level  of the Lanzhou East 750kV transformer is relatively low, making its PD  test essentially a "quasi-withstand" test. The 12 on-site PD  tests on the Phase A transformer likely caused some cumulative insulation  stress. Future 750kV transformers should have an insulation level of at  least 900kV.

• Outlook: Research and planning for  China’s 1000kV AC ultra-high-voltage transmission have been completed, and  demonstration projects are under construction. Given the even smaller insulation margin of 1000kV transformers, research into on-site  commissioning tests should be initiated early to provide technical support  for practical applications.