Selecting Proper Corona Protection Using E-Field Modeling

Corona

E-field modeling of insulator & hardware applications has become a valuable tool to determine proper corona protection for a transmission assembly before submitting it to laboratory testing or for installation and energization in the field. Among the benefits of E-field models is that multiple combinations of corona rings, shields, hardware and insulators can be assessed to obtain desired customer requirements, quickly and efficiently. This can be completed for new projects at the development stage or can prove a tool for troubleshooting existing applications that may have been applied with insufficient corona protection.

For example, a 138 kV braced post model was designed and supplied without corona rings at a time when this was standard industry practice and when corona rings were added only for 230 kV applications and higher. Over approximately 16 years in service, some of these applications have raised concern about corona-related ageing of the sheath, specifically on the line post insulator of the braced post assembly. To determine the ideal corona protection for this braced post design, e-field modeling can be employed to compare various different corona ring combinations.

E-Field Model Assumptions:
• Test laboratory simulation
• Voltage applied to line end and hardware: 80 kV
• Voltage applied to tower and hardware: 0 kV (Ground)
• Single phase at 22 ft (circa 6.7 m) ground clearance
• 138 kV braced post (BP) –
a) BP model with no corona ring
b) BP model with 8” corona ring (CR) on brace insulator
c) BP model with 8” CR on brace and 6” CR on line post insulator
• The brace and line post insulators are connected using a hot line fitting that is unique to this design. (In this case study, this hotline fitting was not changed since the focus was on adding corona rings for easier upgrades of existing applications.

Fig. 1: E-field model set-up.

Objective
Model the ideal braced post arrangement to satisfy current recommended industry performance criteria for E-field stress limits.

Performance Criteria
1. Limit of E-field stress on end fittings and grading rings: 2.0 kV/mm
2. Limit of average E-Field stress along sheath surface: 0.42 kV/mm

Note: Results and conclusions for the proposed 138 kV braced post models are specific to the phase-to-ground clearance defined by the customer. The tighter the phase spacing, the higher E-field stresses may become.

Case Study Background
The stresses on the line post insulator are different than those on the brace insulator. The brace, or suspension, insulator is designed with stacked sheds. As such, even without a corona ring, the sheath is protected from higher electrical stresses at the line end of the application. In addition, the large corona ball feature present on the brace end fitting also helps reduce the stress on the sheath.
The case study aims to identify the E-field stresses for 3 different braced post configurations in order to determine the ideal corona ring applications for 138 kV braced post designs.

Fig. 2: Brace insulator stress plot.
Fig. 3: Line post stress plot.

For well-defined comparable results, each configuration will require measurements taken at the same positions along the insulator so as to observe levels of stress. Data points need to be captured along the end fitting, sheath, sheds and corona rings.

Attend the 2022 INMR WORLD CONGRESS in Berlin where insulator design expert, of MacLean Power Systems, will explain how this process is accomplished using experience from this detailed case study involving a 138 kV braced post application originally designed and applied without corona rings but which no longer meets today’s corona performance requirements.