Thesis Report to Partial Discharge Analysis in HVDC Gas-Insulated Substations (GISs)

In the late 19th century, with the rise of the Electrical Power Industry, a fervent debate emerged over the methods of generating and distributing electricity. This debate shaped the modern power grid. Central figures in this dispute, often referred to as the "Battle of the Currents," were Nikola Tesla, advocating for Alternating Current (AC), and Thomas Edison, who championed Direct Current (DC).

Despite the historical outcome of the "battle" between AC and DC, there is a resurgence of DC power systems today, not just in transmission. DC applications now encompass renewable energy integration, such as solar systems, electric vehicle charging, data centre power supplies, and extensive HVDC interconnections, among others.

The initial modern HVDC interconnections emerged with the Moscow-Kashira system and the Gotland-Sweden mainland connection in 1954. Subsequently, significant advancements have been made in the development of HVDC converters, driven by a variety of technical and economic factors, including:

• Lower overall investment;

• Lower losses, due to only active power flow;

• Increased stability and improvements in power quality;

• Less expensive circuit breakers on the AC side and simpler bus-bar arrangements in the switchyard due to lower short-circuit currents;

A recent Pike Research report indicates that the HVDC transmission market is among the fastest-growing in the utility sector. The report forecasts a 44% increase in investment over the next five years, rising from USD 8.4 billion in 2010 to USD 12.1 billion by 2015. This surge is attributed to the demand for extensive interconnections, primarily in China and India, as well as in Brazil. Significant growth is also expected in the US and European markets.

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Gas Insulated Substations (GIS) cater to both the growth of the electrical grid and the limited availability of sites for constructing substations. GIS are complex systems where SF6 gas, known for its insulating properties, is the primary insulating medium. This allows for a significant reduction in the size of the substation compared to a conventional open-air installation.

Guide to Partial Discharge Analysis in HVDC Gas Insulated Substations, photo by Hitachi Energy

The GIS systems feature a sealed metal enclosure that maintains the gas under pressure, preventing the leakage of SF6, a potent greenhouse gas. Figure 1 depicts a section of a GIS.

The system comprises both primary equipment, such as circuit breakers, and secondary equipment, like current transformers. Its compact dimensions allow for installation in open-air settings or within buildings.

While GIS systems require low maintenance, their reliability can be compromised by unwanted metal particles. When subjected to intense electric fields, these particles can initiate Partial Discharges (PDs), which are a leading cause of various failure mechanisms in Gas Insulated Substations (GIS).

Among other issues, a free-moving particle nearing a conductor might initiate a flashover, or if resting on a spacer, could result in the carbonization of the spacer.

Partial discharges are also responsible for generating corrosive by-products of SF6, which can be detrimental to both spacers and conductive components.

Figure 1 – Representation of a Gas Insulated System:

where,

1. Earthing Switch;

2. Busbar disconnector;

3. Circuit breaker;

4. Spring operating mechanism;

5. Current transformer;

6. Feeder disconnector;

7. Cable termination enclosure;

8. Voltage transformer. The images refer to a B105 Alstom GIS.

The primary PD sources leading to GIS failures include fixed protrusions, free-moving particles, floating electrodes, particles fixed on the spacer surface, and voids in insulators.

The principal PD sources responsible for failures of GIS are:

1. Fixed protrusion;

2. Free moving particle;

3. Floating electrode;

4. Particle fixed on the spacer surface;

5. Void in insulators.

A study on service experience, as detailed by C. Neumann in "PD measurement on GIS of different design by non-conventional UHF sensors," presented at Cigre' in Paris, 2000, highlights the primary causes of dielectric failure in both 123 kV and 420 kV GIS.

Figure 2 – A pie chart illustrating the primary causes of failures in 123 kV and 420 kV Gas Insulated Substations (GIS), based on a study conducted by several German utilities.

Figure 2 indicates that at least 50% of the failure causes for both 123 kV and 420 kV GIS are attributable to defects detectable by PD diagnostics, particularly those related to particles on surfaces, enclosures, and HV conductors.

Additionally, between 60% and 70% of the failures might have been identified by monitoring systems if they were adequately sensitive.

These statistics highlight the significant role of PD monitoring as a valuable tool for preventing failures and planning maintenance schedules.

Partial Discharge Analysis in HVDC Gas-Insulated Substations

Document:	Partial Discharge Analysis in HVDC GIS (Gas Insulated Substations) – Thesis by Mr. Roland Piccin at Delft University of Technology
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