Kds Tutorial

Gas Insulated Substations Dr. K. D. Srivastava December 2012

Topics Covered Section 1. Background Section 2. Field Experience and Persistent Design Challenges Section 3. Operational Experience and Practices Section 4. Recent Developments Bibliography

Section 1 Background

• 1970s-1990s: Gas-filled (SF6) short lengths installed. Many lab models for higher voltages, including three phase designs in a single duct. Also, SF6/N2 • 1990s: 500 kV mass impregnated paper for submarine DC systems in the Baltic Sea • 1970s-1990s: Low temp. cryogenic/supercon. designs tried. 1990s witnessed the phenomenal growth in HTS technology

Energy and Industrial Culture •

Post World War II, energy (all forms) usage was growing at the rate of ~3% per

year, in industrial nations •

But in industrial nations electricity usage

was growing by more than 7% by displacing other forms of energy

•

With oil crisis of 1970s and the growing environmental movement, the energy

picture is very different now! •

In Europe (Western) and North America

the electricity usage is almost constant. In developing countries, however, the

usage is growing between 7 and 10% per year.

•

Compressed gas cable technology has matured over the last 30 years, but its potential for bulk power transport is yet to be exploited and developed.

•

High temperature superconductor technology is developing rapidly but [is] not yet fully commercially viable for bulk power transport.

•

None of the above three are free from technological areas of concern!

•

However, near urban centres overhead lines are no longer acceptable to the communities for environmental and aesthetic reasons.

•

What are the alternatives?

•

Three choices in technology:  Conventional underground power cables  Compressed gas cables (SF6 - Sulphur Hexa-fluoride)  Superconducting cables.

Why GIS?

Why GITL?

•

Land costs in urban areas

•

Aesthetically “superior” to air insulated substations

•

Not affected by atmospheric pollution

•

Completely sealed (metal-clad) permits

very low maintenance •

Demand for higher energy usage in urban

areas requires increased transmission voltages; for example, 420 kV

GITL •

In addition to the advantages listed above

for GIS, there is a need for non-aerial transmission lines near urban areas.

•

There are currently only two alternatives:  Underground cables–conventional or superconducting, or  Gas Insulated Transmission Lines (GITL)

•

GITL, compared to underground cables, have the additional advantage of reduced ground surface magnetic fields.

Design Features of GIS/GITL •

GIS/GITL installations have the usual components: 1. Circuit breakers; disconnect, earthing/grounding switches

2. Current and voltage measuring devices 3. Busduct sections

4. Variety of diagnostic/monitoring devices

•

Installations from distribution voltages right up to the highest transmission voltages (765 kV) have been in service for 30 years or more. Both isolated-phase and

three-phase designs are in use.

•

SF6 is the insulating medium at a pressure of 4 to 5 atmospheres. GITL units are factory-assembled in lengths of 40 to 50 feet.

•

The phase conductor is almost always of aluminium. The outer enclosure is also of aluminium, although earlier designs used

mild steel. For lower voltages, stainless steel has also been used.

•

Usually busducts are of rigid design although flexible and semi-flexible designs have been proposed. None are in use.

Typical Cable Section

Growth of GIS

Growth of GIS Installations Before 1985 January

After 1985 January

Voltage

GIS

CB-Bay-Yrs.

GIS

CB-Bay-Yrs.

1

230

28669

731

28215

2

227

21252

382

12808

3

123

10362

147

5678

4

45

3870

65

2904

5

26

3252

37

1273

6

-

-

2

200

751

67,405

Total

Voltage Class 1

60 – 100 kV

2

100 – 200 kV

3

200 – 300 kV

4

300 – 500 kV

5

500 – 700 kV

6

>700 kV

51,078

5.

Current Transformer

6.

Potential Transformer

7.

Bus Section

8.

Cable Termination

Expansion joint

Main Components of GIS • Busbar and enclosure • Busduct sections

• Bushing • Circuit-breakers

• Disconnectors • Earthing/grounding switches

• Current and voltage transformers and

measuring devices • Expansion joints

• Diagnostic/monitoring devices • GIS grounding and control wiring • Termination modules

Persistent Insulation Challenges Notwithstanding the high reliability of GIS technology, both manufacturers and users have to be aware of certain HV insulation problems inherent in the GIS design. These are:

1.Reliability of support spacers. 2.Generation of VFTO by disconnect switch operation. 3.Contamination of SF6 gas by metallic particles. 4.Arcing/discharge by-products in SF6. 5.Environmental “green house” effects of SF6.

Applied voltage: 300kV, 0.4 MPa (SF6) (81kV/div, 20 ns/div

FTO waveform measured by 1-GHz surge sensor Source: M.M. Rao & M.S. Naidu, III Workshop on EHE Technology, Bangalore, India, 1995.

• Diagnostic methods for identifying defects in a GIS installation have been proposed by CIGRE. Many gross assembly errors and poor quality assurance procedures can give rise to significant partial discharges (PD), which in the presence of moisture may lead to toxic by-products in the SF6 gas. • Automated insulation condition monitoring systems, with innovative sensors, are being developed and installed on GIS and other HV power apparatus. • New techniques for PD detection/location are perhaps the most significant developments in GIS condition monitoring.

n = n0 exp αx Collisional Ionization in NitrogenUniform Electric Field n0 = electrons initially at x = 0 n = electrons at x α = ionization coefficient for the gas

Effective Ionization Coefficient α′ as a function of Electric Field Strength and Pressure

Molecular Formula

BP °C

Relative Electric Strength

SF6

-63.8

2.5/760 mm

C4F6

-5

3.9/730 mm

C5F8

25

5.5/600 mm

C5F10

22

4.3/600 mm

CF3CN

-63

3.6/753 mm

C2F5CN

-30

4.7/735 mm

C3F7CN

1

5.8/550 mm

C8F16O

101

6.3/760 at 180°C

Environmental Impact of SF6 • SF6 is a gas specifically mentioned in Kyoto protocol. Search is on for a replacement gas or gas mixture. 80% of SF6 manufactured is used by the electrical industry. Leakage rates are

Classification Decision

A General Procedure for PD Diagnostics in Power Equipment Clearly, our “Decisions” are as good as our “Data Base”. Lots of experimentation has been done and a lot more is needed. Expertise of disciplines new to power engineering is being brought to bear on GIS technology.

So, How Good is our Data Base? • We know some of the most common sources of PD in GIS, e.g., • Metallic Particles - free moving

• Metallic Particles on spacers • Protrusions on inner/outer conductors

• Void in a spacer • Floating metal objects • -------• --------

• SF6 Related Info. – Pressure – Moisture – Breakdown By-products • PD Data • Sensor Locations • Data Acquisition • Data Reduction • Data Analysis Using Present and Historical Data INTEGRATED CONTROL, MONITORING AND DIAGNOSTIC SYSTEM

What are Detailed Aspects of UHF PD Detection in GIS? • The Resonant Frequencies • What Freq. Range you Select? • What Type of Coupler? – – – –

Internal External New GIS Existing GIS

• Coupler Location – Signal/Noise Ratio – Propagation Through GIS

• Software Design – Customized – Signal Analysis – Data Bank – Expert or Neural Systems – Calibration – Comparisons With Other Data

Partial Discharge Testing of GIS

Purpose: • Developmental tests • Type tests • Production tests • Commissioning tests • Monitoring/Diagnostic • PD - very early local breakdown of gas. May lead to failure in time. Corona stabilization makes voltage level for PD much lower than that for breakdown, except for LI and VFTO.

• Quality control is essential for all the components that go into a GIS

• Possible techniques are: – Electrical – Acoustic – Chemical – Optical

• Optical techniques are best suited for the developmental and type test stage. However, an adequate number of windows are [is?] essential for visual checks during service. • Chemical methods are best suited for the developmental, type test and perhaps as a back-up in the field.

• In practice it is the ratio of downstream stable products SO2F2/SOF2 which offers discrimination as to the source of discharges, for example, tests at CESI show: Phenomena

Time

SO2F2

SOF2

Ratio

PD

260 hrs

15 ppml

35

0.43

Disconnector Cap.

200 oper

5

97

0.05

Switching

400 oper

21

146

0.14

Cir. Break.

5 oper @ 31kA