Gas Handling and Assessment of Gas Quality in Gas-Insulated Switchgear Containing Clean Air

Presented By:
Bernhard Lutz
Siemens Ag, Transmission Products
Mark Kuschel
Siemens Ag, Transmission Products
Karsten Pohlink
Siemens Ag, Transmission Products
Roland Kurte
WIKA Alexander Wiegand Se & Co. Kg Germany
TechCon 2020

Abstract

SF₆ gas is nowadays the most applied medium in gas-insulated electric power equipment for energy transmission and distribution networks. Nevertheless, when emitted from electric power equipment, SF₆ will contribute to Global Warming due to its significant Global Warming Potential (GWP). Recently, SF₆-free gas-insulated switchgear (GIS) was introduced to the market containing Clean Air (i.e. 80% N2 + 20% O2) as insulating gas and vacuum interrupter units as circuit breakers. Clean Air has excellent long-term stability, liquefaction temperature below -50°C, Global Warming Potential of zero, no toxicity and positive life cycle aspects.

This paper provides information on how Clean Air can be handled during manufacturing, installation and commissioning, maintenance and end-of-life of GIS and how the quality of Clean Air can be assessed. Suitable methods for measuring the gas humidity, concentration of contaminants and gas leakage are described. Results of gas quality measurements after switching tests with earthing and disconnecting switches are presented as well as results of gas leakage measurements. Methods for filtering of decomposition products are described. Finally, relevant environment, health and safety aspects as well as considerations on life cycle costs during lifetime of GIS containing Clean Air are discussed and compared to aspects relevant for GIS containing SF₆.

Keywords
Clean Air, GIS, SF₆-Free, Gas Handling, Vacuum Circuit Breaker

1. Introduction

In the last decades, state-of-the-art gas-insulated substations design has been optimized with respect to both, low amount of installed SF₆ gas and very low leakage rates. Nevertheless, SF₆ contributes to the Global Warming Potential (GWP), whenever emitted. Restrictions and guidelines for the responsible application of SF₆ gas are given by the revised European F-gas directive (binding since 2014). Latest political developments like the Paris Treaty or the coming revision of the European F-gas directive in 2020 indicate that even stricter regulations for the application of F-gases are expected in the future.

That is why the demand for even more environmentally friendly substation design is increasing and several alternative pilot solutions for high-voltage products have been presented during the last 20 years. For example, products from Asian and European manufacturers are already available with Dry Air/CO2 insulation. Applications with CO2 as arc-quenching gas have also been presented and installed. At CIGRÉ 2014 and following CIGRÉ 2016, alternative gases based on fluoroketones or fluoronitriles with mixtures of N2, O2, or CO2 were presented for arc-quenching and insulating purposes.

Another alternative technology to SF₆ for switching is the utilization of vacuum interrupter units for arc-quenching, which is a proven technology in medium voltage for several decades. Vacuum interrupter units have proven reliable making and breaking capabilities in many switching operations with increased electrical endurance compared to gas circuit breakers.

Siemens has started to implement the Blue GIS technology based on fluorine-free gas insulation with Clean Air (i.e. 20% oxygen with 80% nitrogen) and vacuum circuit breakers. Latest developments show first applications of the BLUE GIS technology for 66 kV offshore wind-turbine applications and for the 145 kV voltage level (Fig. 1). This paper provides an overview on all aspects to be considered for proper quality assessment and safe handling of Clean Air during the lifetime of Blue GIS.

2. Definition of Clean Air

Clean Air consists of 80% ± 1% nitrogen and 20% ± 1% oxygen and can either be purchased in bottles (typically 110 Lbs [50 litres] bottles at 2900 Psi [200 bar] absolute) or generated by dedicated air purification devices. According to ISO 8573-1: 2010(E) [3], the contaminants and purity classes of compressed air at reference conditions (20°C, 100 kPa) are defined in Table 1.

Typically, for application in gas-insulated switchgear, compressed air with a purity class ≤ 4 is used.

Clean Air is a mixture of pure nitrogen and oxygen having a purity class 0 according to ISO 8573-1.

It is important to note that the purity requirements of compressed air originating from air purification devices or bottles can differ significantly from the purity requirements of compressed air inside gas-insulated switchgear. The latter are design-specific and must be defined by the original equipment manufacturers. During the lifetime of GIS containing Clean Air and vacuum circuit breakers, heavily arced Clean Air with significant amount of gaseous and solid by-products can only occur in case of internal arc faults or making of short circuit currents by make-proof earthing switches. Details are given in section 5.

3. Clean Air Handling During Lifetime

This section describes the methods and the equipment used for handling Clean Air during the lifetime of gas-insulated switchgear. The differences and benefits compared to handling of SF₆ gas will be highlighted. Table 2 provides an overview of gas handling equipment for SF₆ and Clean Air. More details are described in the following subsections.

*only required for heavily arced gas, i.e. after internal arc fault or making of short circuit currents with make-proof earthing switches

**Clean Air can be released to the atmosphere; vacuum pump is only needed for evacuation of gas compartments before filling with Clean Air

3.1 Handling during manufacturing and testing of Blue GIS

During manufacturing and testing of Blue GIS, handling of Clean Air in a closed pressure system is favorable in order to limit the total consumption of gas. Handling of Helium in a closed pressure system as tracer gas for routine tightness testing is helpful as well to reduce manufacturing times and costs.

For this purpose, service carts with specific design for handling Clean Air and Helium were installed. Since thermal properties of Clean Air and Helium differ from SF₆, service carts with different compressor design are required. Both gases are stored in CE-certified containers with a capacity of 6 m3 and maximum permissible pressure of 16 bar. For tightness testing, Helium gas and Helium sniffing devices with a detection limit better than 10-5 mbar l/s are used.

3.2 Handling during installation and commissioning (I&C) of Blue GIS

For installation and commissioning of Blue GIS on-site, Clean Air is preferably provided in gas bottles (50 l, 200 bar absolute) and filled via pressure regulators directly into the compartments of the gas-insulated switchgear up to the rated filling pressure/density. Alternatively, dedicated air purification devices can be used to generate compressed air with a specified purity class. Before filling, gas compartments which are not pre-filled with Clean Air need to be evacuated from ambient air by a vacuum pump. Different from SF₆, no storage containers are needed for handling of Clean Air which can be released to the atmosphere, if needed. Tightness testing on-site can be done by filling Helium as a tracer gas and using Helium sniffing devices. To locate abnormal leaks, leak spray is an alternative way without additional gas handling efforts. Other alternatives are the use of CO2 or ozone sniffing devices (see section 3.4) or the application of digital gas density monitoring systems.

3.3 Handling during maintenance and end-of-life of Blue GIS

For maintenance and end-of-life of Blue GIS, no recovery of the insulating gas is required and no storage containers are needed as Clean Air can be released to the atmosphere. A vacuum pump is only needed after maintenance to evacuate the gas compartments before filling Clean Air again from bottles or air purification devices into the GIS. Tightness testing after maintenance can be done by filling Helium as a tracer gas and using Helium sniffing devices. To locate particular leaks, leak spray is an alternative way without additional gas handling efforts. Other alternatives are the use of CO2 or ozone sniffing devices (see section 3.4) or the application of digital gas density monitoring systems.

3.4 Alternative Clean Air leakage measurement methods

Since sniffing of Clean Air in an air environment is not feasible and the accuracy of leak spray, as well as acoustic and optic methods, is limited, tracer gases must be used for accurate leakage detection on Blue GIS. To avoid additional gas handling by using Helium as a tracer gas, two alternative methods have been developed which will be explained as follows:

1. Measurement of the CO2-concentration (cumulative test method according to [4])
2. Measurement of the ozone concentration (sniffing or probing test method according to [4])

1. Measurement of the CO2-concentration

Clean Air is a mixture of N2 and O2 only and thus contains zero CO2 concentration. Contrarily, the CO2 concentration in the atmosphere is greater than zero, typically around 400 ppmv if no other CO2 sources are present. For that reason, leakage of Clean Air reduces the CO2 concentration in the vicinity of the leaking spot, particularly if the cumulative test method is applied. Fig. 2 shows an exemple of a CO2 sniffing device and the calculated temporal change of the CO2 concentration in different capture volumes used for the cumulative test method. For the calculation, a leak with a leakage rate of 10-3 mbar l/s was assumed (note: this equals 0,5% per year for a gas compartment with a volume of 790 litres). Using a CO2-sniffing device with accuracy of some ppmv, tests have shown that such low leakage rates can be measured during 72 hours with capture volumes up to 20 litres.

It is important to note for the cumulative test method, that enclosure materials with low permeability against CO2 shall be used to suppress the impact of the atmospheric CO2 concentration on the measurement.

2. Measurement of the ozone-concentration

For the leakage measurement at Blue GIS on-site, a certain amount of ozone can be injected into gas compartments containing Clean Air and afterwards the sniffing procedure can be started immediately.

The use of ozone (O3) as a tracer gas for leakage detection has major advantages:

• Portable sniffing devices are available with high sensitivity down to some ppb (Fig 3. left)
• Ozone is an unstable molecule and will decompose to oxygen after several tens of minutes
• No additional efforts for gas handling are required after ozone has been injected into GIS compartments due to its self-decomposition.

To verify this sniffing test method, a copper wire with diameter of about 0.3 mm and a hair (diameter about 0.06 mm) were pressed between the flange and the sealing ring of a lid (Fig. 3). Please note that the leakage rates of those leaks were not quantified in separate gas tightness tests according to IEC 62271-1. However, according to literature data those leaks are assumed to induce leakage rates greater than 10-3 mbar l/s.

The filling pressure for the sniffing tests was 0.5 bar gauge. Different positions of the portable sniffing device (Fig 3. Left) in the vicinity of the artificial leaks were tested. The Personal Ozone Monitor (POM) was used for ozone sniffing having a detection limit of 3 ppb, a gas flow rate of about 0.8 l / min and adjustable measurement intervals from 2 s up to 10 s. An ozone generator with a specified ozone generation rate of 20 000 mg/h was placed inside a closed gas compartment while power supply was established via a gas-tight cable plug. That way, 90 s after switching on the generator a maximum ozone concentration of 1500 ppmv is estimated inside the gas compartment (neglecting ozone decomposition). The results of the ozone sniffing tests are shown in Fig. 3 (right).

Two artificial leaks could be detected when positioning the sniffing device close to the location of the leak. While moving the sniffing device either around the flange or away from the leak location, the measured ozone concentrations dropped significantly. The measured ozone concentrations differed by more than a factor of 10 between the different leaks. Thus, the investigated ozone sniffing method shows good sensitivity against leaks with a leakage rate greater than 10-3 mbar l/s and can principally be used for on-site leakage detection. Portable ozone generators are available for application on-site, but the procedure of ozone injection into gas compartments must be clearly defined based on the specific GIS design.

4. Methods for Clean Air Analysis

This section provides information about methods for the quality assessment of Clean Air during its lifetime. Table 3 shows an overview of different measurement devices and principles which are suitable to analyze Clean Air. More details are given in the following subsections.

*if compressed air with purity class ≥1 is used, additional measurements of oil concentration and number of particles may be required

4.1 Humidity and purity measurements Humidity measurements are helpful to check the quality of Clean

Air during the lifetime of Blue GIS. Basically, all measurement principles suitable for humidity measurements in SF₆, typically chilled mirror or capacitive sensors, can also be used for humidity measurements in Clean Air. Tests in Clean Air with standard SF₆ equipment have proven its compatibility. Besides, capacitive sensor technology used for other industries, like compressed air applications, have also been validated for humidity measurements in Clean Air.

Purity measurements in Clean Air are only needed if closed-loop gas handling is performed, like during manufacturing and testing of Blue GIS. For Clean Air according to the definition in section 2, the following measurements are recommendable:

1. Measurement of oxygen content in Clean Air

Closed loop gas handling may lead to change of mixing ratio N2 / O2 e.g. by accumulation of contaminants. Higher concentration of contaminants would lead to lower oxygen concentration which is monitored inside the storage containers mentioned in section 3.1. Several measurement principles are available for oxygen content measurements, like paramagnetic sensors, electrochemical sensors or optical sensors.

2. Measurement of Helium Concentration in Clean Air

Since Helium is handled in a closed loop during manufacturing and testing of Blue GIS, Helium can potentially accumulate in Clean Air by concentrations greater than 1% after some time. Thus, a regular check of the Helium content in the closed Clean Air loop might be recommendable. Furthermore, if GIS design allows for Helium mixtures with Clean Air for tightness testing on-site, a check of remaining Helium content in gas compartments containing Clean Air might be recommendable as well. Helium concentration measurements can be performed based on different principles, like mass spectrometry, thermal conductivity or speed of sound measurements.

4.2 Measurement of decomposition products

During the lifetime of Blue GIS, significant concentrations of gaseous Clean Air decomposition products can only occur in case of internal arc faults or during making of short circuit currents with make-proof earthing switches. The main gaseous decomposition products of Clean Air are nitrogen oxides (particularly NO2, NO) and ozone. While gaseous ozone has only a limited lifetime of some tens of minutes before decomposing to oxygen, nitrogen oxides (especially NO2) are more stable.

The concentration of nitrogen oxides as main decomposition products of Clean Air can be measured by different methods, like Fourier Transform Infrared Spectroscopy, chemiluminescent detectors or indicator tubes. For a quick and easy indication of nitrogen oxides on-site, indicator tubes are recommendable for example Dräger tubes with the following specifications:

• Nitrogen Dioxide NO2 0.1/a (scaled from 0.1…30 ppmv)
• Nitrous Fumes 2/a (scaled from 2…100 ppmv)

The concentration of nitrogen oxide (NO) can be calculated by subtracting the concentration measured with Nitrogen Dioxide NO2 from the concentration measured with Nitrous Fumes. For quantitative measurements, the gas flow needs to be controlled e.g. by using a needle valve in combination with a gas flow meter. If no gas flow control can be established, the indicator tubes can only indicate whether nitrogen oxides are present or not.

The following tests were performed to create Clean Air decomposition products by electric arcing [7] and the results of measurements with indicator tubes are shown in Table 4:

• Bus Transfer Current (BTC) Switching Tests according to IEC 62271-102 (2003) Annex B
• Test to prove the short-circuit-making performance (Imake) of earthing switches according to IEC 62271-102 (2003), subclause 6.101
• Decomposition by partial discharges (PD)

The investigations have shown that Bus Transfer Switching operations of disconnector switches does not generate considerable decomposition products while NOx concentrations up to 20 ppmv were measured after making of short-circuit currents with make-proof earthing switches. Drying agent was only used for the test “decomposition by partial discharges”. That means that concentrations of decomposition products inside disconnecting switches and earthing switches including drying agent will be even lower than the values in Table 4 during operation of Blue GIS. Moreover, the effectiveness of standard filter equipment used in service carts against nitrogen oxides was proven during the PD test. The NOx concentration was reduced from about 0.5 ppmv to zero after being filtered by a standard filter of a service cart (typically molecular sieve and/or aluminum oxide).

5. Life Cycle Aspects

5.1 Environment, health and safety aspects

Compared to SF₆, the application of Clean Air in gas-insulated switchgear leads to a significant reduction of environmental, health, and safety (EHS) considerations. The main EHS aspects to be considered are described in the latest committee draft (CD) of the standard IEC 62271-4 (2019) Annex C [8].

Clean Air, as well as the major by-products (see subclause C.9 in [5.1a]) from application in electric power equipment, do not contribute to the destruction of the stratospheric ozone layer because they do not contain either chlorine or bromine. Clean Air does not contribute to the greenhouse effect and has a global warming potential of zero. Clean Air is the main gas mixture of our ecosystem and does not harm it. It is not toxic and has no reported potential to be acute or chronic ecotoxic. It presents no danger to surface and groundwater or the soil. A biological accumulation in the nutrition cycle has no impact. Moreover, Clean Air is:

• not carcinogenic: not causing cancer;
• not mutagenic: not causing damage to the genetic constitution;
• not nitrifying: no enrichment in the food chain;
• moderate solubility in water, but without environmental impact.

The positive environmental impact of Blue GIS compared to GIS containing SF₆ has been shown by life cycle assessments according to ISO 14040/44. First results have been published in [9] and [10].

For operation of Blue GIS, no specific training and certification with respect to gas handling and the minimization of gas emissions during handling of Clean Air is required. Used Clean Air can be released to the atmosphere and special safety equipment and procedures are only required if heavily arced Clean Air must be handled, i.e. only after internal arc faults or making of short circuit currents with make-proof earthing switches.

Based on the results discussed in section 4.2, the concentrations of the main decomposition product NOx after different switching tests are shown in Fig. 4 together with the following limit values:

• LC50: 50% lethal concentration (LC50 (NO2) = 115 ppm [11])
• Emission limit value defined by the German Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit (= 183 ppm for NO2 [12])

Fig. 4 indicates that neither the LC50 nor the emission limit values are exceeded during the normal operation of Blue GIS containing Clean Air (note that the presence of a drying agent will further reduce the values in Fig. 4).

5.2 Considerations on life cycle costs

This section will describe an approach of assessing the life cycle costs of 145 kV GIS containing SF₆ and 145 kV Blue GIS containing Clean Air. Here, the product costs (CAPEX) are neglected and only the operational costs (OPEX) are considered. The following assumptions are made for calculations:

Based on the example assumptions in Table 5 applied to four 145 kV GIS cable bays with single busbar, the life cycle costs of 145 kV Blue GIS containing Clean Air are by more than 40% lower compared to GIS containing SF₆ (Table 6). Life cycle costs of GIS containing Clean Air are further reduced by about 9% if the credits for material recycling after end-of-life are considered as well.

Table 6 shows that most of life cycle costs accumulate during operation stage, which is mainly due to CO2 reporting costs on a yearly basis. Thus, the life cycle costs will vary significantly between different countries and depend very much on future legal or political constraints with respect to handling of SF₆.

6. Conclusion

This paper provides an overview of methods for proper gas handling and gas quality assessment during the lifetime of gas-insulated switchgear containing Clean Air (i.e. 80% nitrogen and 20% oxygen). Compared to handling of SF₆, handling of Clean Air enables time and cost savings during the lifetime, especially during installation and commissioning, operation, maintenance and end-of-life. Future political and legal constraints on handling of SF₆ would bring further benefits with respect to life cycle costs of GIS containing Clean Air. Different aging and switching tests with GIS containing Clean Air have shown that no critical concentrations of decomposition products occur during normal operation and that no special safety precautions must be considered.

Thus, Clean Air shows many positive aspects with respect to life cycle costs and environment, health and safety aspects during the lifetime of GIS and is thus an environment-friendly and sustainable alternative to SF₆.

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[8] 17/1051/CD:2019-04 – IEC 62271-4, “High-voltage switchgear and control gear – Part 4: Handling procedures for gases and gas mixtures for interruption and insulation“, Ed. 2.0, April 2019.

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[12] Erste Allgemeine Verwaltungsvorschrift zum Bundes–Immissionsschutzgesetz (Technische Anleitung zur Reinhaltung der Luft – TA Luft), Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit, Seite 59, vom 24.07.2002.