Sulfur hexafluoride (SF6) gas is widely used in electrical equipment due to its excellent insulation properties, strong arc-quenching ability, and good cooling performance. It is a stable chemical compound that plays a crucial role in maintaining the safety and efficiency of high-voltage systems.
According to SF6 management regulations and the requirements of the "pre-regulation," there are specific procedures for monitoring and maintaining SF6 gas in electrical equipment. These include:
1. After installation, the humidity and air content in the SF6 gas chamber must be rechecked before commissioning, ideally 24 hours after inflation.
2. Once the equipment is energized, SF6 humidity tests and leak detection should be conducted every three months. After stabilization, these checks are performed every 1–3 years. If significant changes in gas quality are detected, the "SF6 Testing Center" should be notified.
3. For equipment with low pressure (<0.35 MPa) and minimal gas consumption (such as circuit breakers below 35kV), if no leaks are found during testing, the humidity level at handover is acceptable, and no further checks are required unless abnormal conditions arise.
These guidelines ensure the safe and reliable operation of SF6-based electrical systems. This section focuses on the methods for testing gas humidity and detecting leaks in the equipment.
**First, Gas Humidity Test**
Gas humidity refers to the amount of moisture present in the SF6 gas. Moisture can cause corrosion of insulation and metal components, leading to potential failures. To minimize these risks, the Pre-Regulation sets strict limits on the moisture content in operational SF6 gas, as shown in Table 2-10.
The method specified in the Pre-Regulation involves electrolysis. The SF6 gas is passed into an electrolytic cell, where moisture is absorbed and decomposed. The water content is determined by measuring the current required for electrolysis. The formula used is:
$$ I = \frac{CPT_0 F_q}{3P_0 TV_0} \times 10^{-4} $$
Where:
- $ I $: Electrolysis current (μA)
- $ C $: Gas moisture content (μL/L)
- $ F $: Faraday constant (96485 C)
- $ P_0 $: Standard atmospheric pressure (101.325 kPa)
- $ T_0 $: Critical absolute temperature (273 K)
- $ V_0 $: Molar volume (22.4 L/mol)
- $ P $: Operating gas pressure (Pa)
- $ T $: Operating gas temperature (K)
- $ q $: Gas flow rate during test (mL/min)
When all variables except $ C $ are constant, $ I $ is directly proportional to $ C $. This allows direct reading of moisture content from the ammeter, calibrated in ppm.
When using a moisture meter, it’s important to:
- Ensure the SF6 gas is at rated pressure before measurement.
- Wait 24 hours after charging before testing.
- Avoid testing on rainy days.
- Pre-dry the system with high-purity nitrogen before sampling.
- Ensure the system is well-sealed to prevent moisture ingress.
**Second, Gas Leak Detection (Leak Rate Test)**
Leak detection can be either qualitative or quantitative. Qualitative methods involve visually inspecting joints to locate leaks, while quantitative methods calculate the annual leakage rate using techniques like the bandaging method or pressure folding algorithm.
**Bandaging Method**
This technique involves sealing the flange and pipe joints with plastic film. The volume of the sealed area is measured, and after a period of time, the SF6 concentration inside the sealed space is tested. Using the following formulas, the annual leakage rate can be calculated:
$$ G = \frac{\kappa V \rho t}{\Delta t} \times 10^{-6} \quad (\text{g}) $$
$$ M = \frac{G}{Q} \times 100\% $$
Where:
- $ \kappa $: SF6 concentration in the dressing
- $ V $: Volume of the sealed space (excluding the dressing device), in liters
- $ \rho $: Density of SF6 gas (6.16 g/L at 20°C, 0.1 MPa)
- $ t $: Year hours (8760 h)
- $ \Delta t $: Binding time (h)
- $ Q $: Amount of gas filled in the chamber (g)
The plastic film should be wrapped tightly around the joint and taped securely. For small equipment, a cover method can be used, and for flanges with a detection hole, a bottle-hanging method is suitable.
**Pressure Folding Algorithm**
This method records the pressure and temperature of each chamber over time. By plotting density against time, trends in gas loss can be identified. The annual leakage rate is calculated using:
$$ M = \frac{(p_0 - p_t)}{p_0} \times \frac{T_r}{T_0} \times 100\% $$
Where:
- $ p_0 $: Initial gas density (g/L)
- $ p_t $: Measured gas density after time $ t $ (g/L)
- $ T_0 $: Time interval between measurements (in same units as $ T_r $)
- $ T_r $: Annual time (12 months or 365 days)
Pressure measurements should be taken between 8:00 and 10:00 AM when the temperature difference between the chamber and the environment is minimal.
**Third, SF6 Gas Composition Analysis**
According to the pre-regulation, SF6 gas composition should be analyzed after maintenance or when necessary to identify failure causes. The specifications include:
- Acidity (μg/g): ≤ 0.3
- Carbon tetrafluoride (mass %): ≤ 0.05 (after overhaul), ≤ 0.1 (in operation)
- Air (mass %): ≤ 0.05 (after overhaul), ≤ 0.2 (in operation)
- Decomposable fluoride (μg/g): ≤ 1.0
- Mineral oil (μg/g): ≤ 10
- Purity (%): ≥ 99.8
- Density (standard state) (kg/m³): 6.16
A density relay is a key component in SF6 equipment. It monitors internal pressure and triggers alarms or locks the system when pressure drops below safe levels. Testing the relay's operating pressure ensures proper functionality and reliability.
The process involves filling the system with SF6, adjusting pressure, and checking the relay's response. If discrepancies are found, the relay should be sent for inspection and adjustment.
By following these procedures, operators can maintain the integrity and performance of SF6-based electrical equipment, ensuring long-term safety and efficiency.
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