Oil Treatment Specifications
7.7.1 Taking Oil Samples for DGA
Sampling procedures and lab handling are usually areas that cause the most problems in getting an accurate DGA. There are times when atmospheric gases, moisture, or hydrogen take a sudden leap from one DGA to the next. As has been mentioned, at these times, one should immediately take another sample to confirm DGA values. It is, of course, possible that the transformer has developed an atmospheric leak or that a fault has suddenly occurred inside. More often, the sample has not been taken properly, or it has been contaminated with atmospheric gases or mishandled in other ways. The sample must be protected from all contamination, including atmospheric exposure.
Do not take samples from the small sample ports located on the side of the large sample (drain) valves. These ports are too small to adequately flush the large valve and pipe nipple connected to the tank; in addition, air can be drawn past the threads and contaminate the sample. Fluid in the valve and pipe nipple remain dormant during operation and can be contaminated with moisture, microscopic stem packing particles, and other particles. The volume of oil in this location can also be contaminated with gases, especially hydrogen.
Hydrogen is one of the easiest gases to form. With hot sun on the side of the transformer tank where the sample valve is located, high ambient temperature, high oil temperature, and captured oil in the sample valve and extension, hydrogen formed will stay in this area until a sample is drawn.
The large sample (drain) valve can also be contaminated with hydrogen by galvanic action of dissimilar metals. Sample valves are usually brass; a brass pipe plug should be installed when the valve is not being used. If a galvanized or black iron pipe plug is installed in a brass valve, the dissimilar metals produce a thermocouple effect, and circulating currents are produced. As a result, hydrogen is generated in the void between the plug and valve gate. If the valve is not thoroughly flushed, the DGA will show a high hydrogen level.
Oil should not be sampled for DGA purposes when the transformer is at or below a freezing temperature. Test values which are affected by water (such as dielectric strength, power factor, and dissolved moisture content) will be inaccurate.
Transformers must not be sampled if there is a negative pressure (vacuum) at the sample valve.
This is typically not a problem with conservator transformers. If the transformer is nitrogen blanketed, look at the pressure/vacuum gauge. If the pressure is positive, go ahead and take the sample. If the pressure is negative, a vacuum exists at the top of the transformer. If there is a vacuum at the bottom, air will be pulled in when the sample valve is opened. Wait until the pressure gauge reads positive before sampling. Pulling in a volume of air could be disastrous if the
transformer is energized. If negative pressure (vacuum) is not too high, the weight of oil (head) will make positive pressure at the sample valve, and it will be safe to take a sample. Oil head is about 2.9 feet (2 feet 10.8 inches) of oil per psi. If it is important to take the sample even with a vacuum showing at the top, proceed as described below.
Use the sample tubing and adaptors described below to adapt the large sample valve to ⅛-inch tygon tubing. Fill a length (2 to 3 feet) of tygon tubing with new transformer oil (no air bubbles) and attach one end to the pipe plug and the other end to the small valve. Open the large sample (drain) valve a small amount and very slowly crack open the small valve. If oil in the tygon tubing moves toward the transformer, shut off the valves immediately. Do not allow air to be pulled into the transformer. If oil moves toward the transformer, there is a vacuum at the sample valve. Wait until the pressure is positive before taking the DGA sample. If oil is pushed out of the tygon tubing into the waste container, there is a positive pressure, and it is safe to proceed with DGA sampling. Shut off the valves and configure the tubing and valves to take the sample per the instructions below.
22.214.171.124 DGA Oil Sample Container – Glass sample syringes are recommended. There are different containers, such as stainless steel vacuum bottles and others. Using only glass syringes is recommended. If there is a small leak in the sampling tubing or connections, vacuum bottles will draw air into the sample, which cannot be seen inside the bottle. The sample will show high atmospheric gases and high moisture if the air is humid. Other contaminates such as suspended solids or free water cannot be seen inside the vacuum bottle. Glass syringes are the simplest to use because air bubbles are easily seen and expelled. Other contaminates are easily seen, and another sample can be immediately taken if the sample is contaminated. The downside is that glass syringes must be handled carefully and must be protected from direct sunlight. They should be returned to their shipping container immediately after taking a sample. If they are exposed to sunlight for any time, hydrogen will be generated, and the DGA will show false hydrogen readings. For these reasons, glass syringes are recommended, and the instructions below include only this sampling method.
Obtain a brass pipe plug (normally 2 inches) that will thread into the sample valve at the bottom of the transformer. Drill and tap the pipe plug for ⅛-inch national pipe thread, insert a ⅛-inch pipe nipple (brass if possible) and attach a small ⅛-inch valve for controlling the sample flow. Attach a ⅛-inch tygon tubing adaptor to the small valve outlet. Sizes of the piping and threads above do not matter; any arrangement with a small sample valve and adaptor to ⅛-inch tygon tubing will suffice. See figure 57.
126.96.36.199 Taking the Sample
♦ Remove the existing pipe plug and inspect the valve opening for rust and debris.
♦ Crack open the valve and allow just enough oil to flow into the waste container to flush the valve and threads. Close the valve and wipe the threads and outlet with a clean dry cloth.
♦ Re-open the valve slightly and flush approximately 1 quart of oil into the waste container.
Install the brass pipe plug (described above) and associated ⅛-inch pipe and small valve, and a short piece of new ⅛-inch tygon tubing to the outlet of the ⅛-inch valve.
♦ Never use the same sample tubing on different transformers. This is one way a sample can be contaminated and give false readings.
♦ Open both the large valve and small sample valve and allow another quart to flush through the sampling apparatus. Close both valves. Do this before attaching the glass sample syringe. Make sure the short piece of tygon tubing that will attach to the sample syringe is installed on the ⅛-inch valve before you do this.
♦ Install the glass sample syringe on the short piece of ⅛-inch tubing. Turn the stopcock handle on the syringe so that the handle points toward the syringe. Note: The handle always points toward the closed port. The other two ports are open to each other. See figure 58.
Open the large sample valve a small amount and adjust the ⅛-inch valve so that a gentle flow goes through the flushing port of the glass syringe into the waste bucket.
Slowly turn the syringe stopcock handle so that the handle points to the flushing port (figure 59). This closes the flushing and allows oil to flow into the sample syringe. Do not pull the syringe handle; this will create a vacuum and allow bubbles to form. The syringe handle (piston) should back out very slowly. If it moves too fast, adjust the small ⅛-inch valve until the syringe slows and hold your hand on the back of the piston so you can control the travel.
♦ Allow a small amount, about 10 cubic centimeters (cc), to flow into the syringe and turn the stopcock handle again so that it points to the syringe. This will again allow oil to come out of the
flushing port into the waste bucket.
♦ Pull the syringe off the tubing, but do not shut off the oil flow. Allow the oil flow to continue into the waste bucket.
♦ Hold the syringe vertical and turn the stopcock up so that the handle points away from the syringe. Press the syringe piston to eject any air bubbles, but leave 1 or 2 cc of oil in the syringe. See figure 60.
Turn the stopcock handle toward the syringe. The small amount of oil in the syringe should be free of bubbles and ready to receive the sample. If there are still bubbles at the top, repeat the process until you have a small amount of oil in the syringe with no bubbles.
♦ Reattach the tygon tubing. This will again allow oil to flow out of the flushing port. Slowly turn the stopcock handle toward the flushing port which again will allow oil to fill the syringe. The syringe piston will again back slowly out of the syringe. Allow the syringe to fill about 80% full. Hold the piston so you can stop its movement at about 80% full.
Close the stopcock by turning the handle toward the syringe. Oil again will flow into the waste container. Shut off both valves, remove the sampling apparatus, and reinstall the original pipe plug.
Do not eject any bubbles that form after the sample is collected; these are gases that should be included in the lab sample.
Return the syringe to its original container immediately. Do not allow sunlight to impact the container for any length of time. Hydrogen will form and give false readings in the DGA.
♦ Carefully package the syringe in the same manner that it was shipped to the facility and send it to the lab for processing.
♦ Dispose of waste oil in the plant waste oil container.