What is Cryogenics?
Cryogenics is the science that addresses the production and effects of very low temperatures. The term cryogenics, according to the National Institute of Standards and Technology (NIST), applies to all temperatures lower than –150°C (–238°F).
Compounds that normally are gases at room temperature condense to liquids at extremely low temperatures. The word “cryogenic” means “producing, or related to, low temperatures,” and all cryogenic liquids are extremely cold. Gases can also be condensed to liquids by exposing them to very high pressures. These gases must be cooled below room temperature before an increase in pressure can liquefy them. Different cryogens become liquids under different conditions of temperature and pressure, but all have two properties in common: they are extremely cold, and small amounts of liquid can expand into very large volumes of gas.
Common cryogenic liquids are liquid nitrogen (LN2), liquid oxygen and liquid helium. Applications that use cryogenics include tool or metal tempering, nuclear research, electromagnetism work and multiple laboratory techniques. Surgeons use cryogenics to treat Parkinson’s disease, destroy brain tumors and arrest cervical cancer. Liquid oxygen fuels rocket engines, and cutting and welding torches. But cryogenic liquids require special precautions.

Effects of Cryogenics on Materials
The extreme temperatures of these liquids cause most solids to become more brittle. Materials such as carbon steel, plastics and rubber should not be used with cryogenic liquids because they can fracture or shatter extremely easily.

Physical Effects of Exposure
Contact with cryogenic liquids to the eyes or the skin can cause serious frostbite injuries. Tissues that have been frozen will be painless while still frozen and might look waxy or yellow. Thawed frostbitten skin will be very painful, red and swollen, and can become infected.
Any flesh that contacts a cryogenically cooled material can stick to that material, similar to the way some children stick their tongues to flagpoles in the winter. However, where cryogenics is involved, metallic materials are not the only ones that cause this risk. It is important to remember that even nonmetallic materials are extremely dangerous to touch. Removing skin from any material can cause a tearing of the flesh. To minimize the chances of freezing materials to skin, all watches and jewelry on the hands and wrists should be removed.
Warming cryogenic liquids reverting to their gaseous states might displace oxygen. Oxygen-deficient atmospheres can cause dizziness, nausea, vomiting, unconsciousness, confusion and death. However, not all symptoms will necessarily be present depending on the speed of the gas expansion. Unconsciousness could occur without any preceding signs of danger.

First Aid
Bodily tissues exposed to cryogenic liquids or gases should be restored to normal body temperature by running warm water (108°F) over the affected part. The water should never be more than 112°F, and do not rub the affected part. Rubbing can cause further damage. The victim should get emergency care as quickly as possible to minimize further damage and for damage assessment.
If oxygen loss overcomes a person working with cryogenic liquids, that person should immediately be moved to a well-ventilated area. A self-contained breathing apparatus (SCBA) might be required for rescue. Apply artificial respiration if the victim’s breathing stops. Apply oxygen if the victim has difficulty breathing, and summon emergency medical help.

Personal Protective Apparel for Cryogenic Safety
Personal protective equipment is critical to cryogenic safety. Always wear chemical-splash goggles and face shields during the transfer and handling process to guard against splashes, vessel rupture and flying debris.
Loose-fitting, insulated cryogenic gloves should also be worn. The gloves should fit loosely enough that if a splash occurs inside the gloves, they can easily and rapidly be thrown off.
To protect all parts of the skin, long-sleeve shirts and trousers are also recommended. Pant legs should go over the tops of footwear so spills cannot get into boots or shoes and cause extreme tissue damage before the footwear can be removed.
If working in an oxygen-deficient atmosphere, you must use an oxygen-supplying respirator, such as an airline respirator with an egress bottle or a self-contained breathing apparatus (SCBA.) A cartridge-style respirator would not be appropriate because the problem with the atmosphere is the loss of oxygen, which cannot be added by filtration.

Environmental Controls and Cryogenic-Specific Equipment
These liquids vaporize extremely rapidly, causing a different danger as surfaces warm up. The liquids are capable of producing huge amounts of gas that could produce explosions or vessel ruptures. Containers should never be plugged or covered. Containers should vent to prevent explosions.
Only use containers that are made specifically for cryogenic liquids such as Dewar flasks. Dewar flasks consist of two flasks with one inside the other. There is a space between the two flasks, which provides a layer of insulation. This insulation keeps the liquid from warming quickly and causing rapid expansion of the gas. And although these products are made to specifically withstand the stress of extreme temperature changes, they should still be filled very slowly to protect the containers from excessive internal stress that can cause damage and weaken the container.
Cryogenic containers should not be filled past 80 percent of capacity to account for the expansion of gases.
This rapid expansion of gas can lead to asphyxiation, except for oxygen, in an enclosed area. The gases can displace the oxygen and a person can rapidly be overtaken. Most cryogenic liquids including liquid nitrogen become colorless, odorless and tasteless gases, which makes them undetectable to human senses. Air with less than 18 percent oxygen can cause dizziness, and continued lower levels quickly progress to unconsciousness and death. Always work with these liquids in a well-ventilated, open area.
Personal, handheld or fixed air-monitoring devices are available to measure oxygen levels in enclosed areas. These units alert people in the room of an oxygen-deficient atmosphere, when oxygen levels decline past 19.5 percent.
Cryogenic liquids boil at room temperatures. This boiling can cause eruptions and splashes, so tongs need to be used when removing anything immersed in the liquid.

Q. Why can’t I wear a cartridge-style respirator to protect from liquid-nitrogen vapors?
A. Liquid nitrogen itself is not a danger. In fact, normal air is 78 percent nitrogen. We don’t need protection from high levels of nitrogen gas. The danger is that liquid nitrogen vaporizes so quickly that it displaces oxygen. Oxygen comprises about 21 percent of normal air. When it falls below 18 percent, we do not have enough oxygen to normally function. Cartridge-style respirators are only made to remove contaminants, so they are inappropriate for an environment that is oxygen deficient.

Q. What kinds of gloves are needed for working with cryogenics?
A. Gloves should be insulated and made to withstand the low temperatures of cryogenic liquids. They should also be loose fitting so they can be easily removed if a spill should go inside the glove.

Q. How cold is liquid nitrogen?
A. Nitrogen condenses to a liquid at –320°F.

Q. I only have 1L of nitrogen gas. Why do I have to worry about being in an enclosed space?
A. A single liter of liquid nitrogen can expand to 24.6 cu. ft. of gas, which is an expansion ratio of 1:696. This can rapidly fill a small space and cause a very dangerous situation.

Sources
Schachtman, Tom. “Absolute Zero and the Conquest of Cold.” Houghton, 1999.
Compressed Gas Association 14501 George Carter Way, Suite 103
Chantilly, VA 20151
Phone: 703-788-2700
Fax: 703-961-1831
(Rev. 1/2012)

reference :
1. Grainger
2. Canadian Center for Occupational Health and safety
3.NIST

Beberapa waktu yang lalu, kami melakukan testing dua transformer kering yang akan dioperasikan paralel. Seperti yang telah kita pahami syarat dari kerja paralel transformer adalah harus memiliki vektor group yang sama. Data sheet tranformer yang yang dibeli adalah 2 x 380/230 VAC 15 kVA Yzn11. Jadi kita harus mengetes apakah transformer tersebut telah diwiring dengan benar.

Dari vendor trafo, mereka memberikan gambaran rangkaian wiring seperti gambar Fig 1 dibawah ini.

Fig 1:Wiring Yzn11

Untuk tujuan pengetesan, kita menjadikan wiring U sebagai referensi, untuk itu kita harus men-jumper terminal [U] di sisi HV ke terminal [x] di sisi LV. Seperti terlihat di Fig 2.

Dari wiring di fig 2, bisa kita terjemaahkan menjadi rangkaian vektor seperti pad fig 3.

 Fig 2 : Test vector Yzn11 Fig 3:Vector Yzn11

Dari vektor diatas, dengan menggunakan persamaan aljabar dan persamaan segitiga, kita bisa menemukan panjang dari tiap vektor, dengan persamaan di bawah ini:
1. $\overrightarrow{Ux} =0$
2. $\overrightarrow{UV}=\overrightarrow{Un}+\overrightarrow{nV}$
3. $|Wz|=\sqrt{|UW|^2+|Uz|^2-\sqrt{3}|UW||Uz|}$
4. $|Wy|=\sqrt{|UW|^2+|xy|^2}$
5. $|Vz|=\sqrt{|Vn|^2+|zn|^2-|Vn||zn|}$
6. $|Wn|=\sqrt{|Wz|^2+|zn|^2+\sqrt{3}|Wz||zn|}$

[catatan: Persamaan diatas hanya berlaku saat $\frac{V_{primary}}{V_{sec}} = \sqrt{3}$]
Dari 5 persamaan diatas maka kita bisa membuat tabel seperti dibawah ini, jika terminal HV kita beri input 400 VAC(hal ini tidak boleh dilakukan jika HV transformer lebih dari 400 VAC):

U V W x y z n
U 0 N/A N/A N/A N/A N/A N/A
V 400 N/A N/A N/A N/A N/A N/A
W 400 400 N/A N/A N/A N/A N/A
x 0 400 400 N/A N/A N/A N/A
y 230 230 230 230 N/A N/A N/A
z 230 231.5 231.4 230 230 N/A N/A
n 132.7 267.3 352.6 132.7 132.7 132.7 N/A

Dari hasil pengukuran saat pengetesan di manufacture didapatkan hasil seperti di bawah.

U V W x y z n
U 0 N/A N/A N/A N/A N/A N/A
V 400 N/A N/A N/A N/A N/A N/A
W 400 401 N/A N/A N/A N/A N/A
x 0 400 399 N/A N/A N/A N/A
y 241 225.5 467.4 241.5 N/A N/A N/A
z 239 227 227 239.4 241.3 N/A N/A
n 138.6 260.5 352.5 139 139.5 138.2 N/A

Timbul pertanyaan, mengapa terjadi perbedaan pada beberapa kolom walau tidak significant. Hal tersebut disebabkan oleh ketidaksempurnaan dalam proses pembelitan trafo. Perbandingan primer dan sekunder tidak ideal yaitu $\frac{400}{241}=1.660$. Perbandingan yang ideal adalah $\frac{400}{230}=1.732$