7월 중순, 한 변전소 엔지니어가 뜨겁고 먼지가 많은 중동 항구 도시의 변압기 야드에 서서 화면에서 경고 신호를 모니터링하고 있습니다. 권선 온도가 118ºC에 도달했으며, 50 MVA 용량의 65 K 온도 상승 변압기에서 계속 상승하고 있습니다. 설치된 지 2년이 채 되지 않았고, 변압기는 끊임없이 뜨겁고 기록적인 여름 동안 높은 부하를 견뎌왔습니다. 변압기의 오일 자연 냉각 시스템이 변압기 탱크의 열을 제거할 수 없는 것으로 보입니다. 엔지니어는 모든 대형 변압기 소유자가 언젠가는 해결해야 할 딜레마에 직면해 있습니다: 그는 변압기의 냉각 시스템을 개조하거나, 용량을 줄이거나, 장치의 수명이 짧아질 준비를 해야 합니다. 냉각은 단순한 유지보수 문제가 아니라는 점에 유의해야 합니다. 이는 변압기 탱크의 용량과 전기 절연 수명을 결정하는 엔지니어링 고려 사항입니다.
이 기사는 ONAN 냉각, ONAF 냉각, OFAF 냉각, ODWF 냉각 및 공기 및 열 파이프 개조 솔루션과 같은 새로운 솔루션을 포함하여 사용 가능한 모든 변압기 냉각 기술을 제시합니다.
간단히 말해: 변압기의 냉각 방법은 IEC 표준의 네 글자 코드로 정의됩니다: ONAN, ONAF, OFAF, OFWF 및 ODWF. 열 한계는 권선 온도의 최대 평균 상승에 의해 결정되며, 일반적으로 IEC 60076-2 및 IEEE C57.12.00에 따라 오일 침수 변압기의 경우 65 K입니다. ONAN 변압기에 강제 공기 팬을 설치하면 일반적으로 약 20-33%의 지속적인 용량 증가를 제공합니다; 강제 오일 냉각을 완전히 적용하면 자가 냉각 장치에 비해 전력 공급을 40-50% 증가시킬 수 있습니다.
냉각이 변압기 출력을 결정하는 이유
변압기는 전기 에너지를 매우 효율적으로 변환하지만, 열로 변환되는 1에서 3 %의 전력은 막대한 손실을 의미합니다. 예를 들어, 50 MVA로 평가된 변압기가 99 %의 효율로 작동할 경우, 약 500 kW의 열 손실이 지속적으로 발생하며, 이는 작은 집을 데우기에 충분합니다. 생성된 열은 오직 하나의 경로로만 이동합니다: 즉, 절연 시스템을 통해 오일이나 공기로, 그 다음 탱크의 벽과 라디에이터를 거쳐 마지막으로 주변의 물이나 공기로 이동합니다. 이 열 제거 과정은 kVA로 측정된 변압기의 지속적인 성능에 대한 상한선을 제공합니다.
열은 변압기의 적입니다. 절연 노화의 주요 원인은 아레니우스 원리에 따릅니다: 예상되는 핫스팟 온도보다 6-10 K 상승할 때마다 절연의 예상 수명이 절반으로 줄어듭니다. 65 K의 평균 온도 상승을 위해 설계된 변압기가 80 K에서 작동하면 성능이 크게 저하될 뿐만 아니라 설계 수명을 매우 빠르게 소모하여 매달 수년의 수명을 잃게 됩니다.
냉각 코드 해독 및 주요 냉각 방법
냉각 과정은 IEC 60076-2 및 IEEE C57.12.00에 정의된 표준 4글자 명명법에 따라 식별됩니다: 첫 번째 글자는 권선과 접촉하는 매우 중요한 매체를 나타냅니다 (o = 오일, k = 플래시 포인트가 300° 이상인 액체, a = 공기), 두 번째 글자는 이 매체가 순환되는 방식을 알려줍니다 (n = 자연적으로, f = 인위적으로 강제 순환, d = 지향 순환), 세 번째 글자는 장치가 상호작용하는 매체를 나타냅니다 (a = 공기, w = 물) 그리고 마지막 글자는 순환이 수행되는 방식을 나타냅니다 (n = 자연적으로, f = 인위적으로).
| 냉각 코드 | 작동 방식 | 일반적인 정격 부스트 대 ONAN | 일반적인 변압기 크기 |
|---|---|---|---|
| ONAN | 오일은 자연 대류에 의해 순환되며; 열은 정지된 공기로 방출됩니다. | 기준선 | ~5 MVA까지 (단일 단계); 대형 장치는 부하가 감소된 상태에서 ONAN으로 정격됩니다. |
| ONAF | 방열기 팬을 추가합니다; 공기가 냉각 표면을 가로질러 불어옵니다. | +20에서 +33 %까지 | 5-100 MVA |
| OFAF | 오일 펌프가 순환을 강제합니다; 팬이 방열기를 통해 공기를 강제합니다. | +40에서 +50 %까지 | 50-300 MVA |
| OFWF | 물로 냉각된 열교환기를 통해 펌프된 오일 | +40에서 +50 %까지, 컴팩트한 면적 | 물이 이용 가능한 대형 장치 |
| ODWF / ODAN / ODAF | 권선 채널을 통한 지향 오일 흐름; 외부 팬 또는 물 | 지향 설계에서 +60 %까지 | 100 MVA 이상 |
다단계 정격은 전력 변압기 구매 시 일반적으로 사용되며, 예를 들어 ONAN/ONAF/ONAF 또는 ONAN/ONAF/ONAF/ONAF의 조합으로 운영자가 부하가 증가함에 따라 팬을 켤 수 있도록 합니다. 예를 들어, 60 MVA의 전력을 가진 변압기는 ONAN으로 40 MVA, ONAF로 50 MVA, 모든 팬이 활성화된 상태에서 60 MVA로 정격될 수 있습니다. 사용되는 팬은 일반적으로 0.75 kW에서 3 kW의 전력을 소비하는 축 방향 팬으로, 지원하는 부하에 비해 무시할 수 있는 양이며, 설치당 수백에서 수천 달러에 달합니다.
ONAN 대 ONAF 대 OFAF 대 ODWF
| 기능 | ONAN | ONAF | OFAF | ODWF |
|---|---|---|---|---|
| 이동 부품 | 없음 | 팬만 | 펌프 + 팬 | 펌프 + 물 시스템 |
| 전형적인 정격 범위 | 5-10 MVA까지 | 5-100 MVA | 50-300 MVA | 100 MVA 이상 |
| MVA당 비용 | 가장 낮음 | 보통 | 더 높음 | 가장 높음 |
| 신뢰성 / 유지보수 | 가장 높음; 고장날 것이 없음 | 팬 모터는 주의가 필요합니다. | 펌프 씰 및 모터 추가 | 물 누수 및 오염 위험 |
| 면적 | 대형 방열기 뱅크 | 컴팩트한 방열기 + 팬 | 펌프가 있는 컴팩트한 | 가장 컴팩트한 |
| 소음 | 가장 낮음 | Fan noise | Pump + fan noise | Pump + water noise |
It’s simple to understand the engineering trade-offs here: every additional forced cooling stage provides additional MVA per unit of tank volume, but results in adding equipment that may fail, consume power or produce noise. ONAN is still the most common option, since it has no failure modes besides the transformer, while ONAF is the default option due to affordable, tested and easily retrofitted fans; OFAF and ODWF are only used for large transformers, when tanks and heat density leave no other option.
Temperature Rise Classes and Limits
| 매개변수 | IEC 60076-2 Typical Value | IEEE C57.12.00 Typical Value |
|---|---|---|
| Max average winding rise (oil-immersed) | 65 K | 55 K (OA), 65 K (FA) |
| Max hot-spot winding rise | 78 K (85°C hot-spot, 7 K ambient allowance) | 80 K |
| Max top-oil rise | 60 K | 55-65 K by class |
| Design ambient temperature | Average 30°C, max 40°C | 30°C average, 40°C max |
| Max hot-spot temperature limit | 98°C (winding), 105°C (oil) | 110°C continuous |
These figures are part of an agreement or contract with a format. If a transformer does not pass the temperature-rise evaluation test at the factory, then it has not followed the conditions of the specification and the certification needs to include the measured temperature rise. When altitude is taken into account (1,000 m or higher) or high ambient temperature (plus or minus 40 degrees Celsius), then the factors are to be checked in IEC 60076-2, the reason for which is that the devices will have to be derated in the hotter climate and thus will be different from the devices that will be used in a moderate climate.
In order to understand how the different stages of cooling influence the process | the same 25-MVA transformer has been considered with different conditions and processes, so that one can realize the peculiarity when the suppliers provide unrealistic offers:
| Cooling Mode | Continuous Rating (25 MVA Tank) | Rise at 25°C Ambient | Rise at 40°C Ambient | Derating Needed at 40°C |
|---|---|---|---|---|
| ONAN (natural) | ~15 MVA | 52 K top-oil | 67 K top-oil | ~12% |
| ONAF (fans on) | ~20 MVA | 48 K top-oil | 63 K top-oil | ~8% |
| OFAF (pumps + fans) | ~25 MVA | 44 K top-oil | 59 K top-oil | ~5% |
| ODWF (directed + water) | ~30 MVA | 40 K top-oil | 55 K top-oil | ~5% |
Retrofitting and Uprating Existing Units
Converting from ONAN to ONAF service is often the preferred upgrade method because it’s usually cheaper than buying a bigger transformer.
Check thermal design. Get confirmation from the manufacturer about whether or not the radiators and hot-spot in the windings can cope with the extra heat. Some tanks will require extra radiator surface area even with the installation of fans.
- Install fan bank. The cost will usually fall within $1500 to $8000 and will include 2-6 axial fans, guards, mounting frames, wiring in addition to contactors, while installation will require 2-4 days of outage.
- Equip with temperature-controlled switching. The fans should start working from a top-oil temperature signal while turning off automatically based on either a timer or a temperature drop in order to avoid quick cyclical operation.
- Update the nameplate and protection. The new combined rating must be included in relay settings as well as maintenance records.
- Commission with heat run. It would be advisable to repeat the temperature-rise test at the new rating in order to ensure that the winding rise of 65 K is followed.
Going beyond ONAF and using forced oil circulation requires more effort, as it involves adding oil pumps and modifying the tank plumbing. This option is considered when dealing with transformers that are exceeding 30 MVA. Water-cooled conversions (OFWF) might be possible, but in reality, they don’t bring that many advantages since they are connected to the risk of leakages.
Cooling of Dry-Type Transformers
Dry-type transformers are characterized by their usage of air for cooling and compliance with particular codes which include AN (Air Natural) or AF (Air Forced). In terms of heat removal, dry-type transformers do not include oil, and so instead rely on the area of their encapsulated or wound coils. The capacity of dry-type transformers is relatively smaller than oil-filled models. For instance, typical cast-resin transformers may reach a maximum capacity of up to 3-10 MVA. Dry-type transformers may be categorized into two temperature rise classes according to the IEC 60076-11 standard: Class F (which rises by 100K to reach a temperature of 155°C) and Class H (which rises by 125K to reach a temperature of 180°C).
The implementation of forced-air cooling adds a couple of hundred dollars for the fan and increases the rated capacity of the transformer by approximately 25 to 30%. The key advantage of dry-type transformers is the absence of oil in the operation process. However, the usage of air as a cooling medium also poses certain disadvantages such as the need for proper ventilation and an expensive system for dust management.
Which Cooling Method for Which Application
- Distribution transformers more than 2.5 MVA – In this case, ONAN is the standard method used for most applications. It may be used, however, in a case when there is a presence of peaky industrial load, allowing for use of ONAF.
- Power transformers – At this point, ONAN method could be used with some fans to provide an appropriate cooling environment.
- Large power transformers (more than 100 MVA) are of an OFAF type, so there are oil pumps and forced flow systems being used for these devices.
- OFFSHORE DISTRICT AND CROWDED PLACES – An OFWF type should be preferred when there are water resources, reducing the size of the system.
- Indoor commercial buildings deal with dry type units while using AN and AF methods, as oil and the indoor fire code do not go well together.
- Hot climate and high altitude bases call for a need to implement additional cooling level and/or derating because surrounding air is not capable of taking much heat.
Brands and Price Comparison
| 브랜드 | Origin | Cooling Technologies Offered | Approx. Price Range (1 MVA ONAN) |
|---|---|---|---|
| ABB | 스위스/스웨덴 | Full range, ODWF and directed-flow experience | $10,000-$25,000 |
| 지멘스 | 독일 | Large power units, staged ONAF/OFAF | $11,000-$28,000 |
| 히타치 에너지 | 스위스 | Power transformers with advanced cooling | $12,000-$30,000 |
| 슈나이더 일렉트릭 | 프랑스 | Distribution and dry-type AF units | $9,000-$22,000 |
| Eaton / GE | 미국 | Distribution ONAN/ONAF | $7,000-$18,000 |
| 장쑤 수비안 전력 | 중국 | ONAN, ONAF, staged ratings, dry-type AN/AF | $5,000-$14,000 |
Market leaders such as ABB, Siemens, Hitachi Energy, and Schneider Electric are the defining players in the large-scale power transformer industry with their ODWF/OFAF designs that are used as benchmarks in the industry. Notably, manufacturers such as Jiangsu Subian Electric Power are known for their ONAN and staged ONAF units designed for the smaller 0.5-10 MVA classes that most customers are interested in. Their units have been tested in accordance with IEC 60076-2, and they receive certificates verifying that the temperature rise laboratories can measure the temperature rise of their transformers. Subian ships its products all over the world; it can produce tanks and radiators according to customer specifications at prices that are 40 to 60% lower than the prices quoted by its European competitors. Customers who need fan bank retrofitting should check with Subian about the technical specifications for radiator surface technology and pump choices, since when the ambient temperature is as high as 40 degrees Celsius, these specifications are more important than the price.
How to Choose the Right Cooling Method
- The first thing is to define the type of load profile. A load profile with constant use has a higher cooling requirement than a load profile with varying use. Therefore, a cooling stage may be optimized for running the fans alone during peak afternoon times.
- Next, you shall consider the effects of ambient. If your ambient is more than 40 degrees Celsius or you are located in an altitude of more than 1000 meters, you will need to derate it according to IEC 60076-2 or provide an additional cooling stage.
- Make sure that you account for the space requirements of the equipment. Radiator banks require yard space but OFWF and ODWF technologies reduce the size of the footprint at the cost of water systems.
- Reliability is of high importance. For critical infrastructure, ONAN stages and simple fans should be favored over pumps whenever possible, as long as the rating allows.
- Be aware of the total cost of ownership. Motors and pumps consume energy and require maintenance. A large ONAN tank may have a lower cost than simple forced cooled motor.
- Do not forget to request a temperature-rise test with 65 K (or desired) winding increases being attested by the test certificate rather than being assumed.
Maintenance and Monitoring
The failure of cooling systems occurs in silence and is typically indicated by an alarm triggered by the drop in temperature. In sound cooling practice, a number of inspections of breakdown parts such as fan blades of the fan, fan motors, fan guards, and so forth are included. In addition to a monthly check of all parts mentioned, a quarterly check of pump seals and oil flow indicators should be performed. Together with annual thermography of radiator banks, there is also a need to carry out a written test of the auto-start of temperature switch controlled fan. As for the large cooling boards, the winding temperature section, and DGA should be analyzed together; if there is a rise of a hot spot with no fan malfunction, it points towards an issue within an internal part of the cooling unit.
The oil amount in the conservator should match the cooling design. The occurrence of low oil level can lead to a situation where cavitation happens and this can be of great danger compared to overheating.
자주 묻는 질문
What do ONAN and ONAF mean on a transformer nameplate?
These are designations that are linked to cooling according to IEC 60076-2. ONAN stands for oil natural and air natural, which means the self-cooled standard design. The ONAF designation includes the radiator fans that help to cool the transformer, and so the ONAF designation denotes the increased operation capacity, which is 20% to 33% higher than that of the self-cooled design. For this reason, ONAN/ONAF transformers are rated twice.
How much does it cost to add forced-air cooling to a transformer?
The retrofit of an ONAF on a distribution-class transformer costs is 1500-8000 dollars since it usually involves the installation of 2-6 fans, as well as the installation of the guards, contactors, and temperature-control switches. Using forced air helps to raise the capacity of the transformer by at least 20-33.
Can I overload a transformer if I turn on the fans?
The overloads are permitted only up to the ONAF rating and only if the air temperature and elevation are within the limits prescribed in the concept. The actual increase of temperature is a requirement; thus, in case of the increase of the average temperature higher than 65K, the life span of the insulation will be reduced.
What is the difference between OFAF and ODWF cooling?
The difference consists in the fact that in OFAF, the oil is circulated through the radiator while in OFDA, the oil is pumped in winding cooling channels by means of water flow through the heat exchanger.
Does altitude really matter for transformer cooling?
Indeed, it does. Above 1000 meters, the air density is decreasing which makes cooling very inefficient.
참고 문헌
- IEC 60076-1 — Power Transformers, General Requirements — Master specification for ratings and losses.
- IEC 60076-2 — Power Transformers, Temperature Rise — The standard that defines cooling codes and temperature-rise limits.
- IEEE C57.12.00 — General Requirements for Liquid-Immersed Transformers — North American cooling code and temperature-rise baseline.
- IEC 60076-11 — Dry-Type Transformers — Thermal classes for air-cooled units.
- Hitachi Energy — Transformer Products — Reference for staged cooling ratings on large power transformers.
- ABB Transformers — Industry reference for cooling technology options and datasheets.
- Siemens Energy Transformers — Cooling and uprating documentation for power transformers.
결론
Coolings of transformers act as the important deciding factors determining the power and life span of the transformer. Whenever one reads the 4-letter code for cooling, understands the specifications of temperature rise, and makes a choice between natural cooling, forced cooling, or adding a fan to the existing cooling systems, all that info has practical use in engineering.
- ONAN is the basic standard; ONAF represents an improvement and consists of 20-33% over the ONAN with extra costs of retrofitting from $1,500 to $8,000.
- OFAF and ODWF are used for transformers of large sizes for the heat densities.
- Temperature shall be proven through the test and certified with the winding temperature rise of 65 K.
- In case of high altitude or temperatures, transformers shall be derated or the cooling shall be reinforced.