초고압직류송전

[智雲]

초고압직류송전(high-voltage, direct current, HVDC)는 전력 그리드 시스템중 하나라고 볼 수 있다.^[1][2][3][4] 즉 간단하게 초고압 직류를 송전하는 것을 줄인 것이다.^[5]

HVDC는 주파수가 상이한 교류전력망 간의 송전이 가능하다. 동-서 전력망의 주파수가 다른 일본 같은 국가가 이를 사용하는데, 직류니까 교류로 다시 바꿔주기 전까진 주파수라는 개념이 없는 것이다. 또한 전자파가 발생하지 않는다.

1 개발

보통 그리드는 고전압을 사용해서 저항 으로 발생하는 에너지 손실을 줄이는데, 전압을 2배 올리면 전류가 절반이 되어 발생하는 열이 줄어들기 때문에 4배의 송전효과를 얻을 수 있다. 무식하게 도체를 늘리는 방법도 있지만 송전탑이 더 많아져야 하고, 걸리는 줄이 더 많아져야 한다는 말이다.

전기가 처음 개발되던 19세기 말의 에디슨 대 테슬라의 전기 전쟁에서는 교류 방식이 장거리 송전에 효율적이라고 판단되었지만, 기술이 발전하면서 직류를 초고압으로 보내면 교류보다 더 효율이 좋다는 것이 판명되었다.

실용적인 직류와 교류의 변환은 머큐리 아크 밸브, 그러니까 고전압을 담당하는 진공관같은 전력장비의 개발과 더불어, 1970년대 시작된 사이리스터라던가, 통합 게이트 정류 사이리스터(IGCT), 절연 게이트 양극성 트랜지스터 (IGBT), MOS-제어 사이스터(MCT)등이 개발되면서 이뤄지게 되었다. 그러니까 앞으로도 계속 개발되고 있고 계속 싸질 예정이다. 근데 이게 단점이라면 단점

2 이점과 단점

먼거리의 HVDC 송전은 일반적으로 같은 교류 송전보다 가격이 저렴하고 에너지 효율이 더 높다. 터미널의 HVDC 변환기는 비용이 엄청나게 깨지지만, 직류 라인은 선이 별로 없어도 상관없어서 훨씬 덜 나간다. 대충 1000km당 3.5%의 전류가 손실되는데, 같은 전압의 교류는 30~40%가 깨진다. 아 물론, 짧은 거리라면 이 변환기로 인해서 가격이 훅 날아가서 잇점이 사라지게 된다. 거기다가 변환장비들은 엄청나게 많아서 신뢰성이 떨어지는데, 약 98.5%의 가용성을 보여준다.

그리고 HVDC에서는 변환소가 또 중요한데, 변환소에 들어가는 물건 또한 비싸고, 과부하도 못거는 상황이 벌어질수 있다. 또한 짧은거리의 경우, 교류보다 더 많은 손실률을 보일수 있다.

거기다가 다중 터미널 시스템도 구축하는게 빡세고, 전류 흐름이 컨버터 제어 시스템에 맡겨져 있는지라 교환소 하나가 터지면 다 터지는 결과를 낳는다. 그리고 현재도 계속 개발중인 물건인지라, 돈이 또 제대로 깨질 가능성이 존재한다는게 흠이다.

그러나, 이걸 뛰어넘는 잇점들이 존재하기 때문에 중국의 경우 동쪽에서 만든 전기를 서쪽으로 송전하기 위해 거대 HVDC를 깔아버리고, 대한민국에서도 시장에 발을 담그기 위해 제주도에 HVDC 라인을 시험 설치했다. 정치적인 문제만 잘만 해결되면 아시안 그리드를 깔아서 발전소도 줄일수 있지만, 정치적인 문제가 해결될까?

▣ HVDC

HVDC (high-voltage direct current) is a highly efficient alternative for transmitting large amounts of electricity over long distances and for special purpose applications. As a key enabler in the future energy system based on renewables, HVDC is truly shaping the grid of the future.

▣ Why HVDC?

Since power stations generate alternating current (AC) electricity, and utilities deliver AC power to consumers, why is it sometimes better to transmit electricity as high-voltage direct current (HVDC)?

It’s an interesting question, because most electrical power transmissions also use three-phase alternating current. So how does DC transmission fit into a modern power network?

AC has been the preferred global platform for electrical transmission to homes and businesses for the past 100 years. And yet high-voltage AC transmission has some limitations, starting with transmission capacity and distance constraints, and the impossibility of directly connecting two AC power networks of different frequencies.

With the dawn of a new energy era and the need to build a smarter grid, HVDC is expected to grow far beyond its traditional position as a supplement to AC transmission. 

HVDC is now the method of choice for subsea electrical transmission and the interconnection of asynchronous AC grids, providing efficient, stable transmission and control capability. HVDC is also the technology of choice for long-distance bulk power transmission, able to send vast amounts of electricity over very long distances with low electrical losses. That makes it a key technology in overcoming a huge problem with renewable generation like wind, solar and hydro – that these resources are seldom located near the population centers that need them. 

The reasons for choosing HVDC instead of AC to transmit power in a specific case are often numerous and complex. Either HVDC is necessary or desirable from a technical point of view, i.e. controllability. Or HVDC results in a lower total investment, including lower losses, and/or is environmentally superior.

In many cases, HVDC links are justified based on a combination of technical, economic and environmental advantages.

Economic and environmental advantages

Beyond the ‘break-even’ distance, HVDC transmission systems cost less, even with the added expense of terminal stations. Meanwhile, an HVDC line has lower power losses than an HVAC of the same capacity in practically all cases, which means more power is reaching its final destination.

HVDC systems also have a lower environmental impact because they require fewer overhead lines to deliver the same amount of power as HVAC systems. And HVDC interconnections enable power systems to use generating plants more efficiently, for example substituting thermal generation with available hydropower resources. 

The technology is a key component in the future energy system based on renewable energy sources, such as wind and solar power which are often both volatile and remotely located.

Positive effects on the power systems

Many HVDC transmissions have been built to interconnect different power systems. The links help existing generating plants tied into a power system operate more effectively, so new power station builds can be deferred. This makes economic as well as environmental sense.

The obvious environmental benefit is not having to build a new power station, but there are even greater gains coming from the operation of an interconnected power system that uses its available generating plants more efficiently. There are great environmental advantages to linking a power system with large hydroelectric resources to a system with mostly thermal generation. You can reduce thermal generation (predominately at peak demand) by tapping the hydro generation, which also helps to run the thermal generation more efficiently at constant output, without having to follow load variations.

Reduced right-of-way

One bipolar HVDC overhead line is comparable to a double circuit AC line from a reliability point of view. Therefore, a single HVDC line with two conductor bundles has less environmental impact than a double circuit AC line with six conductor bundles - it requires less space and has less visual impact

With HVDC Light it is possible to use extruded polymer cables for DC transmission. This has made the use of buried land cables an interesting alternative to traditional overhead lines.

Lower losses

HVDC transmission losses are lower than AC transmission losses in practically all cases. An optimized HVDC power transmission line has lower losses than AC lines of the same capacity. Losses in the converter stations must also be added and they are about 0.6 percent for HVDC Classic and below 1 percent for HVDC Light of the transmitted power in each station.

Hence, in a side-by-side comparison, total HVDC transmission losses are still lower than the AC losses in practically all cases. HVDC cables also have lower losses than AC cables. The diagram below shows a comparison of the losses in 1,200 MW overhead line transmissions using AC and HVDC.

Lower investment cost

An HVDC transmission line costs less than an AC line for the same transmission capacity. However, it is also true that HVDC terminal stations are more expensive due to the fact that they must perform the conversion from AC to DC, and DC to AC. But over a certain distance, the so called "break-even distance" (approx. 600 – 800 km), the HVDC alternative will always provide the lowest cost.

The break-even-distance is much smaller for subsea cables (typically about 50 km) than for an overhead line transmission. The distance depends on several factors (both for lines and cables) and an analysis must be made for each individual case.

The break-even distance concept is important, but only one of a number of factors, such as controllability, that are important to consider in choosing an AC or HVDC transmission system. 

HVDC is a proven technology with many distinct technical advantages. For example, it enables bulk power transmission over very long distances, with higher efficiency and lower electrical losses per 1,000 km.

HVDC enables secure and stable asynchronous interconnection of power networks that operate on different frequencies, or are otherwise incompatible. In addition, HVDC provides instant and precise control of the power flow.

Once installed, HVDC transmission systems become an integral part of the electrical power system, improving the overall stability, reliability and transmission capacity.

Asynchronous grids

A number of HVDC links interconnect two AC systems that are not synchronous. When AC systems are to be connected, they must be synchronized. This means that they should operate at the same voltage and frequency, which can be difficult to achieve. Since HVDC is asynchronous it can adapt to any rated voltage and frequency it receives. Hence, HVDC is used to connect large AC systems in many parts of the world.

For example, the Nordel power system in Scandinavia is not synchronous with the UCTE grid in western continental Europe, even though the nominal frequencies are the same. And the power system of the eastern USA is not synchronous with that of western USA, Texas or Quebéc. There are also HVDC links between networks with different nominal frequencies (50 and 60 Hz) in for example Japan and South America. 

Long distance water crossings

There are no technical limits to the potential length of a HVDC cable. In a long AC cable transmission, the reactive power flow due to the large cable capacitance will limit the maximum possible transmission distance. With HVDC there is no such limitation; this is why, for very long cable links, HVDC is the only viable technical transmission alternative.

The 580-km long, ABB-built NorNed

Several HVDC links with very long submarine cables are being considered today, mainly in Europe. One example is Iceland - Europe

Controllability

A fundamental advantage of HVDC technology is the ease of controlling active power in the link.

In most HVDC links, the main control is based on constant power transfer. This property of HVDC has become more important in recent years, given the shrinking margins of power networks as electricity markets in many countries are deregulated.

In many cases, an HVDC link can also improve the performance of AC power systems by means of additional control facilities. Normally these controls are activated automatically as certain criteria are fulfilled. Automatic HVDC control functions include constant frequency control, redistribution of the power flow in the AC network, damping of power swings in the AC networks, etc. In many cases such additional control functions can make possible the safe increase of power transmission capability in AC transmission lines where stability is a limitation.

Today's advanced semiconductor technology, used in both power thyristors and microprocessors for control systems, has created almost unlimited control possibilities in HVDC transmission systems. Different software programs are used for different studies supporting these control options.

Normally a positive sequence program, for example PTI’s PSS/E program is used for load-flow and stability studies. For more detailed investigations of the performance of inner control loops of the converter and its interaction with a nearby network,  a simulation is created in a full three-phase representation program such as PSCAD/EMTDC

Low short circuit currents

An HVDC transmission does not contribute to the short circuit current of the interconnected AC system.

When a high-power AC transmission is constructed from a power plant to a major load center, the short circuit current level will increase in the receiving system. High short circuit currents are becoming an increasingly difficult problem for many large cities, which may result in the need to replace existing circuit breakers and other equipment if the rating is too low.

But if new generating plants are connected to the load center using a DC link, the situation is quite different. The reason is an HVDC transmission does not contribute to the short circuit current of the interconnected AC system.