The increasing demand for power and the deregulation of the power industry continue to produce problems for transmission and distribution system operators who are faced with curtailing investment and the NIMBY (Not In My Back Yard) phenomena. The power demand in South Korea is now increasing by about 4% to 5% annually, and the peak demand in 2010 is expected to exceed 67,000 MW. It is also expected that the load density within the metropolitan area will increase more rapidly than in other areas.

To satisfy the growth of power demand and maintain system reliability, it is imperative to construct a well-designed transmission network and to install sufficient generation capacity. High-temperature superconductor (HTS) cable can help meet these objectives, as well as provide an alternative to overcome the predicted shortage of power network facilities and environmental problems. The Center for Applied Superconductivity Technology (CAST; Changwon, South Korea) has prepared a basic plan for the installation of 22.9-kV HTS cable that it has developed for the distribution system in a downtown metropolitan area, such as in Seoul, South Korea. CAST basically investigated the application of the 22.9-kV HTS cable as an alternative to 154-kV conventional cable. The first stage was to study the application methodology of 22.9-kV HTS cable for practical power networks by qualitative analysis and examine the technical reasons for applying 22.9-kV HTS cable in the future.

Korean Power Network Prospects

As investment in the power industry infrastructure declines, the route length per unit demand should decrease from 0.6 circuit-km/MW (0.37 circuit-miles/MW) at present to 0.53 circuit-km/MW (0.33 circuit-miles/MW) in 2010. This suggests that the capacity of the power network is insufficient to meet the increasing demand in the future, so the distribution and transmission capacity per route should be increased as well as the load transfer capacity. Figure 1 shows the expected route length per unit demand in the future.

Currently, the general rating of the 22.9-kV underground distribution system in South Korea is concentric neutral cross-linked polyethylene cable with a cross-sectional area of 325 mm2 (0.5 in2), having a capacity of 10 MW. In 2002, it was 197,660 circuit-km (123,000 circuit-miles) in length with an annual increase of 8.7%. Table 1 shows the route length and annual growth rate of 154-kV underground cable in South Korea from which it will be expected to increase annually from 6.8% in 2000 to 11.6% in 2010 and 13.4% in 2020.

Due to the increasing load density within the metropolitan area, the demand for 154-kV substation and underground cable construction also increases, resulting in excessive investment and an adverse impact on the environment. To solve these problems, the distribution and transmission capacity per unit route should be increased, and this is why the application of HTS cable to supply downtown areas in metropolitan cities should be considered.

Application of HTS Cable

For a metropolitan city like Seoul, it is vital to establish a bulk-capacity underground cable network capable of supplying increasing demand with a high-reliability source. However, there is a limit to the capacity per underground circuit line. The multicircuit underground cable system introduces difficulties that include securing a cable route in an overcrowded city and managing the heavy burden of public construction costs. Along with underground cable expansion, it demands the construction of new substations in downtown areas that also has adverse effects on the environment, in addition to increasing the cost of supply.

HTS cable can transmit bulk power at low voltage, reducing the capital investment and power loss. Therefore, HTS cable offers an alternative plan to enlarge the capacity of the T&D network and to reduce the substation facility in a metropolitan city. Hence, if the HTS cable is used to supply metropolitan areas, 154-kV substation construction projects are avoided and the installation costs of the underground cable route can be reduced, because the HTS cable has a smaller cross-section. The overall benefits when the HTS cable is considered for downtown areas are:

  • Reduction of power loss (increases the transmission/distribution efficiency)

  • Possibility of low-voltage, bulk-capacity transmission

  • Reduction in cable site laying cost

  • Skipping construction projects of 154-kV substation and the possibility of changing to a switching station

  • Benefits in light of investment efficiency and environment.

Basic Characteristics of HTS Cable

The basic characteristics of HTS cable in terms of system operation should be analyzed. However, the exact technical parameters of the HTS cable development project in South Korea are difficult to determine.

Table 2 compares the electrical characteristics of existing conventional cable with those of HTS cable, assuming that the transmission capacity of HTS cable is several times larger than that of conventional cable on the same voltage level. Although the characteristic impedance of HTS cable is slightly smaller than that of conventional cable, the voltage drop of HTS cable is higher because the capacity of HTS cable is several times larger than that of conventional cable. Furthermore, if HTS cable is installed, it is expected to cause power flow redistribution among transmission lines. However, problems like voltage drop and power flow redistribution, which can occur when HTS cable is applied, can be covered by technical countermeasures and proper selection of the application site.

Probability of 22.9-kV HTS Cable Application

The 22.9-kV HTS cable can have various applications, and the CAST study considers utility and customer system applications. The application methodology for the customer system is to find a specific application site by considering the condition of an individual customer. Table 3 describes the application methodology of 22.9-kV cable for a customer system.

With a utility system, which differs from a customer system, it is necessary to find the general methodology that coincides with specific conditions. Table 4 describes several application methodologies for a 22.9-kV HTS cable.

Alternative to 154-kV Conventional Cable

The application methodology for replacing a 154-kV conventional cable (existing or planned) with a 22.9-kV HTS cable effectively changes the role of an existing 154-kV substation in a downtown metropolitan city to function as a 22.9-kV switching station, supplying power via a 22.9-kV HTS cable from a remote 154-kV substation. This application has the following advantages:

  • Changes the roles of the T&D system in a metropolitan city by replacing a 154-kV substation with a 22.9-kV switching station

  • Avoids the NIMBY movement

  • Reduces substation site and construction costs.

As it is practically impossible to change an entire 154-kV substation in a downtown area into a 22.9-kV switching station at once, a step-by-step strategy must be used. The following four steps outline a strategy that should be considered:

Step 1. Replace one 154-kV conventional cable between a 154-kV substation with a 22.9-kV HTS cable. Leave the existing 154-kV substation unchanged, replacing the 154-kV conventional cable that connects two substations with a 22.9-kV HTS cable. Do this by connecting the 22.9-kV busbars at each substation with a single 22.9-kV HTS that has an equivalent current rating (Fig. 2). An HTS cable capacity of around 200 MVA is similar to that of a 154-kV cable.

Step 2. Change the conventional 154-kV substation into a 22.9-kV switching station. To change a 154-kV substation in a downtown area that is connected to HTS cable from a remote 154-kV substation to a 22.9-kV switching station requires all the 154-kV cable circuits in this switching station to be changed to 22.9-kV HTS cable circuits (Fig. 3). In this case, the benefits are a huge reduction in site area and equipment costs in the down switching station.

Step 3. Replace a 154-kV substation located in the vicinity of a 22.9-kV switching station with a 22.9-kV switching station. A 154-kV substation connected to a 22.9-kV HTS cable from an adjacent 22.9-kV switching station can be changed to a 22.9-kV switching station simply by overlaying the existing 154-kV cable with a 22.9-kV HTS cable (Fig. 4). This design change would also produce financial benefits attributable to the reduction in substation site area and equipment costs.

Step 4. Replace a 22.9-kV conventional cable (existing or planned) with a 22.9-kV HTS cable in the downtown area. This scenario requires a sequential program of installing new 22.9-kV HTS cables (Fig. 5). Because a 22.9-kV HTS cable has a much higher current rating than a conventional 22.9-kV cable with the same cross-sectional area, economies may be possible because fewer 22.9-kV HTS cables may be required. It is reasonable to select the capacity of the 22.9-kV HTS cable at around 50 MVA, some five times higher than the 10-MVA conventional cable rating. Assuming the load density in the downtown area in 2020 will be 2 to 2.5 times above the current value, theoretically a single 22.9-kV HTS cable will be able to supply consumers in an area twice the size of a 22.9-kV conventional cable

Summary

As a result of successful collaboration between KERI and CAST, the research and development work on the design and potential merits of HTS cable are likely to lead to the use of this cable technology for future power systems in Korea. The benefits evaluated include:

  • HTS cable, which can transmit bulk-capacity power with low voltage and lower power loss, is as an epochal alternative plan to solve the problem of future power systems in metropolitan areas.

  • The comparison to 154-kV conventional cable shows that the use of 22.9-kV HTS cable to supply downtown areas has epochal benefits, attributable to reduced 154-kV substation and cable construction costs.

  • The future R&D program on 22.9-kV HTS cable seeks to develop a 200-MW, 22.9-kV HTS cable to replace a 154-kV conventional cable and 50-MW, 22.9-kV HTS cable to replace the 22.9-kV conventional cable currently used for distribution.

  • 22.9-kV HTS cable used to replace 154-kV conventional cables would solve several technical problems such as excessive voltage drop and power flow redistribution.

Finally, as the HTS technology has developed, it is now regarded as the most effective alternative to solve future power network constraints, especially in downtown areas of metropolitan cities such as Seoul.

Acknowledgement

This research was supported by a grant from the Center for Applied Superconductivity Technology of the 21st Century Frontier R&D Program, funded by the Ministry of Science and Technology, Republic of Korea.

Dr. Yoon Jae-young heads the Power System Research Group at the Korea Electrotechnology Research Institute (KERI). He received the BS, MS and PhD degrees in electrical engineering from PUSAN National University. Since 1987, he has been working in the research field of power system analysis, including custom power systems. Currently, he is managing a research project, the application of HTS-equipment such as cables, current limiting reactors and transformers, and is also playing a key role in the research project related to the Northeast Asia System Interconnection that includes North Korea. jyyoon@keri.re.kr

Jong-yul Kim received the BS and MS degrees in electrical engineering from PUSAN National University in 1997 and 1999, respectively. Since 2001, he has been working at KERI, where his research interests include power system analysis, using PSS/E and EMTDC especially for voltage-sourced converter applications. He is now a leader of research projects linked to the application of HTS devices and AI to practical power systems. jykim@keri.re.kr

Seung-Ryul Lee received the BS and MS degrees in electrical engineering from KOREA University in 1999 and 2001, respectively. He joined KERI's Power System Group in 2003. His research interests are in the areas of power system analysis and the application of HTS devices such as fault current limiters, cables and transformers. srlee@keri.re.kr

Table 1. Route Length and Annual Increase of 154-kV Underground Cable
Year 2000 2005 2010
Total 154-kV route length in circuit-km (circuit-mile) 16,747
(10,410)
20,475
(12,725)
22,453
(13,955)
Route length of 154-kV cable installed in circuit-km (circuit-mile) 1143
(710)
2207
(1372)
2599
(4182)
Percentage increase in circuit -length 6.8% 10.8% 11.6%
Table 2. Basic Technical Parameters (Assuming the same voltage level but several times larger capacity than conventional cable)
Items HTS Cable Conventional Cable Note
System constants R ≈ 0.00 (Superconducting) 100%
≈ RQ (Quenching) 100%
L 75% or so 100%
C 50% or so 100%
Z = √L/C 82% or so 100%
SIL = V4/Z 122% or so 100%
Power loss Nearly zero 2% to 3% (or so)
Voltage drop Negative Positive Opposite result with same capacity
Voltage variation Light load: negative
Heavy load: positive
Light load: negative
Heavy load: positive
Conclusion refer to SIL value
Fault current Positive Negative Opposite result in case of substituting 22.9-kV HTSC for existing 154 kV
Stability Positive Negative No big difference
Power flow redistribution Necessary to exhaustive study for specific application cases
Table 3. Application Methodology of 22-kV Cable for Customer System
Methodology Reason for Application Note
Specific place Being inevitable reason for HTS cable Specific case
More than 10-MW 22.9-kV customer No necessity to use 154-kV cable when 22.9-kV HTS cable is applied Specific case
Table 4. Application Methodology of 22.9-kV HTS Cable for Utility System
Methodology Reason for Application Note
Newly established power plant IPB Low voltage, bulk capacity Specific site
Distributed generation plant on the seashore Low voltage, bulk capacity Specific site
Composite thermal plant nearby Seoul Low voltage, bulk capacity Specific site
Substitution 22.9-kV HTS cable for existing and planned 22.9-kV cable • Replacement of existing retired cable
• Increasing capacity of planned cable
Suitable type for applying
Substitution 22.9-kV HTS cable for existing and planned 154-kV cable • Replacement of existing retired cable
• Omission of 154-kV substation in downtown area
Suitable type for applying