koojy@hanyang.ac.kr

Development of the HVDC ±80kV XLPE Cable System and Construction of Test-bed in KOREA

T. H. Lee*, J.W. Choi, E.H. Jung, J.H. Nam, J.N. Kim, I.H. Lee, S.I. Jeon,

D.H. Kim**, H.S. Park and J.W. Kang LS Cable & System*, KEPCO**

KOREA

SUMMARY This paper describes the development of the HVDC ±80kV XLPE cable system capable of 60 MW allowable for polarity reversal operation, and construction of test-bed for DC XLPE cable system in Jeju Island in Korea, together with the results of type test. Polyethylene (PE) has excellent dielectric properties, thus cables insulated with cross-linked polyethylene (XLPE) have been applied to AC cables. However DC power cables with extruded insulation have not been adopted yet. The insulating performance of extruded insulating material used for AC power cable is not suitable for DC operation due to the phenomena of space charge accumulation in the material. Thus many researches have been conducted to find an effective solution, which decrease a space charge accumulation in the materials, such as adding a type of conductive inorganic filler or polarized inorganic filler to cross-linked polyethylene (XLPE). We developed nano-composited compound for DC XLPE cable insulation by adding nano sized inorganic filler. The developed insulation materials show that a space charge accumulation is decreased dramatically. In order to evaluate an electrical performance at cable state, we manufactured a mini-model cable which is 4mm insulation thickness and 400 ㎟ conductor, and we found DC and impulse breakdown characteristics through statistical data processing. KEPCO, LS Cable & System, LS Industrial Systems and some companies have been developing HVDC technology which is system design technology, operation technology and products such as converter system, overhead line, underground cable system, and so on. To verify long term reliability and stability of those system, test-bed will be constructed in Jeju Island until the end of March 2012, and it will be started field test. DC ±80kV XLPE cable shall be applied for this project. The application of XLPE cable for DC transmission is the first project in Korea. We determined the nominal insulation thickness of cable as 12 mm to endure BIL (Basic Insulation Level) which is determined 325kV. Jeju test-bed transmission line is complex hybrid lines which are consisted of about 4.8km long overhead line and 500m long underground cable. DC overhead line will be installed in one pylons carrying AC 154kV circuit. To determine BIL, transmission lines including overhead line and underground cable

21, rue d’Artois, F-75008 PARIS B1-304 CIGRE 2012 http : //www.cigre.org

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are simulated by EMTP (Electro Magnetic Transient Program), and BIL is determined by simulation result added margins. In order to verify the mechanical and electrical performances of DC XLPE cable system, we carried out the type test according to CIGRE recommendations (Electra TB No. 219 : Recommendations for testing DC extruded cable systems for power transmission at a rated voltage up to 250 kV[1]). The specimen for electrical tests was consisted of 50m long DC XLPE cable and terminations. This specimen was subject to mechanical preconditioning bending test according to IEC 60840 before starting the electrical test. The electrical tests consist of load cycle test, polarity reversal test, superimposed switching impulse test, superimposed lightning impulse test and DC voltage test. These test items were executed sequentially on the same specimen. By the result of type test, it was confirmed that the developed cable system has superior mechanical and electrical performances for HVDC transmission lines. Moreover, to evaluate long term reliability and stability, the cable systems will be subjected to the field test KEYWORDS HVDC – XLPE Cable – Type test

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1. INTRODUCTION HVDC transmission systems are used in many projects all over the world. In particular, the demand for long-distance HVDC power transmission lines is increasing to interconnect power grids, such as the interconnection between main land and island and windfarm grid connection. With the increasing demands of the DC cables, the need for the development of new insulation materials has been strengthened. Numerous research efforts are being made in Europe and Japan in order to develop the insulation materials for HVDC cable to improve transmission capacity. The HVDC material technology is generally divided into two based on the converter station and cable system. The research on this area has been carried out with the resin manufacturers in Europe and with cable makers in Japan. The technology for the XLPE insulation material is mostly focused on obtaining the long term stability and optimizing the transmission efficiency by reducing space charges which are accumulated during DC transmission. In Korea, we have been carrying out a government-funded research to apply nano-technology and nano-materials to XLPE materials in collaboration with materials specialists and cable specialists in academia and industry. Through this consortium, the synthesis of nano-materials, surface treatment of nanoparticles, compounding of nanocomposite for XLPE materials have been researched. As the results of this extensive research effort, the required properties of XLPE nano-composites applicable to high voltage DC have been obtained. KEPCO, LS Cable & System, LS Industrial Systems and some companies have been developing HVDC technology which is system design technology, operation technology and products such as converter system, overhead transmission line, underground cable system, and so on. To verify long term reliability and stability of those systems, test-bed will be constructed in Jeju Island. HVDC

±80kV XLPE cable system shall be applied for this project. 2. JEJU HVDC TEST-BED PROJECT OVERVIEW The Jeju HVDC test-bed is HVDC ±80kV transmission system, which is to transmit 60MW power from the Han-Lim converter station to the Gum-ak converter station through an approximately 4.8 km long overhead line and an approximately 0.5km long DC XLPE cable. It will be constructed until the end of March 2012. The system configuration of Jeju HVDC test-bed is shown in Figure 1.

Figure 1. HVDC Cable System Diagram

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DC overhead transmission line will be installed in one pylons carrying AC 154kV circuit. To determine BIL(Basic Insulation Level), transmission lines including overhead transmission line and underground cable are simulated by EMTP (Electro Magnetic Transient Program), and BIL is determined by simulation result added margins. The underground cable system consists of 2 pole cables, 1 return cables, and 1 optical communication cables with 48 cores. In this project, HVDC ±80kV HVDC XLPE cables and 20kV XLPE cables are used as the pole and return cable, respectively.

3. DEVELOPMENT OF THE HVDC ± 80 KV XLPE CABLE 3.1 Electrical properties of nano XLPE Nano-particles used in this study were manufactured by the wet process. The surface of nano-particles was modified by various surface treatment materials in order to have a compatibility with polyethylene. Many researchers have studied how to improve the dispersion of nano-particles in nano-composites and develop an effective analytical tool for dispersion. We have also struggled to solve these problems in this research, so we used various analytical tools to evaluate the distribution of nano-composite for XLPE insulation. Since the dispersion of nano-particles in nano-composites is critical for the performance and properties of materials, it was very important to ensure the reasonable dispersion by analytical methods. The dispersion of the treated nano-particles was determined by SEM (Scanning Electron Microscopy) and TEM (Transmission Electron Microscopy) in this study. Figure 2 and 3 show SEM image and TEM images of nano-particles dispersed in XLPE. The average particle size was around 200 nm as can be seen in these two images.

Figure 2. SEM image of nano-XLPE composite Figure 3. TEM image of nano-XLPE Composite

Figure 4 shows the volume resistivity as a function of applied electric field for two insulation materials at different measuring temperatures: (a) general AC XLPE (XLPE-A), and (b) developed nano-DC XLPE (XLPE-C). It can be clearly seen that, in case of general AC-XLPE, increasing applied electric field results in dramatically decreased volume resistivity, especially at higher temperature (above 25ºC). In the case of nano-DC XLPE, on the other hand, gradual decrease of volume resistivity was obtained as the applied electric field was increased even at high operating temperature (90ºC).

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(a) AC XLPE (b) Nano DC XLPE

Figure 4. Volume Resistivity Characteristics of AC and DC XLPE compounds

To evaluate the charge accumulation behavior in the insulation materials under high DC electric field, Pulse Electro Acoustic (PEA) measurement has been carried out and the result was presented in Figure 5. As illustrated in the figure, after charging for 1 hour under positive electric field of 150kV/mm, hetero-charges were formed at the counter electrodes in the case of normal AC XLPE, whereas only small amount of homo-charges were generated in DC XLPE nano-composites. Therefore, the performance of nano-DC XLPE composite in suppressing space charge generation is expected to be beneficial in DC XLPE cables.

(a) Normal AC XLPE (b) DC nano-composite

Figure 5. Space charge profiles of AC and DC XLPE compounds

3.2 Design of Cable

Figure 6. and Table 1 show the structure and dimension of the HVDC ±80 kV 325 ㎟ XLPE cable. As an insulation material, we used nano-composite XLPE compound.

Figure 6. Construction of HVDC ±80kV XLPE Cable

Conductor

Inner semi-conductive layer

Insulation (XLPE)

Outer semi-conductive layer

Metallic Sheath

PE Sheath

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Table 1. Dimensions of HVDC ±80kV XLPE cable (325 [㎟])

Item Dimension

Thickness [㎜] Diameter [㎜]

Conductor - 21.7

Inner semi-conductive layer 1.2 25.98

Insulation 12.0 49.98

Outer semi-conductive layer 1.0 51.98

Metallic Sheath 2.3 58.38

Polyethylene sheath 3.5 65.78

Table 2. Electrical stress of HVDC ±80kV XLPE cable

Operating condition

Current = 0 A Current = 800 A Conductor

E-field

[㎸/㎜]

Sheath E-field

[㎸/㎜]

Conductor E-field

[㎸/㎜]

Sheath E-field

[㎸/㎜]

Rated Voltage (U0=80 ㎸) 7.83 5.82 7.07 6.35

Superimposed lightning impulse

(DC 80kV +L.I 325kV)

D.C voltage of same polarity 36.66 20.80 35.90 21.34

D.C voltage of opposite polarity 39.82 18.95 40.58 18.42

Superimposed switching impulse

(DC 80kV +S.I 114kV)

D.C voltage of same polarity 15.36 9.73 14.60 10.27

D.C voltage of opposite polarity 18.52 7.88 19.28 7.35

We determined the insulation thickness as 12 mm to ensure that the working stress at high temperature is lower than dcE =20 kV/mm and impdcE =50 kV/mm, and validated by calculating electrical stress as

shown in Table 2. The electrical stress under rated DC voltage was calculated as follows [2]:

])/(1[

)/(

00

10

,

RRR

RrVE

idcr

(1)

1

2

iO

iO

C

RRV

RRVTW

(2)

Where,

dcrE , : Stress at radius r [kV/mm]

V : Working voltage [kV]

iR : Conductor screen radius [m]

oR : Insulation screen radius [m]

cW : Conductor loss [W/m]

T : Thermal resistivity of insulation [K.m/W]

: Temperature coefficient of volume resistivity [/℃](=0.12)

: Stress coefficient of volume resistivity [/kV/mm] (=0.18)

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In case of superimposed impulse, electrical stress was calculated by sum of DC stress and impulse stress as follows:

i

Oimpr

R

Rr

VE

ln, (3)

imprdcrimpdc EEE ,, (4)

4. TYPE TEST We prepared HVDC ±80kV XLPE cable 50m long as the specimen and conducted the mechanical pre-conditioning and electrical tests sequentially. The test items and conditions for HVDC ±80kV XLPE cable are shown in Table 4., as are specified in CIGRE recommendations (Electra TB. No. 219). Before the electrical tests, cable was subjected to bending test.

Figure 5. Set up for electrical test

Table 4. Test items, conditions and the results

Test item Test condition Result

Mechanical Pre-conditioning

Bending Test Diameter : 2.3m Good

Electrical Test

Loading Cycle test

-148kV : 8 Cycles +148kV : 10 Cycles 1 Cycle : Heating 8 hrs/ Cooling 16 hrs

No B.D

Polarity reversal test

±116kV : 10 Cycles Polarity reversal : every 8 hr (Within 2 min.) 1 Cycle : Heating 8 hrs/ Cooling 16 hrs

No B.D

Superimposed impulse test

Lightning impulse : DC (-/+) 80kV +L.I(+/-)325kV 10 times for each polarity

Switching impulse : DC (-/+) 80kV +S.I(+/-)114kV 10 times for each polarity

No B.D

Subsequent DC Test

-148kV / 2hrs No B.D

During all of the electrical, the temperature of the conductor was controlled to be more than

343K(70℃) by applying AC current to the conductor. The cable system specimen has passed the type test successfully, no breakdown occurred during load cycle test, polarity reversal test and superimposed impulse test.

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5. CONCLUSION

The HVDC ±80kV XLPE cable with 325 ㎟ conductor and termination were developed. The excellent performance of the cable system was confirmed by the mechanical and electrical type test according to CIGRE recommendations. And we performed the dissection of specimen after electrical test. As the result of visual inspection, we could not found any damage by mechanical and electrical stresses. From the result of type test, it was confirmed that the developed cable system has excellent performances for HVDC transmission lines. To evaluate long term reliability and stability more severely, the cable system will be installed test-bed in Jeju Island. BIBLIOGRAPHY [1] Working Group SC 21.01 CIGRE. “Recommendations for Testing DC Extruded Cable Systems

for Power Transmission at a Rated Voltage upto 250kV”, (Electra number 219, Feb. 2003 [2] G. F. Moore, Electric Cables Handbook 3rd Ed., BICC Cable