etep628

7/29/2019 etep628

1/11

Overvoltages and insulation coordination of 1000-kV ACtransmission systems in China

Gu Dingxie*,, Zhou Peihong, Dai Min, Xiu Muhong and He Huiwen

High Voltage Research Division, State Grid Electric Power Research Institute of China, Wuhan, China

SUMMARY

Overvoltages and insulation coordination requirements of 1000-kV AC transmission systems in China areintroduced. On the basis of the single-circuit transmission project, which has been put into operation since2009, and the planned double-circuit transmission projects, the control of temporary overvoltage andswitching overvoltages and the determination of the rated voltage of metal oxide arrester are presented. The

lightning protection measures for transmission lines and substations are given. Determination of insulationlevel of substation apparatus and clearance of external insulation for transmission lines and substations areprovided. Copyright# 2011 John Wiley & Sons, Ltd.

key words: China; 1000-kV transmission system; overvoltage; insulation coordination

1. 1000-kV AC TRANSMISSION SYSTEMS IN CHINA

The schematic diagrams of two typical 1000-kV transmission systems are presented in Figures 1 and 2,

respectively. The 1000-kV AC demonstration project in China shown in Figure 1, JindongnanNanyang

Jingmen, has been put into operation since January 2009. It is a single-circuit transmission system. The

planned HuainanShanghai 1000-kV double-circuit transmission system is shown in Figure 2.

2. POWER FREQUENCY TEMPORARY OVERVOLTAGES AND METAL OXIDE ARRESTER

PARAMETERS

2.1. Amplitude and duration of temporary overvoltage

For the single-circuit transmission lines, the elevated voltages as a result of load rejection with no fault

and with single line-to-ground fault should be mainly taken into consideration during temporary

overvoltage (TOV) calculation.

For the double-circuit transmission lines, the situation with one of the circuits out of service also

should be considered, besides the situation with two operation circuits. In addition, it is worth noting

that six-phase load rejection should be considered [1,2].

Most of the future 1000-kV lines in China will be relatively long, and generally, shunt reactors willbe installed for such transmission lines. It is the major measure to control the TOVs. The highest TOV

may usually be caused by single-phase-to-ground fault with load rejection. The following two kinds of

circumstances will lead to the situation described previously with the single-phase reclosing, which is

adopted in China.

(1) The three-phase circuit breakers (CBs) are opened by relay protection because of the unsuccessful

single-phase reclosing.

*Correspondence to: Gu Dingxie, High Voltage Research Division, State Grid Electric Power Research Institute,

Wuhan, China.E-mail: [email protected]

Copyright# 2011 John Wiley & Sons, Ltd.p

EUROPEAN TRANSACTIONS ON ELECTRICAL POWEREuro. Trans. Electr. Power 2012; 22:8393Published online 6 September 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/etep.628

7/29/2019 etep628

2/11

(2) In live working process, single-phase reclosing is usually required to be locked. The three-

phase CBs will be opened when single-phase-to-ground fault occurs.The power frequency phase-to-ground TOVs are limited to 1.3 per unit at the bus-side terminal of

the CB and 1.4 per unit at the line-side terminal in China.

The TOV duration plays an important role in the determination of the metal oxide arrester (MOA)-

rated voltage and the equipment insulation level. In China, a linkage method for opening CBs at the

sending and the receiving end is adopted, which means that CBs at both ends will be opened

synchronously if one of the CBs is opened for any reasons. This method is efficient to shorten the

duration of TOV and reduce the energy absorbed by MOA. The maximum switching time difference

between CBs at both ends can generally be controlled within 0.2s, and the TOV duration will not be over

0.5s even if the CB at one of the ends failed to be opened and the backup protection is carried out [1,2].

2.2. Metal oxide arrester parameters

The traditional principle for determining Un of MOA is that Un should be not less than TOV, where

Un is the rated voltage. However, because of good performance of MOA for withstanding overvoltage

in short time, the 1000-kV projects of China do not follow the principle, and Un is allowed to be lower

than TOV for a short time. Un of MOA for 1000-kV system in China has been selected as 828kV,

which is equivalent to 1.3 per unit and less than 1.4 per unit. According to the test data from MOA

manufacturers in China, MOA is able to withstand 1.4 per unit TOV with the duration up to 10s [2].

Main electrical parameters for MOA (Un is equal to 828kV) installed at the bus-side and line-side

terminals of CB or at the transformer side are listed in Table I.

The energy absorption capability is 40MJ [3]. The calculation results show that the actual maximum

absorption energy of MOA will be less than 10MJ afterit withstands TOV, which lasts 0.5s, and absorbs

2 +600MW

2 +600MW

2 +600MW

30km

12km

3km

Bengbu

2+720MVA

2+210Mvar

1+210Mvar 1+210Mvar

1+210Mvar

2+720MVA

2+720MVA

Tianji

Pingwei

500kV

UHV-T1

Huainan

720Mvar

720Mvar 720Mvar

720Mvar

Double circuit line322km 146km

Double circuit line

720Mvar720Mvar

Wannan Zhebei Huxi

Double circuit line164km

720Mvar 720Mvar

UHV-T2 UHV-T3

500kV

UHV-T4

500kV

500kV

Figure 2. Schematic diagram of the planned 1000-kV AC double-circuit transmission system in China.

JingdongnanPower Plant

North China

500kV Power Grid

358km 283km

Jindongnan Nanyang Jingmen

500kV Power Grid

Central China

Figure 1. Schematic diagram of the 1000-kV AC demonstration project (single circuit) in China.

G. DINGXIE ET AL.84

Copyright# 2011 John Wiley & Sons, Ltd. Euro. Trans. Electr. Power2012; 22:8393

DOI: 10.1002/etep

7/29/2019 etep628

3/11

injected energy produced by twice closing switching overvoltages (SOVs) before TOV. Therefore, there

is still sufficient margin of MOA absorption capability with 828-kV Un. At the same time, the reduction

of MOA Un will be beneficial to lowering the lightning and switching impulse protection levels as well

as the requirements of the equipment insulation level. The overvoltages on the lines can also be limited to

a certain extent.

3. SWITCHING OVERVOLTAGE

The important sources of SOVs are associated with the following events [4,5]:

(1) Line energization and re-energization(2) Ground-fault occurrence

(3) Ground-fault clearing

(4) Transformer switching at no load

3.1. Line energization and re-energization

Switching overvoltages because of line energization and re-energization are control factors for the

insulation design of 1000-kV lines in China. The main measure to control the overvoltages is the use

of CBs with pre-insertion resistors. In China, the 600- pre-insertion resistors are used, and the pre-

insertion time is 9.51.5ms.The maximum phase-to-ground statistical SOVs along a 1000-kV line shall be not more than 1.7 per

unit in China. The maximum phase-to-ground statistical SOVs in a 1000-kV substation shall be not more

than 1.6 per unit and the maximum phase-to-phase statistical SOVs shall be not more than 2.9 per unit [6].The times-to-crests of SOVs as a result line energization and re-energization are generally 1000~

3000ms, which may greatly influence the clearance design of transmission line [6].

3.2. Ground-fault occurrence

For single-circuit 1000-kV transmission lines in China, only the SOVs as a result of single-line-to-

ground fault occurrence are taken into account. The double-line-to-ground fault event can be ignored.

For double-circuit 1000-kV transmission lines, it is sufficient to consider only the SOVs as a result of

single-line-to-ground fault occurrence on one of the circuits. The event of single-line-to-ground faults on

both circuits can be ignored, or the SOVs on health phases are allowed to exceed 1.7 per unit.

However, the probability of simultaneous single-line-to-ground faults on both circuits is close to 0

according to the calculation results of lightning protection for 1000-kV transmission lines and theoperating experience of 500-kV double-circuit lines for over 10years. Thus, this kind of events can be

neglected during insulation coordination [7].

Switching overvoltages as a result single-line-to-ground fault occurrence are low, with the

maximum 2% overvoltage below 1.51 per unit [6]. It is not the control factor that determines the line

insulation level.

3.3. Ground-fault clearing

Switching overvoltages on a transmission line can be caused by ground-fault clearing on the adjacent

lines. The number of grounded phases may significantly influence the crest of such overvoltage. The

Table I. Main electrical parameters of the 1000-kV substation metal oxide arrester (unit, kV).

Systemvoltage

Ratedvoltage Un

(RMS)

Continuousoperation voltage

(RMS)

30/60s and 2kAswitching impulse residual

voltage

8/20s and 20kAlightning impulse residual

voltage

1000 828 638 1460 1620

RMS, root mean square.

1000-KV AC TRANSMISSION SYSTEMS IN CHINA 85


DOI: 10.1002/etep

7/29/2019 etep628

4/11

SOVs can be within the allowable range while clearing a single-phase ground fault and may exceed

the allowable values while clearing a two-phase or three-phase ground fault. Opening resistor can be

used to control this kind of SOVs.

For JindongnanNanyangJingmen 1000-kV transmission line in China, the following points

should be taken into consideration:

(1) The installation of opening resistors will increase the cost and CB fault probability because of

the mechanical complexities of the breaker mechanisms.

(2) The probability of two-phase or three-phase ground faults on 1000-kV transmission lines is very low.

(3) The maximum overvoltage occurs on transmission lines rather than in substations. Probably,

the SOVs may result in line insulation flashover. However, it will not threaten the equipment in

substations.

So, it is not necessary to install opening resistors for JindongnanNanyangJingmen 1000-kV

transmission lines.

3.4. Transformer switching at no load

Both calculation research and field tests made in China indicate that the pre-insertion resistors are not

necessary for the 500-kV CBs during transformer switching at no load at the 500-kV side. In such

case, no excessively high resonance overvoltage will be caused, and both amplitudes of inrush currentand overvoltage are within the allowable range.

The possibility of relatively high resonance overvoltage and the inrush current caused by transformer

switching at no load at the 1000-kV side is greater than that at the 500-kV side. The adoption of pre-

insertion resistors is beneficial for reducing resonance overvoltage and inrush current. However, it is not

effective for all system wiring configurations and may lead to higher cost and lower reliability. It is

recommended to install the pre-insertion resistors in CBs, which are used for energizing both 1000-kV

unloaded transformer and 1000-kV unloaded transmission line. And for the CBs, which are used only for

energizing 1000-kV unloaded transformer, the pre-insertion resistors are not necessary.

To effectively restrict the harmonic resonance overvoltage and make it possible to use CB with no pre-

insertion resistors at B13 for switching 1000-kV transformer, the following measures can be adopted:

(1) Energizing the transformer at the 500-kV side.(2) If measure 1 cannot be achieved, switch the unloaded transformer by line CB B12 with pre-

insertion resistors, which are in the middle position of the CB branch, as shown in Figure 3 and

pointed by the arrow.

4. VERY FAST TRANSIENT OVERVOLTAGE

The operation of disconnect switch (DS) within a gas-insulated-switchgear (GIS) system generates

very fast transient overvoltage (VFTO) with very steep wave front and very high amplitude. Such

VFTO will probably damage insulation of the following equipment:

1000kV 500kV

B11

B12

B12

B51

B52

B53

Figure 3. Schematic diagram of the transformer switching at no load.

G. DINGXIE ET AL.86


DOI: 10.1002/etep

7/29/2019 etep628

5/11

(1) GIS bodies;

(2) Equipment with windings, such as transformers;

(3) Secondary equipment.

Very fast transient overvoltage should be more worthy of attention for 1000-kV GIS. Generally,

the higher the system-rated voltage is, the lower the ratio of the lightning impulse withstand

voltage (LIWV) of equipment and the system-rated voltage is. For example, the rated voltage of

1000-kV GIS and the crest VFTO are two times higher than that of 500-kV GIS; however, the

LIWV of the 1000-kV GIS equipment is only 1.55 times higher than that of 500-kV GIS

equipment. Therefore, VFTO may be more harmful to 1000-kV GIS equipment than to 500-kV

GIS equipment.

The calculation research on VFTO has been carried out for 1000-kV GIS. The results show the

following [8,9]:

(1) Shunt resistors are necessary for DSs in GIS to effectively limit VFTO. The resistance is 500, and

the crest VFTO can be limited to 1.13 per unit from 3.11 per unit (1 per unit is equal to 898kV).

(2) The crest VFTO caused by the DS operation can be up to 2.15 per unit in the hybrid GIS, which

is within allowable range. Therefore, shunt resistors are not necessary for the DSs of the hybrid

GIS from a safe and economic point of view.

(3) The wiring configurations of GIS, such as bus length, will affect the crest VFTO, and both the

initial wiring configuration and the future wiring configuration should be considered.

Because the transformers of GIS in China are usually connected via overhead lines with a distance

from several 10 to 100m, the amplitude of VFTO will attenuate fast and the wave front will be less

steep. The crest VFTO at the transformers is low (the calculated crest value is 925kV) with slow rising

speed (the calculated time-to-crest is longer than 1.5ms). There should be no any harm to the

transformer insulation.

5. INCOMING LIGHTNING OVERVOLTAGE TO SUBSTATION AND THE ARRESTERS

LAYOUT

The incoming lightning overvoltage to 1000-kV substation is the control factor for the equipmentinsulation design.

While calculating the incoming lightning overvoltages to 1000-kV substations of China, two wiring

configurations with rigorous conditions should be taken into consideration (as shown in Figure 4).

(1) Single line with CBs opened

(2) Single line single transformer wiring configuration

The maximum incoming lightning overvoltage to substations is caused by the shielding failure in

the entrance section of the transmission line. Two kinds of measures for limiting incoming lightning

overvoltage have been adopted in China.

(1) Limiting the maximum lightning shielding failure current in the entrance section of the

transmission line

Ground wire protection angle less than 4 and three ground wires (as shown in Figure 5) in theentrance section of the transmission line are effective methods to limit the current.

(2) Optimizing the layout of MOAs

The layout with less MOAs has been adopted. One group of three-phase MOA is installed at the

entrance of each circuit. One group is installed for each busbar. One group is installed beside the

transformer. The amplitudes of overvoltages in different substations are different. The calculated typical

maximum lightning overvoltages are 2040kV for GIS, 1854kV for shunt reactors, and 1796kV for

transformers. The LIWVs of the transformers and the shunt reactors are 2250kV and that of other

equipment are 2400kV in China. The allowed LIWVs of equipment are higher than the maximum



DOI: 10.1002/etep

7/29/2019 etep628

6/11

incoming lightning overvoltage and can meet the requirements of safety margin of internal (15%) andexternal (5%) insulation. Because of the small probability of single-line wiring configuration, the safety

margin of internal insulation can be reduced to 10% for this kind of wiring configuration.

Probabilistic or statistical method is also used to calculate the incoming lightning overvoltage to

substation in addition to deterministic method. The mean time between failures of substation as a

result of lightning is required to be more than 1500years while using statistical method.

6. LIGHTNING PROTECTION OF LINES

Because of the importance and the higher insulation level of the 1000-kV transmission lines, their

expected lightning trip-out rate should be lower than that of the 500-kV lines by adopting control

measures. According to the operating experience of 500-kV transmission lines in China, the average

lightning trip-out rate is 0.14 times per 100km/year. So, the expected lightning trip-out rate of 1000-kVtransmission lines can be set as 0.1 times per 100km/year, which is equal to 70% of the trip-out rate of

500-kV transmission lines. The schematic diagrams of towers of 1000-kV single-circuit and double-

circuit transmission lines are shown in Figure 6.

MOA

Single transmission line

Bushing

CB whichis openedis openedCB which

Transformer

Incoming lightningwave u(t) wave u(t)

Incoming lightning

Transformer

CB whichis closed

is closedCB which

Bushing

Single transmission line

MOA

MOAMOA

Figure 4. Substation wiring configurations considered during the incoming lightning overvoltagecalculation, (a) single line with CBs opened, (b) single line single transformer wiring configuration.

Figure 5. Three ground wires adopted in the entrance section of the transmission line.

G. DINGXIE ET AL.88


DOI: 10.1002/etep

7/29/2019 etep628

7/11

Operating experience shows that the proportion of the lightning backflashover rate is becoming less

and less to the total lightning trip-out rate as the line insulation level improved. The proportion is less

than 10% for 500-kV transmission lines in China. The calculation results show that the lightning back

flashover at 1000-kV line will not occur. Lightning shielding failure is the main cause of lightning trip

out. Therefore, to prevent lightning shielding failure is the key point of lightning protection of 1000-kV

lines.

The main method for calculating the shielding failure rate is the improved electrical geometric

model (EGM). The influence of terrain along the transmission line, correction coefficient of the

lightning striking distance to the earth, and the probability distribution of the intruding lightning

angle of lightning leader have been considered to improve the EGM model. At the same time, the

study on the leader propagation model has been carried out in China. However, the calculation

results are quite different with different parameters and criterions, and the determination of these

parameters and criterions is difficult so far. Therefore, the calculation results from EGM are the main

design basis for the lightning protection, and the results from leader propagation model are taken as

reference.

Reducing the ground wire protection angle a can reduce the shielding failure trip-out rate, and the

trip-out rate is affected by terrain along the lines significantly too. The following principles for ground

wire protection angle a of 1000kV lines should be used.

(1) For single-circuit transmission lines, a used in plain area is less than 6 and less than 4 in themountain area.

(2) For double-circuit transmission lines, a used in plain area is less than 3 and less than 5 inthe mountain area.

(3) For jumper line at the strained angled towers, a of single-circuit transmission lines in plain areais less than 6 and not more than 0 for the single-circuit or the double-circuit transmission linesin the mountain area.

The protection angle mentioned previously refers to the angle between the connecting line from

the ground wire to the outermost sub-conductor and the perpendicular line of the horizontal

surface.

With long air gap clearances, higher insulation level and no arcing horn installed on China 1000-kV

transmission lines, it is possible to limit the lightning trip-out rate to the expected value.

Because the 1000-kV AC demonstration project (JindongnanNanyangJingmen) in China was put

into operation in January 2009, no lightning trip-out fault has occurred until now.

a)Typical towers of single-circuittransmission lines

(b)Typical tower of double-circuittransmission lines

Figure 6. Schematic diagrams of towers of 1000kV single-circuitand double-circuit transmissionlines (unit, m),(a) typical towers of single-circuit transmission lines, (b) typical tower of double-circuit transmission lines.



DOI: 10.1002/etep

7/29/2019 etep628

8/11

7. INSULATION COORDINATION

7.1. Principles of insulation coordination

In the insulation coordination process, safety margin of internal insulation (15%) and external

insulation (5%) are required in China [2,10].

To determine the air gap clearance of 1000-kV AC lines and substations, lightning and switching

surge impulse and power frequency voltage testing using a full-scale model are conducted, and a serial

of critical 50% flashover voltage curves was obtained, which is important for insulation coordination

process [2].

7.2. Insulation levels of substation equipment

The insulation levels of the major equipment in 1000-kV substations of China are shown in Table II.

In general, the equipment insulation levels of China are lower than those of Russia and higher than

those of Japan [11]. They are determined by the overvoltage levels to which 1000-kV transmission

systems are exposed and the equipment manufacturing experience of China.

7.2.1. Transformer insulation level. The transformer insulation levels are shown in Table III.

The insulation level of 1000-kV transformer is determined by its LIWV and the power frequencyvoltage withstand. LIWV 2250kV of China can meet the requirement of safety margin, for the

lightning overvoltage incoming to the transformer terminal will be not so high with low lightning

impulse protection level of MOA and reduced maximum shielding failure lightning current.

Operating experience indicates that most transformer faults occurred under the normal operating

voltage. The power frequency voltage withstand testing is conducted to check the existence of partial

discharges in transformer, and it is strictly considered in China. In China, the testing power frequency

voltage is 1100kV, and the duration of its application is 5 min, which is more strict than 1 min

recommended by the International Electrotechnical Commission [12].

7.2.2. Testing voltage on longitudinal insulation of CB and DS. The lightning impulse testing voltage

on longitudinal insulation of 1000-kV CBs and DSs of China is 2250+900 (kV), in which 900kV is

the amplitude of the power frequency component of opposite polarity.However, the amplitude of the power frequency component of opposite polarity recommended by

the IEC 60071-1 is 0:7 Um ffiffiffi

2p

=ffiffiffi

3p

, which refers to the peak value of the operating voltage

multiplied with the coefficient 0.7, and with this coefficient, about one-fourth cycle of the operating

voltage cannot be covered and the guaranteed coverage probability is 0.75.

Table II. Rated insulation withstand voltages for 1000-kV equipment (unit, kV).

Equipment Lightning impulsewithstand voltage

Switching impulsewithstand voltage

Power-frequency short-duration withstand voltage

Transformer and reactor 2250 (choppedlightning impulse:

2400)

1800 1100 (5min)

GIS (CB, DS, pipe) 2400 1800 1100 (1min)Post insulator and DS

(open type)2550 1800 1100 (1min)

Capacitive voltagetransformers

2400 1800 1300 (5min)

Bushing of transformerand reactor

2400 (choppedlightning impulse:

2760)

1950 1200 (5min)

Bushing of GIS 2400 1800 1100 (1min)Longitudinal insulation

of switching device2400+900 1675+900 1100+635 (1min)

GIS, gas-insulated-switchgear; DS, disconnect switch.

G. DINGXIE ET AL.90


DOI: 10.1002/etep

7/29/2019 etep628

9/11

In China, the coefficient is specified as 0.7 ~1.0 in the National Standard Insulation Coordination

Part 2: Application Guide for Insulation Coordination for High Voltage Transmission and Distribution

Equipment (GB311.1-1997), and the coefficient is set as 1 for 1000-kV CBs and DSs. For the

importance of the 1000-kV equipment, the coefficient is set as 1 and the amplitude of the power

frequency component of opposite polarity is the peak value of the operating voltage.

7.3. Minimum air clearances of substation

The minimum air clearances of 1000-kV substations of China with altitude above sea level below

1000m are listed in Table IV. In Table IV, A1 is the minimum clearance between conductor and

framework, A100 is the minimum clearance between equipment and the framework, and A2 is the

minimum clearance between two phases.

7.4. Line insulation levels

To determine the minimum number of insulators, contamination withstand voltage method and

specific creepage distance method can be used. The insulator selection of 1000-kV AC demonstration

project in China is regulated as follows:

(1) Suspension string. For area with pollution level II medium, where equivalent salt deposit

density (ESDD) is 0.06~ 0.10mg/cm2, 54 units of double-shed disk-type insulators (for 300 kN)

are required. For areas with pollution level III heavy (ESDD=0.10~0.25mg/cm2) and IV very

heavy (ESDD=0.25~0.35mg/cm2), the composite insulator with total length of 9750mm and

creepage distance of 30300mm is required [10].

(2) Tension string. For area with pollution levels II, III, and IV, 44, 54, and 60 units of disk-typeinsulators with creepage distance of 700mm (for 550kN) is required, respectively.

The minimum air clearances for 1000-kV transmission lines in China are listed in Table V.

For single-circuit transmission lines, the minimum conductor-to-tower clearances of the middle

phase are determined to withstand the expected maximum switching overvoltages, and the clearances

of the two outer phases are determined to withstand the operating voltage at high wind speed. So, the

minimum air gap clearances for lightning impulse withstand have not been regulated.

However, towers of double-circuit transmission lines are higher. They are more likely to be struck

by lightning because of the larger lightning attraction area and weakened shielding effect of earth.

Table III. Insulation levels of ultra-high voltage transformers of China and Japan (unit, kV).

Country Lightning impulse withstandvoltage

Switching impulse withstandvoltage

Power frequency voltagewithstand

China 2250 1800 1100(5min)Japan 1950 1425 1100(5min)

Table IV. Minimum air clearance for the 1000-kV substation with altitude above sea level below 1000 m

(unit, m).

Type of voltage applied A1 A2

A1 A100

Power frequency 4.2 6.8Switching impulse 6.8 7.5 10.1 (corona ringcorona

ring) 9.2 (four-bundledconductorsfour-bundledconductors) 11.3 (tubularbustubular bus)

Lightning impulse 5.0 5.5



DOI: 10.1002/etep

7/29/2019 etep628

10/11

Because shielding failure will not be caused by heavy current, appropriate improved insulation can

effectively reduce the number of lightning shielding failures. One of the main measures for insulation

improvement is to increase the air gap clearances between the phase conductors and the towers,

especially the clearances between the conductors and the tower trusses under them. Thus, the

minimum air gap clearances for lightning impulse withstand have been regulated.

To determine the air gap clearance of 1000-kV AC lines considering the influence of tower

width, full-scale switching surge impulse testing was conducted. The times-to-crest of switching

impulse is 1000ms. The influence of the number of parallel gaps on the discharge voltage has been

considered in the insulation coordination process, and the flashover rate of the entire line insulation

under SOV has been calculated. During the calculation, the variation of SOVs amplitude at differentpositions along the line and the probability distribution of SOVs at each position are taken into

account. It is regulated that the flashover rate of the entire line insulation under SOV must be not

higher than 0.01 times per year.

8. CONCLUSIONS

(1) The power frequency phase to ground TOVs are limited to 1.3 per unit at the bus-side-terminal

of the CB and 1.4 per unit at the line side terminal in China, and generally, the TOV duration

can be controlled within 0.2s and will not be more than 0.5s.

(2) Rated voltage Un of MOA for 1000-kV system in China has been selected as 828 kV.

(3) The maximum phase-to-ground statistical SOVs along a 1000-kV line shall be not more than1.7 per unit in China. The maximum phase-to-ground statistical SOVs in a 1000-kV

substation shall be not more than 1.6 per unit, and the maximum phase-to-phase statistical

SOVs shall be not more than 2.9 per unit.

(4) Shunt resistors of 500 are necessary for DSs in GIS to effectively limit VFTO.

(5) The maximum incoming lightning overvoltage to substations is caused by the shielding failure

in the entrance section of the transmission line. Limiting the maximum lightning shielding

failure current in the entrance section of the transmission line and optimizing the layout of

MOAs can limit the overvoltage. Ground wire protection angle less than 4 and three groundwires in the entrance section of the transmission line are measures adopted in China to limit the

maximum lightning shielding failure current.

Table V. Minimum clearances for 1000-kV transmission lines in China (unit, m).

Type of voltage applied Type of lines Minimum clearances

Power frequencyvoltage

Single circuit anddouble circuit

2.7 at elevations 500m above sea level2.9 at elevations 1000m above sea level3.1 at elevations 1500m above sea level

Switching

impulse

Single circuit outer phase: 5.9, middle phase: 6.7* or 7.9** at

elevations 500m above sea levelouter phase: 6.2, middle phase: 7.2* or 8.0** atelevations 1000m above sea level

outer phase: 6.4, middle phase: 7.9* or 8.1** atelevations 1500m above sea level

Double circuit 6.0 at elevations 500m above sea level6.2 at elevations 1000m above sea level6.4 at elevations 1500m above sea level

Lightningimpulse

Single circuit Not to be specifiedDouble circuit Plain area: 6.7; mountain area: 7.0 at elevations 500m

above sea levelPlain area: 7.1; mountain area: 7.4 at elevations 1000m

above sea levelPlain area: 7.6; mountain area: 7.9 at elevations 1500m

above sea level

*These values are air gap clearances between the middle phases and the tower trusses above them.**These values are air gap clearances between the middle phases and the tower trusses below them.

G. DINGXIE ET AL.92


DOI: 10.1002/etep

7/29/2019 etep628

11/11

(6) To prevent lightning shielding failure is the key point of lightning protection of 1000-kV lines.

Reducing the ground wire protection angle a can reduce the shielding failure trip-out rate

effectively.

(7) The equipment insulation levels of China are lower than those of Russia and higher than those

of Japan.

9. LIST OF ABBREVIATIONS AND SYMBOLS

TOV temporary overvoltageCB circuit breakerMOA metal oxide arresterSOV switching overvoltagesDS disconnect switchGIS gas-insulated-switchgearVFTO very fast transient overvoltageLIWV lightning impulse withstand voltageEGM electrical geometric model ground wire protection angleESDD equivalent salt deposit density

REFERENCES

1. Liu Zhengya. Overvoltage and insulation coordination of UHV AC transmission system. Electric Power Press,

Beijing: China, 2008; 1017. (in Chinese)

2. National standard of P.R.C. GB/Z 248422009. Overvoltage and insulation corordination of 1000kV UHV AC

transmission project, 2009; 3. (in Chinese)

3. National standard of P.R.C. GB/Z 248452009. Specification of metaloxide surge arresters without gaps for

1000kVac system, 2009; 6. (in Chinese)

4. Gu Dingxie, Dai Min, et al. Study on overvoltage and insulation of 1000kV UHV AC double-circuit line

transmission system. State Grid Electric Power Reseach Instituite 2008; 2340. (in Chinese)

5. Gu Dingxie, Zhou Peihong, Dai Min. Comparison and analysis on overvoltage and insulation coordination of UHV

AC transmission systems in China and Japan. High Voltage Engineering 2009; 35(6): 12481253. (in Chinese)

6. Gu Dingxie, Zhou Peihong, Xiu Muhong, et al. Study on overvoltage and insulation coordination for 1000kV AC

transmission system. High Voltage Engineering 2006; 32(12): 16. (in Chinese)

7. Gu Dingxie, Dai Min, et al. Analysis of failure trip rate of 1000kV double circuits on the same tower. High VoltageEngineering 2009; 35(3): 441444. (in Chinese)

8. Gu Dingxie, Xiu Muhong, Dai Min, et al. Study on VFTO of 1000kV GIS substation. High Voltage Engineering

2007; 33(11): 2732. (in Chinese)

9. Yamagata Y, Tanaka K, Nishiwaki S, et al. Suppression of VFT in 1100kV GIS by adopting resistor-fitted

disconnector. IEEE Transactions on Power Delivery 1996; 11(2): 872880.

10. IEC60071-2 1996 International standard Insulation Co-ordination-Part 2: Application guide, 1996; 6971.

11. Watanabe T, Yamagata Y, Zaima E. Insulation Coordination for UHV System. CIGRE 1998. Session 33101.

Paris, 1998, 110.

12. Yamagata Y, Tamaki E, Ikeda M, et al. Development andfield test of 1000kV 3000 mva transformer, CIGRE 1998;

Session 12303, Paris, 1998, 16.



DOI: 10.1002/etep

etep628

Documents

Transcript of etep628