prEN IEC 63042-102:2020

UHV AC transmission systems - Part 102: General system design

This part of IEC 63042-102 specifies the procedure to plan and design UHV transmission project and the items to be considered.

Prenosni sistemi UHV AC - Načrtovanje splošnih zahtev

General Information

• Status: Not Published
• Publication Date: 08-Aug-2022
• Withdrawal Date: 08-Feb-2023
• ICS: 29.240.01 - Power transmission and distribution networks in general
• ICS: 29.240.10 - Substations. Surge arresters
• Technical Committee: CLC/SR 122 - UHV AC transmission systems
• Drafting Committee: IEC/TC 122 - UHV AC transmission systems
• Current Stage: 3098 - Decision to drop project - Standstill maintained / released - Consensus building
• Start Date: 02-Sep-2021
• Completion Date: 23-Sep-2025
• Ref Project: oSIST prEN IEC 63042-102:2020 - UHV AC transmission systems - General system design

Overview

prEN IEC 63042-102:2020 (CLC) provides procedural guidance for planning and designing Ultra High Voltage (UHV) AC transmission systems. The standard addresses the sequence of studies and design decisions needed for large-capacity, long-distance transmission projects and highlights the practical trade-offs between efficiency, reliability and cost. It emphasises minimizing right-of-way (ROW) and switchyard size while addressing system impacts on lower-voltage networks (e.g., 420 kV, 550 kV).

Practical value: use this document to structure project planning, identify required studies and select configuration options that reduce losses, improve fault current conditions and limit environmental impact.

Key Topics

• Planning procedure and studies: system planning studies, system impact/detailed design studies, and implementation-phase analyses. Typical analyses include power flow, short-circuit, stability, harmonic and electromagnetic transient studies.

• Primary design decisions: capacity, system voltage and route selection are treated as the three critical conceptual-stage choices.

• Reactive power management: address long-line phenomena such as the Ferranti effect and surge impedance loading (SIL). The standard provides a flowchart approach for defining shunt or series compensation and reactive power devices.

• Voltage and insulation design: consider insulation coordination and voltage regulation to limit temporary overvoltages (TOV) and secondary-arc extinction issues. IEC 63042-101 is cited as a normative reference for voltage regulation and insulation design.

• Protection, control and reclosing: schemes and settings must reflect UHV-specific behaviours, including reclosing strategies and mitigation of zero-offset phenomena. Table-driven specifications and configurations (e.g., fast earthing switch concepts) are included for design guidance.

• Reliability and topology: network connectivity and redundancy are evaluated through scenario planning across peak/off-peak and seasonal load/generation cases.

Applications

This standard is applicable to:

• National or regional planners evaluating the introduction of UHV AC overlays on existing networks.
• Transmission project teams preparing feasibility, system impact and detailed design studies.
• Asset owners and OEMs selecting substations, compensation equipment and insulation coordination strategies.

Key benefits for projects adopting the guidance:

• Reduced ROW and tower materials: e.g., an 1100 kV line can replace multiple 550 kV lines, lowering construction footprint and costs.
• Improved system stability and fault-current management across voltage levels.
• Lower transmission losses and environmental impact through optimized topologies.

Related Standards

• IEC 63042-101 - Voltage regulation and insulation design (normative reference within this part).
• Other IEC and TC inputs: the document notes coordination with committees and standards addressing EMC, environment, quality assurance and safety.

Use prEN IEC 63042-102:2020 as a framework to sequence studies, document assumptions, and align technical choices (voltage, route, compensation and protection) with project objectives for efficient, reliable UHV AC transmission deployment.

Standards Content (Sample)

SLOVENSKI STANDARD
01-december-2020
Prenosni sistemi UHV AC - Načrtovanje splošnih zahtev
UHV AC transmission systems - General system design
Ta slovenski standard je istoveten z: prEN IEC 63042-102:2020
ICS:
29.240.01 Omrežja za prenos in Power transmission and
distribucijo električne energije distribution networks in
na splošno general
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

122/94/CDV
COMMITTEE DRAFT FOR VOTE (CDV)
PROJECT NUMBER:
IEC 63042-102 ED1
DATE OF CIRCULATION: CLOSING DATE FOR VOTING:
2020-09-25 2020-12-18
SUPERSEDES DOCUMENTS:
122/92/CD, 122/93/CC
IEC TC 122 : UHV AC TRANSMISSION SYSTEMS
SECRETARIAT: SECRETARY:
Japan Mr Eiichi Zaima
OF INTEREST TO THE FOLLOWING COMMITTEES: PROPOSED HORIZONTAL STANDARD:

TC 8,TC 14,SC 17A,SC 17C,TC 37,TC 99
Other TC/SCs are requested to indicate their interest, if any, in this
CDV to the secretary.
FUNCTIONS CONCERNED:
EMC ENVIRONMENT QUALITY ASSURANCE SAFETY
SUBMITTED FOR CENELEC PARALLEL VOTING NOT SUBMITTED FOR CENELEC PARALLEL VOTING
Attention IEC-CENELEC parallel voting
The attention of IEC National Committees, members of
CENELEC, is drawn to the fact that this Committee Draft
for Vote (CDV) is submitted for parallel voting.
The CENELEC members are invited to vote through the
CENELEC online voting system.
This document is still under study and subject to change. It should not be used for reference purposes.
Recipients of this document are invited to submit, with their comments, notification of any relevant patent rights of which they
are aware and to provide supporting documentation.

TITLE:
UHV AC transmission systems – General system design

PROPOSED STABILITY DATE: 2024
NOTE FROM TC/SC OFFICERS:
electronic file, to make a copy and to print out the content for the sole purpose of preparing National Committee positions.
You may not copy or "mirror" the file or printed version of the document, or any part of it, for any other purpose without
permission in writing from IEC.

63042-102/Ed1/CDV  IEC (E) – 2 – 122/94/CDV

1 CONTENTS
2 FOREWORD . 3
3 INTRODUCTION . 5
4 1 Scope . 6
5 2 Normative references . 6
6 3 Terms and definitions . 6
7 4 Objective and key issues of UHV AC transmission application . 6
8 4.1 Objective . 6
9 4.2 Key application issues . 7
10 5 Required studies on UHV AC system planning / design . 7
11 5.1 General . 7
12 5.2 Required studies . 7
13 5.3 Required analyses tools . 8
14 6 UHV AC system planning. 9
15 6.1 General . 9
16 6.2 Scenario planning . 11
17 6.3 Scenario procedure . 11
18 6.3.1 Power transmission capacity. 11
19 6.3.2 System voltage . 12
20 6.3.3 Route selection . 12
21 6.3.4 Series compensation . 13
22 6.4 Required parameter . 13
23 6.5 Transmission network (topology) . 14
24 6.6 Reliability . 14
25 7 UHV AC system design . 15
26 7.1 General . 15
27 7.2 Reactive power management . 15
28 7.3 Reclosing schemes . 15
29 7.4 Zero offset phenomenon . 17
30 7.5 Protection and control system . 17
31 7.6 Insulation design (cost effectiveness) . 18
33 Figure 1 Analysis tool by time domain . 8
34 Figure 2 The flowchart for reactive power compensation configuration . 10
35 Figure 3 Π equivalent circuit . 11
36 Figure 4 Four-legged reactor . 16
37 Figure 5 Fast reclosing using High Speed Earthing Switches (HSES) . 17
38 Figure 6 Procedure for Insulation Design . 19
40 Table 1 Specification to reclosing scheme . 17
41 Annex A (informative) History of development of UHV AC transmission technologies ………. 20
42 Annex B (informative) Experience of UHV AC transmissions (Use case) ……….…………….23
43 Annex C (informative) Summary of system technologies specific to UHV AC transmission
44 systems ……….……….……….……….……….……….……….……….……….………………49
63042-102/Ed1/CDV  IEC (E) – 3 – 122/94/CDV
46 INTERNATIONAL ELECTROTECHNICAL COMMISSION
47 ____________
49 UHV AC TRANSMISSION SYSTEMS –
51 Part 102: General system design
53 FOREWORD
54 1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
55 all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
56 co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
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80 expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC Publications.
81 8) Attention is drawn to the normative references cited in this publication. Use of the referenced publications is
82 indispensable for the correct application of this publication.
83 9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent
84 rights. IEC shall not be held responsible for identifying any or all such patent rights.
85 IEC 63042-102 has been prepared by IEC technical committee TC122: UHV AC transmission
86 systems.
63042-102/Ed1/CDV  IEC (E) – 4 – 122/94/CDV

88 The text of this International Standard is based on the following documents:
FDIS Report on voting
XX/XX/FDIS XX/XX/RVD
90 Full information on the voting for the approval of this International Standard can be found in the
91 report on voting indicated in the above table.
92 This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
93 The committee has decided that the contents of this document will remain unchanged until the
94 stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
95 the specific document. At this date, the document will be
96 • reconfirmed,
97 • withdrawn,
98 • replaced by a revised edition, or
99 • amended.
63042-102/Ed1/CDV  IEC (E) – 5 – 122/94/CDV
108 INTRODUCTION
109 Large-capacity power sources including large-scale renewable energy have recently been
110 developed, which are generally located far away from the load center. To meet the requirements
111 for large power transmission, some countries have introduced, or are considering introducing,
112 ultra high voltage (UHV) transmission systems, overlaying these on the existing transmission
113 systems at lower voltages such as 420 kV and 550 kV.
114 The objective of UHV AC power system planning/design is to achieve both economic efficiency
115 and high reliability, considering its impact on systems at lower voltages such as 420 kV and
116 550 kV.
117 Moreover, UHV AC transmission systems requires comparatively large space so that how to
118 realize to minimize and optimize the size and structure of UHV AC transmission lines and
119 substation apparatus is another important issue.
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121 UHV AC TRANSMISSION SYSTEMS –
123 Part 102: General system design
126 1 Scope
127 This part of IEC 63042-102 specifies the procedure to plan and design UHV transmission
128 project and the items to be considered.
129 2 Normative references
130 The following documents are referred to in the text in such a way that some or all of their content
131 constitutes requirements of this document. For dated references, only the edition cited applies.
132 For undated references, the latest edition of the referenced document (including any
133 amendments) applies.
134 IEC 63042-101, Voltage regulation and insulation design
135 3 Terms and definitions
136 For the purposes of this document, the following terms and definitions apply.
137 ISO and IEC maintain terminological databases for use in standardization at the following
138 addresses:
139 – IEC Electropedia: available at http://www.electropedia.org/
140 – ISO Online browsing platform: available at http://www.iso.org/obp
141 4 Objective and key issues of UHV AC transmission application
142 4.1 Objective
143 Recently, large-capacity power sources including large-scale renewable energy have been
144 developed, in most cases, far away from the load centres. To fully utilise these facilities, it is
145 important to transmit power generation from these sources efficiently. Evacuation through Extra
146 High Voltage (EHV) network enhancements would need more lines (Right-of-way, ROW) and
147 substation, increasing transmission losses and worsening fault current problems.
148 UHV transmission systems are characterized with large capacity over long distances and can
149 provide a solution to address above issues by minimising ROW and Switchyard requirements,
150 effectively less losses, improvement of fault current conditions etc.
151 For example, the transmission Surge Impedance Loading (SIL) capacity of an 1100 kV
152 transmission line can replace 4 to 5 of 550 kV lines, effectively reducing one-third of the tower
153 materials and one-half of the wires. It can save construction cost of the power lines and
154 substations.
155 UHV transmission system has many features as follows:
156 – Large- capacity, long-distance and high-efficiency power transmission
157 – Decrease of ROW required for transferring per unit GW
158 – Improvement of fault current condition and system stability
159 – Reduction of environmental impact
160 – Reduction of transmission losses
161 Many UHV projects have been launched and are in commercial operation.

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162 Note: Introduction of an UHV AC transmission systems can lessen or solve fault current problems in most cases,
163 however, the effect of UHV on short circuit issues should be seen in totality.
164 4.2 Key application issues
165 UHV AC transmission systems are capable of transmitting large amounts of electric power.
166 However, if a failure occurs in an UHV AC system, the system influence can be severe from the
167 viewpoints of reliability and overall security of the supply of the power system. Especially, UHV
168 AC transmission systems design should be considered to improve lightning and switching
169 protection performance.
170 In UHV AC transmission systems, typical phenomenon depends on the length of the
171 transmission line. For the phenomenon due to the long transmission line, reactive power issues
172 such as voltage rise due to the Ferranti effect and geometrical mean distance for increasing
173 surge impedance loading (SIL) should be taken care. For high voltage issues, secondary arc
174 extinction, TOV (Temporary Over-Voltage) at load shedding, and DC time constant of short-
175 circuit currents are also necessary to be considered.
176 In addition, size and cost of equipment are large so that system design should consider
177 minimizing visual impact, construction/maintenance costs and transmission losses, and
178 increasing the network connectivity by forecasting generation and load scenarios.
179 5 Required studies on UHV AC system planning / design
180 5.1 General
181 Early strategic system planning is conducted to meet their load growth and power source
182 development planning. Once it is determined that a new transmission line is required in the
183 system, preliminary economic feasibility study and project design begin.
184 In the term of project design, three primary decisions must be addressed in a transmission-line
185 project at the conceptual stage at first capacity, voltage, and route.
186 Furthermore, strategic planning, as it relates to the environmental permitting process, is often
187 overlooked or viewed as being of secondary importance. Early strategic planning for the project-
188 specific environmental review process can avoid significant effects on a project’s schedule,
189 costs, and ultimate success.
190 5.2 Required studies
191 The analytical studies can be divided into three types, corresponding to chronological phases
192 of a project’s planning, design, and implementation:
193 1) System planning study
194 In the planning stages, wherever new lines are needed, the voltage and current ratings, and
195 major auxiliary equipment such as shunt compensation are determined. At this stage, system
196 contingencies are considered. Further studies need to be carried out for various power demand
197 and generation scenarios, typical ones including peak demand, off peak demand for various
198 seasons (Summer/Winter/Rain), to check adequacy of the proposed transmission system. The
199 basic study is a power flow calculation for which positive sequence parameters are adequate.
200 2) System impact study or detailed system design study
201 The impact of new planned transmission or generation on the power system should be
202 evaluated by the system impact study. Based on the impact study, the high-level specification
203 has to be determined. The system impact study may result in some adjustments, or mitigations
204 applied to the system.
63042-102/Ed1/CDV  IEC (E) – 8 – 122/94/CDV

205 Study topics include harmonic resonance, short-circuit currents, transient stability, voltage
206 stability, and system relaying. The study tools include short circuit, stability, and harmonic
207 analysis programs, and in some cases an electromagnetic transient analytical program to
208 explore resonant overvoltages. The modelling needs to vary from lumped parameter to
209 distributed parameter, from positive sequence to three-phase unbalanced representation, and
210 from DC to a few kHz, depending on the subject. Models are often generic in early studies, later
211 progressing to specific models for particular equipment.
212 3) Equipment and system design study
213 Detailed protection and operating procedures for the switchgear, shunt compensation, and
214 related equipment are established. The basic study tool is an electromagnetic transient
215 analytical program.
216 Accurate frequency dependent models are preferable and sometimes necessary for many of
217 these studies.
218 5.3 Required analyses tools
219 The main considerations are power flow, fault current, voltage control, dynamic stability and
220 operational criteria that include reliability and system security.
221 Once the high-level specification (number and type of conductors, voltage level, current rating,
222 and reactive power compensation) has been determined, a more detailed design phase follows
223 to specify equipment, such as circuit breakers, shunt reactors, and surge arrestors. No
224 foreseeable problem should affect the reliable and safe operation of the system.
225 The analysis tool by time-domain mentioned in Figure 1.
227 Figure 1 Analysis tool by time domain
228 Line Constants – a program that calculates and represents electrical RLC parameters in a
229 matrix form for a general system of tower and conductors, over a range of frequencies, and
230 using either transposed or full unbalanced assumptions. This function may be bundled with
231 another tool, or used separately.

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232 Power flow – calculate steady-state voltages and currents based on a positive sequence model,
233 with nonlinear loads. The line model is symmetric and transposed. Power flow is the basic tool
234 for transmission planning.
235 Short-Circuit – a program that solves for voltage and current during faults, especially three-
236 phase and single-phase-to-ground faults. The model is linear, symmetric, and assumes phase
237 transposition. An auxiliary protection function simulates the response of relays to fault current
238 and voltage.
239 Dynamics – a time-domain simulator based on numerical integration of differential equations. It
240 differs from an Electromagnetic Transients Program (EMTP) in focusing on (slower)
241 electromechanical and control system transients, rather than electromagnetic transients. The
242 models are sometimes linear and balanced. The program usually includes eigenvalue analysis,
243 or other functions for small-signal stability.
244 Harmonics – a frequency domain program that solves voltage and current over a range of
245 frequencies, using linear or non-linear load and source models, and balanced or unbalanced
246 impedances. The frequency-scan function outputs driving point impedance, as obtained from
247 the bus voltage for a unit current injection.
248 EMTP – a time-domain or transient simulator based on numerical integration of differential
249 equations, including non-linear component models, unbalanced impedances, and frequency-
250 dependent RLC parameters. An EMTP can also perform frequency scans, and may include an
251 AUX program of EMTP Cable Constants.
252 Electromagnetic Field Program – A program can compute electric and magnetic fields in the air
253 and soil, as well as electric potentials, and the current distribution in the soil and in the
254 conductors.
255 6 UHV AC system planning
256 6.1 General
257 Generally, the planning study is proceeded by the following steps. As UHV AC system planning
258 has specific requirements, some considerations are necessary in each steps.
259 6.1.1 Transmission capacity considering their routes and line types to use
260 In the planning and design of power grid, increasing the voltage level of the transmission line
261 to UHV not only increases the transmission capacity, but also reduces the cost of the
262 transmission system and increases the corridor utilization rate of the transmission line.
263 The economic transmission distance of UHV transmission lines can reach 1 000~1 500 km or
264 even longer. The single line transmission capacity with 8 bundled wires can reach to 12 000
265 MW. In the selection of UHV transmission capacity, the economic benefits of the entire power
266 grid should be considered, rather than being limited to the economic benefits of a transmission
267 line project.
268 6.1.2 Reactive power management issues
269 In the planning of the power system, the planning of reactive power supply and reactive power
270 compensation facilities must be included. In the engineering design of UHV AC transmission,
271 the design of reactive power supply and reactive power compensation facilities should be
272 carried out.
273 Appropriate amount of reactive power supply should be planned and installed in UHV AC system
274 to meet the system voltage regulation requirements and reduce the unintended reactive power
275 transfer between different network nodes.

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276 Sufficient amount of reactive power supply with flexible adjustable capacity, as well as reserve
277 capacity of reactive power should be maintained.
278 The configuration of reactive power compensation and equipment type selection should be
279 technically and economically compared.
280 Planning and design of reactive power compensator for UHV AC system should meet the
281 overvoltage limiting requirement of UHV AC transmission systems.
282 The process of configuring reactive power compensation for UHV AC system is as follows:
283 1) Identify the range of likely active power flow across the UHV line, calculate and analyze the
284 characteristics of reactive power and voltage profiles along the UHV line, taking into account
285 of charging reactive power produced by UHV AC lines and reactive power loss under
286 different power flows. (Step 1)
287 2) Select UHV transformer tap position to avoid overvoltage under a range of operating
288 conditions taking into account UHV substation location, number of transmission lines
289 connected, and system operation mode. (Step 2)
290 3) Select capacity and location of UHV shunt reactor with consideration of limiting temporary
291 overvoltage and reducing secondary arc current, and balancing charging power of lines and
292 flexibly controlling bus voltage. (Step 3)
293 4) Identify total and unit capacity of compensator installed in tertiary side of the transformer.
294 The total capacity should be selected to reduce the reactive power exchange between
295 different voltage levels and maintain bus voltage in an admissible range; the selection of
296 single bank capacity should consider the voltage fluctuation induced by switching of single
297 group capacitor or reactor within a reasonable range. (Step 4)
298 5) Check if the dynamic reactive power reserve provided by generators is adequate within their
299 reactive power capability range. If it is adequate, then the process stops, otherwise go back
300 to Step 4. (Step 5)
301 Figure 2 shows the process of configuring reactive power compensation
303 Figure 2 The flowchart for reactive power compensation configuration

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304 6.1.3 Environmental issues
305 The environmental impact of power transmission project, generally includes the impact on the
306 ecological environment, electromagnetic fields, land occupation, visual landscape, and so on.
307 At present, the public's awareness of the quality of the environment in which they live has been
308 strengthened, and more and more attention are paid to the environment impact of power
309 transmission project. Related environment laws and regulations in each country should be
310 complied with.
311 During the UHV AC system planning and feasibility research, environmental issues must be
312 included. UHV AC transmission has advantages in saving the total width of transmission
313 corridor, as a result of its huge transmission capacity. However, because of its higher voltage
314 and rated current, it may cause more serious electromagnetic fields and related problems, which
315 include power frequency electric field, power frequency magnetic field, corona phenomenon,
316 radio interference, audible noise. Corresponding countermeasures should be considered during
317 the substation and transmission line design. Appropriate test and measurement should be
318 carried out to verify the effect of the countermeasure, during research and system
319 commissioning.
320 6.2 Scenario planning
321 System planning mainly include power load forecast, power source development planning and
322 power grid planning. System planning is formulated considering the load growth demand, site
323 selection of power source, and paths and networking how to connect the demand side and the
324 supply side. Then it power grid planning depends on the power development source planning.
325 Construction of power plants requires several years but its prerequisite is that the corridors and
326 required network enhancement is prepared.
327 Generally speaking, construction period of UHV transmission line may be comparatively longer
328 than those of lower voltage level and is required more restrictions for so that it is necessary to
329 determine when and how to introduce UHV AC transmission systems based on the accumulated
330 experiences and findings as well as demand forecast and then formulate planning scenario to
331 consider the required construction period and the timing of power plants commissioning.
332 6.3 Scenario procedure
333 6.3.1 Power transmission capacity
334 Under steady-state balanced AC conditions, a power line can be represented by the simple¸π
335 equivalent circuit shown in Figure 3.
337 Figure 3 π equivalent circuit
338 In Figure 3, the subscript “S” on the voltage and current applies to the sending-end and the
339 subscript “R” to the voltage and current at the receiving-end of the line. R is the series
340 resistance, L is the series inductance and C is half the total shunt capacitance of the line.
341 Shunt conductance provides a resistive path in parallel with both shunt capacitors. However,
342 since the basic insulation for transmission lines is air, the shunt conductance is assumed to be
343 zero and is ignored.
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344 An analysis of a loaded line shows that, if line losses can be regarded as small in comparison
345 with the power transferred by the line, the maximum power that the line can transmit is given
346 by Equation (1)
347 PL= V V sin(δ)/X ……(1)
S R
349 Where:
350 PL is the power limit of the line, power transmission capacity.
351 V and V are the rms values of the sending-end and receiving-end voltages, respectively.
S R
352 X is the series reactance of the line.
353 δ is phase difference between sending end and receiving end.
354 6.3.2 System voltage
355 The maximum power that a line can transmit is directly proportional to the product of sending-
356 and receiving-end voltages. Typically, in most transmission systems, sending-end and
357 receiving-end voltages are more-or-less the same and hence the power limit is proportional to
358 the square of the system voltage. Then higher voltages are used to increase power to be
359 transmitted. As the voltage increases, the change in reactance is generally small. Though
360 increases in voltage require greater phase spacing and more insulation, wider rights-of-way, or
361 servitudes, the relationship is not linear, and the economics of line design as well as the
362 environmental impact are, usually, in favor of increasing the voltage instead of placing
363 additional parallel lines in the same right-of-way.
364 However, to upgrade the voltage, the insulation to ground and between phases has to be
365 increased. In addition, the conductor surface gradient must to be maintained below certain
366 levels to prevent the generation of audible noise and radio and television interference.
367 Frequently these requirements lead to larger towers and conductors.
368 System voltage is described in IEC 60038 which shows 2 voltage levels (1 100 kV and 1 200 kV)
369 so that the introduced voltage level should be selected to meet the individual network topology.
370 6.3.3 Route selection
371 Transmission-line routing is the selection of a corridor for a proposed line based on optimizing
372 engineering, environmental, and economic criteria.
373 During the route selection process, there will be numerous tradeoffs between some factors
374 listed previously. For example, a delta configuration tower may be desirable from an electric-
375 field standpoint and smaller right-of-way, but taller and difficult for construction and high cost.
376 It is clear that each major design factor needs to be evaluated from its impact to the environment
377 which line is to be routed. Sophisticated digital techniques, such as composite map with
378 computer graphics, could visually indicate optimum and alternate transmission line easily. In
379 addition, specific circumstance each area has to be considered. For example, the geographical
380 area under consideration may have restraints concerning population density, transportation line
381 routes, preservation of natural habitats and areas of historical significance, and the like, that
382 could prohibit or severely restrict any transmission line construction.
383 There will be trade-off between environmental concerns and economics involved in line
384 delineation. With the soaring cost of land in some areas of the country, corridor length become
385 a crucial issue. Another factor contributing to line routing costs is the clearing method itself. If
386 the optimum line goes through surrounding vegetation, there might be additional cost to clearing
387 and maintenance. Also, right-of-way, such as legal fees, must be considered.

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388 Route selection must begin early stage in the strategic of plan. This will allow optimum solution
389 to be thoroughly documented, compared with alternatives, and presented in a convincing
390 manner to governmental private bodies as well as to the general public.
391 6.3.4 Series compensation
392 The power limit is inversely proportional to the series reactance of the line. This reactance is
393 directly related to the phase separation and the dimensions and configuration of the phase
394 conductors as well as the line length. For a given length of line, the power limit can be increased
395 by reducing the series reactance. This involves a reduction of phase spacing. However,
396 considering the restriction of clearance of phase-to-phase, phase spacing couldn’t be reduced
397 much.
398 For long lines, it is necessary to reduce the series inductance electrically by means of series
399 capacitive compensation.
400 During the project planning and feasibility research, the reactance, rated current and location
401 should be considered.
402 The reactance of series capacitor is selected as a fixed percentage of the reactance of
403 transmission line. The percentage is selected from criteria like system power flow, stability,
404 subsynchronous resonance and so on. The cost of the equipment should be considered also.
405 40% series capacitive compensation degree has been used in Chinese UHV AC projects.
406 The rated current of series capacitor should be selected based on the research of continuous,
407 emergency and swing current requirement of transmission line. The cost is an important issue
408 also.
409 Normally, series capacitors are installed at terminals of transmission line in substations. If
410 there is not enough space in substations, it can be installed at appropriate point of the line.
411 However, power supply and other auxiliary equipment have to be equipped for these standalone
412 series capacitor station.
413 The electrical resonance produced by the series arrangement is always below power frequency
414 so that resonance at power and harmonic frequencies will be avoided. Series capacitor
415 compensated transmission system connected with thermal generator, sub-synchronous
416 resonance risk must be analyzed carefully. Accordingly after analyzing results, some specific
417 suppression and protection technology and equipment may be necessary.
418 6.4 Required parameter
419 To formulate the feasible plan, the assessment by both technical and economic aspects is
420 required so that various analyses can be carried out. In such studies, it is important to use
421 adequate parameters so that the referential or typical parameters are prepared better in
422 advance.
423 The typical required data for feasibility study are as follows.
424 – Line data for power flow analysis (R, X /km positive sequence)
425 – Line data for fault current analysis (positive sequence impedance, negative sequence
426 impedance, zero sequence impedance/km)
427 – Load data
428 – Transformer data (e.g. reactance, impedance, grounding method)
429 – Generator data
430 – Generators’ model for dynamic simulation (e.g. governor model, generator model)
431 – Unit price of the transmission line and substation

63042-102/Ed1/CDV  IEC (E) – 14 – 122/94/CDV

432 6.5 Transmission network (topology)
433 Transmission network should be well coordinated between transmission lines and devices in
434 the substations which depends on the individual system criteria, grid codes and guidelines as
435 well as geographical characteristics. Adequate systems also depend on system configuration
436 of their subsystem. As a result, network requirements are dominated by such a topology.
437 Considering the expectation for UHV AC transmission systems, reliability and flexibility are key
438 issues. Especially, how to maintain the reliability and increase flexibility for the integration of
439 renewable energy generation is important.
440 In the beginning and transition period, the coordination in voltage control and system operation
441 between UHV and the lower voltage level should be considered.
442 In addition, the interaction is another consideration. As for the interaction between AC systems
443 and DC transmission systems, it is necessary to consider how to decrease the impact when DC
444 transmission systems have troubles and provide little impact to DC transmission systems when
445 AC systems have problems. Same as DC-AC interaction, inter-tie should be considered.
446 To introduce UHV AC transmission systems, it is important to consider such a transmission
447 network topology as well as technical requirements
448 6.6 Reliability
449 Feasible plan should keep the system reliability to some degree. Especially, UHV AC
450 transmission systems require high reliability to avoid widespread influence of their system
451 trouble due to their characteristics.
452 As for the system reliability, following aspects might be checked.
453 1) Overloading
454 Most of UHV AC transmission systems projects are targeted to deal with mass power to avoid
455 the troubles spreading to whole systems.
456 Then it is necessary to make the plan not to overload the facilities under the contingency.
457 Generally single contingency (N-1) is considered to check overload situation but UHV AC
458 transmission systems require high reliability so that targeted contingency should be considered
459 according to the individual network topology.
460 2) Fault current levels
461 Fault current level affects switchgear duty and electrical life time. Increased penetration in
462 transmission systems can increase the prospective fault current levels.
463 3) Voltage stability
464 UHV AC transmission systems provide more reactive power than EHV. Switched or variable
465 shunt reactors associated with the system may provide more flexibility in reactive power control.
466 However, the loss of these components must be considered as a contingency in voltage stability
467 analyses.
468 4) Dynamic stability
469 Installation of higher voltage is favourable to angle stability. On the other hand, generally in
470 UHV AC transmission systems long distance transmission lines are used which worsen the
471 angle stability. For due diligence, stability should be checked to determine whether UHV
472 installation has any significant impact to transmit large amount of electric power.
473 5) Ferroresonance
63042-102/Ed1/CDV  IEC (E) – 15 – 122/94/CDV
474 Ferroresonance is sensitive to the amount of capacitance isolated with a transformer and shunt
475 reactor. Due to the long distance and bundles wires of UHV AC transmission lines, the phase
476 to phase and phase to ground capacitance of UHV AC are larger, which generates much
477 capacitive reactive power. When a large amount of capacitive reactive power passes through
478 the inductive components (e.g. transformers and transmission lines,) of the system, the voltage
479 will increase at the end of the line, which appears as "capacitive effect" or "ferroresonance"
480 phenomenon. Transformers and long distance UHV transmission lines should not be switched
481 together. To reduce the effect of “ferroresonance”, shunt reactor is generally selected to limit
482 the overvoltage of UHV AC transmission system. Shunt reactor, generally connects between
483 the middle or end of the UHV/EHV transmission line and the ground which is parallel to the grid
484 for compensating capacitive current. Relying on the inductive reactive power of the shunt
485 reactor to compensate the capacitive charging power on the line, so as to reduce the increase
486 of power frequency voltage.
487 6) Angle stability
488 Angular separation between adjacent buses is used to check reliability for system studies
489 (steady state analysis) for transmission planning. The value below 30 degree is often used for
490 the angular separation under N-1 contingency condition. Accordingly, same is assessed while
491 carrying out system studies.
492 7 UHV AC system design
493 7.1 General
494 In the design stage, one of the most important issues is how to counter overvoltage under
495 various system conditions due to UHV system characteristics. Then UHV AC transmission
496 systems require many kinds of analyses to keep their high reliability. The following items are
497 typical issues:
498 1) Voltage control and reactive power management
499 2) Overvoltage analysis and insulation coordination
500 3) Switching transients (energization, fault clearing, etc)
501 4) Influence of TOV and energy duty on arrester specifications
502 5) Lightning transients and arrester placement
503 6) Impact of auto-reclosing on system availability and reliability could be considered in the
504 planning study
505 7) Shunt reactor energization and zero-offset phenomenon when high compensation degrees
506 are deployed
507 7.2 Reactive power management
508 Reactive power compensation at the UHV side (primary side) refers to equipment that is directly
509 connected to the UHV AC line or bus, including fixed capacity and controllable shunt reactors.
510 UHV shunt reactive power compensation is mainly used to compensate the charging reactive
511 power of a UHV transmission line, limit temporary overvoltage and limit voltage to below the
512 maximum operation voltage in transmission line energization. In addition, a shunt reactor with
513 a neutral point reactor may be used to limit secondary arc current.
514 A shunt reactor connected to UHV transmission lines is used for reactive power compensation
515 and overvoltage limiting. For substations with some short lines, the shunt reactor is normally
516 connected to the bus, which is mainly used to compensate the charging reactive power of the
517 UHV transmission line.
518 7.3 Reclosing schemes
519 The secondary arc is generally extinguished within several hundred ms. in 550 kV system and
520 below a
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