3.16 protective levels of an arrester for each voltage class, residual voltage that appears between the terminals of an arrester during the passage of a discharge current corresponding
Scope
This part of IEC 60071 provides guidance on the procedures for insulation co-ordination of high-voltage direct current (HVDC) converter stations, without prescribing standardized insulation levels
This standard is specifically designed for HVDC applications within high-voltage AC power systems and does not pertain to industrial conversion equipment It provides principles and guidance solely for insulation coordination purposes, and it does not address requirements related to human safety.
Additional background
The implementation of power electronic thyristor valves in series and/or parallel configurations necessitates careful attention to overvoltage protection in converter stations, differing from traditional a.c substations This standard details the assessment procedures for overvoltage stresses on converter station equipment, which are exposed to a mix of d.c., a.c power frequency, harmonic, and impulse voltages Additionally, it outlines the criteria for establishing protective levels of surge arresters in these configurations to ensure effective protection.
The basic principles and design objectives of insulation co-ordination of converter stations, in so far as they differ from normal a.c system practice, are described
This standard focuses on metal-oxide surge arresters without gaps, specifically designed for modern HVDC converter stations It outlines the essential characteristics and requirements of these arresters, as well as the evaluation process for maximum overvoltages they may encounter during operation Additionally, the article presents typical protection schemes for arresters, the stresses they experience, and the methods used to determine these stresses.
This standard addresses the insulation coordination of equipment linked between the converter a.c bus—comprising a.c harmonic filters, the converter transformer, and circuit breakers—and the d.c line side of the smoothing reactor It also encompasses line and cable terminations that impact the insulation coordination of converter station equipment.
The standard primarily addresses conventional HVDC systems with the commutation voltage bus located at the a.c filter bus Additionally, it includes guidelines for insulation coordination related to capacitor commutated converters (CCC), controlled series compensated converters (CSCC), and various other specialized converter configurations in the annexes.
This standard discusses insulation co-ordination related to line commutated converter (LCC) stations The insulation coordination of voltage sourced converters (VSC) is not part of this standard
This document references essential documents that are crucial for its application For references with specific dates, only the cited edition is applicable, while for undated references, the most recent edition, including any amendments, is relevant.
IEC 60060-1, High-voltage test techniques – Part 1: General definitions and test requirements IEC 60071-1:2006, Insulation co-ordination – Part 1: Definitions, principles and rules
IEC 60071-2:1996, Insulation co-ordination – Part 2: Application guide
IEC 60099-4:2004, Surge arresters – Part 4: Metal-oxide surge arresters without gaps for a.c systems
IEC 60633, Terminology for high-voltage direct current (HVDC) transmission
IEC TS 60815-1:2008, Selection and dimensioning of high-voltage insulators intended for use in polluted conditions – Part 1: Definitions, information and general principles
IEC TS 60815-2:2008, Selection and dimensioning of high-voltage insulators intended for use in polluted conditions – Part 2: Ceramic and glass insulators for a.c systems
IEC TS 60815-3:2008, Selection and dimensioning of high-voltage insulators intended for use in polluted conditions – Part 3: Polymer insulators for a.c systems
For the purposes of this document, the following terms and definitions apply
NOTE Many of the following definitions refer to insulation co-ordination concepts (IEC 60071-1), or to arrester parameters (IEC 60099-4)
Insulation coordination involves selecting the appropriate dielectric strength of equipment based on the operating voltages and potential overvoltages in the system This selection must consider the service environment and the characteristics of available protective devices.
3.2 nominal d.c voltage mean value of the direct voltage required to transmit nominal power at nominal current
3.3 highest d.c voltage highest value of d.c voltage for which the equipment is designed to operate continuously, in respect of its insulation as well as other characteristics
3.4 overvoltage voltage having a value exceeding the corresponding highest steady state voltage of the system
Note 1 to entry: Table 1 presents (as per IEC 60071-1) the classification of these voltages which are defined in 3.4.1 to 3.4.2.3
3.4.1 temporary overvoltage overvoltages of relatively long duration (ranging from 0,02 to 3 600 s as per IEC 60071-1)
Note 1 to entry: The overvoltage may be undamped or weakly damped
3.4.2 transient overvoltage short-duration overvoltage of a few millisecond or less, oscillatory or non-oscillatory, usually highly damped
3.4.2.1 slow-front overvoltage transient overvoltage, usually unidirectional, with time to peak 20 às < T p ≤ 5 000 às, and tail duration T 2 ≤ 20 ms
For insulation coordination, slow-front overvoltages are categorized based on their shape, independent of their source Despite significant variations from standard shapes in real systems, this standard typically allows for the classification and peak value description of such overvoltages in most instances.
Table 1 – Classes and shapes of overvoltages, standard voltage shapes and standard withstand voltage tests
Fast-front overvoltage refers to the overvoltage occurring at a specific location in a system, typically caused by a lightning discharge or other factors For the purpose of insulation coordination, this overvoltage can be characterized by a waveform that resembles the standard impulse defined in IEC 60060-1, which is utilized for lightning impulse testing.
Note 1 to entry: Fast-front overvoltage is defined as transient overvoltage, usually unidirectional, with time to peak 0,1 às < T 1 ≤ 20 às, and tail duration T 2 ≤ 300 às in IEC 60071-1:2006, 3.17.3.2
For insulation coordination, fast-front overvoltages are categorized based on their shape, independent of their source Despite significant variations from standard shapes in real systems, this standard typically allows for the classification and peak value to adequately describe such overvoltages.
3.4.2.3 very-fast-front overvoltage transient overvoltage, usually unidirectional, with time to peak Tf < 0,1 às, and with or without superimposed oscillations at frequency 30 kHz < f < 100 MHz
3.4.2.4 steep-front overvoltage transient overvoltage classified as a kind of fast-front overvoltage with time to peak
Note 1 to entry: A steep-front impulse voltage for test purposes is defined in IEC 60700-1
Note 2 to entry: The front time is decided by means of system studies
3.4.2.5 combined overvoltage overvoltage consisting of two voltage components simultaneously applied between each of the two-phase terminals of a phase-to-phase (or longitudinal) insulation and earth
Combined overvoltage encompasses various types, including temporary, slow-front, fast-front, and very-fast front overvoltages It is categorized based on the component that exhibits the highest peak value.
U rp overvoltages assumed to produce the same dielectric effect on the insulation as overvoltages of a given class occurring in service due to various origins
Note 1 to entry: In this standard it is generally assumed that the representative overvoltages are characterized by their assumed or obtained maximum values
RSFO voltage value between terminals of an equipment having the shape of a standard switching impulse
Note 1 to entry: This note applies to the French language only
RFFO voltage value between terminals of an equipment having the shape of a standard lightning impulse
Note 1 to entry: This note applies to the French language only
The RSTO voltage value features a standard shape with a crest time that is shorter than a standard lightning impulse, yet longer than that of a very-fast-front overvoltage, as specified by IEC 60071-1.
Note 1 to entry: A steep-front impulse voltage for test purposes is defined in Figure 1 of IEC 60700-1:2008 The front time is decided by means of system studies
Note 2 to entry: This note applies to the French language only
U cw for each class of voltage, value of the withstand voltage of the insulation configuration, in actual service conditions, that meets the performance criterion (IEC 60071-1)
The withstand voltage test is crucial for ensuring that insulation can endure specific overvoltages during its service life This test determines the required withstand voltage, which is based on the coordination withstand voltage and is defined according to the conditions of the standard test used for verification.
U w test voltage suitably selected equal to or above the required withstand voltage (U rw )
For alternating current (a.c.) equipment, withstand voltage values (U w) are standardized according to IEC 60071-1 In contrast, high voltage direct current (HVDC) equipment lacks standardized withstand voltage values, which are instead rounded to practical values for convenience.
The standard impulse shapes and test procedures for equipment withstand tests are outlined in IEC 60060-1 and IEC 60071-1 For certain d.c equipment, such as thyristor valves, these impulse shapes may be adjusted to better represent actual operating conditions.
SIWV withstand voltage of insulation with the shape of the standard switching impulse
Note 1 to entry: This note applies to the French language only
LIWV withstand voltage of insulation with the shape of the standard lightning impulse
Note 1 to entry: This note applies to the French language only
3.8.3 steep-front impulse withstand voltage
STIWV withstand voltage of insulation with the shape specified in IEC 60071-1
Note 1 to entry: This note applies to the French language only
3.9 continuous operating voltage of an arrester
U c permissible r.m.s value of power frequency voltage that may be applied continuously between the terminals of the arrester
3.10 continuous operating voltage of an arrester including harmonics
U ch r.m.s value of the combination of power frequency voltage and harmonics that may be applied continuously between the terminals of the arrester
Note 1 to entry: It may be noted that this definition only pertains to coordination of arrester protective levels and not to assessing arrester energy duty
3.11 crest value of continuous operating voltage
CCOV highest continuously occurring crest value of the voltage at the equipment on the d.c side of the converter station excluding commutation overshoots
Note 1 to entry: This note applies to the French language only
3.12 peak value of continuous operating voltage
PCOV highest continuously occurring crest value of the voltage at the equipment on the d.c side of the converter station including commutation overshoots and commutation notches
Note 1 to entry: This note applies to the French language only
3.13 equivalent continuous operating voltage of an arrester
The ECOV represents the root mean square (r.m.s.) value of sinusoidal power frequency voltage at a metal-oxide surge arrester It is defined under the condition of being subjected to an operating voltage of any waveform that produces equivalent power losses in the metal-oxide materials as the actual operating voltage.
Note 1 to entry: This note applies to the French language only
3.14 residual voltage of an arrester peak value of voltage that appears between the terminals of an arrester during the passage of a discharge current
3.15 co-ordination currents of an arrester for a given system under study and for each class of overvoltage, the current through the arrester for which the representative overvoltage is determined
Note 1 to entry: Standard shapes of co-ordination currents for steep-front, lightning and switching current impulses are given in IEC 60099-4
Note 2 to entry: The co-ordination currents are determined by system studies
General
This article presents a selection of commonly used symbols and abbreviations, with visual representations provided in Figure 1 and Table 2 For a comprehensive list of symbols utilized in HVDC converter stations and insulation coordination, please consult the standards mentioned in the normative references (Clause 2) and the Bibliography.
Subscripts
0 (zero) at no load (IEC 60633) d direct current or voltage (IEC 60633) i ideal (IEC 60633) max maximum (IEC 60633) n pertaining to harmonic component of order n (IEC 60633)
Letter symbols
U c continuous operating voltage of an arrester
U ch continuous operating voltage of an arrester including harmonics
U di0 ideal no-load direct voltage (IEC 60633)
1 Numbers in square brackets refer to the Bibliography
U di0m maximum value of U di0 taking into account a.c voltage measuring tolerances, and transformer tap-changer offset by one step
U s highest voltage of an a.c system (IEC 60071-1 and 60071-2)
U m highest voltage for the equipment
U v0 no-load phase-to-phase voltage on the valve side of converter transformer, r.m.s value excluding harmonics
U cw co-ordination withstand voltage
U w standard withstand voltage α delay angle (IEC 60633); “firing angle” also used in this standard β advance angle (IEC 60633) γ extinction angle (IEC 60633) à overlap angle (IEC 60633)
Abbreviations
CSCC controlled series compensated converter
CCOV crest value of continuous operating voltage
PCOV peak continuous operating voltage
ECOV equivalent continuous operating voltage
RSFO representative slow-front overvoltage (the maximum voltage stress value)
RFFO representative fast-front overvoltage (the maximum voltage stress value)
RSTO representative steep-front overvoltage (the maximum voltage stress value) RSIWV required switching impulse withstand voltage
RLIWV required lightning impulse withstand voltage
RSTIWV required steep-front impulse withstand voltage
SIPL switching impulse protective level
LIPL lightning impulse protective level
STIPL steep-front impulse protective level
SIWV switching impulse withstand voltage
LIWV lightning impulse withstand voltage
STIWV steep-front impulse withstand voltage p.u per unit
5 Typical HVDC converter station schemes
Figure 1 illustrates a typical HVDC converter station featuring two 12-pulse converter bridges arranged in series The diagram also indicates potential locations for arresters as specified in this standard, although some of these arresters may be deemed redundant and can be omitted based on the specific design requirements.
Figure 2 illustrates a single line diagram and arrester configuration for a back-to-back converter station Alternative arrangements with various earthing connections, such as earthing at the midpoint between two six-pulse bridges, are also prevalent Additionally, the placement of the smoothing reactor may vary based on these configurations.
The a.c and d.c filter configurations could be more complex than those shown in these figures Table 2 presents the graphical symbols used in this standard
The thyristor valves being voltage sensitive require strict overvoltage protection, which is provided by valve arresters that are connected directly across the valve terminals
Valve arresters, when used alongside other arresters, effectively protect transformer valve windings Typically, phase-phase and phase-earth arresters are not included However, it may be beneficial to consider phase-to-earth arresters for transformer valve windings.
800 kV and above to lower the insulation levels especially to the top valve group
Each voltage level and component is protected by either a single arrester or a combination of series or parallel connected arresters
Arrester designations and details on their design and specific roles are presented in Clause 8
A: AC bus arrester FA: AC filter arrester
FD: DC filter arrester EL: Electrode line arrester
E1: DC neutral bus arrester EM: Metallic return arrester
EB: Converter neutral arrester B: Bridge arrester (6-pulse)
V: Valve arrester CB: Converter unit d.c bus arrester
T: Transformer valve winding arrester DB: DC bus arrester
DR: Smoothing reactor arrester DC: DC cable arrester
DL: DC line arrester CM: Arrester between converters
CL: LV converter unit arrester MH: Mid-point bridge arrester (HV bridge) CH: HV converter unit arrester ML: Mid-point bridge arrester (LV bridge)
Figure 1 – Possible arrester locations in a pole with two 12-pulse converters in series
A: AC bus arrester FA: AC filter arrester
Figure 2 – Possible arrester locations for a back-to-back converter station
Valve (commutation group) Valve (one arm)
Arrester Reactor Capacitor Transformer with two windings Earth
6 Principles of insulation co-ordination
General
The primary objectives of insulation co-ordination are:
• to establish the maximum steady state, temporary and transient overvoltage levels to which the various components of a system may be subjected in practice,
• to select the insulation strength and characteristics of equipment, including the protective devices, used in order to ensure a safe, economic and reliable installation in the event of overvoltages.
Essential differences between a.c and d.c systems
Insulation coordination in HVDC converter stations shares principles with a.c substations but requires special considerations due to several factors Key aspects include the need for different insulation levels for series-connected valve groups and surge arresters, the topology of converter circuits that limit exposure to external overvoltages, and the presence of reactive power sources and harmonic filters that can lead to overvoltages and resonance conditions Additionally, long overhead transmission lines or cables without switching stations pose risks of resonance on the d.c side Converter transformers, which may not be directly earthed, introduce complexities such as d.c voltage offsets and composite voltage wave shapes Control malfunctions can result in valve misfires and current extinction, while fast control actions help mitigate overvoltages The constant polarity of d.c stress increases contamination risks, necessitating greater creepage and clearance requirements compared to a.c insulation Furthermore, the interaction between a.c and d.c systems, especially in weak a.c networks, and the various operating modes of converters, such as monopolar and bipolar configurations, highlight the absence of standard insulation levels for d.c systems.
Insulation co-ordination procedure
The investigation method for an HVDC converter station involves several key steps: first, selecting the d.c circuit configuration, which includes determining the placement of d.c smoothing reactors, d.c side earthing, and the converter transformer valve winding connection (either star or delta) to the higher d.c voltage terminal Next, the arrangement of arresters is chosen based on the selected d.c circuit configuration Additionally, it is crucial to evaluate the characteristics of the a.c system at the commutation bus and the d.c system, along with their interaction, to identify various representative overvoltages and the current/energy stresses on surge arresters Finally, the design is optimized through iterative assessments of equipment insulation and arrester requirements.
Comparison of withstand voltage selection in a.c and d.c systems
According to IEC 60071-1, the insulation coordination procedure consists of four key steps: first, the determination of representative overvoltages (U rp ); second, the identification of coordination withstand voltages (U cw ); third, the assessment of required withstand voltages (U rw ); and finally, the determination of standard withstand voltages (U w ).
Table 3 is a flow chart showing the procedure in selecting the withstand voltages (U w ) in both a.c (Figure 1 of IEC 60071-1:2006) and d.c systems with the differences in the d.c case being identified
The individual steps involved in the selection process are detailed in IEC 60071-1 for the a.c system application and in Clause 9 of this standard for the d.c system
Table 3 – Comparison of the selection of withstand voltages for a.c equipment with that for HVDC converter station equipment
Determination of rated or standard insulation levels for three-phase a.c equipment according to IEC 60071-1
Deviations from IEC 60071-1 in the selection of withstand voltages for HVDC converter station equipment
Representative voltages and overvoltages (U rp )
Selection of the insulation meeting the performance criterion, data inaccuracies, arrester separation distance
HVDC converter equipment necessitates precise protection through the coordination of arresters, where withstand voltages are established based on the calculation of standard coordination currents.
Application of factors to account for the differences between type test conditions and actual service conditions
The selection of standard rated withstand voltages for a.c side equipment is guided by IEC 60071-1 For d.c side equipment, the specified insulation voltages are adjusted to more practical values.
7 Voltages and overvoltages in service
Continuous operating voltages at various locations in the converter station
In an HVDC converter station, the continuous operating voltages vary from those in an a.c system, as they are not limited to just the fundamental frequency voltages Instead, these voltages can include a mix of direct voltage, fundamental frequency voltage, harmonic voltages, and high-frequency transients, depending on the specific location within the station.
Figure 3 illustrates an HVDC converter station featuring a single 12-pulse converter configuration for each pole Typically, phase-earth arresters are not included on the valve side of the converter transformer (T) for HVDC systems operating at voltages up to 600 kV.
Figure 1 shows an HVDC scheme with two 12-pulse converters per pole configuration, which has been used for the early 600 kV scheme and some of the recent 800 kV schemes
Figure 4 illustrates the typical waveforms of continuous operating voltages at different locations within the HVDC converter station, excluding commutation overshoots, either to earth (G) or to another point, as depicted in the configuration of Figure 3 The node numbers and arrester designations in Figure 3 are identified by corresponding numbers and letters These waveforms were generated using a simulation tool that takes into account standard d.c parameters.
Note that Figures 1, 2 and 3 show possible arrester locations, and some of them may be eliminated because of specific designs
M: Mid-point bridge arrester EM: Metallic return arrester
E: DC neutral bus arrester EL: Electrode line arrester
V: Valve arrester B: Bridge arrester (6-pulse)
T: Transformer valve winding arrester C: Converter unit arrester
DR: Smoothing reactor arrester DB: DC bus arrester
DL: DC line arrester DC: DC cable arrester
FA1, FA2: AC filter arresters FD1, FD2: DC filter arresters
Figure 3 – HVDC converter station with one 12-pulse converter bridge per pole
Loc (1-G) Loc (2-G) Loc (5), (6) (ph-ph)
Figure 4 – Continuous operating voltages at various locations (location identification according to Figure 3)
The harmonics produced on the alternating current (a.c.) side are presumed to be eliminated by the connected filters, resulting in a voltage at point (1-G) that resembles a pure sine wave of fundamental frequency, free from any harmonics.
The voltage shape at points (1-2) primarily resembles a fundamental frequency sine wave, with harmonics superimposed The harmonic content is significantly influenced by the filter configuration, tuning frequencies, and the operating conditions of the converters, usually comprising less than 30% of the fundamental frequency.
The d.c voltages across the 6-pulse bridges (Loc 7-8 and 9-7) are derived from approximately 60° segments of line-line a.c voltages, reflecting both the duration of these arcs and the average line-line voltages.
The voltage at the 6-pulse bridge to earth (Loc 7-G) can match the voltage at Loc (7-8) when the station is properly earthed and operates symmetrically in a bipole configuration However, during unsymmetrical bipolar or monopolar operations, an additional direct current (d.c.) offset will be introduced.
The voltage across the 12-pulse bridge (Loc 9-8) comprises of 30° arcs of line-line a.c voltages with superimposed influence of firing delay and overlap angles
The voltage at the 12-pulse bridge to earth (Loc 9-G) may match that of Loc (9-8) or may exhibit an additional DC offset, similar to the factors affecting Loc (7-G).
The voltage waveforms of locations (5b-6a) and (5c-6a) illustrate the voltage between two distinct phases of the two six-pulse groups This specific wave shape is applicable solely to three-phase, three-winding transformers.
The voltage at Loc (10-G) is the smoothed out voltage due to the influence of the smoothing reactor and d.c filter, if applicable
The voltages at locations (6-8) and (9-5) represent the voltages across a valve in rectifier mode, highlighting the valve's conduction period and commutation within its own row, as well as in the adjacent row of thyristors in a 6-pulse bridge configuration.
The voltage across the transformer valve winding phase-phase is illustrated in locations (5) and (6) The zero voltage indicates the commutation process of the valves linked to the two corresponding phases, while the notches represent the commutation involving valves connected to one of the phases.
Neutral bus voltage at location 8-G, along with the voltages across the filters, reflects standard voltage levels influenced by the parameters of the electrode circuit and filters Additionally, location 8-G may exhibit a direct current (d.c.) offset, particularly during monopolar metallic return operations.
The voltage at the n-G location consists of a direct current (d.c.) component that is three-quarters of the pole voltage at Loc 10-G, in addition to the ripple effect from the lower 6-pulse bridge and half of the ripple from the upper 6-pulse bridge.
Peak continuous operating voltage (PCOV) and crest continuous operating
The switching action of valves generates high-frequency turn-on and turn-off commutation transient voltages that overlay the commutation voltage This overshoot during turn-off elevates the transformer valve-side winding voltage, particularly affecting the off-state (reverse-blocking) voltage across the valves and their associated arresters The magnitude of this overshoot is influenced by several factors, including the inherent characteristics of the thyristors, the distribution of recovered charge in a series-connected string of thyristors, the damping resistors and capacitors at individual thyristor levels, the various capacitances and inductances within the valve and commutation circuit, as well as the firing and overlap angles, and the valve commutation voltage at the moment of turn-off.
Special attention shall be paid to the commutation overshoots, including wave shape with respect to power dissipation in the valve arresters and other arresters on the d.c side
The continuous operating voltage waveform across the valve (Loc 6-8 and 9-5) and valve arrester (V), during rectifier operation, is shown in Figure 5
The CCOV (defined in Clause 3) is proportional to the U di0m, and is given by: v0 di0m 2
Refer to 4.3 for the definition of U di0m and U v0
Operation with large delay angles α increases the commutation overshoots, and special care shall be taken that these do not overstress the arresters
Figure 5 – Operating voltage of a valve arrester (V), rectifier operation
The continuous operating voltage waveforms across the mid-point arrester (M) (Loc 7-G) and across the converter bus arrester (CB) (Loc 9-G) are shown in Figures 6 and 7, respectively
Figure 6 – Operating voltage of a mid-point arrester (M), rectifier operation
Figure 7 – Operating voltage of a converter bus arrester (CB), rectifier operation
Sources and types of overvoltages
Overvoltages on the a.c side can arise from various sources such as switching, faults, load rejection, or lightning Evaluating these overvoltages requires consideration of the dynamic characteristics of the a.c network, including its impedance and effective damping at key transient oscillation frequencies, as well as accurate modeling of converter transformers, static and synchronous compensators, and filter components Additionally, when busbar lengths in the a.c switchyard are considerable, they must be factored into the assessment of lightning and fast-front overvoltages, including distance effects and the placement of arresters.
Overvoltages on the d.c side may originate from either the a.c system or the d.c line and/or cable, or from in-station flashovers or other fault events
When evaluating overvoltages, it is essential to consider the configurations of both a.c and d.c systems, along with the dynamic performance of valves and controls, as well as plausible worst-case scenarios, as outlined in Clauses 8 and 10.
Impacts on arrester requirements are discussed in Clause 8
While the origin of overvoltages can result from different phenomena (switching, fault and lightning) as described above, the overvoltages are categorized according to their shape and duration as:
• temporary overvoltages (power frequency overvoltage of relatively long duration),
• transient overvoltages (short-duration overvoltage of few milliseconds or less, oscillatory or non-oscillatory, usually highly damped)
Transient overvoltages can be further classified as:
Temporary overvoltages
General
Temporary overvoltage refers to an oscillatory overvoltage that lasts for a relatively long duration and is either undamped or only weakly damped These overvoltages can arise from both the alternating current (a.c.) side and the direct current (d.c.) side.
Temporary overvoltages on the a.c side
Temporary overvoltages are typically caused by switching operations or faults, with the most significant occurrences linked to sudden load loss due to faults in either the a.c or d.c systems while a.c reactive sources remain connected When reactive elements and the a.c system create resonance conditions, the severity of these temporary overvoltages can increase, impacting both the magnitude of the overvoltage and the energy duty of arresters.
Together with the highest a.c operating voltages (U s ), the temporary overvoltages will be decisive for setting the rated voltage of a.c bus arresters (A)
Temporary overvoltages together with high firing or extinction angles should also be considered for valve arresters (V)
Temporary overvoltages from a.c side faults lead to asymmetrical and distorted a.c voltages, generating second harmonic voltages on the d.c side This phenomenon subsequently causes third harmonic voltages on the a.c side, which places stress on the a.c filter arresters (FA) Additionally, when converters are blocked with firing pulses directed to by-pass pairs, the arresters across the non-conducting valves may be subjected to phase-phase voltages.
Temporary overvoltages on the d.c side
An uncontrolled energization of the rectifier with the far end being blocked could result in high overvoltages, especially for a cable transmission system
High current blocking of an inverter without activating a bypass pair can lead to overvoltages This situation applies fundamental frequency voltage to the inverter, and if the DC circuit resonates near this frequency, it may cause significant overvoltages that stress the DC bus arrester (CB).
Slow-front overvoltages
General
Slow-front and temporary overvoltages on the a.c side are crucial for understanding arrester applications These overvoltages, along with the highest a.c operating voltages (U s), establish the overvoltage protection and insulation levels for the a.c side of HVDC converter stations Additionally, they play a significant role in valve insulation coordination.
Slow-front overvoltages on the a.c side
Slow-front overvoltages in the a.c bus of an HVDC converter station can arise from various sources, including the switching of transformers, reactors, static var compensators, a.c filters, and capacitor banks, as well as from fault initiation and clearing, and the closing and reclosing of lines These overvoltages typically exhibit high amplitudes during the first half cycle of the transient, with significantly reduced amplitudes in subsequent cycles Additionally, slow-front overvoltages originating from remote locations in the a.c network tend to have lower magnitudes compared to those generated by events occurring near the converter a.c bus.
Throughout the operational lifespan of equipment, frequent switching of devices linked to the AC converter bus can take place While the overvoltages resulting from these routine switching actions are typically less intense than the slow-front overvoltages triggered by faults, there are rare instances where the circuit breaker’s switching-off can lead to a restrike phenomenon, resulting in overvoltage.
When selecting a.c arresters for HVDC converter stations, it is crucial to account for existing parallel arresters in the a.c network This consideration helps prevent the overloading of these existing arresters during slow-front and temporary overvoltages.
7.5.2.2 Overvoltages due to switching operations
To protect equipment during frequent operations, it is essential that surge arresters do not absorb significant energy This can be achieved by minimizing slow-front overvoltages through the use of circuit breakers with closing and/or opening resistors, synchronizing the operation of circuit breaker poles, or installing arresters across the poles Additionally, the HVDC control system can effectively dampen temporary overvoltages.
Energizing transformers leads to inrush current due to saturation effects, primarily featuring second-order and other low-order harmonics When these harmonic currents encounter resonant conditions in a low-damping network, they can generate high harmonic voltages, resulting in overvoltages In HVDC converter stations, the presence of AC filters and capacitor banks exacerbates these resonant conditions, lowering the resonance frequency and potentially introducing second or third harmonic resonances.
The temporary overvoltages can last for several seconds, or in rare cases up to a minute
Asymmetric faults in a.c networks lead to transient and temporary overvoltages on healthy phases, significantly influenced by the zero sequence network In solidly earthed systems, commonly found in networks linked to HVDC converter stations, transient overvoltages typically range from 1.4 p.u to 1.7 p.u., while temporary overvoltages range from 1.2 p.u to 1.4 p.u.
Transformer saturation can be caused by both symmetric and asymmetric faults The impact of this saturation on overvoltages is influenced by the timing of the fault's onset and its clearance Consequently, it is essential to examine different fault conditions when investigating this phenomenon, which is elaborated on in Clause 8.
Temporary overvoltages are most likely to occur during sudden three-phase faults and complete load rejection, especially when converters block simultaneously without disconnecting filters The combination of filters, capacitor banks, and the a.c system can lead to low resonance frequencies, exacerbating the severity of temporary overvoltages and increasing arrester energy stresses Utilizing filters tuned or damped at frequencies between the second and fifth harmonic can effectively reduce voltage distortion and alleviate stresses on arresters Additionally, AC active filters can serve a similar purpose in mitigating these issues.
Slow-front overvoltages on the d.c side
The insulation coordination on the d.c side for slow-front and temporary overvoltages is primarily influenced by fault-generated slow-front overvoltages, aside from the a.c side overvoltages transmitted through the converter transformers.
Key events to consider include d.c line-to-earth faults, d.c side switching operations, and occurrences that lead to an open earth electrode line Additionally, the generation of superimposed a.c voltages can arise from faults in the converter control, such as complete loss of control pulses, misfiring, commutation failures, earth faults, and short-circuits within the converter unit For a more detailed discussion of these contingencies, refer to Clause 8.
It is crucial to consider the energization of the d.c line when the remote inverter terminal is open, especially if precautions have not been implemented to prevent this situation, as it can occur at the peak d.c output voltage of the rectifier.
In HVDC converter stations featuring series-connected converter bridge units, it is crucial to consider events like bypass operations on one converter while the other is active, especially during inverter operation Insulation coordination of parallel-connected converter bridge units requires special attention Additional details on these and other unique converter configurations can be found in Annex C.
Fast-front, very-fast-front and steep-front overvoltages
The different sections of HVDC converter stations should be examined in different ways for fast-front and steep-front overvoltages The sections include:
• a.c switchyard section from the a.c line entrance up to the line side terminals of the converter transformers;
• d.c switchyard section from the line entrance up to the line side terminal of the smoothing reactor;
• converter bridge section between the valve side terminal of the converter transformers and the valve side terminal of the smoothing reactor
The converter bridge section is isolated from the other sections by series reactances, including the inductance of the smoothing reactor and the leakage reactance of the converter transformers Lightning-induced traveling waves on the a.c side of the transformer or the d.c line beyond the smoothing reactor are attenuated due to the interplay of series reactance and shunt capacitance to earth, resembling slow-front overvoltages Therefore, these waves should be integrated into the slow-front overvoltage coordination strategy.
The a.c and d.c switchyard sections exhibit low impedance relative to overhead lines, primarily due to the inclusion of filters and shunt capacitor banks Unlike traditional a.c switchyards, these sections feature a.c filters, d.c filters, and potentially large shunt capacitor banks, which can mitigate incoming overvoltages.
Steep-front overvoltages resulting from earth faults in HVDC converter stations, particularly within the valve hall, are critical for insulation coordination, especially concerning the valves These overvoltages generally exhibit a front time ranging from 0.5 µs to 1.0 µs and can last up to 10 µs It is essential to determine the specific values and waveshapes through digital simulation studies, as both the peak magnitude and the maximum rate of change of voltage are significant factors.
In the a.c switchyard, very-fast-front overvoltages, characterized by front times ranging from 5 ns to 150 ns, can be triggered by the operation of disconnectors or circuit breakers within gas-insulated switchgear (GIS) Additional details regarding the impact of GIS are provided in Clause C.6.
Arrester characteristics
Since the late 1970s, metal-oxide surge arresters have been the primary choice for overvoltage protection in HVDC converter stations due to their superior protection characteristics and reliable performance in various configurations The arrangement of these arresters is determined by the specific layout of the HVDC converter station and the transmission circuit type The key criterion is to ensure that each voltage level and its connected equipment are adequately protected while balancing cost with the desired reliability and equipment withstand capability.
Metal-oxide surge arresters without gaps are increasingly utilized in modern HVDC converter stations, replacing traditional arresters in existing systems They offer superior overvoltage protection due to their low dynamic impedance and high energy absorption capability When connected in parallel with closely matched characteristics, these arresters can effectively share discharge energy, allowing for customizable energy capability Additionally, multiple parallel paths within a single arrester unit or across several units can be established to meet specific energy requirements, and this configuration can also help reduce the residual voltage of the arrester when necessary.
For metal-oxide arresters, the variation of voltage U with current I can be represented by the equation:
The relationship between current (I) and voltage (U) in a material is expressed as \$I = k \times U^\alpha\$, where \$k\$ is a constant and \$\alpha\$ is a non-linearity coefficient that varies based on the disk formulation and the current range For zinc oxide materials, this coefficient is notably high, typically ranging from 10 to 50, in contrast to silicon carbide elements used in gapped arresters, which generally have a coefficient around 3.
The protective characteristics of an arrester are determined by the residual voltages it experiences during maximum steep-front, lightning, and switching current impulses Typical current waveforms for defining arrester protective levels include 8/20 µs for the LIPL and 30/60 µs for the SIPL, as per IEC 60099-4, while the STIPL is defined for a 1 µs front time impulse The voltage waveforms across the arrester vary due to the high non-linearity coefficient of the arrester block material The coordination current, which specifies the amplitude for the protective level, is selected based on different current waveforms and the locations of the arresters, and these currents are determined through detailed studies conducted during the design phase.
Arresters on the a.c side are specified based on their rated voltage and maximum continuous operating voltage, similar to those in standard a.c systems The rated voltage represents the highest permissible r.m.s value of power frequency voltage between the terminals for proper operation, determined through operating duty tests Additionally, the maximum continuous operating voltage serves as a key parameter for defining the operating characteristics of the arresters.
In HVDC converter stations, the continuous operating voltage for arresters on the d.c side is defined by the complex voltage waveshape, which often includes superimposed direct, fundamental, and harmonic components, along with potential commutation overshoots The specifications for arrester voltages include peak continuous operating voltage (PCOV), crest value of continuous operating voltage (CCOV), and equivalent continuous operating voltage (ECOV), as outlined in Clause 3 Consequently, testing protocols for these arresters must be tailored to their specific applications, differing from the standard tests typically used for a.c arresters Additionally, the energy capability of the arresters must account for the relevant waveshapes, amplitudes, durations, and the frequency of discharges.
For filter arresters, the higher losses due to harmonics shall be taken into account.
Arrester specification
The residual voltage of an arrester refers to the peak voltage that occurs between its terminals when a discharge current flows through it The maximum residual voltages are defined for specific arrester currents, known as coordination currents, as shown in Table 7.
The coordination currents are determined through system studies conducted by the supplier, which consider factors such as energy duty in arresters, the number of parallel arrester columns, and the peak current in each arrester The peak current selected for the arresters corresponds to the coordination current, which is associated with the residual voltage that leads to the representative overvoltage for directly protected equipment The goal is to achieve an optimal balance between the specifications and design of the arresters and the voltage withstand requirements of HVDC converter equipment, all of which hinge on the selection of coordination currents.
For arrester testing purposes and protection levels assessment, standard shapes defined in IEC 60099-4 for switching, lightning and steep current impulse are applied to the co-ordination currents
In HVDC converter stations, it is crucial to assess the arrester coordination current for lightning stresses, particularly for outdoor valves, while taking into account the design of the station's shielding The maximum current resulting from shielding failure can be evaluated using methodologies outlined in references [11] or [14].
Arrester discharge currents during contingencies can vary in duration, necessitating careful consideration of both the amplitude and duration when specifying arrester energy capability Repetitive current impulses, occurring over multiple cycles of fundamental frequency, are treated as a single discharge with equivalent energy content and duration based on the accumulated values of actual energy impulses From a thermal stability perspective, these repetitive impulses should be evaluated over an extended timeframe Additionally, it is important to note that the energy withstand capability of metal-oxide arresters diminishes with shorter pulse durations, specifically those less than 200 µs.
When determining the capability of an arrester, it is essential to incorporate a calculated energy value that includes a reasonable safety factor, typically ranging from 0% to 20% This factor accounts for tolerances in the input data, the modeling approach employed, and the likelihood of critical fault sequences resulting in greater stresses than those analyzed in previous studies.
Arrester stresses
General
In a two-terminal bipolar HVDC scheme featuring a 12-pulse converter per pole, the typical arrester arrangement between the a.c side of the converter bridges and the d.c transmission circuit is illustrated in Figure 3 It's important to note that not all arresters may be necessary, as their usage depends on the overvoltage withstand capability of the connected equipment and the protection provided by other arresters at the same location For instance, the d.c bus can be safeguarded by utilizing a series combination of the bridge (B) and mid-point d.c bus (M) arresters, rather than relying solely on the converter unit d.c bus arrester (CB).
Protective arrangements can be applied to stations featuring two 12-pulse converters per pole or back-to-back stations In back-to-back configurations, only valve arresters (V) are typically required on the valve side due to the significantly lower operating voltage compared to line or cable transmission systems However, mid-point bus (M) or bridge (B) arresters may also be incorporated.
For HVDC converter stations connected directly to d.c cables, the d.c bus/line arresters (DB and DL) may be deleted since the pole may not be exposed to fast-front overvoltages
On the a.c side of the HVDC converter station, phase-to-earth arresters (A) are normally provided to protect the converter a.c bus and the a.c filter bus
Arresters are also normally connected across both a.c and d.c harmonic filter reactors or from the high-voltage terminals of the filter reactors to earth, as shown in Figure 3
In systems involving a combination of d.c cables and/or overhead lines, arresters may be needed at the cable terminations to protect them from overvoltages originating from the overhead line
The basic principles when selecting the arrester arrangement are that:
• overvoltages generated on the a.c side should, as far as practicable, be limited by arresters on the a.c side The main protection is given by the a.c bus arresters (A);
Overvoltages on the direct current (D.C.) line or earth electrode line must be effectively limited using D.C bus, D.C line/cable arresters (DB and DL/DC), converter bus arresters (CB), and neutral bus arresters (E).
To safeguard against overvoltages in HVDC converter stations, it is essential to install arresters close to critical components Valve arresters (V) should be used to protect thyristor valves, while a.c bus arresters (A) are necessary for the line side windings of transformers The valve side of transformers typically requires a series connection of arresters, including bridge arresters (B), mid-point arresters (M), and valve arresters (V) Additionally, measures must be taken to protect transformer valve windings in cases where the transformers may be disconnected from the bridges.
AC bus arrester (A)
The a.c side of an HVDC converter station is safeguarded by arresters positioned at the converter transformers and other strategic locations, as illustrated in Figure 3 These arresters are specifically engineered for a.c applications to effectively limit overvoltages on both the line side and the valve side of the converter transformers They account for overvoltages that may be transferred from the line side to the valve side through inductive and stray capacitance coupling.
The substantial size of shunt capacitors and filter banks can restrict the duty experienced by arresters from switching and lightning overvoltages in the a.c system Nevertheless, significant energy duty may arise from the discharges of the charged shunt reactive banks.
These arresters are engineered to handle extreme fault clearing scenarios and subsequent recovery, addressing issues such as transformer saturation overvoltages, overvoltages from load rejection, and potential restrikes of circuit breakers during the opening process.
Because of possible saturation overvoltages of high amplitude and long duration, this arrester may need to be designed for high energy duty
It is essential to coordinate A arresters with existing a.c arresters located near the commutating bus Due to the station layout, long separation distances may require the installation of a.c bus arresters at multiple locations.
Using arresters to limit temporary overvoltages during load rejection in weak a.c systems, particularly under low order resonance conditions, necessitates high energy duty, which may require multiple columns for effective performance.
AC filter arrester (FA)
The a.c filter reactors and resistors may be protected by a.c filter arresters
The continuous operating voltage of an a.c filter arrester includes a power frequency voltage along with superimposed harmonic voltages that align with the filter branch's resonance frequencies The ratings for these arresters are typically based on transient events, and it is essential to account for the harmonic voltages, as they can lead to significant power losses in the arrester, influencing its overall rating.
When evaluating filter arrester duties, it is essential to consider slow-front and temporary overvoltages on the a.c bus, as well as the discharge of filter capacitors during earth faults on the filter bus The slow-front overvoltages determine the required SIPL, while the capacitor discharge influences the LIPL and energy discharge requirements Additionally, high energy discharge duties may arise from low order harmonic resonance or non-characteristic harmonics generated by unbalanced operations during a.c system faults.
The arrester energy duties must be determined by the highest values among the following criteria: a) the maximum fundamental frequency phase-to-earth voltage charged in filter capacitors, b) the a.c bus charged to the switching surge protective level before a fault occurs, and c) temporary overvoltages that may arise during load rejection in weak a.c system conditions, particularly under low-order resonance scenarios affecting low-order harmonic filters.
Transformer valve winding arresters (T)
Valve arresters, when used alongside other arresters, effectively protect transformer valve windings Typically, phase-earth arresters are not included on the valve side of the converter transformer (T) for HVDC schemes operating at voltages up to 600 kV.
At voltages of 800 kV and above, incorporating phase-earth arresters linked to the valve winding of the top 6-pulse transformer can help lower the phase-earth insulation requirements for this winding.
Valve arrester (V)
Valve arresters (V) are installed, close to the valves, in parallel with each valve
The valve arrester is crucial for safeguarding thyristor valves against excessive overvoltages, serving as a key component in their overvoltage protection Additionally, protective firing of thyristors in the forward direction enhances this protection Given that the cost and power losses of the valves are closely linked to the insulation level, it is vital to maintain a low insulation level and, consequently, a low arrester protective level.
The continuous operating voltage of the valve arrester is characterized by sine wave sections that include commutation overshoots and notches, as illustrated in Figure 5 Ignoring the commutation overshoots, the crest value of the continuous operating voltage (CCOV) is directly proportional to \( U di0m \) and is defined according to section 7.2 as \( v0 \).
When determining the reference voltage of the arrester, it is essential to consider the peak continuous operating voltage (PCOV), which accounts for the commutation overshoot This overshoot is influenced by the firing angle α, necessitating careful attention to operations involving large firing angles.
For normal firing angles (alpha and gamma) typical values of commutation overshoot range between 15 % to 25 % of the CCOV for a duration of 100 às to 300 às
8.3.5.3 Temporary and slow-front overvoltages
Maximum temporary overvoltages typically occur on the a.c side during fault clearances combined with load rejections near the HVDC converter station It is important to consider only contingencies that involve either no blocking or partial blocking of the converters, as valve arresters experience reduced stress when the valve is blocked and the by-pass pair is extinguished.
Significant valve arrester currents of a switching nature can arise from several events: a) an earth fault occurring between the converter transformer and the valve within the commutating group at the highest potential; b) the clearing of an AC fault near the HVDC converter station; and c) the extinction of current in a single commutating group, if relevant.
8.3.5.3.2 Earth fault between the converter transformer and the valve
A phase-to-earth fault on the valve side of a converter transformer at the highest d.c potential imposes significant stresses on the valve arresters in the upper commutation group The resulting discharges consist of two current peaks: the first is a steep-front surge from the discharge of stray and damping capacitances, affecting the valve connected to the faulty phase The second discharge, characterized by a slow-front overvoltage lasting approximately 1 ms to crest, occurs through the d.c reactor and transformer leakage reactance, potentially stressing arresters connected to the other phases with high current and energy Key parameters, including d.c voltage at the fault moment, d.c reactor inductance, transformer leakage inductance, and line/cable characteristics, influence which of the three upper arresters experiences the most stress and the magnitude of that stress In d.c schemes with parallel-connected converters, this fault scenario leads to additional stresses as the unfaulted converter continues to supply current to the earth fault until protection systems trip the converters, making this fault case critical for the energy and current rating of the arresters across the upper three valves.
In the case of a phase-to-earth fault, the calculated stresses are significantly influenced by the d.c bus voltage level It is advisable to utilize the maximum d.c voltage that can be sustained for several seconds Additionally, this scenario may necessitate an arrester with a high energy discharge capability Ultimately, the decision should take into account the likelihood of voltages exceeding the maximum operating voltage in conjunction with an earth fault.
Excessive overvoltages in the a.c network occur primarily when converters are blocked during fault clearing If the converters remain operational after a fault, they help dampen these overvoltages, resulting in significantly reduced total discharge energy The scenario that typically leads to maximum arrester energy involves the converter being permanently blocked with by-pass pairs This blocking may also indicate that the converter transformer breakers are opened a few cycles later.
After a fault is cleared, the arresters remain unaffected by any operating voltage When calculating the transferred overvoltages from the line side, it is essential to use a realistic tap changer position for the relevant load flow Unfavorable system conditions may lead to ferroresonance involving the a.c filter or shunt capacitor, the converter transformer, and the a.c network impedance To account for variations in transformer saturation, it is important to vary both the fault inception and the fault clearance instants.
A simultaneous extinction in all three valves of a commutating group can significantly impact the energy rating of the arrester, even while the series-connected valves continue to conduct current This situation forces the current to shift to one of the parallel arresters linked to the non-conducting valves If the current is not rapidly diminished to zero, the energy dissipated in this arrester can be considerable.
Possible contingencies leading to the extinction of current in a single commutating group of valves include firing failures in a valve, often caused by issues in the valve control unit, and the blocking of all valves in a converter without the firing of the by-pass pairs Such scenarios can result in the converter current dropping close to zero during transient conditions, ultimately extinguishing current in one of the series-connected commutating groups This situation is particularly critical during inverter operation.
The consideration of current extinction as inconceivable leads to its exclusion from discussion The perception of whether current extinction is conceivable hinges significantly on the level of redundancy and the nature of the control or protection systems in place.
8.3.5.4 Fast-front and steep-front overvoltages
The valves and valve arresters in the converter area are isolated from the a.c and d.c switchyards by large series reactances, including converter transformers and smoothing reactors These components help attenuate travelling waves from lightning strikes, reducing their magnitude to resemble slow-front overvoltages However, in systems with large transformer ratios, such as back-to-back stations, capacitive coupling may become significant Generally, the valve and valve arresters are only exposed to fast-front and steep-fronted overvoltages due to back-flashovers and earth faults within the converter area Direct lightning strikes are a concern only if they bypass the shielding system, but such occurrences can often be mitigated in high-voltage HVDC converter stations equipped with effective shielding and earthing systems.
Steep-front overvoltages are most critical during an earth fault on the valve side of the converter transformer associated with the highest d.c potential A detailed circuit model, including stray capacitances and bus inductances, is utilized to estimate this scenario effectively.
In designing thyristor valves, it is crucial to account for the scenario where the valve experiences a forward overvoltage and is fired during this condition, leading to the immediate transfer of arrester current from the arrester to the valve It is important to note that the arrester current relevant for this commutation does not necessarily align with the specified coordination current for the valve arrester, which typically pertains to reverse overvoltage Instead, for forward overvoltage situations, a coordination current of switching character, corresponding to the protective firing level across the valve, is sufficient Additionally, when estimating the arrester current, one must consider the tolerances in arrester characteristics and redundant thyristors.
Bridge arrester (B)
A bridge arrester can be installed between the d.c terminals of a six-pulse bridge, offering essential protection These arresters can be positioned across both the lower and upper six-pulse bridges The combination of the upper bridge arrester and the mid-point arrester safeguards the d.c bus from potential earth faults.
The continuous operating voltage (CCOV) remains consistent with the valve arrester as outlined in section 8.3.5.2, excluding commutation overshoots However, when determining the reference voltage of the arrester, it is essential to consider the peak continuous operating voltage (PCOV), which accounts for these overshoots The magnitude of the commutation overshoot is influenced by the firing angle α, necessitating careful attention during operations involving large firing angles.
Events that can generate switching impulse type arrester currents include the clearing of an AC fault near the HVDC converter station and the extinction of current in the associated six-pulse bridge, if relevant.
The switching overvoltages transferred from the a.c side normally result in low arrester currents since the bridge arrester is then connected in parallel with a valve arrester.
Converter unit arrester (C)
A converter unit arrester may be connected between the d.c terminals of a 12-pulse bridge, arrester C in Figure 3
The maximum operating voltage is composed of the maximum direct voltage from one converter unit plus the 12-pulse ripple
The theoretical maximum operating voltage for zero values of the firing delay and overlap angles is given by the following expression:
In practice the CCOV is smaller and can be estimated during the preliminary design stage using the following equation:
Digital simulations can be used to determine the CCOV under possible steady state operating conditions
The commutation overshoots should be considered in the same way as for the valve arrester when the arrester is specified
Converter unit arresters typically do not encounter high discharge currents from switching activities However, in series-connected converters, the creation of a by-pass pair during valve group blocking or unintentional closing of the by-pass switch can put stress on the arrester While the arrester can mitigate overvoltages caused by lightning that affect the valve area, these lightning stresses are not the primary concern for the arrester's performance.
Mid-point d.c bus arrester (M)
A mid-point d.c bus arrester is utilized to lower the insulation level of the upper converter transformers in a 12-pulse converter system This arrester is typically connected from the mid-point of the 12-pulse converter to the ground, as illustrated by arrester M in Figure 3 and MH and ML in Figure 1.
The mid-point arrester CCOV is determined by adding the valve arrester CCOV to an offset that accounts for the voltage drop in the return path during inverter operation It is essential to consider commutation overshoots similarly to how they are addressed for the valve arrester when specifying this arrester.
Significant arrester stresses of a switching nature can occur during current extinction in the lower six-pulse bridge Additionally, the operation of bypass switches can induce stresses in series-connected converter units Furthermore, shielding failures may lead to lightning stresses.
Converter unit d.c bus arrester (CB)
A d.c bus arrester can be installed between the bus and earth to safeguard equipment linked to the high voltage d.c pole on the converter side of the smoothing reactor.
The operating voltage for the inverter is comparable to that of the converter unit arrester, taking into account the voltage drop in the earth electrode line during inverter operation.
The arrester typically remains shielded from high discharge currents caused by slow-front overvoltages due to its elevated protective level However, moderate lightning stresses can occur as a result of shielding failures.
DC bus and d.c line/cable arrester (DB and DL/DC)
The d.c bus arrester DB is essential for safeguarding d.c switchyard equipment connected to the d.c pole To ensure comprehensive protection across various station areas, multiple arresters may be necessary, with the arrester at the line entrance identified as the d.c line (cable) arrester DL (DC) In HVDC transmission systems that include both overhead lines and cable sections, it is crucial to install surge arrester DC at the junction between the cable and overhead line to mitigate the risk of excessive overvoltages on the cable.
In HVDC converter stations with direct connections between the d.c cable and the converter indoor bus, the use of d.c bus/cable arresters (DB and DC) is often unnecessary, as the pole is typically shielded from fast-front overvoltages.
The maximum operating voltage is almost a pure d.c voltage with a magnitude dependent on the converter and tap-changer control and possible measurement errors
Lightning stresses primarily affect these arresters By carefully selecting parameters in the main circuit, critical slow-front overvoltages can often be mitigated, preventing harmful resonances In a bipolar overhead d.c line, a pole-to-earth fault on one pole induces an overvoltage on the healthy pole, with the overvoltage magnitude influenced by the fault's location, line length, and termination impedance Typically, these overvoltages do not pose a significant risk to terminal insulation.
High switching surge overvoltages may arise at the converter terminal opposite a faulted cable junction, particularly when the cable length is short.
In HVDC transmission systems utilizing long cables, the energy rating of cable arresters is determined by the maximum voltage the cable may reach during a contingency This typically leads to lower discharge currents, but can result in significant energy discharge through the arresters Key contingencies to consider include valve misfires and the total loss of firing pulses at one of the stations, which can initiate the rectifier against an open or blocked inverter.
In a line/cable junction, the lightning stresses on DC cable arresters are minimal due to the cable's low surge impedance, provided that the overhead line is well-shielded and the towers maintain low footing resistance for several spans from the junction.
Neutral bus arrester (E, EL, EM in Figure 3, EB, E1, EL, EM in Figure 1)
The neutral bus arrester safeguards the neutral bus and connected equipment, and when used alongside valve arresters, it can also protect the bottom converter transformers To ensure comprehensive protection across various sections of the station, it may be necessary to install arresters at multiple locations, depending on the separation distance between the arresters and the protection point.
The normal operating voltage of the arrester EB (with a smoothing reactor on the neutral line), would consist of ripple voltages and could be substantial
For the rest of the neutral bus arresters E1, EL, EM the operating voltages are normally low At balanced bipolar operation they will be practically zero
However, during monopolar or metallic return operation the operating voltages on all these arresters EB, E1, EL and EM increase by the d.c offset
These arresters are provided to protect equipment from fast-front overvoltages entering the neutral bus and from the overvoltages described below
Arresters must be designed to handle significant energy discharges during earth faults on the d.c bus or line, particularly between the valves and the converter transformer In monopolar operation, a loss of return path can lead to excessive energy ratings, making sacrificial arresters a preferable option An earth fault on the d.c bus may cause the d.c filter to discharge through the neutral bus arrester, resulting in a high but brief current peak, influenced by the configuration of the d.c filter and arrester The critical assumption is that the pre-fault voltage of the filter is typically set to the maximum operating d.c voltage Following the rapid discharge of the d.c filter, a slower fault current from the converter occurs, with the rate of rise primarily constrained by the d.c reactor The fault current is distributed between the earth electrode line and the neutral bus arrester, while in metallic return operation, the impedance in parallel with the arrester encompasses the entire d.c line impedance.
In the event of an earth fault occurring on a phase between the valve and the converter transformer, the alternating current (a.c.) driving voltage is divided between the converter transformer impedance and the earth electrode line impedance The most critical scenario arises at the terminal with the longest earth electrode line, particularly in metallic return operation at the unearthed terminal The most severe conditions are observed when the station functions as a rectifier, due to the polarity of the driving voltage.
In metallic return operations, the neutral bus arrester faces high demands, making it beneficial to choose a higher arrester rating for unearthed stations compared to earthed ones This consideration is particularly relevant for long electrode lines, typically those exceeding 50 km in distance.
Recent schemes have incorporated neutral bus capacitors primarily to meet harmonic filtering requirements and to mitigate overvoltages on the neutral bus However, their presence will affect the stresses on the neutral bus arrester, necessitating their inclusion in the study model Additionally, the stresses on the neutral bus arrester are influenced by the converter control and the protective measures implemented during faults.
In cases where energy ratings indicate an overly complex design under rare circumstances, a sacrificial arrester may be utilized This approach is especially favored when replacing the arrester does not greatly affect outage duration Additionally, in bipolar systems, it is essential to position sacrificial arresters to prevent bipolar outages.
When incorporating a smoothing reactor in the neutral bus, it is crucial to carefully coordinate the reference voltages and energy requirements of the neutral arresters (EB, E1, EM, EL) Additionally, if a neutral blocking filter is present, it must also be factored into the coordination of the arresters.
DC reactor arrester (DR)
The DR arrester provides terminal-to-terminal protection for the smoothing reactor
The smoothing reactor serves as a protective buffer between the direct current (d.c.) line and the converter station, effectively managing lightning surges from the d.c pole To maintain its protective function, it is essential to keep the insulation level of the arrester and smoothing reactor as high as possible, ensuring optimal performance and safety.
The operating voltage of the d.c reactor arrester consists only of a small 12-pulse ripple voltage from the converter
The arrester will experience lightning overvoltages that are of opposite polarity to the operating voltage of the converter d.c bus, known as subtractive lightning impulses It is essential to consider the potential for lightning stresses to be transmitted through the arrester to the thyristor bridge.
In certain configurations, the d.c reactor arrester may be eliminated if the insulation level of the reactor satisfies the voltage requirements set by the d.c line arrester, along with the maximum operating voltage of the opposing polarity.
DC filter arrester (FD)
The d.c filter reactors and resistors are protected by the d.c filter arresters FD
The d.c filter reactor arrester operates at a low normal voltage, typically comprising one or more harmonic voltages that align with the resonance frequency of the specific filter branch Due to the significant power losses associated with these harmonic voltages, it is essential to factor them into the rating of the arresters.
Arrester duties are mainly determined by filter capacitor discharge transients resulting from earth faults on the d.c pole, and occasionally due to lightning surges.
Earth electrode station arrester
To protect the equipment at the earth electrode station, such as distribution switches, cables, and measuring devices, it is essential to guard against overvoltages that may enter through the earth electrode line Installing an arrester at the line entrance can effectively mitigate these risks The continuous operating voltage is minimal, while the arrester is specifically designed to handle lightning stresses that may come through the overhead line.