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ADVANCED CONVERTER BASED FAULT-TOLERANT NETWORK IN HVDC

TRANSMISSION SYSTEM

C. Narachandrasekhar PG Student [EPS], Dept. of EEE,

SITS, KADAPA, Andhra Pradesh, India

Seetha. Chaithanya Assistant Professor, Dept. of EEE,

SITS, KADAPA, Andhra Pradesh, India

G. Venkata Suresh Babu Associate Professor, HOD,Dept. of EEE,

SITS, KADAPA, Andhra Pradesh, India

ABSTRACT:-This paper proposes a brand new kind of high-voltage dc (HVDC) transmission systems supported on a hybrid multilevel voltage source converter (VSC) with ac-side cascaded H-bridge cells. The projected HVDC system offers the operational flexibility of VSC based systems interms of active and reactive power management, black start capability, additionally to improved ac fault ride-through capability and also the distinctive feature of current-limiting capability throughout dc aspect faults. In addition, it offers options like smaller footprint and a bigger active and reactive power capability curve than existing VSC-based HVDC systems, as well as those using modular construction converters. May be the feasibleness of the projected HVDC system, this paper assesses its dynamic performance throughout steady-state and network alterations, as well as its response to ac and dc aspect faults. Index Terms—DC fault reverse blocking capability, hybrid multilevel converter with ac side cascaded H-bride cells, modular multilevel converter, voltage-source-converter high-voltage dc (VSCHVDC) transmission system..

INTRODUCTION

In the last decade, voltage-source-converter high-voltage dc (VSC-HVDC) transmission systems have evolved from simple two-level converters to neutral-point clamped converters so to true structure converters like standard converters. This evolution aimed to lower semiconductor losses and increase power- handling capability of VSC-HVDC transmission systems to the extent comparable to that of conventional HVDC systems supported thyristor current-source converters, improved ac side wave form quality so as to attenuate or eliminate ac filters, reduced voltage stresses on convertor transformers, and reduced convertor overall value and footprint. With increased demand for clean energy, facility networks got to be reengineered to be additional economical and versatile and strengthened to accommodate increased penetration of renewable power while not compromising system operation and responsibility.

A VSC-HVDC transmission may be a candidate to satisfy these challenges because of its operational flexibility, like provision of voltage support to ac networks, its ability to work freelance of ac network strength so makes it appropriate for affiliation of weak ac networks like offshore wind farms, suitableness for multiterminal HVDC network realization as active power reversal is achieved without dc link voltage polarity amendment, and resiliency to ac side faults (no risk of commutation failure like line-commutating HVDC systems). However, vulnerability to dc side faults and absence of reliable dc circuit breakers capable of operational at high-voltage prohibit their application to point-to-point connection.

A present VSC-HVDC transmission system depends on their converter station control systems and effective resistivity between the point-of-common-coupling (PCC) and therefore the device terminals to ride-through dc side faults. With present converter technology, the dc fault current includes the ac networks contribution through device free-wheeling diodes and discharge currents of the dc side capacitors (dc link and cable distributed capacitors). The magnitude of the dc-side capacitors’ discharge current decays with time and is larger than the ac networks contribution. For this reason, dc fault interruption may need dc circuit breakers that may tolerate high let-through current which will flowing the dc side throughout the first few cycles once the fault, with high current breaking capability and quick interruption time. Recent HVDC converter topologies with no common dc link capacitors, like the modular multilevel converter (M2C), could minimize the magnitude and period of the discharge current first peak.

There are two approaches to help VSC-HVDC transmission systems to ride-through dc side faults. The primary approach is to use a quick acting dc electrical fuse, with significantly high let-through current to tolerate the high dc fault discharge current that will flow within the dc side. This breaker should be capable of operational at high voltage and isolates temporary or permanent dc faults, and have a comparatively high-current-breaking capability. Reference presents a prototype 80-kV dc electrical fuse with dc current breaking capability of 9 kA within 2ms.However, this beginning is insufficient, because the operational

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voltage of present VSC-HVDC transmission systems reach 640 kV pole-to-pole (or 320 kV for a bi-polar configuration), with power-handling capability of 1 GW. This breaker approach might introduce vital steady-state losses attributable to the semiconductors within the main current path.

Fig.1. Hybrid voltage multilevel converter with ac side

cascaded H-bridge cells

The second approach is to use converter stations with dc fault reverse-blocking capability. every device station should be able to block current flow between the ac and dc sides throughout a dc fault, permitting dc-side capacitor discharge current, is the is that the major element of the dc fault current, to decay to zero and so isolate the fault. Many device topologies with this inherent feature are planned, as well as an H-bridge modular multilevel device, an alternate arm modular multilevel device, and a hybrid multilevel device with ac-side cascaded H-bridge cells. However, the disadvantage is that the active power exchange between the ac networks reduces to zero throughout the dc fault amount.

Commensurate with the second approach, this paper presents a replacement HVDC transmission systems supported a hybrid-voltage-source multilevel convertor with ac-side cascaded H-bridge cells. The adopted converter has inherent dc fault reverse-blocking capability, which might be exploited to improve VSC-HVDC resiliency to dc side faults. With coordination between the HVDC converter station control functions, the dc fault reverse-blocking capability of the hybrid convertor is exploited to attain the following: • eliminate the ac grid contribution to the dc fault, thus minimizing the danger of convertor failure owing to uncontrolled over current throughout dc faults; • facilitate controlled recovery without interruption of the VSC-HVDC system from dc-side faults without the necessity for opening ac-side circuit breakers; • alter dc fuse style owing to a reduction within the magnitude and period of the dc fault current; and • improve voltage stability of the ac networks as converter reactive power consumption is reduced throughout dc-side faults.

Section II of this paper describes the operational

principle and management of the hybrid voltage source multilevel converter with ac-side cascaded H-bridge cells.

Section III describes the HVDC system management style, specifically, ac current controller in synchronous system, dc link voltage, and active power, and ac voltage controllers. A detailed diagram that summarizes however completely different management layers of the projected HVDC transmission system area unit interfaced is conferred.

Section IV presents simulations of a hybrid convertor HVDC transmission system, that demonstrate its response throughout steady-steady and network disturbances enclosed are simulations of four quadrant operation, voltage support capability, and ac and dc fault ride-through capabilities.

II. HYBRID MULTILEVEL VSC WITH AC-SIDE

CASCADED H-BRIDGE CELLS

Fig. 1 shows one section of a hybrid multilevel VSC with H-bridge cells per phase. It will generate voltage levels at device terminal “a” relative to provide midpoint “0.” so, with a large variety of cells per phase, the device presents close to pure sinusoidal voltage to the device transformer as represented in Fig. 1. The two-level converter that blocks high-voltage controls the basic voltage using selective harmonic elimination (SHE) with one notch per quarter cycle, as shown in Fig. 1. Therefore, the two-level converter devices operate with 150-Hz switch losses, hence low switch losses and audible noise are expected. The H-bridge cells between “M” and “a” are operated as a series active power filter to attenuate the voltage harmonics made by the two-level converter bridge. These H-bridge cells are controlled using level-shifted carrier-based multilevel pulse width modulation with a 1-kHz switch frequency. to reduce the conversion losses within the H-bridge cells, the number of cells is reduced such that the voltage across the H-bridge floating capacitors add to .

This might lead to a tiny converter device station, as a result of the quantity of H-bridge cells needed per device with the projected HVDC system is one quarter of these needed for a system supported the modular based on device. With a large variety of cells per section, the voltage wave generated across the H-bridge cells is as shown in above fig and an efficient switch frequency per device of but 150 Hz is feasible. The dc fault reverse-blocking capability of the projected HVDC system is achieved by inhibiting the

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gate signals to the device switches, so no direct path exists between the ac and dc side

through freewheel diodes, and cell capacitor voltages can oppose any current flow from one side to another. Consequently, with no current flows, there's no active and reactive power exchange between ac and dc side throughout dc-side faults. This dc fault side means that transformer coupled H-bridges can't be used. The ac grid contribution to dc-side fault current is eliminated, reducing the danger of device failure as a result of exaggerated current stresses within the switch devices throughout dc-side faults. From the grid point of view, the dc fault reverse-blocking capability of the projected HVDC system might improve ac network voltage stability, because the reactive power demand at device stations throughout dc-side faults is considerably reduced. The ac networks see the nodes wherever the device stations are connected as open circuit nodes throughout the complete dc fault amount. However, operation of the hybrid multilevel VSC needs a voltage-balancing theme that ensures that the voltages across the H-bridge cells are maintained at below all operational conditions, wherever is that the total dc link voltage. The H-bridge cells voltage balancing theme is complete by rotating the H-bridge cell capacitors, taking into consideration the voltage magnitude of every cell capacitor and phase current polarity. a further PI regulator is employed to confirm that the cell capacitors be maintained at as delineate in Fig. 2(b) (inner control layer).

Fig. 2. (a) Representation of VSC station and (b)

schematic diagram summarizing the control layer of the hybrid multilevel converter with ac side cascaded

III. CONTROL SYSTEMS

A HVDC transmission supported a hybrid multilevel VSC with ac-side cascaded H-bridge cells needs 3 three layers. The inner management layer represents the modulator and capacitance voltage-balancing mechanism that generates the gating signals for the convertor switches and maintains voltage balance of the H-bridge cell capacitors. The intermediate management layer represents the current controller that regulates the active and reactive current parts over the complete operative vary and restraints convertor station current injection into ac network throughout network disturbances like ac and dc side faults. The outer management layer is that the dc voltage (or active power) and ac voltage (or reactive power) controller that offer set points to this controllers. The inner controller has solely been mentioned to tier acceptable to power systems engineers. The intermediate and outer management layers are given very well to administer the reader a way of HVDC system quality. This, power, and dc link voltage controller gains are chosen using root locus analysis, supported the applicable transfer functions. a number of the controller gains obtained using root locus analysis offer smart performance in steady state however didn't offer acceptable network disturbance performance. Therefore, the simulation final gains used are adjusted within the time domain to produce satisfactory performance over a large operative vary, together with ac and dc side faults. Fig. 2 summarizes the management layers of the hybrid multilevel VSC.

TABLE I

TRANSMISSION PARAMETERS

TABLE II CONVERTER STATION PARAMETERS

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TABLE III CONVERTER TRANSFORMER PARAMETERS

However, the gains for the ac voltage controllers are

obtained using a trial and-error search method that automatically runs the overall system simulation several times in an attempt to find the gains that produce the best time domain performance. The gains for all of the controllers and test network parameters used in this paper are listed in Tables I–III.

IV. PERFORMANCE EVALUATION

The viability of the VSC-HVDC system that uses a

hybrid multilevel VSC with ac-side cascaded H-bridge cells is investigated here, with stress on its dynamic performance throughout network alterations. Within the steady state, the test network in Fig. 3(a) is employed to assess its power management and voltage support capabilities. To any illustrate the benefits of structure device throughout ac and dc network disturbances, an equivalent test network is subjected to a three-phase ac-side fault and a pole-to-pole dc-side fault at locations delineate in Fig. 3(a), each for a 140-ms period. device stations 1 and a couple of in Fig. 3(a) are described by careful hybrid VSC models with seven cells per part, with the controllers in Fig. 2(b) incorporated. Seven cells per arm are utilized in this paper so as to realize acceptable simulation times without compromising result accuracy, as every system

element is described in careful. Also, the hybrid device with seven H-bridge cells per part generates 29 voltage levels per part, is the is that the same because the two-switch standard structure device with 28 cells per arm, for an equivalent dc link voltage such devices in each converters expertise an equivalent voltage stresses. The converters ar designed to control active power exchange and dc link voltage, and ac voltage magnitudes at and respectively. The test system in Fig. 3(a) is simulated within the MATLAB Simulink atmosphere.

A. Four-Quadrant Operation and Voltage Support

To demonstrate four quadrant operation and

voltage support capability of the bestowed VSC-HVDC system, device station one is commanded to extend its output power export from grid to from 0 to 0.5 pu (343.5 MW) at 2.5 pu/s. At time 1 s it's commanded to reverse the active power flow so as to import 343.5 MW from grid, at 2.5 pu/s. At a load of is introduced to, illustrating the voltage support capability of device station 2 throughout network alteration.

Fig. 3(b) and (c) show converters 1 and 2 active and reactive power exchange with and severally. The converters are ready to regulate their reactive power exchange with and so as to support the voltage throughout the complete operative amount. Fig. 3(c) and (d) show that device 2 adjusts its reactive power exchange with once the load is introduced at 2 s to support the voltage magnitude. Fig. 3(e) and (f) show that device 2 injects and presents high-quality current and voltage waveforms into with no ac filters installed). Fig. 3(g) demonstrates that the voltage stresses across the H-bridge cell capacitors of converter 1 are controlled to the required point throughout the complete amount. Fig. 3(h) displays the whole dc link voltage across converter 2, that regulates the dc link voltage. supported these results, the planned VSC-HVDC system is ready to satisfy basic steady-state necessities, like provision of voltage support and 4 quadrant operation while not compromising the voltage and current stresses on the converters switches. B. AC Network Faults

To demonstrate the ac fault ride-through capability

of the bestowed HVDC system, the test network is subjected to a 140 ms three-phase fault to ground at the situation shown in Fig. 3(a). Throughout the fault

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amount the facility command to converter 1 is reduced in proportion to the reduction within the ac voltage magnitude (this is achieved by sensing voltage). this is to attenuate the two-level device dc link voltage rise due to the trapped energy within the dc side, since power can't be transferred because the voltage at collapses.

Fig. 4 displays the results once the test network

exports 0.5 pu (343.5 MW) from grid to and is subjected to the three-phase fault at. Fig. 4(a) shows the active and reactive powers device 1 exchanges with. Note that device 1 matches its active power export to so as to attenuate the increase of device 2 dc link voltage as its ability to inject active power into grid reduces with the voltage collapse at , as shown in Fig. 4(d) and stated above. Fig. 4(b) shows the active and reactive powers that device 2 injects into. The system is ready to recover as shortly because the fault is cleared, and device 2

Fig. 3.a. Test network and waveforms demonstrating the steady-state operation of HVDC system based on hybrid voltage source multilevel converter with ac side cascaded

H-bridge cells.

3(b) Active and reactive power converter station 1 exchange

with pcc1

3(c) Active and reactive power converter station 2 exchanges with pcc2

3(d )Voltage magnitude at pcc2

3(e) Voltage waveforms at pcc2

3(f) Current waveforms converter station 1 exchanges with

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pcc1

3(g) Voltage across 21 cell capacitors of the three phases of converter 1

3(h) Voltage across the dc link of converter station 2 adjusts its reactive power exchange with grid in order to support voltage. The transients shown of active and reactive powers at PCC2 are related to the reaction of

the ac voltage controller that regulates the ac voltage at Fig. 4(c) shows that the voltage magnitude at remains unaffected confirming that the hybrid voltage source multilevel converter does not compromise the HVDC

transmission system’s decoupling feature despite adopting active power matching at converter 1, as

explained

4(a) Active and reactive power converter 1 exchanges with

pcc1

4(b) Active and reactive power converter 2 injects into pcc2

4(c) Voltage magnitude at pcc1

4(d) Voltage magnitude at pcc2

4(e) Current waveforms converter 2 injects into pcc2

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4(f) Converter 2 dc link voltage

4(g) Voltage across 21 H-bridge cells of the converter 2

4(h) Line-to-line voltage waveform at the terminal of converter 1

Fig. 4(e) shows that converter 2 restrains its contribution to the fault current to less than full load current despite the voltage at collapsing to 20% of its rated voltage, due to converter 2’s current controller. Fig. 4(f) shows that coordination of the HVDC controllers, as illustrated, minimizes the impact of ac-side faults on the transient. Results in 4(i) – 4(o) demonstrate the case when the converter stations operate close to their maximum active power capabilities (power command at converter 1 is set to 0.75 pu, which is 515 MW) and system is subjected to a three-phase fault with a 300-ms duration.

4(i) Active and reactive power at PCC1

4(j) Active and reactive power at PCC2

4(k)Voltage magnitude at PCC1

4(l) Voltage magnitude at PCC2

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4(m) Current waveforms converter 2 injects into PCC2

4(n) Converter 2 dc link voltage

4(o) Voltage across the 21 H-bridge cell capacitors of converter 2

C. DC Network Faults

The inherent current-limiting capability of the hybrid structure VSC with ac-side cascaded H-bridge cells that allows the VSC-HVDC system to ride-through dc-side faults are going to be incontestable here. The check network is subjected to a 140 ms solid pole-to-pole dc-side fault at the placement indicated in Fig. 3(a). throughout the dc-side fault amount, active power exchange between the 2 grids and is reduced to zero. This facilitates uninterruptable system recovery from the temporary dc fault with least inrush current, since the facility ways between the convertor’s ac and

dc sides are blocked (by inhibiting all converter gate signals) to eliminate a grid contribution to the dc fault. Fig. five shows the results once the check network is subjected to a brief solid pole-to-pole dc fault at the center of the dc link. Fig. 5(a) and (b) shows the active and reactive powers that convertor stations 1 and 2 exchanges with pcc1 and pcc2. Observe zero active and reactive power exchange between the convertor stations and ac grids G1 and G2 throughout the fault period; therefore there's no current flow within the switches of converters 1 and 2. However, an outsized surge in active and reactive power is determined once the gating signals to converters 1 and 2 are rebuilt once the fault is cleared, so as to restart the system. Fig. 5(c) and (d) shows that the present surge practiced by each convertor stations causes noticeable voltage dipping at and owing to increased consumption of reactive power throughout system start-up and dc link voltage build-up following fault clearance. The surge in active and reactive powers in each convertor stations happens because the dc side capacitors try and charge from each ac sides; this causes an outsized current flow from each ac sides to the dc side to charge the dc link capacitors and cable distributed capacitors as shown in Fig. 5(e) and 5(f). The ends up in Fig. 5(e) and 5(f) additionally demonstrate the advantages of dc fault reverse block capability inherent during this hybrid system, because the convertor switches expertise high current stresses solely throughout dc link voltage build-up. Fig. 5(g) shows that convertor 2 dc link voltage recovers to the pre-fault state once the fault. Notice the recovery amount for the dc link voltage is comparatively long; this can be the main disadvantage of the planned HVDC systems because it uses a typical dc link capacitance. Fig. 5(h) expands the dc fault current and shows the 60-kA peak decays to zero in but four cycles (for fifty Hz) once discharge of dc link and cable distributed capacitors. This result confirms the likelihood of eliminating dc circuit breakers to isolate permanent dc side faults in dc networks that use HVDC converters with current limiting capability. Fig. 5(h) additionally shows the ac grids begin to contribute to the dc link current once the fault is cleared, to charge the dc side capacitors. Fig. 5(i) and (j) shows the voltage across the twenty one H-bridge cells of the convertor stations 1 and 2 (each cluster of traces represent voltages across seven H-bridge cell capacitors in every phase). The voltage across the H-bridge cell capacitors remains unaffected throughout the whole fault amount because the converters are

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blocked. The cell capacitors begin to contribute energy to the most dc link capacitors throughout dc link voltage build-up once restoration of the convertor gating signals. This contribution creates an obvious reduction within the cell capacitance voltages throughout system restart. The cell capacitors of convertor 2 that regulate dc link voltage, expertise a bigger voltage dip than convertor one, that regulates active power. However, the reduction in H-bridge cell capacitance voltages is decreased if giant capacitance is employed. Fig.5. Waveforms demonstrating dc fault ride through capability of HVDC transmission systems supported hybrid voltage multilevel convertor with ac side cascaded H-bridge cells.

5(a) Active and reactive power converter 1 exchanges with

pcc1

5(b) Active and reactive power converter 2exchanges with pcc2

5(c) Voltage magnitude at pcc1

5(d) Voltage magnitude at pcc2

5(e) Current waveforms converter 1 exchange with grid G1at pcc1

5(f) Current waveforms converter 2 exchange with grid G2at pcc2

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5(g) Converter 2 dc link voltage

5(h) Zoomed version of dc link current demonstrating the benefits of dc fault reverse blocking capability

5(i) Voltage across the H-bridge cell capacitors of converter 1

5(j) Voltage across the H-bridge cell capacitors of converter 2

V. CONCLUSION

This paper conferred a brand new generation VSC-HVDC transmission system supported a hybrid multilevel converter with ac-side cascaded H-bridge cells. the most blessings of the planned HVDC system are: • Potential little footprint and lower semiconductor

losses compared to gift HVDC systems. • Low filtering necessities on the ac sides and presents high-quality voltage to the device electrical device. • doesn't compromise the benefits of VSC-HVDC systems like four-quadrant operation; voltage support capability; and black-start capability, that is important for association of weak ac networks with no generation and wind farms. • Standard style and device fault management (inclusion of redundant cells in every section might enable the system to work usually throughout failure of many H-bridge cells; wherefrom a cell bypass mechanism is required). • Resilient to ac aspect faults (symmetrical and asymmetrical). • inherent dc fault reverse obstruction capability that permits device stations to dam the ability ways between the ac and dc aspects throughout dc side faults (active power between ac and dc sides, and reactive power exchange between a device station and ac networks), hence eliminating any grid contribution to the dc fault current.

REFERENCES

[1] G. P. Adam et al., “Network fault tolerant voltage-source-converters for high-voltage applications,” in Proc. 9th IET Int. Conf. AC and DC Power Transmission, London, U.K., 2010, pp. 1–5. [2] Y. Zhang et al., “Voltage source converter in high voltage applications: Multilevel versus two-level converters,” in Proc. 9th IET Int. Conf. AC and DC Power Transmission, London, U.K., 2010, pp. 1–5. [3] G. P. Adam et al., “Modular multilevel inverter: Pulse width modulation and capacitor balancing technique,” IET Power Electron., vol. 3,pp. 702–715, 2010. [4] M. M. C. Merlin et al., “A new hybrid multi- level voltage-source converter with DC fault blocking capability,” in Proc. 9th IET Int. Conf.AC and DC Power Transmission, London, U.K., 2010, pp. 1–5. [5] V. Naumanen et al., “Mitigation of high -originated motor overvoltage’s in multilevel inverter drives,” IET Power Electron., vol. 3, pp. 681–689, 2010. [6] H. Abu-Rub et al., “Medium-voltage multilevel converters: State of the art, challenges, and requirements in industrial applications,” IEEE Trans. Ind. Electron., vol. 57, no. 8, pp. 2581–2596, Aug. 2010.

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[7] G. P. Adam et al., “Modular multilevel converter for medium-voltage applications,” in Proc. IEEE Int. Conf. Electr. Mach. Drives Conf., 2011, pp. 1013–1018. [8] G. P. Adam, S. J. Finney, A.M.Massoud, and B.W.Williams, “Capacitor balance issues of the diode-clamped multilevel inverter operated in a quasi-two-state mode,” IEEE Trans. Ind. Electron., vol. 55, no. 8, pp.3088–3099, Aug. 2008.

C.Narachandrasekhar has received the B.Tech (Electrical and Electronics Engineering) degree from Sri Venkateswara Institute of Science and Technology, Kadapa in 2012 and pursuing M.Tech (Electrical Power Systems) in Srinivasa Institute of Technology and

Science, Kadapa.

Seetha. Chaithanya has 4 years experience in teaching in both Graduate and Post graduate level and he is presently working as Assistant Professor in department of EEE in SITS, Kadapa.

G. Venkata Suresh Babu has 12 years experience in teaching in both Graduate and Post graduate level and he is presently working as Associate Professor and HOD, department of EEE in SITS, Kadapa.