Part A
Thyristor Based HVDC with Forced Commutation
Abstract—This paper presents a novel converter configuration, based solely on conventional thyristors and aimed for the use with High Voltage DC (HVDC) Transmission. The converter utilizes resonant turn off and forced commutation with
auxiliary thyristors to aid commutation in the converter switches.The PSCAD/EMTDC simulation confirms that the HVDC inverter is capable of operating with a wide range of firing angles, including operation with reactive power export. Further simulation proves that the system is immune to commutation failure even in the case of most severe close single-phase faults. The harmonic generation is comparable to the conventional converter and also the thyristor voltage stress is not significantly increased. The auxiliary thyristors do increase costs, but this is offset by the elimination of reactive power support and the improvements in performance.
Index Terms— HVDC transmission, HVDC converters, commutated circuits, reactive power.
I. INTRODUCTION
HVDC has found many applications in countries worldwide since the first installation in 1954. There are however well known limitations that have hampered further proliferation of conventional HVDC transmission[1] :
bull; The need for reactive power support and in particular variable reactive support which increases costs.
bull; Even modest (around 10%) voltage decrease at the inverter AC terminals will cause commutation failure.
Commutation failure causes a short-circuit on DC voltage and thus power transfer is interrupted.
bull; The inability to operate with weak inverter AC systems, which is indirectly caused by poor control capabilities at the inverter side and the above two issues.
Recently, HVDC transmission based on Voltage Source Converters has been introduced and some medium-power systems have been installed [2]. VSC Transmission has none of the above issues with conventional HVDC, however other notable limitations are present:
bull; The rating of the converters is limited, presently to around 360MW.
bull; The losses are noticeably higher than with conventional HVDC, caused primarily by the increased switching losses.
bull; The cost of this technology is high.
In parallel with the developments in VSC transmission,there has been a stream of research on improving conventional, thyristor based, HVDC. In particular the concept of Capacitor Commutated Converters (CCC) has shown promising results[3-7] . The researchers in[3] have concluded that only series capacitor configuration might be of practical importance for improving commutation in HVDC converters. The CCC technology was further investigated and improved in a series of projects [4-7] and recently it has been installed in a practical back-to-back HVDC at Garabi station Brasil.[8]. The CCC concept offers HVDC converters that could operate at an improved power factor and with reduced commutation failure probability. However the following issues with CCC.HVDC have been identified:
bull; The occurrence of ferromagnetic resonance[7].Research shows that this can be resolved using thyristor controlled capacitors or advanced feedback control[5,7], but at the increased costs.
bull; Typically CCC achieves around 10-15% reactive power consumption. In order to operate with a leading power factor, and to completely eliminate commutation failure, very large series capacitors would be needed. This increases voltage stress, harmonics and costs.
bull; The increased voltage stress on thyristors, which can be 2 p.u. or even 3p.u with very large capacitors.
bull; The insulation of series commutating capacitors.
bull; The harmonic generation is somewhat increased, and inparticular DC harmonics are significantly larger.
It is clear that there is incentive and the scope for further development of HVDC transmission and that the potential benefits are significant. Ideally, the research on HVDC converters would improve HVDC technology:
bull; To operate at higher firing angles enabling reactive power export.
bull;To eliminate commutation failure issues, if possible including most severe single-phase faults.
bull; Not to significantly increase harmonic pollution, or thyristor voltage stress or costs.
In this paper, a new HVDC converter that potentially meets the above goals is studied. The concept is based on combining forced commutation and the resonant turn off methods. The basic principles of these techniques are known since they have been used with some low power DC-DC choppers[9-11] , although such techniques have not been employed in high power, three-phase circuits. The paper will further give detailed analysis of operation and the method of calculating optimum parameters. A wide range of PSCAD/EMTDC simulation results will be presented to evaluate performance of the new scheme in terms of: reactive power exchange, commutation failure resilience, harmonic generation and stability aspects.
II. CIRCUIT DESCRIPTION
Figure 1 shows the proposed converter and the AC system at the inverter side of an HVDC. There are two six-pulse bridges in Y and transformer connections. Thyristors TY1-TY6 and T1-T6 are the main thyristors in the conventional Graetz connection. Each main thyristor has one auxiliary thyristor to aid commutation (TY1a-TY6a and T1a-T6a). There are also three commutating capacitors Cc and a single resonant capacitor Cs (per 6-pulse bridge).
The complete HVDC test system is based on the CIGRE HVDC Benchmark model and all parameters can be found in. This is a monopolar 12-pulse HVDC system with
1000MW, 500kV, 2000A rating, and with weak AC systems at both ends. It is desired to keep the test systems as close as possible to the original CIGRE model, to enable comparisons. Some minor modifications are made, however. Since the new converters enable zero reactive power excha
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Thyristor Based HVDC with Forced Commutation
Abstract—This paper presents a novel converter configuration, based solely on conventional thyristors and aimed for the use with High Voltage DC (HVDC) Transmission. The converter utilizes resonant turn off and forced commutation with auxiliary thyristors to aid commutation in the converter switches.The PSCAD/EMTDC simulation confirms that the HVDC inverter is capable of operating with a wide range of firing angles, including operation with reactive power export. Further simulation proves that the system is immune to commutation failure even in the case of most severe close single-phase faults. The harmonic generation is comparable to the conventional converter and also the thyristor voltage stress is not significantly increased. The auxiliary thyristors do increase costs, but this is offset by the elimination of reactive power support and the improvements in performance.
Index Terms— HVDC transmission, HVDC converters, commutated circuits, reactive power.
I. INTRODUCTION
HVDC has found many applications in countries worldwide since the first installation in 1954. There are however well known limitations that have hampered further proliferation of conventional HVDC transmission[1] :
bull; The need for reactive power support and in particular variable reactive support which increases costs.
bull; Even modest (around 10%) voltage decrease at the inverter AC terminals will cause commutation failure.
Commutation failure causes a short-circuit on DC voltage and thus power transfer is interrupted.
bull; The inability to operate with weak inverter AC systems, which is indirectly caused by poor control capabilities at the inverter side and the above two issues.
Recently, HVDC transmission based on Voltage Source Converters has been introduced and some medium-power systems have been installed [2]. VSC Transmission has none of the above issues with conventional HVDC, however other notable limitations are present:
bull; The rating of the converters is limited, presently to around 360MW.
bull; The losses are noticeably higher than with conventional HVDC, caused primarily by the increased switching losses.
bull; The cost of this technology is high.
In parallel with the developments in VSC transmission,there has been a stream of research on improving conventional, thyristor based, HVDC. In particular the concept of Capacitor Commutated Converters (CCC) has shown promising results[3-7] . The researchers in[3] have concluded that only series capacitor configuration might be of practical importance for improving commutation in HVDC converters. The CCC technology was further investigated and improved in a series of projects [4-7] and recently it has been installed in a practical back-to-back HVDC at Garabi station Brasil.[8]. The CCC concept offers HVDC converters that could operate at an improved power factor and with reduced commutation failure probability. However the following issues with CCC.HVDC have been identified:
bull; The occurrence of ferromagnetic resonance[7].Research shows that this can be resolved using thyristor controlled capacitors or advanced feedback control[5,7], but at the increased costs.
bull; Typically CCC achieves around 10-15% reactive power consumption. In order to operate with a leading power factor, and to completely eliminate commutation failure, very large series capacitors would be needed. This increases voltage stress, harmonics and costs.
bull; The increased voltage stress on thyristors, which can be 2 p.u. or even 3p.u with very large capacitors.
bull; The insulation of series commutating capacitors.
bull; The harmonic generation is somewhat increased, and inparticular DC harmonics are significantly larger.
It is clear that there is incentive and the scope for further development of HVDC transmission and that the potential benefits are significant. Ideally, the research on HVDC converters would improve HVDC technology:
bull; To operate at higher firing angles enabling reactive power export.
bull;To eliminate commutation failure issues, if possible including most severe single-phase faults.
bull; Not to significantly increase harmonic pollution, or thyristor voltage stress or costs.
In this paper, a new HVDC converter that potentially meets the above goals is studied. The concept is based on combining forced commutation and the resonant turn off methods. The basic principles of these techniques are known since they have been used with some low power DC-DC choppers[9-11] , although such techniques have not been employed in high power, three-phase circuits. The paper will further give detailed analysis of operation and the method of calculating optimum parameters. A wide range of PSCAD/EMTDC simulation results will be presented to evaluate performance of the new scheme in terms of: reactive power exchange, commutation failure resilience, harmonic generation and stability aspects.
II. CIRCUIT DESCRIPTION
Figure 1 shows the proposed converter and the AC system at the inverter side of an HVDC. There are two six-pulse bridges in Y and transformer connections. Thyristors TY1-TY6 and T1-T6 are the main thyristors in the conventional Graetz connection. Each main thyristor has one auxiliary thyristor to aid commutation (TY1a-TY6a and T1a-T6a). There are also three commutating capacitors Cc and a single resonant capacitor Cs (per 6-pulse bridge).
The complete HVDC test system is based on the CIGRE HVDC Benchmark model and all parameters can be found in. This is a monopolar 12-pulse HVDC system with
1000MW, 500kV, 2000A rating, and with weak AC systems at both ends. It is desired to keep the test systems as close as possible to the original CIGRE model, to enable comparisons. Some minor modifications are made, however. Since the new converters enable zero reactive power exchange, implying higher AC voltage, the
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