METHOD AND SYSTEM FOR MITIGATION OF TRANSFORMER SATURATION AND GROUND ELECTRODE POLARIZATION IN A HIGH VOLTAGE DC

TRANSMISSION SYSTEM

FIELD

[0001] The field of the disclosure relates generally to high voltage DC transmission systems. More specifically, the disclosure relates to methods and systems for mitigating transformer saturation in a high voltage DC transmission system particularly when operating in ground return mode, and for mitigating potential ground electrode polarization problems.

BACKGROUND

[0002] High voltage direct current (HVDC) transmission systems generally use one conductor for positive voltage and current and another conductor for negative voltage and an equal and opposite return current. Such a HVDC configuration is called a bi-polar transmission system. Each pole, positive and negative, is equipped with its own monopolar converter bridges. Under normal operating conditions, substantially all of the return current flows through the second pole while essentially no current flows through the ground. Some HVDC transmission systems have been built with a single pole such that all of the return current flows through the ground or a metallic ground wire. In a typical HVDC transmission system when one line is out of service, the other can continue to operate in a monopolar ground return mode. In this state, the transmission system operates at half power. Thus, bipolar HVDC lines can operate at half power in a monopolar ground return mode with a line out of service. To convert an alternating current (AC) line to direct current (DC), all of the transmitted power is passed through an AC/DC converter operating as a rectifier at the sending terminal and a DC/AC converter operating as an inverter at the receiving terminal.

[0003] Significant DC currents flowing from ground into the neutrals of transformers have been observed in HVDC transmission systems. These DC

currents are sufficient to magnetically saturate the transformers and to potentially damage the transformers. For this reason, the transmitted DC current in a monopolar ground return mode may require reduction in order to avoid the magnetic saturation. To increase the ground return current in the monopolar mode without damage to transformers, DC blocking devices (BDs), may be used. The BDs eliminate DC current in the neutrals of the transformers to which they are fitted. However, the transmitted ground return current when operating in ground return mode may still not allow achievement of maximum current without subjecting other transformers in the overall power network to the risk of DC magnetic saturation. In principle, the complete elimination of DC current in all transformers would require separate BDs in the neutrals of all power transformers in the path of the DC ground current. In real world applications, this is an impractical proposition based on cost and maintenance of the BDs.

[0004] Thus, there is a need for a method and a system for simultaneously mitigating saturation of all transformers affected by the ground return current of a HVDC transmission system operating in ground return mode. There further is a need for a method and a system to support operation in ground return mode at a full rated current without transformer saturation.

SUMMARY

[0005] A method and a system are provided which provide continuous operation in ground return mode at full rated current. A method of operating a HVDC transmission system when operating in ground return mode includes periodically reversing the polarity of the current in the ground and in the transmission line while the direction of the average power flow remains in the same direction. For a bipolar HVDC transmission system in ground return mode with a line out of service, the positive and negative converters at each end may be bidirectional 4-quadrant converters. The positive and negative converters at each end may be re- configurable as 4-quadrant converters from unidirectional converters. The periodic reversal may be achieved by control of the bidirectional converters. The periodic

reversal may be achieved using phase control of the 4-quadrant converters. The polarity of the line current and the voltage is periodically reversed at a frequency sufficient to mitigate a saturation effect of a transformer while minimizing the average loss of transmitted power associated with each reversal. The duration of the polarity reversal process is fast enough to minimize voltage flicker in the AC receiving system, but slow enough to avoid unacceptable transient oscillations in the system. Reconfiguration of the converters as 4-quadrant converters in ground return mode can also be used to set a constant polarity of ground return current in a selected direction to mitigate an electrode polarization problem.

[0006] In an exemplary embodiment, a method of operating a high voltage, direct current transmission system in a ground return mode is provided. The method includes, but is not limited to, periodically reversing a current polarity in a ground return path while maintaining an average power flow in a direct current (DC) transmission system in a constant direction, wherein the periodic reversal is sufficient to mitigate saturation of a transformer operably coupled with the DC transmission line.

[0007] In another exemplary embodiment, a method of operating a high voltage, direct current transmission system in a ground return mode is provided. The method includes, but is not limited to, periodically reversing a current polarity in a ground return path while maintaining an average power flow in a direct current (DC) transmission system in a constant direction, wherein the periodic reversal is sufficient to mitigate an electrode operational problem.

[0008] In still another exemplary embodiment, a method of high voltage, direct current transmission system in a ground return mode with a line out of service is provided. The method includes, but is not limited to, configuring a first converter and a second converter at a sending end of a direct current (DC) transmission line as a four-quadrant converter; configuring a third converter and a fourth converter at a receiving end of the DC transmission ϋne as a four-quadrant converter; and setting the ground current to a constant polarity to mitigate an electrode operational problem.

[0009] In yet another exemplary embodiment, a method of operating a high voltage, direct current transmission system in a ground return mode with a line out of service to mitigate an electrode operational problem is provided. The method includes, but is not limited to, identifying a first line out of service of a direct current (DC) transmission system; reducing the current through a first converter at a sending end of a DC transmission line to zero using phase control of the first converter; reducing the current through a second converter at a receiving end of the DC transmission line to zero using phase control of the second converter; disconnecting the first converter and the second converter from the DC transmission system; and connecting a third converter at the sending end and a fourth converter at the receiving end to feed a second line of the DC transmission system to set a current in a ground return path to a constant polarity.

[0010] Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Exemplary embodiments of the invention will hereafter be described with reference to the accompanying drawings, wherein like numerals denote like elements.

[0012] Fig. 1 depicts a circuit diagram of a HVDC transmission system in a bipolar operational mode in accordance with an exemplary embodiment.

[0013] Figs. 2a and 2b depict circuit diagrams indicating a positive and a negative current flow, respectively, in a reconfigured HVDC transmission system with a negative line out of service in accordance with an exemplary embodiment.

[0014] Figs. 3a and 3b depict circuit diagrams indicating a negative and a positive current flow, respectively, in the reconfigured HVDC transmission system of Figs. 2a and 2b with a positive line out of service.

[0015] Fig. 4 depicts a circuit diagram of the DC elements of the reconfigured HVDC transmission system of Figs. 2a, 2b, 3a, and 3b indicating positive line converters at each end conducting with a negative line out of service.

[0016] Fig. 5 depicts a circuit diagram of the DC elements of the reconfigured HVDC transmission system of Figs. 2a, 2b, 3a, and 3b indicating negative line converters at each end conducting with a negative line out of service.

[0017] Fig. 6 is a waveform diagram illustrating converter control of the reconfigured HVDC transmission system to reverse the line current and voltage in accordance with a first exemplary embodiment.

[0018] Figs. 7a-7g depict a switching sequence for reconfiguring the converters of the HVDC transmission system of Fig. 1 for four-quadrant operation with a line out of service in accordance with an exemplary embodiment.

[0019] Fig. 8 depicts a second switching arrangement for reconfiguring the converters of the HVDC transmission system of Fig. 1 for four-quadrant operation with a line out of service in accordance with an exemplary embodiment.

DETAILED DESCRIPTION

[0020] With reference to Fig. 1 , a bipolar HVDC transmission system 100 is shown in simplified form in accordance with an exemplary embodiment. In the exemplary embodiment of Fig. 1 , transmission system 100 includes a sending end 102 and a receiving end 104. A positive transmission line 106 is connected for current flow between sending end 102 and a receiving end 104. A negative transmission line 108 is connected for current flow between sending end 102 and receiving end 104. A ground return 110 grounds the positive and negative poles of sending end 102 and a receiving end 104. Under normal operating conditions, all of the return current flows through negative transmission line 108 while no significant current flows through ground return 110. The positive and the negative line currents are controlled by the positive and negative converters respectively so that the two line currents are nominally equal. In practice, some discrepancy occurs

between the positive and negative line currents, and the small difference current flows through the ground. This current generally is less than approximately 1 % of a full rated current. To convert an AC line to DC, the transmitted power is passed through an AC/DC converter operating as a rectifier at sending end 102 and a DC/AC converter operating as an inverter at receiving end 104.

[0021] Sending end 102 includes a first AC bus 112, a first pair of transformers or transformer sets 114, a first positive converter 116, a second AC bus 118, a second pair of transformers or transformer sets 120, and a first negative converter 122. AC buses 112 and 118 may be connected together. Receiving end 104 includes a third AC bus 128, a third pair of transformers or transformer sets 126, a second positive converter 124, a fourth AC bus 134, a fourth pair of transformers or transformer sets 132, and a second negative converter 130. AC buses 128 and 134 may be connected together. The terms "positive" and "negative" applied to the converters are taken to mean converters that in normal operation serve the normal positive and negative transmission lines respectively. In an exemplary embodiment, the positive and negative converters 116, 122, 124, 130 are 12-pulse converters rated at 2000 Amps and 500 kilovolts (kV) for a total rated power capacity of 2000 megawatts (MW). With one converter out of service at one or both ends and with both positive transmission line 106 and negative transmission line 108 operational, a continuous monopolar operation may be provided using a metallic return mode such that no reduction of the return current is needed.

[0022] First AC bus 112 connects with the first pair of transformers or transformer sets 114. The output of the first pair of transformers or transformer sets 114 is input to first positive converter 116. A first end of first positive converter 116 connects at a first node 150 and a second end of first positive converter 116 connects to a first inductor 154. Positive transmission line 106 connects to first inductor 154. Second AC-bus 118 connects with the second pair of transformer or transformer sets 120. AC buses 112 and 118 may be connected together. The output of the second pair of transformers or transformer sets 120 is input to first negative converter 122. A first end of first negative converter 122 connects at first

node 150 and a second end of first negative converter 122 connects to a second inductor 156. Negative transmission line 108 connects to second inductor 156.

[0023] Positive transmission line 106 connects to a third inductor 158. A first end of second positive converter 124 connects to third inductor 158 and a second end of second positive converter 124 connects at a second node 152. The output of second positive converter 124 is input to the third pair of transformers or transformer sets 126. Third AC bus 128 connects with the third pair of transformer or transformer sets 126. Negative transmission line 108 connects to a fourth inductor 160. A first end of second negative converter 130 connects to fourth inductor 160 and a second end of second negative converter 130 connects at second node 152. The output of second negative converter 130 is input to the fourth pair of transformers or transformer sets 132. Fourth AC bus 134 connects with the fourth pair of transformer or transformer sets 132. AC buses 128 and 134 may be connected together.

[0024] Positive transmission line 106 connects to first inductor 154 through a first sending end terminal point 138 and connects to third inductor 158 through a first receiving end terminal point 140. Negative transmission line 108 connects to second inductor 156 through a second sending end terminal point 142 and connects to fourth inductor 160 through a second receiving end terminal point 144. First node 150 is connected to ground through a third sending end terminal point 123 which connects to a first sending end ground electrode terminal point 146 to a sending end ground electrode. Second node 152 is connected to ground through a third receiving end terminal point 136 to a first receiving end ground electrode terminal point 148 to a receiving end ground electrode. It is noted that for simplicity only the major elements of a typical bipolar HVDC transmission system are shown in Fig 1. For example, though not shown, DC filters are typically connected between each HVDC line and ground, and AC filters and VAR capacitors are typically connected at the AC sides of the transformers at each end.

[0025] With reference to Figs. 2a and 2b, a monopolar HVDC transmission system 200 operating in ground return mode is shown in accordance with an

exemplary embodiment with negative transmission line 108 out of service. A positive line current and voltage is indicated on positive transmission line 106 in Fig. 2a. A negative line current and voltage is indicated on positive transmission line 106 in Fig. 2b. Monopolar HVDC transmission system 200 is formed by reconfiguring the positive and negative converters 116, 122, 124, 130 at each end 102, 104 of transmission system 100 to operate as composite 4-quadrant "dual converters". In an exemplary embodiment, the reconfiguration is formed using mechanical switches as will be described with reference to Figs. 7a-7g. The reconfiguration of monopolar HVDC transmission system 200 continues in ground return mode until the mechanical switches are switched back to form bipolar HVDC transmission system 100. At the end of the ground return mode of operation, the converters 116, 122, 124, 130 are reconfigured for normal bipolar operation.

[0026] In the exemplary embodiment of Figs. 2a and 2b, monopolar HVDC transmission system 200 operating in monopolar ground return mode includes a sending end 202 and a receiving end 204. Sending end 202 includes first AC bus 112, the first pair of transformers or sets of transformers 114, first positive converter 116, second AC bus 118, the second pair of transformers or sets of transformers 120, and first negative converter 122 arranged as described with reference to Fig. 1. Receiving end 204 includes third AC bus 128, the third pair of transformers or sets of transformers 126, second positive converter 124, fourth AC bus 134, the fourth pair of transformers or sets of transformers 132, and second negative converter 130 also arranged as described with reference to Fig. 1. Sending end 202 further includes a first link 206, which, if such connection is not already in place for normal operation, connects first AC bus 112 with second AC bus 1 18, and a second link 208. Second link 208 connects to positive transmission line 106 at a first end between first inductor 154 and first sending end terminal point 138 and to negative transmission line 108 at a second end between second inductor 156 and second sending end terminal point 142. Receiving end 204 further includes a third link 210 and a fourth link 212 which connects third AC bus 128 with fourth AC bus 134, if such connection is not already in place for normal operation. Third link 210 connects to positive transmission line 106 at a first end between third inductor 158

and first receiving end terminal point 140 and to negative transmission line 108 at a second end between fourth inductor 160 and second receiving end terminal point 144.

[0027] With reference to Fig. 2a, arrows indicate positive current flow from first positive converter 116 through positive transmission line 106 through second positive converter 124 and through ground return 110. With reference to Fig. 2b, arrows indicate negative current flow from first negative converter 122 through ground return 110 through second negative converter 130 and through positive transmission line 106.

[0028] With reference to Figs. 3a and 3b, monopolar HVDC transmission system 200 operating in monopolar ground return mode is shown in accordance with an exemplary embodiment with positive transmission line 106 out of service. A negative line current and voltage is indicated on negative transmission line 108 in Fig. 3a. A positive line current and voltage is indicated on negative transmission line 108 in Fig. 3b. The only essential difference between Figs. 2a and 2b and Figs. 3a and 3b are the line which is out of service. Thus, the same switch positions (to replace the simple links 208 and 210 and to be exemplified by reference to Figs 7a to 7g apply for either line 106, 108 out of service.

[0029] Figs. 4 and 5 depict circuit diagrams of the DC elements of monopolar HVDC transmission system 200 operating in monopolar ground return mode of Figs. 2a and 2b indicating a positive line converter and a negative line converter conducting, respectively and with negative line 108 out of service. A 4-quadrant converter is capable of operation with either polarity of dc voltage and with either polarity of dc current. As known to those skilled in the art, the 4-quadrant converter formed by combining the positive and negative converters at each end 202, 204 of the lines 106, 108 can be controlled so that the polarity of current and voltage on the line can be reversed periodically during operation in ground return mode. Because the current and voltage both are reversed, the power flow remains in the same direction, for example, from the sending end to the receiving end.

[0030] As reported in Saturation Time of Transformers under DC Excitation. Electric Power Systems Research, L. Bolduc et al., Vol. 56, Iss. 2, Nov. 2000, time to saturation for large transformers with DC current is typically in the range of 0.5 seconds for high levels of applied DC current and may be up to a minute or more, when the DC voltage applied to the magnetizing inductance is very low. The frequency at which the line voltage and current are reversed is chosen to be as low as possible, consistent with keeping the transformers 114, 120, 126, 132, as well as other transformers throughout the power network, out of saturation. Additionally, the frequency at which the transmitted voltage and current are reversed may be in the range of one cycle every ten seconds, perhaps longer which may require the use of blocking devices at transformers that have high prospective unwanted magnetization due to ground current, so that these transformers are blocked, leaving unblocked other transformers that are subject to relatively lower levels of prospective ground current. In an exemplary embodiment, the frequency at which the transmitted voltage and current are reversed may be in the range of one full cycle every 60-120 seconds. Thus, the transmitted voltage and current may be reversed every 30-60 seconds. The periodic reversal of transmitted voltage and current may be done by static phase control of the converters 116, 122, 124, 130. Thus, no dynamic mechanical switching is needed for the periodic reversal of transmitted voltage and current.

[0031] If the process of reversing the line voltage is too rapid (distinct from the intervals between reversals of voltage ), unacceptably high discharging and charging currents and undesirable oscillation due to the DC line filters, line capacitance, and other components may occur. The converter voltages should be controlled sufficiently slowly to minimize these undesirable effects. Transition from full transmitted power at a given polarity of voltage and current, to full transmitted power with the opposite polarity of voltage and current may need to span a period of about 0.2 seconds - 0.3 seconds. This interval is referred to here as the reversing interval. In general, the reversing interval should not be any longer than necessary because the transmitted power falls transiently to zero during this time.

If the reversing interval is too long, an unacceptable transient voltage and a reduction in power and/or frequency may result in the AC system.

[0032] The waveforms in Fig. 6 illustrate in exemplary idealized form how the converter voltages and current could be controlled within the reversing interval. Before time t1 , the line voltage is positive, a sending end converter voltage 602 and a receiving end converter voltage 604 are positive, and sending end converter voltage 602 is higher than receiving end converter voltage 604, causing a positive line current 606 to be transmitted. The process of reversing the line voltage and current is initiated at a first time t ₁ , at which sending end converter voltage 602 starts to be decreased, while receiving end converter voltage 604 is for the time being kept at it's previous value. The reduced voltage difference between the sending end and receiving end converters causes line current 606 to start to decrease. At a second time t ₂ , sending end converter voltage 602 at the sending converter becomes less than receiving end converter voltage 604 at the receiving converter, the polarity of the voltage between the sending and receiving converters reverses, and the rate of decrease of line current 606 increases. At a third time t ₃ , the voltage difference between the sending and receiving end converters is sufficient to cause line current 606 to decrease at a desired rate. Ramp-down of receiving end converter voltage 604 is initiated, and proceeds in coordination with ramp down of sending end converter voltage 602, such that the difference between the sending and receiving end converter voltages achieves the desired rate of decrease of line current 606. Due to the 4-quadrant operation of the dual- converters, the voltages and currents at each end can be ramped smoothly through zero, then increased in the reverse direction, regardless of instantaneous polarities of voltage and current. Smooth transition of the current through zero may be enhanced if necessary by operating each dual converter in "circulating current" mode just before and just after the zero-crossing of the current, as is well known. The circulating current between the positive and negative converters at each end may also be controlled during the reversing interval to regulate the reactive power and ac voltage at each end. At a fourth time t ₄ , t sending end converter voltage 602 reaches full value in the reverse direction. Line current 606 and receiving end

converter voltage 604 reach their full reverse values shortly thereafter, at a fifth time t ₅ . The reversing interval extends from first time ti to fifth time t ₅ .

[0033] With this particular embodiment, within the reversing interval, the instantaneous transmitted power 612 falls transiently from full value to zero then back to full value. An average power during the reversing interval is about 33% of the full power transmitted during the periods between the reversing intervals. The transient power shortfall during the reversing interval may be supplied by the short- term overload capacity of the receiving system, and is likely to result in some transient AC voltage reduction. With the assumption that the reversing interval is 0.2 second, and the period T 610 between reversals is ten seconds, the average transmitted power 608 over period T 610 including the transient loss of power during the reversing intervals is about 99% of a steady instantaneous power 612 transmitted during the constant-power intervals between the reversing intervals. It may be preferable that the AC filters and VAR compensation capacitors for each converter at each end are kept in service during ground return operation. Maintaining all available VAR compensation in service may reduce AC voltage dips during the reversing periods. For example, rotating synchronous condensers where installed can help to reduce transient AC voltage reduction during the reversing interval. With the assumption that the reversing interval is 0.2 second, and the period T 610 between reversals is thirty ten seconds, the average transmitted power 608 over period T 610 including the transient loss of power during the reversing intervals is more than 99% of a steady instantaneous power 612 transmitted during the constant-power intervals between the reversing intervals. It may be preferable that the AC filters and VAR compensation capacitors for each converter at each end are kept in service during ground return operation. Maintaining all available VAR compensation in service may reduce AC voltage dips during the reversing periods.

[0034] The above exemplary description is idealized. Other control strategies may be utilized as known to those skilled in the art. For example, the voltage reversal can be accomplished ahead of the current reversal, or vice versa, and

there is no fundamental need for linear ramping of the voltage and current waveforms as depicted in Fig. 6.

[0035] With reference to Fig. 7a, a switching arrangement which supports the reconfiguration of the converters from a bipolar operating mode to a monopolar operating mode is shown in accordance with an exemplary embodiment. Sending end 202 includes a first switch 702, a second switch 706, a third switch 710, a fourth switch 714, a fifth switch 718, a sixth switch 722, and a seventh switch 726. First switch 702 is on a first line connected between a third node 150 and first terminal point 123. Third node 150 is on a line between first positive converter 116 and first negative converter 122. Second switch 706 is connected between a fourth node 742 and second inductor 156. Third switch 710 is connected between first node 730 and first terminal point 123. Fourth switch 714 is connected between fourth node 742 and first node 730. Fifth switch 718 is connected between a fifth node 738 and first node 730. Sixth switch 722 is connected between third node 150 and first node 730. Seventh switch 726 is connected between a fifth node 738 and first inductor 154.

[0036] Receiving end 204 includes a first switch 704, a second switch 708, a third switch 712, a fourth switch 716, a fifth switch 720, a sixth switch 724, and a seventh switch 728. First switch 704 is on a first line connected between a sixth node 152 and third receiving end terminal point 136. Sixth node 152 is on a line between second positive converter 124 and second negative converter 130. Second switch 708 is connected between a seventh node 744 and fourth inductor 160. Third switch 712 is connected between second node 734 and third receiving end terminal point 136. Fourth switch 716 is connected between seventh node 744 and second node 734. Fifth switch 720 is connected between a eighth node 740 and second node 734. Sixth switch 724 is connected between sixth node 152 and second node 734. Seventh switch 728 is connected between an eighth node 740 and third inductor 158.

[0037] In an exemplary embodiment, each of the switches is rated for the line to ground voltage or half the line to line voltage. Switches 706, 708, 710, 712, 714,

716, 718, 720, 726, and 728 may be installed for operation in metallic return mode as known to those skilled in the art. Switches that are used for other conventional functions, but do not have a role in reconfiguring the DC connections of the converters at each end into 4-quadrant converters are not shown. The switch positions shown with reference to Fig. 7a are for normal bipolar operation. Switches 702, 704, 714, 716, 718, and 720 are open with the remaining switches closed. With reference to Fig. 7b, negative transmission line 108 is out of service. All switches 702-728 remain in the same position as in Fig. 7a, but the current in the positive line returns through ground return 110 instead of through negative transmission line 108. This is typically the case when the negative line first opens, which forces the already established current to flow through the ground.

[0038] With reference to Fig. 7c, first switches 702 and 704 are closed. The ground return current now flows partially through sixth switches 722, 724 and partially through first switches 702 and 704. Second switches 706, 708 also are opened to temporarily disconnect the presently dormant negative converters 122, 130.

[0039] With reference to Fig. 7d, third switches 710 and 712 and sixth switches 722, 724 are opened. The ground return current now transfers completely to flow through first switches 702 and 704. The firing angles of the negative line converters 122, 130 are adjusted to reverse the prospective voltages of these converters so that their DC terminal voltages are set to "match" the voltages of the companion positive converters 116, 124 when the switching step shown with reference to Fig. 7e is taken. Alternatively, the negative converters 122, 130 may preferably remain blocked (i.e., without firing pulses for the thyristors) until required to carry current.

[0040] With reference to Fig. 7e, fifth switches 718 and 720 and fourth switches 714, 716 are closed. With reference to Fig. 7f, second switches 706, 708 are closed thereby configuring the positive and negative converters 116, 122, 124, 130 at each end as 4-quadrant converters at each end. The ground return current still flows in the same direction as indicated in Fig. 7b. Fig. 7g shows the path for reversed direction of ground return current, when the converter voltages are

reversed for example using the converter control sequence of Fig. 6. Power still flows in the same direction. For the purpose of reconfiguring the converters at each end from the normal operating mode into 4-quadrant converters for ground return operation, the switches 702 and 704 are not needed and could be replaced by solid shorting connections. However, switches 702 and 704 may be needed to fulfill other conventional functions, in which case their inclusion has relevance in the foregoing description.

[0041] With reference to Fig. 8, a switching arrangement which supports the reconfiguration of the converters from a bipolar operating mode to a monopolar operating mode is shown in accordance with a second exemplary embodiment. Sending end 202 includes second switch 706, fourth switch 714, fifth switch 718, seventh switch 726, an eighth switch 800, and a ninth switch 812. Second switch 706, fourth switch 714, fifth switch 718, and seventh switch 726 are connected as described with reference to Figs. 7a-7g. Eighth switch 800 of sending end 202 is connected between a ninth node 802 and tenth node 804. Ninth node 802 is connected between first positive converter 116 and third node 150. Tenth node 804 is connected between fifth switch 718 and fourth switch 714. Ninth switch 812 is connected between third node 150 and first terminal point 123. Receiving end 204 includes second switch 708, fourth switch 716, fifth switch 720, seventh switch 728, a tenth switch 806, and an eleventh switch 814. Tenth switch 806 of receiving end 204 is connected between an eleventh node 808 and twelfth node 810. Eleventh node 808 is connected between second positive converter 124 and second node 152. Twelfth node 810 is connected between fifth switch 720 and fourth switch 716. Eleventh switch 814 is connected between second node 152 and third receiving end terminal point 136.

[0042] In an exemplary embodiment, each of the switches is rated for the line to ground voltage or half the line to line voltage. Switches 706, 708, 714, 716, 718, 720, 726, 728, 812, and 814 may be installed for operation in metallic return mode as known to those skilled in the art. For example, switches 812 and 814 may be the metal return transfer breakers, which do not have a role in reconfiguring the

connections at each end into 4-quadrant converters, and switches 714, 716, 718, 720 may be the ground return transfer switches that are used for other conventional functions, but also have a role in reconfiguring the DC connections of the converters at each end into 4-quadrant converters. The switch positions shown with reference to Fig. 8 are for normal bipolar operation. Switches 714, 716, 718, and 720 are open with the remaining switches closed. Assuming a negative transmission line 108 is out of service as described with reference to Figs. 7a-7g, all switches 706, 708, 714, 716, 718, 720, 726, 728, 800, 806, 812, and 814 initially remain in the same position as shown in Fig. 8, but the current in positive transmission line 106 returns through ground return 110 instead of through negative transmission line 108. This is typically the case when the negative line first opens, which forces the already established current to flow through the ground.

[0043] To reconfigure the converters from a bipolar operating mode to a monopolar operating mode with negative transmission line 108 open, switches 800 and 806 are opened and negative converters 122 and 130 at each end 102, 104 of transmission system 100 are blocked. Current continues to flow in a positive direction through positive transmission line 106 and returns through ground return 1 10. Switches 714, 716, 718, and 720 are closed, connecting the positive and negative converters 116, 122, 124, 130 at each end in back-to-back configuration to form 4-quadrant converters. The negative converters 122 and 130 remain blocked, and current continues in a positive direction through positive transmission line 106, returning through ground return 110.

[0044] The positive converters 116 and 124 are blocked, and the negative converters 122 and 130 are controlled to send current in a negative direction through positive transmission line 106 and back through ground return 1 10. The transition from "positive" ground current to "negative" ground current is achieved solely by static phase control of the positive and negative converters 116, 122, 124, 130 at each end. No mechanical switching occurs for as long as periodic reversing of the ground current remains in progress. When returning to normal bipolar operation, switches 714, 716, 718, and 720 are opened during a period when these

switches do not carry current. For example, with negative transmission line 108 out of service, this occurs when a positive current is being transmitted through positive transmission line 106. Conversely, with positive transmission line 106 out of service, this occurs when a negative current is being transmitted through negative transmission line 108.

[0045] The foregoing description addresses the case in which one line of a bipolar HVDC system is out of service, but the converters at each end are operational. If one converter (or more) at each end is out of service, it is not possible to reconfigure the converters for bidirectional operation, which is a key requirement in order to transmit periodically reversing DC current in the ground to mitigate transformer saturation. A bipolar HVDC system typically has two 6-pulse converters that form a positive 12-pulse converter (116,124), and two 6-pulse converters that form a negative 12-pulse converter (122,130) at each end of the line. One or more separate uncommitted spare 6-pulse unidirectional converters can be provided at each converter station 102 and 104, with switching arrangement that can substitute the uncommitted spare converter or converters for whichever normally operational converter is out of service. This allows periodic reversal of the polarity of the line voltage and current in ground return mode, even if a converter and/or line are out of service. In the ground return operating mode, each unidirectional converter carries current with substantially 50% duty cycle. Thus, the spare uncommitted converter could use thyristors that are smaller than the thyristors in the converters for normal continuous bipolar operation. The size of each spare uncommitted 6-pulse converter would be perhaps 18% of the combined size of the 4 6-pulse converters used for normal continuous operation at each station.

[0046] The converter control sequences of Fig. 6 or any other sequence may be implemented in or include hardware, firmware, software, or any combination of these methods. Thus, a control system configured to execute the converter control sequence of Fig. 6 or any other control sequence may include circuitry that can

implement the processes indicated in the form of hardware, firmware, and/or a processor executing instructions embodied in software.

[0047] In some bipolar HVDC systems, the earth electrode may not be allowed to assume an optional polarity. Such optional polarity is, however, a normal requirement of a bipolar HVDC system when operating in ground return mode, depending upon which line is out of service. As stated in U.S. Patent No. 6,141 ,226, "an earth electrode located in clay ground is for example not allowed to function as anode, since there is a risk of explosions as a result of osmosis and gas formation, while it does not matter if a cathode is arranged in clay ground." The objective of U.S. Patent No. 6,141 ,226 is that in the event of either one of the transmission lines being out of service, initiating ground return operation, the polarity of the current in the ground electrode is defined to keep the same given polarity, i.e. the electrode at one station will always be an "anode", and the electrode at the other station will always be a "cathode", regardless of which line fails. In the exemplary embodiment described in U.S. Patent No. 6,141 ,226, the converters at each end connected to the normally positive line are conventional unidirectional 2-quadrant converters, but the converters at each end connected to the negative line are bidirectional 4-quadrant converters. If the negative line fails, instigating operation with ground return current, the positive line stays positive, the receiving ground electrode operates as an "anode", and the sending electrode operates as a "cathode". The 4-quadrant converters for the negative line remain inoperative, because the negative line is out of service. If the positive line fails, the unidirectional converters for the positive line are inoperative, and the bidirectional converters for the negative line are controlled so that the polarity of the current and voltage of the normally negative line is reversed by the bidirectional converters to become continuously positive, thereby keeping the same direction of continuous power flow as in bipolar operation, while the receiving ground electrode remains as "anode" and the sending electrode remains as "cathode".

[0048] A significant disadvantage of U.S. Patent No. 6,141,226 is that one of the two converters at each HVDC station must be bidirectional; therefore, these

converters must employ either bidirectionally controlled thyristors or conventional thyristors connected back-to-back. The technique herein for forming 4-quadrant converters in ground return mode from the unidirectional converters already installed for normal operation is advantageous because this does not require any additional or special thyristors. When one line is out of service, existing 2-quadrant converters at each station are reconfigured into bidirectional converters by means of a relatively simple switching arrangement.

[0049] Of course, to achieve unidirectional electrode current of the required polarity in ground return operation, periodic reversing of the ground current could not be implemented, so there is no mitigation of transformer saturation. As an example, if the negative line goes out of service, the electrode at the receiving end may be required to operate as an anode and the electrode at the sending end may be required to operate as a cathode in ground return operation. The return current of the positive line from the positive converters naturally transfers to the ground with the polarity that results in the receiving electrode operating as an anode and the sending electrode operating as a cathode. If the positive line goes out of service, the return current of the negative line from the negative converters transfers to the ground with the polarity that results in the receiving electrode operating in undesired cathode mode, and the sending electrode operating in undesired anode mode. In this case, the converters can first be reconfigured as four-quadrant converters by operation of mechanical switches as previously described, and the ground current and line voltage can be reversed by phase-control that transfers the line current to the positive converters and blocks the negative converters. Once the transfer of current to the positive converters is completed, the negative converters can optionally be physically disconnected for example by opening switches 706 and 708 in Fig. 8 for as long as the positive line is out of service.

[0050] An alternative procedure when the positive line goes out of service would be first to reduce the current through the negative converters to zero by phase control of the negative converters, then disconnect the negative converters, for example by opening switches 706 and 708 in Fig. 8. The positive converters can

then be connected to feed the negative line, by opening switches 800 and 806 and then by closing switches 714, 716, 718, and 720 in Fig. 8. Additionally, an alternative procedure when the negative line goes out of service would be first to reduce the current through the positive converters to zero by phase control of the positive converters, then disconnect the positive converters, for example by opening switches 728 and 726 in Fig. 8. The negative converters can then be connected to feed the positive line, by opening switches 800 and 806 and then by closing switches 714, 716, 718, and 720 in Fig. 8.

[0051] It is possible, however, that periodic reversal of the ground current to mitigate transformer saturation also may mitigate the electrode problem, allowing both the transformer saturation and electrode polarization problems to be simultaneously mitigated. Mitigation of the electrode problem might be enhanced by introducing a slight imbalance between the "positive current" and the "negative current" intervals sufficient to create a favorable DC bias for the electrode, but small enough not to significantly jeopardize mitigation of transformer saturation.

[0052] In a first exemplary embodiment, a method of operating an HVDC transmission system operating in ground return mode includes periodically reversing the polarity of the current in the ground while the direction of the average power flow remains in the same direction.

[0053] In a second exemplary embodiment, an HVDC transmission system which, when operating in normal steady state full power operation, operates with substantially steady direct current in each of the transmission lines and with preferably substantially zero current in the ground, and when operating with at least one line out of service in ground return mode, the polarity of the current transmitted through the ground is periodically reversed while the average power transmitted remains in the same direction. In normal operation, the positive and negative line currents preferably are controlled so the net sum of the line currents results in substantially zero current in the ground. Sending and receiving end converters may be bidirectional 4-quadrant converters which can deliver either polarity of voltage and current. Periodic reversal of ground current may be achieved by control of the

bidirectional converters to mitigate DC saturation of transformers and/or electrode operational problems.

[0054] In a third exemplary embodiment, an HVDC transmission system which, when operating in normal steady state full power operation, operates with preferably substantially steady direct current in each of the transmission lines, and with substantially zero current in the ground, and when operating in ground return mode with at least one line out of service, bidirectional 4-quadrant converters are formed by circuit reconfiguration of individual converters that are otherwise used for unidirectional current flow in normal operation. The circuit reconfiguration may be implemented using switches. The ground current may be periodically reversed and may be achieved by control of the bidirectional converters to mitigate DC saturation of transformers and/or electrode operational problems. The polarity of the ground current may be set continuously in a selected direction to mitigate electrode operational problems. At least some of the switches may also be used to perform other duties unrelated to mitigation of DC saturation of transformers and/or electrode operational problems such as osmosis and gas formation.

[0055] In exemplary embodiments, at least one converter may be provided to substitute for an out of service converter using one or more switches.

[0056] In a fourth exemplary embodiment, an HVDC transmission system which, when operating in normal steady state full power operation, operates with preferably substantially zero current in the ground, and when operating with at least one line out of service in ground return mode, forms bidirectional 4-quadrant converters by circuit reconfiguration of individual converters that are otherwise used for unidirectional current flow during normal operation.

[0057] The foregoing description of exemplary embodiments of the invention have been presented for purposes of illustration and of description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and as practical

applications of the invention to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as suited to the particular use contemplated.