High voltage direct current transmission and distribution system

A direct current to alternating current inverter sub-system is for a HVDC distribution system. The DC to AC inverter sub-system includes an enclosure and a DC to DC galvanically isolated buck converter having a DC input electrically connectable to a HVDC cable and a DC output. A DC to AC inverter includes a DC input electrically connected to the DC output of the DC to DC galvanically isolated buck converter and an AC output electrically connectable to an AC transmission line. The DC to AC inverter is mounted in an enclosure with the DC to DC galvanically isolated buck converter, in order that the DC output of the DC to DC galvanically isolated buck converter is directly electrically connected within the enclosure to the DC input of the DC to AC inverter.

BACKGROUND

The disclosed concept pertains generally to power distribution and, more particularly, to high voltage direct current transmission and distribution systems, such as, for example, such systems for above ground, below ground or subsea applications. The disclosed concept further pertains to circuit interrupters for high voltage direct current distribution systems.

2. Background Information

Alternating current (AC) power distribution systems are well known.

High voltage, direct current (DC) power distribution systems have less energy losses and require less costly transmission cables than corresponding AC distribution systems. In long transmission lines and, in particular, in ones that use cable, high voltage DC power transmission may be the only feasible method of power transmission because using AC will cause debilitating instability and excessive losses.

When connecting a DC voltage source to a relatively long cable transmission line when the cable capacitance is discharged, relatively large oscillatory currents occur which, in turn, generate relatively large voltage spikes along the cable length. These voltage spikes can compromise the insulation of the cable itself as well as the insulation of any device electrically connected thereto.

Furthermore, the relatively large oscillatory currents can cause nuisance tripping of protection devices of the transmission line.

There is room for improvement in high voltage direct current transmission and distribution systems specifically regarding, for example, the cable charging process when the transmission line is energized.

There is also room for improvement in circuit interrupters for such systems.

SUMMARY

These needs and others are met by embodiments of the disclosed concept in which a direct current to alternating current voltage source inverter is mounted in an enclosure with a corresponding galvanically isolated direct current to direct current converter, in order that a direct current output of the galvanically isolated direct current to direct current converter is directly electrically connected within the enclosure to the direct current input of the direct current to alternating current voltage source inverter.

These needs and others are also met by embodiments of the disclosed concept in which a circuit interrupter for a power circuit of a high voltage direct current distribution system comprises a controller cooperating with the series combination of a solid-state switch and an electromechanical isolation switch to open, close and trip open the power circuit, the controller being structured to repetitively turn on and turn off the solid-state switch when the electromechanical isolation switch is closed, in order to control charging of the power circuit from zero volts to a high direct current voltage.

In accordance with one aspect of the disclosed concept, a high voltage direct current transmission and distribution system comprises: an alternating current to direct current converter including an alternating current input and a direct current output; a first high voltage direct current cable including a first end electrically connected to the direct current output of the alternating current to direct current converter and an opposite second end; and a distribution system comprising: a number of high voltage direct current circuit breakers, each of the number of high voltage direct current circuit breakers including a first portion electrically connected to the opposite second end of the first high voltage direct current cable and a second portion; a number of second high voltage direct current cables, each of the number of second high voltage direct current cables including a first end electrically connected to the second portion of a corresponding one of the number of high voltage direct current circuit breakers and an opposite second end; a number of galvanically isolated direct current to direct current converters, each of the number of galvanically isolated direct current to direct current converters including a direct current input electrically connected to the opposite second end of a corresponding one of the number of second high voltage direct current cables and a direct current output; a number of direct current to alternating current voltage source inverters, each of the number of direct current to alternating current voltage source inverters including a direct current input electrically connected to the direct current output of a corresponding one of the number of galvanically isolated direct current to direct current converters and an alternating current output; a number of alternating current transmission lines, each of the number of alternating current transmission lines including a first end electrically connected to the alternating current output of a corresponding one of the number of direct current to alternating current voltage source inverters and an opposite second end; and a number of alternating current loads, each of the number of alternating current loads electrically connected to the opposite second end of a corresponding one of the number of alternating current transmission lines, wherein each of the number of direct current to alternating current voltage source inverters is mounted in an enclosure with a corresponding one of the number of galvanically isolated direct current to direct current converters, in order that the direct current output of the last such corresponding one of the number of galvanically isolated direct current to direct current converters is directly electrically connected within the enclosure to the direct current input of a corresponding one of the number of direct current to alternating current voltage source inverters.

As another aspect of the disclosed concept, a direct current to alternating current inverter sub-system is for a high voltage direct current distribution system. The direct current to alternating current inverter sub-system comprises: an enclosure; a direct current to direct current galvanically isolated buck converter including a direct current input electrically connectable to a high voltage direct current cable and a direct current output; and a direct current to alternating current voltage source inverter including a direct current input electrically connected to the direct current output of the direct current to direct current galvanically isolated buck converter and an alternating current output electrically connectable to an alternating current transmission line, wherein the direct current to alternating current voltage source inverter is mounted in the enclosure with the direct current to direct current galvanically isolated buck converter, in order that the direct current output of the direct current to direct current galvanically isolated buck converter is directly electrically connected within the enclosure to the direct current input of the direct current to alternating current voltage source inverter.

As another aspect of the disclosed concept, a circuit interrupter for a power circuit of a high voltage direct current distribution system comprises: a first terminal; a second terminal; an electromechanical isolation switch; a solid-state switch electrically connected in series with the electromechanical isolation switch between the first and second terminals; and a controller cooperating with the solid-state switch and the electromechanical isolation switch to open, close and trip open the power circuit, the controller being structured to repetitively turn on and turn off the solid-state switch when the electromechanical isolation switch is closed, in order to control charging of the power circuit from zero volts to a high direct current voltage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As employed herein, the statement that two or more parts are “connected” or “coupled” together shall mean that the parts are joined together either directly or joined through one or more intermediate parts.

As employed herein, the term “low voltage” shall mean any alternating current voltage that is less than about 1000 VRMS(i.e., a low alternating current voltage), or any direct current voltage that is less than about 1500 VDC (i.e., a low direct current voltage).

As employed herein, the term “medium voltage” shall mean any alternating current voltage greater than a low alternating current voltage and in the range from about 1 kVRMSto about 38 kVRMS(i.e., a medium alternating current voltage), or any direct current voltage greater than a low direct current voltage and in the range from about 1500 VDC to about 50 kVDC (i.e., a medium direct current voltage).

As employed herein, the term “high voltage” shall mean any suitable alternating current voltage greater than a medium alternating current voltage (i.e., a high alternating current voltage), or any suitable direct current voltage greater than a medium direct current voltage (i.e., a high direct current voltage). A high direct current voltage is also equivalently referred to as high voltage direct current (HVDC) herein.

Referring toFIG. 1, a high voltage direct current (HVDC) transmission and distribution system2includes an alternating current to direct current (AC/DC) converter4, an HVDC cable6, and a distribution system7including a number of HVDC circuit breakers8for a number of branch loads, a number of HVDC transmission lines10, a number of galvanically isolated direct current to direct current (DC/DC) converters12, a number of voltage source inverters14, a number of three-phase transmission lines16, and a number of AC loads, such as a number of AC motors18.

In one example embodiment, the AC/DC converter4and a portion of the HVDC cable6are above the surface of the sea or above ground, and the remaining portion and the rest of the system2are subsea or below ground.

In another example embodiment, all of the system2is subsea or below ground.

The AC/DC converter4includes an AC input4A and a DC output4B. The HVDC cable6includes a first end6A electrically connected to the DC output4B of the AC/DC converter4and an opposite second end6B. Each of the number of HVDC circuit breakers8includes a first portion8A electrically connected to the opposite second end6B of the HVDC cable6and a second portion8B. Each of the number of HVDC cables10includes a first end10A electrically connected to the second portion8B of a corresponding one of the number of HVDC circuit breakers8and an opposite second end10B. The number of DC/DC converters12are galvanically isolated to avoid ground currents in relatively long HVDC cables, such as10, and to permit the grounding of one of the phases of the DC to AC output14B. Each of the number of galvanically isolated DC/DC converters12includes a DC input12A electrically connected to the opposite second end10B of a corresponding one of the number of HVDC cables10and the DC output12B. Each of the number of DC/AC voltage source inverters14includes a DC input14A electrically connected to the DC output12B of a corresponding one of the number of galvanically isolated DC/DC converters12and an AC output14B. Each of the number of AC transmission lines16includes a first end16A electrically connected to the AC output14B of a corresponding one of the number of DC/AC voltage source inverters14and an opposite second end16B. Each of the number of AC loads18is electrically connected to the opposite second end16B of a corresponding one of the number of AC transmission lines16. Each of the number of DC/AC voltage source inverters14is mounted in an enclosure20with a corresponding one of the number of galvanically isolated DC/DC converters12, in order that the DC output12B thereof is directly electrically connected within the enclosure20to the DC input14A of a corresponding one of the number of DC/AC voltage source inverters14.

The AC/DC converter4can be installed on or above the surface of the sea or underwater (e.g., subsea) and is structured to ramp the DC voltage of the DC output4B at a limited rate in order to avoid high voltage transients in the transmission line of the HVDC cable6during its energization. In the case of underwater installation, there is an isolation transformer (not shown) on a platform (not shown) and a multi-pulse transformer and converter (not shown) underwater. This configuration minimizes the ground current at the source.

The HVDC circuit breaker8, the HVDC cable10, the galvanically isolated DC/DC converter12, the DC/AC voltage source inverter14, the enclosure20, the AC transmission line16, and the AC load18are either subsea or below ground.

As a non-limiting example, the typical output voltage of the AC/DC converter4is +/−5 kV to +/−30 kV and the AC/DC converter4can be connected in series (FIG. 2), in parallel (FIG. 3) or in series and in parallel (FIG. 4), in order to scale the output power and/or voltage of the DC output4B. The AC/DC converter4increases the HVDC at a rate that will prevent any high voltage transients from forming during the transmission line energization. Being an active converter, the AC/DC converter4is capable of shutting down in case of a short circuit fault in the HVDC cable6or in the first input portion8A of the HVDC circuit breaker8. Hence, a fault current generated due to a short circuit in the transmission line of the HVDC cable6or in the HVDC circuit breaker first input portion8A is contained by the AC/DC converter4. If there is such a short circuit, then the AC/DC converter power semiconductor switches (not shown) are turned off to prevent excessive energy to flow in the downstream power circuit.

The AC electrical energy from a generator (not shown) is transformed into DC electrical energy using the AC/DC converter4, which can be implemented using diodes or active semiconductor switches (not shown). Normally, a multi-pulse AC input4A is employed in order to reduce the harmonic distortion footprint in the utility. Whether it uses the diodes or the active semiconductor switches, such as IGBTs, the AC/DC converter4can have its DC output voltage slowly climb from zero to its rated value (e.g., without limitation, 60 kVDC), although the methods used for each type to achieve the slow output voltage climb are very distinct. The speed of the voltage climb affects the number of frequency components applied to the transmission line of the HVDC cable6. The faster the voltage climb is, the higher its frequency components. High frequency voltage content will cause the transmission line reactive components to generate voltage transients that normally reach twice the applied voltage (e.g., in the example of a 60 kVDC transmission voltage, the voltage spike generated by a fast voltage climb is about 120 kVDC). This will play havoc with the life of the components in the transmission lines as well as over stress the cable insulation. To avoid this high voltage transient, the disclosed concept reduces the AC/DC converter output DC voltage climb from zero to rated value. This DC voltage climb rate can be adjusted to the cable type and transmission line length if the fastest possible voltage climb is desired or to a predetermined longer rate that will cover any cable type and length.

Preferably, the AC/DC converter4is an active converter structured to shut down responsive to a short circuit condition operatively associated with the HVDC cable6and/or the circuit breaker8, and structured to ramp the DC voltage at the DC output4B thereof at a predetermined rate (e.g., without limitation, 10 kVdc per second for distances up to 100 km; 10 kVdc per two seconds for distances from 100 km to 200 km; 10 kVdc per three seconds for distances greater than 200 km) from zero volts to a suitable HVDC. The same rates are applicable for the HVDC circuit breaker8.

The HVDC cable6employs bipolar transmission, and can be partially above ground and partially below ground, partially above sea and partially subsea, entirely subsea, or entirely below ground. For example,FIG. 5shows another system2′ in which the HVDC cable6′ is partially above sea and partially subsea. No unipolar transmission is allowed in subsea or below ground applications due to galvanic corrosion of components of the enclosures involved.

The HVDC circuit breaker8includes a cable pre-charge function to prevent high voltage transients in the transmission line of the HVDC cable10. The HVDC circuit breaker8protects a number of branch circuits (e.g.,FIG. 5shows a plurality of branch circuits of plural HVDC cables10) derived off of the main HVDC transmission line of HVDC cable6′, and pulse width modulates the output8B to charge the downstream load side transmission line of the HVDC cable10to prevent high voltage transients due to cable length. A fault current generated due to a short circuit in the transmission line of the HVDC cable10or the input12A of the galvanically isolated DC/DC buck converter12will be contained and isolated by the HVDC circuit breaker8. As a result, the DC circuit breaker solid-state switches (e.g.,26ofFIG. 16) are turned off to protect the downstream power circuit from excessive energy.

The transmission line of the HVDC cable10contains resistive10R, inductive10L and capacitive10C components (FIG. 16). The impedance values of these components are proportional to the cable length. When a voltage pulse is applied to the transmission line, the cable resistive and inductive components limit the current magnitude and its rate of change while its capacitive component stores the electric charge transferred by the voltage pulse and defines a voltage magnitude across the cable terminals10T (FIG. 16). For a given cable length, the magnitude of the current and how quickly it climbs from zero to its rated value is defined by its resistance and inductance, and the amount of voltage across the cable terminals10T is defined by the cable capacitance.

When the voltage pulse is first applied, the current in the HVDC cable10climbs from zero to a maximum value. The rate of the current climb is limited by the cable inductance. If the voltage pulse is long enough, such that the current will reach its maximum value, this maximum value is defined by the cable resistance. As this current flows, in time, it defines an electric charge that is accumulated in the cable capacitance and is reflected in the cable terminals10T with a voltage magnitude.

By assuming a maximum cable length and, thus, a maximum resistance, inductance and capacitance, a minimum voltage increase can be calculated across the cable terminals10T when a given voltage pulse is applied. This means that with a given voltage pulse width, a minimum voltage should appear at the cable terminals10T. If the voltage does not appear or is smaller than the calculated value, it means that there is a short circuit in the transmission line or at the load input.

It also means that a fault in the system2can be detected without allowing hundreds or thousands of amperes to flow in the transmission line of the HVDC cable10before the fault is detected. This is called a look ahead function (e.g., function22ofFIGS. 13-16), which looks for the fault before fully energizing the transmission line. The rate of change of the current (di/dt) is defined by the cable inductance (L):
di/dt=V/L

The steady state current magnitude (i) is defined by the cable resistance (R):
i=V/R

The voltage (V) produced by the voltage pulse is defined by the cable capacitance (C) and the current that is defined by the cable resistance and inductance:
V=i*t/C
wherein:

i*t is calculated from the double time integral of the di/dt=V/L plus the time integral of the current, i=V/R, after the current reaches the rated value defined by R and for the rest of the duration of the voltage pulse.

Referring toFIGS. 13-16, the hybrid HVDC circuit breaker8provides both a short circuit detection, look ahead function22and a cable pre-charge function24. The circuit breaker8includes a solid-state switch26in series with an electromechanical switch28. The electromechanical switch28is employed solely to galvanically isolate the upstream primary circuit27from the downstream secondary circuit29being switched on or off solely when no electric current is flowing through it, since the solid-state switch26can have leakage current flowing through its semiconductor junction when it is turned off. The advantage of using solid-state switches (e.g., without limitation, IGBT; IGCT), such as26, is that they can switch on and off much faster than electromechanical switches, such as28. For example and without limitation, a high voltage IGBT can turn on or off within five microseconds, while an electromechanical switch can take dozens of milliseconds to do the same.

The circuit breaker8includes an inductor30in series with the solid-state switch26and the electromechanical switch28to limit the rate of change of the current. Utilizing the switching speed of the solid-state switch26and the inductor30, a relatively simple but effective control strategy is employed to detect a short circuit or a relatively low insulation value in the DC power circuit under protection, such as the example load29.

Cable transmission lines exhibit an impedance which is composed of inductive, capacitive and resistive components. If the load end is disconnected or the load equipment is turned off, when the circuit breaker8is turned on, the DC power source at27has to first charge the capacitance of the transmission line of the HVDC cable10to reach the source voltage level. The differential equation that defines the voltage across the transmission line capacitance as a function of the injected DC current is shown in Equation 1.
Cc*[dV/dt]=i(Eq. 1)
wherein:

Cc is the cable capacitance which increases linearly with cable length as a function of its distributed capacitive components10C;

dV/dt is the derivative of the voltage across the cable positive and negative terminals10T; and

i is the current in the HVDC cable10.

This means that a DC voltage step increase at the cable terminals10T can be attained by injecting a current, i, during a time t, as is shown by Equation 2.
dV=[i*dt]/Cc(Eq. 2)
When the solid-state switch26is turned on and off, it generates a current impulse via the inductor30. For the bipolar transmission line, there are two solid-state switches26and there are two inductors30as shown inFIG. 16. The current impulse has a duration which is determined by how long the solid-state switch26was kept turned on (e.g., i*dt of Equation 2). For a given cable capacitance Cc, the voltage step dV is achieved if there are no leakages due to insulation degradation or due to a short circuit between the positive and negative cable terminals10T.

If the cable capacitance Cc is known, then the voltage step dV can be determined by turning the solid-state switch26on and then off by a known amount of time using Equation 2.

If the cable capacitance Cc is not known, then the current impulse amplitude can be controlled through the inductors30by turning the solid-state switches26on and then measuring and monitoring the current amplitude by a controller32using current sensor34. When the current amplitude reaches a predetermined value, which should be equal to or less than the solid-state switch maximum current capability, then the controller32turns off the solid-state switches26. The di/dt value is limited by the inductance of the inductors30(e.g., L1 and L2) as shown by Equation 3.
di/dt=V/[L1+L2]  (Eq. 3)

If there is no short circuit between the positive and the negative cable terminals10T, then after the current impulse above, there will be a voltage step developed in the HVDC cable10. This voltage step is then measured and monitored by the controller32using voltage sensor36. If the voltage is steady, then its value corresponds to the unknown cable capacitance Cc. If a confirmation is needed, then a second current impulse can be injected in the same manner as was described above and the resulting voltage step should be twice as large as the first voltage step.

The above approach can also be used for the known cable capacitance case.

If the monitored voltage step starts to decay within a few seconds, as detected by the controller32, then this means that there is a current leakage due to an insulation degradation somewhere in the downstream power circuit (e.g., in the HVDC cable10or any downstream load such as the DC/DC converter12).

The disclosed concept uses the controller32with the above capabilities to generate the current impulse, limit the current amplitude, and measure and monitor the voltage step as a consequence of the current impulse. The measurement of current and voltage can be achieved using any suitable current and voltage sensors, such as34,36.

By verifying that the voltage step is present and that it does not change within a couple of seconds, the controller32infers that the cable insulation is healthy and that no short circuit is present. This is the look ahead function22.

The electromechanical isolation switch28is in series with the solid-state switch26, such as for example and without limitation, a power semiconductor switch. The solid-state switch26controls and protects against overload and fault currents while the electromechanical switch28is used to isolate the downstream protected branch circuit29from the upstream power circuit27or mains. The electromechanical switch28opens and closes at no load. The series combination of the switches26,28is electrically connected between first input portion8A and the second output portion8B of the HVDC circuit breaker8. When both of the switches26,28are closed, the second portion8B charges the downstream HVDC cable10to or toward a HVDC.

The controller32forms a modulation element structured to repetitively turn on and turn off the solid-state switch26when the electromechanical isolation switch28is closed, in order to control charging of the downstream HVDC cable10from zero volts to a HVDC from the first input portion8A of the HVDC circuit breaker8. This modulation element ramps a duty cycle of the solid-state switch26being on from zero to one hundred percent, as will be described.

The controller32also provides a trip mechanism structured to detect a fault downstream of the second output portion8B of the HVDC circuit breaker8responsive to a failure to charge the downstream HVDC cable10. As will be described, the trip mechanism applies a predetermined current pulse and detects a corresponding predetermined voltage increase of a voltage of the downstream HVDC cable10. The failure to charge can be caused by a short circuit or an insulation failure of the downstream HVDC cable10.

As was indicated above, the circuit breaker8also includes the cable pre-charge function24. To resolve the problems of nuisance tripping, relatively large oscillatory currents, and relatively large voltage spikes along the cable length when connecting a DC voltage source to a relatively long cable transmission line, the disclosed hybrid DC circuit breaker8can be used to charge the cable capacitance Cc by utilizing the relatively fast switching capability of the solid-state switches26and the current limiting characteristic of the inductors30.

After the controller34verifies that there is no short circuit and no cable insulation degradation, the controller34employs further current impulses to continue to charge the cable capacitance Cc, such as formed by the distributed capacitance components10C. Then, after the controller34detects that the HVDC cable10is fully charged, the solid-state switches26will remain turned on.

A relatively longer HVDC cable10will take relatively more current impulses and will take a relatively longer time to charge to the rated source DC voltage with the solid-state switches26.

In one embodiment, the controller32predefines the current impulse frequency and duty cycle, and then adjusts them by the thermal capacity of the hybrid circuit breaker8and by the solid-state switch maximum current capability. In other words, the current impulse amplitude should be equal to or smaller than the solid-state switch maximum current, and the frequency of the impulses should not be so high that it will cause the solid-state switches26to overheat.

InFIG. 16, the voltage source27provides a relatively constant DC voltage amplitude to the HVDC cable10and to the downstream load29via the hybrid circuit breaker8as long as its output current capability is not exceeded.

The disclosed concept can be employed for both unipolar and bipolar DC power transmission even though for subsea or underground applications the system should be bipolar to avoid galvanic corrosion effects when using the ground or the water as the return path for the electric DC current. Unipolar is the transmission mode where the earth (or sea water) is used as the return path. In this case, only one pole is transmitted in an isolated cable. The return path is usually a buried graphite rod (not shown) on both ends of the transmission line coupled to a cable (not shown) that is then electrically connected to the equipment, such as29. The bipolar transmission mode, shown inFIG. 16, uses two conductor cables38,40, one for the positive pole and one for the negative pole.

Every DC current impulse from the HVDC circuit breaker8adds to the cable voltage by accumulating electric charge in the cable capacitance Cc (Equation 2), but the impulse duration is short enough to prevent the current from reaching an amplitude that will cause high voltage transients. The current amplitude is advantageously limited by: (1) the impulse duration (which is controlled by the controller32); (2) the HVDC bus voltage amplitude; and (3) the inductance of the inductors30(e.g., reactors).

The cable capacitance Cc varies linearly with the cable length and, thus, as the cable length increases, the cable capacitance Cc increases proportional to the length increase. For a given current impulse (amplitude and time), the voltage across the cable terminals10T decreases inversely proportional to the cable length increase.

This means that for a particular current impulse pattern (if fixed by the controller32), relatively more impulses are needed to charge the HVDC cable10to its desired or rated voltage as the cable length increases. On the other hand, if the cable capacitance Cc is preloaded (e.g., without limitation, manually) to the controller32, then it can adjust the current impulse to keep the pre-charge time the same. This can also be achieved automatically by applying a predetermined current impulse and expecting an ideal voltage step as a consequence. If the measured voltage is much lower than expected and it does not decay rapidly within a couple of seconds, then this means that the cable capacitance Cc is relatively higher than expected. Otherwise, if the measured voltage is much higher than expected as a consequence of the predetermined current impulse, then this means that the cable capacitance Cc is relatively lower than expected. In either case, the controller32can suitably adjust the current impulse to achieve the ideal or desired cable charging time.

FIG. 13shows the first pulse42of a pre-charge cycle43of the pre-charge function24, which is also employed for the short circuit detection, look ahead function22ofFIG. 16. The desired DC bus voltage in this example is 60 kVDC. The first plot44shows the DC bus voltage after the circuit breaker8switches at the cable input10A. The example voltage reached after the first pulse42of the second plot46is 2.6 kVDC. Otherwise, if the cable insulation is compromised, then this voltage will decrease rapidly. The second plot46shows the voltage pulse42generated when the circuit breaker solid-state switches26are turned on and then off. The example voltage pulse amplitude is 60 kVDC. The third plot48shows the current pulse49generated by the voltage pulse42and through the inductors30. The example current pulse amplitude of 200 A is limited by the controller32. The fourth plot50shows the circuit breaker solid-state switches26gate command signal52from the controller32.

FIG. 14shows the first fifty example pulses54of the pre-charge cycle43. Again, the DC bus voltage before the circuit breaker8switches in this example is 60 kVDC. The first plot56shows the DC bus voltage after the circuit breaker8switches at the cable input10A. The example voltage reached after the first fifty example pulses54is 9 kVDC. The second plot58shows the voltage pulses54generated when the circuit breaker solid-state switches26are turned on and then off The example voltage pulse amplitude is 60 kVDC. The third plot60shows the current pulses61generated by the voltage pulses54and through the inductors30. The example current pulse amplitude of 200 A is limited by the controller32. The fourth plot62shows the circuit breaker solid-state switches26gate command signals52from the controller32.

FIG. 15shows the full pre-charge cycle43. Again, the DC bus voltage before the circuit breaker8switches in this example is 60 kVDC. The first plot64shows the DC bus voltage after the circuit breaker8switches at the cable input10A. The voltage reached after the pre-charge cycle is 60 kVDC. The second plot66shows the voltage pulses54generated when the circuit breaker solid-state switches26are turned on and then off. The voltage pulse amplitude is 60 kVDC. The third plot68shows the current pulses69generated by the voltage pulses54and through the inductors30. The current pulse amplitude of 200 A is limited by the controller32. The fourth plot70shows the circuit breaker solid-state switches26gate command signal52from the controller32. The command signal52stays high or turned-on fully at71after the pre-charge cycle43is complete.

For the pre-charge function to work on a relatively long cable, the inductors30ofFIG. 16are not needed per se because the cable inductance will limit the di/dt. However, the inductors30are paramount during a short-circuit (e.g., without limitation, directly right at the output8B of the HVDC circuit breaker8in which case no cable inductance will limit the di/dt).

The cable pre-charge function24of the HVDC circuit breaker8prevents high voltage transients in the HVDC transmission line of the HVDC cable10. The DC energy from the output4B of the AC/DC converter4is transmitted to the HVDC circuit breaker8via the HVDC cable6(e.g., without limitation, which can be dozens of miles long). The voltage transient in this segment while it is energized is mitigated by the relatively slow climb of the AC/DC converter output voltage. This potential arrives at the input8A of the HVDC circuit breaker8whose output8B feeds the HVDC cable10that feeds the corresponding galvanically isolated DC/DC converter12, which powers the corresponding DC/AC voltage source inverter14.

To avoid the same issue described above in connection with the AC/DC converter4ramping the DC voltage at a limited rate as to avoid high voltage transients in the transmission line of the HVDC cable6, the output8B of the HVDC circuit breaker8is not switched on normally. Instead, the HVDC circuit breaker solid-state switches26are commanded on and off at relatively very short time segments. The solid-state switches26intended for the use in60kVDC circuit breakers can be switched on or off within a few microseconds. This feature allows for the HVDC circuit breaker output8B to be modulated to permit a controlled charging of the cable capacitance Cc and consequently a slow climb of the cable voltage to avoid the two times voltage transient. Moreover, with this feature, the HVDC circuit breaker8can also detect a fault in the transmission line of the HVDC cable10or at the input12A of the DC/DC converter12by turning on and off. Therefore, the pre-charge and the look ahead functions are operating simultaneously.

As one non-limiting example, the controller32limits the current to 200 A or less when the 60 kVdc voltage is switched on and off, and the current derivative is limited by the inductors30in series with both positive and negative cables. When the HVDC cable10is fully discharged, its voltage Vc is equal to zero volts between the positive and negative cables, and the first pulse will have a current derivative defined by:
di/dt=(60kVdc−Vc)/L
wherein:

L is the sum of the inductances of both inductors30(L1 and L2);

Vc is the instantaneous cable voltage;

dt is the time required to reach di; and

60 kVdc is the input voltage from the AC/DC converter4.

dt represents the on-time of the circuit breaker solid-state switch26and, thus, with the corresponding off-time, defines the modulation duty cycle. As the pulses build the voltage Vc in the cable capacitance, the difference (60 kVdc−Vc) diminishes, which increases dt and, thus, the duty cycle. When Vc reaches 60 kVdc, the difference (60 kVdc−Vc) equals zero and dt goes to infinity, which means that the solid-state switch26is continuously turned on.

The galvanically isolated buck DC/DC converter12allows for the motor output14B of the DC to AC voltage source inverter14to have one of its three phases grounded. The DC/DC converter12is isolated and reduces the HVDC coming from the source to a DC voltage level that is compatible with the motor insulation rating. This reduced voltage is fed into the input DC bus14A of the DC/AC voltage source inverter14whose output14B feeds the AC transmission line16and subsequently the AC motor18. The galvanic isolation reduces the effective zero sequence currents that may flow to the three-phase cable16and the AC motor18. This isolation allows shorting to ground of one of the motor phases with no operational interruption. The DC/AC voltage source inverter14is an active converter and, thus, the output14B thereof is protected against short circuit faults.

The galvanically isolated DC/DC buck converter12in the same enclosure20as the DC/AC voltage source inverter14allows for the transmission of power at high voltage up to the input14A of the voltage source inverter14, which reduces the current levels and consequently the voltage transients due to current changes in the HVDC transmission line of the HVDC cable10and better utilization of the cable conductor cross section. Also, a fault current generated due to a short circuit in the input14A of the voltage source inverter14will be contained by the DC/DC buck converter12.

The high power AC motor18is designed for medium voltage to reduce the copper cross section of its windings and of the corresponding supply cable16. The “buck” aspect of the galvanically isolated DC/DC buck converter12reduces the high direct current voltage from the corresponding HVDC cable10to a medium direct current voltage. Typically, for subsea applications, the AC motors18are rated, for example and without limitation, for 4160 V or 6600 V input. The insulation rating for a 6600 V motor is 20 kVDC or 14,200 VACRMS. If power is transmitted from a surface platform (see the AC/DC converter4ofFIG. 5) to the motor18subsea at about 60 kVDC, then this voltage is three times higher than the motor's insulation voltage rating. Therefore, at some point in the transmission line, the 60 kVDC has to be reduced to well below 20 kVDC. For the inverter14to produce a 6600 VAC output, its input voltage needs to be around 10 kVDC. Hence, the DC/DC converter12reduces the transmitted 60 kVDC, in this example, to 10 kVDC.

Subsea cables are relatively expensive, such as about $1000 per meter not counting the installation cost. The largest cables for subsea applications would be ideally limited to around 240 square millimeters due to sheer size and weight which affect the cost and difficulty of installation. This means that the electric current is limited by the copper cross section. For 240 square millimeters, the maximum payload current is between 400 A to 500 A depending on the top side and water temperature conditions.

In order to transmit power effectively, a high voltage is needed as close to the load18as possible, in order to reduce the amount of current in the transmission line and thus reduce the conductor cross section, which in the subsea case, has a limit due to installation restrictions. This is provided by the inclusion of the DC/DC buck converter12with the inverter14whose output14B feeds the AC motor18. In the above example, the 60 kVDC is transmitted all the way to the point of delivery and the system2can benefit from the lower current up to that point. Continuing to apply this concept to the above example, with a 450 A maximum cable current and 60 kVDC, the transmission line can carry 27 megawatts. With the same 450 A limitation and 10 kVDC, the maximum power transmitted is 4.5 megawatt.

The disclosed “in the same box” configuration of the DC/DC buck converter12, the inverter14and the enclosure20reduces current levels and consequently voltage transients due to current changes in the transmission line of the HVDC cable10and provides better utilization of the cable conductor cross section. Better utilization of the cable conductor cross section was explained, above. The reduction of voltage transients is a consequence of the reduction of current magnitude. The formula for voltage transients in a transmission line can be simplified to obtain:
Vtrans=L*di/dt
wherein:

Vtrans is the peak voltage transient which goes above the DC transmission voltage;

L is the inductance of the transmission line; and

di/dt is the current transient which is higher in the presence of solid-state switching devices switching loads on or off

Therefore, for the same switching time, dt, a smaller current magnitude, di, will produce a smaller transient voltage.

The galvanically isolated DC/DC buck converter12allows for the output to the AC motor18to be grounded in one of the phases. The system grounding is complex when it comes to DC transmission. The generators on a surface platform (e.g.,FIG. 5) will be generally high impedance grounded. The output of the AC/DC converter14is also high impedance grounded via its neutral point. When using a DC/DC converter12that is isolated from the source on the platform, it means that the inverter circuit connected to its output12B is electrically floating with reference to the source. Therefore, if any one of the inverter's output terminals14B is connected to ground, then there is no ground current that flows in the source.

If there is a short circuit operatively associated with the DC input14A of the corresponding voltage source inverter14(e.g., without limitation, in the inverter components (not shown)), then the power semiconductor switches (not shown) of the galvanically isolated DC/DC buck converter12are turned off to shut down the converter12, thus, protecting the inverter14from excessive fault energy.

The voltage source inverter14inputs from the DC input14A and outputs AC from the output14B to the AC motor18. Due to the possibility of relatively long electrical connections between the inverter output14B and the AC motor18, there can be a suitable filter (not shown) to minimize common mode and harmonic currents. The inverter14protects the three-phase transmission lines16and the motor windings (not shown) in case of a short circuit fault. The inverter output14B has a filter (not shown) to suppress the pulse width modulation transients allowing for a relatively long transmission line cable16to the AC motor18. The inverter14employs a selective harmonic elimination modulation method to minimize the filter size.

A fault current generated due to a short circuit in the transmission lines16or the AC motor18will be contained by the inverter14.

The voltage source inverter14has a relatively large capacitor (not shown) connected between the positive and the negative terminals of its input DC bus14A. In contrast, a current source inverter (not shown) does not have such as capacitor, but does have a relatively large inductor (not shown) in series with its DC bus positive and/or negative terminals (not shown) of its input DC bus.

The distribution system7is divided into sections that are protected by different elements in the circuit. The AC motor18is protected by the inverter14. If there is a short circuit in the three-phase transmission lines16between the inverter14and the motor18or in the motor18itself, then the inverter semiconductor switches (not shown) are turned off to stop the fault current that is detected by the inverter control circuit (not shown).

The alternating current output14B of the voltage source inverter14can be a low alternating current voltage, a medium alternating current voltage, or a high alternating current voltage.

The three-phase transmission lines16can employ low, medium or high voltage AC.

The AC motor18can be a low, medium or high voltage AC motor, such as a three-phase AC motor.

The enclosure20is preferably compensated for subsea pressure. For example and without limitation, U.S. Pat. No. 6,822,866 discloses a power conversion system that is void of air by the vacuuming and immersion of power conversion system components in a dielectric gel, oil, gas or in vacuum. This permits the high voltage circuit to become much smaller than it would be in air. To build a pressure compensated power conversion system, its components are immersed in a fluid that: (1) fills all gaps and voids in the converter structure in order to allow the pressure in the enclosure20to be equalized with the external pressure; (2) is dielectric in nature and inert to prevent chemical corrosion of components immersed therein; and (3) transports heat energy generated in the DC/DC converter components to the walls (not shown) of the enclosure20.

The disclosed concept permits the cable size of the three-phase transmission lines16to be relatively smaller and to get increasingly smaller as it gets closer to the AC motor18due to the decreasing sea water temperature as the cable goes deeper. One umbilical cable is for 20,000 MW (e.g., maximum cable size has to do with its temperature rating and the limitations imposed by the I-tube (e.g., without limitation, a straight vertical tube that is typically 220 feet long, encases the power cable (or “umbilical”), is physically attached to the platform structure, and protects the cable against sea-induced motions), which supports and holds the cable that leaves the oil platform at sea level as it goes down into the sea bed; there is a length of cable that the I-tube grabs and thus constrains the cable cooling and thus requires the worst case conductor cross section; as the transmission line increases in the subsea region, the water temperature drops dramatically and the cable could have its conductor cross section reduced from a given splice point). This permits transmission of DC power until a few meters from the drive/motor system at the voltage source inverter14. The voltage drop due to relatively long cable length and load power variation is compensated by the AC/DC converter4and the DC/DC converter12regulates the DC bus voltage of the inverter14to provide full 6.6 kV to the AC motor18regardless of the cable length or load variation. The voltage source inverter14to AC motor18cable distance can be reduced to less than 150 feet. Performance and control are improved and there is no need for expensive filters.

The disclosed concept can employ a surface or sea-bed installed AC to DC (typically 10 kVDC to 240 kVDC scalable) two-wire (+, − configured) power conversion assembly fed by a single umbilical cable to a HVDC switchgear assembly comprised of one or multiple solid-state protected and controlled HVDC circuit breakers8in a pressure compensated enclosure (not shown but see Example 9) to withstand, for example, a minimum of 300 bar of pressure and 3 kM of sea water depth.

The individual sea-bed switchgear breakers8or single group mounted switchgear breakers8could feed various AC or DC loads, such as the example AC motors18, on the sea-bed including the voltage source inverter14up to and exceeding 13.8 kVAC output voltage.

FIGS. 6A-6Bshow an example distribution system7according to the disclosed concept, as powered by the AC/DC converter4and the HVDC cable6, including the subsea switch gear8, plural cables10, plural drives each formed by the DC/DC buck converter12and the voltage source inverter14, plural cables16and plural AC motors18. Example electrical and cable parameters are also shown. Due to the example system disclosed here, the transmission and distribution cables never conduct a current higher than the 450 A limit.

FIGS. 7 and 8show differences between a prior system2′ and a non-limiting example of the disclosed system2, except simplified to remove the HVDC circuit breaker8. The case inFIG. 7shows the consequence of reducing the DC voltage midway and the other case inFIG. 8shows the advantages of having the DC/DC converter12in the same enclosure20(FIG. 1) as the inverter14. The example cable parameters accurately reflect real world parameters. In both cases, the AC/DC converter4transmits 132 kVdc and 401 A via a 270 mile cable inFIG. 7and a 309 mile cable inFIG. 8. Due to the resistive losses in the cable inFIG. 7, the input voltage at the input of the DC/DC converter12is 97.9 kV while inFIG. 8it is 92.9 kVdc. By transmitting the high DC voltage on the HVDC cable10up to the drive enclosure20inFIG. 8, which includes the buck DC/DC converter12(which reduces 92.9 kVDC to 11 kVDC), this avoids any substantial length of cable13′ (FIG. 7) carrying a current of 3480 Adc and needing a ten times higher cross section than the 240 sq. mm. HVDC cable10in the example. Instead, the relatively short conductor bus (e.g., a few feet of length)13(FIG. 8) carries 3,480 ADC at 11 kVDC to transmit the same amount of load power.

FIG. 9shows a supply cable102(e.g., such as HVDC cable6ofFIG. 1) electrically connected to a line (e.g., supply) plug104of a single circuit breaker enclosure106having three example branch circuit breakers108,110,112, each of which has a load plug114,116,118, respectively. The example branch circuit breakers108,110,112are HVDC circuit breakers, the same as or similar to the HVDC circuit breaker8ofFIG. 1, and are enclosed by the example single enclosure106. The single line plug104is externally electrically connected to the opposite second end120of the HVDC cable102and is internally electrically connected to a first input portion124of the HVDC circuit breakers108,110,112. Each of the load plugs114,116,118is externally electrically connected to a first input end126of a corresponding HVDC cable128.

FIG. 10shows the supply cable202electrically connected to a line (e.g., supply) plug204of a distribution module206having an input207and three example output plugs208,210,212, each of which engages a separate HVDC branch circuit breaker214,216,218, each of which has a load plug220and is enclosed by a corresponding enclosure215,217,219, respectively.

FIG. 11is similar toFIG. 10except that the distribution module206′ is interconnected with the HVDC branch circuit breakers214,216,218by three external cables302,304,306, respectively. The distribution module206′ has an input308electrically connected to the opposite second end310of the supply cable202and a plurality of outputs312,314,316. Each of the outputs312,314,316is electrically connected to the input portion318of a corresponding one of the HVDC circuit breakers214,216,218.

FIG. 12shows a supply cable402electrically connected to a line (e.g., supply) plug410of a circuit breaker enclosure406having one branch circuit breaker408including the line plug410, a load plug412and a daisy chain plug414. Two other circuit breaker enclosures416,418, each having one branch circuit breaker420including a line plug422, a load plug424and a daisy chain plug426, are interconnected with the first circuit breaker enclosure406by daisy chain cables428electrically connected between the daisy chain plug414or426of a prior circuit breaker enclosure406,416and the line plug422of another circuit breaker enclosure416,418.

The line plug410of the enclosure406is externally electrically connected to the opposite second end404of the HVDC cable402and internally electrically connected to the plug414and the input portion408A of the HVDC circuit breaker408. For the HVDC circuit breakers420, the jumper cables428electrically connect the plugs414and422or426and422. Each of the load plugs412,424is externally electrically connected to the first end430of a corresponding HVDC cable432.