In the field of high voltage direct current power transmission networks, a voltage source converter comprises first and second DC terminals for connection to a DC electrical network, and a plurality of single-phase limbs. Each single-phase limb includes a phase element, and each phase element includ
In the field of high voltage direct current power transmission networks, a voltage source converter comprises first and second DC terminals for connection to a DC electrical network, and a plurality of single-phase limbs. Each single-phase limb includes a phase element, and each phase element includes at least one switching element configured to interconnect a DC voltage and an AC voltage. An AC side of each phase element is connectable to a respective phase of a multi-phase AC electrical network, and each single-phase limb is connected between the first and second DC terminals. The voltage source converter further comprises a controller configured to determine independently of one another an amount of active power (Pref) that the voltage source converter should exchange with the AC electrical network and an amount of reactive power (Qref) that the voltage source converter should exchange with the AC electrical network.
대표청구항▼
1. A voltage source converter comprising: first and second DC terminals for connection to a DC electrical network;a plurality of single-phase limbs, each single-phase limb including a phase element, each phase element including at least one switching element configured to interconnect a DC voltage a
1. A voltage source converter comprising: first and second DC terminals for connection to a DC electrical network;a plurality of single-phase limbs, each single-phase limb including a phase element, each phase element including at least one switching element configured to interconnect a DC voltage and an AC voltage, an AC side of each phase element being connectable to a respective phase of a multi-phase AC electrical network, each single-phase limb being connected between the first and second DC terminals; anda controller configured to determine independently of one another an amount of active power that the voltage source converter should exchange with the AC electrical network and an amount of reactive power that the voltage source converter should exchange with the AC electrical network, the controller being further configured to establish a respective phase current reference for each single-phase limb which is independent of each other respective phase current reference, the respective phase current reference for each single-phase limb defining the current each single-phase limb is required to draw from or pass to a corresponding phase of the AC electrical network to effect the determined active power and reactive power exchanges with the AC electrical network;wherein the controller includes a first control block comprising an active power control block to determine the amount of active power to be exchanged with the AC electrical network, and a reactive power control block to determine the amount of reactive power to be exchanged with the AC electrical network,wherein the active power control block includes: a power control section which outputs the amount of active power to be exchanged, the power control section including a selectively operable DC voltage control portion and a selectively operable DC power control portion, the DC voltage control portion operating when the voltage source converter is configured to transfer energy from the AC electrical network to the DC electrical network and the DC power control operating when the voltage source converter is configured to transfer energy from the DC electrical network to the AC electrical network; anda fault detection section configured to modify the output of the power control section in the event of the currents flowing in the single-phase limbs becoming unbalanced in order to alter the amount of active power that the voltage source converter exchanges with the AC electrical network. 2. The voltage source converter according to claim 1, wherein the fault detection section includes a voltage level fault detector configured to detect a decrease in the AC voltage of the AC electrical network and thereafter temporarily reduce the amount of active power that the voltage source converter exchanges with the AC electrical network. 3. The voltage source converter according to claim 1, wherein the fault detection section includes a single phase fault detector configured to detect a fault to ground in a single phase of the AC electrical network and thereafter temporarily reduce the amount of active power that the voltage source converter exchanges with the AC electrical network. 4. The voltage source converter according to claim 1, wherein the fault detection section includes a multi-phase fault detector configured to detect a fault to ground in each phase of the AC electrical network and thereafter temporarily reduce to zero the amount of active power that the voltage source converter exchanges with the AC electrical network. 5. The voltage source converter according to claim 1, wherein the fault detection section includes a manipulation module operable at a first time constant and a second time constant, the manipulation module altering the operation of the fault detection section whereby any reduction in the amount of active power that the voltage source converter exchanges with the AC electrical network takes place at the first time constant and any subsequent increase in the amount of active power that the voltage source converter exchanges with the AC electrical network takes place at the second time constant. 6. The voltage source converter according to claim 5, wherein the first and second time constants differ from one another. 7. The voltage source converter according to claim 1, wherein the reactive power control block includes a selectively operable AC voltage control portion and a selectively operable fault control portion, the AC voltage control portion operating when the currents flowing in the single-phase limbs are balanced, and the fault control portion operating when the currents flowing in the single-phase limbs are unbalanced. 8. The voltage source converter according to claim 1, wherein the controller includes a second control block which has a current reference generator that establishes the respective phase current reference for each single-phase limb based on the amounts of active and reactive power determined by the first control block. 9. The voltage source converter according to claim 8, wherein the current reference generator further establishes the respective phase current reference for each single-phase limb based on positive and negative voltage sequence components within a stationary α-β reference frame, the positive and negative voltage sequence components being derived from an actual phase voltage of each phase of the AC electrical network. 10. The voltage source converter according to claim 8, wherein the second control block also includes a transformation module configured to transform components within a stationary α-β reference frame to individual phase components. 11. The voltage source converter according to claim 8, wherein the second control block also includes a current limiter module configured to limit the maximum value of each phase current reference the current reference generator is able to establish. 12. The voltage source converter according to claim 8, wherein the controller includes a third control block which has a plurality of phase controls, each of which corresponds to a respective phase of the AC electrical network, and each of which is configured to generate an AC voltage demand that the corresponding single-phase limb is required to provide in order that the current flowing in the corresponding single-phase limb tracks the established phase current reference. 13. The voltage source converter according to claim 1, wherein the controller includes a positive and negative sequence generator to derive positive and negative voltage sequence components within a stationary α-β reference frame from the actual phase voltage of each phase of the AC electrical network. 14. The voltage source converter according to claim 2, wherein the fault detection section includes a single phase fault detector configured to detect a fault to ground in a single phase of the AC electrical network and thereafter temporarily reduce the amount of active power that the voltage source converter exchanges with the AC electrical network. 15. The voltage source converter according to claim 2, wherein the fault detection section includes a multi-phase fault detector configured to detect a fault to ground in each phase of the AC electrical network and thereafter temporarily reduce to zero the amount of active power that the voltage source converter exchanges with the AC electrical network. 16. The voltage source converter according to claim 2, wherein the fault detection section includes a manipulation module operable at a first time constant and a second time constant, the manipulation module altering the operation of the fault detection section whereby any reduction in the amount of active power that the voltage source converter exchanges with the AC electrical network takes place at the first time constant and any subsequent increase in the amount of active power that the voltage source converter exchanges with the AC electrical network takes place at the second time constant. 17. The voltage source converter according to claim 9, wherein the second control block also includes a transformation module configured to transform components within a stationary α-β reference frame to individual phase components.
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