Control method for a parallel MMC unit of a LCC-MMC hybrid cascade converter station

Abstract

Provided is a control method for a parallel MMC unit of a LCC-MMC hybrid cascade converter station. The control strategy includes: 1) numbering all MMC units connected in parallel in a MMC valve manifold; (2) for a MMC unit using a constant direct-current voltage control manner, calculating a direct-current instruction value of the MMC unit according to a direct-current measurement value; (3) for a MMC unit using a constant active power control manner, calculating an active power instruction value of the MMC unit according to the rated capacity of the MMC unit and a direct-current instruction value of a system rectifier station; (4) for the MMC unit using the constant direct-current voltage control manner, correcting a direct-current voltage instruction value of the MMC unit by using the direct-current instruction value and the direct-current measurement value, and controlling the MMC unit according to the corrected direct-current voltage instruction value.

Claims

1. A control method for a parallel Modular Multilevel Converter (MMC) unit of a Line Commutated Converter (LCC)-MMC hybrid cascade converter station, comprising following steps: (1) numbering all MMC units connected in parallel in a MMC valve manifold of the LCC-MMC hybrid cascade converter station according to a control manner and a rated capacity of each MMC unit of the MMC units, wherein a specific implementation process of the step (1) is as follows: firstly, dividing all MMC units in the MMC valve manifold into two types, that is, MMC units controlled in the constant direct-current voltage control manner and counted to be N.sub.1, and MMC units controlled in the constant active power control manner and counted to be N.sub.2; and numbering the MMC units controlled in the constant direct-current voltage control manner from 1 to N.sub.1 according to an order of rated capacities from small to large, and numbering the MMC units controlled by the constant active power from N.sub.1+1 to N.sub.1+N.sub.2 according to an order of rated capacities from small to large; (2) in a case where a MMC unit of the all MMC units connected in parallel uses a constant direct-current voltage control manner, calculating a direct-current instruction value of the MMC unit according to a direct-current measurement value; (3) in a case where a MMC unit of the all MMC units connected in parallel uses a constant active power control manner, calculating an active power instruction value of the MMC unit according to the rated capacity of the MMC unit and a direct-current instruction value of a system rectifier station; (4) in a case where the MMC unit of the all MMC units connected in parallel uses the constant direct-current voltage control manner, correcting a direct-current voltage instruction value of the MMC unit by using the direct-current instruction value and the direct-current measurement value, and further controlling the MMC unit according to the corrected direct-current voltage instruction value; and (5) in a case where the MMC unit of the all MMC units connected in parallel uses the constant active power control manner, controlling the MMC unit according to the active power instruction value.

2. The control method of claim 1, wherein in the step (2), the direct-current instruction value of the MMC unit is calculated by using a following formula: $i_{d\mathrm{cref\_k}}=\frac{S_k}{i_{d\mathrm{cba\_k}}\underset{l=1}{\overset{N_1}{.Math.}}{S}_i}\underset{i=1}{\overset{N_1}{.Math.}}\left(i_{\mathrm{dci}}.Math.i_{\mathrm{dcba\_i}}\right)$ wherein i.sub.dcref_k is a direct-current instruction value of a k-th MMC unit in the MMC valve manifold, S.sub.k is a rated capacity of the k-th MMC unit, S.sub.i is a rated capacity of an i-th MMC unit, i.sub.dci and i.sub.dcba_i are a direct-current measured value and a direct-current reference value of the i-th MMC unit respectively, i.sub.dcba_k is a direct-current reference value of the k-th MMC unit, i and k are natural numbers, 1≤i≤N.sub.1, 1≤k≤N.sub.1, and N.sub.1 is a number of MMC units controlled in the constant direct-current voltage control manner in the MMC valve manifold.

3. The control method of claim 1, wherein in the step (3), the active power instruction value of the MMC unit is calculated by using a following formula; $P_{\mathrm{ref}\_r}=\frac{S_r}{i_{\mathrm{dcba}\_r}\underset{j=1}{\overset{N_1+N_2}{.Math.}}{S}_j}{i}_{\mathrm{ba}\_\mathrm{rec}}.Math.i_{\mathrm{ref}\_\mathrm{rec}}$ wherein P.sub.ref_r is an active power instruction value of an r-th MMC unit in the MMC valve manifold, i.sub.ba_rec and i.sub.ref_rec are a direct-current instruction value and a direct-current reference value of a rectifier station respectively, S.sub.r is a rated capacity of the r-th MMC unit, S.sub.j is a rated capacity of a j-th MMC unit, i.sub.dcba_r is a direct-current reference value of the r-th MMC unit, N.sub.1 is a number of MMC units controlled in the constant direct-current voltage control manner in the MMC valve manifold, N.sub.2 is a number of MMC units controlled in the constant active power control manner in the MMC manifold, r and j are natural numbers, 1≤j≤N.sub.1+N.sub.2, and N.sub.1+1≤r≤N.sub.1+N.sub.2.

4. The control method of claim 1, wherein a specific implementation process of the step (4) is as follows: firstly, subtracting a direct-current instruction value i.sub.dcref_k from a direct-current measurement value i.sub.dck of a k-th MMC unit to obtain a corresponding current error value; then, introducing the current error value into a proportional control stage and an amplitude limiting stage in turn to obtain a direct-current voltage correct value of the k-th MMC unit; and finally, adding the direct-current voltage correct value to an original direct-current voltage instruction value of the k-th MMC unit to obtain a corrected direct-current voltage instruction value, and controlling the k-th MMC unit in the constant direct-current voltage control manner according to the corrected direct-current voltage instruction value, wherein 1≤k≤N.sub.1, and N.sub.1 is a number of MMC units controlled in the constant direct-current voltage control manner in the MMC valve manifold.

5. The control method of claim 4, wherein a proportional coefficient of the proportional control stage is set to 0.1, and a maximum output limit value and a minimum output limit value of the amplitude limiting stage are set to 0.1 p.u. and −0.1 p.u. respectively.

6. The control method of claim 1, wherein according to the control strategy, active power of a MMC unit for controlling active power is calculated, and a direct-current voltage instruction value correction stage is added in a constant direct-current voltage MMC unit, a direct-current is reasonably distributed in the MMC units connected in parallel.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1A is a structural diagram of a power transmission system in which an LCC converter station is used on a rectifier side and an LCC-MMC hybrid cascade converter station is used on an inverter side according to the present disclosure;

(2) FIG. 1B is a structural diagram of a MMC valve manifold on an inverter side a system according to the present disclosure;

(3) FIG. 2 is a flowchart of steps of a control method according to the present disclosure;

(4) FIG. 3 is a block diagram of calculation principle of a direct-current voltage instruction value of an MMC unit using a constant direct-current voltage control manner;

(5) FIG. 4 is a simulation waveform diagram of direct-current voltages of outlets of MMC units obtained by simulation using a method of the present disclosure;

(6) FIG. 5 is a simulation waveform diagram of a direct current of an inverter station obtained by simulation using a method of the present disclosure; and

(7) FIG. 6 is a simulation waveform diagram of direct currents of MMC units obtained by simulation using a method of the present disclosure.

DETAILED DESCRIPTION

(8) In order to describe the present disclosure more specifically, the following detailed description of the present disclosure is made with reference to the accompanying drawings and the detailed description of the present disclosure.

(9) As shown in FIG. 2, a control strategy suitable for a parallel MMC unit of a LCC-MMC hybrid cascade converter station according to the present disclosure includes following steps.

(10) (1) All MMC units connected in parallel in a MMC valve manifold of a converter station are numbered according to a control manner and a rated capacity of each MMC unit of the MMC units.

(11) Supposing that N.sub.1 MMC units among parallel MMC units of an inverter station use a constant direct-current voltage control manner, and N.sub.2 MMC units use a constant active power control manner, the N.sub.1 MMC units controlled in the constant direct-current voltage control manner are numbered from 1 to N.sub.1 according to an order of rated capacities from small to large, and the MMC units controlled by the constant active power are numbered from N.sub.1+1 to N.sub.2+N.sub.1 according to an order of rated capacities from small to large.

(12) (2) For a MMC unit using a constant direct-current voltage control manner, a direct-current instruction value of the MMC unit is calculated according to a direct-current measurement value.

(13) Supposing that the parallel MMC units controlled by the constant direct-current voltage manner have rated capacities of S.sub.1 to S.sub.N1 respectively, a direct-current instruction value (p.u.) of a k-th MMC unit in the MMC valve manifold is calculated by using a following formula:

(14) $i_{\mathrm{dcref}\_k}=\frac{S_k}{i_{\mathrm{dcba}\_k}\underset{i=1}{\overset{N_1}{.Math.}}{S}_i}\underset{i=1}{\overset{N_1}{.Math.}}\left(i_{\mathrm{dci}}.Math.i_{\mathrm{dcba}\_i}\right)$

(15) where, i.sub.dci and i.sub.dcba_i are a direct-current measured value (p.u.) and a direct-current reference value of the i-th MMC unit respectively (1≤i≤N.sub.1).

(16) (3) For a MMC unit using a constant active power control manner, an active power instruction value of the MMC unit is calculated according to the rated capacity of the MMC unit and a direct-current instruction value of a system rectifier station.

(17) Supposing that N2 parallel MMC units controlled by the constant direct-current voltage manner have rated capacities of S.sub.N1 to S.sub.(N1+N2) respectively, an active power instruction value Pref_r (p.u.) of an r-th (N.sub.1≤r≤N.sub.2+N.sub.1) MMC unit in the MMC valve manifold is calculated by using a following formula:

(18) $P_{\mathrm{ref}\_r}=\frac{S_r}{i_{\mathrm{dcba}\_r}\underset{j=1}{\overset{N_1+N_2}{.Math.}}{S}_j}{i}_{\mathrm{ba}\_\mathrm{rec}}.Math.i_{\mathrm{ref}\_\mathrm{rec}}$

(19) where i.sub.ba_rec and i.sub.ref_rec are a direct-current instruction value (p.u.) and a direct-current reference value of a rectifier station respectively, S.sub.i is a rated capacity of the i-th MMC unit (1≤i≤N.sub.1+N.sub.2), i.sub.dcba_r is a direct-current reference value of the r-th MMC unit (N.sub.1+1≤r≤N.sub.1+N.sub.2).

(20) (4) For the MMC unit using the constant direct-current voltage control manner, a direct-current voltage instruction value of the MMC unit is corrected by using the direct-current instruction value and the direct-current measurement value, and the MMC unit is controlled according to the corrected direct-current voltage instruction value; and for the MMC unit using the constant active power control manner, the MMC unit is controlled according to the active power instruction value calculated in step (3).

(21) As shown in FIG. 3, a direct-current instruction value i.sub.dcref_k is subtracted from a direct-current measurement value i.sub.dck of a k-th (1≤k≤N.sub.1) MMC unit to obtain a corresponding current error value; the current error value is introduced into a proportional control stage and an amplitude limiting stage in turn to obtain a direct-current voltage correct value ΔU.sub.dcrefk of the k-th MMC unit; the direct-current voltage correct value is added to an original direct-current voltage instruction value U.sub.dc0k (p.u.) of the k-th MMC unit to obtain a corrected direct-current voltage instruction value U.sub.dcrefi (p.u.).

(22) Preferably, a proportional coefficient of the proportional control stage is set to 0.1, and a maximum output limit value and a minimum output limit value of the amplitude limiting stage are set to 0.1 p.u. and −0.1 p.u. respectively.

(23) A structure of a direct-current transmission system according to this embodiment is as shown in FIG. 1A, where the rectifier station is constituted by using two 12-pulsating LCCs, and a high voltage valve bank of an inverter station is constituted by using a 12-pulsating LCC, and a low voltage valve bank uses three MMC units connected in parallel to constitute a MMC valve Bank (MMCB), as shown in FIG. 1B, all devices are controlled in the constant direct-current voltage control manner, and parameters of the direct-current transmission system are as shown in Table 1:

(24) TABLE-US-00001 TABLE 1 Parameter Value Type Name Rectifier side Inverter side Basic Rated capacity (MW) 5000 4750 Parameters Rated direct-current voltage (kV) 800 760 Rated direct current (kA) 6.25 6.25 LCC direct-current voltage (kV) 800 380 MMC direct-current voltage (kV) 380 Active value of alternating-current 500 500 voltage system voltage (kV) MMC Rated capacity (MW) 833 Parameters Number of bridge arm HBSMs 182 Capacitance in a HBSM (mF) 15 Bridge arm reactance (mH) 55 Transformer LCC Winding type Y0/Y parameters connection Transformation Rectifier side: 500/175 transformer ratio/(kV/kV) Inverter side: 500/162 Capacity/MVA 1500 MMC Winding type Y0/Δ connection Transformation 500/200 transformer ratio/(kV/kV) Capacity/MVA 1000 Direct-current Line length/km 2100 line Resistance/Ω 6.4 parameters Inductance/mH 1620

(25) A corresponding simulation platform is built in the electromagnetic transient simulation software PSCAD/EMTDC, and the three-phase metallic short-circuit fault of Bus-2 is simulated on the platform. In the simulation, it is assumed that a 1.5 s failure occurs in 1.5 seconds and the failure lasts 0.1 seconds. FIG. 4 shows waveforms of direct-current voltages of 3 parallel MMC units, FIG. 5 shows a waveform of a direct current of an inverter station, FIG. 6 shows waveforms of direct currents of 3 parallel MMC units, and simulation results prove effectiveness of the present disclosure.