Control of HVDC transmission system based on mmc with three level flying capacitor submodule

Control of HVDC transmission system based on mmc with three level flying capacitor submodulePresented by : ANAND P P s7 f roll no : 6 1contentsINTRODUCTIONWHAT IS HVDC TRANSMISSION SYSTEM?MMCSYSTEM DESCRIPTION AND MODELINGDESCRIPTION OF SYSTEMDecoupled current control modelControl and modulation of mmc- hvdc systemAC and DC current control for MMC- BAC and DC current control for MMC- AArms Energy Balance ControlModulation SchemeAdvantagesDisadvantagesCONCLUSIONSREFERENCES

2IntroductionHVDC stands for High Voltage Direct Current and is today a well-proven technology employed for power transmission all over the world. HVDC technology is used to transmit electricity over long distances byoverhead transmission linesor submarine cables.

It is also used to interconnect separate power systems, where traditional alternating current (AC) connections can not be used.

3What is hvdc transmission system?

4Modular multilevel converter ( mmc )Modular multilevel converter ( MMC ) based HVDC system is the most popular High modularity , high power quality , common dc bus and capable to manage DC faults 2L HB submodule are widely used .it is also possible to use multilevel submodules A 3L FC submodule is considered over 2L HB , which has 2 floating capacitors with different voltage rating

5CONT. During normal operation of MMC , submodule capacitors voltage need to be regulated at its nominal value . In steady state the imbalance in capacitor voltage has a severe impact on the converter performance therefore a control strategy is required to regulate the capacitor voltage along with active and reactive power flowVerified using MATLAB/SIMULINK software 6SYSTEM DESCRIPTION AND MODELINGA . DESCRIPTION OF SYSTEM

FIG 1:- HVDC SYSTEM BASED ON MODULAR MULTILEVEL CONVERTER7

Cont.IT CONSIST OF 2 AC systems , 2 MMC HVDC station consist of 3L- FC submodule and a DC cables which connects two stations modelled as a T-modelIt has six arm and each arm is composed by large number of submodule connected in series .arms are connected between DC terminals and each AC phaseEach 3L-FC submodule has 2 flying capacitors (c1 and c2) . The outer capacitor voltage (vc1) is twice that of inner capacitor voltage (vc2)The switching state and the direction of the arm current decides the charging and discharging of submodule capacitor

8Cont.

FIG 2 :- POWER CIRCUIT OF THREE PHASE MMC9CONT.FIG 3:-Modular MultiLevel Converter submodule states

FIG 4:- CIRCUIT TOPOLOGY 3L-FC

10B. Decoupled current control model11CONT.12Control and modulation of mmc- hvdc systemThe HVDC systems are composed by two AC stations, where one station controls the total DC-line voltage the other station regulates the total transmitted active and reactive power.In this case, the MMC-B regulates the total transmitted active and reactive power the MMC-A maintains the total DC-line voltage

13A . AC and DC current control for MMC-B

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FIG 5 :- BLOCK DIAGRAM OF MMC B CONTROL SYSTEM

Cont.The MMC-B control the total transmitted powerTo control these currents , a PI controller in rotating reference frame synchronized with the output frequency of the MMC-B is used The current idcB is a continuous DC-signalPI controller is used to generate the DC component of the modulation signal (mdcB)The reference DC current component idcB is generated by using a total energy control loop

15B. AC and DC current control for MMC- AThe MMC-A regulates the total DC-line voltage

FIG 6 :- BLOCK DIAGRAM OF MMC - A CONTROL

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c. Arms Energy Balance ControlTo ensure the average DC voltage in positive and negative arms of MMC-A and MMC-B, an arm energy balance controller is used in the DC component of the modulation index control loopFrom fig :- the actual total energy of the arm (Eyx) is compared with its reference (Ex). The resultant error is controlled by a PI-controller. The resultant output is denominated as mdcyx and it is added to the DC modulation index (mdcyx)

FIG 7:- BLOCK DIAGRAM OF ARM BALANCING CONTROL

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17D . Modulation Scheme18AdvantagesReactive power requirementSystem stabilityLesser Corona Loss and Radio interferenceIndependent Control of ac systemFast change of energy flowLine losses in a dc line are lower than the losses in an ac linesThe towers of the dc lines are narrower, simpler and cheaper compared to the towers of the ac lines.

19DisadvantagesExpensive converters with limited overload capacityHigher losses in static converters at smaller transmission distancesHigh voltage DC circuit breakers are difficult to build20CONCLUSIONSA control strategy to control the active power, reactive power and total DC-line voltage in a MMC-3L-FC based HVDC transmission system. With the proposed control strategy, the independent control of the power flow, submodule capacitors voltage in each MMC station and total DC-line voltage are achieved. The control and modulation strategies are easily extended to any number of submodules per arm21REFERENCESM. Perez, R. Lizana F, and J. Rodriguez, Decoupled current control of modular multilevel converter for hvdc applications, in Industrial Electronics (ISIE), 2012 IEEE International Symposium on, May 2012 , pp. 19791984.S. Cole and R. Belmans , Transmission of bulk power, Industrial Electronics Magazine, IEEE, vol. 3, no. 3, pp. 19 24, sept. 2009https://en.wikipedia.org/wiki/HVDC_converter

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