Super Capacitor Energy Storage Based MMC for Energy ...€¦ · DC bus capacitor. An energy storage-enabled MMC seems an ideal solution for the abovementioned issues, since the super

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Article

Super Capacitor Energy Storage Based MMC forEnergy Harvesting in Mine Hoist Application

Xiaofeng Yang 1,* ID , Piao Wen 1, Yao Xue 1, Trillion Q. Zheng 1 and Youyun Wang 2

1 School of Electrical Engineering, Beijing Jiaotong University, Beijing 100044, China;[email protected] (P.W.); [email protected] (Y.X.); [email protected] (T.Q.Z.)

2 Tianshui Electric Drive Research Institute Co. LTD, Tianshui 741020, China; [email protected]* Correspondence: [email protected]; Tel.: +86-10-5168-7064

Received: 23 August 2017; Accepted: 13 September 2017; Published: 17 September 2017

Abstract: This paper proposes a super capacitor energy storage-based modular multilevel converter(SCES-MMC) for mine hoist application. Different from the conventional MMCs, the sub-modulesemploy distributed super capacitor banks, which are designed to absorb the regenerative energyof mine hoist and released in the traction condition, so as to improve energy utilization efficiency.The key control technologies are introduced in detail, followed by analysis of the configuration andoperation principles. The feasibility of the proposed SCES-MMC topology and the control theory arealso verified. Simulation results show that SCES-MMC can adapt to the variable frequency speedregulation of the motor drive, which shows good application prospects in the future for medium-and high-voltage mine hoist systems.

Keywords: super capacitor energy storage (SCES); modular multilevel converter (MMC); mine hoist;state of charge; regenerative energy; energy harvesting

1. Introduction

The mine hoist conveyor is typically one of the largest consumers of electric power, besides of theexcavator systems, of all equipment in the mining industry. As the only inoue-to-downhole access,the output performance of mine hoist electric transmission systems may not only play key roles ineconomical mine production, but may also affect equipment and personal safety [1,2]. In view of thelarge power rating requirements for mine hoist electric transmission systems, medium-voltage sourceconverters (VSC) have increasingly gained importance due to their high power density, excellentefficiency, and high reliability [3–5]. For the last two decades, three-level neutral-point-clamped (NPC)VSCs have been the standard solution in the medium-voltage range for industrial applications [6,7].Since the regenerative energy in an electric mine hoister and excavator may be as high as 60 percent ofthe motoring power [8,9], active front-end rectifiers (AFE) are usually adopted to feed the regenerativeenergy back into the power distribution grid in order for it not to be wasted. However, the practicalconstraints in the controller bandwidth may place restrictions on the system’s regenerative powerhandling. Therefore, protective circuits, such as DC choppers and crowbars, are usually added to thesystem for suppressing the DC bus over-voltage during regeneration [10].

Compared with NPC-VSC, modular multilevel converter (MMC) provides advantages suchas high modularity, lower switching efficiency, high efficiency, better output voltage performance,etc. Thus, in addition to its successful commercial implementation in high voltage direct current(HVDC) transmission projects [11–13], it also shows good application prospects in fields such aspower quality control [14,15]. However, AC drive-based MMCs are only suitable for industrial fans,pumps and other applications of small speed range close to the rated value. In order to solve thelow-frequency torque ripple issues of conventional MMC motor drive systems, many scholars have

Energies 2017, 10, 1428; doi:10.3390/en10091428 www.mdpi.com/journal/energies

Energies 2017, 10, 1428 2 of 11

carried out extensive research and proposed control strategies for common mode voltage/currentmulti-component injection and pulse optimization [16–19]. However, the above method does notfundamentally eliminate the limitations of MMC variable frequency-speed regulation control. On theother hand, domestic mine accidents have been frequent in recent years. If the high-voltage convertercould continue to operate for a short time under outage conditions, the reliability of the mine hoistequipment would be significantly improved. In addition, when the mine hoist is braking, the electrictransmission system will be in the generation condition, and the regenerating energy will charge theDC bus capacitor. An energy storage-enabled MMC seems an ideal solution for the abovementionedissues, since the super capacitors show better power density and a longer service life compared totraditional batteries. Thus, the super capacitor energy storage based MMC (SCES-MMC) is moresuitable for mine hoist application. However, published technical papers focus mainly on batteryenergy storage system (BESS)-based MMCs [20,21]. In view of the above issues, this paper adopts theSCES-MMC for energy-harvesting applications. The super capacitor banks within the SCES-MMCoperates as a power source to ensure the mine hoist keeps working for a short time when powerfailures occur unexpectedly. As a result, the safety of the mine hoist equipment can be significantlyimproved to avoid mine accidents. In addition, when the mine hoist is braking, the regenerative energyis quickly transferred to the SCES to avoid DC bus overvoltage.

This paper is organized as follows. Section 2 presents the structural characteristics and operationprinciples of SCES-MMC topology. In Section 3, the system control strategies including motor frequencycontrols and state of charge (SOC) balancing controls are explored in detail. To validate the feasibilitiesand effectiveness of the proposed topology and theory, extensive simulation results are demonstratedin Section 4. Finally, Section 5 reports the main conclusions.

2. Topology and Operation Principles

2.1. Topology Analysis

A typical topology for three-phase SCES-MMC is shown in Figure 1, where Ls is the outputequivalent AC inductance, o is the neutral point of reference potential between the positive P andnegative N of the DC bus. The SCES-MMC is three-phase balanced, and its phase-leg consists of 2nsub-modules SMjk (phase j =a, b, c, SMs number k = 1, 2, . . . , 2n) and leg inductance La. Figure 1bshows the basic building blocks-SCES-SM, which consists of two IGBTs (T1, T2), two anti-paralleldiodes (D1, D2), a capacitor CSM, a super capacitor bank interface circuit.

Energies 2017, 10, 1428 2 of 11

component injection and pulse optimization [16–19]. However, the above method does not fundamentally eliminate the limitations of MMC variable frequency-speed regulation control. On the other hand, domestic mine accidents have been frequent in recent years. If the high-voltage converter could continue to operate for a short time under outage conditions, the reliability of the mine hoist equipment would be significantly improved. In addition, when the mine hoist is braking, the electric transmission system will be in the generation condition, and the regenerating energy will charge the DC bus capacitor. An energy storage-enabled MMC seems an ideal solution for the abovementioned issues, since the super capacitors show better power density and a longer service life compared to traditional batteries. Thus, the super capacitor energy storage based MMC (SCES-MMC) is more suitable for mine hoist application. However, published technical papers focus mainly on battery energy storage system (BESS)-based MMCs [20,21]. In view of the above issues, this paper adopts the SCES-MMC for energy-harvesting applications. The super capacitor banks within the SCES-MMC operates as a power source to ensure the mine hoist keeps working for a short time when power failures occur unexpectedly. As a result, the safety of the mine hoist equipment can be significantly improved to avoid mine accidents. In addition, when the mine hoist is braking, the regenerative energy is quickly transferred to the SCES to avoid DC bus overvoltage.

This paper is organized as follows. Section 2 presents the structural characteristics and operation principles of SCES-MMC topology. In Section 3, the system control strategies including motor frequency controls and state of charge (SOC) balancing controls are explored in detail. To validate the feasibilities and effectiveness of the proposed topology and theory, extensive simulation results are demonstrated in Section 4. Finally, Section 5 reports the main conclusions.

2. Topology and Operation Principles

2.1. Topology Analysis

A typical topology for three-phase SCES-MMC is shown in Figure 1, where Ls is the output equivalent AC inductance, o is the neutral point of reference potential between the positive P and negative N of the DC bus. The SCES-MMC is three-phase balanced, and its phase-leg consists of 2n sub-modules SMjk (phase j =a, b, c, SMs number k = 1, 2, …, 2n) and leg inductance La. Figure 1b shows the basic building blocks-SCES-SM, which consists of two IGBTs (T1, T2), two anti-parallel diodes (D1, D2), a capacitor CSM, a super capacitor bank interface circuit.

ab

cLa

P

N

SMa1 SMb1 SMc1

O

UDC

2

UDC

2

SMa2

SMan

SMa(n+1)

SMa(n+2)

SMa(2n)

SMb2

SMbn

SMb(n+1)

SMb(n+2)

SMb(2n)

SMc2

SMcn

SMc(n+1)

SMc(n+2)

SMc(2n)

Usc

Usb

UsaLs

Ls

Ls

isa

isc

isb

La iPc

iNc

iPb

iNb

iPa

iNa

IDC

La

La

La

La

Phase-leg Upper/lower arm

M Load :motor

iSM

Interface

CSC

X1

X2

CSM

T1

T2

D1

D2

USC

(a) (b)

Figure 1. Configuration of three-phase SCES-MMC and its sub-module: (a) Three-phase SCES-MMC topology; (b) SCES-SM. Figure 1. Configuration of three-phase SCES-MMC and its sub-module: (a) Three-phase SCES-MMCtopology; (b) SCES-SM.

Energies 2017, 10, 1428 3 of 11

Figure 2 demonstrates three widely-applied topologies for SCES-SM interface circuits: directconnection, bidirectional buck/boost converter and dual active bridge converter. The main differencebetween the latter two is the isolating transformer. To simplify the analysis, Figure 2a is selected asthe SCES-SM interface circuit, but this does not affect the correctness of the theory. Additionally,the distributed super capacitors CSC results in an SCES-MMC with a highly modular structureand redundant capability, which further improves the reliability of the SCES-MMC for mine hoistapplications. What is more, the circulating energy of SCES-MMC is exchanged between phase-legsthrough the common DC bus. Therefore, the SCES-SMs’ state of charge (SOC) may also be applied forsystem controls.

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Figure 2 demonstrates three widely-applied topologies for SCES-SM interface circuits: direct connection, bidirectional buck/boost converter and dual active bridge converter. The main difference between the latter two is the isolating transformer. To simplify the analysis, Figure 2a is selected as the SCES-SM interface circuit, but this does not affect the correctness of the theory. Additionally, the distributed super capacitors CSC results in an SCES-MMC with a highly modular structure and redundant capability, which further improves the reliability of the SCES-MMC for mine hoist applications. What is more, the circulating energy of SCES-MMC is exchanged between phase-legs through the common DC bus. Therefore, the SCES-SMs’ state of charge (SOC) may also be applied for system controls.

Figure 2. Topology of SCES-SM interface circuit: (a) Direct connection (b) Bidirectional buck/boost converter (c) Dual active bridge converter.

2.2. Operation Principles of SCES-MMC

The switching states of the SCES-SM are shown in Table 1, and the output voltage of SCES-SM is switched between zero and USC by controlling T1 and T2, where USC is the DC voltage of the SCES-SM. At the same time, SOC control of the corresponding super capacitor banks (SCBs) may also be implemented in this process.

Table 1. Switching states of SCES-SM.

Mode T1/D1 T2/D2 iSM Output Voltage SOC Charging 0/1 0/0 >0 USC increasing

Discharging 1/0 0/0 <0 USC decreasing Bypass 0 1 >0 0 maintaining Bypass 0 1 <0 0 maintaining

According to the reference direction shown in Figure 1, AC current of phase-j during normal operation is expressed as

P DC s Z1 13 2j j ji I i i= + + (1)

N DC s Z1 13 2j j ji I i i= − + (2)

s P Nj j ji i i= − (3)

C DC Z13j ji I i= + (4)

USC

CSC

USC

CSC

LSM

Interface

CSC

USC

USC

CSC

(a) (b)

(c)

Figure 2. Topology of SCES-SM interface circuit: (a) Direct connection (b) Bidirectional buck/boostconverter (c) Dual active bridge converter.

2.2. Operation Principles of SCES-MMC

The switching states of the SCES-SM are shown in Table 1, and the output voltage of SCES-SMis switched between zero and USC by controlling T1 and T2, where USC is the DC voltage of theSCES-SM. At the same time, SOC control of the corresponding super capacitor banks (SCBs) may alsobe implemented in this process.

Table 1. Switching states of SCES-SM.

Mode T1/D1 T2/D2 iSM Output Voltage SOC

Charging 0/1 0/0 >0 USC increasingDischarging 1/0 0/0 <0 USC decreasing

Bypass 0 1 >0 0 maintainingBypass 0 1 <0 0 maintaining

According to the reference direction shown in Figure 1, AC current of phase-j during normaloperation is expressed as

iPj =13

IDC +12

isj + iZj (1)

iNj =13

IDC − 12

isj + iZj (2)

isj = iPj − iNj (3)

iCj =13

IDC + iZj (4)

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where iPj and iNj are the upper and lower arm currents, respectively; IDC is the DC bus current. The armcurrent flows through both the upper and lower legs consist of half of the AC output current isj and thecommon-mode current iCj, which consists of the DC component IDC/3 and the circulating componentiZj. The former refers to the active power for charging and discharging the SM capacitors, while thelatter indicates the reactive power causing the SM capacitor voltage ripples. Similarly, the resultingAC and DC voltages are determined by

uPj =UDC

2− uj − La

diPj

dt− RaiPj (5)

uNj =UDC

2+ uj − La

diNj

dt− RaiNj (6)

uj =uNj − uPj

2− La

2dij

dt− Ra

2ij (7)

UDC = uPj + uNj + 2LadiCj

dt+ 2RaiCj (8)

where uj is the AC output phase voltage, UDC is the rated DC bus voltage, Ra is the equivalent seriesarm resistor, and uPj and uNj denote the upper and lower arm voltages, respectively.

2.3. Power Flow Analysis

Figure 3 illustrates the power flow modes, where PSCES refers to the power absorbed/releasedby SCES, the load side power Pload is represented by Ptrac when under traction conditions, or by Preg

when under regeneration conditions. The fundamental power exchange relation is as follows

PLoad = Pdc + PSCES (9)

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where iPj and iNj are the upper and lower arm currents, respectively; IDC is the DC bus current. The arm current flows through both the upper and lower legs consist of half of the AC output current isj and the common-mode current iCj, which consists of the DC component IDC/3 and the circulating component iZj. The former refers to the active power for charging and discharging the SM capacitors, while the latter indicates the reactive power causing the SM capacitor voltage ripples. Similarly, the resulting AC and DC voltages are determined by

PDCP a a P

d2 d

jj j j

iUu u L R it

= − − − (5)

NDCN a a N

d2 d

jj j j

iUu u L R it

= + − − (6)

N P a ad2 2 d 2j j j

j j

u u iL Ru it

−= − − (7)

CDC P N a a C

d2 2

dj

j j j

iU u u L R i

t= + + + (8)

where uj is the AC output phase voltage, UDC is the rated DC bus voltage, Ra is the equivalent series arm resistor, and uPj and uNj denote the upper and lower arm voltages, respectively.

2.3. Power Flow Analysis

Figure 3 illustrates the power flow modes, where PSCES refers to the power absorbed/released by SCES, the load side power Pload is represented by Ptrac when under traction conditions, or by Preg when under regeneration conditions. The fundamental power exchange relation is as follows

Load dc SCESP P P= + (9)

Figure 3. Illustration of power flow: (a) Conventional VSC without SCES; (b) Traction (discharging) condition; (c) Regeneration (charging) condition; (d) Fault operation mode.

With regard to bidirectional power flow characteristics, SCES-SM charging/discharging is realized through different operation modes. The two basic operation modes in SCES-MMC are defined as follows.

M

Load

Pdc >0 Ptrac

Preg Power

Consum-ption

DCSource

VSC

SCES-MMC

M

Load

Pdc >0 Ptrac

PSCES>0Power Consum-

ption

DCSource

SCES

SCES-MMC

M

Load

PSCES>0Power Consum-

ption

DCSource

Preg

SCES

Ptrac

Preg

PSCES>0 PSCES<0

Pdc =0 SCES-MMC M

Load

Power Consum-

ption

DCSource

SCES

(a) (b)

(c) (d)

Figure 3. Illustration of power flow: (a) Conventional VSC without SCES; (b) Traction (discharging)condition; (c) Regeneration (charging) condition; (d) Fault operation mode.

Energies 2017, 10, 1428 5 of 11

With regard to bidirectional power flow characteristics, SCES-SM charging/discharging is realizedthrough different operation modes. The two basic operation modes in SCES-MMC are definedas follows.

1. Normal operation mode: |PLoad| > |Pdc|

Mode (a): PLoad = Pdc, PSCES = 0;

Mode (b): Pdc > 0, PSCES > 0 (discharging), Ptrac > 0 (traction condition);

Mode (c): Pdc < 0, PSCES < 0 (charging), Preg> 0 (regeneration condition);2. Fault operation mode: Pdc = 0

Mode (d): PLoad = PSCES.

In mode (a), SCES does not participate in the work. As a result, the operation principles ofSCES-MMC are similar to the conventional MMCs. Load side power is fed exclusively from the DCbus of the SCES-MMC. Notably, when the mine hoist motor is braking, the regenerative energy maybeconsumed on the DC bus due to the uncontrolled rectifier, as shown in Figure 3a.

In mode (b), the load side motor is under traction conditions, and the SCES functions as the DCsource; thus, SCBs discharge to provide part of the AC load power, as shown in Figure 3b.

In mode (c), the load side motor is under regenerative conditions, and the SCES absorbs part of theregenerative power from the mine hoist motor. However, extra regenerative power is still consumedon the DC bus.

In contrast to conventional MMCs, when power outage faults occur, SCES will play the role ofsole DC source, and feed the AC side load under similar control strategies as those shown in Figure 3d;thus, the mine hoist motor system is capable of continuing to work for a short time. This is a majoradvantage of SCES-MMC in terms of enhancing system reliability.

3. System Control Strategies

The SCES-MMC operates differently from regular MMCs. Since the SCBs within each SCES-SMare able to act as the DC source for supplying the AC load, the power will be delivered not only fromthe DC bus but also the SCES. Therefore, the SCES-MMC control strategy, when applied to the minehoist motor drive system, includes two main parts, namely, motor variable frequency speed regulation(VFSR) control and SOC control.

3.1. Variable Frequency Speed Control

The typical operating states of the mine hoist include acceleration, uniform speed, anddeceleration; thus, speed regulation is one of the main control targets. Based on the active flux observer(AFO)-based variable frequency speed regulation control proposed in [22,23], the SCES-MMC VFSRcontrol strategy proposed in this paper is shown in Figure 4, where ω* and ω are the reference minehoist rotor speed and the observed rotor speed obtained by the active flux observer (AFO) proposedin [22,23], respectively. The reference current i*q, i*d is calculated according to the mathematical modelof synchronous motors, as proposed in [22]. Proportional integral regulator PI refers to the currentloop controller. Consequently, the upper- and lower-arm voltages u∗

Pj and u∗Nj are calculated based on

Equations (5) and (6).

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1. Normal operation mode: |PLoad| > |Pdc| Mode (a): PLoad = Pdc, PSCES = 0; Mode (b): Pdc > 0, PSCES > 0 (discharging), Ptrac > 0 (traction condition); Mode (c): Pdc < 0, PSCES < 0 (charging), Preg> 0 (regeneration condition);

2. Fault operation mode: Pdc = 0 Mode (d): PLoad = PSCES.

In mode (a), SCES does not participate in the work. As a result, the operation principles of SCES-MMC are similar to the conventional MMCs. Load side power is fed exclusively from the DC bus of the SCES-MMC. Notably, when the mine hoist motor is braking, the regenerative energy maybe consumed on the DC bus due to the uncontrolled rectifier, as shown in Figure 3a.

In mode (b), the load side motor is under traction conditions, and the SCES functions as the DC source; thus, SCBs discharge to provide part of the AC load power, as shown in Figure 3b.

In mode (c), the load side motor is under regenerative conditions, and the SCES absorbs part of the regenerative power from the mine hoist motor. However, extra regenerative power is still consumed on the DC bus.

In contrast to conventional MMCs, when power outage faults occur, SCES will play the role of sole DC source, and feed the AC side load under similar control strategies as those shown in Figure 3d; thus, the mine hoist motor system is capable of continuing to work for a short time. This is a major advantage of SCES-MMC in terms of enhancing system reliability.

3. System Control Strategies

The SCES-MMC operates differently from regular MMCs. Since the SCBs within each SCES-SM are able to act as the DC source for supplying the AC load, the power will be delivered not only from the DC bus but also the SCES. Therefore, the SCES-MMC control strategy, when applied to the mine hoist motor drive system, includes two main parts, namely, motor variable frequency speed regulation (VFSR) control and SOC control.

3.1. Variable Frequency Speed Control

The typical operating states of the mine hoist include acceleration, uniform speed, and deceleration; thus, speed regulation is one of the main control targets. Based on the active flux observer (AFO)-based variable frequency speed regulation control proposed in [22,23], the SCES-MMC VFSR control strategy proposed in this paper is shown in Figure 4, where ω* and ω are the reference mine hoist rotor speed and the observed rotor speed obtained by the active flux observer (AFO) proposed in [22,23], respectively. The reference current i*q, i*d is calculated according to the mathematical model of synchronous motors, as proposed in [22]. Proportional integral regulator PI refers to the current loop controller. Consequently, the upper- and lower-arm voltages *

Pju and *Nju

are calculated based on Equations (5) and (6).

Figure 4. Block diagram of variable frequency speed control.

i*q

_

+ _

PI

PIω* ∆ω

ω

u*j

Setting point of the speed

i*q calculation according [22]

isa

isb

isc

id

iqabc dq

ω

+

u*q

u*di*d

ω

u*Pj

u*Nj i*d calculation according [22]

Equation 5Equation 6

dq

abc

Figure 4. Block diagram of variable frequency speed control.

3.2. SOC Control

SOC control is one of the main differences from conventional MMCs. It is essential to control thecirculating current of SCES-MMC to maximize the efficiency of the SOC controls. According to theconfiguration features, The SOC control structure of SCES-MMC generally includes SOC control ofupper/lower arm sub-modules and SOC balancing control of phase-legs, as shown in Figure 5, whereK2 to K5 refer to closed-loop controllers such as PI controllers.

Energies 2017, 10, 1428 6 of 11

3.2. SOC Control

SOC control is one of the main differences from conventional MMCs. It is essential to control the circulating current of SCES-MMC to maximize the efficiency of the SOC controls. According to the configuration features, The SOC control structure of SCES-MMC generally includes SOC control of upper/lower arm sub-modules and SOC balancing control of phase-legs, as shown in Figure 5, where K2 to K5 refer to closed-loop controllers such as PI controllers.

Figure 5. Block diagram of SOC control: (a) Individual SOC control (upper arm); (b) Individual SOC control (lower arm); (c) SOC balancing control.

Figure 5a,b shows the block diagram of individual SOC balancing controls, so as to make the SCES-SM SCBs SOC on the same leg equal to its corresponding average arm SOC ( PjSOC , NjSOC ). sign() denotes the signum function, and the average arm SOC PjSOC , NjSOC are expressed as follows:

1

1 n

Pj jkk

SOC SOCn =

= (10)

2

1

1 n

Nj jkk n

SOC SOCn = +

= (11)

The arm SOC balancing control aims to eliminate the deviation between the average arm SOCs. Similarly, the SOC balancing control of the phase-legs aims to eliminate the deviation between the average SOC ( ,j aveSOC ) of each phase and the three-phase average SOC ( aveSOC ). Therefore,

,avejSOC and aveSOC are given by:

2

,1

12

n

j ave jkk

SOC SOCn =

= (12)

,13

c

ave j avej a

SOC SOC=

= (13)

SOCave

SOCj,ave

K3

SOCPj

SOCNj

K4

+ i*zj1

i*zj2

++-

i*zj

+

-

+-

+iPj

iNj

+

iZj

K5

U*j,cir

0.5Individual SOC balancing control of phase-legs

SOC balancing control of upper and lower arms

Circulating current control

SOCNj

SOCjk

K2+

-iNj

sign()

U*Njk,ind

(k=n+1,…,2n)

U*Pjk,indSOCPj

SOCjk

K2+

-iPj

sign()(k=1,…,n)

(a) (b)

(c)

Figure 5. Block diagram of SOC control: (a) Individual SOC control (upper arm); (b) Individual SOCcontrol (lower arm); (c) SOC balancing control.

Figure 5a,b shows the block diagram of individual SOC balancing controls, so as to make theSCES-SM SCBs SOC on the same leg equal to its corresponding average arm SOC (SOCPj, SOCNj).sign() denotes the signum function, and the average arm SOC SOCPj, SOCNj are expressed as follows:

SOCPj =1n

n

∑k=1

SOCjk (10)

Energies 2017, 10, 1428 7 of 11

SOCNj =1n

2n

∑k=n+1

SOCjk (11)

The arm SOC balancing control aims to eliminate the deviation between the average arm SOCs.Similarly, the SOC balancing control of the phase-legs aims to eliminate the deviation between theaverage SOC (SOCj,ave) of each phase and the three-phase average SOC (SOCave). Therefore, SOCj,aveand SOCave are given by:

SOCj,ave =1

2n

2n

∑k=1

SOCjk (12)

SOCave =13

c

∑j=a

SOCj,ave (13)

The above SOC control is implemented by the control of the circulating current, and the referencecirculating current i∗Zj is calculated from the outputs of the above two SOC balancing control i∗Zj1 andi∗Zj2, as follows

i∗Zj = i∗Zj1 + i∗Zj2 (14)

The controller of the circulating current inner loop aims to make the actual circulating current izjequal to the reference current i∗Zj, and then to realize the SOC balancing control, with the referencevoltage U∗

j,cir being the output of the controller.In summary, system control structures for the SCES-MMC are shown as Figure 6, where the

reference voltage of the phase arm is calculated as follows:

U∗Pjk =

u∗Pj

n+ U∗

Pjk,ind + U∗j,cir +

U∗DCn

(15)

U∗Njk =

u∗Nj

n+ U∗

Njk,ind + U∗j,cir +

U∗DCn

(16)

Energies 2017, 10, 1428 7 of 11

The above SOC control is implemented by the control of the circulating current, and the reference circulating current *

Zji is calculated from the outputs of the above two SOC balancing

control *1Zji and *

2Zji , as follows

* * *1 2Zj Zj Zji i i= + (14)

The controller of the circulating current inner loop aims to make the actual circulating current izj equal to the reference current *

Zji , and then to realize the SOC balancing control, with the reference

voltage *,j cirU being the output of the controller.

In summary, system control structures for the SCES-MMC are shown as Figure 6, where the reference voltage of the phase arm is calculated as follows:

* ** * *

, ,Pj DC

Pjk Pjk ind j cir

u UU U Un n

= + + + (15)

* ** * *

, ,Nj DC

Njk Njk ind j cir

u UU U Un n

= + + + (16)

Figure 6. Overview of system control structures for SCES-MMC.

4. Verifications of SCES-MMC

To verify the feasibility of the proposed SCES-MMC mine hoist system and control strategies, a simulation model of a nine-level SCES-MMC was created, as shown in Figure 7. The multi-pulse rectifier and chopper cell are employed to supply the constant DC bus voltage UDC. The simulation model parameters of SCES-MMC are listed in Table 2, and the initial SOC value of the SCBs is 100%. The super capacitor parameter design and selection method are similar to [24], and the modulation method adopted in this simulation is carrier phase-shifted sinusoidal pulse-width-modulation, combined with the third harmonic injection method of [25]. In addition, assuming that the synchronous motor reaches the rated speed at 0.6 s and starts decelerating at 0.9 s, the SCES-MMC output performance is shown in Figure 8.

Figure 8a,b shows that the frequency of the voltage and the current will gradually rise to its rated value under variable frequency speed regulation control. The SCES-MMC comes to its steady state at 0.6 s, and then gradually decays when the system enters the deceleration condition at 0.9 s. From the speed characteristics of the synchronous motor shown in Figure 8c, it can be seen that, before 0.6 s, the SCBs begin to release energy when the motor speed is rising and the system SOC is decreasing, as shown in Figure 8d. Then, the motor speed starts to drop when the synchronous motor brakes at 0.9 s, and the regenerative power in this process will also help charge the SCB of each SCES-SM, hence its SOC will rise.

SM SOCcontrol

Figure 5a

Phase-legSOC

balancingFigure 5b

Upper/lower legSOC

balancingFigure 5b

Circulating current

suppressorFigure 5b

Variable frequency

speed regulationFigure 4 PWM

generatorSCES-MMC

SOCjkU*Pjk,ind

U*Njk,ind

u*Pj

u*Nj

U*j,cir

U*Pjk

U*Njk

ij*

I*Zj2

SOCPj

SOCNj

SOCPj

SOCNj

SOCj,ave

SOCave

I*Zj1

ω*

SOC balancing control VFSR control

SOCjk

SOC average

calculationEq.(9)-(12)

Reference voltage

allocationEquation 15

and Equation 16

Figure 6. Overview of system control structures for SCES-MMC.

4. Verifications of SCES-MMC

To verify the feasibility of the proposed SCES-MMC mine hoist system and control strategies,a simulation model of a nine-level SCES-MMC was created, as shown in Figure 7. The multi-pulserectifier and chopper cell are employed to supply the constant DC bus voltage UDC. The simulationmodel parameters of SCES-MMC are listed in Table 2, and the initial SOC value of the SCBs is 100%.The super capacitor parameter design and selection method are similar to [24], and the modulationmethod adopted in this simulation is carrier phase-shifted sinusoidal pulse-width-modulation,

Energies 2017, 10, 1428 8 of 11

combined with the third harmonic injection method of [25]. In addition, assuming that the synchronousmotor reaches the rated speed at 0.6 s and starts decelerating at 0.9 s, the SCES-MMC outputperformance is shown in Figure 8.

Figure 8a,b shows that the frequency of the voltage and the current will gradually rise to its ratedvalue under variable frequency speed regulation control. The SCES-MMC comes to its steady state at0.6 s, and then gradually decays when the system enters the deceleration condition at 0.9 s. From thespeed characteristics of the synchronous motor shown in Figure 8c, it can be seen that, before 0.6 s,the SCBs begin to release energy when the motor speed is rising and the system SOC is decreasing, asshown in Figure 8d. Then, the motor speed starts to drop when the synchronous motor brakes at 0.9 s,and the regenerative power in this process will also help charge the SCB of each SCES-SM, hence itsSOC will rise.

Table 2. Simulation parameters of SCES-MMC.

System Parameters Value

AC line-to-line voltage 1.45 kVDC link voltage 3.6 kV

Rated power 5.3 MWLeg inductance 5.0 mH

SM capacitor voltage 0.45 kVSuper capacitor capacitance 20 F

Rated speed 52 rpm

Energies 2017, 10, 1428 8 of 11

Table 2. Simulation parameters of SCES-MMC.

System Parameters ValueAC line-to-line voltage 1.45 kV

DC link voltage 3.6 kV Rated power 5.3 MW

Leg inductance 5.0 mH SM capacitor voltage 0.45 kV

Super capacitor capacitance 20 F Rated speed 52 rpm

Figure 7. Configuration of SCES-MMC simulation model.

Figure 8. Output performance of SCES-MMC: (a) AC output voltage; (b) AC output current; (c) Motor speed; (d) Average SOC of SCBs; (e) Voltage of the SCBs—phase a; (f) Voltage of the SCBs—phase b.

Three-phase SCES-MMC

ab

cLa

P

N

SMa1 SMb1 SMc1

O

UDC

2

UDC

2

SMa2

SMan

SMa(n+1)

SMa(n+2)

SMa(2n)

SMb2

SMbn

SMb(n+1)

SMb(n+2)

SMb(2n)

SMc2

SMcn

SMc(n+1)

SMc(n+2)

SMc(2n)

Usc

Usb

UsaLs

Ls

Ls

isa

isc

isb

La iPc

iNc

iPb

iNb

iPa

iNa

IDC

La

La

La

LaRectifire 1

Rectifire m

AC grid

3-phase transformer

M

multi-pulse rectifire

Synchronous motor

Chopper

(a) (b)

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

(d)

99.6

99.7

99.8

99.9

100

SOC

(%)

-100

102030405060

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

(c)

Spee

d (r

pm)

time (s) time (s)

(a) (b)

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

(d)

99.6

99.7

99.8

99.9

100

SOC

(%)

-100

102030405060

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

(c)

Spee

d (r

pm)

time (s) time (s)

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8-2.0-1.5-1.0-0.5

00.51.01.52.0

AC

vol

tage

(kV

)

time (s)

usa uscusb

-6.0-4.0-2.0

02.04.06.0

AC

curr

ent (

kA)

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8time (s)

isa iscisb

(e) (f)

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.80.2

0.3

0.4

0.5

UcA

,SM

(kV

)

time (s) 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.80.2

0.3

0.4

0.5

UcB

,SM

(kV

)

time (s)

Figure 7. Configuration of SCES-MMC simulation model.

Due to the high capacitance of the SCBs, even if a large amount of energy is released/absorbed,there is no significant fluctuation in the sub-module capacitor voltage USM, as shown in Figure 8e,f.

Figure 9 illustrates the power distribution laws of the proposed SCES-MMC. From theabove-mentioned analysis, it can be concluded that the SCBs in the SCES-MMC will be discharged toprovide about 69% of the AC load power requirement. Thus, less DC power is required comparedto those without super capacitor banks. Since SCES is involved in absorbing part of the regenerativepower from the mine hoist motor, under regeneration conditions, the power dissipated in theSCES-MMC is also less than for conventional MMCs. This makes it possible to minimize the capacityrequirements of the AC grid and energy loss of the mine hoist system.

Energies 2017, 10, 1428 9 of 11

Energies 2017, 10, 1428 8 of 11

Table 2. Simulation parameters of SCES-MMC.

System Parameters ValueAC line-to-line voltage 1.45 kV

DC link voltage 3.6 kV Rated power 5.3 MW

Leg inductance 5.0 mH SM capacitor voltage 0.45 kV

Super capacitor capacitance 20 F Rated speed 52 rpm

Figure 7. Configuration of SCES-MMC simulation model.

Figure 8. Output performance of SCES-MMC: (a) AC output voltage; (b) AC output current; (c) Motor speed; (d) Average SOC of SCBs; (e) Voltage of the SCBs—phase a; (f) Voltage of the SCBs—phase b.

Three-phase SCES-MMC

ab

cLa

P

N

SMa1 SMb1 SMc1

O

UDC

2

UDC

2

SMa2

SMan

SMa(n+1)

SMa(n+2)

SMa(2n)

SMb2

SMbn

SMb(n+1)

SMb(n+2)

SMb(2n)

SMc2

SMcn

SMc(n+1)

SMc(n+2)

SMc(2n)

Usc

Usb

UsaLs

Ls

Ls

isa

isc

isb

La iPc

iNc

iPb

iNb

iPa

iNa

IDC

La

La

La

LaRectifire 1

Rectifire m

AC grid

3-phase transformer

M

multi-pulse rectifire

Synchronous motor

Chopper

(a) (b)

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

(d)

99.6

99.7

99.8

99.9

100

SOC

(%)

-100

102030405060

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

(c)

Spee

d (r

pm)

time (s) time (s)

(a) (b)

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

(d)

99.6

99.7

99.8

99.9

100

SOC

(%)

-100

102030405060

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

(c)

Spee

d (r

pm)

time (s) time (s)

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8-2.0-1.5-1.0-0.5

00.51.01.52.0

AC

vol

tage

(kV

)

time (s)

usa uscusb

-6.0-4.0-2.0

02.04.06.0

AC

curr

ent (

kA)

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8time (s)

isa iscisb

(e) (f)

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.80.2

0.3

0.4

0.5

UcA

,SM

(kV

)

time (s) 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.80.2

0.3

0.4

0.5

UcB

,SM

(kV

)

time (s)

Figure 8. Output performance of SCES-MMC: (a) AC output voltage; (b) AC output current; (c) Motorspeed; (d) Average SOC of SCBs; (e) Voltage of the SCBs—phase a; (f) Voltage of the SCBs—phase b.

Energies 2017, 10, 1428 9 of 11

Due to the high capacitance of the SCBs, even if a large amount of energy is released/absorbed, there is no significant fluctuation in the sub-module capacitor voltage USM, as shown in Figure 8e,f.

Figure 9 illustrates the power distribution laws of the proposed SCES-MMC. From the above-mentioned analysis, it can be concluded that the SCBs in the SCES-MMC will be discharged to provide about 69% of the AC load power requirement. Thus, less DC power is required compared to those without super capacitor banks. Since SCES is involved in absorbing part of the regenerative power from the mine hoist motor, under regeneration conditions, the power dissipated in the SCES-MMC is also less than for conventional MMCs. This makes it possible to minimize the capacity requirements of the AC grid and energy loss of the mine hoist system.

Figure 9. Power distributions.

5. Conclusions

A super capacitor energy storage-based modular multilevel converter (SCES-MMC) topology for mine hoist applications has been investigated in this paper. In contrast to conventional MMCs, the sub-modules employ distributed SCBs, which are designed to absorb the regenerative energy of the mine hoist, and release it under traction conditions. Due to the high power density of the super capacitor, the sub-modules’ capacitor voltage will not significantly fluctuate. The configuration and operation principles, together with control technologies were studied in detail. Moreover, the feasibility of the proposed SCES-MMC topology and the control theory were also verified. Simulation results show that SCES-MMC makes reasonable use of the energy of the system through the distributed SCBs, improving the energy utilization efficiency, and shows good application prospects in future medium-/high-voltage mine hoist systems.

Acknowledgments: The authors gratefully acknowledge technical support from the National Key Research and Development Program of China (Award Number: 2016YFE0131700) and the State Key Laboratory of Large Electric Drive System and Equipment Technology (Award Number: SKLLDJ042016005).

Author Contributions: Xiaofeng Yang contributed to the concept, undertook significant theory analysis, and wrote the paper. Piao Wen and Yao Xue helped perform the simulation with constructive discussions, Trillion Q. Zheng supplied guidance, and Youyun Wang provided the field testing parameters and edited the manuscript. All authors read and approved the manuscript.

Conflicts of Interest: The authors declare no conflicts of interest.

-2.5

-2.0

-1.5

-1.0

-0.5

0

0.5

Pow

er/M

W

1.0

1.5

1.0time (s)

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0

Without SCSCES-MMC SCBs

SCBs discharging

SCBs charging

power dissipation

DC power supply

Figure 9. Power distributions.

5. Conclusions

A super capacitor energy storage-based modular multilevel converter (SCES-MMC) topologyfor mine hoist applications has been investigated in this paper. In contrast to conventional MMCs,the sub-modules employ distributed SCBs, which are designed to absorb the regenerative energyof the mine hoist, and release it under traction conditions. Due to the high power density of thesuper capacitor, the sub-modules’ capacitor voltage will not significantly fluctuate. The configurationand operation principles, together with control technologies were studied in detail. Moreover, the

Energies 2017, 10, 1428 10 of 11

feasibility of the proposed SCES-MMC topology and the control theory were also verified. Simulationresults show that SCES-MMC makes reasonable use of the energy of the system through the distributedSCBs, improving the energy utilization efficiency, and shows good application prospects in futuremedium-/high-voltage mine hoist systems.

Acknowledgments: The authors gratefully acknowledge technical support from the National Key Research andDevelopment Program of China (Award Number: 2016YFE0131700) and the State Key Laboratory of Large ElectricDrive System and Equipment Technology (Award Number: SKLLDJ042016005).

Author Contributions: Xiaofeng Yang contributed to the concept, undertook significant theory analysis, and wrotethe paper. Piao Wen and Yao Xue helped perform the simulation with constructive discussions, Trillion Q. Zhengsupplied guidance, and Youyun Wang provided the field testing parameters and edited the manuscript. All authorsread and approved the manuscript.

Conflicts of Interest: The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:

AC alternating currentDC direct currentHVDC high voltage direct currentVSC voltage source converterNPC neutral-point-clampedAFE active front endSCES super capacitor energy storageMMC modular multilevel converterSM sub-moduleSCB super capacitor bankSOC state of chargeIGBT insulated gate bipolar transistorVFSR variable frequency speed regulation

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