VOLTAGE LIMIT CONTROL OF MODULAR MULTILEVEL

CONVERTER BASED UNIFIED POWER FLOW

CONTROLLER UNDER UNBALANCED GRID CONDITIONS

SK NAWAAZ 1, B. MADHAVA2

K. SRINIVAS 3

1PG Student, Dept of EEE, SSN Engineering College, Ongole, AP, India.

2Assistant Professor, Dept of EEE, SSN Engineering College, Ongole, AP, India.

3Associate Professor & HOD, Dept of EEE, SSN Engineering College, Ongole, AP, India

Abstract— The modular multilevel converter

(MMC) is being developed as a core technology for

the next generation of high-power, voltage source

converters (VSCs). Voltage fluctuation and power

losses in the distribution line are problems in

distribution networks. The modular multilevel

converter-based unified power flow controller

(MMC-UPFC) is able to operate under unbalanced

grid conditions with symmetric component

decoupling. However, the constraint of the voltage

limit of UPFC is not considered and no protection

schemes are investigated to protect the UPFC from

over modulation under unbalanced grid conditions.

To solve this problem, this project proposes the

cascaded control scheme for MMC-UPFC based on

voltage limit control and symmetric component

decoupling to balance the ac current of the

transmission line.

The Modular Multilevel Converter

(MMC) is an emerging power converter technology

that has caught widespread attention mainly

because of several technical and economic benefits

such as modular realization, easy scalability, low

total harmonic distortion, fail-safe operations etc.

With appropriate transformer connections for

MMC-UPFC, the negative- and zero-sequence

currents are suppressed by the corresponding inner

current loops. Considering the voltage limit of

MMC, the operating ranges of UPFC-MMC under

balanced and unbalanced grid conditions are

investigated in different control strategy methods.

Finally, the final cascaded control structure is

constructed. The simulation results obtained in

MATLAB/SIMULINK are provided to validate the

effectiveness and robust performance of the

proposed control strategies.

I. INTRODUCTION

Environmental constraints limit the

expansion of transmission networks and generation

near load centres. This has a negative influence on

power system voltage stability. Simultaneous

growth in the use of electrical power without a

corresponding increase of transmission capacity

has bought many power systems closer to their

voltage stability limit. Voltage stability is

concerned with the ability of a power system to

maintain acceptable voltages at all buses under

normal conditions and after being subjected to a

disturbance. The assessment of voltage stability has

become more complicated due to the complexity of

power systems. For example, voltages do not

indicate the proximity to voltage collapse points in

heavily compensated systems even in heavy

loading conditions. The main cause of voltage

collapse affecting the power system is faults on the

power system, often resulting from weather

conditions, e.g. lightning, wind, and ice hitting

overhead lines. On underground cables, typically

used in urban areas, insulation problems and the

operation of excavators are the main causes of

voltage interruptions. The conventional voltage

source converter (VSC) based UPFC is limited to

its poor output waveform, and is difficult to extend

due to the limited voltage and current ratings of

switching devices.

In the latter case, the ac currents of

transmission line are not balanced and therefore the

spread of ac fault cannot be inhibited. So far, the

constraints of the voltage limit of UPFC are not

considered and thus no effective protection

schemes are investigated to protect the vulnerable

switches of UPFC from various unbalanced grid

conditions. The general concept of unbalanced grid

conditions includes the following scenarios: ac

voltage unbalance caused by faults, asymmetrical

transmission lines and load imbalance. Instead of

the local control for MMC, the focus of the

analysis on the power flow under unbalanced grid

conditions considering the voltage limit constraint

of UPFC. The novelty of this project is: 1)

choosing appropriate transformer connection

scheme for MMC-UPFC considering the third

order magnetizing currents (TOMC) and zero-

sequence currents (ZSC); 2) investigating the

operating range of MMC-UPFC under the

proposed control scheme by mathematical

derivation and point scanning approaches; 3)

presenting the voltage limit control and

constructing the cascaded control structure to

protect MMC-UPFC from over- modulation or

over-voltage/current. Besides protecting UPFC

from over-modulation, the additional benefit of the

proposed control scheme is obvious: 1) the ac fault

ride-through ability of MMC-UPFC is greatly

enhanced and the spread of ac fault can be

effectively inhibited; 2) MMC-UPFC can operate

with the maximum operating region under

unbalanced grid conditions.

II. SYSTEM MODELING

With the subject to protect MMC-UPFC

from over modulation and over-voltage/current

under unbalanced grid conditions, this project

proposes the cascade control scheme for MMC-

UPFC based on the voltage limit control and

symmetric component control.

Fig. 1. Configuration of MMC-UPFC.

Fig. 1 shows the basic configuration of

MMC-UPFC, which consists of a shunt MMC and

a series one connected back to back. The shunt

MMC usually provides the stiff dc-link voltage and

also controls the shunt reactive power to support

the ac bus voltage u1 . The series one generates one

controllable ac voltage and couples it to the

transmission line through the series transformer, to

regulate the power flow of the line. TBS and

mechanical bypass switch (MBS) are used to

bypass MMC-UPFC temporarily and for a long

time, respectively. In this project, both MMCs

employ conventional half-bridge sub-modules

(HBSM). Since it is already well-known to all, no

details of HBSM are given in this project. The

control strategies in this project are also applicable

to MMC-UPFC with other SM topologies. In the

case of unbalanced grid condition, MMC can be

divided into positive- and negative-sequence

subsystem. In some situation, zero sequence may

exist. In the rotating synchronous frame (RSF), the

positive- and negative-sequence model of MMC is

investigated. The equivalent circuit of MMC-UPFC

is illustrated in Fig. 2. Corresponding to

aforementioned three scenarios of unbalanced grid

conditions, asymmetrical transmission lines can be

represented by unbalanced impedance Zline; load

imbalance can be represented by asymmetric load

impedance ZL, and the asymmetric ZL eventually

causes unbalanced ac bus voltage ur at the

receiving end; when ac fault occurs, ur will also

become unbalanced.

Fig. 2. Equivalent circuit of MMC-UPFC.

To improve performance of UPFC-MMC

under unbalanced grid conditions, the general

control principle is to compensate the unbalanced

components of ur or Zline by regulating use (u12)

coupled from the series MMC, and the control

objective is to obtain balanced ac currents of

transmission line by suppressing the negative- and

zero-sequence currents. In details, the negative

sequence current (NSC) and ZSC can be

suppressed by injecting the corresponding

negative-sequence voltage (NSV) and zero-

sequence voltage (ZSV) into use. In particular, the

circuit of ZSC mainly depends on the connection of

transformers.

A. Control of Shunt and Series MMC

According to Fig. 1, the shunt MMC

adopts the dc voltage control and ac voltage

control, which are identical with the conventional

one, and only positive-sequence control loop is

implemented for the shunt MMC. The series MMC

is responsible for compensating NSV and ZSV in

the transmission line. Therefore, the positive-,

negative and zero-sequence control loops are all

necessary for the series MMC, while only positive-

sequence loop is used for power flow regulation.

As converter currents are identical with ac currents

of transmission line in per unit value, to fully

suppress the NSC, the references of the negative-

sequence and zero-sequence inner-current loops are

directly set as zero.

B. Connection of Transformers

As each primary winding of series

transformer is connected in series with each phase

of transmission line, the main concerns of

transformer connections are the third-order

magnetizing current (TOMC) and ZSC of

fundamental frequency. To provide the flowing

path for TOMC, the secondary windings of series

transformer should be with delta or star-grounding

connection. However, the delta connection will

also provide the flowing path for ZSC of

fundamental frequency and make it impossible to

inhibit ZSC under unbalanced grid conditions. The

only option for the secondary windings of series

transformer is star-grounding connection.

Fig. 3. Zero-sequence equivalent circuit with the

proposed transformer connections.

On the other side, the shunt transformer

should be with delta star grounding connection to

constitute the circuit for TOMC of series

transformer. Fig. 3 illustrates the equivalent zero

sequence circuit with the transformer connections

employed in this project: the secondary windings of

series transformer is of star grounding connection

while the shunt transformer is with delta-star

grounding connection. In Fig. 3, Xm, X1 and X2

represent the magnetizing reactance, leakage

reactance at primary side and secondary side of the

series transformer, respectively; the superscript

denotes the corresponding variables of shunt

transformer; Ra and La represent the equivalent

arm resistance and inductance of MMCs. In this

way, ZSC will flow through the leakage reactance

X1 and X2 of series transformer, the arm resistance

Ra and inductance La of series and shunt MMCs

and the leakage reactance X_1 and X_2 of shunt

transformer, as shown in Fig. 3. The total

impedance of zero-sequence circuit is a dozen

ohms or below for TOMC. Under unbalanced grid

conditions, ZSC of fundamental frequency will be

suppressed by zero-sequence inner-current loop of

series MMC with constant zero reference input.

Besides providing the flowing path for TOMC, the

transformer connections in Fig. 3 bring more

benefits as follows:

1) providing the grounding points at the converter

sides of series and shunt transformers;

2) being able to couple ZSV generated by series

MMC in series with transmission lines;

3) avoiding the fault at the side of series MMC

from disturbing the power flow of transmission line

too much.

On the other hand, ZSC of fundamental

frequency under unbalanced grid conditions can

also get effectively suppressed if the shunt

transformer is grounded through relatively large

impedance. The value of impedance has to be

designed elaborately by the trade-off between

providing flowing path for TOMC and suppressing

ZSC of fundamental frequency. This section will

quantitatively analyze the controllability of power

flow under balanced and unbalanced grid

conditions with MMC-UPFC based on Fig. 1, and

compare the operating regions of power flow in

both cases. The results from mathematical

derivation and point-scanning approaches will

validate each other.

Fig. 4. Schematic of proposed cascade control.

To be noticed, other auxiliary control

modules in the control of series MMC, such as

circulating current suppression and DC voltage

ripple suppression, are not depicted in Fig.4 for the

purpose of simplification.

III. SIMULATION RESULTS

CASE 1: ASYMMETRICAL TRANSMISSION

LINE PARAMETERS:

fig: (a) active and reactive power

fig: (b) ac currents of transmission line.

Fig. 5. Waveforms for the case of asymmetry of

transmission line parameters: (a) active and

reactive power, (b) ac currents of transmission line.

In this section, the asymmetry of

transmission lines is modeled by the parallel layout

of the underground cables. The depth and spacing

of the cables is 1 m and 0.12 m, respectively.

Before UPFC starts operation at 0.8 s, the original

power flow is (175MW,−125MVar) with visible

ripples because of the unbalanced currents, as

shown in Fig. 5(a). After 0.8 s, UPFC starts to

regulate the power flow at its reference (215 MW,

0 MVar). Fig. 5(b) shows the current waveform

before and after UPFC is activated. With the

proposed control, UPFC reduces the unbalance

factor of currents from about 15% before 0.8 s to

0.22% after 0.8 s.

CASE 2: LOAD IMBALANCE:

(a) voltage at receiving end

(b) ac currents of transmission line

Fig.6. Waveforms for the case of load imbalance:

(a) voltage at receiving end, (b) ac currents of

transmission line.

The load imbalance is modeled by the

imbalance of load impedance ZL. Meanwhile, the

cables are laid in triangle to assure of the balance

of transmission impedance Zline. Fig. 6 illustrates

the waveforms of currents and voltages at the

receiving end. Due to load imbalance, the

unbalance factor of voltages at the receiving end

reaches 2.8%, which causes the unbalance factor of

currents up to 73.1% without UPFC. After UPFC

puts into service at 0.8 s, the unbalance factor of

currents reduces to 0.13%.

CASE 3: AC FAULT:

(a) voltage at receiving end

(b) ac currents of transmission line

Fig.7. Waveforms for the case of ac fault: (a)

voltage at receiving end, (b) ac currents of

transmission line.

Following the event in case 2, a remote ac

fault occurs to the receiving end at 1.2 s. Fig. 7

illustrates the waveforms of currents and voltages

at the receiving end. Even though voltages at the

receiving end become unbalanced, ac currents get

through the fault quickly and still keep balanced

with the help of UPFC, as shown in Fig. 7.

CASE 4: SLIGHT AC FAULT:

(a) active and reactive power

(b) ac currents of transmission line

(c) ac voltages of series MMC

Fig.8. Waveforms for the case of slight ac fault: (a)

active and reactive power, (b) ac currents of

transmission line, (c) ac voltages of series MMC.

Before UPFC is activated at 0.8 s, the

power flow of the transmission line is (185 MW,

−128 MVar), as shown in Fig. 8. At 0.8 s, UPFC

starts operation with the reference (150 MW, 0

MVar) within the controllable region. An ac faults

occurs to the receiving end at 1.1 s and lasts for 0.3

s. The unbalance factor of voltages at the receiving

end is 3.4%. In this case, the set power point

becomes out of the controllable region

corresponding to this unbalanced condition. By the

online calculation block, the corresponding original

point is obtained as (240MW, −70 MVar).With the

proposed voltage limit control, UPFC shifts the

power flow from the reference point (150 MW, 0

MVar) to the actual point (168 MW, −14 MVar) on

the boundary of Um. As shown in Fig. 8, the

maximum voltage amplitude among three phases

corresponding to (168 MW, −14 MVar) is 10 kV.

When the fault is cleared at 1.4 s, UPFC regulates

the power to come back to (150 MW, 0 MVar)

quickly and steadily.

CASE 5: SEVERE AC FAULT:

(a) active and reactive power

(b) ac currents of transmission line

(c) ac currents of MMC

(d) ac voltages of series MMC

Fig. 9. Waveforms for the case of severe ac fault:

(a) active and reactive power, (b) ac currents of

transmission line, (c) ac currents of MMC, (d) ac

voltages of series MMC.

With the same initial condition with case

4, the reference power is set to be (350 MW, 0

MVar) which lies out of the controllable region

even under balanced grid condition. The voltage

limit control will reduce the power to the actual

point (320MW, −12 MVar). Meanwhile, the

magnitude of voltages is equal to 10 kV, as shown

in Fig. 9. When a severe ac fault occurs at 1.1 s, the

UPFC is bypassed once the conditional statement

of U−∗ 12 > U− limit is satisfied. The protection

scheme of the transmission line will take action at

1.2 s to clear the fault. After the fault is cleared at

1.3 s, UPFC starts to recover to the pre-fault state

steadily.

IV. CONCLUSION

This project investigates the controllable

regions of power flow under unbalanced grid

conditions in mathematical derivation and point-

scanning methods, respectively. The results from

the two methods are completely consistent and thus

validate each other. The final cascaded control

scheme for series MMC of UPFC is constructed

based on the proposed voltage limit control and

basic symmetric component control. In the

cascaded control scheme, the suppression of NCS

and ZCS is endowed with the first priority over

power flow regulation. With the voltage limit

control, MMC-UPFC can avoid from over-

modulation and maximize its controllability by

operating on the boundary defined by the voltage

limit. The principle of voltage limit control is also

applicable to other applications of VSCs under

unbalanced grid conditions. The simulation result

shows that with the proposed control strategies

MMC-UPFC can effectively operate under

unbalanced grid conditions and exhibit very good

robust controllability. It can be concluded that the

proposed control strategies improves the ac fault

ride-through ability of MMC-UPFC and also can

help to inhibit the spread of ac fault.

REFERENCES

[1] B. Mwinyiwiwa, B. Lv, and B. T. Ooi,

“Multiterminal unified power flow controller,”

IEEE Trans. Power Electron., vol. 15, no. 6, pp.

1088–1093, Nov. 2000.

[2] H. Fujita, H. Akagi, and Y.Watanabe,

“Dynamic control and performance of a unified

power flow controller for stabilizing an AC

transmission system,” IEEE Trans. Power

Electron., vol. 21, no. 4, pp. 1013–1020, Jul. 2006.

[3] A. Lesnicar and R. Marquardt, “An innovative

modular multilevel converter topology suitable for

a wide power range,” in Proc. Power Tech Conf.,

Bologna, Italy, 2003, pp. 3–6.

[4] Q. Tu, Z. Xu, and L. Xu, “Reduced switching-

frequency modulation and circulating current

suppression for modular multilevel converters,”

IEEE Trans. Power Del., vol. 26, no. 3, pp. 2009–

2017, Jul. 2011.

[5] M. Saeedifard and R. Iravani, “Dynamic

performance of a modular multilevel back-to-back

HVDC system,” IEEE Trans. Power Del., vol. 25,

no. 4, pp. 2903–2912, Oct. 2010.

[6] Z. Yuan, S. W. H. de Haan, J. B. Ferreira, and

D. Cvoric, “A facts device: Distributed power-flow

controller (DPFC),” IEEE Trans. Power Electron.,

vol. 25, no. 10, pp. 2564–2572, Oct. 2010.

[7] F. Z. Peng, Y. Liu, S. Yang, S. Zhang, D.

Gunasekaran, and U. Karki, “Transformer-less

unified power-flow controller using the cascade

multilevel inverter,” IEEE Trans. Power Electron.,

vol. 31, no. 8, pp. 2461–5472, Aug. 2016.

[8] S. Yang, Y. Liu, X. Wang, D. Gunasekaran, U.

Karki, and F. Z. Peng, “Modulation and control of

transformer-less UPFC,” IEEE Trans. Power

Electron., vol. 31, no. 2, pp. 1050–1063, Feb. 2016.

[9] J. Yuan, L. Liu, W. Fei, L. Chen, B. Chen, and

B. Chen, “Hybrid electromagnetic unified power

flow controller: A novel flexible and effective

approach to control power flow,” IEEE Trans.

Power Del., Oct. 2016, early access.

[10] A. Rajabi-Ghahnavieh, M. Fotuhi-Firuzabad,

M. Shahidehpour, and R. Feuillet, “UPFC for

enhancing power system reliability,” IEEE Trans.

Power Del., vol. 25, no. 4, pp. 2881–2890, Oct.

2010.