Low Voltage Ride Through Control of Modular Multilevel Converter based HVDC systems

Ghazal Falahi FREEDM Systems Center

Electrical and Computer Engineering Department North Carolina State University

Raleigh US

Alex Huang FREEDM Systems Center

Electrical and Computer Engineering Department North Carolina State University

Raleigh US

Abstract Low Voltage Ride Through (LVRT) is an important grid requirement for Voltage Source Converter (VSC) based HVDC links. This paper studies the performance of the modular multilevel converter (MMC) VSC based HVDC systems during faults or voltage dips and proposes a new control strategy to improve the LVRT performance. The proposed algorithm controls the system to generate the required active and reactive powers that are calculated mathematically based on the ratings of the MMC-HVDC system and LVRT requirements. The injected active and reactive power values obey the LVRT guidelines and are adaptable to different grid codes. The mathematical calculations are presented and EMTDC/PSCAD simulation evaluates the performance of the proposed method.

Keywords-MMC, HVDC, low voltage ride through

I. INTRODUCTION

Voltage source converter (VSC) based HVDC is a relatively new type of transmission system incorporating controllable switching converters. HVDC systems can control active and reactive power independently therefore they improve power transfer capability and network stability. With the development of remote and large renewable energy farms, VSC HVDC transmission systems are very attractive to deliver power to demand centers. VSC based HVDC systems are becoming a competitor of classical thyristor-based HVDC systems and they may replace the classic HVDC systems one day [1].

VSC-HVDC systems can produce active and reactive power independently, which allows them to operate with small or no AC support. As the power plants increase in size and provide a larger share of the supply, they should stay operational and connected to the grid to support the grid by injecting reactive power during and after voltage sags [2]. When a fault occurs due to short circuit or any other issue in the grid the voltage decreases at AC output terminals, which reduces the active power. The disconnection of a large amount of power may result in serious power stability problems therefore many grid codes require the LVRT capability, which infers the system should be able to supply power to the network even during grid faults, and voltage sags [2].

Low voltage ride-through also known as fault ride through (FRT) requirement states that the system should stay connected when the AC grid voltage is temporarily reduced due to a fault or large load change in the grid. The voltage may reduce in one, two or all three phases of the ac grid. Grid operators define low voltage ride-through (LVRT) requirement to maintain high availability, stability and to reduce the risk of voltage collapse [3]. Restraining the power delivered to the HVDC DC link during voltage sags is a key point in achieving LVRT requirements. LVRT-capable systems enrich overall system stability by injecting reactive power during system events [2-5].

This paper studies the dynamic performance of a transformer-less MMC-VSC based HVDC system and proposes a LVRT conforming control technique to improve the performance during faults or voltage sags. The paper also presents some mathematical calculations based on LVRT curve and grid code. The MMC control system is modified based on the calculations to fulfill the LVRT requirements.

In this paper section II covers MMC mathematical model and operation principal, part III shows the VSC-HVDC system diagram and studies MMC-HVDC control, operation and dynamic performance, part IV proposes a LVRT control methodology to improve the system performance under faults and voltage sags. The control diagram is shown and the PSCAD simulation verifies the performance of the proposed control method.

II. MMC MATHEMATICAL MODEL AND OPERATION PRINCIPAL

The three-phase MMC converter configuration used in HVDC applications is shown in Fig. 1. The converter is composed of three-phase legs each consisting of two arms made by series connection of some identical half-bridge modules called cells or sub-modules (SM). The SMs are inserted or bypassed based on the switching state of the two switching device in each half bridge. The two switches are complementary and table 1 shows the sub-module output voltage in different switching states. The currents in phase-k (k=a, b, c) consist of ik-up and ik-

low which are the upper and lower arm current in each phase. Applying KVL to the arms of the converter yields to the following:

(1)

(2)

(3)

(4)

Where uk is the phase k voltage, u0 is the potential to ground DC side neutral point and the inductance of each arm equals 2L. The MMC grid connection dynamic is described as

(5)

Control system for the each MMC converter shown in Fig. 1 includes four parts [6]:

1. An individual capacitor voltage controller, which keeps the sub module capacitor voltages on their reference values.

2. The averaging controller that controls the total capacitor voltages in each leg to follow the reference value which is 2Vdc.

3. The system controller, which controls active and reactive power. The reference of active power is determined by network demanded active power or the DC-link voltage controller. The reference for reactive power is either determined by ac voltage regulator or by the utility issued reactive power demand.

4. The PWM voltage command generation, which adds up the output of three mentioned controllers to build the modulation waveform. The overall control structure is shown in Fig. 4. The ac voltage command or modulation index for each phase is calculated by adding the output of three mentioned controllers as shown in the following equations.

(j:1-n) (6)

(j: n+1 - 2n) (7)

The voltage command is normalized by each dc-capacitor voltage Vcju and compared with a triangular waveform having a maximum value of unity and minimum value of zero with a carrier frequency of fc. Carrier shifted PWM modulation

TABLE I. SWITCHING STATE OF SUB-MODULES

State S1 S2 Vsm 1 ON OFF Vc 2 OFF ON 0 3 OFF OFF 0

Fig. 1: Modular multilevel converter

Fig.2 HVDC MMC single line diagram

technique is used for switching and carrier waveform of each cell is phase shifted by 360/n [6].

III. HVDC MMC OPERATION PRINCIPAL

The single line diagram of an MMC-HVDC system is shown in Fig. 2, which consists of two MMC converter stations connected to the utility grid on one side and to the DC transmission line on the other side. The DC link voltage in this system is maintained by the sum of sub-module voltages in the converter leg and there is no bulky DC link capacitor as in other VSC-HVDC systems. The two MMC converters can regulate active and reactive power by changing the amplitude and phase of the converter line currents with respect to PCC voltage and there are usually two control loops. The outer control regulates the power transfer between the AC and DC systems and the inner or faster controller is responsible for tracking the generated references by the power control, DC or AC voltage control loop. MMC 2 is usually the grid side converter and MMC 1 is the generator side converter. The main objective of the controller is to keep the DC-link voltage constant while keeping sinusoidal grid currents.

Cell 2

Cell n

Cell (n+1)

Cell (n+2)

Cell 1

Cell (2n)

DC link

va

vb

vcDC link

u0

ia-up

ia-low

Udc/2

Udc/2

ia

ib

ic

2L

2L

2L 2L

2L 2L

La

Lb

Lc

ib-up ic-up

ib-low ic-low

ua

ub

uc

S1

S2VSM

Vc

The MMC-HVDC system is simulated to study the effect of fault and voltage sag. The dynamic performance of the modeled MMC-HVDC system during active and reactive power command reversal is shown in Fig. 3. The active power reversal command is sent at t=0.4 secs and reactive power is reversed at t=0.5 secs and P1, P2, Q1 and Q2 are plotted. Fig. 4 shows the step change in DC link reference voltage.

Fig. 3 Active and reactive power reversal test

Fig. 4 Step change in DC link voltage

Fig. 5 MMC individual and total capacitor voltage controller

IV. PROPOSED LVRT CONTROL STRUCTURE

As soon as a fault occurs in the MMC-2 AC terminal the exported active power will reduce significantly according to the voltage drop. On the other side, generator side continues to generate the normal active power therefore there is an enormous amount of energy imbalance. Due to the energy imbalance the DC-link voltage starts increasing uncontrollably which will damage both generator and grid side converters [7-9]. Improved control must be developed to address this issue.

The proposed LVRT control diagram is shown in Fig. 6, which implements Eq. (8) and LVRT code. In normal operation reference for active current in the MMC-2 side is given by systems demanded active power and DC voltage controller gives the active current command for MMC-1 side. On the other hand, the reactive current given/absorbed to/from the grid is regulated by q component independently and the reference is specified either by reactive power demand or the output of the ac voltage regulation controller.

During fault or voltage sag, PCC voltages decrease quickly and the currents increase due to the energy imbalance in the HVDC system besides the utility demands to meet the LVRT requirements. Hence LVRT requirement specifies the references for active and reactive power in MMC-2 side and the controller structure changes to avoid system instability, damage or disconnection.

A voltage vs. time curve defines the demanded low voltage ride through requirement and represents the minimum required immunity of the system at point of common coupling (PCC).

Fig. 7 shows a typical LVRT requirement curve, which consists of four areas; area 1 is a condition where voltage-sag duration is less than tmin, the voltage magnitude at PCC is equal or more than Vmin and the system should remain connected. Areas 2 and 3 define the voltage recovery specifications. In a certain time, t1-tmin or t2-t1 if the recovered voltage reaches the defined levels of v1 and v2 the system should remain connected. Area 4 states the minimum operational voltage that system can handle without disconnection for any duration moreover if voltage is greater than v2 after t2 seconds the protection should not operate. To summarize, gray area defines the safe operating area (SOA) of the transmission system so when PCC voltage is above the curve the system should remain connected. The voltage levels and time values may be different in different grid codes [2].

Once the voltage sag is detected, the internal controller references switch from normal operating condition values to the commanded values by LVRT requirements. The new active and reactive power references are found by the LVRT curve and (8). Fig 8 shows the LVRT curve, based on United States FERC voltage code for grid fault tmin is 625ms and Vmin is 0.15 pu. Moreover if the duration of the voltage sag is less than 150msec and voltages are equal or smaller than 0.15 pu system should stay connected.

The ratio of required reactive current to the rated current is defined by (8) according to Fig.8, where vac is the ac voltage. The numbers in this curve may change based on different grid codes however the concept stays the same. If fault or voltage sag is detected in the system and the operating voltage is less than the defined criteria by the grid code the LVRT controller changes the active and reactive power references to the values found by (8) accordingly besides reactive power is injected in the system to maintain the voltage.

0 0.1 0.2 0.3 0.4 0.5 0.6-4

-3

-2

-1

0

1

2

time (sec)

P1

&P

2 (p

u)

P1P2

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-0.3

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0

1

2

3

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5

Vdc

(pu)

Vdc refVdc

PIVc*

Vcju (j=1-2n)

-1 :-Ik-up , Ik-low 0+1 :-Ik-up , Ik-low 0

VBju*

Individual DC voltage controller

PI PI

1/2

Vc*

VcuIk-low

Ik-up

Icir*

Icir

VAu*

Total DC voltage controller

Fig. 6 Power controller

vac 0.85 pu

0.5< vac 0.85pu (8)

vac 0.5 pu

An MMC-HVDC system with specifications in Table II is simulated in PSCAD to evaluate the proposed LVRT controller. The simulation results for the unmodified typical controller and the modified controller with LVRT is shown in Figs. 9 and 11. The fault has been applied in MMC-2 AC side at t=0.2secs seconds and the AC voltage drops to 0.3 pu for 0.1 secs and return to 1 pu. In MMC VSC there is no centralized DC link capacitor and sub module capacitors maintain the DC link voltage, and an additional DC voltage controller is used in MMC-1 to control the DC link voltage. The DC link voltage increases by about 30%. When the proposed LVRT controller is used, the DC link voltage increase is reduced by about 50% as shown in Fig.11.

TABLE II. MMC-HVDC SYSTEM SPECIFICATIONS

The AC currents in the MMC-2 side are shown in Fig. 9 and Fig.11 for both cases. With typical control system AC currents increase to 1.4 pu during the voltage sag however the LVRT controller limits the AC currents to 1pu. Active and reactive powers during LVRT control are also shown in Fig.11. When the voltage drops, the active power is decreased to around 0.1 pu promptly and injected reactive power is boosted to around 0.9 pu to maintain the voltage.

The LVRT profile on Fig. 7 is applied to the MMC-HVDC system and the three phase voltages, output powers of both sides and reactive power is plotted in Fig. 10. With regular power control strategy, the AC currents drop and become unbalanced after voltage sag is detected. The LVRT controller commands the system to generate reactive power proportional to the voltage drop to maintain voltages and active powers are reduced to avoid overcurrent in the system, which enables the protection devices. The LVRT controller retains the side ones currents during the sag period and maintains the power flow in the HVDC system.

Fig.7 Low voltage ride through curve

Fig. 8 The required percentage of reactive current during LVRT

I. CONCLUSION This paper studied the dynamic performance of transformer-less MMC-HVDC and proposed a LVRT control method for the grid side converter. The mathematical equations were derived base on LVRT requirements to indicate the required reactive power to be injected in the system during the voltage sag period. The proposed controller changes references according to derived active and reactive power values form LVRT control and increases stability by avoiding frequent disconnection. The simulation results were presented and compared for both cases, which confirm that incorporating the proposed control strategy improves the performance of the MMC-HVDC system.

V*ac

Vmk(k=a,b,c)

PIi*odrefVod

iod

PIi*qref

0Leq

0Leqioq

Voq

3dq/abc

System controller

Normal operationLVRT

Pref

Pref-LVRT

PIV*dc

Vdc

Q*ref

PI

Vac Qref-LVRT

Voltage

2

3

4

Time

Vmin

tmin t1 t20

V1

V2

Vnom

1

Ireactive/Irated(%)

Voltage(%)

0.2pu

50 85

1pu

105

Normal operation

Fault operation

System parameters Values AC system 138 kV-LL rms Power 150 MW Fundamental frequency 60Hz MMC switching frequency 180Hz Source inductance Ls 2mH Arm inductance 7mH Submodule capacitor 2500uF DC link voltage 340 KV

Fig. 9 System voltages, currents and DC link voltage with typical control, voltage sag applied at t=0.2 sec

Fig. 10 Dynamic response of the system when LVRT profile in Fig.7 is applied to MMC-2 side., three phase voltages, active powers and reactive

powers respectively

Fig. 11 System reactive power, currents, DC link voltage and active power with LVRT control, voltage sag applied at t=0.2 sec

ACKNOWLEDGMENT

This work used ERC shared facilities supported by the National Science Foundation.

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u)

iaibic

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link

volta

ge

(pu)

Vdc-refVdc

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u)

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ive

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er (p

u)

P refP

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