ICSET 2008

Abstract— Reactive power control using converter interfaced energy sources is possible and considered advantageous in certain grids. However, in some cases this is not enough to ensure good voltage quality at the connection point (PCC). It has been shown in this paper that the control of active power, along with the reactive power, can help in relieving the constraints over the injected reactive power and at the same time adds more control flexibility. The control of active power has been realized by using a simple current chopper at the DC-link. It has been shown that with combined active and reactive power control, the voltage quality at both the PCC and DC-link has been enhanced. Moreover, the number of the estimated trips of the converter-interfaced energy source is reduced.

I. INTRODUCTION tilizing a DC-link is beneficial in adjusting the performance of an energy source to match the grid

requirements. For instance, an HVDC link could be implemented to connect an offshore wind farm to the grid, where the front-end converter is mainly responsible for the synchronization of the whole farm as a unit to the grid and hence isolating the dynamics caused by the individual turbine units. Moreover, implementing the DC-link using voltage source converters (VSC), as shown in Fig. 1, where IGBTs with pulse width modulation are utilized, provides a high controllability and good power quality at the connection point. The rectifier in the figure is responsible for maintaining a constant DC-link voltage while DC filters are normally equipped at the DC-link in order to buffer the power (or current) oscillations produced by the turbines. These are important features for a correct operation of the VSC on the grid side (inverter) and for complying with the grid codes.

Offshorewind park VSC VSC

DC smoothingfilters

DC transmissioncable

Filter andtransformer

filter andtransformer

Utilitygrid

Fig. 1 HVDC connection of wind farm to the grid.

Besides wind farms interface, front-end converters are implemented with different types of energy sources, particularly renewables, in order to adjust the power of the

This work was financially supported by the Swedish wind energy research organization (Vindforsk-II). The authors are with STRI, a Swedish consultancy firm; e-mails first autor: fainan@stri.se, second author: mikael.wamundson@stri.se, and third author: math.bollen@stri.se.

source to the grid requirements. Moreover, the converter interfaced energy source can introduce some ancillary services to the grid through the control of the front-end converter.

The front-end converter enables a fast control over reactive power and hence could perform voltage control at the connection point [1][2]. However in order to fully achieve an interactive operation, the active power needs also to be controlled (e.g. to perform frequency control) [3][4]. Yet, in order to control the active power, the primary energy source needs also to be controllable or storage on the DC-link is needed. In [4] and [5], the active power control has been considered for a converter interfaced distributed generation (DG). However, the DC link voltage controller has not been implemented, since a constant DC voltage source has been assumed. This, however, is not a convenient assumption when studying the possibility of the active power control in the converter-interfaced DG.

In this paper, both the injected active and reactive powers are controlled at the connection point of a current controlled converter interfaced energy source. The reactive power control is achieved through the control of the amplitude of the reactive current reference used by the current controller of the converter. On the other hand, the active current control is achieved using an inductive DC-current chopper at the DC-link. The active current controller is activated in order to compensate for the fast dynamics in the grid voltage. The proposed controller has been tested regarding the voltage dips ride-through capability and the mitigation of the oscillations in the grid-voltage amplitude.

II. SYSTEM UNDER FOCUS

As the dynamics of the energy source are much slower than the time constant of the controller of the front-end converter, a constant DC current source can be assumed to decouple the dynamics of the primary source from the dynamics of the grid. The configuration in Fig. 2, referred to afterwards as DC1, is then a valid simplification in order to study the impact of the grid voltage quality on the VSC operation.

The controller of the front-end converter (or voltage source converter; VSC) calculates the current reference that is required to keep the DC-link voltage regulated in case of transients at the grid and to compensate for the voltage at the point of common coupling (PCC). The DC-link voltage regulation criterion depends on injecting the maximum available active power (and hence current Iin) into the grid.

Combined Active and Reactive Power Control with Converter Interfaced Energy Sources

Fainan Hassan, Member, IEEE, Mikael Wämundson, and Math Bollen, Fellow Member, IEEE

U

690978-1-4244-1888-6/08/$25.00 c© 2008 IEEE

This would allow limited reactive power to be available for the grid voltage compensation without hitting the compensation limit (e.g. VSC current limit).

vscinverter

idc

Iin

PCC

udc

DC-link

AC-filterUtilityGrid

Cdc

Fig. 2 DC-link injecting maximum available input power (DC1).

In a way to relieve this constraint and allow higher reactive power limit, a simple DC-link current chopper is proposed as shown in Fig. 3 (referred to afterwards as DC2). The current chopper controller is responsible for the regulation of the DC-link voltage, where the actual injected active power (hence the current idc) can be lowered when needed. The size of the chopper determines, eventually, how much active power could be reduced and for how long. This is, however, considered as a first measure in order to have a fast control over the active power. Further measures could also be encountered (e.g. through the control of the energy source or extra storage) in case the problem at the grid persists. These are, though, considered as secondary control measures with longer time constants and are not further discussed in this paper.

vscinverter

idc

Iin

PCC

udc

DC-link

AC-filterUtilityGrid

Cdc

Ldc

Fig. 3 DC-link with active power control possibility (DC2).

III. MAIN CONTROLLER DESCRIPTION

Generally the controller of the converter-interfaced energy sources aims at injecting the maximum power available from the primary source into the grid through the control of the injected current. Moreover, reactive power support to the grid is also possible through the control of the angle of the injected current. In order to control the active and reactive powers (hence current) independently, a vector current controller is implemented.

A. Vector current controller A vector current controller (VCC) is implemented in the

rotating synchronous reference frame (dq-frame), which is synchronized with the grid voltage frequency using a phase locked loop (PLL). Hence, the grid voltages at the connection point (PCC) and the DG injected currents as three-phase quantities are measured and transformed into constant vector quantities in the dq-frame. The different blocks of the converter current-controller are depicted in Fig. 4. The VCC is basically a PI-controller that compares the

reference current vector *dqi with the actual injected current

vector dqi . The error is then used to modify the reference

voltage vector *dqu , and a reference voltage limit is applied

in order to avoid the saturation of the pulse width modulator (PWM). The delay block in the figure represents the inherited one sample time delay of the digital controller and is compensated for through the controller within the delay-

predictor block. The reference voltage vector *dqu is then

transformed into three-phase quantities and then used in the pulse width modulator (PWM) to generate the switching pattern that would change the injected current in a way to remove the error. The plant block in Fig. 4, represents the PWM, VSC, AC filter, and transformer. This controller can also be described in both negative and positive sequence frames for a proper operation in case of grid voltage imbalance.

Limitation ofreference voltage

Delaykp

Delaypredictor

Plant

Integrator

i*

dq( )k

idq( 1)k-

idq( )k^

idq( -1)k^

+

_+

_

FFdq( )k

�uIdq( )k

u*

dq++

+

edq( )k

idq

udq( )kFeed-forward

vector calculation

VCC

+�

currentlimit

Fig. 4 Vector current controller.

B. Dynamic current limitation The injected VSC current is limited, in order to protect the

converter operation, by limiting the current reference vector *dqi . The current limitation method impacts the transient

performance of the DG as will be shown later. In order to investigate the impact, it will be assumed that at nominal operation the DG is only injecting active current, as shown in Fig. 5 by the current vector at a. This current vector reflects a full-power operation of the DG, which is not always the case yet considered here as the worst case for the required dynamic capability.

In case of a certain disturbance at the grid, it is assumed that the current vector will move to b in order to compensate for the disturbance. There are generally two methods to instantly limit this vector; either by prioritizing the active current or prioritizing the reactive current. In both cases, the active current should be reduced in order to provide some reactive current (or power) during the transients. This is implemented here using a DC-link current-chopper (DC2), as shown in Fig. 3. The amount of the active current reduction, ξ, is limited by the chopper size.

Two different limitation algorithms are considered, as shown in Fig. 5.

L1: Limiting the q-component of the current reference *qi , which represents the reactive current component, in a

way to reduce the injected current amplitude, while the

required reduction of the d-component of the current *di is

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reduced. Since the q-component of the current is responsible for controlling the amount of the injected reactive power into the grid, as explained in the next subsection, the current limitation, in this way, presents also a limitation over the value of the voltage drop (or rise) at the connection point to be compensated for. Referring to Fig. 5, the current vector at b will be limited to c.

L2: Limiting both of the dq-components of the current reference with preference to injecting more reactive current;

*qi . This method gives preference to the voltage control on

the expense of reduced active power. Both methods, however, will be also limited by the chopper size ξ.Referring to Fig. 5, the current vector at b will be initially limited to d using L2. However, since that will require an active current reduction higher that ξ, the current vector will be limited to e instead.

Active current

Re

active

cu

rre

nt

Maximum currentamplitude

b

a

c

d

e

�

L1

L2

Fig. 5 Reference current limitation.

C. Reactive power control A reactive power controller will generally be activated in

case that the voltage at the PCC deviates from its set value. The controller is implemented in the dq-frame. The instantaneous reactive power generated by the DG at the PCC is described as

( ) ( ) ( ) qPCCddPCCqPCC ieieq −= (1)

Where ed represents the voltage amplitude at the PCC, eq

reflects the error in the voltage angle that is assumed here to be zero, id is the active current component, and iq is the reactive current component. Hence, the reactive current reference is generated to compensate the error in the voltage amplitude using a PI-controller, as follows

( ) ( ) ( )−+==

k

nn

T

Tkkkki

1e

ir

sprepr

*q 1εε

(2)

( ) ( ) ( )kekek *dde −=ε (3)

Where k is the sampling instant, kpr is the proportional gain

of the PCC-voltage regulator, Tir is the integral time, and *de

is the set value of the amplitude at the PCC.

D. Performance of dynamic current limit The performance of the above-mentioned two current

reference limitation methods is evaluated with regards to the transient operation in case of voltage dips at the grid. Implementing the reactive power controller and applying voltage dips with different amplitudes, the regulation capability curve is shown in Fig. 6. In the figure, the solid line is related to the case when using the current limitation method referred to as L1, while the dashed line is related to the case when using the current limitation referred to as L2. For a voltage dip of 0.4 p.u. at the grid (before compensation), the voltage at the connection point of the VSC will be compensated to about 0.7 p.u. using L1 and about 0.9 p.u. using L2. Considering a sensitive load that is connected at the same connection point or in a close proximity as the converter interfaced source, it might be beneficial to apply L2 to ride through the dip period. That would also depend on the dip duration and statistics (how frequent it is) as compared to the extra cost related to the investment on the DC chopper.

Fig. 6 Performance of VSC controller with two current limitation methods; L1 (solid) and L2 (dashed).

IV. PCC VOLTAGE REGULATION LIMIT

The above discussion concerning the current limitation algorithm represents a limitation of the capability of the VSC to regulate the voltage at the connection point (PCC). Another limitation is imposed from the grid side that is related to both the injected active power of the VSC unit and the grid impedance as seen from the PCC. A simple network, shown in Fig. 7, is used for the demonstration. In the figure, the impedance Rs + j Xs represents the Thévenin equivalent impedance of the grid as seen by the PCC. It is further assumed here that Rs is negligible and the Thévenin equivalent voltage E is constant and equal to 1 p.u. Assuming that the local loads are disconnected, the power flow from the VSC to the grid can easily be derived, and a relation between the injected power Pin and the voltage at the PCC (UPPC) can be derived. This relation is realised in Fig. 8 using Xs is equal to 0.84 p.u.

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Grid

R Xs s+j

P Qin in,

P QL L,UPCCE �

localloadsGrid

loads

VSC+Energy Source

Bus R

Fig. 7 Simplified network.

The curve in Fig. 8 represents the voltage collapse limit in case that the VSC controller is not regulating the PCC voltage, otherwise it represents the limit for the full regulation area. For a voltage drop of 0.5 p.u. and injected active power of 0.7 p.u., the PCC voltage will not be fully regulated regardless of the amount of the injected reactive power. In order to compensate for such a voltage drop, the injected active power should be reduced to about (or below) 0.55 p.u. This emphasizes, once more, the need to implement a current chopper that provides the transient capability for reducing the active current.

Voltage drop [p.u.]

1.1

1

0.9

0.8

0.7

0.6

Act

ive

pow

er [p

.u.]

0.5

0.4

0.3

0.2

0.10.90.80.70.60.50.40.30.20.10

Full regulation area

←Increased Xs

→ Decreased Xs

Fig. 8 Full regulation region depending on the grid impedance as seen from the PCC.

V. ACTIVE AND REACTIVE POWER CONTROL (DC2) Combined active and reactive power control is possible

with the modified DC-link referred to as DC2. A current chopper consisting of an inductor Ldc and a thyristor switch is implemented in order to regulate the DC voltage and in the same time to provide the capability of injecting oscillating active current into the grid. The design value of Ldc is then dependent on the DC capacitance. The design criteria for both are explained in the following:

DC-link capacitor design The size of the capacitor is determined from the

constraints on the maximum allowable DC voltage ripple dcuΔ . A design expression of the capacitor size, which is

based on a simplified analysis of the instantaneous power flow, is used here [8]:

ndcdc

ndc uu

SC

ω21

* ⋅Δ

= (4)

where Sn is the rated power of the VSC, and nω is the fundamental angular frequency at the grid.

Current chopper design The following steps are proposed to design the current

chopper: 1. The chopper inductance Ldc is designed, regarding

the worst case, to hold the capacitor energy and the maximum DC current:

2max

2dc

dcdc Iu

CL = (5)

where udc is assumed equal to its reference value. The maximum current Imax is equal to the input current plus the maximum value of the allowed ripple:

maxr,inmax III += (6) 2. The difference between the input current and the

average value of the output current, which represents the average value of the inductor current ( ξ ), should be small, since the switching frequency depends on it in the way that less ξ results in higher switching frequency and hence lower oscillations in the DC-link voltage.

The choice of the value of ξ affects the dynamic performance of the energy source. The impact of the value ofξ on the ride through capability is investigated regarding the estimated number of trips per year for the energy source in case of a remote fault at the grid resulting in a voltage dip at the connection point.

Since the number of dips varies strongly between different locations in the power system, it is not possible to give general information on the number of dips that can be expected without having details on the network supplying that location and the number of faults in that network. However, a simple radial approximation of the network results in the following expression for the number of voltage dips, due to faults, with residual voltage V (in per-unit) less than the nominal voltage (1 p.u.) [6]

( )V

VkVN

−×=

1xdips (7)

where kx is a location dependent factor. Comparison in [6] and [7] with both measurements and simulations has shown that this expression is an acceptable first approximation, where no other information is available, even for strongly-meshed networks. Using (7), and different values of chopper size, ξ, the estimated number of trips per year (divided by kx)as a function of the PCC voltage is shown in Fig. 9. For instance, with a voltage immunity of 0.8 p.u., the DG would have twice the number of the estimated trips if ξ = 0.2 p.u. instead of 0.3 p.u. It is worth noting that after increasing the chopper size beyond a certain limit no difference will be noticed regarding reducing the estimated number of trips. This is shown by the two cases when ξ= 0.3 p.u. and 0.4 p.u., where the two curves coincide. Referring to Fig. 5, this

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corresponds to the case when the current vector at d lies within the operational area of the chopper.

Fig. 9 Estimated number of trips with different chopper size; ξ= 0.1 p.u. (solid black), ξ = 0.2 p.u. (dashed blue), ξ = 0.3 p.u. (green with asterisk), and ξ = 0.4 p.u. (dash-dotted red that coincides with the curve ξ = 0.3 p.u.).

It is worth to note here, once more, that the result in Fig. 9 is an estimation and it highly depends on the location of the DG and the grid impedance as seen from the DG connection point

3. The maximum of the output current should be less than (or equal to) the input current.

According to the above design guidelines, ξ is set to 20%

of the input current ( in2.0 I=ξ ). This value of ξ allows 20% peak value of injected current oscillations. The injected DC current (hence the active current reference) is accordingly:

rindc 8.0 iIi += (8)where ir is the ripple current that depends on the oscillations

at the PCC voltage as: *d

dr 1

uui −= .

The DC-link current in (8) is set equal to the active current reference *

di for the VCC. This active current equation is linearly related to the voltage amplitude at the PCC, since the non-linear part of the power flow equations (relating Pin and Qin to the voltage amplitude) is compensated for using the reactive power controller.

VI. CASE STUDY: GRID VOLTAGE AMPLITUDE VARIATIONS

To investigate the impact of voltage amplitude variations on the VSC, a fast fluctuating load is considered connected at the remote bus; R as designated in Fig. 7. The load is however not modelled in details. Instead actual measurement data of the voltage at an arc furnace connection point is used and imposed as the voltage source at bus R. The measured voltage at bus R is shown in Fig. 10.

Fig. 10 Voltage at remote bus (actual measurement close to an arc furnace).

A. Impact on DC-link voltage (DC1) The controller with the DC-link configuration DC1 is first

examined. In this case, only the reactive power control is incorporated while the active power is controlled (from the energy source side) in a way to inject maximum available input power from the energy source. The voltage at the VSC connection point (at PCC) and the DC-link voltage are both oscillating as shown in Fig. 11. The amplitude of the oscillations at the PCC is affected by both the oscillations at the remote bus R and the feeder parameters. Since the DC input current in this case is not allowed to oscillate, the oscillations of the PCC voltage penetrate to the DC-link as oscillations imposed on its nominal voltage. Regarding 0.05 p.u. allowed oscillation, as has been used in the design criterion, the DC-link overvoltage protection might trip in this case.

Fig. 11 Voltage amplitude at the PCC (upper) and DC-link voltage (lower); with only reactive power control (DC1).

B. Improved DC-link (DC2) In this case both active and reactive power control (from

the grid side) is possible, and the DC-link configuration referred to as DC2 is implemented. The PCC-voltage amplitude and the DC-link voltage are shown in Fig. 12, where the effect of the better regulation of the PCC-voltage is reflected as well on the DC-link voltage oscillations that

694

have been significantly reduced.

Fig. 12 Voltage amplitude at the PCC (upper) and DC-link voltage (lower); with both reactive and active power control (DC2).

VII. CONCLUSIONS

Combined active and reactive power control for converter-interfaced energy sources has been realised through the implementation of a simple DC-link current chopper. The chopper has been proposed, designed, and implemented in order to provide the capability to reduce the active power during grid voltage dynamics. This allows for more reactive power to be injected into the grid in order to compensate for different power quality phenomena, which will eventually result in better interface capability and good performance of the energy source. Adding the possibility of active power control, the full PCC voltage regulation area is

increased (regarding both the grid and the source limitations) and better voltage quality of both the AC voltage at the connection point and the DC-link voltage of the converter is achieved. Moreover, the number of estimated trips per year (mainly due to voltage dips) is reduced.

VIII. REFERENCES

[1] E. Twining, M. J. Newman, P. C. Loh, and D. G. Holmes, “Voltage Compensation in Weak Distribution Networks Using a D-STATCOM,” at Power Electronics and Drive System Conference (PEDS’03), Nov. 17-20, 2003, vol.1, pp. 178-183.

[2] G. Joos, B. T. Ooi, D. McGillis, F. D. Galiana, and R. Marceau, ”The Potential of Distributed Generation to Provide Ancillary Services,” at IEEE Power Engineering Society Summer Meeting, 16-20 July 2000, vol. 3, pp. 1762-1767.

[3] F. Magueed Hassan, A. Sannino, and J. Svensson, “Design of Robust Converter Interface for Wind Power Applications,” in Wind Energy Journal, vol. 8, no. 3, 2005, pp. 319-332.

[4] M. Dai, M. N. Marwali, J-W. Jung, and A. Keyhani, “Power Flow Control of a Single Distributed Generation Unit,” in IEEE Trans. on Power Electronics, vol. 23, no. 1, January 2008, pp. 343-352.

[5] P. Rodriguez, A. V. Timbus, R. Teodorescu, M. Liserre, and F. Blaabjerg, “Flexible Active Power Control of Distributed Generation Systems During Grid Faults,” in IEEE trans on Industrial Electronics,vol. 54, No. 5, October 2007, pp. 2583-2592.

[6] M. H. J. Bollen, “Fast Assessment Methods for Voltage Sags in Distribution Systems,” in IEEE Transactions on Industry Applications, Nov. 1996, vol. 31, no. 6, pp. 1414-1423.

[7] M. R. Qader, M. H. J. Bollen, and R. N. Allan, “Stochastic Prediction of Voltage Sags in a Large Transmission System,” in IEEE Transactions on Industry Applications, Jan. 1999, vol. 35, no. 1, pp. 152-162.

[8] M. Bojrup, Advanced Control of Active Filters in a Battery Charger Application, Licentiate Thesis, Lund University, Sweden, 2003.

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