Droop Control

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Droop control in LV-Grids Alfred Engler, Nikos Soultanis

Remote electrific electrification ation with island island supply systems, systems,  Abstract  Abstract — Remote the incre increasi asing ng accept acceptanc ancee of the micro microgri grids ds conce concept pt and the penetration of the interconnected grid with DER and RES require the application of inverters and the development of new control algorithms algorithms.. One promising promising approach approach is the impleme implementatio ntation n of  conventional f/U-droops into the respective inverters, thus down scaling the conventional grid control concept to the low voltage grid. Despite contradict contradicting ing line parameters, parameters, the applicabil applicability ity of  this this proce proceedi eding ng is outlin outlined ed and the bounda boundary ry condit condition ionss are are derived. droops,  Index Terms— droops,

low voltage voltage grids, grids, micro micro grids, control, control, distribute distributed d generatio generation, n, DER, RES, VSI.

I. I NTRODUCTION

R

EMOTE electrification with island supply systems, the increasing acceptance of the microgrids concept [1] and the penetration of the interconnected grid with DER and RES require require the applicati application on of invert inverters ers and the developm development ent of  new control algorithms. One promis promising ing approa approach ch is the imple implemen mentat tation ion of conconventional f/U-droops into the respective inverters, thus down scaling the conventional conventional grid control concept to the low voltage voltage grid. By this methodology a superior system architecture is enabled, providing redundancy, enabling expandable distributed systems and avoiding vast communication expense. With the development of the control algorithm selfsync the operability of droops in inverters has been proven. Being based on conventional droops this control concept can be derived from inductive coupled voltage sources. A voltage source combined with an inductance represents a high voltage line with a stiff stiff grid or a synchronous synchronous machine machine (generator). (generator). Here Here the reacti reactive ve power power is relate related d with with the voltage voltage and the activ activee power power with with the phase phase shift shift or respec respecti tivel vely y with with the freque frequency ncy.. This This change changess with with the low low volta voltage ge line line and its resisti resistive ve character character,, where reactive reactive power is related related with the phase shift and active power with the voltage. Nevertheless the droop concept is still operable due to its “indirect operation”, which will be outlined below. TM

of the compon component ents. s. Such Such approa approach ch result resultss in the follo followin wing g features: • •

• • •

simple expansion of the system increased redundancy, as the system does not rely on a vulnerable bus system for optimisation a simple bus system is sufficient a simplified supervisory control more complex control tasks in the components.

Additional Additional redundancy redundancy in grids can be achieved achieved by using using voltage source inverters (VSI) in parallel. This approach avoids the master/slave operation. In fact, all VSIs form the grid. The inverters are coupled via the inductances resulting from their filters for the pulse suppression and of decoupling chokes (s. Fig. 2). But the configuration in Fig. 2 is difficult to handle as will be shown. The active power P  and the reactive power Q of the voltage sources can be calculated as follows:

P 1

=

Q1

=

U 1,eff  · U 2,eff  sin δ  ωN (L1 + L2 ) U 12,eff  U 1,eff  · U 2,eff  ωN (L1 + L2 )

−

ωN (L1 + L2 )

(1)

cos δ 

A phase shift δ  between two voltage sources causes active power transmission. Reactive power transmission is due to the voltage difference U 1 − U 2 . Assuming standard values for the inductance L1 and L2 results in very sensitive systems, where even smallest deviations of the phase and the magnitude cause high currents between the inverters. This sensitivity is the reason why fixed frequency and fixed voltage controlled inverters can’t operate in parallel. There is always a voltage difference due to tolerances of the sensors, references, temperature drift f 

u

f 0

u

0

Df 

Du

-1%

-4%

I I . D ROOP CONTROL

Fig. 1.

1P PN

0

-1

In expandable distributed inverter inverter systems communication and/or extra cabling can be overcome if the inverters themselves set their instantaneous active and reactive power. In [2], [3] a concept has been developed using reactive power/voltage and active power/frequency droops for the power control of  the inverters. The droops are similar to those in utility grids (s. Fig. 1). The supervisory control just provides parameter settings for each component, which comprise the idle frequency, the idle voltage, the slopes of the droops and basic commands. This way expensive control bus systems are replaced by using the grid quantities voltage and frequency for the co-ordination

(2)

QN

Conventio Conventional nal droops in in the interconnec interconnected ted grid

L1

U1

L2

~

~

equivalent circuit

Fig. Fig. 2.

1Q

0

-1

Inducti Inductive ve coupled coupled voltage voltage sources sources

U2

U1

d

U2

phasor diagram

2

and ageing (e. g. 1 - 5%) and also crystals are not equal. The frequency errors of the crystals are integrated over the time, resulting in hazardous angle differences (s. Eq. 1). The obvious method for implementing frequency droops is to use P  as a function of  f . But in a real system obtaining an accurate accurate measurement measurement of instantan instantaneous eous frequency is not straight-forward. Measuring instantaneous real power is easier. It has therefore been proposed [2] a control with f  to be a function of  P : the VSI output output power power is measur measured ed and this this quantity is used to adjust its output frequency. f 0 i

P Tmech

st1

f 

st'1

j

u ref 

Fig. 4. Two battery inverters inverters SunnyIsland  TM by SMA Regelsysteme GmbH, Kassel, Kassel, Germany Germany operating operating in paralle parallell (rated (rated power power 4.2 kW, clock 16 kHz, coupling inductor 0.8 mH)

u0 U

300

u

Q

st2

Texcit e

IuI

200    A    /    0 100    1    *       l      a        t      o

 power  acquisition

0

   I t  ,    V−100    /    U

voltage reference

droops

decoupling 

I

−200 −300 5

Fig. 3.

5.01

5. 02

5.03

Control Control approach approach selfsync TM by ISET e. V., Kassel, Germany [4]

5. 05

5.06

5.07

5. 08

5. 05

5.06

5.07

5. 08

t/s

I

6

TM

Firstly Firstly this control control approach, approach, named selfsync , was was imimplemented plemented into the battery battery inverte inverterr SunnyIsland  for rural electrification (s. Fig. 4). For an experiment [5] three of these inverter inverterss programmed programmed with this scheme were connected connected on a single phase to an ohmic load, each via a thin low voltage cable. The frequency droop of the inverters denoted by L1 , L2 in Fig. 5 was set to 1 Hz/rated power. The inverter denoted with L3 was set to 2 Hz/rated power. It is evident that this method allows L3 to supply a smaller proportion of power. The load sharing corresponds to the settings. L1 , L2 are equal, L3 half of it. Noticeable is the phase shift of  L3 to L1 , L2 which is due to the different loading of the cables, causing a slight voltage difference between the inverters, which results in reactive power flow. The compatibility of  selfsync with rotating generators [6] and compatibility with the grid [7] will be outlined in the full paper.

5.04

L1, L2

4       A       /

I

2

       3        L

TM

L3

   I

 ,

0

       2        L

   I

 ,

       1 L

   I

−2 −4 −6 5

5.01

5. 02

5.03

5.04 t/s

Fig. Fig. 5. 3 kW  steady state operation; load sharing of three SunnyIslands running in parallel

TM

The active power P and the reactive power Q of resistive coupled voltage sources - here an inverter and a grid - can be calculated as follows with the notation according to Fig. 6:

TM

Qinv

=

P inv inv

=

U inv inv,eff  · U grid grid ,eff  sin δ  Rline 2 U inv U inv inv,eff  · U grid grid,eff  ,eff  Rline

−

Rline

(3)

cos δ .

(4)

III. I MPLICATIONS OF LINE PARAMETERS  A. Power transmission in the low voltage grid 

Table able I shows shows the typica typicall line line parame parameter terss R  , X  and the typica typicall rated rated curren currentt for the high-, high-, mediummedium- and low low voltage lines. Assuming inductive coupled voltages sources for representing the droop controlled inverters and the distribution system system would be only only correc correctt for the high high voltag voltagee level level.. A medium voltage line has mixed parameters and the low voltage line is even predominantly resistive. TABLE I T YPICAL LINE

PARAMETERS PARAMETERS

[8]

Type of line

R’ Ω /km

X’ Ω /km

A

low voltage line medi medium um volta oltage ge line line high voltage line

0.642 0.16 0.161 1 0.06

0.083 0.19 0.190 0 0.191

142 396 396 580

I N 



R X

7.7 0.85 0.85 0.31

R line

Ugrid

~

~

equivalent circuit

Fig. Fig. 6.

Uinv 

d

Ugrid

Uinv 

phasor diagram

Resisti Resistive ve coupled coupled voltage voltage sources sources

Eq. 4 reveals that the active power flow and the voltage is linked linked in the low voltage voltage grid. A phase difference difference between between the volta voltage ge source sourcess causes causes reacti reactive ve power power flow flow (s. Eq. 3). This fact suggests suggests to use active active power/vo power/voltag ltagee and reactive reactive power/frequency droops - hereinafter called “opposite droops” - in the low voltage grid instead of reactive power/voltage and active active power/fr power/frequenc equency y droops - hereinafte hereinafterr called called “conven“conventional droops”.

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 B. Comparison of droop concepts for the low voltage level

In the following the advantages and disadvantages of using conventi conventional onal or inverse inverse droops on the low voltage level are discussed. The boundary conditions for applying conventional droops in low voltage grids will be outlined afterwards. In the low voltage grid the voltage profile is linked with the active power distribution. Reactive power is not suited for voltage control. From a system’s view the voltage control and the active power dispatch are the major control issues. Table II shows pros and cons of using these two droop concepts.

ACKNOWLEDGMENT We would like to express our thanks to the European Commission for their support in the MicroGrids -project ENK5-CTENK5-CT2002-00610. R EFERENCES

TABLE II C OMPARISON OF DROOP CONCEPTS FOR THE LOW VOLTAGE VOLTAGE LEVEL

compatible with HV-level compatible with generators direct voltage control active power dispatch

Still the question of voltage control remains open, which should be supported by the grid layout. However, in order to improve improve the situatio situation n the partial compensation compensation of lines lines has been successfully demonstrated by means of simulation.

conven conventio tional nal droop droop

opposit oppositee droop droop

yes yes no yes

no no yes no

As one can see from the Table II the only advantage of using the inve inverse rse droops is the direct direct volta voltage ge contro control. l. But if one would control the voltage this way, no power dispatch would be possible. Each load would be fully supplied by the nearest generator. As this generally is not possible, voltage deviations would remain in the grid. Using conventional droops results in connectivity to the high voltage level, allows power sharing also with rotating generators and a precise power dispatch. The voltage deviations within the grid depend on the grid layout, which is today’s standard.

MicroGrid power networks , Cogener [1] J. Lynch, Lynch, MicroGrid Cogeneratio ation n & On-Site On-Site Power Production, James & James, London, May-June 2004. [2] A. Engler Engler,, Regelung von Batteriestromrichtern in modularen und erweiterbaren Inselnetzen, Dissertation.de, Dissertation.de, Berlin, May 2002, ISBN 3-89825439-9. [3] A. Engle Engler, r, Control of Parallel Operating Battery Inverters , 1st PV Hybrid Power Systems Conference, Aix-en-Provence, September 2000. [4] A. Engler, Engler, Vorrichtung orrichtung zum gleichberecht gleichberechtigten igten Parallelbet Parallelbetrieb rieb von einoder dreiphasigen dreiphasigen Spannungsquellen Spannungsquellen, German German patent patent No. 101 40 783.1 (pending), European patent No. 02 018 526.26 (pending), US Patent No. US 6,693,80 6,693,809 9 B2, Feb. 17, 2004 (granted (granted), ), Japanese Japanese patent patent No. 20022002240991 (pending). (pending). [5] A. Arulampalam, Arulampalam, M. Barnes, A. Engler, Engler, A. Goodwin, N. Jenkins, Jenkins, Control of Power Electronic Interfaces in Distributed Distributed Generation , International International Journal of Electronics, London, XXXX 2004. [6] A. Engler, Engler, C. Hardt, Ph. Strauss and M. Vandenbergh, Vandenbergh, Parallel Operation of Generators for Stand-Alone Single Phase Hybrid Systems , EPVSEC, Munich, October 2001. [7] [7] S. Papath Papathan anass assio iou, u, D. Georg Georgak akis, is, N Hatzia Hatziarg rgyr yrio iou, u, A. Engle Engler, r, Ch. Ch. Operation on of a proto prototype type Micro-gr Micro-grid id system system based based on micromicroHardt, Operati sources equipped with fast-acting power electronic electronic interfaces , 31th PESC, Aachen, June 2004. Elektrische Energieversorgu Energieversorgung ng, [8] Klaus Klaus Heuk, Heuk, Klaus-Di Klaus-Dieter eter Dettmann Dettmann,, Elektrische Vieweg, 3rd edition.

IV. IV. I NDIRECT OPERATION OF DROOPS Basicall Basically y, the conventi conventional onal droop is operable operable in the low voltage grid due to the generator’s voltage variability by means of exchan exchangin ging g reacti reactive ve power power.. The reacti reactive ve power power of each each generator is tuned the way that the resulting voltage profile satis satisfies fies the desire desired d activ activee power power distri distribu butio tion. n. In the low low voltage grid the reactive power is a function of the phase angle (s. Eq. 3). This is adjusted with the active power / frequency droop. The control sense of the entire loop has to be consistent. Four stable operating points result, two of which make sense, depending on the slopes of the droops. A mathematical derivation and respective simulations will be presented in the full paper, explaining the effectiveness of  the “indirect operation” of the droop control in LV-grids. V. CONCLUSION It has been shown that the droops, used in the interconnected grid, grid, can be used used effec effecti tivel vely y on the low low volta voltage ge level level due to their “indirect operation”. So far, this effect has not been reported about. The only boundary condition is the same sign for the freque frequency ncy as well well as for the voltag voltagee droop droop factor factors. s. As a conseq consequen uence ce of this this outcom outcomee the contro controll strate strategy gy of  the conventional grid can be down scaled to the low voltage level without any restrictions. This coherence will support the introduction of DER and RES on the low voltage level and concerns about grid stability and safety can be alleviated.

Dr.-Ing. Alfred Engler is head of the group “Electricity tricity Grids” of ISET’s division division “Engineering and Power Power Electron Electronics” ics”.. He has been with with ISET e.V., e.V., Kassel Kassel,, German Germany y, which which he joined joined in 1995 1995.. He receiv received ed his Dipl.-I Dipl.-Ing. ng. (Master’ (Master’s) s) in 1995 1995 from the Techn Technical ical Univer University sity of Braunsch Braunschweig weig with with a thesis thesis on control control of inducti induction on machine machines. s. In 2001 2001 Dr. Engler received received the degree Dr.-Ing. (Ph.D.) for the development development of control algorithms algorithms for inverters in modular and expanda expandable ble island island systems. systems. He is mainly involved involved with inverter inverter control, control, island grids, micro micro grids, grids, power power quality quality and grid integrat integration ion of wind wind power power.. He has presente presented d the results results of his work in about about 40 publica publicatio tions ns and patents. patents. He regularly lectures in distributed generation and control of power electronics.

Nikos Soultanis is