Ancillary Services of Distributed Power Generation Systems Marco Liserre [email protected] Ancillary Services of Distributed Power Generation Systems Marco.

Ancillary Services of Distributed Power Generation Systems

Marco Liserre [email protected]


Marco Liserre

[email protected]



Agenda

Definition of ancillary services

Active and reactive capability of DPGS

Power Flow through a line

Droop Control

Services at Load Level

Services at Electric Power System (EPS) Level



The IEEE 1547.3 “IEEE Guide for Monitoring, Information Exchange, and Control of Distributed Resources Interconnected with Electric Power Systems” defines ancillary services only those provided by DPGS at the Electric Power Systems Area

They are: load regulation, energy losses, spinning and non-spinning reserve, reactive supply.

However future ancillary services may include also power quality enhancement

Moreover they can also be defined at Load Level (e.g. UPS functionality)

Ancillary services contribute to a systemic approach to the management of the new power system characterized by an higher inflow from DPGS

Anyway ancillary services could facilitate the penetration of RES in power systems




Ancillary services are based on the specific characteristic of inverter-based DPGS that can be used to inject active, reactive power and harmonics in the grid (the second and third even if the energy source is not available)

Some of the ancillary services are similar to those that traditional power plant provide to contribute to the safe and stable system operation

However DPGS ancillary services are mainly considered at a distribution level hence part of the features of active distribution grid (smart grid) and not at transmission level (where traditional power plants are usually connected)

Moreover when a DPGS is LV-connected the grid frequency and the grid voltage cannot be controlled independently since low-voltage distribution lines have a not negligible resistance

Hence these issues can not be considered as a mere transposition of known concepts at a different level since they are involved in a wider change of the power system




Active and reactive capability of the inverter-based DPGS

P

0gI

V

E##############

Q

P

gI

V

LV

E##############

Q

P

gI

V

LVE

##############Q

P

gI

V

LV

E##############

Q

P

gI

V

LV

E##############

Q

P

gI

V

LV

E##############

Q

(a)

LV

(b)

(c) (d)

(e) (f)

Active and reactive power management of the DPGS depends on the grid converter

P

Q

P

Q

full power converter half power converter (DFIG)



In case of an inductive line

The previous equations can be used for:

controlling the power sharing among different systems feeding/absorbing power (parallel operation of inverters for UPS applications or micro-grid)

Support of voltage profile and frequency in the EPS area where the DPGS is connected

Support of the load by DPGS (UPS functionality)

Power Flow through a line

21cos cos sin sinP EV E EV

Z

21cos sin sin cosQ EV E EV

Z

sinX

EVP

2cosEV EQ

X

The equations are the basis of the droop control that is the most straightforward way of relating P,f and Q,V



Using short-line model and complex phasors, the analysis below is valid for both single-phase and balanced three-phase systems.

At the section A

For a mainly inductive line

I

VA

Z

Vd

VBI

VB0

VA

Vq

RIXI

BA

cos cosA A BA

V V VP

Z Z

2

sin sinA A BA

V V VQ

Z Z

cos A Ad A B

A

RP XQV V V

V

sin A Aq B

A

XP RQV V

V

A

A B

XP

V V A

A BA

XQV V

V

Droop control technique: inductive line



The angle δ can be controlled regulating the active power P whereas the inverter voltage VA is controllable through the reactive power Q.

Control of the frequency dynamically controls the power angle and, thus, the real power flow.

Thus by adjusting P and Q independently, frequency and amplitude of the grid voltage are determined

From another point of view to share the P and Q among several units working in parallel

0 0pf f k P P 0 0qV V k Q Q

Q

V

V0

Q0 P

f

f0

P0

Droop control technique: inductive line

Q1 Q2

V

P1 P2

f



Implementation of the droop characteristics

* *

* *

( ) ( )

( ) ( )

p

q

G s P P

V V G s Q Q

Vc , Ic voltage and current of the converter

Ig grid current

Gp(s) , Gq(s) compensators transfer functions



Droop control technique: RL line Distribution lines have a not negligible resistive nature, hence more complex formula have to be considered

In case the aim is to control the active power injected by the DPGS and the reactive power exchanged with the grid the following equations can be used

However they are dependent on the grid impedance nature

1cos sinCP E V E EV

Z

1( sin cosGQ E V E EV

Z

tan /X R



Droop control technique: P/Q decoupling In order to remove from active and reactive powers the dependence of line impedance the following transformation is proposed:

that leads to

hence the control laws can be formulated as

' sin cosC GP P Q

' cos sinC GQ P Q

' sinEV

PZ

2' cosEV

Q EZ

In these formula active and reactive powers are clearly related to the phase (frequency) and amplitude of the voltage how it was in the pure inductive case

* *( ) sin cosp C C G GG s P P Q Q

* * * *( ) cos sinq C C G GV V G s P P Q Q

that express the reference amplitude and phase of the voltage produced by the DPGS to have the desired P and Q



Implementation of the droop characteristics with P Q decoupling

( ) i pp

m m sG s

s

( )q pG s n



Stability and robustness analysis

11

22

5

42

11

2

3

Root locus for 0.00002 < mp < 0.001 and mi=0.0002

Root locus for 0.000002 < mi < 0.0018 and mp=0.00006

Root locus diagram for grid inductance variations: 8.5 mH

< Lg’< 5000 mH

Using the small-signal analysis it is possible to check stability margin and robustness respect to parameter variation such as the grid inductance

2

2 2

ˆˆsin ( ) cos ( )ˆ ( )

2o

measo o

E v s VE sp s

Zs s

2

2 2

ˆˆcos ( ) sin ( )ˆ ( )

2o

measo o

E v s VE sq s

Zs s

ˆ ˆˆ( ) sin ( ) cos ( )i pm m ss E v s VE s

s

ˆˆ ˆ( ) cos ( ) sin ( )pv s n E v s VE s



Voltage support provided by the DPGS at load level

CURRENT CONTROLLEDgrid-feeding component adjusting reactive power according to grid

voltage variations

VOLTAGE CONTROLLEDgrid-supporting component

controlling its output voltage in order to stabilize load voltage

Vload

Ic

IloadIg

E

Vload

Vc

Iload

Ig

E

Lg

Lg



normal conditions

compensation of a voltage dip of 0.15 pu

Vload

E

Iload

Ic

Ig

VLg

Vload

E

Iload = Ic

Ig = 0

VLg

In normal conditions the shunt controller provides a current IC = Iload

In case of voltage dips it provides the active power required by the load and it injects the reactive power needed to stabilize the load voltage

The amount of reactive power is inversely proportional to the grid impedance

A large inductance will help in mitigating voltage sags although it is not recommendable during normal operation

Voltage support provided by the DPGS at load level



PV system with shunt-connected multifunctional converter

The voltage sags compensation requires a large-rated converter

However the PV shunt-connected converter is already rated for supplying full power

An inductance Lg* of 0.1 pu is placed

on the grid line (inductive line)

It is possible to control the voltage frequency and amplitude adjusting active and reactive power independently.

PVarray

PV converter

Iload

Ic

E

Ig

Vc ’

Lg*

LOAD



The droop controller provides the reference for the voltage control

Multifunctional PV inverter



Voltage control

The PV inverter is voltage controlled

The current injection is controlled indirectly

The voltage error is pre-processed by the repetitive controller (the periodic signal generator of the fundamental component and of the selected harmonics)

The PI controller improves the stability of the system

The voltage in the PCC is constant and equal to the desired value

In presence of a voltage dip Ig is forced to be phase-shifted by almost 90° with respect to the corresponding grid voltage

Iload

Ic

E Ig

PIRepetitive

control

Ic

Vref +

- -+

Iref

Vc’

LOAD

the 3rd and the 5th harmonics are compensated

1

0

2 2cos

h

N iDFT ai k NF z h i N z

N N



Simulation results: grid normal conditions

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

-10

0

10

time[s]

curre

nt[A

]

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5-10

0

10

time[s]

curre

nt[A

]

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

-2

0

2

time[s]

curre

nt[A

]

0 1 2 3 4 5-200

0

200

400

600

800

1000

1200

1400

1600

1800

time[s]

P[W

], Q

[Var

]

P

Q

active and reactive power provided by the PV inverterinverter current Ic (top), load current Iload (middle),

grid current Ig (bottom).



Simulation results: voltage sag of 0.15 pu

active and reactive power provided by the PV inverter

E = grid voltage Ic = inverter current

Ig = grid current Iload = load current



Voltage harmonic mitigation

Experimental results obtained in case of distorting load and without shunt converter: A grid voltage [300V/div], C load voltage [300V/div], 1 load current [10V/div].

Experimental results in case of distorting load and shunt converter connected to the grid: A) grid voltage [300V/div], C) load voltage [300V/div], 1) load current [10V/div].

Without compensation (Black bar), with compensation (white bar)



DC power supply

LC

AC power SourceFilterGrid ConverterDC source

1 x 240 V1-ph VSI

LOAD

Load Lg*Isolation

transformer

acquisition board and

control

PWM

Experimental setup: Politecnico di Bari



Experimental results: voltage dip duration equal to 1.5 s

grid voltage E (top) : voltage dip of 0.15 pu, load voltage Vload (middle), grid current Ig (bottom)

without droop control



High wind condition + reactive power compensation

High wind condition

upgrade for 600 kW WT systems using 300 kW back-to-back converter

Reactive power compensation at the PCC

reduction in mechanical stress

reactive power control without capacitor banks



Voltage support provided by the DPGS at EPS area

The reactive power injection by grid-connected systems can enhance the voltage profile

The goal is to reduce the active power supplied by the low-voltage feeder, injecting reactive power to support the voltage amplitude decreasing the current and as a consequence the losses

without reactive power injection

with reactive power injection

grid inverter grid inverter

f (Hz) 50 50 50 50

E (V) 228 228 228 230

P (kW) 24.5 0 21.3 0

Q (kVar) n.a. 0 n.a. 16.5



Active filter operation

Only current is compensated

The WTs are cleaning the line current

Power Quality Enhancement provided by the DPGS at EPS area



Ancillary services are considered as those services that the DPGS can provide to the grid in order to behave more similarly to a traditional power plant

However DPGS are connected at distribution network level, hence new problems and new possibilities arises

Moreover DPGS are usually installed to meet demand of local loads hence they could be used to provide ancillary services also to the loads

In general if the full power of the DPGS is managed by a PWM inverter, full control on active and reactive power and on the harmonics can be used to make DPGS contributing to the stabile and safe operation of the EPS area where it is connected and even to allow micro-grid operation

Since all the ancillary services are connected to the management of the power flow in the grid hence the power flow theory and the consequent droop control are useful to guarantee the desired dynamic performances and robustness against grid impedance variation

Conclusions



References1. IEEE 1547.3, IEEE Guide for Monitoring, Information Exchange, and Control of Distributed Resources

Interconnected with Electric Power Systems, 2007

2. Tsai-Fu Wu, Hung Shou Nien, Hui-Ming Hsieh, Chih-Lung Shen “PV power Injection and Active Power

Filtering with Amplitude-Clamping and Amplitude-Scaling Algorithms”, IEEE Transactions on Industry

Applications, vol. 43, no. 3, May/June 2007.

3. Josep M. Guerrero, José Matas, Luis García de Vicuña, Miguel Castilla, Jaume Miret, “Wireless-Control Strategy for Parallel Operation of Distributed-Generation Inverters”, IEEE Transactions on Industrial Electronics , vol.53, no.5, Oct. 2006, pp. 1461-1470.

4. Josep M. Guerrero, José Matas, Luis García de Vicuña, Miguel Castilla, Jaume Miret, “Decentralized Control for Parallel Operation of Distributed Generation Inverters Using Resistive Output Impedance”, IEEE Transactions on Industrial Electronics, vol.54, no.2, April 2007, pp. 994-1004.

5. K. De Brabandere, B. Bolsens, J. Van den Keybus, a. Woyte, J. Driesen, R. Belmans, “A Voltage and Frequency Droop Control Method for Parallel Inverters”, IEEE Transactions on Power Electronics,vol.22, no.4,July 2007, pp.1107-1115.

6. P. Wang, N. Jenkins, M.H.J. Bollen, “Experimental investigation of voltage sag mitigation by an advanced static VAR compensator”, IEEE Transactions on Power Delivery, Vol.13, No. 4, October 1998.

7. P. Mattavelli, F. Pinhabel Marafao, “Repetitive-Based Control for Selective Harmonic Compensation in Active Power Filter”, IEEE Transactions on Industrial Electronics, vol. 51, no. 5, October 2004, pp. 1018-1024.



References8. R. A. Mastromauro, M. Liserre, T. Kerekes, A. Dell’Aquila, “A Voltage Controlled Grid Connected

Photovoltaic System with Power Quality Conditioner Functionality”, accepted for publication on IEEE Transactions on Industrial Electronics, forthcoming issue.

9. M. Bollen, Understanding Power Quality Problems: Voltage Sags and Interruptions; Piscataway, NJ: IEEE Press, 1999.

10. M. Routimo; M. Salo; H. Tuusa; “Current sensorless control of a voltage-source active power filter”, Applied Power Electronics Conference and Exposition, 2005. APEC 2005. Twentieth Annual IEEE, vol.3, Iss., 6-10, March 2005 pp. 1696- 1702.

11. R. R. Sawant and M. C. Chandorkar, “Methods for multi-functional converter control in three-phase four-wire systems”, IET Power Electron., vol. 2 , no. 1, Jan. 2009, pp. 52-66.

12. S.-J. Lee, H. Kim, S.-K. Sul, F. Blaabjerg, “A novel control algorithm for static series compensator by use of PQR instantaneous power theory,” IEEE Trans. Ind. Electron., vol. 19, no. 3, May. 2004, pp.814-827.

13. J. M. Guerrero, L. García de Vicuña, J. Matas, M. Castilla, J. Miret, “Output impedance Design of Parallel-Connected UPS Inverters With Wireless Load-Sharing Control”, IEEE Trans. Ind. Electron., vol.52, no.4, Aug. 2005, pp. 1126-1135.

14. J. M. Guerrero, L. García de Vicuña, J. Matas, M. Castilla, Jaume Miret, “A Wireless Controller to Enhance Dynamic Performance of Parallel Inverters in Distributed Generation Systems,” IEEE Trans. Power Electron., vol. 19, no. 5, Sep. 2004, pp. 1205-1213.

15. J. C. Vasquez, R. A. Mastromauro, J. M. Guerrero, M. Liserre, “Voltage Support Provided by a Droop-Controlled Multifunctional inverter”, accepted to be published on IEEE Transactions on Industrial Electronics, forthcoming issue.

16. C.-C. Shen and C.-N. Lu, “A voltage sag index compatibility between equipment and supply”, IEEE Trans. On Power Delivery, vol. 22, no. 2, April 2007, pp. 996-1002.

Ancillary Services of Distributed Power Generation Systems Marco Liserre [email protected] Ancillary Services of Distributed Power Generation Systems Marco.

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