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Distributed Power Generation Units and Their Impact on the Power Network

ABB Switzerland Ltd.

Christoph Haederli

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Large scale DG interconnection

System parameters Protection Stability “Plug and Power”

“Virtual DG” in small European town

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Case Study with Small European Town Study in CALPOS

Real network (ca. 1 km2)

“random” placing of DG

5 MVA load / 7 MVA DG

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Considered Network Configurations

Meshed network configuration Three MV lines

MV transformers connected on LV level (All LV lines connected in selected area).

Redundancy for MV lines

Station to station (non meshed) network configuration

Three MV lines

No connection between MV transformers on LV level

No redundancy for MV lines

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Voltage Profile

All these nodes are LV-nodes. In the MV-network, the voltage profile is in the admissible range for all calculated cases.

Load profiles applied Admissible range: -10 / +6 % (Germany)

DG off 1 0.9 0.8

meshednon meshed0

20

40

60

80

100

120

140

number of nodes out of

profile

Cos(phi)

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Voltage Rise in Station to Station LV-grid

0,00

1,00

2,00

3,00

4,00

5,00

6,00

7,00

8,00

9,00

10,00

0 0,02 0,04 0,06 0,08 0,1 0,12

S DG /S" kPCC

du [%] DG

SMT = 0.25 MW S”kPCC = 5.5 MVA

SMT = 0.1 MW S”kPCC = 1.2 MVA

SMT = 0.5 MW S”kPCC = 4.2 MVA

SMT = 0.5 MW S”kPCC = 5.5 MVA

12

cos2111"""

kPCC

DGDGkPCC

kPCC

DGDGkPCC

kPCC

DG

S

Sψ

S

Sψje

S

SΔu

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Voltage Rise in meshed LV grid

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Voltage Rise in Station to Station LV-grid

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Loading of LV Network

DG off 1 0.9 0.8

MeshedNon meshed0

2

4

6

8

10

12

14

number of overloaded

cables

Cos(phi)

All these lines are LV-lines.

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Comparison of Network Configurations

Meshed LV grid with multiple MV strands higher reliability

more balanced voltage profile

smaller voltage rise at load steps

lower equipment loading (in normal operation)

lower losses

can handle more DG without alterations in grid

Non-meshed LV grid with station to station supply higher capacity with the same amount of resources

simple design, easier to plan and extend

Cheaper

No reverse power relays needed

Lower short circuit power

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Impact on Protection

Qualitative Reverse power flow

Additional functionality required (Loss of mains detection, etc.)

Quantitative Relay parameter

settings

Increased short circuit power

Selective tripping schedule of distance protection relays with infeed effect

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Reverse Power Relays

DG may trip reverse power relay under normal operating conditions

MV Line 1

Meshed LV grid

Reverse Power Relay

MV Line 2

Reverse Power Relay

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Ik” Indicators on MV Line (Small Town Study)

( Ik” = short circuit current )

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Stability: Inverter Based DG Configuration

DG-power conditioner

=

~ =

~

~

Prim e M over

G enerator

R ectifier C onverter

EM C filter

G rid

D C link

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Stability: Equivalent Network with Multiple DG

Paralleling of inverters in LV grid

Line parameters from small town case study

Transmission line

Stiff generator

s V

l Z

DG-Inverter 2

l l V f I

DG-Inverter 1 l I

l l V f I l V

l I

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Robust Stability Analysis

Robust stability analysis

H

lZ

+

-

lV lIsV

e

+

+

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Stability Analysis, Parallel Units

Number of units allowed for stable operation (Generic 100kW DG model used for calculations)

Stability depends very much on filter and control Other configurations may allow higher numbers of

parallel units

0 75 150 285 385 475

0

1

2

3

4

5

Units

Distance from MV/LV transformer (m)

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Conclusions

“Plug and power” for small units and small aggregated power.

Limited “Plug and power” for intermediate Power

No “Plug and power” for large units and a lot of aggregated power. Advanced planning

Economical barriers

Important factors Network configuration

DG type

Stability depends strongly on network impedance, filter design and control of DG

A significant share of power can be supplied locally by DG without reverse power flow into the network

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