impacts of varying penetration of distributed resources...

Managed by UT-Battelle for the Department of Energy

Impacts of Varying Penetration of Distributed

Resources with & without Volt/Var Control:

Case Study of Varying Load Types

D. Tom Rizy1, Senior Member,

Huijuan Li2, Member,

Fangxing Li1,3, Senior Member,

Yan Xu1, Member,

Sarina Adhikari3, Student Member,

Phil Irminger2, Student Member

2011 IEEE PESGM

July 26, 2011

1ORNL, Power & Energy Systems Group 2Oak Ridge Associated Universities 3University of Tennessee, Knoxville

2 Managed by UT-Battelle for the Department of Energy

Background

Follow-up to 2010 paper “Properly Understanding the Impacts of Distributed Resources (DR) on Distribution Systems”

Addresses how DR impacts vary in regards to both DR voltage regulation capability and load mix

Focuses on impacts to distribution capacity, losses and voltage regulation with DR penetration

Comparison of DR with and without volt/var control on 10MVA feeder example with two DRs

Inverter-based volt/var controls based on ORNL R&D work at the DECC Lab.


Impacts Of Distributed Resources (DR)

On Distribution System

The connection of DR systems to the distribution system

will have an impact on

Feeder Capacity

Line Losses

Voltage Regulation

System Protection

Safety

A steady-state analysis of DR impacts may not be

adequate for addressing the full impacts of DR in a

distribution system.


Dynamic versus Steady-State Analysis

of DR Impacts

Standard approach is to use a power-flow-based program to calculate the network voltages with different DR sizes, penetration and feeder loadings.

Used to be quite difficult to dynamically model a small system not to mention a large distribution system.

However, new tools such as EMTP-RV make the modeling of large systems possible along with their dynamic behavior.


Issues that Merit Consideration for DR

Impact Assessment

Voltage Sensitivity of the Feeder Loads

Impacts on feeder capacity, losses and voltage regulation depend on feeder load voltage sensitivity.

To what degree?

Load mix and distribution for each phase may be important.

Variable Distributed Resource (DR) Output

Concern with the variability of renewable DR (i.e., wind and PV) that does not have an energy storage component.

May not be a concern with penetration level lower than 10%

Penetration level of 20% or greater, intermittent DR may quickly change the feeder voltage profile.


Issues that Merit Consideration for a

DR Impacts Assessment (cont.)

Representation of Multiple DR

A big question is a usable and accurate aggregation model.

Protection Changes with DR

Not expected to be an impact at low DR penetration (i.e. 10% or less)

Becomes a concern at higher DR penetration especially if the DR type is a generator-based system.

Inverter-based DRs are inherently current limited; but may need new modeling and protection methods for high penetration.

DR with Reactive Power Capability

DRs not allowed (i.e., per 1547) to regulate voltage on the distribution system unless authorized by the utility.

Being amended by both IEEE Standards (1547.8) and NIST (P2030).


Advantages of Allowing DR to Provide

Volt/Var Control

Provides reactive power locally instead of delivery over transmission lines from central power plants.

Provides reactive power needed to maintain a steady voltage profile at the load.

Respond to voltage transients (i.e., motor starts or load step changes) to maintain voltage.

Improves feeder capacity by reducing reactive current flow from substation to load.

Reduces line losses along with line flows due to local reactive power injection for voltage regulation.


Impacts Study Approach

DR with and without voltage regulation capability

No regulation – only active power injection from DR

Regulation – both active and reactive power injection (to maintain voltage reference)

Distribution feeder impacts with increasing DR levels

Total line flow (used capacity)

Line losses

Voltage profile

Repeated for different feeder load compositions

Constant Power – served as the benchmark

Constant Impedance

Constant Current

ZIP – equal combination of the previous ones


Example Used to Evaluate DR

Impacts on Distribution System

Inverter-based DR controls1 used to compare impacts of DR with and without voltage regulation capability.

DR penetration level (% of total DR output to feeder capacity) varied to analyze impacts.

Repeated for the four different loading cases.

1Developed and tested by ORNL

Substation

bus

1 32 4

INVERTER ICONTRO

LLER

DE

INVERTER CONTROLLER

65

INVERTER ICONTRO

LLER

DE

INVERTER CONTROLLER

DR1 DR2

10MVA Feeder

Total Load:

5.1MW, 3.7MVar, 0.8pf


Inverter-Based DR Voltage Control

Power System

(Controlled System)

Voltage

ReferenceControllerCompare

Error

Voltage

(Controlled Variable)

Measure

vc

DR: Distributed Energy Resource

Control variable: the PCC voltage

Reference: the desired value of the PCC voltage

Error: difference between reference and measured PCC voltage

Fixed control:

PI control with Kp and Ki fixed

Kp and Ki typically by trial & error

Incorrect gains result in under-performance, oscillation, or instability

Adaptive control:

Kp and Ki values are initially conservative but adjusted in real-time to achieve desired system response time

Voltage stability is ensured

Load

Controller

ic

vdc

switching

signals

vt

(PCC)is i

l

Lc

ic

vc

is

vt

vdc

il

Ls R

sv

s

DEDR


DECC Lab interfaced with Actual Distribution

System Supports Volt/Var Control

Development and Testing.

DECC Lab


Voltage Regulation with both

Fixed and Adaptive Gain Control

(b) Voltage regulation with fixed gains. (a) No voltage regulation.

(c) Voltage regulation with adaptive gains.

Response to two- volt (2V) local voltage transient.

Tested on ORNL distribution system at DECC Lab.

Faster voltage regulation achieved with adaptive gains.

The voltage scales are different since under different distribution

system operating conditions on different days.


0%

5%

10%

15%

20%

25%

30%

35%

5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55%

% O

utp

ut

by

Each

DR

Total DR Penetration Level (%)

DR1 %

DR 2 %

Study Assumptions

10MW Radial Feeder with 63% used capacity

DR penetration from 5% to 55% in 5% increments

Feeder load is fixed for each loading case

DRs provide -1.5MVAr to 1.5MVAr to regulate voltage

0.980pu for DR at bus 4

0.975pu for DR at bus 6

Capacitors both at substation and on circuit assumed to be fixed

DR active power scaled up instead of adding more DRs


Voltage Feeder Profiles for the

Constant Power Load Case

Voltage profiles with increasing DR without voltage regulation

Voltage profiles with increasing DR with voltage regulation

0.960

0.965

0.970

0.975

0.980

0.985

0.990

0.995

1.000

1.005

Bus 1 Bus 2 Bus 3 Bus 4 Bus 5 Bus 6

Vo

lta

ge (p

er

un

it)

Substation to End of Feeder

0%5%10%20%25%30%35%40%45%50%55%

DR

Pen

etra

tio

n L

evel

s

0.960

0.965

0.970

0.975

0.980

0.985

0.990

0.995

1.000

1.005

Bus 1 Bus 2 Bus 3 Bus 4 Bus 5 Bus 6

Vo

lta

ge (p

er

un

it)

Substation to End of Feeder

0%5%10%20%25%30%35%40%45%50%55%

DR

Pen

etra

tio

n L

evel

s

Direction of Increasing DR

Penetration


DR Impacts on Distribution

Losses for Constant Power load

Active power losses with and without DR voltage regulation

Reactive power losses with and without DR voltage regulation

40

50

60

70

80

90

100

110

120

130

0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60%

Act

ive

Po

we

r Lo

sse

s (k

W)

DR Penetration Level (%)

No Regulation

Regulation

40

50

60

70

80

90

100

110

120

130

0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60%

Re

acti

ve P

ow

er

Loss

es

(kV

Ar)


No Regulation

Regulation

Dashed line shows where DR with voltage regulation compared to DR

without voltage regulation no longer provides benefit. Losses increase.


DR Output with Voltage

Regulation for the Load Cases

DR active/reactive power with increasing penetration

DR power factor with increasing penetration

0.0

0.5

1.0

1.5

2.0

2.5

3.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60%

Act

ive

Po

we

r O

utp

ut

(MW

)

Rea

ctiv

e P

ow

er O

utp

ut

(MV

ar)


Q_CP - VR

Q_CI - VR

Q_CZ - VR

Q_ZIP - VR

P_ALL

Injecting Reactive Power

Absorbing Reactive Power

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60%

DR

Po

we

r Fa

cto

r


CP_pf

CI_pf

CZ_pf

ZIP_pf

Injecting Reactive Power

Absorbing Reactive Power

Dashed line shows where DR with voltage regulation absorbs reactive

power instead of injecting reactive power.


2.0

2.5

3.0

3.5

4.0

4.5

5.0

0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60%

Rea

ctiv

e P

ow

er L

ine

Flo

w (

MV

ar)


CP - No VRCP - VRCI - No VRCI - VRCZ - No VRCZ - VRZIP - No VRZIP -VR

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60%

Act

ive

Po

wer

Flo

w (

MW

)


CP - No VR CP - VR

CI - No VR CI - VR

CZ - No VR CZ - VR

ZIP - No VR ZIP -VR

DR Impacts on Distribution Power

Flow for the Load Cases

Active power flow with and without DR voltage regulation

Reactive power flow with and without DR voltage regulation

Dashed line shows where DR with voltage regulation compared to without

voltage regulation no longer provides benefit. Reactive load increases.

VR

No VR


Voltage Profiles for the Four

Different Load Cases

5% DR

Penetration

35% DR

Penetration

20% DR

Penetration

55% DR

Penetration

VR

No VR

VR

No VR

VR

No VR

VR

No VR


Results

Observed slight differences in distribution impacts for the four load cases.

Most difference was in losses - maximum variation of 8.3% between load cases.

Maximum variation of 6.0% for line flows between load cases.

Similar voltage responses for the four load cases.

Reach a point of diminishing return using high penetration DR to provide local voltage regulation.


Results (cont.)

At 35% or greater DR penetration:

Active power from DR causes mode change.

DR voltage regulation mode reverses from injecting to absorbing reactive power.

DR becomes additional reactive load.

Penetration trigger point could be higher if voltage references are allowed to be higher.

However at 30%, reactive power losses increase above no regulation case.

Analysis is based on our specific example and on fixed DR voltage references.


Summary

A number of distribution system impacts merit consideration for DR interconnection (i.e., losses, feeder capacity, voltage regulation, protection and safety).

If some DRs can provide reactive power in addition to active power, they can provide voltage regulation and support.

If voltage regulation is dynamic, DRs can respond to a voltage transient in 0.5 or less and be transparent to system voltage controls.

Local voltage regulation done correctly can lower feeder line flows and losses and increase capacity.

Standards are being developed to determine when DR volt/var control is appropriate.


Possible Future Considerations

Impacts as loading and distribution is varied (rather than fixed) with high DR penetration.

Impacts of intermittent DR (i.e., PV) penetration.

Impacts of air-conditioning stall and DR impacts together.

Impacts of multiple inverter-based DR control to address possible interaction.

More complex distribution network.

Managed by UT-Battelle for the Department of Energy

Q&A


OAK RIDGE NATIONAL LABORATORY Managed By UT-Battelle for the Department of Energy

D. Tom Rizy, Research Staff

Power & Energy Systems Group

Energy & Transportation Science Division

One Bethel Valley Road, MS-6070

Oak Ridge, Tennessee 37831-6070

(865) 574-5203 Voice, 575-7643 Fax

(865) 207-6769 Cell

Email: [email protected]

www.ornl.gov, www.ornl.gov/sci/decc

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