closed-loop operation of power distribution systems for...

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Closed-loop operation of Power Distribution

Systems for Integration of High Penetration

Levels of Distributed Energy Resources

New Orleans, LA

Jan. 10, 2016

Julio Romero Agüero, H. Lee Willis, Johan Enslin,

Farid Katiraei, John Spare, Valentina Cecchi


Introduction

Distribution systems are under an unprecedented evolution driven

by the need to accommodate and manage growing penetration

levels of renewable Distributed Generation (DG). The intermittent

nature of photovoltaic and wind generation technologies is

impacting all aspects of distribution systems planning and

operations

Intermittent DG impacts volt-VAr control and may cause voltage

violations and voltage fluctuations. Other impacts include reverse

power flow, protection system issues, excessive loading of system

components, etc

These issues can be addressed, to a certain extent, through a

combination of conventional and Smart Grid technologies


Introduction

On the Smart Grid side, there is a need for robust real-time control

algorithms that allow operating this highly dynamic system within

the quality, reliability, efficiency, and security requirements imposed

by modern and future grids

However, as DG penetration levels increase control-based solutions

are bounded by physical limitations imposed by radial distribution

feeders, e.g., feeder capacity and stiffness

Therefore, integrating growing amounts of DG also requires

identifying alternative operation modes of distribution feeders


Closed-loop operation

At a conceptual level, most of the problems caused by high

penetration levels of DG are prompted by the equivalent impedance

at the point of interconnection (POI), i.e., by system stiffness

Expectedly, in radial systems stiffness is inversely proportional to

the distance from the substation, therefore, the farther the POI from

the substation the more severe the impacts

“Looping” two radial feeders has the immediate and direct effect of

decreasing the POI impedance of the combined system and

increasing its stiffness, which solves numerous issues and

augments the maximum penetration level of DG that can be allowed

without deteriorating performance

Furthermore, if adequate control and protection and automation

systems are put in place, closed-loop operation of distribution

feeders leads to increased reliability and efficiency, and available

feeder capacity is utilized in a more efficient manner


Closed-loop operation modes

Three types of closed-loop operation modes, with increasing levels

of complexity, have been identified: 1) feeders from the same

substation transformer, 2) feeders from different transformers of the

same substation, and 3) feeders from different substations

Several considerations must be made when upgrading from radial

to closed-loop operation for each one of these operation modes,

reference [1] discusses this in detail.

Factors to be considered include:

Substations: short circuit currents, capacities, and voltage levels

Substation transformers: ratings, impedances, configurations,

loadings, and load characteristics

Feeders: size, length, loading, load distribution, and load

characteristics


Existing practices

It is worth noting that non-radial operation of distribution feeders is

already utilized in urban distribution to attain premium reliability

levels, e.g., spot networks, secondary networks, ring operation

However, in suburban and rural areas such operation had not been

considered an alternative since lower load densities did not justify

the required investments and additional complexities. However, the

need to mitigate DG impacts provides an additional justification

Suburban and rural feeders are experiencing significant proliferation

of DG. Suburban feeders are natural candidates for implementation

of closed-loop operation to mitigate voltage increase and fluctuation

issues and facilitate DG integration. Rural feeders generally

experience more severe DG-driven impacts, however, they are

constrained by the availability of fewer feeder ties


Example

S3

S2 S1

G1G2

G3

Substation

Two 12.47 kV radial feeders

(same substation transformer)

and three 2 MW DGs

Three normally open tie-switches

suitable for “close-looping” or

“meshing” both feeders

Goal is studying voltage profiles

and potential voltage fluctuations

due to DG interconnection


Results – voltage (S1 closed & no DG)

Low voltage


Results – radial feeders


Results – radial feeders & 3 DG units (6 MW)


Results – meshed feeders (S1 closed)


Results – meshed feeders & 3 DG units (6 MW)


Results – summary

VA VB VC Balanced

Radial 0.964 1.000 0.962 0.975

Radial+DG 0.997 1.030 0.994 1.007

Difference 0.033 0.030 0.032 0.032

Meshed 0.983 0.992 0.987 0.987

Meshed+DG 1.000 1.009 1.004 1.005

Difference 0.017 0.017 0.017 0.017

Reduction 49% 43% 47% 46%

Voltage at G1 POI (PU)Case


Radial operation:

Results show major voltage unbalance before and after DG

interconnection

Results also show significant voltage increase along the feeder,

particularly at POI-DG1 (average increase is about 0.032 PU or

3.84 V on a 120 V base)

Closed-loop operation:

Results show noticeable reduction in voltage unbalance before

and after DG interconnection

Results also show clear reduction in voltage increase along the

feeder. Voltage increase at POI-DG1 is about 0.017 PU or 2.04

V on a 120 V base. This represents an average reduction of

46% with respect to radial feeder operation

Results – summary


Results – S1, S2, S3 closed & 3 DG units (6 MW)


The results show that closed-loop operation has evident

advantages for DG integration over radial operation

Further mitigation of DG impacts on voltage can be attained by

combining closed-loop operation with volt-VAr control, for instance,

with non-unity power factor operation of DG units

Additional advantages of closed-loop operation include loss

reduction and increased reliability

Results – summary

Ploss Qloss Sloss

S1 94% 96% 95%

S2 88% 93% 92%

S3 93% 94% 94%

S1+S2+S3 87% 88% 88%

LossesCase


Implementation of closed-loop operation requires more complex

protection systems, and it may require more robust (and expensive)

equipment since fault duties are higher than radial operation

It is worth noting that the technology required is already available,

moreover, there are industry success stories of (non DG) closed-

loop operation, e.g., International Drive [5]

Most important, closed-loop operation requires the utilization of

updated planning guidelines and operations practices

It is necessary to answer questions such as how many loops are

required? Should closed-looping of feeders from different

transformers or different substations be allowed? How are capacity

limits and reliability requirements defined?

Planning and operations


DG proliferation is leading to increasing impacts on radial feeders.

These impacts can be alleviated by a combination of conventional

and Smart Grid solutions. As DG penetration increases planners

are left with less options, even Smart Grid solutions are bounded by

the physical limitations of existing radial feeders

Closed-loop operation represents the next natural step in the

evolution of the distribution system towards a highly efficient and

reliable grid. Its advantages include improved voltages profiles,

capacity utilization and reliability, and more efficient operation. It

also requires more complex protection systems, more robust

equipment, and updated planning and operations philosophies.

Since the technology required to overcome these issues is already

available, the industry is encouraged to consider closed-loop

operation as a viable alternative to achieve the reliability and

efficiency goals set by the Smart Grid

Conclusions


1. T.H. Chen et al, Feasibility Study of Upgrading Primary Feeders

From Radial and Open-Loop to Normally Closed-Loop Arrangement

2. J. Romero Agüero, Improving the Efficiency of Power Distribution

Systems through Technical and Non-Technical Losses Reduction,

in Proc. of 2012 IEEE PES T&D Conference and Exposition, May

2012

3. G. Celli et al, Meshed vs. Radial MV Distribution Network in

Presence of Large Amount of DG, in Proc. of 2004 IEEE PES

Power Systems Conference and Exposition (PSCE), Oct. 2004

4. N. Hadjsaid et al, Novel architectures and operation modes of

Distribution Network to increase DG integration, in Proc. of 2010

IEEE PES General Meeting, Jul. 2010

5. B. Pagel, Energizing International Drive, T&D World, Apr 2000

References

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