Frankfurt (Germany), 6-9 June 2011 Gerhard Theil Technical University Vienna Institute of Energy...

Frankfurt (Germany), 6-9 June 2011

Gerhard TheilTechnical University Vienna

Institute of Energy Systems and Electric Drives [email protected]

Andreas TheilWien Energie Stromnetz

[email protected] Theil

[email protected]

THEIL – Austria – Session S5 – Paper 0314

MEDIUM VOLTAGE NETWORK RELIABILITY EVALUATION:

SIMULATION OF PRACTICALLY APPLIED

SUPPLY RESTORATION STRATEGIES FOR DOUBLE-FAILURE EVENTS


Objective

Reliability evaluation of medium voltage overhead-line and cable networks

Main topics• Assessment of multiple-fault frequency in networks

exposed to progressed component aging • Simulation of efficient supply restoration procedures for

cases when fault-clearing personnel has to expect occurrence of more than one fault location per failure-event



Multiple-fault frequency model

Approach:• Modeling short-circuit exposition levels caused

by past-time failure events for individual network components – “stress factors”.

• Multiple outage frequency of network components is determined by “stress factors”.



Classical multiple-fault frequency model

Classical conditional probability approach for frequency of second-order fault:

(primary event at component i secondary event at component k)

fe Frequency of the primary event, e.g. single-phase-faultpe Conditional probability of the second event given a first one has

occurredli, lk Line lengths

Substitution of conditional probability by the stress-factor results in the:

Stress-based multiple-fault frequency model


kiki lpelfef ...,,2


Stress-based multiple-fault frequency model

Sm Stress factor representing the second-order-event sensitivity of component k with respect to a first-order event occurring at component i,

tp Age of component k ch Calibration factor

Stress factor is dependent on: Expectation of fault number accumulated during age of component k Position of components i and k within network structure.

(Components positioned near an injection source usually carry larger short-circuit-currents than components in remote positions).


kikiki lltpSmchtpf .).(.)( ,,,2


Medium-voltage network reliability evaluation

System reliability index: Not supplied energy

nc Number of network componentsnn Number of consumer substations affected by the failure eventf Component outage frequenciestU Interruption time of the i-th consumer affected by the failure eventLU Not supplied load of the i-th consumer


nnn

UUncki

kinci nnn

UUiU nkinkininiLtfLtfE )()(

,,,,,,,

,,2


Fault management strategies

Failure-search procedureA) Fault-clearing personnel activities are conducted by minimization of

not supplied energy.

B) Binary search.

Supply restoration1.) Switching-in of lines for supply restoration is performed during the

entire fault-clearing procedure irrespective of the number of switching actions.

2.) Supply restoration actions are performed for identified intact parts of the line only.



Fault management strategies

Single-phase-fault handling

- Line remains in operation, short-time consumer interruptions occur because of switching performed during failure search.

+ Line is switched off immediately after fault indication. Short-time switching-in actions are performed for fault locating. Thus, exposure time to failure effects is lower than in case '-'.

* Single-phase-faults are transformed to short-circuit-faults by activating low-impedance-grounding. Subsequently, information provided by fault-locators is used during failure-location procedure.



Study System

Total line length: 41 kmNumber of supply stations: 100Single-outage frequency: overhead-line: 0,069/km.a cable: 0,021/km.aDouble-outage frequency: 10-9 to 10-5 /a



Comparison of LOLE for different supply restoration strategies – 2 lines

Line 1 Line 2

Simulation of single and double component outages Upper dark column parts: Contribution of double-faults


0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

LOLE(2)

LOLE(1)

LOLE[h/yr.]

A1- A2- A1+ A1* A2* B1- A1- A2- A1+ A1* A2* B1-


Comparison of LOLE for different supply restoration strategies

RemarksA1-, B1-

Proportions of reliability indices are dependent on line structure and load distribution along line. For lines without lateral branches (line L1), strategy A is the preferable one. For lines with branches (line L2) binary search procedure is converted to strategy A.

A1-, A2-Reduction of stress exposition (A2) leads to longer supply interruption times.

A1-, A1+Disconnecting the line immediately after fault occurrence (A1+) possibly leads to reduction of the probability of double and multiple faults. Provided that this reduction is sufficiently large, improvement of system reliability can be reached. Otherwise an increase of LOLE has to be expected.

A1-, A1*Activating low-impedance-grounding can result in similar effects.

A1+, A1*Information delivered by fault-locators (A1*) is available for acceleration of fault-location and supply-restoration procedures. Additional reduction of LOLE can be observed (line 1).



Conclusions

Multiple-fault frequency in networks exposed to progressed component aging is modeled by taking stress accumulated during component lifetime into account.

Algorithms for simulation of supply restoration procedures handling multiple failure events were developed and implemented into medium-voltage network reliability evaluation software.



Conclusions

Taking solely first-order-failure events into consideration, supply restoration procedures without limitation of not successful switching actions result in the lowest total not-supplied energy. However significant lifetime reductions of components may be the consequence.

Taking second-order-failure events into consideration, strategies resulting in lower times of exposure to single-phase-faults seem to be more profitable. Possibility of significant multiple-outage frequency reductions and substantial system reliability improvements exists.

Low-impedance-grounding activated immediately after occurrence of single-phase-faults seems to be a promising measure.


Frankfurt (Germany), 6-9 June 2011 Gerhard Theil Technical University Vienna Institute of Energy...

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