Review on Protection Issues with Penetration of Distributed Generation in Distribution System
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Drivers Of Distribution Generation(DG) Growth
Limiting green house gas (GHG) emissions
Avoidance of the construction of new transmission circuits and large generating plants
General uncertainty in electricity markets favors small generation schemes
DG is a cost effective route to improved power quality and reliability
Diversification of energy sources to enhance energy security
Support for competition policy
CHALLENGES TO DISTRIBUTED GENERATION
Voltage rise effect
REVIEW ON PROTECTION ISSUES
Classical protection schemes for power distribution system are developed for passive networks, i.e. networks containing only loads
Highly affects the selectivity and sensitivity as the range of fault current changes
Blinding of protection
Prohibition of automatic reclosing
In traditional distribution system, the protection system is designed for radial structure of distribution network, with the assumption that the system has a single source
Since the distribution system penetrated with DG is no longer single source, the assumptions for both of these aspects of fault location fail.
Fault currents through protective devices would change after the introduction of DG.
The impact of certain factors, for example the voltage control, on EMS functions (economic dispatch) is possible. However, this solution would work only if DG penetration is low
Algorithm based on fault current amplitude difference of zone. The algorithm realizes direction detection using the difference of short - circuit capacity between system source and DG.
CHARACTERISTICS OF PROTECTION DEVICES
Circuit breakers and reclosers are normally equipped with inverse-time over current trip devices
t and Ipickup stand for the trip time and the relay current set point respectively
Two inverse-times over current characteristic named Minimum Melting (MM) and Total Clearing (TC)
BLINDING 0F PROTECTION UNIT
The fault current measured by the feeder relay decreases due to the contribution of fault current by DG.
This may cause the delayed operation of the relay or even faults which remains undetected.
This reduction in current may cause false acknowledgement of over-current relays and relay may not operate.
FALSE TRIPPING OF FEEDERS
When a fault occurs at another feeder, the operating device located in that faulted feeder should operate
Nevertheless, the circuit breaker or fuse at the feeder of the DG may operate because of over-current fed by the DG unit and cause unreasonable electricity interruption.
EXCEEDING THE INTERRUPTION CAPACITY OF CIRCUIT BREAKER
The short-circuit fault occurs in point F.
When DG connects to bus 2 the fault current through circuit breaker 2 (IF) will increase.
Due to the increasing fault current levels, the interruption capacity of switching equipment should be changed for new operation point.
Without the DG connection (IDG = 0) and
IF = IS = ICB2
With the DG connection
IF = IS + IDG and ICB2 = IF
However, ICB2 ≥ IS
RELAY MAL OPERATION
For correct operation of relay it is also important that the relay measures the real fault current.
Fig. shows a distribution feeder with a distributed generator supplying part of the local loads.
Assuming a short circuit at point F, the generator will also contribute to the total fault current and we have the following equation.
IF = IS + IDG
But the relay R1 will only measure the current coming from the network.
MISS COORDINATION OF PROTECTION DEVICES
Relay-Relay miss coordination
R1, R2 and R3 are inverse overcurrent relays used for protection
for maximum fault current in point F, time of operation of relay R2 is larger than time of operation of R3
When DG is connected, the fault current increases. And thus the coordination fails
2. Fuse-Fuse miss Coordination
the fuses are coordinated for all fault currents levels below IFmax.
After installing DG, it is possible that the fault current level exceeds IFmax and the coordination between two fuses is lost
3. Fuse-Recloser miss Coordination- For coordination between reclosers and fuse, the recloser must protect the fuse from temporary faults and fuse must operate for permanent faults. -This requires the recloser fast curve should be below than the MM curve of the fuse
if an island forms, repair crews may be faced with unexpected live wires
End-user equipment damage:
customer equipment could theoretically be damaged if operating parameters differ greatly from the norm. In this case, the utility is liable for the damage.
Ending the failure:
Reclosing the circuit onto an active island may cause problems with the utility's equipment, or cause automatic reclosing system to fail to notice the problem.
Reclosing onto an active island may cause confusion among the inverters.
Superconducting Fault Current Limiters (SFCLs)
Utilize superconducting materials to limit the current directly
Supply a DC bias current that affects the level of magnetization of a saturable iron core
nonlinear response to temperature, current and magnetic field variations
When a fault occurs, the current increases and causes the superconductor to quench thereby increasing its resistance exponentially
The current level at which the quench occur depends on operating temp, and the amount and type of superconductor
Ideally, the incipient fault current is limited in less than one cycle.
MULTI-CRITERIA SFCL PLACEMENT METHOD USING SCENARIO OPTIMIZATION WITH ENTROPY MEASURE
Step 1) Perform the load flow to obtain the maximum load current in a power system with DGs
Step 2) Create SFCL placement scenarios with various maximum numbers and locations of SFCLs. The number of scenarios m is given as
NSFCL = maximum number of SFCLs;
Nlocations =the number of candidate locations for the SFCLs to be installed; and
k =number of SFCLs to be installed in each scenario
Step 3) Calculate the near-end fault currents seen by the primary and backup relays for each scenario and the reduction in the average fault current
Step 4) To facilitate comparison of different criteria, perform normalization of all data in each scenario using
where s = 1,2,3,…m and b = 2,3,…n
n = total no. of criteria
Step 5) Determine weight wb for each criterion by calculating its entropy, i.e., Eb, using
Step 6) Reduce the number of scenarios to only the feasible scenarios within the lowest 10% as ranked using
Step7) Solve the scenario subproblem, i.e., the overcurrent relay coordination problem, and then perform the EM method (Step 4 and Step 5) with the data for all criteria including the results obtained from the scenario subproblem.
Step 8) Solve the tracking problem to determine the best scenario, i.e., the number and locations of SFCLs. If all scenarios are found to be infeasible in the tracking problem, extend the region of the feasible scenarios in the scenario subproblem, go back to the Step 7, and repeat the algorithm
The main disadvantage of DG is producing fault current that are more than breaking capacity of circuit breakers and fuses that already exist in the distribution system.
Location, number and capacity of DG are essential factors for studying the effect of DGs on protection system
Due to different problems raised by the penetration of DG, both fault detection and localization is affected
The addition of generation usually requires that feeder protection be upgraded to permit directional supervision of overcurrent functions
Intelligent protective system is highly essential which will enhance the fast estimation and full exploitation of DGs in active power distribution network
Superconducting fault current limiters (SFCLs) can be used to help reduce the fault currents within the breaking capacity of the protective devices
The agent-based relay coordination has the ability to self-check and self-correct and rapidly acts