Abstract- Cascading failures and blackouts are the most significant threats for power system security. If the process of cascading failure proceeds due to further line tripping then the system will face uncontrolled islanding situation. Establishment of uncontrolled islands with deficiency in MW or MVAr power balance, are the main reason for system blackout. In order to prevent system blackout due to uncontrolled islanding, intentional islanding has been considered as a preventive strategy against system blackout. In this paper, for recognizing proper islanding scenarios, a new search algorithm based on the ant search mechanism is proposed to find controlled islands. The security constraints considered for finding proper islanding scenarios, are load-generation balance and avoiding line overloading which are evaluated by linear programming and DC load flow. The principle of search algorithm for finding controlled islands is based on the ant search mechanism. The proposed algorithm is applied on IEEE 39-bus network and simulation results show its ability and efficiency for finding proper islanding scenarios.
Keywords: blackout, intentional islanding, power balance, line overloading, load shedding
Introduction
Security is a vital requirement for power systems which could be defined as its ability for preventing blackout following initiation of the process of cascading failures. Cascading failures is the main mechanism for pushing power system toward blackout. Cascading failure is a complicated process consisting of a sequence of events and line tripping which take place due to network weaknesses and local function of protection relays. In the process of cascading failure, if line tripping proceeds further, then, network splitting into uncontrolled islands will be inevitable. The uncontrolled islands always suffer from load-generation unbalance which may cause angle, voltage or frequency instability leading to blackout. Establishment of uncontrolled islands in the network is the dominant characteristic of power system dynamic during the process of cascading failure which is known as the main cause for blackout. In order to avoid uncontrolled splitting of power network and reducing the risk of blackout, intentional islanding has been considered as a preventive action in the defense strategy of power systems. System splitting which is known as controlled islanding is intentional separation of the entire network by tripping properly selected lines, into two or several islands. After system splitting, the whole power system will be under island operation and each island including its own load and generation should remain stable [1]. In such situation, although the power system is operating in an abnormal degraded state, customers are continuing to be served.
In order to apply the strategy of controlled islanding, three tasks should be carried out sequentially.
Recognizing the proper operating instant at which applying intentional splitting is inevitable otherwise the system will be separated into uncontrolled islands
Identifying the proper islands in the network for intentional network splitting such that each island be able to preserve its power balance and stability
Implementation of the planned islanding scenario in a proper way without any dynamic and transient consequence causing large oscillation and instability for islands
However, regarding tasks 1 and 3 concerning recognition of the proper time for applying splitting scenario and how to split the whole system into islands, less works have been reported. When islanding scenario is identified and decided to be applied, the most important task is implantation of the scenario by proper tripping of the lines between islands. For this purpose, the priority of line tripping is very important which dominate system stability. Most reported works are mainly focused on the task 2 concerning identification of the proper islanding scenario for network splitting [3]-[7]. Detecting islands and determining asynchronous groups of generators have been investigated in [8]-[10]. Enhancing functions and operation schemes of relays for reducing their contribution in system blackout is worked in [7], [11]. However, still for recognition of controlled islanding strategies to prevent blackouts, limited studies have been reported [1], [2], [12], [15]. Algorithms based on the technique "Ordered Binary Decision Diagram" (OBDD) has been used to determine proper splitting strategies [1], [2], [12], [13]. In [1], a three phase method has been used to determine proper islanding scenario in the network. In Phase-1, a much simpler reduced network of the original power network is constructed by graph theory; then in Phase-2, the verification algorithms based on OBDDs can efficiently narrow down the strategy space and can give enough splitting strategies satisfying "load-generation balance" constraints. In Phase-3, by using power-flow calculations the possibility of line overloading is checked, and final proper splitting strategies will be given. Also in [2] a two phase method has been used to find proper islanding schemes. The method narrows down the strategy space using highly efficient OBDD-based algorithm in the first phase, then finds proper splitting strategies using power-flow analysis in the reduced strategy space in the second phase. In [12], in addition to load-generation balance constraint and overloading constraint in transmission lines, some other constraints such as synchronism between generators in each island and stability of islands after splitting are considered for finding proper islands. In [12], a three phase method has been used to satisfy these constraints in order to find proper islands. In the first phase power network will be split into separated sub networks; in the second phase, the constraints of load-generation balance and groups of synchronized generators will be checked. Finally in the third phase, constraints of overloading in transmission lines and stability of separated islands will be checked.
It is noteworthy that, all works and studies reported mainly focused on determining proper strategies for islanding. However, recognition the suitable condition for applying intentional islanding and also implementation of islanding schemes in real operational environment of power system are left with no work.
The main aims of network islanding could be numerated as follows;
Separating the vulnerable parts of network from other parts in order to avoid spreading weakness of the system to other parts
Splitting the whole system into small subsystems for easy handing
Separated islands must be able to maintain their power balance and stability by using generation control and load shedding. In this paper, a new algorithm based on the ant search mechanism is proposed for recognizing controlled islanding scenario. Here, it is assumed that the first task has been done and decision for islanding has been taken; so concerning the second task, and in order to identify proper islanding scenario, this approach is proposed. The third task is not considered in this paper and it is the subject of another study.
Intentional and controlled islanding
In the process of power system blackout forced by cascading failures, some parts of network may experience angle or voltage instability. In such situation, trying to maintain system integrity and operate the system as a whole is very difficult and may cause propagation of local weaknesses to other parts of system. In critical conditions, continuation of cascading failures threats integrity of whole network and breakdown the network into uncontrolled islands. Therefore, in such situation, intentional separation of power system into controlled islands is recognized as an effective defense strategy.
Intentional controlled islanding of power systems is justified based on the following advantages:
Separating weak and vulnerable parts from stable parts of the system
Easy handling and controlling small subsystems with respect to the whole system in critical conditions
After formation of the forced islands, load-generation imbalance, line overloading, voltage, angle and frequency instabilities are the phenomena which can threat the stability and integrity of each island. Therefore, islands must be formed in such way that being able to manage and maintain static and dynamic stability of their own region independently. For this purpose, each island must have adequate recourses of active and reactive powers and sufficient control facilities like load shedding and generation reserve. The number of islands for an intentional islanding is better to be as much as less but, it depends on the operating condition of the system at the instant of islanding, network structure, propagation of vulnerability into network, synchronous groups of generators and control equipments in the network. As mentioned, the process of intentional islanding consists of three phase 1-islanding decision, 2-identifying islanding scenario and 3-implementing islanding scenario. Decision depends on the criticality of operational conditions. Whenever system operator recognizes system inability for preserving its integrity a proper islanding strategy can be applied as a defensive solution. In this paper, based on the decision made to split network, a method for identifying the proper islanding scenario is proposed.
Criteria for Intentional Islanding
After decision made for intentional islanding, the most important step is identifying the islanding scenarios. The criteria for finding proper and stable islanding scenarios can be categorized as follows;
Integrity criterion: All buses inside an island must be connected as an integrated subsystem.
Static criteria: in each island, the following criteria should be satisfied to guarantee its feasibility and steady state stability;
Criteria 1-ability to preserve load-generation balance including generation control and load shedding capacities in each island. Equations (1) and (2) check island ability for balancing load-generation in the cases of generation shortage or generation surplus respectively.
Where:
PGri : Controllable generation reserve capacity at bus 'i' which can be positive or negative.
PLsi : load shedding capacity at bus 'i'
PGi : Active generation power at bus 'i'.
PLi : Active load power at bus 'i'.
n : Number of buses in the island
Criteria 2-line overloading constraint in islands.
Criteria3- Voltage drop constraint in islands.
Criteria4- Voltage security margin constraint
Dynamic criteria: during transient period of island formation due to tripping boundary lines, the following criteria should be satisfied in each island which guarantee dynamic stability of the island
Generators should remain in synchronism with damped oscillation
Voltage should remain dynamically stable
Frequency should remain stable within acceptable limit.
In this paper, for finding stable islanding scenario, the integrity criteria and the two first criteria of the static criteria are considered and checked. For this purpose, a new algorithm based on the ant search mechanism is proposed. It is noteworthy that for practical implementation of an islanding scenario all static and dynamic criteria should be fulfilled.
Proposed Approach
It should be noted that practical implementation of an islanding scenario require satisfaction of all criteria. For this purpose, as the search process proceeds further with respect to the fulfillment of more criteria, the search space becomes narrower until it reaches to final scenarios. Therefore, the proposed algorithm in this paper is able to find any number of islanding scenarios with respect to the integrity criteria and the two first static criteria. These scenarios could be used as an initial feasible search space for further search until the point that all criteria will be checked and fulfilled in the final scenario. The developed algorithm in this paper for finding islanding scenarios can be explained through four steps as follows.
Step 1: structural integrity
The first and the most important step for identifying islanding scenarios is to find all possible separation schemes in the network satisfying the integrity criteria. The proposed algorithm for finding separation schemes in the network is derived from ant search algorithm combined with a probabilistic movement mechanism.
Step 2: load-generation balance
In this step, by using equations (1) or (2) the ability of each separation scheme for preserving load-generation balance is verified. Separation schemes not satisfying this constraint even for only one separated area will be omitted from the search space for islanding schemes. This step is implemented within the first step, so the integrity and power balance criteria are verified simultaneously.
Step 3: line overloading constraint
In this step, for separation schemes passed from steps 1 and 2, the ability of each separated area for preventing line overloading is verified. For this purpose, the generation reserve capacity for generation rescheduling and also load shedding capacity are used as control parameters for preventing line overloading. Calculation of this step is carried out by means of linear programming using equations (3)-(7) which are based on DC load flow equations.
Where:
PGi : active generation at bus #i at the moment of islanding.
PLi : active load at bus #i at the moment of islanding.
PGri : active generation control at bus #i
PLsi : active load shedding at bus #i
Fk : active power flow of line #k.
Fkmax : maximum power capacity of line #k
PGrimax >0: maximum generation increase at bus #i
PGrimin <0: minimum generation decrease at bus #i
PLsimax : capacity of load shedding at bus #i.
C : matrix relating line power flow to bus active power
m : number of lines
At this step, the ability of each separated area for preventing line overloading is verified. Even if only one island of an islanding scenario failed to prevent line overloading, that islanding scenario will be omitted from search space. In this algorithm, the objective function is merely based on the minimization of load shedding. Equations (4) and (5) represent power balance and line overloading constraints based on DC load flow equations respectively.
Step 4: final verification
After finishing step 3, those separation schemes which have fulfilled all criteria will be selected as acceptable islanding scenarios. For each islanding scenario, the lines connecting the islanded areas and their power transfer prior to tripping are recognized and evaluated. In this step, departing from dynamic criteria, some additional criteria can be defined to merely verify easy implementation of islanding scenarios as follows.
Number of lines connecting islands
Power flow of lines connecting islands
It is clear that the scenario with less boundary lines is more easier for implementation of islanding scenario. However, it is understood that implementation of an islanding scheme is a very complicated process which requires several dynamic and protective considerations in the network. For example, the following points are part of problems which must be fully considered.
The order of islands to be separated
The order of line tripping to establish islanding
The control actions which should be taken
Principles of Search Algorithm
In this paper, the principle for finding islanding scenarios is developed based on a probabilistic search mechanism denoted as ant search algorithm. In this algorithm, the number of islands in each islanding scenario could be adopted arbitrarily prior to search. Search for finding each islanding scenario starts simultaneously from a number of initial points equal to the number of islands. Basically the number of islands could be adopted arbitrarily but, from view point of practical considerations as less the number of islands as easy would be implementation of islanding scenario. However, practical constraints like the number of asynchronous groups of generators could be used as a guideline for this purpose. Search algorithm could be repeated many times and in each time, the algorithm may either converge to an islanding scenario or not. Whenever the algorithm converged, one islanding scenario will be found. Principles of search algorithm can be explained in the following three steps.
Pre Processing Network Structure
In order to speed up the search process for finding islanding scenarios, it is necessary to reduce the search space by removing unnecessary points (buses). For this purpose, a group number (GN) is assigned to each bus. The initial default value for GN is 0 for all buses indicating no dependency between them. Buses with the same GN value rather than 0, represent a group of dependent buses which must appear in one island. The points at the ends of radial lines take same GN as the first points. The points intended to be grouped in one island take the same individual GN value rather than 0.
Initial points for search
The search for finding each islanding scenario starts simultaneously and in parallel from a number of initial nodes equal to the number of intended islands. Initial nodes are adopted based on network structure and operational constraints. For example, in the case of existing asynchronous groups of generators, the initial points should be selected within the area of each asynchronous group. In this paper, in order to increase the probability of search convergence and avoiding interference of search mechanism for islands, initial points are selected randomly in sequence. After random selection of each initial point, a corresponding forbidden area around that point is defined such that the next initial point is not allowed to fall within forbidden areas of other initial points. The forbidden area corresponding to each initial point consists of buses connected directly or indirectly to the initial point. The approximate size of the forbidden area determined based on the size of whole network and the number of expected islands as follows.
(8)
Where, Nbus and Nisland are number of total buses and expected islands respectively and Nf is approximate number of buses in each forbidden area.
Ant movement for finding islands
After choosing the initial points, from each point an ant starts to search for finding an island according to the following rules. In order to speed up the search activity, all criteria mentioned in the section IV are implemented within the ants search for island identification.
Initial status of all points are set to 0 indicating their availability in the search space for occupation by ants
For each ant searcher an ID number is assigned corresponding to an island
when a point is occupied by an ant its status changes to the ID number of the ant indicating its removal from search space and its attribution to the ant's island.
Each ant only moves toward unoccupied points with status 0 which are connected to its current position.
When the status of a point changes to an ID, the status of all points with the same GN also change to that ID indicating their location in the same island.
The next point for ant movement is randomly selected from all available points connected to the current position based the highest probability.
An ant stops searching when there is no point with status 0 connecting to it.
All points occupied by an ant with the same ID constitute an island corresponding to the ant
When all ants stop searching, the remained points with status 0 will be connected to the neares islands
Whenever an island is established, equations 1 and 2 of the criteria 1, will be checked and if they are not satisfied the search process for all ants will be terminated.
For islanding scenarios which passed criteria 1, criteria 2 using equation (3)-(7) will be checked and those scenarios satisfying these criteria constitute initial islanding scenarios for further examination.
The proposed algorithm has been coded in Matlab.
Simulation Studies
In order to demonstrate the effectiveness and ability of the proposed algorithm for finding islanding scenarios, IEEE 39-bus test system is adopted for simulation studies. The proposed algorithm is applied for a critical operating condition with total load of 7613MW in which unit G34 and line 25 are tripped due to disturbances and unit G36 has decreased its output by 300MW. In this study, it is decided to separate the entire network into four islands. For each generator, 20 percent of its nominal capacity is considered as controllable reserve and 20 percent of each load is considered for load shedding which are shown in table 1. The algorithm has been run on a PC with 2.26 GHZ CPU, 2GB RAM and 3MB Cache with a time calculation about 3 seconds for finding all island scenarios. For the given operating point, 10 islanding schemes have been found. Fig. 1 shows system splitting for the 8th islanding scenario. Table 1 shows generation, load, generation reserve and load shedding capacities in each island of the 8th islanding scenario. Table 2 shows transmission lines between islands with total power transfer between each two islands a prior to splitting. In fact in order to create islands these lines must be tripped. Tables 3-6 show the amount of load shedding and generation rescheduling carried out in four established islands. In each island load-generation balance and constraints of line overloading have been satisfied first by means of generation control and finally by proper load shedding.
Table (1)- Generation, load, generation reserve and load shedding in the islands of the 8th islanding scenario
DPG + Pshed (MW)
DPG (MW)
PBC (MW)
PShed (MW)
PResev (MW)
PLoad (MW)
PG (MW)
No. of Buses
Island No.
573
0
-573
484
185
2421
1848
11
1
1196
0
-119
221
98
1103
984
7
2
236
0
-236
446
199
2230
1995
10
3
5
0
-5
372
185
1859
1854
11
4
Fig. 1. IEEE 39-bus system splitting for 8th islanding scenario
Conclusion
In this paper, a new algorithm based on ant search mechanism has been proposed for identifying feasible islands in a controlled splitting strategy. The proposed algorithm is based on a probabilistic search mechanism which starts from random initial points and principally is able to split the network into any number of controlled islands. However, the feasible number of islands should be decided based on security and operational constraints. In this approach, the ability of islands for balancing load-generation and avoiding line overloading are considered as islanding constraints. But, it is noteworthy that due to the nature of search mechanism, further static and dynamic constraint can be easily implemented in islanding scenarios. The algorithm is able to identify several islanding scenarios, but the simulation results show that in spite of random initialization of search activity for each islanding scenario, often definite number of feasible islanding scenario will be found. The proposed algorithm has been applied on IEEE 39-bus network and simulation results clearly demonstrate its ability and efficiency.
Table (2)- Transmission lines connecting islands prior to splitting in the 8th islanding scenario including their transfer power
Island
No.
No. of Buses
Island 1
Island 2
Island 3
Island 4
1
11
--
0
0
1-7-17-36
(144)
2
7
0
--
21
(-319)
30
(84)
3
10
0
21
(-319)
--
20
(-384)
4
11
1-7-17-36
(144)
30
(84)
20
(-384)
--
Table (3)- Load-generation , generation control and load shedding in island 1
ΔPG (MW)
Load shed (MW)
Final controlled island
(MW)
Just after splitting
(MW)
Bus
Gen
Load
Gen
Load
0
0
0
0
0
0
1
106
225
1167
1114
1061
1339
39
0
0
0
0
0
0
9
0
100
0
533
0
633
8
0
0
0
0
0
0
5
0
0
0
0
0
0
6
0
0
0
0
0
0
11
0
0
0
0
0
0
10
79
0
866
0
787
0
32
0
19
0
146
0
165
12
0
45
0
239
0
284
7
185
388
2033
2033
1848
2421
total
Table (4)- Load-generation , generation control and load shedding in island 2
ΔPG (MW)
Load shed (MW)
Final controlled island (MW)
Just after splitting
(MW)
Bus
Gen
Load
Gen
Load
0
4
0
337
0
341
27
0
0
0
0
0
0
17
0
0
0
0
0
0
34
0
5
0
164
0
169
26
0
7
0
337
0
344
29
0
5
0
245
0
250
28
98
0
1082
0
984
0
38
98
21
1082
1082
984
1103
total
Table (5)- Load-generation , generation control and load shedding in island 3
ΔPG (MW)
Load shed (MW)
Final controlled island (MW)
Just after splitting
(MW)
Bus
Gen
Load
Gen
Load
0
5
0
370
0
374
24
0
5
0
394
0
399
16
0
0
0
0
0
0
19
0
15
0
810
0
825
20
79
0
866
0
787
0
33
0
7
0
294
0
300
23
0
5
0
327
0
332
21
0
0
0
0
0
0
22
40
0
437
0
398
0
36
81
0
891
0
810
0
35
199
36
2194
2194
1995
2230
total
Table (6)- Load-generation , generation control and load shedding in island 4
ΔPG (MW)
Load shed (MW)
Final controlled island (MW)
Just after splitting
(MW)
Bus
Gen
Load
Gen
Load
0
0
0
0
0
0
13
0
0
0
0
0
0
14
0
0
0
607
0
607
4
0
0
0
391
0
391
3
0
0
0
0
0
0
2
-19
0
448
0
466
0
30
0
0
0
272
0
272
25
0
0
0
388
0
388
15
0
0
0
192
0
192
18
31
0
746
10
715
10
31
-7
0
666
0
673
0
37
5
0
1859
1859
1854
1859
total
