Analysis of the Maximum Available Time to Switchthe Operation Control Mode of a DistributedGeneration During an Islanding Occurrence

Rodolpho V. A. Neves, Elian. J. Agnoletto, Giann B. Reis, Ricardo Q. Machado, Vilma A. OliveiraLaboratory of Control Systems

Universidade Sao PauloSao Carlos, SP, Brazil

E-mail: rodolphon@sc.usp.br

Abstract—The operation of distributed generation systems(DG) is subjected to events such as unintentional islandingoccurrences. To prevent the activation of the DG’s protectionsand power supply interruption, when an islanding occurs, theDG must change from grid-connected operation mode to isolatedoperation mode and keep the voltage and the frequency withinacceptable operational levels. The available time to switch theoperation control mode should be investigated to avoid falseislanding detections and to minimize the negative impact onthe quality of the generated energy. This paper analyzes themaximum time for switching the operation control mode of theDG, after an unintentional islanding occurrence. Simulationswere carried out on the PSCAD/EMTDC. The simulated DGsystem is based on a diesel generator and distinct loads. Usinga classical control strategy (CCS) and a fuzzy control strategy(FCS) for voltage, frequency, active and reactive powers, themaximum available switching time is analyzed. The FCS providedlonger available times than a strategy using classical controllers.

Index Terms—Distributed generation, Fuzzy control, Islandedmode operation.

I. INTRODUCTION

Alternative energy sources in DG can improve voltagelevels and reduce investments on long distance transmissionlines, since, DGs are built near to costumers [1]. Despite theadvantages of connecting the DG to the grid, it is necessaryan evaluation about the DG’s location, operation schedule andthe type of load to be fed, otherwise, DGs may not be soadvantageous [2].

The DG operation is largely affected by islanding occur-rences, i.e., when the DG disconnects from the utility bus[3]. When an unintentional islanding happens, the DG mustkeep the islanded operation, if the DG has enough power,controlling the frequency and the voltage on the bus. Afteran occurrence of islanding, the DG could be connected toan not supported load and the DG’s protections would beactivated, ending on an island without power [4]. Otherwise,

The authors would like to thank the Fundacao de Amparo a Pesquisado Estado de Sao Paulo (FAPESP) for the financial support under grant2011/02170-5.

the DG could provide power to the island and continue to beadvantageous for the electric power system.

In [4], the authors have analyzed the maximum availabletime, called Ts, for switching the operation control modeof a synchronous generator moved by a steam turbine andconnected to a medium voltage level bus. The limit time Ts

is defined as the maximum time the DG can switch fromgrid-connected to isolated operation mode after the islandingoccurrence and before activating the frequency and the voltageprotections and disconnecting the DG. Also in [4], the authorshave shown the switching must occur in a short time, to notdamage the power quality of the supplied energy or to causethe instability of the island, and in a large time, in case of afalse detection of an islanding.

The DG’s exciter system regulates the terminal voltage level(when the DG is on an isolated operation mode) and thereactive power (when the DG is on a grid-connected operationmode), by the injection or absorption of reactive power in thegrid [5]. Thus, after an islanding occurrence, the switchingfrom the reactive power control to the terminal voltage controlmust be performed to keep a safe operation and power quality.Also, after the islanding occurrence, the DG must switch theoperation control mode from active power control to speedcontrol [2].

In [2], the authors presented results for a diesel enginegenerator managed by controllers based on fuzzy logic. Avoltage-reactive power coordinated PD+I fuzzy controller actson the exciter system of a synchronous generator, regulatingthe terminal voltage and the supplied reactive power at thesame time. The coordination is set by a correction factor,which penalizes the reactive power control when there aredisturbances on the terminal voltage. Using the coordinatedcontroller, the switching between terminal voltage and reactivepower controllers, after an islanding occurrence or connectionto the grid, is not needed. However, the switching from theactive power controller to the speed controller must occur.

The main objective of this paper is to investigate thecritical switching time to change DG’s operation control modeafter an unintentional islanding occurrence without activating

Fig. 1. Diagram of the DG and its control loops. The DG and grid data were taken from [2].

frequency or voltage protections and using classical PIDcontrollers or the coordinated fuzzy controllers. Also, theinfluence of the control strategy and the power imbalance onthe maximum available time to switch the operation controlmode is investigated. To obtain the critical time, after theislanding occurrence, the switching time is changed until oneof the DG’s protections is activated. Simulations were carriedout on the PSCAD [6].

II. DESCRIPTION OF THE CONTROL STRATEGIES

A diesel based DG has two main control loops, a mechanicaland an electrical. The mechanical control loop is in charge ofregulating the speed or the active power generated by the DG.On the other hand, the electrical control loop regulates theterminal voltage and the reactive power supplied by the DG.Fig. 1 shows the control loops of the DG for each operationcontrol mode.

The mechanical controller output indicates how much thediesel engine fuel valve should be opened. The operationcontrol mode is given by the logical signal denoted S1. Ifthe breaker S1 is closed, the grid-connected operation modeis selected and the DG operates supplying active and reactivepowers. On the other hand, if S1 is opened, the DG regulatesthe frequency and the terminal voltage of the island.

This paper has considered two different control structuresfor the terminal voltage and reactive power: a standard cascadestructure [7] and a coordinated structure proposed in [2].For easy reference, the standard cascade and the coordinatedstructures are shown in Figs. 2 and 3, respectively. The cascadestructure is composed by the reactive power PI controller andthe terminal voltage PID controller.

When the DG is operating on the grid-connected mode,the S1 logical signal is true and the reactive power controllerprovides a term VVAr which is added to the voltage reference.If the DG operates on the isolated mode, S1 logical signal is

Fig. 2. Terminal voltage PID and reactive power cascade PI controllers.

false, the voltage PID controller works based on the referencesignal Vref .

In Fig. 3, the regulation of the terminal voltage and the reac-tive power are made partially independently. The upper fuzzycontroller acts over the terminal voltage. If there is a smallterminal voltage error (eVt

), the reactive power controllerworks as usual. Otherwise, the bottom control loop is weightedby eVt through the term α. The coordinated controller output isgiven by the sum of the upper and bottom controller’s outputsand the initial Ef0. Details on the PD+I fuzzy controller andfuzzy rules can be found in [8].

Without any switching, the coordinated controller regulatesthe reactive power to follow Qref . If the DG is operatingas grid-connected mode, Qref can be controlled as usual.However, if the DG operates as isolated mode, Qref must beequal to the reactive power of the load.

A PID speed and a PI active power controllers are part ofa classical control strategy (CCS) used for comparison. ThePID voltage and the PI reactive power cascade controllers arealso included in the CCS. A PD+I fuzzy speed controller, aPI fuzzy active power controller and the coordinated fuzzycontroller compose the fuzzy control strategy (FCS).

Fig. 3. Terminal voltage and reactive power coordinated PD+I fuzzy con-troller.

III. MAXIMUM AVAILABLE TIME ASSIGNMENT

All the simulations were carried on the PSCAD. Fig. 4shows the PSCAD environment, the DG and its measurementpoints, the point of common coupling and the break that willsimulate an unintentional islanding. Three different cases weresimulated using the same DG and the same utility grid. TheDG’s parameters can be found in [2]. First, the DG providesactive and reactive powers, as shown in Fig. 5. The amount ofactive and reactive powers is different for each simulated case.After the unintentional islanding occurrence, the maximumavailable time Ts is measured for different local loads. Foreach simulation, the switching time is changed. When oneof the protection criteria is activated, the switching time isassigned as the Ts for that local load and that case. Thecriteria to select the maximum available time Ts have beenadapted from [9] and they are presented in Table I. Localloads are defined in Table II. Switching times were assignedto several load conditions and power imbalances based on theDG’s synchronous generator’s capability curve presented inFig. 5 as shown next.

Fig. 4. Overview of the simulated system on PSCAD environment.

A. Supplying active power to the grid and providing reactivepower to the local loads

In this case, the DG supplies 0.8 pu of active power tothe grid and provides reactive power to the local loads. Theobtained Ts are presented in Fig. 6. It is noticeable that Ts

increased from Load 1 to Load 3 and from Load 4 to Load 6

Fig. 5. The capability curve of the DG’s synchronous generator and thesimulated cases’ active and reactive power ranges. DG’s nominal power is1.12 MVA. On the first simulation, the DG operated over the line A-B. Onthe second and the third simulations, the DG operated on the points C and D,respectively.

TABLE IMAXIMUM AVAILABLE TIME PROTECTION CRITERIA

Criteria Values (pu) Time (s)

Over-frequency f ≥ 1.1 Instantaneous1.01 ≤ f < 1.1 > 10

Under-frequency f ≤ 0.9 Instantaneous

Over-voltage Vt ≥ 1.3 Instantaneous1.1 ≤ Vt < 1.3 > 10

Under-voltage Vt ≤ 0.5 Instantaneous

TABLE IINOMINAL POWER OF THE LOCAL LOADS

Inductive loads Capacitive loadsLoad 1 Load 2 Load 3 Load 4 Load 5 Load 6

P (pu) 0.32 0.6 0.72 0.32 0.6 0.72Q (pu) 0.16 0.3 0.36 0.16 0.3 0.36

for both control strategies. However, the CCS resulted in loweravailable time than the FCS. The activated criterion was theover-frequency instantaneous criterion. This is due to the factthat the reactive power generated by the DG was consumedby the load, but there was an active power imbalance betweenthe DG and the local loads. Also, the average Ts for theLoads 1, 2 and 3 are close to the average Ts for the Loads 4, 5and 6, respectively, because they have the same active powerimbalance.

B. Supplying active and reactive power to the grid and localloads

In this case the DG supplies 0.8 + j0.56 pu to the gridand local loads. The obtained Ts are presented in Fig. 7.There are active and reactive power imbalances between theDG and local loads. The FCS reached higher Ts for allloads, but Load 4. The load imbalance after the islandingis the cause of an increasing Ts from Load 1 to 3 for both

Fig. 6. Maximum available Ts for the DG operating over the line A-B.

strategies. The highest Ts was achieved for the third localload and it happened due to the smaller load imbalance afterthe islanding. The over-frequency instantaneous protection wasactivated for Loads 1 to 4. The time Ts for Loads 5 and 6 didnot reach more than 1 ms for both strategies. The capacitiveloads and the DG, acting as a synchronous condenser, havecaused a large reactive power imbalance between generationand consumption. For Loads 5 and 6, the CCS and FCSactivated the over-voltage instantaneous protection due to theexcess of reactive power (the synchronous condenser and thecapacitive load were providing reactive power to the island)after the islanding occurrence.

Fig. 7. Maximum available Ts for the DG operating on the point C.

C. Supplying active power and absorbing reactive power fromthe grid and local loads

In this case, the DG supplies 0.8 pu and absorbs j0.56 pufrom the grid and local loads. The obtained Ts are presentedin Fig. 8. Again, there are power imbalances between theDG and local loads but with the DG absorbing the reac-tive power. The greatest difference among the times in thiscase was for the Load 3 (55 ms). Capacitive loads allowlonger Ts than inductive loads when the DG absorbs reactivepower for both strategies. The activated criterion for theCCS simulations, Loads 1 to 3, was the instantaneous under-voltage criterion. For all the others, the activated criterion wasthe instantaneous over-frequency. Fig. 9 shows the terminalvoltage and frequency variations for the Load 3. The CCSactivated the under-voltage limit (0.5 pu) and the FCS activated

Fig. 8. Maximum available Ts for the DG operating on the point D.

Fig. 9. Terminal voltage and frequency responses for operating point D andLoad 3. The under-voltage limit (UVL) is 0.5 pu and the over-frequency limit(OFL) is 1.1 pu. The Ts for the CCS is 1 ms and the Ts for the FCS is56 ms.

the over-frequency limit (1.1 pu). The maximum available timedifference between the CCS and the FCS for this load was55 ms.

D. Average Available Time

Analyzing the results for Ts, power imbalance is an issuewhich affects the maximum available time for switching theoperation control mode, independently of the control strategy.Fig. 10 shows how the average available time is affectedby the power imbalance. For each local load, the higher isthe power imbalance, the lower is Ts. For example: Load 3reached the highest average Ts (1343 ms) for the operatingpoint C, when the power imbalance was 0.08 + j0.2 pu, andTs = 28.5 ms for operating point D, when the power imbalancewas 0.08− j0.92 pu.

IV. CONCLUSION

This paper has analyzed the maximum available time tochange the operation control mode of a DG, after an uninten-tional islanding occurrence. The power imbalance is the mainissue after the islanding. Also, the results have shown that theDG’s control strategy and the type of load, if capacitive orinductive loads, affect the maximum available time.

Both control strategies activated the frequency or voltageprotections, but, for some cases, the FCS avoided the voltageprotection because of the coordinated action, which prioritizes

Fig. 10. Average available time for the simulated cases and local loads.

the terminal voltage instead of reactive power. Simulated caseswith large reactive power imbalance have shown that theDG would keep operating after the islanding using the FCSproposed here, but not the CCS. For the most of simulatedcases, the maximum available time was higher when usingthe FCS.

It can be concluded that an efficient control strategy mayallow higher switching times. As a result, it is possible to useslower islanding detection methods to change the operationmode control and maintain the terminal voltage and frequencyof the island within quality operating standards.

REFERENCES

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