Published in: Proceedings of the 39th Annual Conference of the IEEE Industrial Electronics Society, IECON 2013

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1 Aalborg Universitet Coordinated Priary and Secondary Control with Frequency-Bus-Signaling or Distributed Generation and Storage in Islanded Microgrids Wu, Dan; Tang, Fen; Dragicevic, Toislav; Quintero, Juan Carlos Vasquez; Zapata, Josep Maria Guerrero Published in: Proceedings o the 39th Annual Conerence o the IEEE Industrial Electronics Society, IECON 213 DOI (link to publication ro Publisher): 1.119/IECON Publication date: 213 Docuent Version Early version, also known as pre-print Link to publication ro Aalborg University Citation or published version (APA): Wu, D., Tang, F., Dragicevic, T., Vasquez, J. C., & Guerrero, J. M. (213). Coordinated Priary and Secondary Control with Frequency-Bus-Signaling or Distributed Generation and Storage in Islanded Microgrids. In Proceedings o the 39th Annual Conerence o the IEEE Industrial Electronics Society, IECON 213 (pp ). IEEE Press. I E E E Industrial Electronics Society. Annual Conerence. Proceedings General rights Copyright and oral rights or the publications ade accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition o accessing publications that users recognise and abide by the legal requireents associated with these rights.? Users ay download and print one copy o any publication ro the public portal or the purpose o private study or research.? You ay not urther distribute the aterial or use it or any proit-aking activity or coercial gain? You ay reely distribute the URL identiying the publication in the public portal? Take down policy I you believe that this docuent breaches copyright please contact us at vbn@aub.aau.dk providing details, and we will reove access to the work iediately and investigate your clai. Downloaded ro vbn.aau.dk on: januar 1, 219

2 This docuent is the preprint version o the paper: D. Wu, T. Dragicevic, J.C. Vasquez, and J.M. Guerrero, Coordinated priary and secondary control with requency-bus-signaling or distributed generation and storage in islanded icrogrids, in Proc. o the 39th Annual Conerence o the IEEE Industrial Electronics Society, IEEE IECON 13, Vienna, Austria, Nov 213. Coordinated Priary and Secondary Control with Frequency-Bus-Signaling or Distributed Generation and Storage in Islanded Microgrids Dan Wu 1, Fen Tang 1,2, Toislav Dragicevic 1, Juan C. Vasquez 1, and Josep M. Guerrero 1 1 Departent o Energy Technology, Aalborg University. Denark {dwu, tdr, juq, joz}@et.aau.dk 2 School o Electrical Engineering, Beijing Jiaotong University, P. R. China tang_nego@126.co Abstract In this paper, a distributed coordinated control schee based on requency-bus-signaling (FBS) ethod or a low-voltage AC three phase icrogrid is proposed. The control schee is coposed by two levels. Firstly a priary local control which is dierent or the DGs and the ESS is proposed. The ESS adopts FBS control which is based on changing slightly the bus requency in the icrogrid when the state-o-charge is near to the liit. This way, the DG controller when detecting that the requency is increasing, will reduce the injected power by using a virtual inertia control loop. Then secondary control is ipleented to restore the requency deviation produced by the priary ESS controller while preserving the coordinated control perorance. Real-tie siulation results show the easibility o the proposed approach by showing the operation o the icrogrid in dierent scenarios. Keywords Coordinated control, virtual inertia, priary control, secondary control, droop control, requency bus signaling, icrogrids I. INTRODUCTION A Microgrid can be considered as a local grid with ultiple distributed generators (DGs), energy storage systes (ESS), and loads, able to operate in either grid-connected or islanded odes, with sealess transition between both odes [1]. In grid connected ode o AC icrogrid, the AC bus requency and voltage are ixed by the ain grid, and all the DGs are working as grid ollowing units to exchange power with ain grid. However in islanded ode, the power exchange aong DGs, ESS and loads should be balanced inside the isolated icrogrid. In the literature, the ost popular control technique used in islanded icrogrids is the droop ethod. It consists on adjusting the requency o the inverters in unction o output active power to achieve power sharing [2-4]. When using ultilayer hierarchical control systes, this ethod is deined inside the priary local controllers [5]. Although this technique works well when we have ully dispatchable generators, such as uninterruptible power supply systes or energy storage integrated units, however when cobining renewable energy sources (RESs) like photovoltaics (PV) or windturbines (WT) the dispatching is liited to the axiu power point tracking (MPPT) and is eaningless to get equal current sharing between RES and ESS. Further, ESSs have liitations in ters o state-o-charge () that have to be respected to avoid daages and ailures. This eans that when the is near to the axiu, the power produced by the DG should be reduced to liit power o ESS. Consequently, in an islanded icrogrid there is a need o coordination between DGs and ESS units. To achieve the coordinated behavior aong DGs and ESS, centralized controllers in energy anageent syste (EMS) are proposed in [6-7]. In the literatures, the upper level controller akes decisions taking into account o and give coands to local controllers. However, the drawback is that syste stability relies on a icrogrid central controller (MGCC) and its counication links [8]. I they are disabled, the whole syste loses the coordination signal. To overcoe the liitation o the centralized coordinated control, decentralized coordinated control can be ipleented to enhance the syste reliability. Aong the control strategies, bus-signaling ethod (BSM) is a very proising way to ipleent since power line is used to transit coordinated signals instead o using external ast counication links. In [9-1], BSM is used as power anageent aong ESS, RESs and loads in DC systes. By changing the bus voltage according to dierent thresholds, the coordination signal is counicated to other units. However, this control law needs the odes changes aong units, which akes the paraeters o units hard to be designed and even ay cause the unstable operation in the dynaic switching process. In this paper, a requency bus signaling (FBS) ethod is proposed or AC islanded icrogrids. When the o ESS is approaching high, the power o ESS is liited; at the sae tie the RESs operate in o-mppt ode autoatically. It is worth noticing that no odes changes is required in this ethod, thereby avoiding the dynaic stability proble ay occur in previous BSM. The paper is organized as ollows. Section II presents the syste description, Section III gives the ESS and RESs control algoriths in priary level. In order to eliinate the AC bus requency deviation produced in priary control, a secondary coordinated control is proposed in Section

3 D. Wu, T. Dragicevic, J.C. Vasquez, and J.M. Guerrero, Coordinated priary and secondary control with requency-bus-signaling or distributed generation and storage in islanded icrogrids, in Proc. o the 39th Annual Conerence o the IEEE Industrial Electronics Society, IEEE IECON 13, Vienna, Austria, Nov 213. IV. Section V gives the detailed description o control ipleentation o ESS and RESs, and inally the real-tie siulation results are presented in Section VI to validate the proposed control strategy. II. MICROGRID SYSTEM DESCRIPTION Fig.1 shows a lexible icrogrid consists o RESs and ESS with batteries which can operate in either grid connected ode or islanded ode according to the state o Intelligent Bypass Switch (IBS). To ake the axiu utilization o renewable energy, the RESs usually operate in MPPT as grid ollowing units in both grid connected ode and islanded ode. However, the roles o ESS are dierent in each operating ode. In grid connected operation, the behavior o ESS should be deterined by both its and tie o use (TOU) o electricity. This eans that besides discharging the power to supply loads, the ESS can also take additional unction as peak shaving in peak period o grid. While in islanded operation, the ESS usually works as grid oring unit to aintain the coon grid bus. Since islanded icrogrid has no power exchange with ain grid, the ESS also has to operate as energy buer to balance the power between sources and loads. Thereore, to obtain a reliable energy anageent unction aong RESs, ESS and loads, the capability o ESS based on is very iportant when coordinate various units in islanded icrogrid. The ollowing description is based on the analysis o islanded icrogrid operation. In order to obtain a coordinated control aong units according to and MPPT, the syste control structure based on FBS is proposed in Fig. 3. The coordinated control strategy can be classiied into priary local level and secondary centralized level. In the priary level, there is no need o counication aong ESS and RESs units. According to estiated, the ESS changes the bus requency as signaling, and RESs receive the signaling in AC bus to change the output power. However the requency deviation will result in this level. Then i requireent or the tight requency range is needed, additional secondary control level can be applied to restore requency in noinal value with low bandwidth counication link, which will be illustrated in section IV. In this part, the priary control level is illustrated based on ESS aster control and RES slave control respectively. d ESS d Low bandwidth Counication Link G sec(s) Priary Control o ESS Centralized Secondary Control d eas2 eas sec P re d PV MPPT RES Priary Control o RES d WT MPPT 1 1 eas Loads Loads Fig. 3. Coordinated control structure Fig. 1. Microgrid syste desctription III. PRIMARY COORDINATED CONTROL The priary control in icrogrid is developed aied at regulating the output power o each unit, at the sae tie aintaining the stability o bus voltage and requency. Usually, the ESS and RESs units are controlled as aster-slave way as Fig. 2 presented. The ESS works as aster unit aintaining AC bus voltage E and requency, and RESs operate as slave units regulating its output power according to MPPT. Master Unit E P 1 Slave Units P 2 A. ESS Master Control-Bus Signaling Control. In [3], it has been illustrated that when the output ipedance o converter is highly inductive, the active power can be controlled alost exclusively by the output requency. Thereore, it akes sense using requency o ESS as signal to coordinate units when approaching high. Fig. 4 shows the diagra o ESS requency signaling. and 1 are the noral requency and axiu requency, and 1 is the threshold o ESS. e and e are the inal value o requency and. When is lower than the up threshold, the ESS regulates its output requency as noinal value. When is higher than the threshold, the requency then increases with slope o to coordinate other units to aintain. The output requency o ESS is deterined as 1 ( 1) 1 where the boost requency coeicient can be deined as (1a) ESS RES 1 RES 2 LOADS 1 1% 1 (1b) Fig. 2. Master-slave control o islanded icrogrid 2

4 D. Wu, T. Dragicevic, J.C. Vasquez, and J.M. Guerrero, Coordinated priary and secondary control with requency-bus-signaling or distributed generation and storage in islanded icrogrids, in Proc. o the 39th Annual Conerence o the IEEE Industrial Electronics Society, IEEE IECON 13, Vienna, Austria, Nov 213. P re 1 e P e 1 e 1% n e 1 eas Fig. 4. Frequency signalling o ESS B. RES Slave Control-Virtual Inertia Control. Having the requency increasing, each RES unit decreases power ro axiu power point. This perorance is siilar with the inertia o the power syste. In this case, the power drop o RESs ay be achieved by adding virtual inertia o syste. Fig. 5 shows the virtual inertia perorance o RESs, where P re is the active power reerence o RESs, and P e reerred to the MPPT point and inal power output o each RESs, and eas is the sensing requency by the phase lock loop (). When the eas not above noinal requency, RES units are working under MPPT state, and aking the ull use o renewable energy. When eas is above, RES units start to decrease output power to liit o ESS coordinately. As the requency relects the inoration, the higher is the bus requency, the lower is the power ro RESs. Finally, when the power absorbed by the ESS is low enough to aintain, the requency will be stable at e, and power ro RES unit has decreased to P e. Thus the active power reerence o each RES can be expressed as ollowing Pre PMPPT eas P P n re MPPT ( eas ) eas where the virtual inertia coeicient n can be deined as n 1 (2a). (2b) Usually the icrogrid requency is easured by the RES by eans o, which can be approxiated as a irst order syste [11]. Hence, the easured requency eas can be expressed as eas 1 s 1 where is the tie constant o the. (3) Fig. 5. Virtual inertia o RES Considering that the syste is working in the condition 1 <<1%, and ro (1) and (2), the RES power reerence takes the or n Pre ( s) PMPPT n( 1 ) (4) s 1 whereas denotes the dierential o requency. Consequently, using sall-signal analysis the closed-loop syste inertia can be calculated according to the ollowing deinition [12] G () s vi Pre (s) n (s) s 1 It can be seen that when n is liited by the axiu requency deviation, tuning independently the tie constant can change the inertia o the RESs. IV. SECONDARY COORDINATED CONTROL As previous analyzed, the bus signaling o ESS with only priary control results in requency deviation. Although this requency deviation can be designed inside the allowable liits, but soe events like reconnection to ain grid or synchronous achines connection to the icrogrid ay require tight requency regulation. The ai o this Section is to propose a secondary control to cancel the requency deviation produced by the priary control, while preserving the autonoous operation o each unit. Fig. 6 shows the bus signal ethod o ESS with the secondary control action. When 1, the curve o the ESS shits downwards in order to regulate the icrogrid requency in steady state. Then, we can odiy the control strategy (1) by adding a shiting-requency ter, thus (1) can be rewritten as 1 d ( 1) 1 where d is the shiting-requency ter, which adjust (6) to achieve = anytie. The secondary control will generate d by using the ollowing centralized controller: (5) (6) 3

5 D. Wu, T. Dragicevic, J.C. Vasquez, and J.M. Guerrero, Coordinated priary and secondary control with requency-bus-signaling or distributed generation and storage in islanded icrogrids, in Proc. o the 39th Annual Conerence o the IEEE Industrial Electronics Society, IEEE IECON 13, Vienna, Austria, Nov 213. kisec d Gsec ( s) ( sec eas 2) k psec ( sec eas 2) s (7) where k psec and k isec are the proportional and integral ters, respectively; sec is the secondary requency reerence; eas2 is the easured requency obtained by secondary control, which can be as in (3) approxiated by the ollowing irst order approxiation: eas 2 1 s 1 where 2 is the tie constant o the o secondary control. 1 e d 2 1 e Priary Secondary 1% Fig. 6. Secondary control action over the requency signalling o ESS. On the other hand, i we restore the requency in the icrogrid, then the eect o the RES priary control will be cancelled, so that we need to change the requency threshold o equation (2), which can be odiied as ollowing: re MPPT eas P P d Pre PMPPT n( eas d ) eas d Notice that now the requency threshold, instead o just using now also incorporates the shiting-requency ter d. Fig 7 shows the adaptive behavior o the requency threshold changing or the virtual inertia unction. Consequently, the RESs will deliver the output power coanded by P re, coordinated with the bus signaling but without requency deviation. For coherency, the secondary requency reerence is selected as sec =, then by cobining (6) and (9), we can obtain the power reerence dynaics o RESs: (8) (9) n Pre ( s) PMPPT n( 1 ) (1) s 1 So that, ro (1) the closed-loop syste inertia with secondary control can be calculated as: G () s vi Pre (s) n (s) s 1 (11) It can be seen that the secondary control can be used to restore the bus requency without changing the local inertia o RESs. P re P e d Priary n 1 Fig. 7. Virtual inertia control o RESs with secondary control V. CONTROL IMPLEMENTATION Secondary eas The ai o this section is to illustrate the control ipleentation in details or each o the control loops previously proposed. The control algorith is shown in Fig. 8. A. Inner control loops The inner loops are designed to control the output voltages and currents o each unit. For ESS, it operates as grid oring unit in voltage control ode (VCM) and regulate the output requency and voltage according to priary control coands. For RESs, they operate as grid ollowing units in current control ode (CCM) to regulate its output currents according to priary control coands with phase lock loop (). To obtain a good transient and steady state perorance siilar with DC syste, both ESS and RESs eploy controllers ipleented in d-q synchronous reerence rae with - reerence rae transoration. B. Priary ESS control The objective o priary ESS control is to change the requency o voltage reerence according to the estiated. When estiated above the threshold, the requency is changed steadily based on (1a). And the slope o is designed according to axiu requency deviation as (1b). C. Priary RES control The priary RES control is aied at changing the output power o each RES unit according to the bus requency signaling ro ESS. When detecting the bus requency above the noinal value, the output power is changed with (2a). Also the inertias o RESs are designed with respect to dierent tie constant o and slope n according to (4). Hence, by changing the output power o each RES, the power dispatching o syste changed, and the power low into ESS can be liited. 4

6 D. Wu, T. Dragicevic, J.C. Vasquez, and J.M. Guerrero, Coordinated priary and secondary control with requency-bus-signaling or distributed generation and storage in islanded icrogrids, in Proc. o the 39th Annual Conerence o the IEEE Industrial Electronics Society, IEEE IECON 13, Vienna, Austria, Nov 213. Secondary Control TABLE II. COORDINATED CONTROL PARAMETERS d eas2 sec AC bus Ite Sybol Value Microgrid requency threshold 1 51Hz ESS V dc i L L in C L o V o upper liit 1 95% Secondary requency reerence Secondary control proportional ter sec 5Hz k p sec.5 V re I re d PWM Secondary control Integral ter k i sec.5 s -1 Tie constant o RES.56 s RES Voltage Reerence Generator I re Current Reerence Generator w E P Q 2p d V dc Fig. 8. Control algorith o ESS and RES V g Priary ESS Control d PWM eas 1 i L L in C s Priary RES Control D. Secondary control The secondary control o whole syste is to eliinate the inal requency deviation when approaching liitation. It is achieved by using (6) and (9), thereby changing the requency deviation setpoint o ESS and RESs. It should be noted that the secondary control is optional in syste since the coordinated control o syste is achieve in priary level even without counication. Then i the islanded icrogrid is required in advance situation like synchronized with ain grid, this secondary control with low bandwidth can be used in addition. TABLE I. n d POWER STAGE PARAMETERS L o V g Ite Sybol Value Noinal output voltage E 23 V Noinal output requency Filter inductor Output inductor 5 Hz L in L o 1.8 H 1.8 H Output capacitor o ESS C 27 F Output capacitor o RES Power o Loads C s 4.7 F P L 1.6 kw LOADS VI. REAL-TIME SIMULATION RESULTS The proposed coordinated control strategy is validated through real-tie siulation based on dspace plator. The described syste is coposed o one ESS and two RES units. The power stage paraeters o ESS and RESs are shown in Table I, the control paraeters are shown in Table II. (%) output power o ESS (W) output power o RES1 (W) output power o RES2 (W) AC bus requency (Hz) 1% 9% 8% 7% Fig. 9. Siulation results o priary coordinated control (a) (b) (c) (d) (e) d 5

7 D. Wu, T. Dragicevic, J.C. Vasquez, and J.M. Guerrero, Coordinated priary and secondary control with requency-bus-signaling or distributed generation and storage in islanded icrogrids, in Proc. o the 39th Annual Conerence o the IEEE Industrial Electronics Society, IEEE IECON 13, Vienna, Austria, Nov 213. Fig.9 shows the siulation results with only priary coordinated control. Beore the ESS is approaching the charging liitation 95% o, ESS operates as VCM, and the RESs are operating as ideal current controlled ode with output power o 1.5kW and 1kW. When is over the charging threshold 95% as shown in Fig. 9(a), then priary coordinated control is activated, the input power o ESS starts to be liited in Fig. 9(b) by coordinated decrease the output power o RESs in Fig.9(c) and Fig. 9(d). Finally, the input power o ESS can be liited to alost zero to aintain its. And as shown in Fig. 9(e), there is a requency deviation about.2hz o AC bus requency in steady state. The siulation results adding secondary control is presented in Fig.1. When is over the charging threshold 95% as shown in Fig. 1(a), not only the priary control is activated, but also the secondary control is enabled to restore requency. Copared to Fig. 9(e), Fig. 1(e) shows that with secondary control, the AC bus requency can be restored to 5 Hz with secondary coordinated control. (%) output power o ESS (W) output power o RES1 (W) output power o RES2 (W) AC bus requency (Hz) 1% 9% 8% 7% (a) (b) (c) (d) (e) VII. CONCLUSION This paper gave a distributed coordinated control strategy aong DGs and ESS in islanded icrogrid based on requency bus signaling ethod. In the proposed control strategy, the priary control is used based on FBS o ESS and virtual inertia o RESs to achieve the power balancing aong ESS and RESs in dierent condition o ESS. Thus the ESS can be prevented ro overcharging by the priary coordinated control. To eliinate requency deviation, a centralized secondary control was ipleented without changing the coordinated control behavior o priary control. The siulation results have validated the proposed control strategy in both priary and secondary level. REFERENCES [1] IEEE Guide or Design, Operation, and Integration o Distributed Resource Island Systes with Electric Power Systes," IEEE Guide or Design, Operation, and Integration o Distributed Resource Island Systes with Electric Power Systes. [2] Tuladhar, A.; Hua Jin; Unger, T.; Mauch, K., "Control o parallel inverters in distributed AC power systes with consideration o line ipedance eect," Industry Applications, IEEE Transactions on, vol.36, no.1, pp.131,138, Jan/Feb 2 [3] Alves Coelho, E.A.; Cortizo, P.C.; Garcia, P.F.D., "Sall signal stability or single phase inverter connected to sti AC syste," Industry Applications Conerence, Thirty-Fourth IAS Annual Meeting. Conerence Record o the 1999 IEEE, vol.4, no., pp.218,2187 vol.4, 1999 [4] Chandorkar, M.C.; Divan, D.M.; Adapa, R., "Control o parallel connected inverters in standalone AC supply systes," Industry Applications, IEEE Transactions on, vol.29, no.1, pp.136,143, Jan/Feb 1993 [5] Guerrero, J.M.; Chandorkar, M.; Lee, T.; Loh, P.C., "Advanced Control Architectures or Intelligent Microgrids Part I: Decentralized and Hierarchical Control," Industrial Electronics, IEEE Transactions on, vol.6, no.4, pp.1254,1262, April 213 [6] Olivares, D.E.; Canizares, C.A.; Kazerani, M., "A centralized optial energy anageent syste or icrogrids," Power and Energy Society General Meeting, 211 IEEE, vol., no., pp.1,6, July 211 [7] Jong-Yul Ki; Seul-Ki Ki; Jin-Hong Jeon, "Coordinated state-ocharge control strategy or icrogrid during islanded operation," Power Electronics or Distributed Generation Systes (PEDG), 212 3rd IEEE International Syposiu on, vol., no., pp.133,139, June 212 [8] Tan, K.T.; Peng, X. Y.; So, P. L.; Chu, Y.C.; Chen, M. Z Q, "Centralized Control or Parallel Operation o Distributed Generation Inverters in Microgrids," Sart Grid, IEEE Transactions on, vol.3, no.4, pp.1977,1987, Dec. 212 [9] Schonberger, J.; Duke, R.; Round, S.D., "DC-Bus Signaling: A Distributed Control Strategy or a Hybrid Renewable Nanogrid," Industrial Electronics, IEEE Transactions on, vol.53, no.5, pp.1453,146, Oct. 26 [1] Vandoorn, T.L.; Renders, B.; Degroote, L.; Meersan, B.; Vandevelde, L., "Active Load Control in Islanded Microgrids Based on the Grid Voltage," Sart Grid, IEEE Transactions on, vol.2, no.1, pp.139,151, March 211 [11] Vasquez, J.C.; Guerrero, J.M.; Savaghebi, M.; Teodorescu, R., "Modeling, analysis, and design o stationary reerence rae droop controlled parallel three-phase voltage source inverters," Power Electronics and ECCE Asia (ICPE & ECCE), 211 IEEE 8th International Conerence on, vol., no., pp.272,279, May June [12] Kundur P.; Power syste stability and control. Tata McGraw-Hill Education, Fig. 1. Siulation results o secondary coordinated control 6