Micro-grid Inverter Parallel Droop Control Method for Improving Dynamic Properties and the Effect of Power Sharing

Transcription

1 2015 AASRI Internatonal Conference on Industral Electroncs and Applcatons (IEA 2015) Mcro-grd Inverter Parallel Droop Control Method for Improvng Dynamc Propertes and the Effect of Power Sharng aohong Zhu Qujng Power Supply Bureau, Qujng , Chna Abstract Parallel operaton control s one of the key technques for hgh performance nverters wth the features of excellent regulaton, modular, hgh relablty, and redundancy. Through theoretcal analyss of mult-nverters parallel operaton model, the paper derved the conventonal droop control method, the stablty lmt of ths method and ts small-sgnal model. the effect of dfferent parameters on system power sharng s analyzed. The smulaton of conventonal droop control method wth PSCAD software s also mplemented. After dscussng the lmts of conventonal droop control method, a novel adaptve droop control method, s proposed, whch can mprove actve power dynamc and reactve power sharng. A small sgnal model s establshed and the advantages of the method over conventonal droop control are also valdated by smulaton. Keywords-nverter; droop control; wreless parallel; PSCAD smulaton. I. INTRODUCTION Wth the gradual falure of conventonal energy sources and the ncrease of envronment polluton, countres around the world began to focus on eco-frendly, effcent and flexble power generatons--dstrbuted Generaton (DG). The rapd development of DG generatons has provded a lot of clean and effcent energy for the communty, but has also brought great challenges to the exstng power system. In order to reduce the adverse mpact on the exstng dstrbuton network bought by DG whle gve play to ts auxlary functon, whch s the effectve use of renewable energy, mcro-grd, as an mportant form of DG has ganed attenton and promoton from many countres around the world. Mcro-grd s an ndvdually controllable system composed of load and dstrbuted power supply, and t provdes electrcty and heat to local load [1]. Mcro-grd combnes and connects varous forms of energy and load nto a power supply network by each controllable nterface (usually power electronc nterfaces). These power electronc nterfaces are mostly nverters. In a mcro-grd formed by multple nverter nterfaces, due to the dfferences n ther output characterstcs, they have a problem of crculaton between nverters, whch ncrease the burden on the current transformer and lne loss, and t wll affect the normal operaton of the mcro-grd n severe stuatons. Meanwhle, wth the rapd development of electrcal equpment n modern socety the demand for capacty of nverters s ncreasng. There are manly two ways to mprove the nverter capacty, one s desgnng hghpower nverters and the other s to use nverter parallel technology to acheve power modular. Usng standard nverter power modules n parallel can not only flexbly compose nverter power system of any desred capacty, whle multple parallel power supply modules share loads and man swtches of each module s n the small current stress; thus the relablty s fundamentally ensured. And f necessary, we can parallel redundant nverters to obtan fault-tolerant redundant power n low cost n order to ensure the stablty of crtcal power. Based on the above understandng, the parallel nverter technology has become a hot research feld of power electroncs, whch has great practcal sgnfcance. II. PRINCIPLES AND LIMITATIONS OF CONVENTIONAL DROOP CONTROL PARALLEL SYSTEM A. Parallel nverters operaton prncples Parallel nverters workng system s manly composed of three parts (Fgure 2-1): nverter modules, lne mpedances and load. Fgure 2-1. Parallel nverters workng system schematc Parallel nverters that supply load, must be carred out wth parallel control n order to meet certan condtons, whch means that each nverter module s output voltage ampltude, frequency and phase need to be consstent to acheve current sharng control of each module. The man contents of ths chapter focus on the model of the parallel nverters, and make analyss of power and crculaton characterstcs of the system, and then get the basc control strategy for parallel operaton, and analyze power dstrbuton and stablty The authors - Publshed by Atlants Press 124

2 B. Control prncples of droop control based parallel nverter system The followng wll be a detaled analyss of the theoretcal prncples of parallel nverter droop control. The smplfed schematc of parallel nverter system s shown n Fgure 3-2.Rl, l and R2, 2 are summatons of each nverter s output connected mpedance and lne nductance. Fgure 2-2. Two parallel nverter system dagram Z θ R + j( 1,2) (2.1) Complex power flowng nto bus can be represented as: Where: P--Real power Q--Reactve power Whose value s [7] S P + jq (2.2) VV V P VV V Q 2 o o o cos( θ ϕ) cosθ Z Z 2 o o o sn( θ ϕ) snθ Z Z Where: o - th nverter output voltage ampltude; φ - th nverter power phase angle; Z - th nverter output mpedance value; Vo - th nverter output mpedance phase; Vo - common bus voltage (2.3) C. Purely nductve output mpedance case The tradtonal assumpton consders the nverter output mpedance s manly nductve (θ 90 ), whch s because that the nverter output flterng nductance s generally large, and also n the long-dstance transmsson, the lne mpedance s manly nductve. In ths case, we can derve the followng expressons for actve and reactve power from (2-3): VV P snϕ VV Q Z o o Z 2 o o cosϕ Vo (2.4) Generally, snce φ s small, ( 1,2) s relatvely small, t can be approxmated that: snφ 0, cosφ 1, thus the actve and reactve power s reduced to: VV o o P ϕ (2.5) Vo( Vo Vo) Q For equaton (2-5), the real power output of the nverter s a functon of ts output voltage ampltude and power angle, dfferental on both sdes, we get: Vo Δ po ( Vo Δ ϕ + ϕ Δ Vo +ΔVo Δϕ ) (2.6) Snce φ s very small, Smplfy Eq.(2-5) we get: VV o o Δ po Δϕ (2.7) Smlarly, dfferental reactve power on both sdes, we get: Vo Δ qo ΔV o (2.8) From Eq.(2-7)&(2-8) we know that the output voltage phase affects ts output real power, whle the output voltage ampltude change ts output reactve power. Thus, to control the nverter output real power and reactve power, ths can smply be acheved by adjustng the ampltude and phase of nverter output voltage. However, due to the phase s not easy to detect, t s generally by adjustng the frequency of the output voltage to regulate the the phase of the output voltage, and thus to regulate the real power output of the nverter. Frequency and ampltude of the output voltage of each nverter vares as the followng equaton, where we get the conventonal droop control equaton: ω ω m P o V V n Q o (2.9) (2.10) Where: M -droop factor of output angular frequency of the th nverter (referred as frequency droop factor) N - droop factor of output voltage ampltude of the th nverter (referred as voltage droop factor) D. Purely resstve output mpedance case In the low-voltage dstrbuton lnes, the mpedance of the lne s manly resstve (θ 0 ). Same as the dervaton n 2.2.1, we can get: 125

3 Vo( Vo Vo) P R VoVo Q R ϕ (2.11) Thus, when the output mpedance s hghly resstve, P / Q droop objects need to be changed: ω ωo + m Q V Vo n P (2.12) Therefore, P-ω and Q-V droop based control strategy should be appled to nductve output mpedance, and for resstve output mpedance, P-V and Q-w droop based control should be appled. E. The power dstrbuton relatonshp of conventonal droop control based nverter 1) Dstrbuton of real power As shown n Fgure 2-2, two parallel nverter system, when steady state s reached, the frequences of two nverters are nevtable equal, otherwse the system wll be n a dynamc adjustment or oscllaton process. So there s: ω ω 1 2 (2.13) Substtutng Eq.(2-18) n the the Eq.(2-9), we can get the expresson of real power dstrbuton of two nverters n steady-state: ω m P ω m P o1 1 1 o2 2 2 (2.14) It shows that real power of two steady-state nverters s only related to ts frequency and frequency droop factor of no-load output voltage, regardless of the output nductance. 2) Reactve power dstrbuton When two nverters reached steady state, the nverter output voltage ampltude s shown n Eq.(2-3). Substtute Eq.(2-10) nto Eq.(2-3), we have: Vo cosϕ Vo Q + ncosϕ V o (2.15) Clearly, n addton to controllable parameters Vo and n, reactve power s also affected by output nductance. Thus, when the two output mpedances of nverters are mbalance, reactve power cannot be well shared. III. THE PROPOSED IMPROVED DROOP CONTROL METHOD A. Lmtatons of vrtual mpedance control The mplementaton of control method of last chapter s too complcate. Dfferentator must be used f vrtual mpedance s added, and dfferentators easly enlarge hgh-frequency nose, whch affect the system s stablty. Moreover, these controls have hgh requrements on controllers speed, whch ncrease the cost of the controllers. Ths chapter proposed two easly mplemented mprovements of control methods. The fgure below s a schematc dagram of the proposed mproved control strategy. Fgure 3.1. Block dagram of the proposed mproved closed-loop droop control system B. Control strategy to mprove the dynamc characterstcs of real power 1) Analyss of ntal phase droop method The phase of the nverter output voltage sφ, usng conventonal droop control, as the equaton: ω ϕ ωdt ( ω mp) dt m ( P) dt m (3.1) In conventonal droop control, the phase control s pure ntegral control, the dynamc followng capablty of sudden power change of pure ntegral control s not as 126

4 good as the dynamc followng capablty of proportonal ntegral control. If the followng real power droop strategy s appled: ω ω mp ϕ kp ϕ (3.2) Where φ s the generated ntal phase of the reference voltage. Then ω ϕ ωdt+ ϕ ( ω mpdt ) kp ϕ m ( Pdt ) kp ϕ m (3.3) After addng the ntal phase droop, the control form of the phase s a proportonal ntegral control. dϕ dϕ dp ω ω mp ω mp kϕ dt dt dt (3.4) For frequency ω, t means the ntroducton of real power dervatve term. Ths control method whch drectly adds the dervatve term s also proposed n some lteratures. However, wth respect to the drect ntroducton of real power dfferental term, the proposed ntal phase droop method of ths work can use a smple lnear expresson to acheve the same effect wthout addng a dfferentator, whch also avods the hghfrequency nose caused by the dfferental terms. 2) Smulaton of ntal phase droop method Man crcuts of ntal phase droop method s smulaton are as follows: Fgure 3-2. Man smulaton crcut (lne mpedances of two nverters are dfferent, 0.5S breaker s closed, sudden load ncrease) The smulaton parameters are shown n Table3-1. TABLE 3-1. SIMULATION PARAMETERS f(hz) m E(k) n k k p k φ Inv Inv As can be seen from the smulaton results n Fgure 3-3, the real power dynamc characterstcs s greatly mproved after addng ntal phase droop control, and the adjustng tme s sgnfcantly reduced. 127

5 (a) C. Control strategy of mprovng reactve power sharng effect Obtaned by the analyss n Secton 3.4, when the two nverter output mpedances are unequal, even by usng droop control cannot share reactve power well. In nductve output mpedances, reactve power can be controlled ether by controllng the output voltage or the output mpedance. In addton, the effect of reactve power sharng can be enhanced by control the voltage droop factor. If the droop factor s not a constant, but ncreases wth the reactve power, as shown n equaton. n n + knq (3-5) (b) Fgure 3-3. Smulaton of real power curve of ntal phase droop (a. Conventonal droop b. Intal phase droop) TABLE 3-2. SIMULATION PARAMETERS f(hz) m E(kV) n k k p k φ k n n Inv Inv Then smulaton below s to used to verfy the effect of mprovng reactve power control. The smulaton parameters are shown n Table 3-2. Wth ths control, two nverters voltage droop factor are no longer equal, as shown n Fgure 3-4. Reactve power lne graphs of before and after addng droop are shown n Fgure 3-5. Clearly, the effect of reactve power sharng has been greatly mproved. (a) Fgure 3-4. Lne graph of droop parameters (b) Fgure 3-5:. Smulaton reactve power lne graph of ncreased droop factors (a. Conventonal droop b. Increased droop factors y-axs:. MVA; x-axs: s) 128

6 IV. CONCLUSION Ths paper analyzed the mpact on droop control and nverter sharng that caused by dfferent stuatons of transmsson lnes, and made mprovement for droop control when transmsson lne nductance values are not equal. Improvements for the former one s power of the outer rng, whch ntroduces reactve power droop control system to ncrease the effect of mprovement of reactve power sharng. And for the slow flow of power rngs and shock easly, ntal phase droop control s ntroduced, whch mproved real power dynamc characterstc. Both control strateges are verfed and smulated by PSCAD. REFERENCES [1] R.Lasseter, A.Abbas, C.Marnay, and et al. Integraton of dstrbuted energy resources. Calforna Energy Commsson, [2] J.M. Guerrero, L.G. de Vcuna, J. Matas, et al. Output Impedance Desgn of Parallel-Connected UPS Inverters Wth Wreless Load- Sharng Control. IEEE Trans. on Industral Electroncs, 2005, 52(4): [3] Chang S J, Chang J M. Parallel control of the UPS nverters wth frequency-dependent droop scheme. IEEE Annual Power Electroncs Specalsts Conference, Vancouver, Canada, 2001 [4] De Brabandere K, Bolsens B, Van den Keybus J, et al. A voltage and frequency droop control method for parallel nverters. IEEE Annual Power Electroncs Specalsts Conference, Aachen, Germany, [5] J.M. Guerrero, L.G. de Vcuna, J. Matas, et al. A Wreless Controller to Enhance Dynamc Performance of Parallel Inverters n Dstrbuted Generaton Systems. IEEE Trans. on Power IEEE Trans. on Power Electroncsvol, 2004, 19(5):