Reference Model Based Power Smoothing for Stand-alone Hybrid PV-Diesel Micro Grid

81 Pages·2013·2.26 MB·English

Reference Model Based Power Smoothing for
Stand-alone Hybrid PV-Diesel Micro Grid
by
Yize Xu

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF
THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF APPLIED SCIENCE
in
The Faculty of Graduate and Postdoctoral Studies
(Electrical and Computer Engineering)

THE UNIVERSITY OF BRITISH COLUMBIA
(Vancouver)
December 2013
© Yize Xu, 2013
Abstract
Photovoltaic (PV) Generator generates clean energy but also brings active power fluctuation to the
network. The thesis investigates the frequency stability issue of a MW level stand-alone hybrid micro
grid which contains PV generator, diesel generator, storage unit and loads. The PV generator can only
generate as much power as the sun provides. The resulting power mismatch between PV generation
and load demand needs to be compensated. The slow responding diesel generator is designed to
compensate for the steady state power mismatch. The battery, as the fast responding storage unit, is
set to reject the power transients.
A battery control method based on the micro grid frequency feedback and PV output feed-forward is
presented to satisfy the requirement of active power compensation in transients. It will be shown
that the method keeps the stand - alone micro grid frequency within a specified region and provides
the diesel generators more margin of time to adjust their output for better diesel efficiency.

ii
Preface
This thesis is original, unpublished, independent work by the author, Yize Xu.

iii
Table of Contents
Abstract .................................................................................................................................................. ii
Preface ................................................................................................................................................... iii
Table of Contents ..................................................................................................................................iv
List of Tables ..........................................................................................................................................vi
List of Figures ....................................................................................................................................... vii
Acknowledgements ............................................................................................................................. xi
1. Introduction .................................................................................................................................... 1
2. Background and literature review ................................................................................................ 3
2.1. Background: Summary of the stand-alone hybrid micro grid ......................................... 3
2.2. Challenges: Active power fluctuation and frequency stability ....................................... 6
2.3. Solutions: Review of battery storage control schemes .................................................. 8
2.3.1. Frequency droop control ...................................................................................... 8
2.3.2. Frequency derivative control ................................................................................ 9
2.3.3. Virtual synchronous generator (VSG) control ......................................................11
2.3.4. Comparison among above schemes ................................................................... 17
3. Reference model based VSG topology for the proposed hybrid micro grid ............................. 18
4. Modeling of the micro grid ..........................................................................................................23
4.1. Model of diesel generator ...............................................................................................23
4.1.1. Physical description of diesel generator .................................................................23
4.1.2. Governor control of diesel generator ................................................................ 24
iv
4.1.3. Voltage control of diesel generator .................................................................... 27
4.2. Model of PV generator and battery ............................................................................... 30
4.2.1. Physical description of the PV generator and the battery ................................ 30
4.2.2. Control signals of the inverter controller ............................................................ 31
4.3. Control system structure of the micro grid ................................................................... 33
4.3.1. Frequency (secondary) control of the diesel generator ................................... 34
4.3.2. Reactive power control of the PV generator ..................................................... 37
4.3.3. Dispatch (tertiary) control of the diesel and PV generator............................... 39
4.4. Micro grid model .............................................................................................................. 41
4.5. Summary .......................................................................................................................... 42
5. Simulation of the hybrid micro grid ............................................................................................ 44
5.1. Case 1: PV output reduction ............................................................................................ 45
5.2. Case 2: Load demand reduction ..................................................................................... 50
5.3. Case 3: PV output increase .............................................................................................. 54
5.4. Case 4: Load demand increase ....................................................................................... 59
5.5. Size of the battery ........................................................................................................... 62
6. Conclusion .................................................................................................................................... 66
Bibliography ......................................................................................................................................... 68
Appendices .......................................................................................................................................... 70
Appendix A: Generator datasheet .............................................................................................. 70

v
List of Tables
Table 1 - Simulation of primary control of diesel generator (Summary) .......................................... 26
Table 2 - Simulation of reactive droop control of diesel generator (Summary) .............................. 29
Table 3 - Simulation of reactive power control of the PV generator and the diesel generator combined
(Summary) ............................................................................................................................ 38
Table 4 - Simulation of the dispatch control (Summary) ................................................................... 40
Table 5 - Components specifications of the hybrid micro grid model ............................................... 41
Table 6 - Micro grid frequency and battery response under a 2MW PV output reduction for different T
values (K=0.5) ....................................................................................................................... 65
Table 7 - Grid frequency and battery response under a 2MW PV output reduction for different K values
(T=5) ...................................................................................................................................... 65
Table 8 - The effect of different control modes under different disturbances................................ 67

vi
List of Figures
Figure 1 - Stand- alone hybrid micro grid ............................................................................................... 1
Figure 2 - Simplified stand-alone hybrid micro grid ............................................................................. 3
Figure 3 - 24Hr PV output profile .......................................................................................................... 4
Figure 4 - Typical fuel efficiency (kWh/liter) vs. output power based on [9] ..................................... 5
Figure 5 - An example of Frequency droop with deadband ................................................................ 8
Figure 6 - Grid frequency disturbance for different Kd (shown as Kvi in the referred figure) values [15]
(permission granted for republication)............................................................................. 10
Figure 7 - Virtual synchronous generator control concept [19] (permission granted for republication) 12
Figure 8 - Virtual synchronous generator example 1[16] (permission granted for republication) .. 12
Figure 9 - Simplified synchronous machine model ............................................................................. 13
Figure 10 - Governor used in paper [16] (permission granted for republication) ............................. 14
Figure 11 - Generic PLL block diagram [21] (permission granted for republication) ......................... 15
Figure 12 - Linearized PLL block representing a generator ................................................................ 16
Figure 13 - Frequency feedback control .............................................................................................. 18
Figure 14 - Feedforwad control of PV generated active power ......................................................... 19
Figure 15 - ‘Battery Control Scheme’ block from the feedforward control diagram ........................ 19
Figure 16 - Proposed battery control scheme that combines frequency feedback and PV power
feedforward ....................................................................................................................... 20
Figure 17 - Response of the proposed battery controller in the case of suddenly losing the PV
generator of rated output ................................................................................................. 21
vii
Figure 18 - Diesel generator structure [19] (permission granted for republication) ........................23
Figure 19 - Block diagram representation of the diesel generator structure ................................... 24
Figure 20 - Droop characteristic of Primary control ...........................................................................25
Figure 21 - Simulation of primary control of diesel generator (circuit) ..............................................25
Figure 22 - Simulation of primary control of diesel generator (Waveform) ..................................... 26
Figure 23 - Vector summation of reactive droop control .................................................................. 28
Figure 24 - Reactive droop curves ...................................................................................................... 28
Figure 25 - Simulation of reactive droop control of diesel generator (Waveform) ......................... 29
Figure 26 - General structure of the PV Generator ............................................................................ 30
Figure 27 - The active power output P of the PV generator (or battery) controlled by the active
out
power reference P ......................................................................................................... 31
user
Figure 28 – P reduction caused by variation of P ........................................................................32
out sun
Figure 29 - Over frequency power reduction[26] ...............................................................................32
Figure 30 - Control system overview of the micro grid ..................................................................... 33
Figure 31 - Frequency (secondary) control ......................................................................................... 34
Figure 32 - Graphical representation of frequency control ............................................................... 35
Figure 33 - Comparison between the response of the diesel generator with and without the secondary
control ................................................................................................................................ 36
Figure 34 - PV Var control .................................................................................................................... 37
Figure 35 - Simulation of reactive power control of the PV generator and the diesel generator
combined (Circuit) ............................................................................................................. 38
viii
Figure 36 - Simulation of reactive power control of the PV generator and the diesel generator
combined (Waveform) ...................................................................................................... 39
Figure 37 - Simulation of the dispatch control (Waveform) .............................................................. 40
Figure 38 - The structure of the complete hybrid micro grid model ................................................. 41
Figure 39 - The order of simulation of different battery control modes .......................................... 44
Figure 40 - Simulation 1: PV output reduction without the battery support ................................... 45
Figure 41 - Simulation 2: PV output reduction supported by the frequency feedback control mode of
the battery ......................................................................................................................... 46
Figure 42 - Simulation 3: PV output reduction supported by the PV power feedforward control mode
of the battery ..................................................................................................................... 47
Figure 43 - Simulation 4: PV output reduction supported by both the PV power feedforward control
mode and the frequency feedback control mode of the battery .................................. 49
Figure 44 - Simulation 1: Load demand reduction without the battery support ............................. 50
Figure 45 - Simulation 2: load reduction supported by the frequency feedback control mode of the
battery................................................................................................................................. 51
Figure 46 - Simulation 3: load reduction supported by the PV power feedforward control mode of the
battery.................................................................................................................................52
Figure 47 - Simulation 4: load reduction supported by both the PV power feedforward control mode
and the frequency feedback control mode of the battery ............................................. 53
Figure 48 - Simulation 1: PV output increase without the battery support ...................................... 54
Figure 49 - Simulation 2: PV output increase supported by the frequency feedback control mode of the
battery................................................................................................................................ 56
ix
Figure 50 - Simulation 3: PV output increase supported by the PV power feedforward control mode of
the battery ......................................................................................................................... 57
Figure 51 - Simulation 4: PV output increase supported by both the PV power feedforward control
mode and the frequency feedback control mode of the battery .................................. 58
Figure 52 - Simulation 1: load demand increase without the battery support ................................. 59
Figure 53 - Simulation 2: load demand increase supported by the frequency feedback control mode of
the battery ......................................................................................................................... 60
Figure 54 - Simulation 3: load demand increase supported by the PV power feedforward control mode
of the battery ...................................................................................................................... 61
Figure 55 - Simulation 4: load demand increase supported by both the PV power feedforward control
mode and the frequency feedback control mode of the battery .................................. 62
Figure 56 - Abnormal frequency withstand times from different generator manufactures [29]
(permission granted for republication)............................................................................ 63
Figure 57 - Battery output curve in p.u. in the case of suddenly losing 2MW PV generator (2MW base)
............................................................................................................................................ 64

x