Photovoltaic Synchronous Generator (PVSG):

Transcription

1 Photovoltaic Synchronous Generator (PVSG): From Grid Following to Grid Forming Professor Alex Huang, Progress Energy Distinguished Professor FREEDM Systems Center, NC State University August 7, 2017

2 Alex Q. Huang, Ph.D. & IEEE Fellow Dula D. Cockrell Centennial Chair in Engineering Department of Electrical and Computer Engineering The University of Texas at Austin Prof. Ross Baldick Prof. Surya Santoso Prof. Hao Zhu Prof. Bob Hebner

3 Presentation Outlines Background and Motivation Proposed Photovoltaic Synchronous Generator (PVSG) Key Benefit 1 Key Benefit 2 Key Benefit 3 Frequency Regulation Improved RoCoF and Power Intermittence Voltage Regulation Hardware Hybrid Energy Storage System 3/22

4 Summary: Today s PV power plant and Synchronous Generators PV panels PV inverter Utility grid Synchronous generator Following the grid: Current source (PQ bus) Follow the grid Inject active (and reactive power) Fast response to the intermittent irradiation levels (no buffer) How about high PV Penetration? Forming (Supporting) the grid: Voltage source (PV bus) Set grid voltage and frequency Provide active and reactive power to the load via voltage Slow response due to large inertia Islanding and weak grid operation 4/22

5 Major Challenge #1: Introduction DRER Intermittence PV Daily Output FREEDM Center Wind Speed with 1-minute Average Output Power from SW Minnesota Wind Power Plant [2] 5/22

6 Major Challenge #2: Introduction Voltage Rise or Sag Phasor Diagram of Grid and PCC voltage, (a) PF = 1.0, (b) PF = -0.9 (c) PF = 0.9 6/22

7 Major Challenge #2: Introduction Voltage Rise or Sag Example One-line diagram of the IEEE 34 node test feeder [3] [3] S.A. Pourmousavi, A.S. Cifala and M.H. Nehrir, Impact of High Penetration of PV Generation on Frequency and Voltage in a Distribution Feeder, /22

8 Existing Solutions Introduction Voltage Rise Problem DRER Side Management Network Side Management Active power control Buffering excess active power Reactive power control STATCOM Inserting a series reactor in service line Appliances power control On load tap changer Active grid voltage control Reducing the primary substation voltage Re-conductoring the network Use only local real power measurement, No MPPT Independent of PCC voltage, battery capacity and cost, communication and data exchange Independent of generation and network operator, higher currents and losses in the feeder, not efficient for high R/X ratio, more expensive oversize inverters, limited by Grid Codes Cheaper than storage devices, limited by Grid Codes Raise X/R ratio, higher power losses Load management, economic impact Lifetime, communication required Starting and recovery voltage settings Not practical for long lines or many distribution transformers involved Expensive and not reasonable and economically 8/22

9 From PV Inverter to Smart PV Inverter ( P P ) Q 2 2 pv curl pv 2 2 Snom ( k * Snom) 1 Parameters 2-Level NPC TNPC Power device number Output voltage quality Low high high Active power capability 25.2kW 67.2kW 28.5kW Total loss for Pmax 384.8W W 330W Loss percentage for P max 1.527% % 1.158% *Supported under the DOE Sunlamp program Reactive power capability 28.8kW 68.7kW 57.6kW Loss percentage Q max 418.8W 476W 597.8W Loss percentage for Q % % 1.038% Reactive power constraint factor k Loss per kvar 14.5W 6.9W 10.37W 9

10 Major Challenge Introduction #3: Frequency Stability Frequency Stability DRER Variability increases fluctuation of net load DRER generation decouples from grid frequency by PLL Lack of Rotational Inertia No Up and Down Reserve Swing Equation ROCOF 10/22

11 Frequency Introduction Response Example Lack of System Inertia [4] 11/22

12 Grid Forming PV System: Photovoltaic Synchronous Generator (PVSG) AC Grid line impedance v g circuit breaker Power Stage L g v C C f i L L f Inverter bridge i out C dc i in + v dc - i PV PV arrays Terminal Energy storages Terminal v C i L abc/dq i Ld PWM d dq/abc d d d q i Lq PVSG v Cd controller v dc v Cq δ i PV Based on virtual synchronous generator concepts [1], [2] Emulate SG behaviors in P and Q Voltage source (amplitude and frequency) instead of a current source Auxiliary energy storages are used to support the functions [1] Qing-Chang Zhong, Phi-Long Nguyen, Zhenyu Ma, and Wanxing Sheng, Self-Synchronized Synchronverters: Inverters Without a Dedicated Synchronization Unit, IEEE Transactions on Power Electronics, vol. 29, no. 2, pp , Feb [2] M. Ashabani, and Y.A.-R.I. Mohamed, Novel Comprehensive Control Framework for Incorporating VSCs to Smart Power Grids Using Bidirectional Synchronous-VSC, IEEE Transactions on Power Systems, vol. 29, no. 2, pp , March /22

13 PVSG Control Diagram Frequency Control Energy Storage v dc V DC -P droop G LPF4 (s) v dc_fes - + v dc_p&o K es + + P es_r P es_ref + - P es PI i es_ref + - i es PI d es DC-DC converter control for energy storages ω R + - ω K ω v dc i PV v dc_ω v dc_p&o Q MPPT control P&O + - v dc_ref V-Q droop control + - Q R v dc G LPF1 (s) v dc_f G LPF2 (s) AC frequency and DC voltage regulation 1 Jvirs ω 1 s E +- K E + + Q f E R δ PVSG control voltage reference generator 2E abc/dq v Cd_ref v Cq_ref + - v Cd v Cq voltage loop K vp K s R C R C vi + - i Ld_ref + - K - K vi vp s i Lq_ref K + K ci cp + - s - i Ld i Lq current loop K R L R L cp K s ci d d + + d q MPPT Voltage Control Dual Loop Control 13/22

14 Battery vs. Ultra Capacitor Battery: High Energy Density, slow charge and discharge process, low power density, degrade over time, slow and steady energy supplier Ultracapacitor: Fast charge and discharge, high power density, no storage capability loss, low energy density, no energy sustainment 14/22

15 Energy Storage Coordination Ultracapacitor Battery 15/22

16 PV power (W) PVSG power (W) PV frequency (rad/s) DC voltage (V) Ultra-cap voltage (V) Simulation results of a 1.5 kw PVSG: Inertia Irradiation decreases Irradiation increases Autonomously support grid frequency at high solar penetration level: Introduce virtual inertia for dynamic response Slow down intermittent PV output Time (s) Ultra Capacitor Based Energy Storage is a good choice the PVSG 16/22

17 Preliminary experimental results of PVSG: Inertia PV shading PV unshading CH1:4A PVSG current CH2:600W PVSG power CH3:60Hz PVSG frequency CH3:180V DC bus voltage CH1: 2A/div CH2: 300W/div CH3: 0.04Hz/div CH4: 10V/div X-axis: time 400ms/div slow down intermittent PV output Autonomously support grid frequency and voltage stability 17/22

18 Preliminary experimental results of PVSG: Inertia CH1: 4 A i PV Ramp rate: CH2: 300W CH3: 120π rad/s CH4: 180V P out CH1: ipv (2A/div); CH2: Pout (300W/div); CH3: ω (0.08π rad/s /div); CH4: vdc (10V/div); X-axis: time t (2s/div) ω v dc Significantly reduce the power ramp rate: 200X from our simulation model Small energy requirement For Virtual inertia: about 1/3*Ppu*1second of energy is needed 18/22

19 Preliminary experimental results of PVSG Primary & secondary Frequency response CH1:6A CH2:900W CH3:60Hz CH4:0A PV current PVSG power PVSG frequency Battery current Accurate frequency measurement that can be used in wide area monitoring and control CH1: 2A/div CH2: 300W/div CH3: 0.04Hz/div CH4: 2A/div X-axis: time 400ms/div 19/22

20 How to Apply PVSG to existing PV System? AC Grid 277/480Vac 3ph v g Line impedance CAPER Project focus 1) Upgrade existing PV System 2) System integration, data collection and analysis the impact Circuit breaker L f 1200V/100A IGBT inverter module v dc DC Bus bi-directional DC-DC converter bi-directional DC-DC converter UCAP Virtual Inertia + Primary f response 40kW PV arrays PVSG system Conventional PV 40kW PV inverter system from AEG i PV same PCC v C C f additional current sensors for PV part Energy storage inverter i L v C i L i PV v C i L v dc 2E Inner fast PE control AC voltage reference Synchronous generator emulation control bi-directional DC-DC converter bi-directional DC-DC converter droop control with distinguish algorithm Battery local DC voltage for each unit v dc Secondary f response + economic dispatch 20/22

21 Simulation results of a 40 kw PVSG: Inertia & Primary Response Primary & Secondary Response UCAP (Primary response) Power (kw) Hz f(pvsg) second Time (s) Battery (Secondary response) 21/22

22 Power (kw) Power (kw) Power (kw) Power (kw) Simulation results of a 40 kw PVSG: inertia T = 1 s p PV p g Integral of the power of the supercapacitor is the energy needed Time (s) 40 p sc When inertia time = 1 s, E = -8 kj, voltage of SC increase from 300 V to 305V with 5F capacitance T = 5 s p PV p g When inertia time = 5 s, E = -44 kj, voltage of SC increase from 300 V to 328V with 5F capacitance p sc Time (s) 22/22

23 Thank you Questions? 23/22