Photovoltaic Synchronous Generator (PVSG)

Photovoltaic Synchronous Generator (PVSG):

From Grid Following to Grid Forming

Professor Alex Huang, Progress Energy Distinguished Professor

FREEDM Systems Center, NC State University

[email protected]

August 7, 2017

mailto:[email protected]

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

[email protected]

Prof. Ross Baldick Prof. Surya Santoso Prof. Hao Zhu Prof. Bob Hebner

mailto:[email protected]

© 2017 by Alex Huang

3/22

Presentation Outlines

Background and Motivation

Frequency Regulation

Key Benefit 1 Key Benefit 3

Improved RoCoF and Power Intermittence

Key Benefit 2

Photovoltaic Synchronous Generator (PVSG)

Proposed

Voltage Regulation Hybrid Energy Storage System

Hardware


4/22

Summary: Today’s PV power plant and Synchronous Generators

Following the grid:

Current source (PQ bus)

Follow the grid

Inject active (and reactive

power)

Fast response to the

intermittent irradiation

levels (no buffer)

PV panels PV inverter Utility grid Synchronous

generator

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

How about high

PV Penetration?


5/22

Introduction Major Challenge #1: DRER Intermittence

PV Daily Output Power @ FREEDM Center

Wind Speed with 1-minute Average Output Power from SW Minnesota

Wind Power Plant [2]


6/22

Introduction Major Challenge #2: Voltage Rise or Sag

Phasor Diagram of Grid and PCC voltage, (a) PF = 1.0, (b) PF = -0.9 (c) PF = 0.9


7/22

Introduction

One-line diagram of the IEEE 34 node test feeder

Example

[3]

Major Challenge #2: Voltage Rise or Sag

[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, 2013


8/22

Voltage Rise Problem

DRER Side Management

Active power control Use only local real power measurement, No MPPT

Buffering excess active power

Independent of PCC voltage, battery capacity and cost, communication and data exchange

Reactive power control

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

STATCOM Cheaper than storage devices, limited by Grid Codes

Inserting a series reactor in service line

Raise X/R ratio, higher power losses

Appliances power control

Load management, economic impact

Network Side Management

On load tap changer Lifetime, communication required

Active grid voltage control

Starting and recovery voltage settings

Reducing the primary substation voltage

Not practical for long lines or many distribution transformers involved

Re-conductoring the network

Expensive and not reasonable and economically

Introduction Existing Solutions

From PV Inverter to Smart PV Inverter

9

Parameters 2-Level NPC TNPC

Power device number 6 18 12

Output voltage quality Low high high

Active power capability 25.2kW 67.2kW 28.5kW

Total loss for Pmax 384.8W 533.24W 330W

Loss percentage for Pmax 1.527% 0.7935% 1.158%

Reactive power capability 28.8kW 68.7kW 57.6kW

Loss percentage Qmax 418.8W 476W 597.8W

Loss percentage for Q 1.4546% 0.6931% 1.038%

Reactive power constraint

factor k 1.129 1.02 2.02

Loss per kVar 14.5W 6.9W 10.37W

2 2

2 2

( )1

( * )

pv curl pv

nom nom

P P Q

S k S

*Supported under the DOE Sunlamp program


10/22

Introduction

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

Major Challenge #3: Frequency Stability


11/22

Introduction Frequency Response Example

Lack of System Inertia

[4]


12/22

Grid Forming PV System: Photovoltaic Synchronous Generator (PVSG)

Lf

Cdc

iout

vg Cf

Lg

vdc

iin

+

-

iL

vC

circuit breaker

AC Grid

Inverter bridgePV

arrays Terminal

PWM

PVSG controller

iLd

vCd vdc

dd dq

Power Stage

abc/dq

dq/abc

d

iLq

vCq

δiLvC

iPV

iPV

Energy storages Terminal

line impedance

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. 617–630, Feb. 2014.

[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. 805–814, March. 2014.


13/22

PVSG Control Diagram

ddiLd_ref

+-

iLd

+-

+-

+-

+iLq_ref

iLq

++ dq

voltage loop

-

vCq

ωR Kω

vdc_P&O

vdc_ref

ω

vdc_f

+-

-

+-

+ ω

vo

ltag

e r

efe

ren

ce g

en

era

tor

vCd

ER

++QR KE+

-

E

vCd_ref

vCq_ref

δ

vivp

KK

s

vivp

KK

s

cicp

KK

s

cicp

KK

s

RL1

s

1

virJ s

vdc

QfQ

AC frequency and DC voltage

regulation

GLPF1(s)

GLPF2(s)

current loop

V-Q droop control

vdc_ω

P&Ovdc

iPV

ab

c/d

q

2E

RL

+-

++

RC

RC

MPPT control

Kes

+-

vdc_P&O

vdc_fes

GLPF4(s)vdc

Pes_R

++

VDC-P droopPes_ref

+-

Pes

PIies_ref

+-

ies

PI des

PVSG control

DC-DC converter control for energy storages

Energy Storage

MPPT Voltage Control

Dual Loop Control

Frequency Control


14/22

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


15/22

Energy Storage Coordination

Ultracapacitor Battery


16/22

Irradiation

decreases Irradiation

increases PV

po

wer

(W)

PV

SG

po

wer

(W)

PV

freq

uen

cy

(rad

/s)

DC

vo

ltag

e

(V)

Ult

ra-c

ap

vo

ltag

e

(V)

Time (s)

Simulation results of a 1.5 kW PVSG: Inertia

Autonomously support grid

frequency at high solar

penetration level:

Introduce virtual inertia

for dynamic response

Slow down intermittent

PV output

Ultra Capacitor Based Energy Storage is a good choice the PVSG


17/22

Preliminary experimental results of PVSG: Inertia

PV shading

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

PV unshading

PVSG current

PVSG power

PVSG frequency

DC bus voltage

CH1:4A

CH2:600W

CH3:60Hz

CH3:180V


18/22

CH1: 4 A

CH2: 300W

CH3:120π rad/s

Pout

ω

vdc

iPV

CH4: 180V

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)

Preliminary experimental results of PVSG: Inertia

Ramp rate:

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


19/22

PV current

PVSG power

PVSG frequency

Battery current

CH1:6A

CH2:900W

CH3:60Hz

CH4:0A

CH1: 2A/div CH2: 300W/div CH3: 0.04Hz/div

CH4: 2A/div X-axis: time 400ms/div

Preliminary experimental results of PVSG

Primary & secondary Frequency response

Accurate frequency measurement that can be used in wide area monitoring and control


20/22

How to Apply PVSG to existing PV System?

vg

AC Grid277/480Vac 3ph

iPV

Line impedance

40kW PV inverter system

from AEG

40kW PV arrays

samePCC

1200V/100A IGBT inverter

module

DC Bus

Circuit breaker

Lf

Cf

iLvC

Synchronous generator emulation

control

iPV

vC

iL

vdc

Inner fast PE control

AC voltage reference2E

vC

iL

additional current

sensors for PV part

vdc

bi-directional DC-DC

converter

Energy storage inverter


converter


converter


converter

droop control with distinguish

algorithmvdc

local DC voltage for each unitPVSG

system

CAPER Project focus 1) Upgrade existing PV System

2) System integration, data collection and analysis the impact

Conventional PV

Virtual Inertia + Primary f response

Secondary f response + economic dispatch

UCAP

Battery


21/22

Simulation results of a 40 kW PVSG:

0.01 Hz

3.5 second

Inertia & Primary Response

Primary & Secondary Response

f(PVSG)

Power

(kW)

0.5 1 1.5 2 2.5 3 3.5 4

Time (s)

0

-5

-10

UCAP (Primary response)

Battery (Secondary response)


22/22 0 200 400 600 800 1000 1200 1400 1600-0.5

0

0.5

1

1.5

psc

0 200 400 600 800 1000 1200 1400 160037.5

38

38.5

39

39.5

40

pg

pPV

T = 5 s

0 200 400 600 800 1000 1200 1400 1600-0.5

0

0.5

1

psc

Po

wer

(kW

)

Time (s)

Simulation results of a 40 kW PVSG: inertia

Po

wer

(kW

)

Po

wer

(kW

)

Po

wer

(kW

)

0 200 400 600 800 1000 1200 1400 160037.5

38

38.5

39

39.5

40

pg

pPV

T = 1 s Integral of the power

of the supercapacitor is the energy needed

When inertia time = 1

s, E = -8 kJ, voltage of

SC increase from 300

V to 305V with 5F

capacitance

When inertia time = 5 s,

E = -44 kJ, voltage of SC

increase from 300 V to

328V with 5F capacitance

Time (s)


23/22

Thank you

Questions?

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Photovoltaic Synchronous Generator (PVSG)