cyber-resilient control of inverter based microgrids · renewable ders in microgrids decouple the...

Cyber-Resilient Control of Inverter Based Microgrids Martine Chlela | © 2016 McGill University. All rights reserved. 1

Cyber-Resilient Control of Inverter Based

Microgrids

IEEE Global Signal & Information Processing

Conference

Martine Chlela

McGill University Electric Energy Systems Laboratory

http://www.power.ece.mcgill.ca/

7-9 December 2016

Cyber-Resilient Control of Inverter Based Microgrids Martine Chlela | © 2016 McGill University. All rights reserved.

Outline

Background

Microgrid Power Electronic Interfaced DERs Types & Control

Microgrid Control Architecture

Cyber Security for Microgrids

Proposed Cyber Resilient Control Strategy

Microgrid Benchmark System & Simulation Results

Conclusion

2


Background – Renewable Energy Penetration

3

Source: Government of Canada


Background – Power Electronic Interfaces

Power electronic inverters are used to interface the extensively deployed

renewable DERs in microgrids

Decouple the rotating masses from the grid i.e. type 4 wind turbine

Interface systems with no inertia i.e. photovoltaics and energy storage

Provide poor V&f response in the event of disturbances due to the

lack of inertia

Control of PEI DERs is a major concern, specially in 100% inverter-

interfaced islanded microgrids

4


Microgrid PEI DERs Types & Control– Renewable DER Current-Controlled VSI

Grid-tie inverters with DC-links fed from DC-DC converters with power

control loops following MPPT curves. No active regulation of V&f

The VSI outer loop generates an inverter current reference to maintain a

DC-link voltage

5

PMPPTSolar irradiance (kW/m2)

MPPT curve

PMPPTWind speed (m/s)

MPPT curve

mabc

Vdc-ref

Vdc-link

ΔV

Imin

PI

Imax

idref

iqrefup

u

lo

y

Iq ref limit

idmeas

iqmeas

ωLRL

RRL

RRL

ωLRL

vdmeas

vqmeas

md

mq

dqabc

θPLL PI

PI


Microgrid PEI DERs Types & Control– Isochronous ESS Voltage-Controlled VSI

The ESS is operated as the isochronous resource to set & regulate the

islanded microgrids voltage and frequency

The VSI outer loop regulates the grid voltage to its reference value. The

inner loop controls the inverter current. The grid-side frequency is

readily imposed by a virtual PLL

6

mabc

Vd ref = 1

Vd meas

Id min

PI

Id max

idref

idmeas

iqmeas

dqabc

θPLL virtual

Vq ref = 0

Vq meas

Iq min

PI

Iq max

iqref

0

PI

1

mdref

0

PI

1

mqref


Microgrid Control Architecture – Reliance on Communications

7

Main Grid

Point of Common

Coupling

Solar Energy Storage Controllable Loads

Microgrid Controller

Communication Network

Wind


Cyber Security for Microgrids – Types of Cyber-Attacks

Block, delay or corrupt communications to make the network resources unavailable

Eavesdrop on communication network to acquire confidential information

Malicious modification or corruption of measurements and commands exchanged in the grid

8

Integrity

Confidentiality

Availability


Cyber Security for Microgrids – Data Integrity Cyber-Attack & Performance Metrics

The cyber-attack tampers with the protection and control commands

sent to the DERs circuit breakers to modify their status causing their

sudden disconnection

A DER disconnection requires a surge in power from the

isochronous generator - more current to be supplied by the PEI

In 100% inverter-interfaced microgrids, active power is proportional

to voltage

Large active power imbalances cause severe voltage excursions at the

grid side that could not be remediated by local controllers

Cyber-resilient & intelligent control algorithms need to be employed

to mitigate the attack

9


Proposed Cyber-Resilient Control Strategy – Supplementary Control Loop for the Voltage-Controlled VSI

In analogy with the virtual inertia concept applied to regulate frequency

excursions, a virtual inertial controller is added to the PEI DERs primary

control loops to provide transient voltage support

The inner current regulation control loop of the ESS VSI modified

∆V 𝐼𝑑 𝑖𝑛𝑒𝑟𝑡𝑖𝑎 𝑟𝑒𝑓 𝐼𝑑 𝑟𝑒𝑓 - The resulting contribution is limited by

the maximum inverter current

10

ΔV

Inertial control

Kinertia Id inertia ref

Local supplementary controller

Vd ref

Vd meas

Id min

PI

Id max

idref

idmeas

0

PI

1

mdref

𝑇𝑤𝑠

𝑇𝑤𝑠 + 1

Vd meas

Vd ref = 1


Proposed Cyber-Resilient Control Strategy – Supplementary Control Loop for the Current-Controlled VSI

The MPPT controller of the WTG is also modified to incorporate virtual

inertia

The virtual inertia active power contribution is added to MPPT reference

to set the WTG power reference. The supplementary control

contribution is limited by the WTG dynamics

11

Pref

Vref

Vmeas

ΔV

Inertial control

Kinertia𝑇𝑤𝑠

𝑇𝑤𝑠 + 1

Pinertia

Local supplementary controller

PMPPT

MPPT curve

ωrotor


Proposed Cyber-Resilient Control Strategy – Conventional Load Shedding Scheme

In the event of large active power disturbances causing the grid-forming

ESS and WTG to saturate, virtual inertial control would not be sufficient

to provide complete compensation and voltage regulation

Traditional UVLS employed to shed % of loads in proportion to voltage

excursions – in analogy to UFLS of the NERC standard.

12

Voltage

Threshold (p.u.) Total time (s)

Load shed at

stage (%)

Cumulative load

shed (%)

0.9750 10.0 3 31

0.9816 0.30 7 28

0.9850 0.30 7 21

0.9883 0.30 7 14

0.9916 0.30 7 7


Proposed Cyber-Resilient Control Strategy – Adaptive Load Shedding Scheme

The adaptive load shedding

scheme developed activate if

1) ∆𝑉𝑝𝑐𝑐 ≤ ∆𝑉𝑡ℎ

2) the current reference of the

isochronous DER VSI reaches

its maximum rated value

Load is shed only when voltage

excursions are due to the

inability of the isochronous

DER to generate more power

and provide balance

13

Start𝑡𝑘 = 𝑡0

𝑉 𝑡𝑘 − 𝑉𝑟𝑒𝑓 ≥ ∆𝑉𝑡𝑕

𝑖𝑑𝐸𝑆𝑆 𝑡𝑘 = 𝐼𝑔𝑟𝑖𝑑 𝑐𝑜𝑛𝑣𝑒𝑟𝑡𝑒𝑟

𝑚𝑎𝑥

𝑘 = 𝑘 + 1;

𝑡𝑘 = 𝑡𝑘−1 + ∆𝑡 NO

YES

YES

NO

𝑃𝐿𝑜𝑎𝑑𝑆𝑕𝑒𝑑

𝑡𝑘 = (𝑖𝐸𝑆𝑆 𝑡𝑘 + 𝑖𝑊𝑇𝐺 𝑡𝑘 + 𝑖𝑃𝑉 𝑡𝑘 ) × ∆𝑉(𝑡𝑘))

Measure 𝑉 𝑡𝑘 at PCC, isochronous generator

converter current 𝑖𝑑𝐸𝑆𝑆 (𝑡𝑘) and grid side

currents of DERs 𝑖𝑊𝑇𝐺(𝑡𝑘), 𝑖𝑃𝑉(𝑡𝑘) and 𝑖𝐸𝑆𝑆(𝑡𝑘)


Microgrid Benchmark System

25kV distribution system adapted from a utility feeder and reconfigured

as a microgrid

Microgrid PEI DERs & control employed:

150 kW WTG

50 kW PV source

125 kW/125kWh ESS – operated as the grid forming DER regulating

& forming the voltage and frequency of the islanded microgrid

Cyber-attack considered: Attacker with valid user credential gains access

to the operator workstation and tampers with the command sent to the

PV unit circuit breaker causing a sudden disconnection of the renewable

source at 20 seconds

14

interfaced through a current controlled grid-tie

inverter, following the corresponding MPPT curves


Impact assessment – Data integrity cyber-attack on Inverter-interfaced microgrids

15

Available active power Shortage of active power

20 30 40 50 60-50

0

50

100

150

200

Time (s)

Active

Po

we

r (k

W)

Load

ESS

WTG

PVPV disconnection

20 30 40 50 600

50

100

150

200

250

Time (s)

Active

Po

we

r (k

W)

Load

ESS

WTG

PV

PV disconnection

20 30 40 50 600.8

0.85

0.9

0.95

1

1.05

Time (s)

Vo

lta

ge

(p

.u.)

20 30 40 50 600.9

0.92

0.94

0.96

0.98

1

Time (s)

Vo

lta

ge

(p

.u.)

(a)

(b)

(c)

(d)


Case Study 1 – Available Active Power Mitigation Performance & Validation

16

20 30 40 50 60-50

0

50

100

150

200

Time (s)

Active

Po

we

r (k

W)

Load

ESS

WTG

PV

18 19 20 21 22 23 24 250.9

0.92

0.94

0.96

0.98

1

Time (s)

Vo

lta

ge

(p

.u.)

No Control

Traditional UVLS + VI

Adaptive UVLS + VI

18 19 20 21 22 23 24 250

0.05

0.1

Time (s)

Active

Po

we

r (p

.u.)

ESS contribution

WTG contribution

10 20 30 40 50 60

0

10

20

30

Time (s)

Lo

ad

Sh

ed

(kW

)

Traditional UVLS

Adaptive UVLS

(a)

(c)

(b)

(d)


Case Study 2 – Shortage of Active Power Mitigation Performance & Validation

17

20 30 40 50 600

50

100

150

200

250

Time (s)

Active

Po

we

r (k

W)

Load

ESS

WTG

PV

10 20 30 40 50 60

0

20

40

60

Time (s)

Lo

ad

Sh

ed

(kW

)

Traditional UVLS

Adaptive UVLS

20 25 30 35 40 45 500.8

0.9

1

1.1

1.2

1.3

Time (s)

Vo

lta

ge

(p

.u.)

No control

VI

Traditional UVLS + VI

Conventional UVLS + VI

20 25 30 35 40 45 500

0.05

0.1

0.15

0.2

Time (s)

Active

Po

we

r (p

.u.)

ESS contribution

WTG contribution

(a)

(c)

(b)

(d)

Adaptive UVLS +VI


Results Summary

Virtual inertia provides sufficient compensation

Unnecessary load shedding due to traditional UVLS

18

Voltage nadir (p.u.) Load shed (kW)

Case 1

No control 0.9139 --

Virtual Inertia 0.9757 --

Traditional UVLS + VI 0.9757 31

Adaptive UVLS + VI 0.9757 0

Case 2

No control 0.8653 --

Virtual Inertia 0.8682 --

Traditional UVLS + VI 0.9655 67.2

Adaptive UVLS + VI 0.9666 17.13

Virtual inertia does not provide sufficient compensation as the DER

capacity limits are reached

Unnecessary load shedding due to traditional UVLS


Conclusion

Impact assessment of data integrity cyber-attacks on the power

management strategies of 100% inverter-interfaced islanded microgrids

Two-layer cyber-resilient control consisting of supplementary local

control loops and load management schemes to enhance resilience to

attacks

Virtual inertial control added to the WTG and the ESS primary

controllers provides transient voltage regulation by smoothing the

ramps streaming from the cyber-attack

Adaptive load management strategy to overcome the DERs rated

capacity limits ensures post-attack active power balance

19


Q&A

20

Thank you!!

Martine Chlela

[email protected]

Electric Energy Systems

Laboratory

http://www.power.ece.mcgill.ca/

McGill University

cyber-resilient control of inverter based microgrids · renewable ders in microgrids decouple the...

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