impact of converter-based technologies on power system ... · and a threat to power system...

pscad.comPowered by Manitoba Hydro International Ltd.

Impact of Converter-based Technologies on Power System Stability: Challenges, Lessons Learnt and Recommendations.

Dharshana MuthumuniYousef PipelzadehWesley Mueller


Abstract: The use of appropriate modelling tools can facilitate insights into the performance of low inertia (weaker) AC systems with high levels of renewable technologies, HVDC links and FACTs devices connected in close proximity. Phenomena’s such as control interactions and sub-synchronous oscillations are a real challenge and a threat to power system stability and security. Phasor-based RMS simulation programme have key limitations to study weak grids with large penetration of renewables and HVDC links, and cannot accurately capture the transient interactions and events that could potentially lead to demand disconnection as recently experienced in the GB and Australian power system. As part of lessons learnt from recent events in Australia, the electricity market operator AEMO, developed new and mandatory modelling requirements and guidelines for using Electromagnetic transient (EMT)-type simulation tools for studying large-scale power systems. Therefore, the entire Australian Power System has been developed in PSCAD to diagnose and reproduce the network response with high fidelity.

Manitoba Hydro International, as the developers of PSCAD™/EMTDC™ have provided consulting services, expert advice and in-depth training on electromagnetic transient (EMT) studies for over 40 years to utilities and manufacturers. This technical seminar highlights the importance of EMT modelling and demonstrates through real case studies the benefits of using EMT studies to address the challenges, lessons learnt and opportunities for accommodation of higher penetration and share of renewable energy in electricity grids.


Manitoba Hydro International (MHI)

• Head Quarter in Canada and Technical Expert Presence in the UK

• Engineering Consulting

• Software development (PSCAD)

Who we are

pscad.comPowered by Manitoba Hydro International Ltd.Outline

General Description of Challenges❖ Inverter based interface to power systems❖ EMT Studies – Background on EMT and RMS❖ Connecting to weak grid locations❖ Impact of transmission network characteristics

Brief Description of Selected Practical Cases (USA, UK and Australia)❖ Control Interaction Issues ❖ Low frequency voltage oscillations❖ Low inertia concerns - South Australian Blackout 2016❖ Resonance issues and inverter response to network voltage and current transients❖ Controls interactions and Torsional interaction (SSTI) concerns

pscad.comPowered by Manitoba Hydro International Ltd.Wind Generators and influence of Transmission

System Characteristics

o The characteristics of wind generators are much different from traditional synchronous machine based generation.o Wind and PV generation is interfaced to the grid through power electronic devices

o Fast control of P and Qo Natural ‘Inertia’ – not available from power electronic interfaced devices. Even with Type 3, the fast controls will regulate

(maintain) the power to pre-set values.

o Nature of AC or HVDC transmission used to connect wind to the transmission grid (long ac cables, filters, weak grids, series compensation)

Synchronous generator based

Wind Type 3Wind Type 4


Wind and Solar PV generation – Based on Power Electronic converter interface – Impact of Network Characteristics

• Weak grid (Low short circuit current, high system impedance)

• T3 and T4 controls depend on system voltage and current measurements as inputs

• Weak grids : Changes in system quantities are harder to track following a system event.

• Specially the change in voltage phase.

• Series compensated systems

• Network resonance points in the sub synchronous frequency range ( < 50 Hz)

Base Case - Case1

Hz 0.0 0.2k 0.4k 0.6k 0.8k 1.0k 1.2k ...

...

...

0.0

0.2k

0.4k

0.6k

0.8k

1.0k

1.2k

1.4k

Z+(o

hm

s)

case0 case1

Zs

System Impedance Vs Frequency Plots

Zs


Wind Generators and Transmission characteristics

• The dynamic characteristics of wind and Solar PV installations are much different from traditional synchronous

machine based generation.

• Nature of AC (or HVDC transmission) used to connect wind to the transmission grid (weak grids, series

compensation, long ac cables, filters) has a significant impact on wind/solar PV response following system events

Inputs to generator controls:RMS and instantaneous inputs to inverters


Wind Generators and Transmission characteristics

Vabc abc→dq vq

vd

PI

0

−

ω0

Reset Input to PLL

PLL Output to Inverter


Integration of wind power to weak grids –Example

Bus voltage following a fault

This voltage must be tracked and phase angle shifts estimated accurately and fast to ensure stable operation

time

P and Q reference

Pref

Qref


Integration of wind to weak grids – Example to highlight importance of system voltage tracking

P controller

d-qTo

ABCQ

controller

Vd*

Vq*

P

P’ref

Q

Q’ref

Voltage angle(PLL)

EMT simulations must be used to accurately represent the response of the PLL and fast controls.This is more of a concern in ‘weak grid interconnections’ and when there is significant penetration of renewables in an area.

pscad.comPowered by Manitoba Hydro International Ltd.Characteristics of Synchronous generators - Inertia

The Synchronous generator response

is determined by

• Machine electrical

characteristics

• Exciter characteristics

• Governor / turbine

• Inertia of the rotating

masses

SG

Network

Exciter

Governor/Turbine

The inertial response immediately follows the

event

• The inertial response is due to the inertia of

large synchronous generators

• Primary control - 20 -30 Sec

• Power electronic based generation does not provide the same style of ‘inertia’


EMT and RMS Simulation –Main Differences


PSCAD/EMTDC – The Industry Standard EMT Simulation Tool

pscad.comPowered by Manitoba Hydro International Ltd.Transients and Steady State

• Transient solution – influenced by

• Harmonics

• Non-linear effects

• Frequency dependent effects

• Steady state solution

• RMS Value of voltages and currents

• Magnitude and phase

Transients are initiated due to a change to the network topology

• Switching Events

• Faults and fault clearance

• Lightning

• Others

Example: Closing the breakers has initiated an electromagnetic transient. The energy exchange between L-C causes the oscillatory transient. Resistance in the circuit acts to damp the transient.

pscad.comPowered by Manitoba Hydro International Ltd.Transients and Steady State

o RMS type simulations • Each solution based on phasor calculations

• 50Hz/60 Hz representation of electric network

• Network dynamics are not considered

o Electro-Magnetic Transient Simulations (EMT)• Direct time domain solution of Differential Equations• Network dynamics are captured in the solution (results

are based on the solution of the differential equations)

)()()()( ILjIRV +=)()()( ti

dt

dLtiRtv +=

2. π. 50

pscad.comPowered by Manitoba Hydro International Ltd.EMT Vs RMS response

Capacitor voltage response

R

CEcap

- Harmonics- DC offset in currents and voltages are represented- Fast controls of inverters can be better represented- Interaction between fast acting power electronic devices

can be studied- However, emt simulations are slow compared to rms type

simulations

pscad.comPowered by Manitoba Hydro International Ltd.EMT Vs RMS response - Example

pscad.comPowered by Manitoba Hydro International Ltd.EMT vs. RMS Simulations

RMS• Assume quasi-steady state• Network transients neglected• Fundamental phasor solution• Positive sequence• Large network possible

EMT• Consider differential equations• Numerical integration substitution• Upper freq. depends on simulation time step (0~MHz)


Weak Grids


Weak Grids – Low Short Circuit Ratio (SCR)

line_6_7_1

line_6_7_2

Bus_6

P+

jQ0.6

/ph

-0.9

/ph

10

#1 #2

300132 / 275

Bus_9

Bus_7

40

Bus_10

P+

jQ6 /

ph

0 /

ph

PI Section

line_7_11_1

Bus_11

line_7_12_1

Bus_12

#1#2

150132 / 66

P+

jQ10.5

/ph

17.0

/ph

Bus_8

line_POI_8_1V

A

V

A PI Section

Bus_POI

P+

jQ7.6

666 /p

h0.6

/ph

P+

jQ7.6

666 /p

h0.6

/ph

Bus_1

#1#2

7566 / 22

Bus_2

line_1_3_1

Bus_3

line_3_5_1

Bus_4

P+jQ1 /ph

0.5 /ph

Bus_5

P+

jQ30 /p

h2.5

/ph

#1

#21

0066

/

22

#1#2

125132 / 66

60 km

45 km

3 km

30 km

1 km

30 km

80 km

40 km each

15

RRL

RRL

RRL

TimedFaultLogicABC->G

Ia

#1#2

100132 / 26

Synch. Gen. a

V

A

V_2 B2B1

10.8

DBlkV

A

DBlkTIME

10.0

BRK1

BRK1

Vw

Type-IV

Wind turbine

A ‘weak’ Point of connection (POC) (‘weak’ grid)

• Low short-circuit current level at POC

• High system impedance

Injection of P and Q (Current) at a weak POC will

lead to voltage variations at the bus

Zsystem

VPOCVTH

VPOC = Iinj x Zsystem + VTH

pscad.comPowered by Manitoba Hydro International Ltd.Weak Grids – Undesirable interactions

Bus_6

P+

jQ0.6

/ph

-0.9

/ph

10

P+

jQ10.5

/ph

17.0

/ph

Bus_8

line_POI_8_1V

A

V

A PI Section

Bus_POI

Bus_1

line_1_3_1

Bus_3

line_3_5_1

Bus_4

P+jQ1 /ph

0.5 /ph

Bus_5

P+

jQ30 /p

h2.5

/ph

#1

#21

0066

/

22

#1#2

125132 / 66

60 km

45 km

3 km

30 km

40 km each

TimedFaultLogicABC->G

Ia

#1#2

100132 / 26

Synch. Gen. a

V

A

V_2 B2B1

10.8

DBlkV

A

DBlkTIME

10.0

BRK1

BRK1

Vw

Type-IV

Wind turbine

Vw

Type-IV

Wind turbine

10.8

Vrms

1pu

Injection of P and Q (Current) at a weak POC

will lead to voltage variations at the bus

Small Q injection ‘large’ Voltage change


Practical Examples


Example 1 – Black System South Australia – September 28, 2016


Example 3 – Black System South Australia –September 28, 2016

pscad.comPowered by Manitoba Hydro International Ltd.Black System South Australia – September 28,

2016

Event Description▪ Extreme weather conditions resulted in five system faults on the SA transmission system in the 87 seconds between 16:16:46 and

16:18:13, with three transmission lines ultimately brought down

▪ In response to these faults, and the resulting six voltage disturbances, there was a sustained reduction of 456 MW of windgeneration to the north of Adelaide.

▪ Increased flows on the Heywood Interconnector counteracted this loss of local generation by increasing flows from Victoria to SA

▪ This reduction in generation and increase of imports on the Interconnector resulted in the activation of HeywoodInterconnector’s automatic loss of synchronism protection, leading to the ‘tripping’ (disconnection) of both of the transmissioncircuits of the Interconnector. As a result, approximately 900 MW of supply from Victoria over the Interconnector wasimmediately lost.

▪ This sudden and large deficit of supply caused the system frequency to collapse more quickly than the SA Under-Frequency LoadShedding (UFLS) scheme was able to act.

▪ Without any significant load shedding, the large mismatch between the remaining generation and connected load led to thesystem frequency collapse, and consequent Black System.


Black System South Australia – September 28, 2016

Fault ride through requirement of wind farms

• All wind farms met the ride through requirement for the number of faults within the short duration

• However, an additional protection that was not known to system operators got activated to trip some of the wind farms (more than 3-4 faults experienced within a pre defined short duration)



Lessons Learned and Recommendations

•Generation Mix and System Inertia:

▪ The system inertia on the SA side was not sufficient to maintain the frequency drop (once theHaywood interconnector tripped) and to make the under frequency load shedding (UFLS)effective.

▪ ‘Must run’ thermal generation may have to be identified.

▪ Synchronous condensers may be investigated as a potential solution if the thermal generationdispatch is expected to be low under specific load conditions.

▪ Load shedding



System Studies – Model validity

Reference:

BLACK SYSTEM SOUTH AUSTRALIA 28 SEPTEMBER 2016 - Report by

AEMO

www.aemo.au


Hornsdale wind farm (300 MW) and Battery Energy Storage System – South Australia

December 2017 – “Hornsdale Power Reserve, constructed by Tesla in South Australia, kicked in just 0.14 seconds (5-7 cycles) after a major plant, the Loy Yang station in the neighboring state of Victoria tripped”


Example 2 –300 MW Wind Farm near Series Compensated 345 kV line – SSCI and Interaction with Network Transients

pscad.comPowered by Manitoba Hydro International Ltd.Wind farms near series compensated lines

The series compensated line is tapped to facilitate the wind farm connection


Issue No.1: Sub Synchronous Control Interaction - SSCI (more an issues with Type 3 wind)

Network Impedance characteristics

Unstable - SSCI


Issue No.2: Wind Inverter response to system transients

The DC offset in the POI voltage caused the inverter DC link voltage to rise.- Poor network side damping- Excessive energy in DC link chopper

resistance (resulted in a trip)


Potential for sub synchronous oscillations between wind farms and series compensated lines in the UK

Series Cap locations

Wind farm locations

-20

-15

-10

-5

0

5

0 10 20 30 40 50 60

Frequency scan of Type 3 wind generator• Dynamic Resistance vs Frequency


Example 3 – Multiple Inverter Based Devices in a Local Area – Control Interactions in a Weak Grid Area


• Fault is applied at 15s for 120ms

• First 10ms duration fault: large rotation in Vd and Vq frame leads to high Iq injection and Low Id injection

• Next 50ms, inverter bring down Id and Iqto allow the PLL to relock to the phase.

• Last 60ms during the fault: inject Iq to support the system

• After fault release, PLL goes unstable and causes large voltage fluctuation


Example 4 – Low Frequency Voltage Oscillations – Weak Grid Issue


Example 2 – Low Frequency Voltage Oscillations (8- 10 Hz range)

Bannerton SF

T[s] 20.0 25.0 30.0 35.0 40.0 ...

...

...

0.0

0.2

0.4

0.6

0.8

1.0

1.2

pu

V_Bannerton SF 66kV

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

pu

P_Bannerton SF

-0.10

-0.05

0.00

0.05

0.10

pu

Q_Bannerton SF

Low Frequency Voltage Oscillations – Interaction between multiple dynamic devices


Example 5 – Line Series Compensation – Fixed Capacitors Or power electronic inverter based solutions


Series compensation – Power electronic based devices

FACTS Devices – Series Compensation

Series compensation of transmission lines - Series Capacitors

System


-1.00E-03

-5.00E-04

0.00E+00

5.00E-04

1.00E-03

1.50E-03

2.00E-03

10 15 20 25 30 35 40 45Dam

pin

g Fa

cto

rFrequency (Hz)

SSTI Impacts on a Gas Power Station

Without SV With SV

System Dynamic Response Study results


Transmission System Expansion and Interconnectors in the UK


Example 6: – Control Interaction and SSTI Impacts -South East England


Potential Control Interaction and SSTI issues -South East England

• South East England is where several HVDC interconnectors land and is a region that has little synchronous plants and even that is being displaced by offshore wind farms.

• Three STATCOMs commissioned to provide voltage support.

• The short circuit ratio is low and reactive current during a fault is sought.

• Control interactions and sub synchronous oscillations concerns given the ‘weak grid’

To Netherlands (BritNed)

To Belgium (Nemo Link)

To France (ElecLink)

To France (IFA)


Potential Control Interaction and SSTI issues -South East England

• Modelling the entire South East England network including all vendor models for all HVDC, STATCOMS, Wind farms in EMT Platforms.


Model Validation of South East England between PSCAD and PF

Active Power flow

x 3.0 4.0 5.0 6.0 ...

...

...

-400.00

-350.00

-300.00

-250.00

-200.00

-150.00

-100.00

-50.00

MW

Blyth - Tynemouth (PF) Blyth - Tynemouth (PSCAD)

20.00 40.00 60.00 80.00

100.00 120.00 140.00 160.00 180.00

MW

Fourstone-Stella West (PF) Fourstone-Stella West(PSC...

0.800k

0.850k

0.900k

0.950k

1.000k

1.050k

1.100k

1.150k

MW

Gretna - Harker (PF) Gretna - Harker (PSCAD)

0.850k

0.900k

0.950k

1.000k

1.050k

1.100k

1.150k

1.200k

MW

Harker - Moffat (PF) Harker - Moffat (PSCAD)

60.00 80.00

100.00 120.00

140.00 160.00 180.00 200.00 220.00

MW

Harker - Stella West (PF) Harker - Stella West (PSCAD)

Reactive Power flow

x 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 ...

...

...

-300.00

-250.00

-200.00

-150.00

-100.00

-50.00

0.00

50.00

MVAr


-50.00 0.00

50.00 100.00 150.00 200.00 250.00 300.00 350.00 400.00

MVAr

Fourstone-Stella W (PF) Fourstone-Stella West (PF)

-160.00 -140.00 -120.00 -100.00 -80.00 -60.00 -40.00 -20.00

0.00 20.00

MVAr


-20.00 0.00

20.00 40.00 60.00 80.00

100.00 120.00 140.00 160.00

MVAr


-100.000

0.000

100.000

200.000

300.000

400.000

500.000

MVAr


Bus Voltage

x 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 ...

...

...

0.00

50.00

100.00

150.00

200.00

250.00

300.00

kV


140.00 160.00 180.00 200.00

220.00 240.00 260.00 280.00 300.00

kV

Fourstone-Stella West (PF) Fourstone-Stella West (PSCAD)

310.00

420.00

kV


310.00

420.00

kV


180.00

290.00

kV


The full National Grid System model in PowerFactory was validated against PSCAD


Example 7: Hornsea 3 Offshore Wind Power - System Interaction Studies

pscad.comPowered by Manitoba Hydro International Ltd.Offshore Wind Power Projects


Hornsea 3 Offshore Wind Power Projects: studies


Example 8: European Transmission Network Model Development for Transient Studies

pscad.comPowered by Manitoba Hydro International Ltd.Transmission Upgrades - Belgium

Network model development for transient studies


Thank You

impact of converter-based technologies on power system ... · and a threat to power system...

Documents