Grid Stability – Large Scale Solar

Mark Parker16 May 2018

1 CONSEQUENCE OF RENEWABLES – TECHNOLOGY INHERENT LIMITATIONS

2 SYSTEM STRENGTH

3 SHORT CIRCUIT CONTRIBUTION

4 INERTIA, FREQUENCY AND ACTIVE POWER

5 CONTROL SYSTEM INSTABILITY

Grid Stability –Solar Challenges

6 MODEL ADEQUACY

CONSEQUENCE OF RENEWABLES- Weakening System -> Reduced Short Circuit Power (Low SCR)- Reduced Inertia -> Displacement of Rotating Machines- Higher Reliance on Control Systems- Reduced Power Quality

- Synchronous Machines- Inherent Response (Inertia, Short Circuit)- Create own speed reference (Grid Leader)

- Asynchronous (e.g. Solar Inverter)- Delayed Response (Control System)- Rely on healthy grid sine wave (Grid Follower)

No technology can retire synchronous machines from the existing grid

1 CONSEQUENCE OF RENEWABLES – TECHNOLOGY INHERENT LIMITATIONS

2 SYSTEM STRENGTH

3 SHORT CIRCUIT CONTRIBUTION

4 INERTIA, FREQUENCY AND ACTIVE POWER

5 CONTROL SYSTEM INSTABILITY

Grid Stability –Solar Challenges

6 MODEL ADEQUACY

SYSTEM STRENGTH

2019 2029 2039

Strong

Weak

1 CONSEQUENCE OF RENEWABLES – TECHNOLOGY INHERENT LIMITATIONS

2 SYSTEM STRENGTH

3 SHORT CIRCUIT CONTRIBUTION

4 INERTIA, FREQUENCY AND ACTIVE POWER

5 CONTROL SYSTEM INSTABILITY

Grid Stability –Solar Challenges

6 MODEL ADEQUACY

SHORT CIRCUIT CONTRIBUTIONSynchronous Machine

- Initial response is electro-mechanically coupled

Asynchronous Machine

- Initial response requires control detection

SCC Typical:Asynchronous :

- Solar Inverter - 1pu

- Statcom - 3 pu

Synchronous:

- Generator - 2.5 pu

- Condenser - ≥ 5 pu

Voltage (kV) Clearance Time (ms)

400kV 80

250kV 100

100kV 120

Asynchronous response too slow at higher voltages for meaningful SCC

NER Fault Clearing Times

SCC – STABILITY EXAMPLES

Loss of Protection Grading

dI

dT

Longer faults -> controller instability

SCC – SOLAR INVERTER RESPONSESNot OK for Transmission1. IQ settling time >70ms

2. Gate Blocking

OK for Transmission1. IQ settling time <70ms

Gate Blocking Delay

V

P

Q

I

V

P

Q

I

Slow P Rise

Delay & Slow Q Rise

1 CONSEQUENCE OF RENEWABLES – TECHNOLOGY INHERENT LIMITATIONS

2 SYSTEM STRENGTH

3 SHORT CIRCUIT CONTRIBUTION

4 INERTIA, FREQUENCY AND ACTIVE POWER

5 CONTROL SYSTEM INSTABILITY

Grid Stability –Solar Challenges

6 MODEL ADEQUACY

INERTIA AND FREQUENCYA

ctiv

e P

ow

er

0

100%

~0.12s ->0.4s(Response Time)

Asynchronous Controller Initiated Response

300s

Inertia

Frequency Response – Synchronous Vs Asynchronous

≥ 3s(Rise Time)

Response Time: ~ 120ms to 400ms depending on technology> Meter processing time 1-2 cycles (~20-40ms)> Frequency filter delay 3-5 cycles (~60ms->100ms)> PLC or Power Plant Controller (PPC) delay (~40ms->200ms)

Rise Time: Will depend on controller response (e.g. PI control, ramp rate, etc...)

Inherent Responsefollowed byPrimary Frequency Response (e.g. Governor, Bypass Valve)

Synchronous Asynchronous

INERTIA & FREQUENCY – EXAMPLE QLDQLD OVER FREQUENCY - LOSS OF QNI

LARGE SCALE SOLAR PLANT REQUIREMENTS:- New GPS to include active power reduction within 3s - Demonstrate frequency response during commissioning (NOT ENABLED)- Run-back schemes

HIGH GENERATOR (Δf) RESPONSE

LOW GENERATOR (Δf) RESPONSE

INERTIA & FREQUENCY – EXAMPLE QLDCONTINGENCY ANALYSIS

System Normal Pre-Event Contingency – N-1

- Lack of frequency response for modelled generation

- Controller instability

SOLAR - ACTIVE POWER VOLATILITY & DIVERSITY

https://anero.id/energy/solar-energy

Curtailment Unconstrained

NEM Solar Average - Diversity Reduced Volatility Unconstrained Average - Diversity Reduced Volatility

INTERCONNECTED NEM REDUCES OVERALL SOLAR VOLATILITY

1 CONSEQUENCE OF RENEWABLES – TECHNOLOGY INHERENT LIMITATIONS

2 SYSTEM STRENGTH

3 SHORT CIRCUIT CONTRIBUTION

4 INERTIA, FREQUENCY AND ACTIVE POWER

5 CONTROL SYSTEM INSTABILITY

Grid Stability –Solar Challenges

6 MODEL ADEQUACY

CONTROL SYSTEM INSTABILITY

Solar Inverter Essentials:

1. Stable Phase Locked Loop (PLL)

2. Operate Plant within Design Capability

CONTROL SYSTEM INSTABILITY - EXAMPLESolar Inverter & Statcom Interaction:- Inverter PLL Instability = Bad response

- Inverter Inductive Q (adverse impact)

- Inverter slow IQ - 300ms (70ms target)

- Inverter Q windup/overshoot on recovery

Inductive Q – Adverse fault impact

Q Overshoot on fault clearance

PLL instability = slow correction

Statcom - Good response

CONTROL SYSTEM INSTABILITY - EXAMPLEPLANT OSCILLATORY INSTABILITY - PPC dispatch greater than MVA capability- Interaction between separate control functions

(ID control & IQ control)

Step in-out of limiter

Command inside MVA capability

Command outside MVA capability

1 CONSEQUENCE OF RENEWABLES – TECHNOLOGY INHERENT LIMITATIONS

2 SYSTEM STRENGTH

3 SHORT CIRCUIT CONTRIBUTION

4 INERTIA, FREQUENCY AND ACTIVE POWER

5 CONTROL SYSTEM INSTABILITY

Grid Stability –Solar Challenges

6 MODEL ADEQUACY

MODEL ADEQUACY

PSS/E Numerical Inaccuracies

- Future grid planning requires model improvement

- PSS/E (RMS) numerical inaccuracies (AEMO PSS/E Model)

- PSCAD (EMTD) more accurate, but also has limitations:- Non-existent PSCAD models of existing NEM Generators

- Significant investment required & lack of maturity (Cost & Delay)

- AEMO rules do not enable sharing of models to new intending participants

- NSP tasked with PSCAD studies -> Biased outcomes (lack of independence)

PSS/E SCR 3 PSCAD SCR 3

PSCAD oscillations

not captured in PSS/E

PSCAD BENCHMARKING CONNECTION STUDY (AEMO PSS/E MODEL)