00_StabilityFundamentals

Power System Stability On Island Networks DIgSILENT GmbH Prepared for IRENA Workshop, 8 - 12 April 2013, Palau

Fundamentals on Power System Stability

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Overview

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Definition of power system stability

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Rotor angle stability

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Frequency Stability

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Voltage stability

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Renewable energy integration and stability

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What is Power System Stability?

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Power System Stability Definition of stability:

Power system stability is the ability of an electric power

system, for a given initial operating condition, to regain a state of operating equilibrium after being subjected to a physical disturbance, with most system variables bounded so that practically the entire system remains intact. Source: IEEE/CIGRE Joint Task Force on Stability Terms and Definitions, “Definition and Classification of Power System Stability”, IEEE Transactions on Power Systems, 2004

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Types of Stability

• Rotor angle stability (transient stability, small-signal stability) • Frequency stability • Voltage stability (short-term, long-term, small disturbance, large disturbance)

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Rotor Angle Stability

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What is Rotor Angle? Rotor angle

Reference Machine Fundamentals on Power System Stability

Synchronous Machine 2 7

Transient Stability

Large signal rotor angle stability (Transient stability) Ability of a power system to maintain synchronism during severe disturbances, e.g. – – –

Short circuit fault Loss of generation Large step loading (or loss of load)

Large signal stability depends on system properties and the type of disturbance (not only a system property) – –

Analysis using time domain simulations Critical fault clearing time

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Transient Stability 1500.00

1.013

1000.00

1.008

500.00

1.003

0.00

0.998

-500.00 0.993

-1000.00 0.00

2.00

4.00

G1: Positive-Sequence, Active Power in MW G1: Positive-Sequence, Reactive Power in Mvar

6.00

8.00

[s]

10.00

0.988 0.00

2.00

4.00

6.00

8.00

[s]

10.00

G1: Speed in p.u.

Left: Active power (red) and reactive power (green) Right: Generator speed

Case 1: Stable Fundamentals on Power System Stability

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Transient Stability 2000.00

1.0325

1500.00

1.0200

1000.00

1.0075

500.00

0.9950

0.00 0.9825

-500.00 0.00

2.00

4.00

G1: Positive-Sequence, Active Power in MW G1: Positive-Sequence, Reactive Power in Mvar

6.00

8.00

[s]

10.00

0.9700 0.00

2.00

4.00

6.00

8.00

[s]

10.00

G1: Speed in p.u.

Left: Active power (red) and reactive power (green) Right: Generator speed

Case 2: Critically Stable Fundamentals on Power System Stability

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Transient Stability 1.90

1.70

1.50

1.30

1.10

0.90 0.00

2.00

4.00

6.00

8.00

[s]

10.00

G1: Speed in p.u.

Left: Active power (red) and reactive power (green) Right: Generator speed

Case 3: Unstable Fundamentals on Power System Stability

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Transient Stability in Island Networks

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Significance of transient stability depends on several factors, e.g. – Distribution of synchronous generation: highly centralised vs highly dispersed – Types of machines and controllers: same type of prime mover, AVR and governor vs completely different types

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Highly centralised power systems with generators of the same make / model are typically more robust against transient instability

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Small Signal Stability

Small signal rotor angle stability (Oscillatory stability) Ability of a power system to maintain synchronism under small disturbances The following oscillatory phenomena are of particular concern: – – – –

Local modes Inter-area modes Control modes Torsional modes

Analysis using modal / eigenvalue analysis

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Small Signal Stability

• •

Td = damping torque Ts = synchronising torque

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Small-Signal Stability in Island Networks

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Most studies suggest that small-signal stability is not a significant issue – In the EirGrid study [1], increased wind penetration actually improved damping in the oscillatory modes – A study by Potamianakis and Vournas [2], which reflects small systems in the Greek isles, also shows that small-signal stability is not a major issue

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Frequency Stability

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Frequency Stability

Frequency stability Ability of a power system to compensate for a power deficit

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Frequency Stability

Source: EirGrid [1]

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Frequency Stability

How a typical power system compensates for a power deficit: 1. Inertial reserve (network time constant) – Lost power is compensated by the energy stored in rotating masses of all generators -> Frequency decreasing 2. Primary control (1s to 15s): – Lost power is compensated by an increase in production of primary controlled units. -> Frequency drop partly compensated

3. Secondary control (15s to 3min): – Lost power is compensated by secondary controlled units. Frequency and area exchange flows reestablished 4. Re-Dispatch of Generation

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Frequency Stability •

Frequency disturbance following an unbalance in active power Frequency Deviation according to UCTE design criterion 0,1 t in s

0 -10

-0,1

0

10

20

30

40

50

60

70

80

90

-0,2 -0,3 -0,4 -0,5 -0,6 -0,7 -0,8 -0,9

dF in Hz

Rotor Inertia

Fundamentals on Power System Stability

Dynamic Governor Action

Steady State Deviation

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Frequency Stability

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Effects of off-nominal frequencies: – Resonances in rotating machines causing mechanical vibration damage – Overheating of transformer and generator core laminations if Volts/Hz ratio is too high – Change in induction machine operating speed – Flicker in lighting equipment – Time error in AC powered clocks

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Frequency Disturbance Example – Ireland 2005

Source: Lalor [3] Fundamentals on Power System Stability

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Frequency Stability in Island Networks

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Frequency stability is a significant issue in small island grids due to low system inertias – Low system inertia => high sensitivity to frequency deviations – Large frequency deviations after a disturbance are more likely – Frequency deviations may cause activation of load-shedding, over/under-frequency or ROCOF relays

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Frequency Stability in Island Networks

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Considerations: – Spinning reserve to cover contingencies and limit frequency deviations • More spinning reserve = higher level of contingency that can be suffered by the system without collapse • More spinning reserve = more inertia = smaller freq deviations • More spinning reserve = higher generator running costs – Minimum loading of thermal generators (e.g. typically 40 – 60% for diesel generators to avoid cylinder bore glazing)

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Voltage Stability

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Voltage Stability

Voltage stability refers to the ability of a power system to maintain steady voltages at all buses in the system after being subjected to a disturbance. • Small disturbance voltage stability (Steady-state voltage stability) – Ability to maintain steady voltages when subjected to small disturbances, e.g. increasing load, change in solar PV output

• Large signal voltage stability (Dynamic voltage stability) – Ability to maintain steady voltages after following large disturbances, e.g. transmission line trip

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Voltage Stability - Analysis

Long-Term

Small-Signal: - Small disturbance

Large-Signal - System fault - Loss of generation

- PV Curves (load flows) - QV Curves - Long-term dynamic models including tap-changers, varcontrol, excitation limiters, etc.

- PV Curves (load flows) of the faulted state. - Long-term dynamic models including tap-changers, varcontrol, excitation limiters, etc.

Short-Term - Typically not a problem and not studied

Fundamentals on Power System Stability

- Dynamic models (short-term), special importance on dynamic load modeling, stall effects etc.

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DIgSILENT

Voltage Stability – QV and PV Curves 1.40

Voltage

1.20

1.00

P=2000MW P=1800MW P=1600MW

0.80

P=1400MW

0.60

x-Achs e:

562.64 SC: SC: SC: SC: SC:

862.64

1162.64

1462.64

1762.64

Reactive power

Blindlei stung in Mvar Voltage in p.u., P=1400MW Voltage in p.u., P=1600MW Voltage in p.u., P=1800MW Voltage in p.u., P=2000MW

DIgSILENT

0.40 262.64

1.00

0.90

pf=1

Voltage

0.80

pf=0.95

0.70

pf=0.9

0.60

0.50 100.00 x-Achs e:

350.00 U_P-Curve: T otal Load of selected loads in MW Klemmleiste(1): Voltage in p.u., pf=1 Klemmleiste(1): Voltage in p.u., pf=0.95 Klemmleiste(1): Voltage in p.u., pf=0.9

Fundamentals on Power System Stability

600.00

850.00

1100.00

1350.00

Active power 28

Voltage Stability: Example (PV Curves) All generators in service

Outage of large generator

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Voltage Stability in Island Networks

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Voltage instability is mainly caused when a power system cannot meet its demand for reactive power. Problem is much the same for islands as for interconnected grids. Factors influencing voltage stability include:

– – – – – –

Weaknesses in the network (subject to local voltage instability) High system loading Distances between generation and load Availability of reactive power support Dynamic effects, e.g. OLTCs, field excitation limiters, SVCs, etc Load characteristics, e.g. induction motors (air-conditioning)

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Renewable Energy Integration and Stability

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Renewable Energy Integration – Key Stability Issues

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Frequency stability: – Renewable energy sources are often connected via a converter interface and have no inertia (as seen from the grid) – Replacing synchronous generators with sources using a converter interface therefore reduces total system inertia and is more sensitive to frequency deviations – Thermal generators may run under minimum load if displaced by renewable energy sources

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Potential mitigation measures: – Minimum system inertia, i.e. minimum number of synchronous generators online (spinning reserve) – Under-frequency load shedding – Energy storage with fast response [4] – Demand side management (DSM), i.e. smart grid technologies

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Renewable Energy Integration – Key Stability Issues

No wind FSIG

DFIG

Source: Lalor [3] Fundamentals on Power System Stability

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Renewable Energy Integration – Key Stability Issues

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Transient stability: – Effects of renewable energy integration on transient stability must be assessed on a case-by-case basis and depends more on distribution of synchronous generators and controller types – Some past studies indicate that for moderate penetrations e.g. 30 – 40%, renewable energy sources do not significantly affect transient stability [1]

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Potential mitigation measures: – Depending on grid characteristics, it may be necessary to limit penetration of renewable energy sources (case-by-case)

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Renewable Energy Integration – Key Stability Issues

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Voltage stability: – Renewable energy sources with limited or no reactive power control (e.g. fixed-speed induction wind turbines, householdscale PV inverters) will decrease voltage stability – Integrating renewable energy sources into weak parts of the grid can actually improve voltage stability

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Potential mitigation measures: – Use renewable energy sources that are capable of reactive power control – Connect renewable energy sources at weak parts of the grid

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References

1. EirGrid, “All Island TSO Facilitation of Renewables Studies”, 2010, http://www.eirgrid.com/renewables/facilitationofrenewables/ 2. Potamianakis, E. G., Vournas, C. D., “Modeling and Simulation of Small Hybrid Power Systems”, IEEE PowerTech Conference, 2003 3. Lalor, G. R., “Frequency control on an island power system with evolving plant mix”, PhD Dissertation, 2005 4. Kottick, D., Blau, M., Edelstein, D., “Battery energy storage for frequency regulation in an island power system”, IEEE Transactions on Energy Conversion, Vol 8 (3), 1993

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