DigSILENT PF - 00 stability fundamentals

Fundamentals on Power System Stability 1
Power System Stability
On Island Networks
DIgSILENT GmbH
Prepared for IRENA Workshop, 8 - 12 April 2013, Palau

• Definition of power system stability
• Rotor angle stability
• Frequency Stability
• Voltage stability
• Renewable energy integration and stability
Overview

What is 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
Power System Stability

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

Rotor Angle Stability

What is Rotor Angle?
Reference Machine Synchronous Machine 2
Rotor angle

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
Transient Stability

Transient Stability
Left: Active power (red) and reactive power (green) Right: Generator speed
Case 1: Stable
10.008.006.004.002.000.00 [s]
1500.00
1000.00
500.00
0.00
-500.00
-1000.00
G1: Positive-Sequence, Active Power in MW
G1: Positive-Sequence, Reactive Power in Mvar
10.008.006.004.002.000.00 [s]
1.013
1.008
1.003
0.998
0.993
0.988
G1: Speed in p.u.

10.008.006.004.002.000.00 [s]
2000.00
1500.00
1000.00
500.00
0.00
-500.00
G1: Positive-Sequence, Active Power in MW
G1: Positive-Sequence, Reactive Power in Mvar
Transient Stability
Case 2: Critically Stable
10.008.006.004.002.000.00 [s]
1.0325
1.0200
1.0075
0.9950
0.9825
0.9700
G1: Speed in p.u.

Transient Stability
Case 3: Unstable
10.008.006.004.002.000.00 [s]
1.90
1.70
1.50
1.30
1.10
0.90
G1: Speed in p.u.

• 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
• Highly centralised power systems with generators of the same
make / model are typically more robust against transient instability
Transient Stability in Island Networks

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
Small Signal Stability

Small Signal Stability
• Td = damping torque
• Ts = synchronising torque

• 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
Small-Signal Stability in Island Networks

Frequency Stability

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

Source: EirGrid [1]
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
Frequency Stability

• Frequency disturbance following an unbalance in active power
Frequency Deviation according to UCTE design criterion
-0,9
-0,8
-0,7
-0,6
-0,5
-0,4
-0,3
-0,2
-0,1
0
0,1
-10 0 10 20 30 40 50 60 70 80 90
dF in Hz
t in s
Rotor Inertia Dynamic Governor Action Steady State Deviation
Frequency Stability

• 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
Frequency Stability

Frequency Disturbance Example – Ireland 2005
Source: Lalor [3]

• 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
Frequency Stability in Island Networks

• 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)
Frequency Stability in Island Networks

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
Voltage Stability

Small-Signal:
- Small disturbance
Large-Signal
- System fault
- Loss of generation
Long-Term - PV Curves (load flows)
- QV Curves
- Long-term dynamic models
including tap-changers, var-
control, excitation limiters, etc.
- PV Curves (load flows)
of the faulted state.
- Long-term dynamic models
including tap-changers, var-
control, excitation limiters, etc.
Short-Term - Typically not a problem and not
studied
- Dynamic models (short-term),
special importance on dynamic
load modeling, stall effects etc.
Voltage Stability - Analysis

Voltage Stability – QV and PV Curves
1762.641462.641162.64862.64562.64262.64
1.40
1.20
1.00
0.80
0.60
0.40
x-Achse: SC: Blindlei stung in Mvar
SC: Voltage in p.u., P=1400MW
P=2000MW
P=1800MW
P=1600MW
P=1400MW
DIgSILENT
1350.001100.00850.00600.00350.00100.00
1.00
0.90
0.80
0.70
0.60
0.50
x-Achse: U_P-Curve: Total 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
pf=1
pf=0.95
pf=0.9
DIgSILENT
Voltage
Active power
Voltage
Reactive power

Fundamentals of Power System Stability 29
Voltage Stability: Example (PV Curves)
Outage of large generator
All generators in service

• 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)
Voltage Stability in Island Networks

Renewable Energy Integration and Stability

• 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
• 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
Renewable Energy Integration – Key Stability Issues

Source: Lalor [3]
No wind
FSIG
DFIG

• 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]
– Depending on grid characteristics, it may be necessary to limit
penetration of renewable energy sources (case-by-case)

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

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
References

DigSILENT PF - 00 stability fundamentals

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