Power System Stability Issues

1. By: Chandan Kumar, Manager
POSOCO
Power System Stability Issues and
Remedial Action

2. “Electricity is really just organized lightning”
-George Carlin
“When the laws of Economics and Physics collide, Physics always wins”
-George C. Loehr

3. Power System Reliability
Reliability
SecurityAdequacy
• Adequacy relates to the existence of sufficient facilities within the system to satisfy the consumer load
demand at all times.
• Security relates to the ability to withstand sudden disturbances
Reliability of a power system refers to the probability of its satisfactory operation over the long run. It denotes the
ability to supply adequate electric service on a nearly continuous basis, with few interruptions over an extended time
period
- IEEE Paper on Terms & Definitions, 2004
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4. Power System Reliability
The North American Electric Reliability Corporation (NERC) defines two components of
system reliability:
• Adequacy – Having sufficient resources to provide customers with a continuous supply
of electricity at the proper voltage and frequency, virtually all of the time. “Resources”
refers to a combination of electricity generating and transmission facilities, which
produce and deliver electricity; and “demand-response” programs, which reduce
customer demand for electricity.
• Security – The ability of the bulk power system to withstand sudden, unexpected
disturbances such as short circuits, or unanticipated loss of system elements due to
natural or man-made causes.
Power system is operated in a reliable and secure manner so that system stability
is not endangered.
Contd….
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5. Power System Stability
Angle stability Voltage stability
Small signal stability Transient stability Large disturbance Small disturbance
Mid term Long term
Study period up to
10 secs
Study period up to
several minutes
Study period up to
tens of minutes
Power System Stability
Frequency stability
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6. • Maintaining Frequency Closer to Nominal Frequency (50 Hz)
1. Frequency Stability
50 Hz
Generation Load
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7.  To arrest change in frequency, control actions are required
 Frequency Control can be divided into three overlapping windows of time
 Primary Frequency Control
 Control provided by Interconnection
 Secondary Frequency Control (AGC)
 Control provided by individual control area
 Tertiary Frequency Control
 Control provided by individual control area in coordination with other control areas
Frequency Control
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8. Frequency Control Actions
df/dt,
UFLS/SP
S
Depletion of Network/Tripping
AGC
Generator
Governor
System
Operator
Power System
Emergency
Control/
Defensive
Mechanism
∆
∆
∆
Demand side controlGeneration side control
∆ + ∆
∆
Source: Royal Institute, KTH, EMS
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9. • Keeping Adequate Synchronizing and Damping Torque in system to
reach new equilibrium point after any small/Large disturbance.
• Lack of Synchronizing Torque : The disturbance on the system is quite severe
and sudden and the machine is unable to maintain synchronism under the
impact of this disturbance.
• Lack of Damping Torque : For inadequate amount of damping torque, the
rotor angle undergoes oscillations with increasing amplitude.
2. Angular Stability
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Source : https://nptel.ac.in/courses/108107028/module6/lecture1/lecture1.pdf

11. • Delayed Fault Clearance Near to CGPL
Power Plant.
• Tc > Critical Clearing Time for CGPL Units
• Generator not able to Push Active power
to System due to Fault and over
speed/accelerate
• Leading to pole slip and out of step
protection operation.
• Centre of swing lies in CGPL causing
instability.
• All Outgoing lines tripped on Power
Swing Protection.
Lack of Synchronizing Torque : Pole Slip/Out of Step
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13. • Ensuring adequate number of lines for full evacuation of power.
• Ensuring Angular separation between adjacent nodes are within limit.
• Checking Stability under various criteria through simulation.
• Using Fast Acting System protection Scheme to reduce the
accelerating power available in the system to maintain
synchronization.
• Finding Critical clearing time (tc) for various operating scenario and
having suitable remedial scheme.
• Improving voltage profile by dynamic Reactive support under large
disturbance.
• Reducing Angular difference in case of contingencies by reducing load/
generation.
Remedial Action
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14. Lack of Damping Torque : Low Frequency Oscillation
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15. • Ensuring adequate number of lines for full evacuation of power.
• Ensuring Angular separation between adjacent nodes are within
limit.
• PSS to be properly tuned and verified with tests.
• Using Fast Acting System protection Scheme to reduce
generation and grid stress.
• Tuning of HVDC/FACTS Power Oscillation Damping (POD)
Controller.
Remedial Action
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16. • Maintain steady acceptable voltages at all buses in the system
• System enters a state of voltage instability during a disturbance
or increase in load demand
• Reason : Inability of a power system to meet the demand for
reactive power.
• Remedial Action :
• Ensure Voltage Stability study for Load rich areas
• Adequate Reactive Support Margin availability
• Ensuring N-1 and N-1-1 Reliability criteria
• Implement UVLS Scheme
3. Voltage Stability
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• Voltage Collapse at Agartala due
to loss of reactive Support from
400 kV System.
• Voltage dependent load got
stalled in Tripura Power System
and Bangladesh (North Camila)

18. Voltage Dip During Fault And FIDVR
• Induction Motor : Power Requirement depend on Frequency and Voltage.
• Air conditioning Load and Industrial Motor Load : Either Stall (In absence or protection) or Trip
(Thermal/Under Voltage/LVRT Protection) during voltage dip
• They will start quickly after Stall/Trip and draws huge reactive power.
• Fault induced Load Loss (FILL) : A Major Portion of Load Can trip or Stall during fault and come
back slowly after fault clearance
• If the LVRT protection is not there and large amount of load stall under fault : After fault
clearance, it requires large reactive power from the system to run (Its Induction Motor)
• Fault Induced Delayed Voltage Recovery (FIDVR) : Large reactive power is drawn, Voltage
remains low for large duration and additional load trip specially which have LVRT.
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19.  3 Phase Symmetrical 220 kV Bus Fault near Industrial Load
Centres In Maharashtra.
 Delayed Fault clearance (Voltage Reduced by 50 %)
 Around 1000 MW load was lost.
 No report of loss in terms of lines/ICT tripping at
distribution/transmission level.
 Key Learning :
• No Tripping is observed yet frequency increased
indicating large quantum of load loss.
• Load Loss can occur when fault is severe and delayed
clearance is there. (Not FIDVR Case)
• Surprise for System Operator.
Case Study 1 : Padghe Event
Voltage
Demand and
Frequency
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20. • Summer Season : High Concentration of Air
condition load in Delhi and nearby area.
• Phase to Phase Fault on 400 kV Transmission
line near Delhi and delayed fault clearance.
• Fault clearance is delayed.
• 3700 MW load was lost based on SCADA and
PMU data. High Shoot up in frequency.
• No report of loss in terms of lines/ICT tripping
at distribution level.
• Slow Voltage recovery and FIDVR Characteristic
Observed.
Case Study 2 : 400 kV Bawana – Mundka - II tripping event (FIDVR)
Voltage
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21. Angular Separation in Grid and frequency
Increased Drastically
Northern Region Demand Reduced by Large
Quantum
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22. FIDVR or Severe Fault Causing Load Loss becoming frequent and act as a challenge for System
Operator.
Characteristic of Such Events
• Any phase to phase or three phase of fault at 220 kV and above level near to load centers :
• Severe nature
• Cause load loss due to tripping of induction motor load on overcurrent/under voltage protection.
• Delayed fault clearance makes it more severe as causes more load loss.
• FIDVR event
• Occur during the summer season when load are majorly of single phase AC units’ load.
• Occur during severe fault near to the load centers along with its delayed clearance from the system.
• Followed by a sharp rise in frequency due to load loss and quick recovery with 5-20 minutes.
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23. • Treatment of Distorted Voltage : Erroneous Frequency Calculation can result in
Tripping of the Inverter (Solar Plant)
• Wind Related LVRT protection setting for DFIG based Turbine having crowbar
arrangement : Limitation with How many times LVRT will operate within a
duration of time.
• Protection Coordination and Islanding
Some Key Issues with Renewable & Inverters
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24. Thanks 
chandan@posoco.in
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