Power system stability issues and Defence Mechanism in Indian Grid

By: Chandan Kumar, Deputy Manager
ERLDC,POSOCO
Power System Stability Issues and
Remedial Action

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

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
11-Jan-19 ERLDC, POSOCO 3

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….

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

• Maintaining Frequency Closer to Nominal Frequency (50 Hz)
1. Frequency Stability
50 Hz
Generation Load

 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

Frequency Control Actions
df/dt,
UFLS/SP
S
Depletion of Network/Tripping
AGC
Generator
Governor
System
Operator
Power System
Emergency
Control/
Defensive
Mechanism
𝑑𝑓
𝑑𝑡
𝑓
∆𝑝
∆𝑓
∆𝑓
𝑈𝑠𝑒𝑐
𝑈 𝑝𝑟𝑖
𝑈𝑓
Demand side controlGeneration side control
∆𝑝 + 𝒌∆𝑓
𝐴𝐶𝐸
∆𝑝
Source: Royal Institute,KTH, EMS

• 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

Source : https://nptel.ac.in/courses/108107028/module6/lecture1/lecture1.pdf

• 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

Fig: CGPL Voltage from Simulation

• 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

Lack of Damping Torque : Low Frequency Oscillation

• 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

• 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

• 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)

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 and 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.

 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

• 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

Angular Separation in Grid and frequency
Increased Drastically
Northern Region Demand Reduced by Large
Quantum

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.

• 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

Thanks 
chandan@posoco.in

Defense Mechanism

1. Operational States of the Grid
Normal
• Equality Constraint : Load+Loss=Generation
• Inequality Constraint : All Parameters within Safe limit

• Normal state :All system variables are within the normal operating range
• Alert state : Security level falls below a certain limit of adequacy because of a
Event
• Emergency state : Severe disturbance
• In Extremis : Cascading outages
• Restorative state

• Ultimate Objective is to not reach the IN EXTREMIS State of Operation.
• Each Level having its own defense mechanism for returning back to previous state.
• Normal state :
• Operational Standards : CEA Standard and IEGC
• Operational Ranges for System Parameters : CEA Standard and IEGC
• Operational Criteria : IEGC and Operational Procedure
• Alert state :
• Primary , Secondary and Tertiary Control
• Automatic Control from FACTS/HVDC devices
• Load Generation Redispatch
• Manual Load Shedding and Generation Revival
• Under Voltage Load Shedding Scheme

• Emergency State
• System Protection Scheme
• Load Trimming Scheme
• Generation Reduction Scheme
• In Extremis
• Under Frequency Load Shedding Scheme
• ROCOF based Load Shedding Scheme
• Islanding Scheme
• Unintended System Separation
• Restorative state
• Black Start and Island Synchronization

2. Need of Defense Mechanism
Power System Operate with following Contingencies in Real Time:
• N : No Element is Out
• N-1: One Important element is Out
• N-1-1 : One element is out followed by other element
• N-K: Multiple elements are out
Real Time Operators ensure that there is no additional insecure operation
after these outages/contingencies. All Electrical Parameters are within their
prescribed band.

However, System Can go from N state to
N-K with Events/Contingencies
That’s why there is a need of Defense Mechanism
2. Need of Defense Mechanism

3. Defense Mechanism and Types
• Need of Some Mechanism to recover the system back to Normal State of Operation.
• Mechanism placed in Grid for Recovering System from Alert/Emergency/ In Extremis
State are called Defense Mechanism for Power System.
• Automated Mechanism
• Quick recovery (Response Time is in millisecond to few seconds)
• Fast, Logic Based, Measurement Based, Reliable.
• Action can be Local or Global depending on design.
• Act as last line of defense before system go for blackout or cascaded tripping.

Defense Mechanism Can be Classified as :
• Based on Type of Action : Generation, Load, HVDC Set Point, FACTS.
• Based on Area of Action : Local and Global
• Based on Parameter : Frequency Based, Voltage Based, Current Based, Active
and Reactive Flow Based
• Based on Time : Instantaneous, Time delayed
3. Defense Mechanism and Types

3.Defense Mechanism in Indian Power System
• Dynamic Control of HVDC/FACTS, PSS Of Generators
• System Protection Scheme (SPS)
• Automated Under Voltage Load Shedding (AUVLS)
• ICT Load trimming Scheme (LTS)
• Automated Under Frequency load Shedding (AUFLS) and ROCOF based Load
Shedding
• Islanding Scheme

4.a. Dynamic Control of HVDC/FACTS, PSS Of Generators
• HVDC/FACTS are having dynamic control which depending of criteria provide
response to reduce the impact of event.
• Frequency Controller of HVDC
• Sub-synchronous Torsional Interaction(SSTI) Controller of HVDC
• Power Oscillation Damping (POD) of HVDC/FACTS
• Dynamic Reactive Support from SVC/STATCOM

4.a. Dynamic Control of HVDC/FACTS, PSS Of GeneratorsTalcher-KolarHVDCFrequencyController

4.a. Dynamic Control of HVDC/FACTS, PSS Of Generators
t/s
-5 0 5 10 15 20 25 30 35
P_LINE1 NA/MW
0
50
100
150
200
t/s
-5 0 5 10 15 20 25 30 35
POD_OUT_DX NA/PU
-0.0
0.2
0.4
t/s
-5 0 5 10 15 20 25 30 35
X_REF_MOM NA/PU
0.75
1.00
1.25
1.50
t/s
-5 0 5 10 15 20 25 30 35
XAPP_R R/PU
0.0
0.5
1.0
1.5
t/s
-5 0 5 10 15 20 25 30 35
XAPP_Y Y/PU
0.0
0.5
1.0
1.5
t/s
-5 0 5 10 15 20 25 30 35
XAPP_B B/PU
0.0
0.5
1.0
1.5
t/s
-5 0 5 10 15 20 25 30 35
P_SWING NA/MW
0
25
50
75
t/s
-5 0 5 10 15 20 25 30 35
ABS_P_SWING NA/MW
0
25
50
75
t/s
-5 0 5 10 15 20 25 30 35
AB
BP_STRAT_ON
POD_ON
POD_CBP
RaipurTCSCPODController

4.b. Automatic Under Voltage Load Shedding Scheme
• Power systems today are much more susceptible
to voltage collapses as increasingly depend on
generation sources that are located remotely from
load centers.
• UVLS relays are meant for shedding of load when
the voltage is going below the operational lower
limit in order to avoid voltage collapse.
• IEGC 5.2.t Mandates : All Users, CTU and STUs shall
provide adequate voltage control measures
through voltage relay as finalized by RPC, to
prevent voltage collapse. and shall ensure its
effective application to prevent voltage collapse/
cascade tripping.

Basic Design Criteria :
1. Designed to coordinate with protective devices and control schemes for momentary
voltage dips, sustained faults, low voltages caused by stalled air conditioners, etc.
2. The time delay should be in seconds, not in cycles
3. Must be on Bus VTs and coordinate with ICT/Lines Load Trimming Scheme.
4. Voltage pick-up points to set reasonably higher than the “nose point” P-V nose curve.
5. Enough load shed to bring voltages to minimum operating voltage levels or higher
Example Setting for UVLS
In case of two stage operation
• Alarm: If Voltage < 375 kV ; Stage 1: If Voltage < 372 kV for 5 sec; Stage 2: If Voltage < 370 kV for 10 Sec
In case of Single stage operation
• If Voltage < 370 kV for 5 seconds
4.b. Automatic Under Voltage Load Shedding Scheme

4.c. Load trimming Scheme (LTS)
• These are also SPS which are local in nature and has designated purpose of
saving elements getting overloaded due to contingencies.
• Implementation for Line and ICT to avoid overloading.
• Several Such LTS scheme are there in Various States across India.
• Their operation during contingencies helps in arresting the event
widespread.
• Basically Designed using Overcurrent Feature of the Relays with PLCC as
Communication Media.
• Need of Coordination between various LTS is very much necessary in an
interconnected system (One LTS Impacts Others): On two occasions
Kolhapur ICTs (3 Nos) and Karad ICTs (3 Nos) tripped One iCT tripping
even after the LTS operation due to Non-Coordination.

• Frequency should be within prescribed band of 49.9-50.05 Hz.
• However, Frequency will fall rapidly with System Islanding/Large
generation complex loss.
• AUFLS and ROCOF (df/dt) based automated load shedding are ways to
arrest gradual as well as fast fall in frequency in the system (Frequency fall
Arrester)
• ROCOF based relay arrest the fast fall in frequency while AUFLS will restrict the
frequency above a certain limit.
• AUFLS is global phenomenon and load shedding occurs as per design in the
complete synchronized grid While ROCOF based load shedding quantum caries
with nearness to Frequency related event source.
• Quantum of such Load connected with AUFLS and df/dt relay changes with system
size and connectivity.
4.d. AUFLS and ROCOF Relays for Load Shedding

Last AUFLS Operation :
Separation of NEW and SR Grid
on 24 May 2015 during High
import by Southern Region.

CGPL Event 12 March 2014
UMPP Loss Largest Contingency in the System
ROCOF Operated in Gujarat (636 MW) and
Maharashtra (334)
ROCOF Relay Operated.

49.9 Hz
ROCOF Relay operation in Gujarat on 18 Jan 2017
• Cause : Momentary Cessation of Full HVDC Mundra
Power Order of 2500 MW to Zero and Auto-Restart to
2500 MW in 10 ms interval causing Large swing in WR
and NR (See Frequency Plot)
• Root Cause : Simultaneous DC line Fault (Rare
Contingency)
• ROCOF operated in Gujarat System near Mundra : 486
MW load Loss
• PMU indicated ROCOF above 0.4 Hz/Sec (Highest Value
of ROCOF)

• When The system is moving towards in Extremis and Blackout , its desirable to
split system having load/generation balance in Island for faster restoration.
• It’s the Last Measure : Islanding should take place only when all other defense
plan have been allowed their full opportunity to bring back and maintain system
integrity and still the health of integrated system is on path of deteoriation
towards failure.
• Large Number of Islanding Scheme is Not Suitable for Integrated Grid.
• Operational frequency band for pre-islanding defense mechanism and islanding
frequency band sufficiently apart : Help in taking care of continued fall of
frequency during the interval between relay pick-up and breaker opening.
• IEGC 5.2.n
4.e. Islanding Scheme

• Islanding to Occur : When all other defense plan ( fault clearance, SPS action, UFLS and df/dt
load shedding etc.) have been allowed their final operational opportunity and the system is
still on the path of deterioration towards collapse.
• Islanding not to Occur : If there is still a chance that a distressed system can be brought back
to emergency condition (or alert condition or normal condition) with operation of the
mechanism planed for integrated and interconnected states of Grid.
• Frequency Band Setting for Islanding
• Devised for Part of system which have Load generation balance : Preserve those areas which
inherently are in a position to achieve load generation balance post islanding.
To be Reviewed after 2 Years
Ramakrishna Committee Recommendation

• Generally Implemented for only those sub-parts of network :
• Connect to rest of grid in an electrically radial manner with only a few
interconnecting lines
• Have their own load generation balance to a large extent, requiring comparatively
smaller exchanges with rest of the grid.
• Features to be incorporated In Islanding Scheme
• Adequate automated mechanism for achieving load generation balance.
• Frequency control of islanded subsystem: Sufficient number/capacity of generating
units in on free governor mode of operation in the island.
• Load connection/disconnection should be possible remotely from the dispatch
centre of the islanded sub-system.
• Health of Monitoring System for all equipment in island System should be good.

MumbaiIslandingScheme
TROMBAY 220 KV
TROMBAY 110 KV
SALSETTE 110 KV
KALYAN 110 KV
SALSETTE 220 KV
BORIVALI 110 KV
BORIVALI 220 KV
UFR - 47.9 Hz
RPUF – 48.0 Hz
RPUF - 47.9 Hz
UFR - 47.9 Hz
UFR - 47.9 Hz
RPUF - 47.7 Hz
47.9Hz RPUF - 47.9 Hz
UFR - 47.9 Hz
UFR - 47.6 Hz
AAREY
VERSOVA
DAHANU
UFR - 47.9 Hz
MSETCL
Tata Power
R-infra
UFR - 47.9 Hz
GHODBUNDER
TROMBAY
KALWA
BORIVLI
BOISAR
BORIVLI
UFR - 47.9 Hz
UFR - 47.9 Hz
UFR - 47.9 Hz

Power system stability issues and Defence Mechanism in Indian Grid

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Power system stability issues and Defence Mechanism in Indian Grid

Similar to Power system stability issues and Defence Mechanism in Indian Grid (20)

More from Chandan Kumar

More from Chandan Kumar (13)

Recently uploaded

Recently uploaded (20)

Power system stability issues and Defence Mechanism in Indian Grid