Wide area monitoring systems (WAMS) are essentially based on the new data acquisition technology of phasor measurement and allow monitoring transmission system conditions over large areas in view of detecting and further counteracting grid instabilities.

1. WAMS Applications

2. CONTENTS
1 WIDE AREA PROTECTION & CONTROLSCHEMES
2 POWER SYSTEM OSCILLATIONS
3 OSCILLATION SPECIFICATIONS AND DAMPING CRITERIA
4 MITIGATION OF POWER SYSTEM OSCILLATIONS
5 MODELING OF VSC-HVDC TRANSMISSIONS
6 SMALL-SIGNAL STABILITY ASSESSMENT IN NETWORKS WITHHVDC
SYSTEMS
7 VOLTAGE STABILITY ANALYSIS FOR LOAD POINTS AND TRANSMISSION
CORRIDORS
8 SPECIAL PROTECTIONS SCHEME
9 DYNAMIC RATING AND REAL-TIME CONGESTION MANAGEMENT
10 LOAD MODEL CHARACTERIZATION

3. 1-WIDE AREA PROTECTION & CONTROL SCHEMES
• Recently some relay manufacturers have implemented
PMU functionalities in distance and overcurrent
protective schemes, these prototype schemes or Phasor
Measurement and Control Units (PMCUs) provide the
assignment of logical variables in devices.

4. • In angular instability triggered-based automatic
generation or load shedding applications the Phasor
Measurement and Control Units PMCUs must take
control actions independently from the GPS signal,
similarly as is done in differential protection schemes.

5. • This new special protection scheme, called “Angular
Difference Protection Scheme”, should be able to
operate as a discrete control scheme and at the same
time transmit measurements at the same sampling
frequency and under the standard IEEE C37.118
protocol (IEEE Sinchrophasors for Power Systems 2006).

6. 2-POWER SYSTEM OSCILLATIONS

7. What oscillation are observed in Electrical
Grid ?

8. Methods

9. Why such oscillations poses threat to the system
• In normal power system state such LFO (Low frequency
Oscillations ) are well damped.
• However they get excited during any small disturbance
in the system and lead to oscillation in power system
parameters like rotor velocity, voltages, currents and
power flows.

10. • Due to oscillations in parameters, protection of
equipments may operate leading to cascade tripping in
power system.
• That’s why the observation of such modes is very
essential in respect to system reliability and security.
• Among these parameters the rotor velocities of the
generators and the power flows in the network are the
most important.

11. • The rotor velocity variation causes strain to mechanical
parts in the power plant and should be limited.
• The power flow oscillations may amount to the entire
rating of a power line as if they are superimposed on
the stationary line flow it will limit the transfer
capacity by requiring increased safety margins.

12. Oscillation Stability Assessment
• Electromechanical oscillations occur in power systems
due to lack of damping torque at the generator rotors.
Oscillations of the generators rotors cause oscillations
of other power system variables, e.g., bus voltage and
frequency, and reactive and active powers on
transmission lines.
3-OSCILLATION SPECIFICATIONS AND DAMPING
CRITERIA

13. • Depending on the number of involved generators and
the size of the power network, power system
oscillations have been reported in the range of 0.05–2
Hz.
• Local oscillations lie in the upper part of the above
range and consist of oscillations of a single generator or
a group of generators against the rest of the system.
Interarea oscillations lie in the lower part of the range
and comprise the oscillations between groups of
generators.

14. • In general, power system oscillations are ever-present,
poorly damped, and not dangerous as long as they do
not become unstable. The objective has been to
develop an algorithm for a real-time monitoring of
oscillations from online measured signals;

15. • in other words, to estimate the parameters
characterizing the electromechanical oscillations such
as frequency and damping and to present this
information to the operator in a user-friendly
environment of the operator station. This kind of
information can hardly be obtained only by watching
the measured signals displayed in the time domain.

16. • Figure shows the proposed approach to the model-
based detection of oscillations. The power system is
excited by a sequence of disturbances represented by
the noise and modeled by the AR model with adjustable
(time-varying) coefficients. The appropriate signal (a
measurement provided by a PMU) is selected either
based on results of modal analysis of the power system
model or consulting an experienced operator in order
to ensure a possibly high observability of the modes of
interest.

17. Basic scheme for the proposed
detection of power system
oscillations.

18. WAMS Benefit:
Detection of oscillation, Assessment of power system
damping, Increase power transfer at defined security, and
Early warning to avoid power system collapse.

19. Actions Required to Damp such Oscillations
• HVDC Damping Controller
• TCSC POD Tuning
• PSS Tuning of generators
• Proper Planning of transmission lines to Strengthen the
transmission network.
• Need to adapt Global PSS Tuning

20. DAMPING OF POWER-OSCILLATIONS USING WAMSAND
FACTS/HVDC
• The most common practice today, is to use power
system stabilizers (PSS) applied as an auxiliary input to
the voltage set-point of the automatic voltage
regulator (AVR) of large synchronous generators.
4- MITIGATION OF POWER SYSTEM OSCILLATIONS

21. • However, any fast power electronic devices that can
modulate directly or indirectly electric power flows
(such as FACTS or HVDC), can be used as actuators to
deal with the problem of poorly damped
electromechanical oscillations in large electric power
systems.

22. • However, the effectiveness of the actuators for damping
control, be it FACTS/HVDC or synchronous generators, largely
depends on the tuning of the damping extensions and on the
location of the actuator.
• Furthermore, generators and transmission network often
belong to different entities with different interests. Thus,
network operators are increasingly considering network
components such as HVDC and FACTS devices to be equipped
with damping controllers having the functionality equivalent
and in some respects surpassing that of power system
stabilizers.

23. • Wide-area signals are used for supplementary modulation of
the HVDC power order driven through a multivariable
controller, see Figure

25. Signal Selection
• Appropriate selection of stabilizing signals for HVDC
modulation controller is of critical importance to
ensure desired damping to the inter-area oscillations.
Wide-area signals are preferred to local signals as they
often have higher modal observability.

26. 5- MODELING OF VSC-HVDC TRANSMISSIONS
• Figure shows a basic structure of a VSC-HVDC link
connected in parallel with an ac transmission line.
Basic Structure of a VSC-HVDC transmission.

27. • From the connection nodes Bus i and Bus j, a VSC-HVDC
transmission can be seen as a synchronous machine
without inertia where the production or consumption of
active power is independent of the production or
consumption of reactive power. This interpretation
leads to modeling a VSC-HVDC transmission as two
controllable voltage sources in series with a reactance,
which represents the impedance of the power
transformer. This modeling is shown in Figure.

28. Modeling of a VSC-HVDC transmission.

29. 6- SMALL-SIGNAL STABILITY ASSESSMENT IN
NETWORKS WITH HVDC SYSTEMS
• Small-signal stability analysis is about power system
stability when subject to small disturbances. If power
system oscillations caused by small disturbances can be
suppressed, such that the deviations of system state
variables remain small for a long time, the power
system is stable.

30. • On the contrary, if the magnitude of oscillations
continues to increase or sustain indefinitely, the power
system is unstable.
• Power system small-signal stability is affected by many
factors, including initial operation conditions, strength
of electrical connections among components in the
power system, characteristics of various control
devices, etc.

31. • Since it is inevitable that power system operation is
subject to small disturbances, any power system that is
unstable in terms of small-signal stability cannot
operate in practice.
• In other words, a power system that is able to operate
normally must first be stable in terms of small-signal
stability. Hence, one of the principal tasks in power
system analysis is to carry out small-signal stability
analysis to assess the power system under the specified
operating conditions.

32. Aim
• With the continuous increase in electricity demand and
the trend for more interconnections, one issue of
concern is the mitigation of low-frequency inter-area
oscillations (LFIO).

33. • Typically, inter-area oscillations occur in large power
systems interconnected by weak transmission lines that
transfer heavy power flows. Usually, these oscillations
are caused by incremental changes, (thus, “small-
signal”) and have the critical characteristic of poor
damping.

34. • When a certain type of swing occurs in such system,
insufficient damping of LFIOs may lead to a limitation
of power transfer capability or, worse than that, a
growth in amplitude of the LFIOs which could possibly
cause a system to collapse.

35. • To enhance transfer capacity while preventing the
system from breaking up, a common countermeasure is
to install power system stabilizers (PSS), which provide
additional damping to the system through generators.
Successful damping, however, relies heavily on the
locations and types of input signals used by the PSS, as
well as the PSS locations.

36. • One of the most common applications of phasor
measurement units (PMUs) is power system monitoring,
especially for monitoring wide-area disturbances and
low frequency electromechanical oscillations.
• PMUs are a solution to increase observability in
traditional monitoring systems and provide additional
insight of power system dynamics.

37. • In recent years, the introduction of synchrophasor
measurement technology has significantly improved
observability of power system dynamics and is expected
to play a more important role in the enhancement of
power system controllability.

38. • Power system stabilizers (PSSs) are the most common
damping control devices in power systems. The PSSs of
today usually rely on local measurements and are
effective in damping local modes.

39. • Carefully tuned PSSs may also be able to damp some
inter-area oscillations; those which can be observed in
the monitored input signals. By appropriately tuning
available PSSs, together with wide-area measurements
obtained from PMUs, it is expected that inter-area
damping can be effectively improved.

40. • Numerous research has investigated one “branch” of
the problem, that is, PSS design using various control
schemes. Before addressing the issue of controller
design, it is important to focus on developing a proper
understanding of the “root” of the problem: system-
wide oscillations, their nature, behavior and
consequences. This understanding must provide new
insight on the use of PMUs which allows for feedback
control of LFIOs.

41. • Study aims to illustrate various aspects of power
transmission system operation with the emphasis on
stability assessment and control.
• A main objective has been to show how proper tuning
of damping controllers can contribute to raising
transmission capacities, and thereby reduce operating
costs.

42. • A small signal stability analysis is carried out on a full
Nordel transmission system model in order to assess
main stability properties and to perform controller
design.

43. • Study has found that SVCs in Southern Norway, as well
as HVDC converters and a number of generators and
FACTS devices show efficient for damping inter-area
oscillations (Sweden and Finland modes).

44. Overview of generation and transmission capacity in the
Nordel power system

45. Case Study of a Multi-Infeed HVDC System
• the area receiving electrical power from multiple HVDC
transmission links is relatively strong due to the
presence of large amount of generation units nearby
there are still some questions that need to be
investigated such as the issues underlining the
operation of such a multi-infeed system, the proper
design of the controls of the HVDC systems and the
system dynamic performance under extreme
contingencies.

46. • The projects have performed small signal analysis of
the system to assess instability associated with the
control modes.
• Electromechanical and voltage stability analysis were
performed for harmful contingencies. Dynamic
performance analysis was also carried out to analyze
the interaction amongst various HVDC inverters during
disturbances.

47. • This paper investigates into an example of such a
multiinfeed HVDC system. The studied system is one of
the planned configurations of the China Southern Grid
(CSG) system which will have five HVDC transmission
links by 2010 feeding 15,800 MW of electrical power
into the load center in Guangdong province.

48. • The authors have performed small signal analysis of
the system to assess instability associated with the
control modes. Electromechanical and voltage stability
analysis were performed for harmful contingencies.

49. • Dynamic performance analysis was also carried out to
analyze the interaction amongst various HVDC
inverters connected to a common AC network during
various disturbances. The purpose of the study is to
investigate if the system under peak load condition
would withstand critical contingencies and to identify
the possible problems and countermeasures.

50. Geographic diagram of CSG network

51. Power angles in critical areas of CSG system

52. Identified critical area in CSG system

53. P-V curves for the five AC inverter busses

54. 7-VOLTAGE STABILITY ANALYSIS FOR LOAD
POINTS AND TRANSMISSION CORRIDORS
• Transmission corridors, between or within
interconnected power systems, probably become
bottlenecks in power transmission and may even cause
voltage instability contingencies. Therefore, it is
necessary to monitor the Available Transfer Capacity
(ATC) of a transmission corridor.

55. • A Thevenin equivalent model is placed on a load bus, to
calculate the strength of the transmission system
relative to the load bus.
• Multiple transmission lines in a transmission corridor
are equaled as one line, with a Thevenin equivalent
model in the generation center and a constant
impedance load model in the load center. However,
this model is too simple to take different load increase
directions into consideration, which are critical to the
load margin calculation.

56. • A new equivalent model based on PMU measurements is
proposed to analyze and predict the voltage stability
of a transmission corridor. As it retains all transmission
lines in a transmission corridor, the method ismore
detailed and accurate than traditional Thevenin
equivalent model widely used.

57. • To estimate the model parameters, Newton method or
the least square estimation method is adopted with
multiple continuous samples of PMU measurements.
With the proposed equivalent model, the actual load
increase direction is calculated on-line with PMU
measurements, and the load margin of the transmission
corridor is calculated accordingly. Such a load margin
calculated is equal to the ATC of a transmission
corridor, which can be used as a voltage stability index.

60. 8- SPECIAL PROTECTIONS SCHEME

61. 9- DYNAMIC RATING AND REAL-TIME
CONGESTION MANAGEMENT

62. • System Operators must address all events, all
constraints – reliability, economic and natural
– Perform future studies to determine safe operating
conditions
– Perform day-ahead studies of expected operating
conditions to test its operating plan
– Operate in real time according to day ahead plan

63. Congestion Management - Tools
• Real-time assessment of power system
• Real time alarm systems for real and contingency
overloads
• Calculation of sensitivity factors of generation flows on
the transmission system
• Determination of redispatch options to maintain
reliable operations

64. • Security-constrained, economic dispatch
• Determine the economic value of generation in real
time at every node on the system based upon:
– transmission system parameters
– generation loading
– transactions

65. Congestion Management - Methods
Transmission Line loading Relief (TLR)
– Identify flowgates (frequently constrained segments of
the transmission system)
– Determine generation impact on flowgates
– Monitor flowgates and generate alarms for overload
conditions (manually)
– Curtail regional transactions impacting loading on
flowgates based upon priority of transmission service

66. 10- LOAD MODEL CHARACTERIZATION