Importance of Angle in Power System Operation : A Practical Approach
Importance of angle in Power system Operation
Angle is one of the important quantities
that determine the various aspects of the power system. Different types of
angles provide vital information ranging from the power factor, direction of
power flow, stress and stability of power system, generating plant stability
etc.
In general, power system engineers come
across three kinds of angles :
- Phase
Angle
- Torque Angle (Load Angle)
- Power Angle
Each of these angles have their own roles
in providing input to power system engineers and operators in controlling and
managing the grid. Better the control is in hand, more safe is power system.
During disturbance, these angles vary significantly and automated/manual action
are utilized to ensure that they recover back within safe operating limit.
Other angles in power system are related
to DC system which are firing angle, commutation angle, advance angle,
extinction angle and margin angle.
Source : EPRI Power system
Dynamic Tutorial, HVDC Power Transmission System by K.R.Padiyar
Types of angles used by Power system operator (PSO) or Grid Operator at ISO/TSOs
In power system, there are basically two
types of power angles which is important for system operator to monitor on day
to day basis. These are adjacent node/bus angle and wide area node pair angle.
1. Adjacent node/bus angle
It refers to angles between two directly connected nodes/buses in power system.
This is basically the angle between voltages between the adjacent nodes
as shown in below figure.
Its importance can be
understood in terms of following key points :
- Adjacent connected bus angle in
power system is important for overall system stability. If this angle
between all adjacent nodes in power system are within safe limit, then
overall power system has better stability. Lower angle refers to good
synchronizing torque in the power system.
- Adjacent node angle is important for
radial connected power plants and loads as it decides their stability
during normal and contingency condition. It also shows the available
stability margin available during any contingency.
- Adjacent node angle helps during
synchronization of transmission lines post any relay and trip operation.
Such limits are defined in advance through studies/practice based on
which synchro check relays allows the closing of the transmission lines or
generators. It also helps in deciding maximum allowable closing angle
(Standing phase angle) for lines/generators up to which the
system/equipment will be stable.
- It also provides feedback to system
planner or operator on any corrective action which is needed to reduce the
phase angle within acceptable limits and when it is safe to close the line
or synchronize the generator.
- As thumb rule it is required that
adjacent bus angle should always be within 30 degrees for better stability
margin.
Major role played
during major transmission line closing and islands synchronization :
- This angle should always be checked
before closing any major transmission line when there is large power flow
in the associated corridor. When angular separations are higher, they can
result in large angular changes, which can put stress on the synchronizing
breaker as well as nearby generators and they can experience large changes
in their load angles as well.
- A low power angle between
synchronizing tie lines is required during island synchronization in order
to reduce jerk between the two islands. If possible, the line should be
closed at angles within ±5 degrees. An incorrect synchronization,
especially one with a large phase angle, will function as a three-phase
symmetrical fault at the breaker at which the synchronization takes place,
causing severe jerks to all generating plants on both islands. This
has the potential of destabilizing both islands.
2. Wide area node pair angle
It refers to angular separation (Power angle) between two distant nodes in a power system. There is a distant meaning in power systems associated with the angular separation between two distant nodes. In order to determine the overall stress level of a system, the wide area node pair angle can be used.
Such node pairs generally are used for
monitoring when power flows unilaterally, i.e., from a large generator area to
a large load center connected by several nodes and transmission lines. As well,
it can be used for flow gates (interfaces) in power systems where multiple
transmission lines in one area carry power in either direction. Thus
generation-load node pair, flow gate area node pair etc. are used widely by the
system operator for checking wide area stability.
Further another way of looking at wide area node pair is by means of selecting all nodes in a power system and checking the maximum angular separation in the system. Such angular separation where all nodes whose angle can be measured with respect to common reference helps in providing key figures on overall system level stress and stability. This also helps in defining a safe limit of system vide wide area angle for the entire interconnected power system.
Such node pair angle or maximum angular
separation within the system do not have a thumb rule limit in general as
compared to adjacent node pair angle. It will depend on equivalent impedance
between areas, voltages, reactive power support etc. However, based on
stabilities studies and baselining of measurement, these limits can be defined
for alerting system operators.
The higher, as well as lower threshold
wide area angle limits, provide critical information to system/grid operators
to get an alert for system stability. Let’s us say for one such wide area node
pair between generation to load area is above threshold alert limit, then it
can mean that the power flow in the system is high OR any tripping of major
transmission line in the flow gate leading to higher loading of lines OR
tripping of major power plant OR low voltages in system OR low reactive power
system etc. This requires immediate attention to the grid operator for checking
any line tripping, utilizing parallel DC link to reduce angular separation,
load generation re-dispatch, additional reactive support etc.
While if the wide area angle is less than
lower threshold limit then it signifies that power flow is quite less and
voltages in the system are on higher side. This also needs immediate
attention in terms of additional reactive power support, reducing parallel HVDC
loading, higher VAR absorption by generators, taking additionally available
reactors in service, switching off capacitor banks and at the end opening
lightly loaded (high gain circuits)
Thus, it can be observed that how wide
area node pair angle can help grid operators in ensuring stability in the
system and getting alerts during any contingency events.
Reference:
Chandan Kumar, Pradeep Kumar Sanodiya, Rajkumar Anumasula, Vivek Pandey, Raj
Protim Kundu, " New Approach
to Reference Node selection for Wide Area Situational Awareness Using PMUs in
Large Grids", CIGRE India Journal, Year: 2019, Volume: 8,
Issue: 2
Online:
Comments
Post a Comment