Voltage Collapse: Effects, Prevention and Modelling in Renewable Energy Sources

Voltage Collapse 1
VOLTAGE COLLAPSE
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INTRODUCTION
This research paper evaluates the effects of voltage collapse which occurs when the source
cannot supply enough reactive power especially in the renewable sources of energy such as solar
PV and wind energy sources. The research also investigates how these effects of power collapse
can be prevented especially in cases where the renewable energy sources have been installed in
the entire network. Voltage instability can be defined as a system of instability which involves
the reactive power demand of the load not being attained as a result of a shortage in the
transmission and generation of reactive power. Appropriate application of control schemes,
protective relaying, as well as other remedial actions can be applied to minimize the probability
of voltage collapse.
Some of the effects of voltage collapse especially in renewable energy sources include heavily
loaded systems, inadequate reactive support, and low voltage profiles. The ability of these large-
scale grids of power to transmit power from generation to customers is hindered by all the
nonlinear physics of power flow and network structure. Presently, the transition to deregulated
and distributed small-scale production and increasing consumer demand are resulting to raised
stress on the system, and the grid’s operator have tough financial incentives to control networks
next to their physical boundaries. When these physical restrictions are reached or approached,
these power systems may show a form of failure of network known as voltage collapse.
VOLTAGE COLLAPSE CONCEPT
During the daily operations of the Solar PV and Wind energy sources, these systems of power
may show both under-voltage and over-voltage violations which can be controlled through
control of voltage/Var. Through regulating the flow, adsorption, and generation of reactive
power at all levels in the system, the voltage can regulate the profile of voltage within the limit
acceptable and minimize the voltage collapse. Reactive power denotes the flow of energy in the
electromagnetic system constituents. This energy is released and store when every AC cycle,
enabling the constituents of the system to usually operate and to enable the movement of
significant active power with low transmission losses [1].
The voltage collapse and reactive power are the main issues in the operation of power systems
because of topological dissimilarities between the transmission and distribution systems. Voltage
instability happens when an increase in load or low transmission or generation amenities results
in dipping voltage, which results in reduced reactive power from line charging and capacitor and
still further reduction in voltage. In case the reduction of voltage continues further, additional
elements may trip resulting in loss of load and a further reduction in voltage. This outcome in
these uncontrollable and progressive voltage declines is that the system incapable to deliver the
required reactive power providing the demands of reactive power [2].
Specific interest has been taken during the establishment of control schemes to prevent the
voltage collapse. Reliable and efficient action of power systems can be attained by ensuring that
the reactive power and voltage control satisfy conditions, namely:
 The flow of reactive power is reduced to minimize utilization of the transmission system
 Voltage stability is improved to optimize the application of the transmission line system
 Voltages at every stations of every apparatus in the system are within the acceptable
limits

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All the above-mentioned conditions ensure that the systems of transmission operates basically
for active power hence the power is supplied by the power system to numerous loads and is
feeding on various generation stations.
The voltage collapse and other instabilities related to the systems have been recognized as the
factors contributing to numerous blackouts of large scale across the country. The major obstacle
in the prediction of voltage instability is the widespread application of capacitor banks to
maintain the voltage level at substations of wind and solar generation plants and along the
transmission lines. This voltage support maintains the system within the operating limitations,
but covers the little margin of stability of the network, resulting in enlarged risks of a shutdown.
The application of statistical mechanics and network theory to networks of power transmission
have heavily focused on the synchronization which is a phenomenon related to the collective
behaviour self-stabilization of synchronous generator [3].
Figure 1: Voltage stability condition for renewable energy sources
Synchronization process used in renewable energy sources is generally maintained by the flow of
active power which is the real power utilized by the load when doing the work. Voltage control
in renewable energy sources is significant for perfect action of the electrical power system to
prevent voltage collapse, maintain the system’s ability to sustain voltage instability, reduce
losses of transmission, and stop destruction like overheating of generators and motors. When the
reactive power supply is of lower voltage, the current must increase as the voltage drops to
maintain supplied power, resulting in the system consuming more power and further voltage
dropping. In the case of the current further upsurges, the lines of transmission overload other
lines and resulting in cascading failure potentially.
The system becomes unstable when there is a continuous overwhelming drop in the voltage’s
magnitude after disturbances, upsurge in demand of load or even the change in conditions for
operations. The major factor that causes the profiles of voltage not acceptable is the distribution
system’s inability to meet the reactive power demand. Under the ordinary conditions of
operation, the magnitude of bus voltage rises as the Q inserted at the same bus rises. [4].

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MODELLING OF VOLTAGE COLLAPSE
Techniques of modelling can be grouped into two major categories which include dynamic and
static. In determining the effectiveness of dissimilar approaches, it is significant t differentiate
between many events which affect the probability and speed of voltage collapse;
Disturbance of topography; which may include outages of equipment, or even faults then
followed by the outages of equipment. Several of these disturbances are same as the ones which
are traditionally associated with the analysis of temporary stability and occasionally the
difference is difficult to make [5].
Load disturbances; these are the fluctuations of the load and maybe dynamic and can be divided
into fast and slow fluctuations of load. Slow fluctuation of load may be regarded as static since
they cause the stable system of equilibrium to slowly move and make it effective to estimate
profile changes of voltages by the discrete sequence [6].
Table 1: Types of disturbances that cause voltage collapse and instability
Voltage collapse proximity indicator
Currently, the static simulations are used widely for operating and planning purposes t determine
the requirements of reactive supports and capacity of loading the system. The time domains
simulation may be applied for the analysis of voltage stability. Historically, early attempts to
evaluate the conditions of voltage instability were based on the attempts to improve the solution
of the program for static loading flow applied to the systems of power that are heavily loaded
with low voltage profile. It was not easy to solve load flow for such systems because, at the point
of voltage collapse, there is no real stable state solution for load flow. Later, the dual solution
was realized to converge to one point to solve the flow of power [7].
Early indicators applied the distance of two points of solutions as the indicator of the collapse
proximity since the distance reduces as the maximum point of loadability approaches.