This document discusses transients in DC circuits and renewable energy systems like wind and solar. It defines transients as momentary changes in voltage or current that occur over a short period of time. Transients can be caused by switching events, lightning strikes, capacitor bank switching, and more. The document explores the transient behavior of circuits containing resistors, capacitors, and inductors. It also discusses transient overvoltages and dynamic overvoltages that have been issues in renewable energy projects, and how transients can impact power system stability and reliability. The paper aims to focus on transient events in DC circuits and wind/solar systems, and techniques to improve transient stability.

1. Transients in DC, Solar and Wind System
Abibe Hyusein
Graduate Degree Program Student
New Jersey Institute Of Technology,
New Jersey, USA
Abstract – Renewable energy generation installations
have experienced various design and performance issues
that have not always been properly addressed during the
development of the projects. These issues are the transient
overvoltages, dynamic overvoltages and transient loads.
Transient stability is a major concern in power system
security and reliability because it is the most common type
of instability and its impacts can cause greatest economic
losses. Maintaining transient stability means keeping
enough of the bits of the power system, i.e., generation,
transmission, and distribution, connected to avoid
interrupting customers after the gridis whacked by Mother
Nature or more nefarious forces.
In this paper we are going to focus on the transient events
in DC circuit, transients occurring in the wind and
photovoltaic systems and more specifically in wind systems,
the main aspects having a possible impact on transient
stability issues and the existing techniques for transient
stability improvement.
I. INTRODUCTION
The use ofrenewable energy increased greatly just after the
first big oil crisis in the late seventies.At that time, economic
issues were the most important factors, hence interest in such
processes decreased when oil prices fell. [1]
Unlike classical sources of energy, wind farms supply real
power variations into the upstreamgrid, and at the same time,
in some types of wind generation systems,the reactive power
consumption is related to the real power production. These
power variations cause voltage variations with consequences
for the electrical power systemand the customers.On the other
hand, the increasing use of power electronics in wind
generation systems introduces voltages and current harmonics
into the power system. As wind energy is a non-controllable
energy source,it can cause problems with voltage stability and
transient stability. [2] Transient stability is a major concern in
power system security and reliability because it is the most
common type of instability and its impacts can cause greatest
economic losses.
Renewable energy generation installations have
experienced various design and performance issues that have
not always been properly addressed during the development
of the projects. These issues have included transient
overvoltages (TOVs), dynamic overvoltages (DOVs) and
transient loads in wind systems
In this paper we are going to focus on the transient events
in DC circuit, transients occurring in the wind and
photovoltaic systems, and more specifically in wind systems,
the main aspects having a possible impact on transient stability
issues and the existing techniques for transient stability
improvement.
For better understanding the transient impact on the power
systemwe can start with reviewing the transient basics.
II. TRANSIENT BASICS
Transients are momentary changes in voltage orcurrent that
occur over a short period of time. They are divided into two
categories which are easy to identify: impulsive and
oscillatory. The most common transient, is the "oscillatory
transient". It is sometimes described as a "ringing transient".
This type of transients is characterized by swings above and
below the normal line voltage (fig.1).
The othertype (impulse) transient,is more easily explained
as a "one-shot" type ofevent,and it is characterized by having
more than 77% of it being one pulse above the line voltage
(fig.2). A lightning strike can be composed of multiple
transients of this type. [3]
A theoretical or "ideal" DC circuit such as illustrated in
Fig.3 contains only resistance. The transient operation of the
circuit, during switch operation, is shown in Fig. 4. As the
switch closes on contact B, the amount of current flowing,
which has previously been zero, will instantly rise to a
maximum level. This will make the current (I) equal to the
battery EMF (E) divided by the resistance (R). That is:
,
E
I A
R
 (1)
Figure 1- Oscillatory Transients.
Figure 2- Impulsive Transients.

2. Figure 3-Theoretical (“ideal”) DC
circuit.
Figure 4 – Transient operation of
the theoretical circuit.
Fig. 4, shows what happens to the voltage and current when
the switch is closed, and then opened again. Both current and
voltage rise immediately to a steady value as the switch is
closed,then fall immediately to zero when the switch is opened.
The voltage across the resistor (VR) while the switch remains
closed is given by [4]:
* ,RV I R V (2)
In circuit, which contains capacitance and resistance (fig.5),
when the switch is changed from position A to position B, the
circuit current rises very rapidly, as the capacitor begins to
charge (fig.6). Although the voltage is still low, its rate of
change is large and the voltage graph is steep,showing that the
voltage is changing in a very short time. [5] If the switch is
now changed to position C, this causes the capacitor to
discharge through R. Immediately maximum current (fig.7)
flows, but this time in the opposite direction to the that during
charging. Again an exponential curve describes the fall of this
negative current back towards zero. The voltage also falls
exponentially during this time, until the capacitor is fully
discharged.
Figure 5- Capacitance and resistance circuit.
Figure 6 –Voltage through capacitor.
Figure 7 – Current through capacitor.
In a circuit which contains inductance (L), as well as
resistance (R), such as the one shown in Fig. 8, when the switch
is closed the current does not rise immediately to its steady state
value but rises in exponential fashion (fig.9). This is due to the
fact that a back electromagnetic field (EMF) is created by the
change in current flow through the inductor. Because the back
EMF opposes the rapid change in current taking place in the
inductor, the rate of change of current is reduced and what
would be a vertical line on the graph (Fig. 9) becomes a slope.
The rate of change of current through the inductor is now less,
so a smaller back EMF is produced. This allows the current to
increase further. The relationship between the changing current
and back EMF produces a curve which always follows a
mathematical law to produce a particular shape of curve i.e. an
exponential curve. When the switch is opened, the current
decays in a similar exponential manner towards zero. Looking
at Fig. 10 which shows the voltage (VL) across the inductor (L)
we can see that at switch on, the voltage immediately rises to a
maximum value. As current begins to flow through L however,
the voltage VL decreases until a point is reached where the
whole of the battery voltage is being developed across the
resistor R and the voltage or potential difference across L is
zero. When the current is switched off, the rapidly collapsing
magnetic field around the inductor produces a large spike of
induced current through the inductor in the opposite direction
to the current that was flowing before switch−off. These rapid
changes in current as the switch opens can cause very large
voltage spikes, which can lead to arcing at the switch contacts,
as the large voltage jumps the gap between the contacts. The
spikes can also damage other components in a circuit,
especially semiconductors. [6]
Figure 8 - Inductance and reactance circuit.
Figure 9 – Current through an inductor.
Figure 10 – Voltage through an inductor.

3. Transients can be generated internally, or they can come into
a facility from external sources. The least common of the two
are externally generated transients.
Lightning is the most well-known of the externally generated
transients. Most lightning transients are not actually the result
of direct lightning strikes. They are most often "induced" onto
conductors as lightning strikes near the power line. The large
electric fields generated during a discharge can couple into the
power system, creating induced transients. Other externally
generated transients may also be imposed on power lines
through normal utility operations. Switching of facility loads,
opening and closing of disconnects on energized lines,
switching of capacitor banks, re-closure operations and tap
changing on transformers can all cause transients.Pooror loose
connections in the distribution system can also generate
transients.They may be caused by high winds, which can blow
one power line into another or blow tree limbs into the lines
causing arcing. Accidents and human error also account for
externally generated transients.
The main culprits of internal sources are device switching,
static discharge, and arcing. Each time you turn on, turn off,
load, or unload an inductive device, transient is produced.
Inductive devices are those devices that use "magnetic mass" to
function. Examples of inductive loads are motors and
transformers. Static electricity (also called "electrostatic
discharge")can generate up to 40,000 volts.This type of hazard
is very dependent upon environmental conditions and areas
with lower humidity have the worst problems. Arcing can
generate transients from a number of sources. Faulty contacts
in breakers, switches, and contactors can produce an arc when
voltage jumps the gap. [3]
III. TRANSIENTS IN SOLAR AND WIND SYSTEMS
Renewable energy generation installations have experienced
various design and performance issues that have not always
been properly addressed during the development ofthe projects.
These issues have included transient overvoltages (TOVs) and
dynamic overvoltages (DOVs). These overvoltages are very
high frequency and short term lasting from less than one cycle
to several cycles. Generally their impact is within the first 30
milliseconds of the event that initiates the transients, and they
are quickly damped in the system. TOVs can be caused by the
following:
 Lightning strikes
 Transmission line switching
 Shunt capacitor bank switching
 Abnormal switching events
 Transformer and reactor switching
Lightning overvoltages on transmission systems result from
three possible causes in order of increasing severity:
− Induced voltages
− Shielding failures
− Backflashes
These three events only happen on overhead transmission or
distribution lines. Since most collector systems for renewable
energy generation are cable systems, lightning has not been a
major problem. Renewable projects are usually connected to
overhead transmission lines through a power transformer in a
substation, but if the substation has proper surge protection
these surges do not results in any significant TOVs on the
collector systems.
The switching or energizing of the collector system cable
feeders at wind and solar generating installations can result in
significant transient overvoltages on the system.
The shunt capacitor banks on the higher voltage system can
cause voltage amplification on the renewable energy collector
system. For example, the switching of a shunt capacitor bank
on a 115-kV bus that connects to the collectorsystemcan result
in the amplification of transient voltages on lower voltage
systems that also have capacitance.The collector systemcables
provide significant capacitance on the 34.5-kV system. This
amplification has been known to result in very high transient
overvoltages.
There are many different systemdisturbances that can result
in dynamic overvoltages (DOVs), which are high 60 Hz
voltages on a system. As defined, DOVs are typically
fundamental frequency overvoltages. They can occur within a
cycle, but they may remain on the system until some other
action is taken. They normally result when an event leaves
capacitance on the system that provides more reactive power
than the systemcan use in its changed configuration. DOVs do
not have as high peak voltages as TOVs, but their duration can
result in equipment damage and failure if they are not reduced.
In some case, DOVs reach the protective levels of the arresters
which can quickly cause the arresters to fail. Some of the typical
causes of DOVs are:
− Transmission line and cable tripping
− Load rejection or interruption
− Isolating shunt capacitors on a weak system
− Open-ended lines and cables
There are several transient mitigation methods available in
the market. Using surge arresters can reduce the TOV caused
by transmission line or cable switching to more acceptable
levels. Synchronized closing or pre-insertion resistors can
minimize the TOV caused by shunt capacitor bank switching,
and current limiting reactors can help minimize the outrush
current during a fault. [7] The one method to mitigate DOV is
to have a fast grounding switch to close and ground each phase
immediately after opening the collector feeder.
There is another transient that needs to be taken under
consideration for the design and performance issues ofthe wind
turbines. A significant amount of work has been done over the
past few years to showthat transient loads,particularly transient
torque reversal loads, are occurring more often than expected.
Transient loads are sudden changes in the magnitude and/orthe
direction of a torque load. Field recordings of torsional loading
in drive systems ofmany different turbine models showthat the
worst torsional vibrations and the worst torque reversals
generally occur during transient events, such as emergency
stops and other hard tripping faults or stops.
Figure 11 shows how aerodynamic braking on a modern
wind turbine can cause this torsional windup in the drivetrain.
As the blades pitch, the systemsees rapid

4. Figure 11 – Aerodynamic braking on a wind turbine.
deceleration of the rotor, continuing through the gearbox to
decelerate the generator as well. As seen in Figure 11, the red
torque trace shows that torsional reversals can be as high as 75
percent of rated torque (in the negative direction). This can
cause significant excitement of vibrational energy as well. Each
spike on the curve is a torsional reversal whose energy must be
dissipated, causing impact loads on bearings throughout the
turbine.
In harder and more critical stops, the caliper disc brake
engages as well. As seen in Figure12a), the braking begins as
aero-only braking in the negative direction and then becomes
positive as the mechanical brake engages.This causes extreme
oscillation of the torque loads. These load swings can be highly
damaging as the bearings are loaded significantly in forward
and reverse in an alternating fashion. Even worse for the turbine
is when the turbine stops since there is a consistentfinalreversal
to the system. This is a high-magnitude torque spike occurring
on these stationary components. These torsional and impact
loads can cause potentially significant damage to the surfaces
of all the bearings, from main shaft to gearbox and even in the
generator.
It is not just stopping or tripping modes that can cause these
events. Field measurements have captured events that were
never recorded by SCADA systems. The torque reversal
(fig.12b) was captured by field monitoring of a wind turbine,
Figure 12 – a) Critical stopof a windturbine andb) Torque reversal ofa wind
turbine.
but SCADA never recorded the anomaly caused by wind
variation. The turbine continued to operate without a fault code
being recorded, despite the fact a significant negative torque
spike had occurred in all of the components throughout the
system. [8]
IV. TRANSIENT STABILITY
Transient stability is a major concern in power system
security and reliability because it is the most common type of
instability and its impacts can cause greatest economic losses.
Generally speaking, transient stability refers to the synchronism
of generators rotor angles in the power system. The result of
transient stability assessment is used for preventing the
occurrence of instability and correcting the potentialdangerous
scenarios to enhance the reliability. [9] Transient stability
analysis answers the question “Does the system return to an
acceptable condition within the first minute of being perturbed
by a generation or transmission line outage?” Figure 13 shows
a widely used visualization of the problem, updated to include
wind and photovoltaic (PV) power. The round masses represent
generators, with the tension on the various springy lines
representing power transfer. The board at the top represents the
academic fiction of an infinite bus—a real, finite power system
is floating. The level at which it is floating is a proxy for
frequency, which must stay very close to 60 Hz. The hands
represent wind and PV. They put tension (inject power) into the
system, but they are all control and no weight. The mission of
these devices, unless taught to do otherwise, is to pull
uniformly, regardless of whether or not the node to which they
are connected is moving. The scissors represent a disturbance,
which might cut a line or disconnect a generator. The rubbery
mass-spring systembounces around. If the event is too severe
or some of the lines are stretched too taught (too much loading),
more lines will break. It is easy to imagine a cascading failure
in which each successive break leads to another failure. A
substantial part of systemplanning is aimed at avoiding such
unacceptable consequences.
Transient stability is dominated by the dynamic behavior of
the essential elements of the power system during the first
minute following a systemdisturbance.The primary concern is
Figure 13 – Visualization of transient stability problema)
b)

5. that the power systemreturn to a near equilibrium state that is
acceptable to customers and equipment. [10]
The main aspects having possible impact on transient
stability issues in power systems are:
1. Wind resources are usually at different locations than
conventional power stations. Hence, power flows are
considerable different in the presence of a high amount of wind
power and power systems are typically not optimized for wind
power transport. This aspect can be more or less severe in
different countries.
2. Wind generators are usually based on different generator
technologies than conventional synchronous generators.
3. Wind generators are usually connected to lower voltage
levels than conventional power stations. Most wind farms are
connected to sub-transmission (e.g.110 kV, 66 kV) or even to
distribution levels (e.g.20 kV, 10 kV) and not directly to
transmission levels (>110 kV) via big step-up transformers as
in case of conventional power stations.
4. Other aspects, especially the fluctuating nature of wind
power have not been relevant to transient stability problems
because wind speed variations are too slow compared to the
time frame relevant to transient stability (one to ten seconds).
However, because of limited predictability of wind speed,
systems with high amount of wind power usually require higher
spinning reserve than conventionalpower systems,which adds
inertia to the systemthat has influence on transient stability. In
this sense wind fluctuations are having an indirect influence on
transient stability issues.
A large number of wind farms connected to the grid,
especially at the end grid, will greatly change the power
distribution and transmission lines loading. In addition, the
wind turbine itself is different from the traditional synchronous
generator with respect to dynamic characteristics; therefore the
systemtransient stability needs to be enhanced.
Existing Techniques for Transient Stability Improvement:
1. Using doubly-fed induction wind generator (DFIWG)
can improve transient stability. At present, large and medium
sized wind power generation systems mostly use asynchronous
generatorunit, in order to capture maximum wind energy under
various speeds. When wind turbine runs with variable speed,
the generator connected to it should be with variable speed
operation, outputting constant voltage and constant frequency
power to the grid. Wound rotor doubly-fed induction generator
is combined with the latest insulated-gate bipolar transistor
(IGBT) inverter technology and pulse-width modulation
(PWM) controller. The AC excitation variable speed constant
frequency (VSCF) wind turbine using doubly fed induction
generator connected to the grid is the optimal one of all
programs. DFIWG is on the basis of ordinary wound induction
generatorin addition a converterconnected in between the rotor
slip ring, the statorand its control system. The generatoroutput
power supplied to the grid is composed of two parts: i.e. the
output powerfrom the statordirectly and the output powerfrom
the rotor through inverter. DIFWG stator is connected to the
grid; the rotor is excited through a converter supplying three
phases slip frequency current, as shown in-Fig.14. When the
Figure 14 - Variable speed wind turbine DFIG connected to the grid
wind speed slowed, the generator speed is less than
synchronous speed of stator rotating magnetic field, the
generator is in sub-synchronous operation, at this time the
converter provides AC excitation to the generator rotor, the
stator sends electric energy to the grid; when the wind speed
increased, the generator speed is greater than synchronous
speed, the generator is in super-synchronous operation, at this
time the generator stator and rotor send electric energy to the
grid at the same time, the energy of converter reversely flows.
When the generator speed is equal to synchronous speed, the
generatoris in synchronous operation,at this time the generator
is running as a synchronous machine, the converter provides
DC excitation to the rotor. It can be seen when the generator
speed changes, if the rotor current frequency corresponding
change is controlled, it will make the stator current frequency
remain constant, and is consistent with the grid frequency, so
VSCF control can be achieved. [11]
2. The SCESS systemconsists ofa supercapacitorand a
DC/DC converter that controls the flow of current from the
supercapacitor. Supercapacitors were introduced since 1960’s,
but the interest has grown recently about utilizing them as an
energy source for static compensators (STATCOMs).
Supercapacitors or ultracapacitors are electrochemical double
layer capacitors with very high capacitance. They have very
large surface area which makes their capacitance much higher
than conventional capacitors. Their power rating is also much
higher than a conventional battery because they can release
energy quickly, while the chemical process in batteries makes
them slower in releasing energy. Ultracapacitors are highly
temperature and vibration resistive. They have a high discharge
cycle, and they have the ability to provide or absorb high
amount of power and at the same time can compensate system
harmonics, suppress voltage fluctuations and flicker
fluctuations. [12]
3. Static compensator (STATCOM) control program
can control reactive power. The basic principle of operation for
the STATCOM is to compare the voltage in the systemand the
terminal voltage on the voltage source converter (VSC), and
control the phase angle and amplitude on the voltage drop over
the transformer inductance. Figure 15 shows STATCOM
connected to a transmission line. When the voltage in the
electric systemis lower than the terminal voltage on the VSC,
the STATCOM will generate reactive power, the STATCOM
works in capacitive mode. If the voltage on the VSC is lower
than the voltage in the electric

6. Figure 15 – STATCOM connected to a transmission line. [14]
network, the STATCOM will absorb reactive power, the
STATCOM works in inductive mode. If the voltage of the
electric network and the terminal voltage on the VSC are equal,
there will not be any reactive flow in the STATCOM. [13]
4 Static compensatorwith battery energy storage system
(STATCOM/BESS) can control both active power and reactive
power. During sudden severe disturbances, such as line sag or
other fault situations, turbine would transmit less power to the
grid. However, due to the imbalance created between the
mechanical and electrical power, the speed ofthe turbine would
eventually increase. This increase in speed of the induction
generator would result in consumption of more reactive power,
which causes the voltage to dip. In such situations,stability can
only be retained if the increased generator speed is below the
prescribed critical speed limit. Thus, in order to sustain stable
operation in the event of such fault conditions of the wind farm,
reactive power must be supplied externally. The benefit of
using a battery in parallel to the wind turbine is that it gives the
chance to produce always as much power as possible and store
the energy that cannot be injected to the grid. A battery
connected to the STATCOM can be the best solution to
maximize the power that can be injected in a weak network in
a distributed generator (DG) system (fig.16). STATCOM +
BESS unit can be applied to load leveling, saving energy at peak
demand, minimizing subsynchronous oscillations, enhancing
transient and dynamic stability. [14]
Figure 16 – STATCOM+BESS connected to the wind power generator
V. CONCLUSIONS
As more renewable energy sources come online, they will
take on greater and greater importance in the overall generation
mix. It is therefore vital to build our understanding of the
engineering challenges these installations present.In this paper,
we have outlined some of the most common sources of system
disturbances and equipment failure in renewable facilities:
overvoltages. Concerns about transient stability and other
dynamic performance impacts of wind generation are often
overstated and, in the possible cases where they occur, can be
mitigated by a spectrum of traditional and nontraditional, but
commercially available, technologies.
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