Download Series Compensation on Medium Voltage Radial Systems

23rd International Conference on Electricity Distribution
Lyon, 15-18 June 2015
Paper 0817
SERIES COMPENSATION ON MEDIUM VOLTAGE RADIAL SYSTEMS
Carlos FIGUEIREDO
AES Sul – Brazil
[email protected]
Mauro SILVEIRA
AES Sul – Brazil
[email protected]
Nelson DE JESUS
GSI – Brazil
[email protected]
Gilnei SANTOS
AES Sul – Brazil
[email protected]
Luciano QUADROS
AES Sul – Brazil
[email protected]
ABSTRACT
This paper presents the general aspects related to the use
and application of series compensation on medium
voltage radial systems. After the definition of basic
concepts, are discussed the main benefits and also the
performance and operational problems of this
compensation technique as well as the characteristics of
its components. Finally, a recent system installed in a
distribution feeder is used as an example of such device.
INTRODUCTION
The philosophy of series compensation in high and extra
high voltage transmission systems is well known and its
purpose is to increase the capacity of transmission lines,
improving the stability in transient and steady states as
well as the power flow control through electronic devices
that allow varying the capacitive reactance according
operational request. The reliability of these systems is
proved by several series compensators in operation in
transmission lines all over the world.
The use of series compensation in distribution systems
has also presented a number of benefits and many
projects confirm the technical and economic advantages
over conventional methods of voltage control, which use
automatic voltage regulators and shunt capacitors.
However, this practice is not widespread and there are
few such facilities in operation.
In radial distribution systems the main objective of series
compensation is to reduce the voltage drop and improve
the voltage regulation. At the same time, other benefits
are obtained from the application of this kind of
equipment such as the support for motor starting and
reduction of voltage flickering.
Despite the advantages of series compensation in radial
distribution systems, its adverse effects are also well
known and among them subsynchronous oscillations that
will be also discussed along this paper.
Figure 1 – Series compensation in radial system
To better understand this technique, first figure 2 presents
a vector diagram of a system without the SCB,
considering a lagging power factor load.
Figure 2 – Vector diagram of a system without series compensation
From the phasor diagram of figure 2 it is possible to
calculate the voltage drop in each phase, approximately
( ∂ < 30° ), by the following equation:
∆V ≅ RI L cos ϕ + X L I L senϕ ≅
R.PR + QR X L
VR
(1)
Then, after the SCB insertion, the equivalent reactance
can be described by:
Xeq = X L − X C
(2)
Replacing XL from the equation (2) by Xeq in (1) it is
obtained:
R.PR + QR X eq
(3)
1. SERIES COMPENSATION CONCEPT
∆V ≅
The fundamental concept of series compensation is quite
simple, based on the reduction of the inductive reactance
of the line (XL) by inserting a capacitor bank in series
(SCB). The figure 1 shows a basic configuration for the
series compensation analysis in radial systems.
So, from the equation (3) it is possible to conclude that:
If XL=XC, then Xeq=0 and, consequently, the parcel
QR.Xeq = 0. In this case it is said that series compensation
reached 100%. The voltage variation (∆V) depends just
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VR
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23rd International Conference on Electricity Distribution
Lyon, 15-18 June 2015
Paper 0817
on the parcel R.PR.
If XC>>XL turning the ratio QR.Xeq equal to R.PR the
VS = 1 + ∆V
(9)
.
voltage variation is zero. In this situation V R modulus
will be equal to VS absolute value.
Figure 3 shows two phasor diagrams. In one of them the
SCB reactance is lower than the line parameter and the
other one the SCB reactance is higher.
Figure 3 – Phasor Diagram with series compensation.
From the figure 3, with the inclusion of SCB, it is
possible to obtain the following phasor relation between
VR and VS voltages:
V&S = V&R + [R + j ( X L − X C )]I&L
(4)
Multiplying the equation (4) by the conjugate of complex
current, it results in a relation in terms of power, as it
follows:
(5)
P + jQ = P + jQ + [R + j( X − X )]I 2
S
S
R
R
L
C
L
The power of SCB depends on the system current and the
required capacitive reactance. By using a three-phase
circuit power ( S = 3VL I L ) and, normalizing with respect
to the phase voltage, it is possible to have:
3VS
3VR
3RI L
3X L I L
3X C I L
=
+
+j
−j
VL
VL
VL
VL
VL
(6)
The result is interesting because it shows the system
component values per unity (p.u.). They are exactly equal
if referred to the voltage or base power [1]. The capacitor
bank definition depends on the acceptable regulation
level of the load power factor and the ratio between the
circuit elements (R e XL). It is important to note that
equation (6) is only valid for radial circuits as it was
shown in the figure 1. If the voltage load is assumed as
1,0 p.u. after the manipulation of previous equation with
the current phasor, it results:
XC = X L − senϕ − VS2 + sen2ϕ − (1+ 2R cosϕ + R2 )
(7)
Due to the dependence, in many cases it is suitable solve
the equation (7) for the system XL/R ratio, by defining the
following relations [1]:
h=
XL
R
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Finally replacing (8) and (9) in (7):
 2X
X2 
XC = XL −senϕ − (1+∆V)2 +sen2ϕ −1+ L cosϕ + 2L 
h 
 h
(10)
From what was exposed so far, it is possible to conclude
that the inter-relation between circuit elements (R e XL),
the power amount drained by the load in the SCB
location point, and the admissible undervoltage in the
system results in the main set of parameters required to
evaluate the performance of the series compensator. From
the equation (1) it is possible to verify that higher is the
system inductive reactance (XL), lower will be the
necessity of compensation to obtain a satisfactory result.
This fact is one of the motives for the large use of series
compensation in high and extra high voltage, as in these
systems the ratio XL/R is usually higher than one. Another
possible conclusion from equation (1) concerns the
amount of reactive power provided to the load, seen from
SCB location point. From the parcel QR.Xeq it observes
that higher the flowing reactive power through the series
capacitor also will be higher its voltage gain. In other
words, lower the power factor at the application point of
the series compensator, better it will perform [2].
In radial distribution systems, where it has a high
resistive component, it is advisable that the power factor
in the series compensator should be the lowest possible.
Thus it is recommended to remove all shunt capacitors
installed downstream the point of series compensator
application [2].
2.
APLICATION
SYSTEMS
ON
DISTRIBUTION
On distribution systems, series capacitor application is
more attractive to supply remote loads connected to long
feeders in systems with low short circuit and power factor
between 0.8 and 0.95. From the previous analysis, the
performance depends on the system characteristics (XL/R
ratio), the compensation level and the power factor in the
installation point. Consider the follow series
compensation system to analyze the distribution system
behavior. In the figure 4 the loads are distributed along
the feeder.
(8)
Figure 4 – Typical distribution system with series capacitor
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Paper 0817
In radial distribution systems it became appropriate to
define the compensation level (KSC) as follows [1]:
K SC =
XC
XP
(11)
Where:
XC – Capacitive reactance of series capacitor (Ω);
XP – Reactance until the series capacitor BSC = XCC+XT
+XL1+XL2+XLN (Ω).
According to past experiences at AES Sul, the maximum
compensation is usually 150%, whereas higher values
may increase the probability of possible adverse
phenomena, which will be presented later.
2.1 Improvement of voltage profile
Application of series compensation differs in terms of
reactive power. Important to note that while shunt
capacitors the power is voltage dependent, the series
capacitor is dependent on the current, being proportional
to the square of its absolute value. In addition, responses
to rapid load variations are totally distinct from switched
shunts banks with significant superior time response.
The major goal of using series compensation is to
improve the profile and system voltage regulation. Below
it has been estimated in a simplified way a distribution
feeder with series compensation, located at feeder 3 from
the Jacuí AES Sul’s substation. In this case an automatic
voltage regulator (32 steps) was replaced by a series
capacitor with a reactance of –j 35 Ω. Thus, the degree of
compensation is equivalent to 180%. After
commissioning and operation for over ten years, it was
not observed any events related to flickering or problems
encountered throughout history with SCB.
2.2 Starting of motors
The reactive power required by motors during starting is
much higher than under steady state and it has very low
power factor, typically between 0.3 and 0.6. The voltage
drop during the starting of high power motors can result
in problems for other customers, or even influence the
motor itself during the acceleration process. With the
addition of series capacitor the voltage drop caused by
high reactive current can be compensated, thus providing
adequate support to the starting of the electric motor.
2.3 Flickering reduction
In the event of voltage flickering due to heavy variation
in loads, the series compensation provided will result in
the improvement of power quality. This characteristic
may be checked by consulting the phasor diagram of
figure 3 and using equation (1). It is important to note
that the improvement in voltage regulation by series
compensation is continuous and instantaneous. The series
compensation is more effective in the reduction of
flickering due to reactive power of the load peaks.
3. PROBLEMS RELATED
COMPENSATION
TO
SERIES
Despite the inherent advantages of the series
compensation, the side effects can be very severe, which
requires a detailed analysis to the final specification of
the project. So it is recommended simulations using
electromagnetic transient programs like ATP (Alternative
Transient Program) to this definition. Below it is briefly
discussed about the main problems that must be evaluated
mainly when overcompensation series is required.
3.1 Steel resonance
Figure 5 - System with series capacitor (Feeder 03 – Jacui Substation)
By using power flow simulation software, where it was
considered a demand of 5 MVA and PF = 0.9, it was
obtained the voltage profile, what allowed to compare the
both performance of voltage regulator and series
capacitor, as it is presented on figure 8.
Figure 6 – Voltage profile (Feeder 03 – Jacuí Substation)
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When an unloaded transformer is energized, the inrush
current remains temporarily higher than the rated current.
This condition will depend on the transformer core and
system, whereas the maximum values occur when the
energization starts next to the zero point of the voltage
wave (excluding the residual flux). In this case, there is
predominantly low order even harmonics, with strong
saturation of the magnetic field. If, however, the
transformer is supplied by a circuit with series capacitor,
a transient or steady nonlinear resonance condition may
occur. Sub-harmonics and oscillatory currents can be
established. This phenomenon is called steel resonance
and causes severe surges.
When the steel resonance occurs, the solution can be the
insertion of a damping resistor in parallel to the series
compensator, usually controlled by an underfrequency
relay. Figure 9 shows the results of steel resonance
simulations, with the magnetizing current and the voltage
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23rd International Conference on Electricity Distribution
Lyon, 15-18 June 2015
Paper 0817
of one series capacitor phase. This case concerns the
energization of a no-load transformer of 500 kVA in a
200% overcompensated system (KSC = 200%).
Figure 7 - Current and voltage during steel resonance
3.2 Self-excitation of induction motors
The combination of the series capacitor and the inductive
reactance results in a series resonant circuit. The natural
frequency in the compensated electrical system is usually
subsynchronous, with values of 10 to 90% of the
fundamental frequency, depending on the series capacitor
reactance. For inductive loads such as induction motors,
the inductance of the motor is an element of the equation
that determines the overall inductive reactance. The
existence of subsynchronous frequency has presented
problems in starting motors under certain conditions.
During starting, the rotor can stabilize and further
develop a speed below the rated, linked to the
subsynchronous oscillation frequency with high sustained
currents. In those circumstances the motor may be
damaged due to excessive vibration and overheating. The
self-excitation is more likely when the rated power of the
motors exceed 5% the system. The presence of active
load in parallel with the engine tends to reduce the
problem. However, even in this case, the possibility of
self-excitation should be evaluated in the project if the
minimum active demand downstream the series capacitor
is less than 10% of the motor rated power.
The figures below respectively show the speed and
current obtained by simulations for a case of selfexcitation of a 200 HP engine in an overcompensated
distribution system [3].
distances and intensive use of electricity for rice
irrigation. Figure 15 shows a simplified diagram of feeder
42 from Uruguaiana 4 substation with the SCB location.
Before applying the series compensator, the system
already had installed three voltage regulators and two
shunt capacitors 600 kvar each. Even with these devices
it used to have poor voltage level and system stability, as
the furthest point from the substation is the Ibicuí river
that serves as the main water abstraction location for the
rice plantation and therefore where are the located the
most powerful engines for pumping water (200 HP and
400 HP rated power).
Usually these motors have soft-starters that control the
voltage during the engine starting, minimizing the inrush
effects. Even so, without series compensation, the
extreme point of the feeder sometimes reached less than
0.85 pu from the rated voltage, caused by inrush current.
In such situation the driven motor could not start.
More than just shut off the pump, this event used to bring
high overvoltages to the system, in the order of 1.4 p.u. as
a consequence of load rejection and also the presence of
large number of voltage regulators which didn’t have
enough response speed to remain the voltage in the
appropriate values before results in damage to the
electrical customer devices.
Figure 9 shows a simplified diagram of feeder 42 from
Uruguaiana 4 substation with the SCB location.
Figure 9 – Simplified diagram – Feeder 42 Uruguaiana 4 substation
From this figure it is possible to note that the SCB did not
replace any other voltage regulator equipment but as a
complement to the current devices.
Figure 10 presents series compensator bank diagram
while figure 11 shows an overview of the physical
structure implemented in the site.
2.000,0
Corrente (A)
400,0
RPM
300,0
200,0
100,0
0,0
-2.000,0
0,0
0,0
1.000,0
Tempo (ms)
2.000,0
0,0
1.000,0
Tempo (ms)
2.000,0
Figure 8- Speed and current with self-excitation
4. EXAMPLE OF SCB IN OPERATION
As an example of a series compensation system in
operation, it is presented a bank installed in one of the
AES Sul’s distribution feeders operating in 23.1 kV and
located in a rural area of Alegrete city near the border of
Brazil and Argentina. This region has large electrical
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Figure10 – Electrical diagram of the series compensator
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23rd International Conference on Electricity Distribution
Lyon, 15-18 June 2015
Paper 0817
5. CONCLUSION
Figure11 – Series compensator bank
In the chosen SCB application point, whose reactance
was –j35Ω, the compensation level (KSC) was around
120%.
This system was connected in normal operation
conditions bringing significant voltage stability gain.
During the motor starting the voltage drop was just 0.95
p.u. allowing the engine starts without it shuts off or even
disconnect the motors in operation nearby. So the SCB
solved the load rejection and its resulting overvoltages.
As it is possible to check in the figure 12, the current on
25/01/2012 (red line) shows a number of attempts to start
the pump motor early in the morning as well as
throughout the day in the feeder 42. This represented
disturbances to the system and the customer himself that
failed starting the engine. On 27/01/2012 (blue), after the
installation of series compensation in the feeder, it had
system stabilization successfully solving the problems.
180,0
160,0
This paper discussed about the application of series
compensation on medium voltage radial systems and
presented a case where the instabilities were completly
solved by the SCB application. Emphasizes that for
subtransmission systems (from 69kV until 138kV) the
analysis is similar. Although the experiences and studies
of series compensation in distribution feeders had origin
in decade of 20’s, where the first systems were
implemented, it still demands refined enginnering
evaluation. This is made by the simulation tools that are
fundamental for the projec succes. It is important to note
that possible problems must be checked and studied
during the project phase, avoiding customer and utility
losses. However because of the power factor correction,
the SCB tends to have limited application, aggravated by
the high resistance of distribution systems, which
generates the need for overcompensation to arrange the
voltage drops. This is a key question, as proved by the
simplified analitic analysis related to this factor being
recomended analysis to the range between 0.8 and 0.95.
Besides the side effects, ie steel resonance and self
excitation, the the greatest challange is to find a standard
for tipical distribution systems, as the case of voltage
regulators. In dynamical systems, joint analysis of
regulation equipment should be checked, in order to
adjust and improve power quality. Anyway, the
technical-economic analysis must complete the process of
propositions for possible applications of SCB in
distribution systems.
REFERENCES
[1] S. A. Miske, 2001, Considerations for the
Application of Series Capacitor to Radial Power
Distribution Circuits, IEEE Trans. On Power
Delivery, Vol. 16, No.2, pp. 306-318.
[2] Figueiredo, C. E. C., 2004, Aplicação de
Compensação Série para Controle de Tensão em
Sistemas Radiais de Distribuição de Energia
Elétrica, Porto Alegre, Brazil.
140,0
[3] H. R. P. M. Oliveira, C. E. C Figueiredo, N. C. Jesus,
2002, “Resultados Obtidos na Operação de Sistemas
de Compensação Série em Alimentadores de Média
Tensão da AES Sul”, Anais do XV SENDI Seminário Nacional de Distribuição de Energia
Elétrica, Salvador/BA.
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Figure 12 – Feeder current before and after the series compensation
[4] P. M. Anderson, R. G. Farmer, 1996, Series
Compensation of Power Systems, California, USA.
This result indicates a convenient SCB application after
all detailed studies for transient and steady states, mainly
if compared to the previous operation system just with
voltage regulators and shunt capacitors.
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