Scientific Paper For Noah Olela Abongo BSc. In Electrical and Electronic Engineering
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The Adoptability of Capacitor – Coupled Substation (CCS) on a Distribution System as Opposed to a
Distribution Transformer Noah Olela Abong’o Michael J. Saulo Kenneth Mukhaya
Mombasa Polytechnic University College Mombasa Polytechnic University College Mombasa Polytechnic University College (A Constituent College of JKUAT) (A Constituent College of JKUAT) (A Constituent College of JKUAT)
[email protected]@kpa.co.ke [email protected]@yahoo.com [email protected]@yahoo.com
Abstract –The capacitive divider technique has been known for quite a while. With this technique, energy is drawn from the electric field of the high voltage transmission line by use of discrete capacitors. However, the main disadvantage of capacitive voltage substations is their transient behavior. Ferroresonance and overvoltage transient problems could occur in these substations during different conditions. Conversely, distribution transformers are pole-type transformers that supply relatively small amounts of power to residences. They are used at the end of an electrical utility’s delivery system. For distribution transformers with single-phase primary configuration, size and cost increases with the number of leads. Three phase distribution transformers are connected in delta or star configurations. Distribution transformers may be removed from service for many reasons, including failure, over-loading or under-loading, power lines upgrades due to voltage changes or rerouting.
Keywords--Electromagnetic coupling, Capacitive-divider Circuit, Ferroresonance, Distribution Transformer, SimPowerSystems®
I. INTRODUCTION
The provision of electric energy to many remote and / or rural people and communities is still today a big challenge for many developing countries, Kenya included. It is common knowledge that the provision of electricity, with whichever means, is one of the greatest enablers that allows the utilization of modern labour saving electric products and appliances, not to mention the basic electricity requirements for lighting and water pumping.
I.1 Power System Overview
The main purpose of power systems is to generate, transmit, and distribute electric energy to customers without interruptions and in the most economical and safe manner. To achieve these objectives, power systems are divided in generation, transmission and distribution subsystems. The distribution consists of supplying energy to customers at a convenient voltage level. [12].
I.2 Objectives of the Research
The main objectives of this research project are to:
1) Identify technologies which may lead to a greater efficiency in the bulk power system distribution, the existing unexploited power system distribution utilization and a reduction in system losses that would otherwise flow to the end user. One such technology which this research work focused on is the conventional power distribution transformer system being compared with the possibility of electrification through the adoption of capacitor – coupled substations. This is done with a view of improving electricity distribution at affordable costs for rural population within the existing Kenya high voltage infrastructure systems.
2) Compare the adoptability of capacitor – coupled substation as opposed to the currently utilized conventional distribution transformer for the same locality.
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The specific objectives of the research work are to determine the capacitor – coupled substation / conventional transformer performance with respect to given load in terms of:
1. Power Stability considerations.2. Efficiency and Cost of Power Supply Systems under
research.3. Undervoltage and Overvoltage Effects.4. Pilot projects using SimPowerSystems Simulations.5. Limitations and Assumptions made on Power
Distribution Systems.
II. RESEARCH METHODOLOGY
According to Jennifer M. Case and Gregory Light in Emerging Methodologies in Engineering Education Research (2011), “Methodology refers to the theoretical arguments that the researcher uses to justify the research methods and project design chosen.
II.1 Data Collection Tools
SimPowerSystems® is one of the main software for modeling and simulating electric power systems in the Simulink®
environment. SimPowerSystems® seamlessly incorporates Simulink multidomain block libraries. The SimPowerSystems® library contains more than 150 blocks distributed in sublibraries.
II.2 The power system stability of capacitor – coupled systems compared to distribution transformer
The stability of a power system is its ability to develop restoring forces equal to or greater than the disturbing forces to maintain the state of equilibrium. Power system stability, therefore, is the ability of an electric power system, for a given initial operating conditions, to regain a state of operating equilibrium after being subjected to physical disturbance, with most variables bounded so that practically the entire system remains intact.
III. RESULTS AND DATA ANALYSIS
In order for any given power system to be efficient the voltage and frequency of supplied power must be stable. Power stability classification is important due to the following reasons:
1. Stability analysis is easier and leads to proper and effective understanding of different power system instabilities.
2. Key factors that lead to instability can easily be identified.
3. Methods can be devised for improving power system stability.
III.1 Voltage stability – definition and concepts
Voltage stability refers to the ability of a power system to maintain steady voltages at all buses in the system after being subjected to a disturbance from a given initial operating condition. Voltage instability results in progressive fall or rise of voltages of some buses. Large scale effect of voltage instability leads to voltage collapse. The voltage stability, sometimes called load stability, however, is now a major concern in planning and operating electric power system. Voltage instability and collapse have in the past resulted in several major system failures or blackouts. [3]
III.2 Frequency stability
Frequency stability refers to the ability of a power system to maintain steady frequency following a severe system upset resulting in a significant imbalance between generation and load. The initiation of ferroresonance phenomena can cause distorted over-voltages and over-currents to be induced into a system. There are generally two main methods of preventing the occurrence of ferroresonance.
a) Avoid any switching operations that will reconfigure a circuit into a sudden inclusion of capacitance connected in series with transformer with no or light load condition.
b) Provide damping of ferroresonance by introducing losses (i.e. load resistance) into the affected transformer. [15]
III.3 Overvoltage of Capacitor – Coupled Substations and the Distribution Transformers.
Switching of capacitive circuits and banks produces overvoltage that can be harmful to the power system. The transient overvoltages may cause any of the following:
a) Interference in the control circuit and wiring of substations.
b) Undesired tripping or damage to sensitive electronic equipment.
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c) Conductors insulation degradation and possible failure of substation equipment
d) Operation of surge arrestors.Several of the means available to reduce or limit the overvoltages generated by the switching of capacitor circuits / banks are outlined below:
a) Current-limiting devices are used to reduce the current transients.
b) Pre-insertion resistors limit the inrush current and remote overvoltages.
c) Controlled switching of circuit breaker means the opening and / or closing of the circuit contacts at certain points on the waveform, by selecting arcing times longs enough to avoid re-ignitions during circuit breaker de-energizing time.
3.1 Pilot Projects on Distribution Transformers and Capacitor – Coupled Substations Conventional for the same locality
powergui
Discrete,Ts = 5e-005 s
V-I M2VabcIabc
A
B
C
abc
V-I M1VabcIabc
A
B
C
abc
Three-Phase Source
A
B
C
Three-PhaseTransformer
(Two Windings)
A
B
C
a
b
c
Three-PhaseSeries RLC Load
A B C
Three-PhasePI Section Line
A
B
C
A
B
C
Scope 4
Scope 3
Scope 2
Scope 1
Scope
Isubc
Isubb
Isuba
Isc
i+ -
Isb
i+ -
Isa
i+ -
Ipc
i+ -
Ipb
i+ -
Ipa
i+ -
Gain 2
0.5
Gain 1
0.5
Gain
0.5
Fig. 3.51 Normal Condition distribution transformer simulation
The above circuit was used to simulate normal power system for different voltages angles and loads as shown in Table 3.51 below.
Table 3.51 Nominal condition transformer measurements
255.66 V -0.22° ---> V-I M1/Va 255.66 V -120.22° ---> V-I M1/Vb 255.66 V 119.78° ---> V-I M1/Vc 20343.40 V -0.17° ---> V-I M2/Va 20343.40 V -120.17° ---> V-I M2/Vb 20343.40 V 119.83° ---> V-I M2/Vc 6.88 A 107.75° ---> V-I M1/Ia 6.88 A -12.25° ---> V-I M1/Ib 6.88 A -132.25° ---> V-I M1/Ic 14.66 A 139.81° ---> V-I M2/Ia 14.66 A 19.81° ---> V-I M2/Ib 14.66 A -100.19° ---> V-I M2/Ic 14.66 A -40.19° ---> Ip1 14.66 A -160.19° ---> Ip2 14.66 A 79.81° ---> Ip3 6.88 A -72.25° ---> Isa 6.88 A 167.75° ---> Isb 6.88 A 47.75° ---> Isc
Capacitive voltage transformer can be an alternate for supplying communities living within the vicinity of high voltage conductor lines. The main drawback is the high impedance introduced between the source and the load, which results in poor regulation for loads with large variations in demand.
powergui
Discrete,Ts = 5e-005 s
VM5
v+-
VM4
v+-
VM3
v+-
VM2
v+-
VM1
v+-
T1
1 2
Source Resistance
Scope 1
Scope 8
Scope 7
Scope 6
Scope 5
Scope 4
Scope 3
Scope 2
Practical Load
Pi Section Line
L2
Current Limiting Resistor
CM3i+ -
CM2
i+ -
CM1i+ -
C2
C1
Breaker
AC Voltage Source
Fig. 3.52 Capacitor– Coupled Circuit (Substation Model)
Figure 3.52 above was used to simulate the capacitive-coupled circuit as a prototype for the capacitor-coupled substation. Voltage measurements were carried out at the source, capacitive-voltage divider and at the load side as shown below.
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Table 3.52 Capacitor-Coupled circuit measurements
2017.93 V -79.78° ---> VM4 253.73 V -79.79° ---> VM3 116125.47 V -36.10° ---> VM2 63282.88 V -35.99° ---> VM1 132000.00 V 0.00° ---> VM5 2.54 A -80.00° ---> CM3 664.04 A 53.76° ---> CM2 795.24 A 54.01° ---> CM1
The simulation of capacitive divider circuit was achieved since the AC voltage supply of 132000.00 V 0.00° (VM5) was successfully divider using a capacitive circuit to 63282.88 V -35.99° (VM1) as can be seen from Table 3.52 above.
3.2 Assumptions made on the Power Distribution Systems under ComparisonA. Distribution Transformers Assumptions
The sharply increased cost of electrical energy has made it almost mandatory for buyers of electrical machinery to carefully evaluate the inherent losses of these items. [13]
B. Capacitive – Coupled Substation Assumptions
Capacitor – coupled substation uses a capacitive divider instead of an inductive transformer to reduce the voltage, and usually incorporates an air-core series inductor and filter circuits for good voltage control and to avoid ferroresonance.
3.3 The Power Distribution Costs and Efficiency Comparisons
Distribution Transformers represent a significant cost to electric utilities, both as a capital investment and as an ongoing operating expense. Unless a transformer first fails due to tank corrosion, the usual mode of failure is deterioration of its insulation capability. However, from voltage stability point of view, many distribution transformers operate with some degree of saturation, causing sufficiently large exciting current to flow at the rated voltage.
3.4 Overvoltage of Capacitor – Coupled Substations and the Distribution Transformers.
Switching of capacitive circuits and banks produces overvoltage that can be harmful to the power system.
Controlled switching is a method for eliminating harmful transients via time controlled switching operations.
3.5 Pilot Projects on Distribution Transformers and Capacitor – Coupled Substations Conventional for the same locality
Figure 3.52 below was employed in simulating the capacitive-coupled circuit as a prototype for the capacitor-coupled substation. Voltage measurements were carried out at the source, capacitive-voltage divider and voltage shown in Table 3.52 displayed.
powergui
Discrete,Ts = 5e-005 s
VM5
v+-
VM4
v+-
VM3
v+-
VM2
v+-
VM1
v+-
T1
1 2
Source Resistance
Scope 1
Scope 8
Scope 7
Scope 6
Scope 5
Scope 4
Scope 3
Scope 2
Practical Load
Pi Section Line
L2
Current Limiting Resistor
CM3i+ -
CM2
i+ -
CM1i+ -
C2
C1
Breaker
AC Voltage Source
Fig. 3.52 Capacitor– Coupled Circuit (Substation Model)
signals
powergui
Continuous
ic
i+ -
ib
i+ -
ia
i+ -
Va
v+- VA2
v+-
V -> pu
-K-V --> pu
-K-
node 992
node 991
Mux
Load10 MW 38 Mvar
A B C
Integrator
1s
Fourier
signalmagnitude
angle
Breaker3
Breaker2
Breaker1
1 MVA 220 kVEquivalent
N
A
B
C
0.15MVA220 -490 kV
Three -PhaseTransformer
A
B
C
a
b
c
Fourier
Iabc
Flux
Fig. 3.53 Three Phase Transformer Simulation Model
The above circuit shows a three-phase transformer with saturable core used for simulation. Both primary and secondary windings are connected in a star grounded configuration. Upon simulation the following surge voltage and inrush wave-forms were observed. The analysis of the Graph 4.10 and Graph 4.11 below are explained in details in the Data Analysis section.
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Graph 4.10 Simulated Three Phase Transformer with Saturated Core
Graph 4.11 Simulated Capacitive Inrush current
powergui
Continuous
Voltage Measurement
v+
-
Scope
RS L
RL Load
75 MW 20 Mvar
R L Current Measurement
i+
-
C2C1
Breaker
132 .8 KV50 Hz
Fig. 4.12 Simulated Capacitive Transformation Power System Model
3.5 Data Analysis
A. Surge voltages / Inrush current of Distribution Systems under study.
From the above discussion, it is apparent that transformers have the greatest exposure to electrical transients. The shape, magnitude and duration of the inrush current depends on several factors such as the type of electrical network employed, transformer construction, circuit breakers characteristics and the residual flux present in the circuit.
The Top waveform of Graph 4.11 shows the 4th harmonic variation of the phase a voltage. The middle waveform shows the inrush current of all the three phases which reduces with time. The bottom waveform is the variation of the flux with changes in the inrush current. The inrush current within the transformer is caused by saturation effects in the iron core of the transformer when it is energized, at the time the circuit breaker is closed. This simulation is of paramount importance since it identifies situations which can result in transient overvoltages and try to investigate the causes of transformer failures, suggesting the best ways to reduce transformer stress situations.
B. Analysis of Surge voltages / Inrush current of Capacitive-coupled circuits.
The simulation and measurements recorded indicate that the majority of surge voltages in low-voltage power systems have an oscillatory wave shape. This is because the voltage surge excites the natural resonant frequency of the wiring system. In addition to being typically oscillatory, the surges
can also have different amplitudes and wave-shapes in the various places of the wiring system as can be observed in Graph 4.11 above.
3.6 Assumptions made on the Power Distribution Systems under Comparison A. Distribution Transformers Assumptions
The sharply increased cost of electrical energy has made it almost mandatory for buyers of electrical machinery to carefully evaluate the inherent losses of these items. [13]The Top waveform of Graph 4.11 shows the 4th harmonic variation of the phase a voltage. The middle waveform shows the inrush current of all the three phases which reduces with time. The bottom waveform is the variation of the flux with changes in the inrush current. The inrush current within the transformer is caused by saturation effects
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in the iron core of the transformer when it is energized, at the time the circuit breaker is closed.
IV. CONCLUSION
Throughout this research work the capacitive divider circuit has been seen to possess several advantages over conventional methods of rural electrification. This conclusion was arrived at because the conventional electromagnetic step down transformer is not required, which effectively reduces the cost of Power Distribution, since a 630kVA Distribution Transformer like the one at Mombasa Polytechnic University College may cost as much as Kshs.1,300,000/=, going by the current market price.
REFERENCES
This clearly explains why rural electrification schemes are usually uneconomical, especially where distribution transformers have been employed due to the high capital and operating costs. Furthermore, the loads are scattered and are characterized by low demand and poor utilization factors.In conclusion, the researcher has come to a critical finding on why capacitor-coupled substations, despite their perceived advantages over the conventional distribution transformers, have not found a wider application and acceptance as an alternative to the distribution transformers as power distribution preference. Capacitor-coupled substation technique leads to important cost reductions when compared with implementation of conventional distribution transformers.
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[2] A.N.Zomers (NLD) and G.Dagbjartsson (CHE) The challenge of rural electrification CIGRÉ’s strategy and organizational approach. [2006].[3] C.L. Wadhwa Electrical Power Systems [6th Edition – 2010]
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Available at: www.jomitek.dk.[17] Prabha Kundur, John Paserba, “Definition and
Classification of Power System Stability”, IEEE Trans. on Power Systems. Vol. 19, No. 2, [2004]
[18] Siemens Energy Sector – Power Engineering Guide – Edition 7.0
[19] Swee Peng Ang [2010] Ferroresonance Studies of Transmission Systems.
[20] The Point – Bulletin of the Institute of Economic Affairs – Issue No.56 [2003] Available at: http://prr.hec.gov.pk/Chapters/543S-2.pdf
[21] Trevor Gaunt (South Africa) et al Paper on the “Innovative Solutions and Best Practices for Rural Electrification of Remote Areas”. [2007].
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