ANALYSIS OF ELECTROMAGNETIC FIELDS IN

U.P.B. Sci. Bull., Series C, Vol. 76, Iss. 1, 2014
ISSN 2286 – 3540
ANALYSIS OF ELECTROMAGNETIC FIELDS IN
INTERCONNECTING SUBSTATIONS BETWEEN WTG AND
NATIONAL POWER SYSTEM
Gabriela Nicoleta SAVA1, Sorina COSTINAŞ2
This paper deals with the analysis of electric and magnetic fields in highvoltage substations connecting renewable energy sources. The development of large
capacity parks based on renewable energy sources required new substations for
interconnection with the National Power System. These new substations have small
sizes and a reduced number of equipment in order to achieve a compact
construction. In addition, a concern regarding the impact of electric and magnetic
fields on environment and occupational health has appeared. The analysis of
electric and magnetic fields must be done in the design stage. The programs
developed based on MATLAB software provide a solution for the analysis of electric
and magnetic fields within electrical substations and in the vicinity of power lines.
Keywords: electric field, magnetic field, substation, electrical installation,
renewable energy sources
1. Introduction
Along with deregulation of the energy market, the interconnection of
renewable energy sources required the development of new substations for
interconnection with the National Power System. The interconnection substations
with wind parks and solar parks to the power system determined a challenge
regarding the influence of electric and magnetic fields on environment and
substation operating personnel.
Nowadays, the environment, health and personnel safety are the main
focus of public society, ecological organizations and media. These are highly
influencing the political priorities, regulations, standards, local authorities,
international treaties, industry trends and many other society factors [1].
The analysis of the influence of low frequency electromagnetic fields on
biological structure represents a difficult task. Studies were initiated in order to
coordinate all available information regarding the influence on biological structure
of low frequency electromagnetic field [1].
1
PhD. Student, Faculty of Power Engineering, University POLITEHNICA of Bucharest,
Romania, e-mail: [email protected]
2
Assoc. Prof., Faculty of Power Engineering, University POLITEHNICA of Bucharest, Romania,
e-mail: [email protected]
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Gabriela Nicoleta Sava, Sorina Costinaş
The high and very high voltage installations, at power frequency (50 Hz),
represent the most common source of electric fields affecting the human beings.
These installations determine values of electric and magnetic fields exceeding the
natural electric field (100 ... 150 V/m) or the natural magnetic field. The effects on
human beings depend on electric field intensity and exposure duration [2].
The electromagnetic energy absorption by the human bodies depends of:
- frequency and polarity;
- environment characteristics (temperature, pressure, humidity);
- biological parameters (dielectric properties, size and form of tissue);
- position of the body in field, as well as some short term conditions (the
existence of any screening or concentration field elements).
For the assessment of personnel exposure risk, a particular importance is
represented by the daily, monthly or annually monitored dose of electromagnetic
fields [3].
In this paper, the level of electromagnetic fields within the interconnecting
substations, between renewable energy sources (wind park) and power system, is
analyzed. These types of substation have certain characteristics like: small sizes
because of the issues raised by the acquisition and land price and a relatively
reduced number of equipment.
2. Effects and limits of electric and magnetic fields for specialized staff
from substations
Simultaneously with the construction of high voltage electric lines and
substations increases their influence on the environment. Hence, the countries
with developed power systems elaborated specific regulations and standards
regarding the environmental impact of power grids.
Some epidemiological studies, with statistical processing of data, highlight
a direct connection between the exposure to electric and magnetic fields, and the
people health.
In the near future, the research in this area will lead to clearer results. As
consequence of this, new criteria in the design of electric installations and
equipment will be defined, such that to not affect the health of operating
personnel [4].
Further studies on human exposure to electromagnetic fields are
determined by:
- the increase of electromagnetic impact in residential areas, in public
places, and in usual occupational environments;
- the occurrence of occupational environments characterized by high
electromagnetic emissions (electric installations, industrial installations
using important electric currents etc.);
Analysis of electromagnetic fields in interconnecting substations between WTG and (…) 251
-
acknowledgement that biological effects are cumulative and a particular
interest should represent the quantity of disturbances in a time interval [5].
Standardization activity about human exposure to electromagnetic fields is
a major current concern. Thus, exposure limits are established by:
- national standards: Berufsgenossenschaft der Feinmechanik und
Electrotechnik – Germania (BFE), Russia, American Conference of
Governmental Industrial Hygienist - S.U.A. (ACGIH), Poland, Japan,
Check Republic and Slovakia);
- projects of international standards: International Commission on NonIonizing Radiation Protection (ICNIRP), Comité Européen de
Normalisation Electrotechnique (CENELEC).
- recommendation of the independent international professional
organizations: US Environmental Protection Agency (USEPA), World
Health Organization (WHO), National Radiological Protection Board
(NRPB), and International Radiological Protection Association
(IRPA) [1].
The exposure limits to electromagnetic fields are regulated by 2004/20/EC
European Union Directive adopted by the Romanian legislation OMSF 1193/2006
and HG 1136/2006. In these documents the exposure limits values considering the
50 Hz power frequency are established [6], [7]:
- 5 kV/m for electric field intensity (public exposure);
- 10 kV/m for electric field intensity (professional exposure);
- 80 A/m for magnetic field (public exposure);
- 400 A/m for magnetic field (professional exposure).
3. Determination of the electromagnetic fields
3.1. Determination of the electric field in the vicinity of bus-bars
within a high voltage substation (method of equivalent charges)
The intensity of the electric field Ej can be determined, at any point P of
the space (Fig.1) in the electric field of a charge qj (charge per length of the busbar) placed at height hj above ground, using [4]:
G
Ej =
G
G
⎛ r jP r j*P
⎜
⋅
−
2 ⋅ π ⋅ ε 0 ⎜⎝ r jP2 r j2*P
qj
⎞
⎟
⎟
⎠
(1)
252
Gabriela Nicoleta Sava, Sorina Costinaş
G
G
where r jP and r j* P represent the distances between the point where the charge +qj
is placed and the investigated point P, respectively between the point where the
charge –qj (the mirror image of charge +qj) is placed and the point P.
G
r jP
qj θjP hj xP P G
E*
hP G
Ez
hj* θj*P G
r j*P
G
Ex
G
E
G
G G
E j = E + E*
− qj Fig. 1. Determination of electric field intensity at a point in the vicinity of bus-bars within a high
voltage substation.
As shown in Fig. 1 [5], the vertical component Ez and horizontal
G
component Ex of the electric field E can be calculated, function of the distance xP
to the ground projection of charge qj and on the height hP of point P:
Ez =
⎡
⎤
h j − hP
h j + hP
⋅⎢
+
⎥;
2 ⋅ π ⋅ ε 0 ⎢⎣ (h j − hP )2 + x P2 (h j + hP )2 + x P2 ⎥⎦
qj
⎡
⎤
xP
xP
⋅⎢
−
Ex =
⎥.
2
2
2
2
2 ⋅ π ⋅ ε 0 ⎢⎣ (h j − hP ) + x P (h j + hP ) + x P ⎥⎦
qj
(2)
3.2. Determination of the magnetic field in the vicinity of bus-bars
within a high voltage substation
The determination of the magnetic field intensity H and the magnetic field
density B requires the knowledge of:
- the geometrical configuration of bus-bars and the environment material
properties, based on the magnetization curves B(H) for linear
environments;
- the electric currents along the bus-bars.
The magnetic field intensity at any point in the vicinity of bus-bars within
a substation, for a single phase configuration can be determined based on the
calculation scheme shown in Fig. 2.
Analysis of electromagnetic fields in interconnecting substations between WTG and (…) 253
G
The magnetic field intensity H in any point P, determined by the electric
current I, can be expressed as [5]:
G
I ⎛⎜ r jP r j*P
H=
⋅
+
2 ⋅ π ⎜⎝ r jP2 r j2*P
⎞
⎟
⎟
⎠
(3)
where rjP and rj*P represents the distances between point P, where the magnetic
field intensity is calculated, and the center of the bus-bar through which the
electric current I flows, respectively towards the equivalent bus-bar where the
current returns through earth.
z Ij hj hP θjP rjP G
H*
P G
G G
H j = H +H*
xj xP x
G
H
0 rj*P θj*P
‐ Ij Fig. 2. Determination of magnetic field intensity at a point in the vicinity of bus-bars within a high
voltage substation.
Based on (3), the vertical component Hz and horizontal component Hx of
the magnetic field intensity in the considered point P are determined using [5]:
I ⎛⎜ sin θ jP sin θ j*P ⎞⎟
⋅
+
=
Hz =
r j*P ⎟⎠
2 ⋅ π ⎜⎝ r jP
(4)
⎤
xP − x j
xP − x j
I ⎡
=
⋅⎢
+
⎥;
2 ⋅ π ⎢⎣ ( x P − x j ) 2 + (h j − hP ) 2 ( x P − x j ) 2 + (h j* + hP ) 2 ⎥⎦
254
Gabriela Nicoleta Sava, Sorina Costinaş
Hx =
I ⎛⎜ cos θ jP cos θ j*P
⋅ −
+
r jP
r j*P
2 ⋅ π ⎜⎝
⎞
⎟=
⎟
⎠
(5)
⎤
h j − hP
h j * + hP
I ⎡
=
⋅⎢
−
.
2
2
2
2 ⎥
2 ⋅ π ⎢⎣ ( x P − x j ) + (h j − hP )
( x P − x j ) + (h j* + hP ) ⎥⎦
The absolute value of the magnetic field intensity is defined by:
H = H x2 + H z2 .
(6)
4. Case study
4.1. Calculation algorithm for electric field
From (1), (2) and based on calculation steps described above, for a three
phase configuration, a computational program is developed. The value of the
electric field, in the area of bus-bars within a 110 kV substation, is determined.
For calculation a real 20/110 kV substation connecting a wind park with a total
capacity of 140 MW is considered. In Fig. 5, the flowchart of the algorithm for
electric field, in the area of bus-bars within the considered substation, is
illustrated. In most cases, the measurement results indicate that the maximum
values of the electric field intensity are obtained in the area of disconnectors and
circuit breakers [8].
The maximum value of electric field intensity in the bus-bars area is
4 kV/m for the case studied (Fig. 3 and Fig. 4). Obviously, this value does not
present a risk for personnel performing maintenance work in the substation.
bus-bars, obtained by applying the algorithm presented in Fig.5
5
E[kV/m]
4
3
2
1
0
0
10
20
x[m]
30
40
Fig. 4. Numerical result for electric
Fig. 3. Electric field distribution [kV/m] in the area of bus- field in the area of 110 kV bus-bars,
bars, obtained by applying the algorithm presented in Fig.5 obtained by applying the algorithm
presented in Fig.5
Analysis of electromagnetic fields in interconnecting substations between WTG and (…) 255
Fig. 5. Flowchart of the algorithm for electric field intensity.
256
Gabriela Nicoleta Sava, Sorina Costinaş
4.2. Calculation algorithm for magnetic field
Based on (3) – (5) and on calculation steps described above, a calculation
program using the MATLAB environment it is developed. The program computes
the value of magnetic field in the area of bus-bars within the 20/110 kV
substation. Using the calculation algorithm for magnetic field, the value of
48 A/m in the area of bus-bars is obtained (Fig. 6 and Fig. 7). The resulted value
is not exceeding the exposure limits, both professional and public, in accordance
with the current regulations for the 110 kV substations. In most cases, the
measurement results indicate that the maximum values of the magnetic field are
obtained in the area of bus-bars and MV cables of power transformers, as well as
into the MV buildings where the electrical currents are higher [8]. During the
computation steps, the value of electrical current is considered I = 1 kA, but in
reality the electrical current is lower.
The magnetic field distribution in the area of bus-bars within the
substation is illustrated in Fig. 8. As shown, the maximum value of the magnetic
field resulted close to the middle phase and this value begins to decrease
significantly at a distance of 4 meters from the area of bus-bars.
50
50
40
45
H-module [A/m]
H [A/m]
Hx
30
Hz
20
35
30
10
0
-10
40
-5
0
x [m]
5
10
25
-10
-5
0
x [m]
5
10
Fig. 6. Numerical results for the components Hx
Fig. 7. Effective value of the magnetic field,
and Hz of the magnetic field [A/m], obtained by
obtained by applying the algorithm developed in
applying the algorithm developed in MATLAB
MATLAB environment
environment
Analysis of electromagnetic fields in interconnecting substations between WTG and (…) 257
Fig. 8. Magnetic field distribution in the analyzed substation [A/m]
258
Gabriela Nicoleta Sava, Sorina Costinaş
Fig. 8. Magnetic field distribution in the analyzed substation [A/m]
5. Conclusions
The development of wind parks required new substations of small sizes,
with a reduced number of equipment and minimum settled distance between them
in order to achieve a compact construction [9]. In most cases, the latter is the
factor that prevails in choosing the substation layout, being necessary the analysis
of electromagnetic fields configuration.
Analysis of electromagnetic fields in interconnecting substations between WTG and (…) 259
Within the substations, the electromagnetic fields determination is
difficult, due to the complexity of metal structures and equipment. Therefore, the
determination of the values for the electric field E and for the magnetic field H is
based on measurements [10]. Calculations performed are very important in the
design phase.
In section 2, the effects and limits of electric and magnetic fields for
operational personnel from substations are presented. The calculation elements of
electric and magnetic fields are investigated in section 3, considering the electric
and magnetic fields from substations connecting WTG to National Power System.
In section 4, the case study for a substation interconnecting a wind park with a
total capacity of 140 MW is conducted. The impact of the electromagnetic fields
in the connection substation with NPS is analyzed.
Considering the bus-bars of a 20/110 kV substation, the results indicate
areas where the electromagnetic fields values are exceeding (middle phase) the
exposure limits established by the existing standards.
The knowledge of the electromagnetic fields level, and the analysis of
areas where the intensities have higher values allow the designer to establish the
most effective measures to limit these field values [11]. The programs developed
in MATLAB software provide a complete solution for the analysis of electric and
magnetic fields within substations and in the vicinity of power lines. In general,
the areas with high values of electric field do not coincide with the areas with high
values of magnetic field.
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[2] Brochure CIGRE 375/2009 – Technical guide for measurement of low frequency electric and
magnetic fields near overhead power lines.
[3] IEC 61786/1998 – Measurement of low-frequency magnetic and electric fields with regard to
exposure of human beings – Special requirements for instruments and guidance for
measurements.
[5] G. Dragan, Tehnica Teniunilor înalte (High voltage Techniques), vol. III, Ed. Academiei
Romane, Bucharest, 2003.
[6] OMSF 1193/29.09.2006 (Official Journal of Romania 895/03.11.2006).
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Gabriela Nicoleta Sava, Sorina Costinaş
[10] EN 62110/2009 – Electric and magnetic field levels generated by AC power systems –
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