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Abstract
—
In this paper, two major international standards on harmonic emissions in power systems have been reviewed. The purpose is to help utility companies and stakeholders take informed decision regarding the safe limit of harmonic emissions and decide on which standard to adopt. The paper describes the principles underlying the srcinal intent of PCC and clarifies some difficulties in relation to application of the standards. It is shown that location of PCC can be selected based on the method of power distribution system.
Index Terms
—Point of Common Coupling, Harmonic Emission, IEEE-519, IEC-61000 Series, Total Harmonic Distortion, HVDS, LVDS.
I.
I
NTRODUCTION
n Ghana, UK harmonic standard G5/4 is recommended for use [1]. The standard is essentially adoption of the IEC 61000 series. However, in practice, the harmonic emission limits are not being enforced. This is perhaps due to misunderstanding on certain aspects of the standards. Point of Common Coupling (PCC) is not clearly defined
−
which side of the service transformer constitutes PCC is not known. Harmonic measurements are done in customer’s premises and where limits are considered high, consumers are advised to reduce their harmonic distortion to 5% per the Electricity Company of Ghana (ECG) design guidelines [1]. Clearly, it can be seen that the PCC, as against the srcinal intention of IEEE 519 [2], has been misunderstood and being wrongly applied. It is not even clear whether the recommended 5% distortion limit refers to voltage or current. Consumers do not even know level of harmonic current that is safe to operate with. As a result, capacitor banks are failing and consumers are ignorantly blaming utility companies for the failures [3]. From personnel communications with colleagues in the profession (from other Africa countries in IEEE fraternity), it appears that the situation is almost the same. Therefore, the main intent of this paper is to add to the stock of knowledge in harmonics and help clarify some of the difficulties in relation to application of harmonic standards. The paper discusses IEEE and IEC standards on harmonic emission limits and
Manuscript received February 22, 2014; revised April 5, 2014. This work was supported in part by the Electricity Company of Ghana. G. Eduful is with the Electricity Company of Ghana, System Planning Division, Project Office, Box AN 5278, Accra-North, Ghana.Tel: +233246132736. georgeeduful@yahoo.com Ebenezer A Jackson is with Jackson Educational Complex, Teacher Training Department. 115 Ayeduase Road, Kumasi-Ghana. e-mail: jacksonea1@yahoo.com J. E. Cole is with Electricity Company of Ghana, Ashanti East Region -Kumasi. Box 1980, Kumasi-Ghana. email: Josephekowcole@gmail.com.
select location of PCC based on the method of power distribution system. It is hoped that the paper will serve as educative material on harmonic standards and help utility companies and stakeholders take informed decision regarding safe harmonic emission limits and decide on which standard to adopt. II.
H
ARMONIC
T
HEORY
A.
Mathematical representation of harmonics
Harmonics are sinusoidal voltages and currents with frequencies that are integral multiplies of the fundamental frequency that is 50 or 60 Hz in a typical power system. This can be expressed mathematically as
f h f
h
(1)
where h denotes the order of the harmonic (h=1,2,3,....n), f is the fundamental frequency of the harmonic. The harmonic order 1 refers to the fundamental frequency. Mathematically, harmonic currents can be expressed as follows:
hhnh DC h
fh I I i
2sin2
(2)
where
ᵟ
h is the phase angle of the harmonic current and I
DC
is the direct component of the current (does not exist always). The equation of the harmonic voltages has the same form, but the current is replaced with voltage.
B.
Harmonic Distortions and their Measurement
Total harmonic distortion (THD) and total demand distortion (TDD) are two most common indices that are used to quantify harmonic distortion in power systems [4]. They are well defined in standards such as IEEE 519-1992 and IEC 61000-series [5][6]. The THD value is the effective value of all the harmonics current or voltage added together, compared with the value of the fundamental voltage or current. Although many of today's test and measurement instruments can provide THD values, it is still important to understand the calculation that derives THD. The basic equation is as follows [7] [8]:
11
21221
rmsrmshhU
U U U U THD
(3)
where
THD
U
represents voltage or current total harmonic distortion (alternatively represented as
TDH
V
and
TDH
I
Harmonic Emission Limits and Selecting PCC Location Based on the Type of Distribution System
George Eduful,
Member, IAENG
Ebenezer A. Jackson, and Joseph Ekow Cole
I
Proceedings of the World Congress on Engineering 2014 Vol I, WCE 2014, July 2 - 4, 2014, London, U.K.ISBN: 978-988-19252-7-5 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)WCE 2014
respectively) and Urms is the rms fundamental voltage or current. Alternatively, rms voltage or current can be represented in terms of total harmonic distortion as given below.
2112
1
U rmshhrmsrms
THDU U U
As distribution system fundamental voltage and current rarely remain static in magnitude at different times throughout the day, the definition of total harmonic distortion may at times provide a misleading value for the harmonic distortion level [8]. This is especially true for distribution system fundamental currents that fall close to zero at certain periods of the day, resulting in a large value of THD
I
. For this reason, a modified index for harmonic distortion is used with harmonic content of the waveform expressed as a percentage of a fixed nominal value rather than the fundamental value. The modified index is referred to as total demand distortion (TDD). The TDD is very much like the total harmonic distortion THD. The only difference is that the TDD calculation compares the measured harmonics with the maximum demand current. Similarly, the individual harmonic current limits are not given in terms of percent of fundamental. Mathematically, TDD is expressed as:
22
211
hhnomU
U U TDD
The difference between THD and TDD is important because it prevents a user from being unfairly penalized for harmonics during periods of light load. During periods of light load, it can appear that harmonic levels have increased in terms of percent even though the actual harmonic currents in amperes have stayed the same or decreased [9].
C.
Effect of Harmonic Distortions
Harmonics have a number of undesirable effects on power system components and loads. The effects can be overheating of transformers, cables, motors, generators and capacitors connected to the same power supply with the devices generating the harmonics. Electronic displays and lighting may flicker, circuit breakers can trip, computers may fail and metering can give false readings [10]. According to [11] when the level of distortion in the supply voltage approaches 10%, the duration of the service life of equipment significantly reduced. According to [12], service life of equipment reduction has been estimated at 32.5% for single-phase machines, 18% for three-phase machines and 5% for transformers. Some computer-controlled are also reported in literature to be sensitive to voltage distortion of 5% [13 ].
III.
S
TANDARDS ON
H
ARMONIC
E
MISSIONS
In order to prevent harmonics from negatively affecting equipment of utilities and their customers, nationals and international standards have been established on limit on harmonic emissions. The IEEE 519-1992 and IEC 61000-series are two major international standards on harmonics that are widely used. This section discusses IEEE and IEC recommended practices and requirement for harmonic limits in power transmission and distribution systems.
A.
IEEE Standard 519
IEEE Standard 519 was introduced in 1981 and was revised in 1992. The standard clearly established limits for both voltage and current distortion and it was intended to provide direction on dealing with harmonics introduced by static power converters and other nonlinear loads. The underlying philosophy of the standard is that [5]: 1.
Customers should limit harmonic currents, since they have control over their loads, 2.
Electric utilities should limit harmonic voltages since they have control over the system impedances. Both parties share the responsibility for holding harmonic levels in check. The standard restricts customer harmonic current emissions to a value derived from a short circuit level at the point of common coupling (PCC) and the size of customer’s nonlinear load. The standard takes into account the total demand distortion, which is the harmonic distortion of current expressed as a percentage of the maximum demand current (15 or 30 minutes demand). Limit of current distortion depend on the size of the load compared with the size of the supply system at the PCC. The recommended harmonic current limits from IEEE 519 are given in Table 1. The limits are intended to be applied at the point of common coupling between the customer and the utility. Within the customer’s facility these limits do not apply, but they are still useful guides for judging harmonic levels within the customer’s facility [9].
Where I
SC
: Available short circuit current, IL: 15 or 30 minute (average) maximum demand current. TDD: Total demand distortion. TDD is identical to THD except I
L
(as defined previously) is used instead of the fundamental current component.
Table 1 defines Total Demand Distortion (current) limits as well as individual harmonic current limits. The limits are most severe for short circuit ratios of less than 20 because this lower ratio indicates a high impedance power system or a large customer or both. Voltage distortion is more likely to develop from current harmonics consumed at a PCC where the short circuit ratio is low, thereby justifying the more severe limits. It is important to note that Table 1 shows limits for odd harmonics only. IEEE 519 addresses even harmonics by limiting them to 25% of the limits for the odd orders within the same range [5]. Even harmonics result in an asymmetrical current wave which may contain a dc component that will saturate magnetic cores. The distortion limits given above are only permissible provided that the transformer connecting the user to the
Table I Current Distortion Limits (in % of IL) for General Distribution Systems (120-69,000V) [ ] Isc/I
L
<11 11
≤
h<17 17
≤
h<23 23
≤
h<35 35
≥
h TDD <20 4 2 1.5 0.6 0.3 5 20-50 7 3.5 2.5 1 0.5 8 50-100 10 4.5 4 1.5 0.7 12 100-1000 12 5.5 5 2 1 15
˃
1000 15 7 6 2.5 1.4 20
(4) (5)
Proceedings of the World Congress on Engineering 2014 Vol I, WCE 2014, July 2 - 4, 2014, London, U.K.ISBN: 978-988-19252-7-5 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)WCE 2014
utility system will not be subjected to harmonics in excess of 5% of the transformer's rated current as stated in ANSI/IEEE C57.12.00-1980 [14] The second set of criteria established by IEEE 519 is for voltage distortion limits. This governs the amount of voltage distortion that is acceptable in the utility supply voltage at the PCC with a consumer. The harmonic voltage limits recommended are based on levels that are low enough to ensure that consumers' equipment will operate satisfactorily. Based on the harmonic current limit given in Table 1, it is assumed that the voltage levels will not exceed those given in Table 2. Any consumer who degrades the voltage at the PCC would be expected to take steps to correct the problem. However, the problem of voltage distortion is one for the entire community of consumers and the utility [5].
As can be seen, the IEEE standard 519 establishes harmonic limits on voltage as 5% for total harmonic distortion and 3% of the fundamental voltage for any single harmonic. It should be noted that even if the voltage distortion limits are met at the PCC, they could very easily be exceeded downstream where connected equipment could be affected. Since voltage distortion is the result of harmonic currents passing through the impedance of the power system, voltage distortion will always be higher downstream where the harmonic currents are generated and where system impedance is highest. This standard also recognizes the sensitivity of electrical equipment in a building as the limiting factor on how much voltage distortion is acceptable. IEEE 519 therefore provides limits for different types of buildings.
B.
Some Misinterpretations in the Application of the IEEE 519-1992
One of the most difficult aspects to applying IEEE standard 519 is determining the location of the Point of Common Coupling or PCC. The standard defines the PCC as the electrical connecting point or interface between the utility distribution system and the customer's electrical distribution system. The concept of PCC has been used to define this point but unfortunately, the existing standard has not provided a clear definition. While simple in concept, identification of this point is sometimes misunderstood; this leads to confusion and misapplication of the specifications in the Table 1. The single-line diagram shown in Fig. 1 represents a small distribution system. Limit of harmonic current distortions to be allowed depends on the size of the load compared with the short circuit power of system at the PCC. By the IEEE definition, the PCC can either be located at the primary side of the transformer (PCC-1) or at the secondary side of the transformer (PCC-2). However, consulting engineers have often resorted to applying the standard on an individual equipment basis [2] by insisting that the current harmonic limits be met at the terminals of the non-linear equipment. It should be noted that high distortion at PCC3 does not necessarily result in out-of-limit distortion on the distribution system.
Fig 1, The single-line diagram of a small distribution system
It should be noted that IEEE 519-1992 is meant to be applied to system harmonic distortion rather than to individual load distortion. The most effective way to meet harmonic distortion limits is to filter the harmonics at each individual load and measure them at the selected PCC [15 ].
C.
IEC Standard on Limit on Harmonic Emissions
The International Electro-technical Commission (IEC) presents standards on electromagnetic compatibility in several publications that cover many disturbing electrical phenomenon. In relation to harmonic current emissions, the IEC standards deal with three different publications: two concern emissions in LV network and one concerns emission in MV and HV networks [8].
IEC 61000-3-2 and IEC 61000-3-4: Harmonic Emission in LV Networks
The relevant standards in the LV networks are IEC 61000-3-2 and IEC 61000-3-4 [16]. The IEC 61000-3-2 considers emissions in LV network of equipment whose input current is less than 16A. With such equipment, the task of reducing harmonic current is entrusted to the equipment manufacturer. The manufacturer has the responsibility to manufacture equipment with emission levels within the standard limits. It should be noted that the standard does not take into account the size of the system at the point of common coupling as proposed in the case of the IEEE 519. For the purpose of harmonic current limitation under IEC 61000-3-2, equipment are classified into 4 classes as follows: Class A: refers to balanced three-phase equipment, household equipment, tools (excluding portable tools), dimmers and audio equipment. Class B: refers to portable tools and arc welding equipment which is not professional equipment. Class C: refers to lighting equipment. Class D: refers to personal computers and personal
T ov ar i o u s l o a d s
PCC-1PCC-2PCC-3
A customer's facilities
Other customers
S o ur c e of p ow er s u p pl y
12
+30
11/0.433DY_1+-1|10ms|0SW1+10|10ms|0SW2LF45kW21kVARLoad2+-1|10ms|0SW3+-1|10ms|0SW4+-1|10ms|0SW5+10|10ms|0SW6+10|10ms|0SW7BUS1BUS2
Table II Voltage Distortion Limits (in % of V
1
) PCC Voltage Individual Harmonic Magnitude (%) THDv
≤
69kV 3 5 69-161kV 1.5 2.5
≥
161kV 1 1.5
A customer’s facilities PCC- 2 PCC- 3 Source PCC- 1 To various loads Other customers
Proceedings of the World Congress on Engineering 2014 Vol I, WCE 2014, July 2 - 4, 2014, London, U.K.ISBN: 978-988-19252-7-5 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)WCE 2014
computer monitors and television receivers having a rated input current less than or equal to 600W. The applicable limits in relation to the above classes can be found in [16]. The IEC 61000-3-4 deals with equipment with input current higher than 16A, connection of the equipment to the supply system needs either notification or consent of the supply authority. The standard shows three stages of assessment for the purposes of making such connection. These stages are based on the magnitude of the admissible harmonic current caused by the equipment [17]. The recommended emission limit for equipment with input current higher than 16A is given in [16].
IEC 61000-3-6: Harmonic Emission in MV and HV networks
IEC 61000-3-6 relates to emissions in MV and HV networks. The standard ensures that allocation of harmonic emission rights to customers is more equitable [8]. In 2001, Australia and New Zealand adopted the standard and by government regulation, transmission utilities and connected loads are required to abide by the harmonic allocations set by the standard [8]. A key concept is that customers with the same maximum demand and at the same PCC are entitled to equal harmonic emission rights. Central to the implementation of the standard is the concept of agreed power Si, a number which describes the size of a particular consumer load. A premise of the standard is that every consumer at the same point of common coupling and having the same agreed power has the right to draw equal distorted current from the supply [18]. Assuming harmonic voltage Gh is available for distribution to all connected and potential future consumers, a strategy is used to allocate a portion of Gh to each consumer to satisfy the equal rights criterion and to take up Gh fully when the supply is fully loaded. For equitable right, the standard uses two summation laws [18]. The second law, also known as the power law, is more general and would be used to explain the equitable right concept of the standard. IEC 6100-3-6 assumes that the harmonic voltage at the MV level is a combination of the emission from loads and the background distortion of the HV transmission system [8]. Thus HV harmonic planning level L
hHV
must be included in the MV harmonic voltage planning level L
hMV
. Harmonic planning levels as suggested by the standard is given in [8] [18]. Example Suppose the supply capability of the MV substation is St, see Fig 2. Let the planning levels at the h
th
harmonic for the HV and MV buses be L
hHV
and L
hMV
respectively. The global harmonic voltage available for MV loads can be estimated using equation (6)
1
hHV hMV hMV
L LG
Where
α
is dependent on a harmonic order and is given in Table 3. Table III Summation exponents
Harmonic order
α
h<5 1 5 h 10 1.2 h
˃
10 2
Harmonic voltage is to be allocated to a customer with maximum demand Si such that the sum of all allocated harmonic voltages is G
hMV
. According to the standard [19], this can be achieved by choosing the h
th
harmonic voltage emission level of customer “i” to be
1
t ihMV Uhi
S S G E
Fig 2, A Simple Distribution System
Let the fundamental supply system impedance seen at the PCC (point of common coupling) be
x
1
. Assuming this impedance at the h
th
harmonic is hx1 gives a harmonic current emission level of
1
hx E E
Uhi Ihi
As can be seen, the standard makes a logical sense and easy to apply. The harmonic voltage is first allocated to the customer and an equivalent harmonic current calculated. Although the standard encourages an equitable allocation of harmonic ‘right’ to all customers having the same maximum demand, where customers see different fault levels the question arises as to whether these rights are to equal harmonic voltage, equal harmonic current or some other right. Other difficulties found in applying the standard has to do with determination of planning levels for specific power systems [8][18][19] IV.
V
IEWS AND
C
OMMENTS ON THE
H
ARMONIC
E
MISSION
S
TANDARDS
Having reviewed the two major standards on the harmonic emission limits, we now compare and present our views on the standards. The IEEE 519 standard determines the limit of harmonic emission based on a ratio of system short-circuit current and maximum demand of a consumer at the point of common
Substation capability StS1S2SnHV bus Planning level: Lh HVMV bus (PCC) Planning level: Lh MVx1
1 2
+ 3 0
PQPQPQ
(7) (8) (6)
Proceedings of the World Congress on Engineering 2014 Vol I, WCE 2014, July 2 - 4, 2014, London, U.K.ISBN: 978-988-19252-7-5 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)WCE 2014

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