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In recent years, sinusoidal waveform of the current and voltage disturbs in the electrical distribution system because of the due to the increasing number of non-linear loads. Many standards of IEC and IEEE standards have been published in order to limit the voltage and current waveform distortion. The operators of the electricity distribution network widely use the power quality monitoring systems at the point of common connection (PCC). It has been identified that there are substantial number of harmonic currents excess of the standards transferred to the grid according to the data obtained from power quality monitoring systems. In case of exceeding the limits specified in the standards, there is a need to determine the network and customer responsibilities for the implementation of required sanctions. In this study, using recorded data at the PCC of a medium voltage electrical distribution system, voltage and current harmonic distortion responsibilities of the network and customer are determined by the improved harmonic current vector method. Up-to-date load model based on field measurement which provides more accurate results has been used instead of the constant load impedance in the proposed method.

In addition to the known harmonics sources, the wide spread use of power electronic devices cause to injection of more harmonic currents in the electrical distribution network. Therefore, voltage waveform distortion is increasing with each passing day. Consisting harmonic currents as a result of connecting of a non-linear load to the network causes the network’s voltage to become harmonic as a result of the harmonic voltage drop at the network impedance. In this case, even if the linear load is connected to the network, harmonic currents will continue to flow reference measurement point [

In case of to be exceeded of the limit values which are determined by the standards, it is required to take precautions in order to reduce the waveform distortion. Some responsibilities in the distortion of the waveform should be determined to realize who is responsible for this distortion. The source side of the PCC is under the network responsibility while the load side of the PCC is under the customer responsibility. Therefore, the field measurements are of great importance for determining the responsibilities of the customer or/and the network. In the case of the determination of responsibilities, it will be able to punish to customer and/or the network according to their responsibility rate at the PCC.

The harmonic analysis studies of three-phase power distribution networks, modeling of loads [

In a study using the value of the reference impedance, changes in the harmonic sources are included by converting the impedance changes into equivalent current sources [

Harmonic Current Method, which is modeled by Norton equivalent circuit of the customer and the network for a specific harmonic degree, is considered theoretically correct, although it shows some practical shortcomings [

While the load impedance is constantly changing, to use the reference impedance method which assumes the load impedance as constant does not provide sufficiently accurate results. Because, switching positions of the filter and compensation capacitors can cause very large changes in the load impedance. Instead of the reference impedance method, use of harmonic current method with the approach to updating the load impedances online by using the field measurements will provide more accurate results. Furthermore, switching positions of the filters and capacitors have been already taken in to account in the proposed algorithm by means of the long-term measurements at the PCC.

This study proposes a new method, which is based on measurement of current and voltage at the PCC, in order to determine the responsibilities of the network and costumer. Moreover, it is provided that the current vector method has been improvement by using constantly changing impedance. In the proposed method, up-to-date load model based on field measurement has been used instead of the constant load impedance.

In order to determine the current and voltage contributions to the PCC, Norton and Thevenin equivalent circuits of the network and costumer have been used with an algorithm. The proposed algorithm is verified in MATLAB by processing of the measurements. After the processing of the data collected from the PCC for a long-time period, the responsibility of the customer and the network at the PCC and current and voltage THD values have been obtained.

Impedance of the customer and the network are of great importance in determining the compliance of the harmonic standards. The contract between the customer and the network is determined by value of the reference impedance [

Harmonic Vector Method is the representation of the circuit of the customer and network sides with Thevenin/Norton equivalent circuits. Harmonic current or voltage at the PCC are separated into two components (customer and network) in order to identify the individual harmonic distribution. The equivalent circuit is composed as h order harmonic current or voltage source for both customer and network circuits.

Although, characteristics of the customer impedance and especially frequency are known rarely, the impedance of the network side is usually particular. Active harmonic impedance of the customer side is very difficult to determine due to the constantly changing characteristics of the customer side. Therefore, equivalent impedances are determined as reference impedance. The main idea is to describe either the customer or the network side impedances as a reference to an additional source of harmonic. In general, resistive component of the customer side is determined as customer reference impedance. This situation depends on the distribution of the current or voltage harmonics [

To determine the contribution of the harmonic voltage, the Thevenin equivalent circuit shown in

As can be seen in

The customer reference impedance is calculated as Equation (1) using the active power P measured at connection point and current at the fundamental frequency

and voltage at the fundamental frequency. Customer harmonic voltage

Generally, customers are connected to the network via transformer. So, reference impedance of the network is determined as the sum of the impedance of the last transformer before connected to the customer and the short-circuit impedance of the network. This impedance composes the basic part of the network impedance and value of this impedance can be found easily. However, networks should provide an impedance close to the current impedance and should avoid from the unnecessary impedances which cause the harmonic distortion.

Additives to voltage value at the customer and network connection point are given Equations (4) and (5) applying the superposition principle as shown in

Harmonic voltage phasor measured at the connection point

where,

Resonance between the load and the network status is a common occurrence. Limits defined in the standards are usually exceeded due to the increasing harmonic distortion in case of resonance. Responsibilities should be determined in order to put leverage on the customer and network sides when the limits are exceeded. Norton equivalent circuit in which network and customer reference impedances are used shown in

Reference impedance calculation is quite difficult due to the continuously changing state of the load. Reference impedances may be determined due to the measured values at the reference connection point (PCC) according to the general impressions among the researchers. Reference impedance of the network is the sum of the short-circuit impedances and the impedance of the first transformer before the customer connection point. Resistive part of the load impedance of the customer

Customer impedance can be calculated as in Equation (8) by using the active power

In the Equation (8), is the angle between measured current and voltage at the fundamental frequency, and

Harmonic current contributions of the network and customer to the

Total Harmonic current

Scalar harmonic contributions of the Customer and Network are calculated as in Equations (13) and (14) by using harmonic currents of customer

where, _{ }and

A distribution system comprises a number of loads conveniently supplied by circuits from the nearest distribution point. The distribution circuits’ configuration depends on the particular load requirements. A considerable number of loads (linear or nonlinear) are located so close together and supplied from the main distribution point, that they can be considered as whole. The aggregate form of various individual loads called general loads. Residential and commercial buildings can be called as general loads since they have a number of linear and nonlinear load combinations rated at low power values. Owing to the fact that some of the loads in buildings switch on and switch off continuously, the buildings as a general load have varying characteristics and mostly operate unbalanced. Moreover, due to harmonic currents drawn by nonlinear loads, the waveform of voltage in the

buildings becomes distorted. It is therefore that the buildings are very sophisticated loads in terms of modeling. Consequently, using the measured voltage and current data to model these types of loads can be an alternative solution.

The generalized time domain load model is based on actual recorded data and is an extension of the ideas proposed in [

Given the measured voltage

where ^{th} harmonic voltage and current respectively. ^{th} harmonic voltage and current respectively. The load parameters R, L, and C are calculated based on the voltage

where

required costumer parameters, mentioned load model has been used. And the calculated parameters are given in Section 5.

In order to determine the responsibilities of the network and costumer, real impedance value of the customer which is calculated by measuring current and voltage at the PCC has been used in proposed model instead of the constant load impedance. To be able to use the proposed method, equivalent model of the grid system should be

prepared after the measurements at the PCC. After the establishment of Norton and Thevenin equivalent, contributions at the PCC can be found precisely.

In this study, superposition method is applied to the equivalent circuit of this grid system by using MATLAB after modeling of a real grid system. Norton equivalent circuit is used for harmonic current contributions while Thevenin equivalent circuit is used for harmonic voltage contributions. It is very important to determine the network and customer sides in a real grid system. Actually, there are lots of customers connected in parallel to each other on a real grid system. In

MATLAB program has been prepared to calculate the fundamental, 5^{th}, 7^{th}, 11^{th}, 13^{th} and 17^{th} harmonic levels of the current and voltage of customer and network sides, individually. Harmonics that are the 3^{th} and multiples of three and that are not found on symmetrical and balanced systems are not taken into account. The circuit, whose field measurements are performed, has been modelled as given in

Decomposition of the network and customer current and the voltage are done at the PCC by using

where,

and customer, respectively. The accuracy of the calculated network and customer currents can be proven with the condition of being equal to each other of the result of Equation (19) and measured value of the PCC current [

In order to realize the contribution of the network and customer voltages to the voltage of PCC in what rate Equation (20) is used.

where,

One-line diagram of the test system where the measurements were recorded with permission of the Bosporus Electricity Distribution company (BEDAŞ) is shown in

Load model were determined with the field measurements. Measurements had been taken and recorded during the 40 hours in order to determine the contributions of the network and customer current and voltage to the PCC. Sampling time of the measurement is 5 minutes. Instant measurement taken from the PCC were processed via MATLAB according to method described Section 3. According to this method the customer load impedance varies between (0.14 + 0.1i) Ω and (0.6 + 1.1i) Ω.

As a result of processing of the measurement data at the PCC shown in

After the measurements of the current and voltage, load impedance was calculated by using (8) and given in

Harmonic voltage contribution is calculated by adapting the impedance value of the customer and network according to rank of the harmonic analysis. As a result of processing of the measured data, voltage harmonic contribution levels between the 5^{th} and 17^{th} of the customer and network to the PCC is given between Figures 17-21, respectively.

As can be seen from the referred graphs, reason for disturbance in voltage quality as well as current is results from the customer. That is, largest share in the disturbance in the waveform of the voltage at the PCC belongs to the customer. It is understood that the 5^{th} and 7^{th} voltage harmonic levels are higher than the other harmonic levels as expected from a medium power energy system.

Harmonic current contribution is calculated by adapting the impedance value of the customer and network according to rank of the harmonic analysis. As a result of processing of the measured voltage data, voltage harmonic contribution levels between the 5^{th} and 17^{th} of the customer and network to the PCC is given between Figures 22-26 respectively. As can be seen from the referred graphs, largest share in the disturbance in the waveform of the current at the PCC belongs to the customer. As shown in ^{th} and 7^{th} current harmonic levels are higher than the other harmonic levels as a result of the voltage harmonics.

Separately calculated average (Avr.) and maximum (instantaneous value) harmonic level values of the current and voltage are given in ^{th} and 7^{th} harmonic levels of the network is higher than the others. And his is very common phenomenon in the power systems. On the other hand, not only 5^{th} and 7^{th} harmonic levels of the costumer are high but also other harmonic levels are very high according to network.

Calculated voltage and current THDs according the measurement given in

Limits of allowable voltage and current distortion set by IEEE 519 [

THD | Current | Voltage | ||
---|---|---|---|---|

Avr. | Max. | Avr. | Max. | |

C | 0.165 | 0.296 | 0.172 | 0.399 |

U | 0.005 | 0.006 | 0.004 | 0.005 |

PCC | 0.023 | 0.038 | 0.055 | 0.195 |

Bus voltage at PCC | Vh, % | THDv, % |
---|---|---|

V < 69 kV | 3.0 | 5.0 |

69 kV ≤ V < 161 kV | 1.5 | 2.5 |

V ≥ 161 kV | 1.0 | 1.5 |

(120 V - 69 kV) | % | |||||
---|---|---|---|---|---|---|

H < 11 | 11 ≤ h < 17 | 17 ≤ h < 23 | 23 ≤ h < 35 | 35 ≤ h | THD | |

<20 | 4.0 | 2.0 | 1.5 | 0.6 | 0.3 | 5 |

20 - 50 | 7.0 | 3.5 | 2.5 | 1.0 | 0.5 | 8 |

50 - 100 | 10 | 4.5 | 4.0 | 1.5 | 0.7 | 12 |

100 - 1000 | 12 | 5.5 | 5.0 | 2.0 | 1.0 | 15 |

>1000 | 15 | 7.0 | 6.0 | 2.5 | 1.4 | 20 |

THD of the customer is 17.2% while maximum voltage THD is 39.9%. Both of these values exceed the voltage distortion limits as seen in

On the other hand, neither maximum nor average voltage THD of the network exceeds the limits. Therefore, penalty sanctions should be imposed to the customer for exceeding the voltage distortion limits.

In this study, a medium voltage electrical distribution system has been investigated by using field measurement in order to determine the customer and network responsibilities. To be able to determine these responsibilities, a new algorithm based on the current vector method with the reference impedance method modified via constantly updated data has been proposed. The proposed algorithm is verified in MATLAB by processing of the measurements and the responsibilities of the customer and the network have been determined, successfully. It is understood that analysis performed with harmonic vector method using updated reference impedance is a method that is very practical and easy to apply in determining the components of network and customer at the PCC. Customer and network’s individual current and voltages and also customer load impedance have been calculated by processing current and voltage data at the PCC with the proposed algorithm. Average and maximum value of the current and voltage THDs has been also calculated.

According to the analysis results of test system, it is determined that overall responsibilities in the current and voltage waveform distortion at the PCC only belong to the customer. Therefore, penalty sanctions should be imposed to the customer in the amount of it exceeds allowable limits. To be able to reduce to impact of the harmonics to the minimum and determine the harmonic contribution rates accurately and also determine the share of responsibility, some of the arrangements can be made as follows:

● The necessary arrangements about the harmonic contribution of the network and the customer should be performed and determined financial penalties should be applied when related limits given in the standards were exceeded.

● To take into account the average values instead of instantaneous values will be satisfy in terms of feasibility and justice while calculating the effect of the network’s and the customer’s harmonic distortion.

● Variable load condition should be taken into account in order to obtain more accurate results. Constantly changing load and also switching positions of the harmonic filters and compensator capacitors have been already taken into account because of the calculated load impedance by using the voltage and current data updated constantly during the measurement period.

In the future, it is planned to use the same algorithm for determining the customer and network distortion contributions in an unbalanced three phase systems. Still, billing is not made for the harmonic contributions in practice. In this regard, a digital recorder including a harmonic contribution algorithm would be useful to determine the harmonic contribution of the each customer in the allowable limits or not.

This study has been financially supported by TUBITAK with 110E134 project number. The authors would like to express their gratitude to BEDAŞ DistCo for facilitating and helping with the measurements at the substation.

Ömer Gül,Tayfun Gündoğdu, (2015) Harmonic Contributions of Utility and Customer Based on Load Model Using Field Measurements. Journal of Power and Energy Engineering,03,11-28. doi: 10.4236/jpee.2015.35002