23rd International Conference on Electricity Distribution Lyon, 15-18 June 2015

Paper 0403

CIRED 2015 1/5

PROBLEMS OF HARMONIC VOLTAGE IN THE LV DISTRIBUTION GRID

Martin KASPIREK David MEZERA Karel PROCHAZKA

E.ON Czech Republic – Czech Rep. E.ON Czech Republic – Czech Rep. EGC EnerGoConsult CB – Czech Rep.

martin.kaspirek@eon.cz david.mezera@eon.cz kprochazka@egc-cb.cz

ABSTRACT

The supply territory of the company E.ON Distribution in

the Czech Republic (operated by the company E.ON

Czech Republic) accounts for approximately 1.5 million

customers. One of the detected problems was with

harmonic voltage in approximately 7% LV (low voltage)

distribution grids. The survey is based on the evaluation

of approximately 600 voltage quality (VQ) measurements

which were made in points of the LV grid with known

phase-earth system impedance. This system impedance

was measured in each point of VQ measurement and was

compared with the reference value according to the IEC

60725 standard. The purpose of this paper is to

determine the average value of each harmonic voltage, to

compare this value with the limit value according to the

EN 50160 standard and to answer the questions: Which

harmonics create problems in the LV distribution grid? Is

the problem detected in the weak grids or even in the

grids with better network impedance than reference value

according to the IEC 60725standard? What is the reason,

that some harmonic voltages violate the limits according

to the EN 50 160 standard?

INTRODUCTION

The electricity market liberalisation brings considerable pressure on the introduction of penalties for insufficient voltage quality parameters. In case of poor voltage quality, these penalties should be of the electricity rebate payment nature. Another issue included in this paper is the responsibility for the poor voltage quality and the consequent damage. The analysis of VQ parameters in the distribution grid is necessary for a definition of an interface between the customer and the distribution network operator (DNO) under the terms of the responsibility for poor VQ. We make approximately nine hundred week VQ measurements a year so we have good practice with VQ measurements in the LV distribution grid. All measurements are systematically archived and it is possible to filter them for example according to the voltage level or exceeded VQ parameters. If possible, the system impedance phase-earth is measured in the point of the VQ measurement. A special measuring instrument Zerotest 46 N is used – see Fig. 1. The system impedance in all three phases is measured and the average value is calculated and used in the following graphs. Values of the measured system impedance are included in the archival system so that the link between VQ measurement, or more precisely values of VQ parameters and system impedance, is available for each VQ measurement.

Fig. 1: The measuring instrument Zerotest 46N for the system impedance (phase-earth) measurement

REFERENCE IMPEDANCE

IEC 60725 [1] sets the reference impedance value, or

more precisely, a reference (relative) short circuit

impedance (short circuit power) value for the LV

distribution network and electric equipment of nominal

current less than 16/75 A. It is assumed that if the

required impedance value is met at the point of network

connection, an electric appliance will not produce any

adverse retroactive impacts on the network therefore no

interference in the network will occur.

Electric equipment with

nominal current

Reference impedance

Zphase-earth

up to 16A 0,47 Ω

up to 75 A 0,35 Ω

Tab. 1: Reference impedance for equipment with current

ratings ≤16A/75A

The question is when the DNO and when the customer is

responsible for the poor voltage quality. . It can be used

as the guideline for the reference impedance according to

the IEC60725 standard [1] however keeping of this value

from the side of DNO will not guarantee a voltage quality

for all LV grids.

HARMONIC VOLTAGE DEPENDENCY ON

THE SYSTEM IMPEDANCE

The dependence of harmonics on phase-earth system

impedance can be demonstrated by means of graphs. The

survey is based on the evaluation of approximately 600

VQ measurements with known (measured) system

impedance phase-earth. The VQ measurements were

evaluated according to the EN 50160 standard [2] so

values of 95% percentile are evaluated in following

23rd International Conference on Electricity Distribution Lyon, 15-18 June 2015

Paper 0403

CIRED 2015 2/5

graphs. The legend for the following graphs is: red

horizontal line – limit value for corresponding VQ

parameter, red vertical line – reference impedance 0.47Ω,

black vertical line - reference impedance 0.35Ω.

Fig. 2: Evaluation of 2

nd voltage harmonic

Fig. 3: Evaluation of 3

rd voltage harmonic

Fig. 4: Evaluation of 4

th voltage harmonic

Fig. 5: Evaluation of 5th

voltage harmonic

Fig. 6: Evaluation of 6th

voltage harmonic

Fig. 7: Evaluation of 7th

voltage harmonic

Fig. 8: Evaluation of 8th

voltage harmonic

Fig. 9: Evaluation of 9th

voltage harmonic

23rd International Conference on Electricity Distribution Lyon, 15-18 June 2015

Paper 0403

CIRED 2015 3/5

Fig. 10: Evaluation of 10th

voltage harmonic

Fig. 11: Evaluation of 11th

voltage harmonic

Fig. 12: Evaluation of 12th

voltage harmonic

Fig. 13: Evaluation of 13

th voltage harmonic

Fig. 14: Evaluation of 14

th voltage harmonic

Fig. 15: Evaluation of 15th

voltage harmonic

Fig. 16: Evaluation of 16th

voltage harmonic

Fig. 17: Evaluation of 17th

voltage harmonic

23rd International Conference on Electricity Distribution Lyon, 15-18 June 2015

Paper 0403

CIRED 2015 4/5

Fig. 18: Evaluation of 18

th voltage harmonic

Fig. 19: Evaluation of 19

th voltage harmonic

Fig. 20: Evaluation of 20

th voltage harmonic

Fig. 21: Evaluation of 21st voltage harmonic

Fig. 22: Evaluation of 22

nd voltage harmonic

Fig. 23: Evaluation of 23

rd voltage harmonic

Fig. 24: Evaluation of 24

th voltage harmonic

Fig. 25: Evaluation of 25

th voltage harmonic

23rd International Conference on Electricity Distribution Lyon, 15-18 June 2015

Paper 0403

CIRED 2015 5/5

HARMONIC CLASSIFICATION

Three groups of harmonics can be designed according to

the analysis in the previous caption.

Class A

Class A includes harmonics with the ample reserve to the

limit value according to the EN 50160 standard. The limit

value according to the EN 50160 standard is exceeded

only exceptionally and only in grids with the system

impedance worse than reference value. Class A includes

the 2nd, 7th, 11th, 13th, 17th, 20th, 22nd, 23rd, 24th and

25th harmonic.

Class B

Class B includes harmonics with the small reserve to the

limit value according to the EN 50160 standard. The limit

value according to the EN 50160 standard is exceeded

only exceptionally and but in grids with the system

impedance better than reference value too. Class B

includes the 3rd, 4th, 5th, 6th, 8th, 9th, 10th, 12th, 14th,

16th, 18th, 19th and 21nd harmonic.

Class C

Class C includes harmonics with zero reserve to the limit

value according to the EN 50160 standard. The limit

value according to the EN 50160 standard is exceeded in

many cases and in the grids with the system impedance

better than reference value too. Class C includes the 15th

harmonic.

CONCLUSION

In Fig. 15 you can see that the 15th harmonic voltage

differs from other voltage harmonics. The problem with

the 15th harmonic was detected in grids with better

system impedance than reference value. This could be

caused by high penetration of appliances producing 15th

harmonics current which isn’t our case, the measurement

was done in standard rural and small city area. The

second reason could be a low limit value for the 15th

voltage harmonic when measurement uncertainty for

generally used power quality analyzers (class S according

to the IEC 61000-4-30:2008 standard [3]) is +/- 0.5% of

nominal voltage. The standard IEC 62749 [5]

recommends limit values up to the 50th

voltage harmonic

and the limits for some harmonics (with the multiple of

three) are only 0.2% of the nominal voltage. Precise VQ

analyzers with low system noise should be used for

measuring of low harmonic voltage values. For some

measuring equipment the internal noise of the instrument

could disqualify VQ analyzer from measuring of such

low harmonic levels [4]. The experience of the DNO

shows that no complaints regarding harmonic voltage

were obtained up till now. Exceeding of the limit value

for the 15th harmonic voltage is hardly perceived by the

customer and has no influence on operation of commonly

used electrical devices. The goal of this paper is to point

out that there are problems with some non-standard VQ

parameters in LV distribution grids and that there are

problems in grids with good (better than reference)

system impedance. The value of reference impedance can

be used for a decision about the responsibility for poor

voltage quality in the distribution grid. If real value of the

system impedance in the delivery point of the customer is

better (smaller) than the reference value and problems

with voltage quality are detected, the customer is

responsible for the impermissible impact on the grid. If

real value of the system impedance in the delivery point

of the customer is worse (higher) than the reference

value, then DNO is responsible for poor voltage quality

in the distribution grid. Because the DNO has a duty to

meet the requirements according to the EN 50160

standard for all customers, the DNO has to do corrective

measures. These measures are based mostly on the

refurbishment of the grid and require investment cost

from the side of DNO. It is necessary to work out

analysis before the decision of VQ parameters. Such a

partial analysis is described in this paper but we suppose

that the working group IEC SC77A/WG8 (61000-2-2)

should define new limits of compatibility with the focus

on harmonics with the multiple of three.

REFERENCES [1] IEC/TR 60725:2012 Consideration of reference

impedances and public supply network impedances for use in determining the distrurbance characteristics of electrical equipment having a rated current ≤75A per phase, International electrotechnical commision, ISBN 978-2-83220-161-9

[2] EN 50160 Ed.3 Voltage characteristics of electricity supplied by public distribution systems. Brussels: European Committee for Electrotechnical Standardization, 2010. 20 p.

[3] IEC 61000-4-30 Ed.3.0 Electromagnetic compatibility (EMC): Part 4-30: Testing and measurement techniques – Power quality measurement methods. Geneva: International Electrotechnical Commission

[4] BILIK, Petr; ZIDEK, Jan. Precise Harmonic Component Analyzer based on Virtual Instrumentation. In Conference proceeding of the International Conference of Applied Electronics APPEL 2010, Pilsen, 8.-9.9.2010. Pilsen : Department of Applied Electronics and Telecommunications, FEE, ZCU Plzen, 2010, p. 39 - 42. ISBN 978-80-7043-865-7

[5] IEC/TS 62749 Ed.1 Assessment of power quality – Characteristics of electricity supplied by public networks