Охрид, 4 - 6 октомври 2009 - МАКО CIGRE · MAKO CIGRE 2009 C4-1R 4/12 maximum length of the feeders. Test assembly of the testing procedure for the devices looks

C4-1R 1/12

6. СОВЕТУВАЊЕОхрид, 4 - 6 октомври 2009

Herbert Haidvogl Roman Lechner EVN Netz GmbH, A-2344 Maria Enzersdorf, EVN Platz 1

Standards on electricity distribution - examples

ABSTRACT

This paper focuses on generally applicable standards and their introduction in the Republic of Macedonia. On the way to become member of EC it is necessary for the candidate country to implement a number of European Standards. They are often prepared on the state of art in EC and require also suitable legal and technical environment. One difference is e.g. electrical networks in EEC are from the technical point of view old and partly overloaded. According to the standards it is necessary to invest in upgrading the network. But also customers of electricity can be influenced by the standards. Old electrical devices often are not able to work properly with new defined values e.g. rated voltage of standards.

Especially standards with influence to the public as EN 50160, EN 61000-3-XX, pr EN 50522 (earthing of power installations) and related topics to protection against electrical shocks should be discussed in detail.

Bodies for standardisation in the field of electricity are IEC (International Electrotechnical Commission) and CENELEC (European Committee for Electrotechnical Standardisation) in the region of EU worldwide IEC and IEEE. Additionally there are national bodies for standardisation. In 2008 about 5425 IEC-standards were available and 483 standards where published. A number of 1976 are in progress. On the European level in 2008 about 5220 European standards and 305 harmonization documents were available. 483 EN and 10 HD where published. In the draft phase are 959. 861 prEN (projet EN) and prHD where published in 2008 [1]. This high number of published documents is a huge challenge for the body of national standardisation institute because it has to act quickly and accurate. This body should also be a representative in the international standardisation organisations.

The main field of electricity has a very spread spectrum. It includes production of electrical energy, electromagnetic compatibility, lighting protection, effect of current, safety, electric apparatuses for different fields of application, insulation coordination and measurements. All this topics imply self-evident standardisation. The focus should now be on “Standard voltages, current ratings and frequency” and also on “Equipment specifications and requirements”.

Keywords: Standards, European Standards, IEC, CENELEC, CE, EE, EN50160, EN61000, IEC 61936, EN 50522, supply quality, power installations, earthing, operational earthing, protection earthing, voltage regulation, voltage drop, three winding transformer, prefabricated transformerstation, neutral point treatment, measurements

1 SUPPLY QUALITY

Standard voltages, current ratings and frequency for low voltage (up to 1 kV) and medium voltage (1 kV up to 35 kV) networks are fixed in IEC 60038 “IEC standard voltages” [2], EN 50160 “Voltage characteristics of electricity supplied by public distribution networks” [3] and also in EN

MAKO CIGRE 2009 C4-1R 2/12

61000-3-3 [4], -3-11 [5], -3-12 [6] all together deal with electromagnetic compatibility and limits for voltage distortion.

1.1 Nominal voltage

In the standard 60038 of IEC nominal voltage of systems, highest and lowest voltages of system voltage (under normal conditions) and also rated and highest voltages for equipment are fixed. Nominal voltages for networks are the “input” for the EN 50160 which generally focuses the main characteristics of voltage at network users supply terminal in public low and middle voltage electricity distribution system under normal operating conditions. The nominal voltage is fixed in this standard to 230/400 V on the other side in some EE countries this value is 220/380 V. If still used older devices of customers are designed for this standard voltage introduction of new EN and IEC standard could cause a lot of problems. Devices which fulfil the requirements of EN 50160 and all other relevant European Standards are signed with the CE mark which is not only a sign for electrical devices.

1.2 Important characteristic of supply quality

Table 1 shows an excerpt of EN 50160 with important values and parameters. In this table the main responsibility for the values either the TSO (Transmission System Operator) or DSO (Distribution System Operator) can be found in the last column.

Table 1: Excerpt of EN 50160

Attribute of the supply voltage

Values Measuring and analysis parameter

resp

onsi

bilit

y

LV MV sample rate

observation period

percent

frequency 49,5Hz … 50,5 Hz 47 Hz … 52 Hz mean value 10 s 1 week 99,5 % 100 % TSO

voltage variation

Un = 230 V ± 10 % Uc ± 10 % r.m.s. 10 min 1 week 95 %

TSO DSO

Interruptions (short and long)

below 1 % of Uc 10 up to 100 10 up to 50

r.m.s. 10 ms 1 year 100 % DSO

rapid voltage variation

5 % of Un max. 10 %

4 % of Uc max. 6 % r.m.s. 10 ms 1 day 100 % DSO

flicker Plt < 1 Flicker- algorithmus 2 h 1 week 100 % DSO

harmonic voltage reference: Un, Uc

THD max. 8 % r.m.s. 10 min 1 week 95 % DSO

supply voltage unbalance

0 % … 2 % special cases 3 % r.m.s. 10 min 1 week 95 % DSO

As mentioned the implementation of a new standard has influence on both the customer’s and the supplier’s side. Tolerance of frequency is small and indicates a deficit or a surplus of electrical power in the grid not only locally but also because of interconnection of national transmission grids in


the UCTE network. Frequency can only be out of range if there are big events in the UCTE network. Such events will be managed according to UCTE handbook [7] and if this fails stability of network can not be ensured and a black out can happen.

Rapid voltage variation in the supply voltage as mentioned in the EN 50160 are mainly caused either by load changes in the installation of the users or by switching in the system.

All this parameters can be influenced by technical solution either on utility’s or customer’s side. On utility side this means upgrading of the power lines and transformers, building of additional substations, distribution transformer stations and new overhead or cable lines. To achieve those standard requirements a lot of investments are necessary and their implementation need many years for realisation due to financing, getting permissions and limited capacity of resources.

1.3 Distribution and Transmission Codes (rulebooks)

Additional to IEC and CENELEC standards it could be helpful to publish guideline or rulebooks e.g. as Distribution Code and Transmission Code. The connection of new power plants or generators, necessary performance of customer equipment, such as electrical motors, welding machines, compensation devices for reactive load and e.g. rules on judging of voltage characteristics should be regulated. In the Republic of Macedonia the Transmission Grid Code and Distribution Grid Code is a good basis but adjustments according to new standards and practical experience should be done.

For example power plants e.g. wind generators could be obliged to stay connected to the network in case of a voltage dip and support actively voltage level during fault time. That could also mean new power plants should be able to manage reactive power in a wide range which is in the moment a big challenge for some type of generator (e.g. solar power plants). In former times the concept was to regulate on the MPP (maximum power point) that means cos(φ) near to one. Now rectifiers should be able to deliver reactive power on request.

National examples for such technical guidelines are the Germany SDLV (Systemdienst-leistungsverordnung) and TR (Technische Richtlinien) of FGW [8] and the Austrian TOR (Technisch Organisatorische Richtlinien) of the Austrian regulatory board [9].

Within rulebooks (Grid codes) guidelines for connection of devices to low voltage and medium voltage networks are defined. There are also specifications for connections to high voltage level and if the rated power of a new generator exceeds 5 MW. For such cases e.g. it is fixed when DSO has to inform the TSO. Both will investigate the case and prepare an economic and technical acceptable proposal.

Investments have also to be covered by network appropriate tariffs and approved by the regulator.

2 GRID IMPEDANCE

EN 61000-3-XX “Electromagnetic Compatibility” regulates the limitation of voltage changes, voltage fluctuations and flicker in low voltage networks public systems. There are differences in rated current per phase. EN 61000-3-3 for rated current per phase ≤ 16 A, EN 61000-3-11 for rated current per phase ≤ 75 A and EN 61000-3-12 currents per phase >16 A and < 75 A. In these standards the testing procedures for devices are regulated. Impedances and corresponding short circuit power are laid down and could be used both for testing and for improving of grid as an aim.

This short circuit power can be seen as an indicator of stiffness of a certain network on short circuit conditions. Stiffness in this case means that the network is robust against flicker and other voltage fluctuations. Aim should be to provide a minimum of short circuit power at each location in power grids. This can be achieved by using overhead and cable lines with conform diameters and a


maximum length of the feeders. Test assembly of the testing procedure for the devices looks like shown in Figure 1.

Figure 1 Test assembly

M … Measurement device

S … test voltage source (includes generator G and reference impedance

Z = RA+jXA)

Evaluation of the relevant values which are described in IEC 61000-3-3 has to be done on the impedance Z = RA+jXA.

3 VOLTAGE REGULATION ON PTR

Remote control of important grid knots should be installed more and more in order to be able to react quickly in case of failures and come back to normal supply conditions. Another important issue is automatic voltage regulation of power transformers in substations. This is a standard and regularly used measure for transformers with two coils. Three winding transformers are often used in EE countries which make automatic voltage regulation more sophisticated. There are solutions offered on the market. A compromise on the secondary or tertiary winding of the transformer has to be accepted.

By using three winding transformers the voltage drop on costumer side could be decreased by the step voltage directly on the transformer. In Figure 2 this fact is shown very imposing. Variation of the secondary and tertiary apparent power load from zero to 100% leads to a characteristic of the voltage drop between these two sides. That means the allowed voltage drop on the low voltage level has to be decreased (in the example case 4.5 %) to fulfil the requirements.

test

sample


Figure 2 Voltage drop of a three winding transformer

This additional voltage drop depends on the used power transformer with its characteristic parameters. To know exactly how to parameterize the automatic voltage regulator it is necessary to calculate each transformer with its expected load separately. Equipment supplier provides different voltage regulation strategies for such transformers e.g. regulating one side manually and monitoring the other. If voltage is equal to a defined limit the regulation process will be stopped. Second strategy is to choose the side to be regulated by monitoring the load. The side with heavier load will be chosen for regulation. The other side will only be monitored.

The most common solution with two winding transformers is to regulate on constant busbar voltage. If compensation of voltage drop of long feeder lines is necessary voltage control at a certain distant location could be carried out. By using communication technology the actual data is transmitted to the regulation device in the substation and there voltage regulation is done by tap changer. To avoid that voltage exceeds allowed range on the other feeders of this busbar monitoring devices have to be used.

4 NEUTRAL TREATMENT OF NETWORKS

In the context of power quality another important topic is grounding of installations but also neutral grounding. Different types of neutral grounding (resistor, impedance, KNOSPE, NOSPE, isolated) define the possibilities of network operation in case of earth fault. Two standards deal with this question. Firstly European HD 637 S1 “power installations exceeding 1 kV a.c.” [10] includes earthing of power installations. This standard was implemented as national standard OVE-E8383 in Austria and as DIN/VDE 0101 in Germany and secondly on IEC level IEC 61936-1 “Power installations exceeding 1 kV a.c.” [11]. These two standards are now in revision and probably in 2010 new standards EN 50522 and a revised version of IEC/EN 61936-1 as shown in Figure 3 will be available.

U1=110 kV

U2= 35 kV

U3=10 kV

S1=31.5 MVA

S2=15.75 MVA

S3=31.5 MVA

uk12 = 10.18 %

uk23 = 5.96 %

uk31= 16.95 %

cos(φ)= 0.9ind


Figure 3 Harmonization Process

EN 50522 in difference to HD 637 S1 covers only the chapter dealing with earthing while all other topics relating to power installations will be kept in EN 61936-1. There are no big changes in both documents apart from the paragraph dealing with double earth faults and design of earthing conductors. The standards allow to operate network energized during earth fault conditions if neutral treatment is isolated (limited network size) or compensated via Petersen coil (impedance neutral grounding). If compensated the applicable current for dimensioning of grounding equipment will be the residual current of earth fault and if the probability of a double earth fault is low. To switch off feeder helps to keep step and touch voltages below limits but quality of supply will decrease. Otherwise if step and touch voltages are kept within tolerable limits e.g. limitation through compensation coil, grid operation could continue and customer will not suffer supply breaks.

EN/IEC 61936-1-2010 is intended as replacement of HD 637 S1 and also IEC 61936-1 2002 which includes a regulative for design and construction of power installations.

Because of differences in earthing philosophy between Europe and North America different chapters will be written. A more global one is included in IEC 61936-1 2010 and for Europe a more precise one will be written separately in EN 50522.

3.1 EVN Experience on earthing systems from Bulgaria

Results of an 20 kV network which consists of about 40 km cable lines and 200 km overhead lines connected to an 110/20 kV substation which was alternately operated with resistance (R) and resonant neutral grounding and with equipment to increase residual current (L) were analysed. These results are given in Figure 4. In case of resonant neutral (L) grounding feeder tripping by earth fault detection system (IE) decreases to zero but tripping by over-current protection I> and I>> increases a little. In case of resistor earthing (R) each earth fault leads to a feeder trip and supply interruption for customers. The using of resonant neutral grounding improves supply quality. Detection of location along feeder by using this method is not simple although affected feeder in substation could be detected automatically. Normally the system operator has to do switching (so called earth fault detection switching) to find and switch off the network parts with earth fault. During this time step and touch voltages have to be kept within given limits. For physical reasons voltage in “two healthy phases” increases by 3 and reaches phase to phase voltage between phase and earth. Weak insulations in grid e.g. old cables or arrestors could be destroyed.

As it can be seen that introduction of resonant neutral grounding increases the quality of supply on the other hand operation of such networks is more difficult than networks where earth faults are switched off within a short time. Step and touch voltages in case of an earth fault have to be decreased to a non dangerous level as can be found in future in EN 50522 and nowadays in HD 637 S1. To reach this aim efficient earthing systems have to be built and checked regularly.


Figure 4 Results neutral grounding

3.2 Earthing system and touch voltage

Earthing systems can be differentiated in the operational earthing of the transformer and low voltage grid and the protection earthing system. In urban areas the two systems are connected and in rural areas they are operated separately. The “Technical Recommendations Nr. 7”of the Republic of Macedonia, which where based on the JUS standards from the former Republic of Yugoslavia chapter 4, 5 and 6, states how earthing systems have to be built. Chapter 4 is for substations 35/10 kV, and chapter 5 is for 20/0.4 kV and 10/0.4 kV transformer stations with cable grid. In this case both earthing systems have to be connected. In chapter 6 rules for construction of earthing in stations 20/0.4 kV or 10/0.4 kV with LV overhead line grid are defined. The following points have to be taken into account.

If the earthing systems are not connected and MV network is operated by resistant earthing:

– The distance between the earthing systems has to be at minimum 20 m.

– The potential difference between the systems has to be less than 40 % and the voltage on the protection earthing system is less than 1200 V.

Operation mode R

Operation mode L

operational

operational

Successfull AR

Switch off

Successfull AR

Switch off


– The area where the station is located has to be urban.

If the MV network is operated with isolated neutral grounding the earthing system of distribution transformer has to be obligatory connected.

3.3 Result of measurement of step and touch voltage in DT

A distribution transformer station in Macedonia was chosen for measurements in order to check if earthing systems of stations described in chapter 5 can be connected and what are the related step and touch voltages

Figure 5 shows the arrangement for the measurements of step and touch voltages. A current was injected in a switched off 10-kV overhead line in a distance and this simulates an earth fault current. For practical reasons 50 A could be injected. Measurements of step and touch voltage were done in the surrounding of transformer station. If earth fault current in reality is 300 amps a calculation of step and touch voltages could easily performed. Earthing resistance was determined by using common measuring methods.

Results are as given below:

– Connected systems 0.83 Ω.

– Separated systems 1.68 Ω for protection earth and 1.15 Ω for earthing of transformer station.

Graphs in Figure 6 show the results of the experiment. The average value of step voltage as

well as of touch voltage decreases if connected. “Technical recommendations Nr. 7” chapter 2.7 defines the acceptable touch voltages for short and long time. Long time touch voltage should not exceed 75 V. This limit is given in HD637 S1 and is in accordance with prEN 50522. Only location 1 (Figure 6) could require improvement (renewing) of earthing system to avoid too high touch voltage.

To find an obligatory answer whether connection of earthing systems on transformerstation with overhead line grids is possible or not additional measurements with a representative number should be carried out. If the results of measurement campaign show that connection is allowed without a deficit on security these regulations could be adapted. This would be in accordance with Austrian experience.


Figure 5 Arrangement for measure step voltages and touch voltages

Touch voltage

0

10

20

30

40

50

60

70

1 2 3 4 5 6 7

measuringpoint

touc

h vo

ltag

e m

easu

red

in V

splitedconnected

Step voltage

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

1 2 3 4 5 6 7 8 9

measuringpoint

touc

h vo

ltag

e m

easu

red

in V

splitedconnected

Figure 6 Characteristics of touch and step voltage

a … TS

b … incoming feeder

x-c … measuring points for step voltage

x-d … measuring points for touch voltage

a

b


5 NEW DISTRIBUTION TRANSFORMER STATIONS

By using transformer station with a high degree of standardisation which also includes standards for the earthing system not only economical but also technical and safety goals could be achieved. Prefabricated transformer stations are described in the European standard EN 62271-202 “High-voltage switchgear and controlgear - Part 202: High-voltage/low voltage prefabricated substation” [12]. This standard includes regulation on

– Earthing (includes also examples for earthing system)

– Calculation and testing (electrical and mechanical)

– Different types of testing routines

– Data for procurement

The big advantage of prefabricated substations is that the assembly on the site is reduced to a minimum. So quality is much less dependent on weather conditions. Only electrical connection to the network (on medium and low voltage side) and to earthing system has to be made. Installation and technologically correct connection of earthing wires are important. Nevertheless they are underground and could not be seen.

To speed up erection time of DT a general approve of standardised types should be established in the Republic of Macedonia. If every individual solution has to be permitted one by one by the local authorities too much time is needed. In Austria, Macedonia and Bulgaria EVN has defined a standard compact transformer station (20/0.4 kV and 10/0.4 kV) and got a general permission for this type in Austria. In Austria type KN1830 (3.0 m x 1.8 m) has been installed over 2500 times within last ten years. In Macedonia and Bulgaria this station is called FK-3. Transformer, medium voltage switchgear and all necessary equipment for the low voltage is installed into the concrete housing in the factory and delivered on site.

To define the requirements of such stations it is necessary to know well the topology of network (medium and low voltage), and the common type of customers (important for choosing the rated power of transformer). Normally SF6 medium voltage switchgear cells are used which can be combined in different variants. The cubicles have to satisfy the corresponding standards of local environment. Low voltage feeders and equipment can be adapted to the customer needs. Housing normally is made of concrete, metal parts should be rustproof.

Summary of advantages:

– Equipment and arrangement of installation is safe for staff and passer-byes (IEC EN 62271-202)

– Assembly on site can be reduced to a minimum

– Small number of different types

– Easy to use materials

– Staff is used to install and operate the transformerstation (less failures)

– Higher availability of spare parts

– Lower acquisition costs because of higher quantities

Possible disadvantages:

– Delivery delays due to overload of production sources


– Finding contractor who can deliver high quality product within requested time

In summary there are more advantages of standardisation of transformer stations although sometimes individual solutions can be needed to cover special requests. Workers who are responsible for building such stations have to be trained periodically how to use the material, how to connect the cables and to install the earthing system.

To apply EN 62271-202 is an important step to guarantee both security of persons and supply quality.

Figure 7 Austrian (top) and Macedonian (bottom) type of prefabricated transformerstation

6 CONCLUSION

On one hand introduction and apply of EN and IEC standards can help to improve both safety for persons and supply quality.

But on the other hand it is necessary to check what the introduction of the standards implies for relating parties. Changes in standardisations and regulations should be prepared carefully and if necessary implementation should be done step by step.


Carefully should be considered:

What has to be upgraded, who has to upgrade or improve and who has to pay for the introduction? What are the costs in detail and what period is needed for introducing the standards? If it is necessary transitional periods have to be defined that all amendments can be done in a good technical and economical way.

7 LITERATURE [1] Deutsche Kommission Elektrotechnik Elektronik Informationstechnik im DIN und VDE. “Die Kunst

der Normung” DKE Jahresbericht 2008. [2] IEC “IEC 60038 IEC Standard voltages Edition 6.2” Geneva, 2002-07 [3] CENELEC “EN 50160 Voltage characteristics of electricity supplied by public distribution systems”

Brussels, 2005 [4] CENELEC “EN 61000-3-3 Electromagnetic compatibility (EMC) - Part 3-3: Limits - Limitation of

voltage changes, voltage fluctuations and flicker in public low-voltage supply systems, for equipment with rated current ≤16 A per phase and not subject to conditional connection” Brussels

[5] CENELEC “EN 61000-3-11 Electromagnetic compatibility (EMC) - Part 3-11: Limits - Limitation of voltage changes, voltage fluctuations and flicker in public low-voltage supply systems - Equipment with rated current ≤ 75 A and subject to conditional connection” Brussels

[6] CENELEC “EN 61000-3-12 Electromagnetic compatibility (EMC) - Part 3-12: Limits - Limits for harmonic currents produced by equipment connected to public low-voltage systems with input current > 16 A and ≤ 75 A per phase” Brussels

[7] UCTE http://www.ucte.org/resources/publications/ophandbook/ Brussels [8] FGW Fördergesellschaft Windenergie e.V. http://www.wind-fgw.de/ Kiel [9] E-Control http://www.e-control.at/de/recht/regulierungsrecht/marktregeln-strom/tor Vienna [10] CENELEC “HD 637 S1 Power installations exceeding 1 kV a.c.”, Brussels, 1999 [11] IEC “IEC 61639-1 Power installations exceeding 1 kV a.c. - Part 1: Common rules” Geneva, 2002-10 [12] CENELEC “EN 62271-202 High-voltage switchgear and controlgear - Part 202: High-voltage/low

voltage prefabricated substation”, Brussels, 2006-06

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