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Rationalized User Specification
ELECTRICITY SUPPLY —
QUALITY OF SUPPLY
Part 7: Application practices for end
customers
DRAFT 0 FOR WG INPUT
Preferred requirements for applications in
the Electricity Supply Industry
N
R S
This Rationalized User Specification is
issued by the NRS Project
on behalf of the
User Group given in the foreword
and is not a standard as contemplated in the Standards Act, 1993 (Act 29 of 1993).
Rationalized user specifications allow user
organizations to define the performance and quality
requirements of relevant equipment.
Rationalized user specifications may, after a certain
application period, be introduced as national standards.
Amendments issued since publication
Amdt No.
Date
Text affected
Correspondence to be directed to
Printed copies obtainable from
South African Bureau of Standards
(Electrotechnical Standards)
Private Bag X191
Pretoria 0001
South African Bureau of Standards
Private Bag X191
Pretoria 0001
Telephone:
Fax:
E-mail:
Website:
(012) 428-7911
(012) 344-1568
[email protected]
http://www.sabs.co.za
COPYRIGHT RESERVED
Printed on behalf of the NRS Project in the Republic of South Africa
by the South African Bureau of Standards
1 Dr Lategan Road, Groenkloof, Pretoria
1
NRS 048-7:2008 (WG Draft 0)
Contents
Page
Foreword .........................................................................................................................................
2
Introduction .....................................................................................................................................
3
Key words ........................................................................................................................................
4
1 Scope ........................................................................................................................................................................
5
2 Normative references .........................................................................................................................................
5
3 Definitions and abbreviations ..........................................................................................................................
6
4 Basis for Defining Dip Tolerance Requirements .......................................................................................
6
5 Immunity and Characterisation .........................................................................................................................
6
6 Testing and Measurement techniques ........................................................................................................
6
7 Immunity Objectives...............................................................................................................................................
6
Annexes
A Guideline to Improve Process Immunity ......................................................................................................
19
NRS 048-7:2008 (WG Draft 0)
2
Foreword
This first edition of NRS 048-7 was compiled by representatives of the South African Electricity Supply
Industry (ESI), in a working group appointed by the Electricity Suppliers Liaison Committee (ESLC).
The working group membership included customer representation, inter alia formal representation of
the Energy Intensive User Group (EIUG).
This edition of NRS 048-4 was prepared specifically to take into account the revised power quality
management framework defined in the Power Quality Directive of the National Electricity Regulator
(NER). In particular, it identifies the requirements of customers supplied by licensees of the NER, and
provides practical guidelines on how these can be implemented by customers.
A key aspect of this edition, is that it for the first time in South Africa provides specific guidelines on
appropriate levels of immunity for customer equipment. This edition also takes into account the latest
edition of NRS 048-2 (Edition 3), and developments in international power quality measurement
specifications (IEC 61000-4-30).
The working group was guided by recommendations in international (IEC and Cigré) standards and
technical reports, and by reports, data, and experience available locally. In particular, many of the
recommendations on equipment voltage dip immunity have been based on testing done at the Eskom
/ Wits voltage dip test facility.
This part of NRS 048 was prepared by the NRS 048 working group, which comprised the following
members:
AJ Dold (Chairman)
DK Bhana
WS Breed
BG Chatterton
S Delport
HJ Geldenhuys (Dr)
K Gibb
A Hepburn
AC Kachelhoffer
I Kekana
RG Koch
DA Kruger
MW Küster
I Langridge
PF Mabuza (Project Leader)
C Mamone
RR McCurrach
M Ngcamu
A Sayed
V Shikoana
I Sigwebela
T Thenga
H Visagie
S Zuma
eThekwini Electricity
Eskom Holdings Ltd (KSACS Division)
South African Bureau of Standards (SABS)
Eskom Holdings Limited (Distribution Division)
Ekurhuleni Metropolitan Municipality
Eskom Holdings Limited (R&S Division)
City Power Johannesburg (Pty) Ltd
Energy Intensive User Group (EIUG)
Tshwane Electricity
Tshwane Electricity
Eskom Holdings Limited (R&S Division)
Chamber of Mines
City of Cape Town
Mondi Paper
NRS Project
Mangaung Municipality (Centlec)
Eskom Holdings Limited (Transmission Division)
Department of Public Enterprises
City Power Johannesburg (Pty) Ltd
Eskom Holdings Limited (Distribution Division)
Eskom Holdings Ltd (Transmission Division)
National Energy Regulator of South Africa (NERSA)
IST OTOKOV (Pty) Ltd
Eskom Holdings Limited (Corporate Audit)
At the time that the ESLC accepted this edition, the ESLC comprised the following members:
NRS 048 consists of the following parts, under the general title Quality of supply standards:
3
NRS 048-7:2008 (WG Draft 0)
Part 2: Voltage characteristics, compatibility levels, limits and assessment methods.
Part 4: Application guidelines for licensees
Part 6: Distribution Network Interruption Performance Measurement and Reporting Standard
Part 7: Application guidelines for end customers
ISBN 0-626-15418-9
NRS 048-7:2008 (WG Draft 0)
4
Introduction
As part of the regulatory framework for the management of power quality, the application of NRS 048
is intended to optimize and minimize the combined cost of supply and use of electricity on an overall
national basis.
This part of NRS 048 provides recommended technical practices for customers of transmission and
distribution companies licensed by the NER. These technical practices are based on the general
requirements of the NER Power Quality Directive, and the specifications in NRS 048-2. The
documented practices are based on the revised regulatory framework on power quality, defined
through a process of broad consultation by the National Electricity Regulator in the process of
developing the NER Power Quality Directive.
In order to meet the voltage quality requirements in NRS 048 Part 2, transmission and distribution
companies will determine a specific customer's fair proportioned allocation of total allowable pollution
(emission limits) at a given point of common coupling (PCC). The methods by which this may be
done are defined in NRS 048-4. This part of NRS 048 provides guidelines on how such emission
limits may be met by customers.
Where voltage dip and interruption limits are not included in NRS 048-2, this part of NRS 048
provided guidelines for the specification of immunity levels of customer equipment.
Several IEC standards and technical reports dealing with power quality have already been adopted as
SANS standards. This part of NRS 048 defines how these standards should be applied by customers
in the context of the South African regulatory framework.
Specific rights of customers with regard to power quality complaints management, as specified in the
NER Power Quality Directive, are also included.
NRS 048 does not cover safety requirements, network design or equipment performance, nor does it
address issues of negligence.
Key words
quality of supply; immunity levels, emission limits; apportioning; power quality management
5
NRS 048-7:2008 (WG Draft 0)
GUIDELINE
Electricity supply – Quality of supply
Part 7: Application practices for end-customers
For application in the Electricity Supply Industry
1 Scope
This part of NRS 048 provides recommended technical practices for end-customers of transmission
and distribution companies licensed by the NER. It supports the management of power quality in
accordance with the requirements of the NER Power Quality Directive (and in particular, to meet the
requirements of NRS 048 Part 2). In order to meet the voltage quality requirements in NRS 048 Part
2, transmission and distribution companies will determine a specific customer's fair proportioned
allocation of total allowable pollution (emission limits) at a given point of common coupling (PCC).
The methods by which this may be done are defined in NRS 048-4. This part of NRS 048 provides
guidelines on how such emission limits may be met by customers. Where voltage dip and interruption
limits are not included in NRS 048-2, this part of NRS 048 provided guidelines for the specification of
immunity levels of customer equipment. Several IEC standards and technical reports dealing with
power quality have already been adopted as SANS standards. This part of NRS 048 defines how
these standards should be applied by customers in the context of the South African regulatory
framework. Specific rights of customers with regard to power quality complaints management, as
specified in the NER Power Quality Directive, are also included.
2 Normative references
The following documents contain provisions which, through reference in this text, constitute provisions
of this specification. At the time of publication, the editions indicated were valid. All standards and
specifications are subject to revision, and parties to agreements based on this specification are
encouraged to investigate the possibility of applying the most recent editions of the documents listed
below. Information on currently valid national and international standards and specifications can be
obtained from the South African Bureau of Standards.
IEC 61000-2-4:1994, Electromagnetic compatibility (EMC) – Part 2: Environment – Section 4:
Compatibility levels in industrial plants for low frequency conducted disturbances.
IEC 61000-3-6:1996, Electromagnetic compatibility (EMC) – Part 3: Limits – Section 6: Assessment of
emission limits for distorting loads in MV and HV power systems.
IEC 61000-3-7:1996, Electromagnetic compatibility (EMC) – Part 3: Limits – Section 7: Assessment of
emission limits for fluctuating loads in MV and HV power systems.
NRS 034-1:1997, Electricity distribution – Guidelines for the provision of electrical distribution
networks in residential areas – Part 1: Planning and design of distribution systems.
NRS 048-2 Ed.2:2003, Electricity supply – Quality of supply – Part 2: Voltage Characteristics,
compatibility levels, and limits.
NRS 048-7:2008 (WG Draft 0)
6
3. Definitions and abbreviations
The definitions and abbreviations given in NRS 048-1 apply.
4. Practices for managing plant immunity to power quality phenomena
4.1. Customer Practices
A customer is expected to:
a) Appropriately take into account the compatibility levels and limits defined in NRS 048-2 (or where
appropriate, specifically contracted levels) at the customer point of connection to the network,
when designing and specifying production plant. It is anticipated that these levels will be the basis
for specifying the normal operation of the plant.
b) Appropriately take into account the impact of voltage dips and interruption during the design and
specification of plant.
c) Take economically reasonable measures to protect process equipment and economic impacts on
the customer’s business, should network conditions arise under which the NRS 048-2 levels are
not met by the wires company, or for parameters for which compatibility levels and limits are not
defined in NRS 048-2 (e.g. in the case of voltage dips, interruptions, and voltage transients).
Note 1. Information on the expected frequency, duration, and severity of such events may be obtained from
the licensee. It is excepted (in terms of NRS 048-4) that the licensee will be in a position to provide such
information.
4.2. Principles – management of plant immunity
The following basic principles apply:
a) The compatibility levels in NRS 048 Part 2 (or where appropriate, contracted levels) define the
minimum licensee performance requirements for the purpose of evaluating performance to a
particular customer plant.
b) In the planning, design, and operation of its plant, technical issues such as the following should be
addressed by a customer:
5. Practices for managing plant emission levels
5.1. Customer requirements
A customer is required to:
a) Meet the emission limits specified by the licensee in terms of NRS 048-4.
b) Limit the number of voltage dips arising from the customer’s plant.
5.2. Principles – managing plant emission levels
7
NRS 048-7:2008 (WG Draft 0)
The following basic principles apply:
a) In order to meet the compatibility levels in NRS 048-2, a licensee will allocate emission limits on a
fair and consistent basis to individual customers, based on the calculation techniques defined in
NRS 048-4 (note 1).
b) In the case of LV customer equipment connected directly to the public network, emission limits
that apply shall be those defined as individual equipment limits in the relevant SANS/IEC
standards in Annex A.
c) It is anticipated that emission limits will generally be agreed and contracted between the parties
prior to connection of the installation or through a revision of the contract in the case of an existing
installation. Where no such prior agreement exists, the calculation methods in Annexes D1 to D5
may be used as the basis for assessing whether measured emission levels from a given
installation are acceptable.
d) In the planning, design, and operation of its plant, technical issues such as the following should be
addressed by a customer:
-
Harmonic emissions, unbalance emissions, flicker emissions, and rapid voltage change
emissions from its plant need to be limited to the contracted values at the point of evaluation
specified by the licensee
-
It may be necessary to further limit such emission levels within the customer plant in order to
meet internal equipment requirements (see Annex B).
-
Harmonic resonances that may arise due to the installation of shunt capacitors, may impact
the harmonic emission levels of a plant, and need to be taken in consideration in evaluating
the expected emission levels at the design stage.
Note 1. Generally this is done through the determination of maximum levels of emission for such plants, as
defined in Annexes D1 to D5 of NRS 048-4. These maximum levels may be agreed upfront and written into
customer contracts as emission limits, or may be generically defined in relation to the size of the customer
and the system capacity.
Note 2. A model format for the inclusion of emission limits in a customer contract is provided in Annex A of
NRS 048-4.
6 Practices for monitoring and reporting of power quality performance
6.1 Customer requirements
There are no specific requirements for a customer to install instrumentation for the purpose of
monitoring power quality.
The principles addressed in this section relate to the case where a
customer would want to undertake such monitoring.
6.2 Basic principles
The following basic principles apply for the measurement and reporting of power quality parameters:
a) For the purpose of evaluating licensee performance, the measurement and assessment criteria
applied shall be those defined in NRS 048-2.
e) Class A measurement methods defined in SANS 61000-4-30 shall apply where a dispute on the
accuracy of a measurement arises. Class B measurement requirements defined in SANS 1816
may be applied in all other applications, such as general monitoring.
NRS 048-7:2008 (WG Draft 0)
8
6.3 Technical considerations
6.3.1 Measurement transducers
The following technical considerations with regard to transducers should be taken into consideration:
a) Electromagnetic voltage transformers may be used for the determination of network harmonic
voltage magnitudes up to the 25th harmonic (see note 1). Capacitive voltage transformers
(CVT) may be used only where special techniques are applied. Under no circumstances
should the (uncompensated) secondary output of the capacitive voltage transformer be used
for voltage measurement. Where compensation techniques have been proved to meet the
above accuracy requirements, the compensated CVT output signal may be used. Highvoltage dividers and capacitive bushing tap-off techniques which meet the required accuracy
may otherwise be used where electromagnetic voltage transformers are not available.
b) Where harmonic measurements are undertaken, care should be taken that any equipment
connected to the secondary winding of a voltage transducers does not generate harmonic
currents (e.g. rectifier power supplies).
c) Where unbalance measurements are undertaken, care should be taken that the loading on
the secondary windings of the VT is equal on the three phases.
d) Use of protection class VT’s are not recommended for unbalance measurements, but may be
applied for harmonic and flicker measurements.
e) The use of CVT’s for the measurement of voltage unbalance should be undertaken with
caution, as damage to capacitors in the capacitor divider stack can significantly affect the
measured voltage.
f)
The use of CVT’s for undertaking flicker measurements should be undertaken with caution,
as low frequency voltage fluctuations may not be accurately reflected at the secondary.
Note 1. The phase changes introduced at higher harmonic frequencies may not be adequate for the calculation of emission
levels at these frequencies. Such calculations may require a better understanding of the frequency characteristics of the
transducers.
6.3.2 Measurement instrument connections
The following technical considerations with regard to the connection of instruments should be taken
into consideration:
a) For general reporting purposes, voltage measurements on solidly grounded systems shall be
undertaken by phase-to-ground, and voltage measurements on all other systems shall be
undertaken phase-to-phase.
b) For evaluating the dip performance to a specific customer plant, the connection method of the
instrument (i.e, phase to phase or phase to ground) should be considered to match the
connection method of sensitive equipment in the plant.
9
NRS 048-7:2008 (WG Draft 0)
7. Basis For Defining Dip Tolerance Requirements
7.1 System Performance Characteristics
NRS 048-2 is the power quality standard applied by the NER [1] as a licence condition in the licences
of the various suppliers in South Africa. The standard categorises voltage dips according to both the
expected frequency of occurrence, and the impact on customer plant. The original NRS-048:1996
(Edition 1) dip characterisation method was based on theoretical considerations]. Figures 1 and 2
show actual measured dip density plots based on national Eskom dip measurements over a period of
4 years since then]. From this data it is clear that as far as large industrial customers are concerned,
voltage dips of less than 30% in magnitude, and duration shorter than 150ms, have a high probability
of occurring in South African HV networks. Many customers are not affected by these events. This
has resulted in the development of a revised dip categorisation method, published in NRS 048-2:2003
Edition 2 [5].
Figure 1: Dip performance for EHV systems.
NRS 048-7:2008 (WG Draft 0)
10
Figure 2: Dip performance for HV systems.
The revised dip categorisation method, summarised in the figure below, represents the consensus
reached by utilities and customers on the most appropriate voltage dip categories, as well as a
minimum level of dip immunity (represented by the shaded area). The categorisation method allows
effective communication on basic network performance and mitigation requirements. (Eskom for
example, uses the dip classifications in determining performance trends). A case study illustrating the
effective use of the curve is discussed in the next section.
Given the relatively large number of short-duration, small-magnitude dips that occur at any given site
in South Africa, NRS 048-2 Edition 2 has defined what it terms as “minimum dip immunity
requirements”, and “desired dip immunity requirements”. The former is based on customer immunity
proposals made to the working groups, and the latter on Eskom proposals. These are illustrated
below.
= mainly customer responsibility to protect against these events
Residual
Voltage
u % of Ud
90> u  80
80> u  70
70> u  60
60> u  40
40> u  0
Duration t
150 t <
600
(ms)
20 t <
150
(ms)
0.6 t < 3
(s)
Z1
S1
X1
X2
Z2
T1
Figure 3: Minimum immunity requirement (shaded area). The desired immunity is
this plus X1-type dips.
Although the NRS’s PQ Directive recognises that some customers may find it difficult to meet either
the desired or minimum immunity requirements1, general reference to these requirements ensures a
continuous drive by customer to at least come closer to meeting these.
1
In particular, it may be difficult to mitigate against asymmetrical dips in the case of large line-commutated
converters such as DC-drives and AC Current Source Inverter drives .
11
NRS 048-7:2008 (WG Draft 0)
8 Immunity and Characterisation
Characterization of equipment voltage dip immunity should be provided by manufacturers as a usual
and expected component of their product specifications. Where performance characterization must
be validated, compliance testing may be appropriate. To determine the dip immunity level of existing
installed equipment, field testing may be required.
8.1 Approach to Characterization and Compliance Testing
Rigorous testing of a representative sample or samples of a particular product model is time
consuming and requires specialized instruments and expertise. Compliance test protocols should be
limited in scope to minimize test expense.
Manufacturers are encouraged to supply as much characterization information as possible. In each
case, the manufacturer should also specify what is meant by successful ride-through. In contrast,
compliance testing protocols must include specific tests and specific pass-fail criteria.
8.2 Properties of Voltage Dips Relevant to Equipment Design
The list of dip properties is summarized in Table 1. Known issues with equipment response or
equipment damage are included as well.
Table 1, Voltage dip properties
Property / characteristic
dip magnitude
Description
The reduced voltage at the equipment terminals during a
period lasting from one or two cycles up to several seconds
dip duration
The time interval over which a voltage magnitude is below an
accepted threshold. Typical durations range from fractionalcycles to several seconds. See point on wave of dip ending.
voltage magnitude unbalance
The difference between voltage magnitude for the three
phase-to-phase or phase-to-neutral voltages
point on wave of dip initiation
The sudden drop in voltage magnitude at the start of the dip.
This drop may take place at different points-on-wave of the
pre-event voltage waveform.
point on wave of dip ending
The sudden rise in voltage magnitude at the end of the dip.
This rise may take place at different points-on-wave of the
post-event voltage waveform.
This point, as well as the voltage rise time and the dip
duration, determines the current inrush magnitude at the
equipment terminals at dip ending.
phase shift at the dip initiation
The change in voltage phase angle associated with the drop
in voltage magnitude
phase shift at the dip ending
The change in voltage phase angle associated with the rise in
voltage magnitude.
maximum during-event phase shift
The maximum change in voltage phase angle during the
voltage dip
multistage
dips/multiple
event- The drop in voltage magnitude may take place in different
segment
steps due to developing faults; this may occur at a sub-cycle
time scale, but also at a time scale up to 1 second
multistage
dips/multiple
event- The rise in voltage magnitude may take place in different
segment
steps due to the differences in breaker-opening instants in the
NRS 048-7:2008 (WG Draft 0)
post-event voltage recovery
the presence of transients
harmonics during the dip
12
different phases and at different locations; this may occur at a
sub-cycle time scale, but also at a time scale up to 1 second.
The delayed voltage recovery and a high waveform distortion
for a short time after the rise in voltage. The waveform
distortion contains a high level of even-harmonic distortion
and The high-level of harmonic distortion during dips due to
transformer energizing, where the waveform distortion
contains high levels of even harmonics
The classification distinguishes between the three general types of voltage dips that may occur at the
terminals of sensitive equipment:
 Dip Type III is a drop in voltage magnitude that is equal for the three voltages.
 Dip Type II is a drop in voltage magnitude that takes place mainly in one of the phase-tophase voltages.
 Dip Type I is a drop in voltage that takes place mainly in one of the phase-to-ground
voltages.
The characteristic voltage has a magnitude and phase angle which are typically different from those
of the pre-event voltage. The difference in magnitude depends on the fault location; the difference in
phase angle depends on the difference in X/R ratio between the source and the faulted feeder. When
the X/R ratios are similar, this will result in a small phase-angle difference. On the other hand, a large
difference in X/R ratio results in a large difference in phase angle. The difference in phase angle
between the pre-event voltage and the (during-event) characteristic voltage is referred to as the
”characteristic phase angle jump”.
In Figure 4 the resulting voltage dips are shown as phasor diagrams for zero characteristic phase
angle jump. The magnitude of the characteristic voltage is in all cases equal to 50%.
Type III, no phase jump
Type II, no phase jump
Typ
Figure 4. Phasor diagrams for different types of voltage dips due to faults that may occur in a three8.3 Dip Immunity Characterization
The aim of characterization is to give information about the performance of equipment to users or
potential users of that equipment. Characterization testing is not ruled by standards and does not
need to be performed in an accredited laboratory. Characterization testing can be done through
experiments, through simulation, or through a combination of the two
8.3.1 Single Phase Equipment
8.3.1.1 Voltage-tolerance curve
Most commonly the performance of equipment during voltage dips is quantified by means of the socalled “voltage tolerance curve”. Note that in this chapter we consider only the voltage-tolerance curve
as quantifying the actual performance of a piece of equipment or of an installation. The term “voltagetolerance curve” is also used as a way of quantifying performance requirements. Those types of
curves are not considered here.
13
NRS 048-7:2008 (WG Draft 0)
The voltage tolerance curve indicates for which combinations of residual voltage and duration the
equipment will not perform as intended. The equipment will not perform as intended only for dips with
residual voltage and duration below the voltage-tolerance curve.
The manufacturer is to specify what is meant by “not perform as intended” when providing
characterization test information.
An example of a voltage tolerance curve is the ITIC curve which describes a voltage envelope that
can typically be tolerated ( in this case defined as no interruption in function) by most information
technology equipment.
Figure 7: ITIC Curve []
When characterizing voltage-dip immunity of equipment, it is recommended to give at least the
voltage-tolerance curve.
 It should be indicated how “operation as intended” is defined.

It should be indicated for which operational state the curve has been obtained. At minimum, a
curve for operation under normal or typical conditions (of load, temperature, humidity, pressure,
etc.) should be provided.
NRS 048-7:2008 (WG Draft 0)

14
The dips used for the characterization testing should have zero phase-angle jump and start and
end at voltage zero-crossing.

The pre-dip and post-dip voltage waveform should be equal to rated voltage magnitude and
rated frequency with low harmonic distortion. Crest factor and THD of pre-dip and post-dip voltage
waveform during the test should be recorded.

The time durations tested should be, at minimum, 1 cycle, 100 ms, 200 ms, 500 ms, 1 second
and 5 seconds. The voltage tolerance should be determined at these durations. The duration of
ride-through at zero volts (with low input impedance) should also be determined. It is desirable to
test at many more durations as well.

The voltage resolution of the curve should be 5% or better.

All test points should be clearly marked. An approximate curve joining these points should be
given.

The vertical scale shall be given in Volt or in percent on nominal voltage with nominal voltage
indicated.
8.3.1.2 Phase-angle jump
When considering phase-angle jumps, the two-dimensional magnitude vs. duration voltage tolerance
curve becomes a three-dimensional surface; if point-on-wave of dip initiation is included as well, the
surface becomes four-dimensional. The effort required to characterize such surfaces could be
justified only if (1) the additional data points provide significant advantage in predicting the number of
equipment trips due to voltage dips and (2) the information can be sufficiently stripped of complexity
to be useful to equipment users. In general there is no need for a manufacturer to provide data on
the influence of the phase-angle jump on the voltage tolerance. Therefore no requirement is set out
here for phase angle jump during voltage dips.
8.3.1.3 Point-on-wave
Equipment be characterized under: (1) start/stop at zero degrees and (2) start/stop at 90 degrees.
8.3.1.4 Multiple event
Equipment may be sensitive to the occurrence of multiple sequential dip events. The time between
multiple dips is most frequently related to the reclosing delay time of reclosing relays. There is a wide
range of reclosing times over various network operators. Characterization for multiple events is not
recommended.
8.3.1.5 Motor starting events
Motor starting events are only a concern in certain industrial environments and in rural areas with
weak distribution systems. Separate tests for these cases are not required. Where needed the
equipment behavior for (non-rectangular) dips due to motor starting should be estimated from the
behavior for rectangular dips as presented in the voltage-tolerance curve.
8.3.1.6 Transformer energizing events
Transformer energizing events occur throughout the power system. Separate tests for these cases
are not required. Where needed the equipment behavior for (non-rectangular) dips due to transformer
energizing should be estimated from the behavior for rectangular dips as presented in the voltagetolerance curve. It should be noted, however, that voltage dips due to transformer energizing are
associated with a high level of second harmonic distortion and that this distortion may affect
equipment operation.
15
NRS 048-7:2008 (WG Draft 0)
8.3.1.7 Pre-event and post-event voltage
Dip immunity is affected by pre-event voltage. For instance, equipment with capacitor energy storage
(proportional to the square of voltage) may have 20% less stored energy available when the pre-dip
voltage is at 90% of rated voltage. Some equipment operators have chosen to improve equipment
immunity by routinely operating at 102% to 105% of rated voltage, at the expense of energy efficiency.
Voltage tolerance curves should represent rated voltage pre-dip and post-dip conditions.
8.3.2 Three-phase equipment
8.3.2.1 Voltage-tolerance curve
It is recommended to give the voltage tolerance curve for three types of dips, corresponding to the
Types I, II and III
The dips are having zero phase-angle jump.
When only one voltage tolerance curve is presented, it should be for dip type I or dip type II, where
the type should be indicated
8.3.2.2 Phase-angle jump
The majority of real-world equipment does not trip because of phase angle jump and so
manufacturers need not test these conditions
8.3.2.3 Point-on-wave
For equipment sensitive to the shape and point-on-wave of voltage recovery, the difference in voltage
recovery between the three (phase-to-phase or phase-to-neutral) voltages should be considered.
8.3.2.4 Multiple events
Same as for single-phase equipment.
8.3.2.5 Motor starting
Same as for single-phase equipment.
8.3.2.6 Transformer energizing
Same as for single-phase equipment.
NRS 048-7:2008 (WG Draft 0)
16
9 Testing and Measurement Techniques
9.1 Test generator
The following features are common to the generator for voltage dips, short interruptions and
voltage variations, except as indicated.
The generator shall have provision to prevent the emission of heavy disturbances, which, if
injected in the power supply network, may influence the test results.
Any generator creating a voltage dip of equal or more severe characteristics (amplitude and
duration) than that prescribed by the present standard is permitted.
9.1.1 Characteristics and Performance of Generators
Table - Generator Specifications
Output impedance shall be predominantly resistive.
The output impedance of the test voltage generator shall be low even during transitions (for
example, less than 0,4 + j0,25 ).
NOTE 1 The 100 resistive load used to test the generator should not have additional inductivity.
17
NRS 048-7:2008 (WG Draft 0)
NOTE 2 To test equipment which regenerates energy, an external resistor connected in parallel to the load can be
added. The test result must not be influenced by this load
9.1.2 Verification of the characteristics of the voltage dips, short interruptions
generators
In order to compare the test results obtained from different test generators, the generator
characteristics shall be verified according to the following:



the 100 %, 80 %, 70 % and 40 % r.m.s. output voltages of the generator shall conform to
those percentages of the selected operating voltage: 230 V, 120 V, etc.;
the 100 %, 80 %, 70 % and 40 % r.m.s. output voltages of the generator shall be measured
at no load, and shall be maintained within a specified percentage of the UT;
load regulation shall be verified at nominal load current at each of the output voltages and
the variation shall not exceed 5 % of the nominal power supply voltage at 100 %, 80 %, 70 %
and 40 % of the nominal power supply voltage.
For output voltage of 80 % of the nominal value, the above requirements need only be verified
for a maximum of 5 s duration.
For output voltages of 70 % and 40 % of the nominal value, the above requirements need only
be verified for a maximum of 3 s duration.
If it is necessary to verify the peak inrush drive current capability, the generator shall be
switched from 0 % to 100 % of full output, when driving a load consisting of a suitable rectifier
with an uncharged capacitor whose value is 1 700 F on the d.c. side. The test shall be carried
out at phase angles of both 90° and 270°. The circuit required to measure generator inrush
current drive capability is given in Figure A.1.
When it is believed that a generator with less than the specified standard generator peak
inrush current may be used because the EUT may draw less than the specified standard
generator peak inrush current (e.g., 500 A for 220 V-240 V mains), this shall first be confirmed
by measuring the EUT peak inrush current. When power is applied from the test generator,
measured EUT peak inrush current shall be less than 70 % of the peak current drive capability
of the generator, as already verified according to Annex A. The actual EUT inrush current shall
be measured both from a cold start and after a 5 s turn-off, using the procedure of Clause A.3.
Generator switching characteristics shall be measured with a 100Ωload of suitable powerdissipation
rating.
NOTE The 100
Ωresistive load used to test the generator should not have additional inductivity.
Rise and fall time, as well as overshoot and undershoot, shall be verified for switching at both
90° and 270°, from 0 % to 100 %, 100 % to 80 %, 100 % to 70 %, 100 % to 40 %, and 100 % to
0 %.
Phase angle accuracy shall be verified for switching from 0 % to 100 % and 100 % to 0 %, at
nine phase angles from 0° to 360° in 45° increments. It shall also be verified for switching
from 100 % to 80 % and 80 % to 100 %, 100 % to 70 % and 70 % to 100 %, as well as from
100 % to 40 % and 40 % to 100 %, at 90° and 180°.
The voltage generators shall, preferably, be recalibrated at defined time periods in accordance
with a recognized quality assurance system.
NRS 048-7:2008 (WG Draft 0)
18
6.1.2 Power source
The frequency of the test voltage shall be within ± 2% of rated frequency.
9.2 Test procedures
Before starting the test of a given EUT, a test plan shall be prepared.
The test plan should be representative of the way the system is actually used.
Systems may require a precise pre-analysis to define which system configurations must be
tested to reproduce field situations.
Test cases must be explained and indicated in the Test report.
It is recommended that the test plan include the following items:
 the type designation of the EUT;
 information on possible connections (plugs, terminals, etc.) and corresponding cables, and
peripherals;
 input power port of equipment to be tested;
 representative operational modes of the EUT for the test;
 performance criteria used and defined in the technical specifications;
 operational mode(s) of equipment;
 description of the test set-up.
If the actual operating signal sources are not available to the EUT, they may be simulated.
For each test, any degradation of performance shall be recorded. The monitoring equipment
should be capable of displaying the status of the operational mode of the EUT during and after
the tests. After each group of tests, a full functional check shall be performed.
9.3 Test report
The test report shall contain all the information necessary to reproduce the test. In particular,
the following shall be recorded:
 identification of the EUT and any associated equipment, e.g. brand name, product type,
 serial number;
 identification of the test equipment, e.g. brand name, product type, serial number;
 any special environmental conditions in which the test was performed, for example shielded
enclosure;
 any specific conditions necessary to enable the test to be performed;
 performance level defined by the manufacturer, requestor or purchaser;
 performance criterion specified in the generic, product or product-family standard;
 any effects on the EUT observed during or after the application of the test disturbance,
andthe duration for which these effects persist;
 the rationale for the pass / fail decision (based on the performance criterion specified in the
generic, product or product-family standard, or agreed between the manufacturer and the
purchaser);
 any specific conditions of use, for example cable length or type, shielding or grounding, or
EUT operating conditions, which are required to achieve compliance.
9.4
Test Instrumentation: Examples of generators and setup
See annex B for examples of generators and setup.
19
NRS 048-7:2008 (WG Draft 0)
10 Immunity Objectives
10.1 Immunity Objectives and Minimum Performance Criteria
The principal objective of this document is to provide industrial customers and plant designers with
guidance on what measures to take in ensuring their plant is not adversely affected by voltage dips.
A further objective is to provide guidance to equipment manufacturers on how to conduct equipment
performance tests and what needs to be considered during the development of equipment (several
check lists in the appendix should provide this guidance).
At the end of the day the manufacturer can also charge more for his equipment when he provides
more performance test results and higher equipment performance than a competitor’s equipment.
In order to reach that objective it is necessary to divide the responsibility, or give guidance on what
party should perform what.
In principal there are 4 parties involved:
 Industrial customers and process system designers

Electrical utilities

Equipment manufacturers

Regulator
10.2 EQUIPMENT PERFORMANCE CHARACTERISTICS
10.2.1 Equipment Classification
The following terminology applies:

FULL OPERATION
Equipment performs at full rated operation within technical specification in terms of required
output (e.g. speed, voltage level…) and the whole process to which the single equipment
belongs rides through the voltage dip.

RECOVERY
Equipment performs NOT within technical specification, but does automatically recover.

ASSIST
Equipment performs NOT within technical specification, and does NOT automatically recover.

FAILURE
Any unplanned stoppage or variance from the specification of equipment operations other
than assists

INTERRUPT
Any Equipment assist or equipment failure

PASS/FAIL CRITERIA
The pass/fail criteria for voltage dip immunity is testing of single and 3-phase equipment shall
be no interrupts as defined above
NRS 048-7:2008 (WG Draft 0)
20
The following voltage immunity classes are introduced:
 A …which provide best ride through capabilities

B
…which provides good ride through capabilities making equipment immune to most
voltage dips.

C
…basic ride through capabilities to meet basic customer responsibility for voltage dip
immunity, basic required immunity performance

D
…no recognition of ride through capabilities – equipment that fails to meet class C
performance as well as untested equipment
10.2.1 SINGLE PHASE EQUIPMENT CRITERIA
Table 3 defines the test levels and durations for single phase equipment.
Table 3 - test levels and durations for single phase equipment
CLASS
D
C
0%
20ms
Test level and Duration
No requirement
70%
500ms
B
0%
20ms
50%
3 sec
A
0%
20ms
40%
3sec
80%
3sec
Voltage expressed as remaining voltage as percentage of nominal
The output of Table 3 is illustrated graphically in Figure 8.
100
90
CLASS C
CLASS C
80
CLASS B
70
CLASS B
60
50
CLASS A
40
CLASS
A,B,C
30
20
10
0
%
10
100
Time [ms]
1000
Figure 9: Single phase equipment classification
Table 4 defines the test and reference conditions
10000
21
NRS 048-7:2008 (WG Draft 0)
Table 4 - test and reference conditions for single phase equipment testing
SINGLE PHASE EQUIPMENT TEST REQUIREMENTS
Reference conditions
Pre-dip voltage
Nominal voltage ± 1%
Loading
No load
Harmonic distortion
< NRS-048-2 LV limits
Test conditions
Dip profile
Square
Dip magnitude accuracy
± 1%
Duration
500ms ± 2.5ms (50Hz)
Phase shift (jump)
0 degrees
Point-on-wave
zero crossing ± 1ms (50Hz)
Note 2: Chatter is not considered for the purpose of defining the tolerance curve.
10.2.2 THREE PHASE EQUIPMENT CRITERIA
Table 5 defines the test levels and durations for single phase equipment.
Table 5 - test levels and durations for 3-phase equipment
CLASS
D
C
B
A
40%
20ms
0%
20ms
0%
20ms
Test level and Duration
No requirement
70%
500ms
50%
3 sec
0%
200ms
The output of Table 5 is illustrated graphically in Figure 10.
80%
3 sec
40%
3sec
NRS 048-7:2008 (WG Draft 0)
22
100
90
CLASS C
CLASS C
80
CLASS B
70
60
CLASS B
CLASS C
50
CLASS A
40
CLASS
A
CLASS
B
30
20
10
0
10
100
1000
Figure 10: Single phase equipment classification
Table 6 defines the test and reference conditions
Table 6 - test and reference conditions for three phase equipment testing
3-PHASE EQUIPMENT TEST REQUIREMENTS
Reference conditions
Pre-dip voltage
Nominal voltage ± 1%
Loading
Harmonic distortion
75% loading
< NRS-048-2 LV limits
Test conditions
Dip profile
Dip type
Dip magnitude accuracy
Phase shift (jump)
Point-on-wave
3 phase fault level (MVA)
Square
Phase-to-phase dips on each of the three phases.
± 2%
0 degrees
zero crossing ± 1ms (50Hz)
minimum 20 x load (kW) under test
10000
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NRS 048-7:2008 (WG Draft 0)
10.3 Process dip immunity assessment and improvement
The following steps are recommended inorder to improve process immunity against voltage dips:
STEP 1.1: SUPPLY PERFORMANCE
Get information about what voltages dips should be expected or are typical at PCC.
STEP 1.2: PROCESS PERFORMANCE REQUIREMENT
Assessment of the number of process trips an industrial customer can tolerate in a typical year of
production.
STEP 1.3: PIT (PROCESS IMMUNITY TIME)
Process assessment to find the critical equipment
STEP 2: PROCESS IMMUNITY REQUIREMENT
With the inputs form the supply performance and the process performance requirement the
required voltage dip immunity curve for the process can be established.
STEP 3: EQUIPMENT PERFORMANCE REQUIREMENT
With the PIT, the required performance criteria (full operation, recovery, assist) and the required
immunity curve for the process the assessment for each individual equipment from the critical
path can be done. Remark: various combinations of voltage tolerance and performance criterion
may work (e.g. equipment full operating during dip or restart if PIT is high).
STEP 4: EQUIPMENT / MITIGATION SELECTION
In the final step the selection how to make the process more immune is taken, the main choices
are to buy commercial available equipment, to specify a new equipment, or buy mitigation
equipment.
NRS 048-7:2008 (WG Draft 0)
24
.
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NRS 048-7:2008 (WG Draft 0)
Annex A
(informative)
Behaviour of Installations and Processes
A.1 Process behaviour
Understanding the behaviour of a process during a voltage dip or short interruption is essential to be
able to take the correct measures to increase the immunity to a desired level. Even experienced
people have to recognize that gathering this understanding takes a lot of effort and is in most cases
only valid for one specific process. In literature, some case studies
When designing a new process, the impact of voltage dips is never the first concern and in most
cases not considered at all. As a result, the awareness (appreciation) of the company to voltage dips
increases as the costs involved with this disturbance grow. The involved cost is the driving factor to
examine the process and to take corrective retrofit measures.
After studying several processes, two categories were identified: processes requiring tight control of
process variables and processes where large parameter variations can be allowed without disturbing
the entire process. Examples of the first group are synchronized movements, tight speed or position
control, accurate temperature control. Examples for the second group are fluid level control and flow
rate control, air fans, ...
In most processes, a mix of both types of processes is present.
As an example, a paper production facility is analyzed. After the wood preparation, the pulp is
produced and transformed to paper in the paper production process.
Wood preparation : Barking drum, Wood chipper
In this part, normally DOL induction motors are used and the impact of voltage dips is related to the
motor contactors used. The impact of voltage dips on the production is low because of the chip
storage pile which gives enough time to restart any motor. The worst part is the restart of wood
chipping motor with logs inside the machine which can create a voltage dip at the restart.
Pulp preparation : Refiner, Blower, Pumps
The refiners are normally large synchronous motors from 1000 HP to 25 000 HP. The blowers and
pumps are typically equipped with ASD’s and induction machines. The immunity of this part depends
on the ASD’s used for the pumps and blowers because the system process will not allow the material
jams and must keep the fluidity in the process.
Paper Production : Press, Pumps, Calendars, Dryers, Reel, Winder
This part is the most vulnerable to voltage dips because of the required synchronization between the
different processes. ASD’s are used in the entire process. They control the speed differential between
the different production stages. Voltage dips can disturb the good operation of this synchronization
and can result in tearing apart the paper sheets. Restarting this process is time consuming and very
expensive..
The paper production process is clearly the most sensitive part of the process because of the need of
synchronized operation within the process. The pulp production and the wood preparation are less
sensitive. They are characterized by buffer capacity to overcome short interruptions. Finally, figure 3-8
shows the impact of voltage dips on the operation of the process.
NRS 048-7:2008 (WG Draft 0)
26
Voltage DIP for Pulps & Paper
Worst phase case
With without lost of production
DIP with lost of prodcution
100%
90%
80%
Voltage (%)
70%
60%
50%
40%
30%
20%
10%
0%
0,001
0,01
0,1
1
10
100
duration (seconds)
Figure 0-8: Typical immunity of a pulp and paper process showing the voltage dip’s affecting the
normal operation of the process.
A.2 Process Immunity Time concept
The purpose of the proposed methodology is to identify the critical equipment within a process. The
framework is centred around the Process Immunity Time concept (PIT). Each single piece of
equipment in the process is linked with the process parameter(s) on which the equipment has an
impact. A piece of equipment is also contains the switchgear which connect it to the supply. For
example, a direct on line induction motor together with the motor starter and protection equipment is
considered as one piece of equipment during the analyses. Figure 3-9 shows how the PIT can be
determined. Starting with the nominal process parameter value pnom, a supply voltage interruption at t1
to the single piece of equipment is considered. As a result, the process parameter starts to move
away from its nominal value. This may happen instantaneously or, as depicted in Figure 3-9, after a
time interval t. This retardation can be explained to the tripping of the equipment t seconds after
the actual supply voltage interruption or to a dead time in the process itself. At time t 2, the process
parameter value crosses the lower boundary plimit for normal operation of the process. Starting from t2
on, the process no longer operates as intended and must be shutdown. The PIT is now defined as the
time interval between the start of the voltage interruption and the moment the process parameter
goes out of spec.
27
Process
parameter
NRS 048-7:2008 (WG Draft 0)
PIT
pnom
plimit
t1 t1+t
t2
Time
Figure 0-9: Definition of the Process Immunity Time (PIT) for a piece of equipment.
Considering a supply voltage interruption for the determination of PIT is a worst case scenario. The
behaviour of the equipment under such conditions is easy to understand. Furthermore, testing for PIT
is simple because generating a supply interruption is easy compared to voltage dip testing2.
The first step in the procedure is to generate a list containing all pieces of equipment within the
process. The list being as complete as possible is essential. From practical experiences it is known
that most of the time tripping of processes is caused by pieces of equipment, such as sensors, ice
cube relays, controls, of which the engineers were not aware of their impact on the process due to a
voltage dip. The process under consideration is split up in functional units or levels. The number of
levels required depends on the complexity of the process.
The lowest level contains the single pieces of equipment. Finally, for every equipment, the process
parameter involved is identified. In table 3-X this methodology is applied for a simplified chemical
reactor process (level 1) being a part of a chemical plant. In level 2, three subsystems are identified:
the cooling system of the reactor vessel, the reaction process and the controls.
The cooling of the reactor is accomplished by means a direct on line induction motor (DOL IM 1)
driving the water pump. This piece of equipment directly affects the reactor cooling water temperature
which is a critical parameter for the overall process. This pumping system also contains a small oil
pump operating at high oil pressure to lubricate the pump. The cooling of the water circuit is realised
by means of a DOL induction motor driving an air fan (DOL IM 2).
The reactor vessel is equipped with a feed pump (DOL IM 3) to control the flow rate of the chemicals.
For a good and homogenous reaction, a speed regulated mixer is required. This is accomplished by
means of an adjustable speed drive (ASD 1). As the reaction also requires oxygen, an adjustable
speed drive is used to control the air inlet (ASD 2). Temperature en oxygen control are required for a
good reaction process. Both temperature and oxygen sensors are connected to a PLC who is taking
care of the control actions. The temperature sensor is fed directly from the PLC. The oxygen sensor
is supplied separately and communicates with the PLC over a fieldbus. The PLC is equipped with an
Uninterruptable Power Supply (UPS) to keep the PLC and the fieldbus up and running during voltage
dips and interruptions.
In this example process, every piece of equipment is related to only one process parameter. In some
systems however, it is possible that one piece of equipment influences more than one process
parameter. In that case, all the process parameters involved are listed. Some process parameters
may also be controlled by multiple pieces of equipment. To correctly interpret these interconnections
and finally to determine the PIT for each equipment – parameter combination, electrical engineers,
process engineers and instrumentation engineers need to sit together to share information and to
understand the limitation of the supply system and of the process. To determine the PIT, it is essential
2
Voltage dip testing requires specific equipment which is expensive and limited in power range.
NRS 048-7:2008 (WG Draft 0)
28
to establish the nominal value and upper and lower limits for each process parameter. This is typically
a process engineers responsibility.
For each combination “equipment – process parameter”, the PIT is determined by considering a
supply interruption to only that piece of equipment. For an existing process, fault recordings from past
voltage disturbances can be most useful at this stage. For new processes to be build, process
simulations, calculations or experience form similar processes can be used. If possible, field test are
the most reliable way to determine the exact PIT values. They are not always possible due to safety
reasons or in case of a continuous process because interrupted are not allowed.
Once all PIT values are determined, a ranking of the most critical pieces of equipment is available.
This ranking can be done for each defined level within the process. At level 3 for the example process
in table 3-X, the oxygen measurement sensor is the most crucial piece of equipment followed by the
oil pump and the ASD controlling the oxygen inlet. The overall PIT of the reactor vessel is dictated by
the oxygen sensor and is as low as 1 second. The DOL IM 2 driving the fans for the cooling of the
cooling water and the PLC with UPS are the least sensitive pieces of equipment. At level 2, the
control system is the most critical system followed by the cooling system and the reaction process
itself.
Table 3-X: Equipment list and PIT values for a simplified chemical reactor process.
LEVEL 1
LEVEL 2
Process
parameter
LEVEL 3
PIT
Priority
Action
Reactor
Cooling
DOL IM 1
(water)
Reactor cooling
water temp
5s
4
Restart 1
Oil pump
Oil pressure
1,5s
2
Crucial
DOL IM 2 – fan
Cooling of the water
circuit
3min
7
Restart 3
DOL IM 3
(feed)
Flow rate
30s
6
Restart 2
ASD 1 (mixer)
Reaction time
6s
5
Restart
ASD 2 (air)
O2
2s
3
Mitigate
Temperature
sensor
Reactor temperature
1h
8
Oxigen
measurement
% O2
1s
1
1h
8
Reaction
Control
PLC with UPS
Mitigate
Using PIT to select the appropriate mitigation strategy
From the PIT analysis, processes, functions and equipment can be divided into two groups.
Processes with high PIT and processes with low PIT. High and low are related to the typical duration
of a voltage dip. Processes with high PIT values are perfectly capable to operate without supply
voltage for a small period of time (e.g. ventilation system, fluid level control). The process contains
natural buffering against short time interruption. The DOL IM 3, controlling the flow rate of the
chemicals of the simplified chemical reactor example has a PIT of 30s. At the occurrence of a voltage
29
NRS 048-7:2008 (WG Draft 0)
dip or a short interruption, this piece of equipment can be shut down or disconnected from the supply.
The motor speed decays. Once the voltage has recovered, the equipment can be reconnected to the
supply. In the case of the DOL IM 3, the motor reaccelerates and restores the process parameter
value. This procedure of disconnecting and restarting the equipment can only be successful if the
duration of the voltage dip tdip or interruption augmented with the restart time and recovery time of the
equipment trestart is less than the PIT value:
Process
parameter
PIT
plimit
tdip
trestart
Time
Figure 0-10: Correct restart of the equipment after a voltage dip with durarion t dip.
Figure 3-10 shows the process parameter behaviour for a controlled stop and restart action. The
process parameter value remains within the allowable range. The time interval between t dip and trestart
can be dictated by the scanning time or response time of the control system governing the restart
procedure.
If the process contains several direct on line motors with high PIT values for the respective process
parameters, shutdown and a coordinated restart procedure is a cost effective mitigation method. In
the case of big motors, starting all motors simultaneously after voltage recovery causes very high
starting currents and may cause a shallow voltage dip. The PIT value can be used here to determine
which motors should be started first.
The second group contains equipment with very small PIT. They trip quickly after the occurrence of a
voltage interruption or a voltage dip (e.g. speed control, synchronized motion control). Controlled shut
down and restart is not an option here. For these processes, the knowledge of the individual
equipment behaviour under dip conditions is required to take the correct measures to harden the
process. The oxygen measurement in table 3-X has a very small PIT. The oxygen measurement is
directly fed from the supply. At the occurrence of a voltage dip, this sensor trips within 1 second. As a
result the value for the oxygen concentration in the reactor vessel is no longer sent to the controlling
PLC. As a result, the controller interprets the lack of information from the oxygen measurement
system as an unpermissible state and decides to shut down the entire reactor process. As the power
requirement for the oxygen measurement is low, an easy solution is to supply this equipment from the
UPS system already available for the PLC system.
The resulting PIT value for the oxygen sensor increases from 1 second up to approximately 1 hour.
The overall PIT for the reactor vessel is now dictated by the PIT of the oil pump in the cooling water
subsystem.
The PIT procedure as described before identifies critical equipment, requiring mitigation techniques
and equipment which can be shutdown and restarted without causing a total process shut down. This
last category uses the natural buffering available within the process. When designing new processes,
one strategy can be to add extra buffers. In most cases, this will be less expensive compared to other
mitigation techniques. In the example reactor process, the reaction process is very sensitive to
interruptions in the air (oxygen) supply. Instead of directly driving the air inlet, an intermediate air
NRS 048-7:2008 (WG Draft 0)
30
buffer (accumulator) between the ASD 2 driving the fan and the reactor vessel. Starting from the
required PIT for this piece of equipment, the required sizing of the accumulator can be realized. The
same strategy can be used to increase the PIT value of the oil pump in the cooling water subsystem.
Integrating voltage dip levels in the PIT procedure
So far, the procedure assumed a supply interruption as a staring condition to determine the PIT. As a
result, the most critical equipment and equipment that can be stopped and restarted are identified. If
typical voltage dip levels are known or a voltage tolerance level for the process is specified, the
procedure can be refined for those pieces of equipment that have shown to be very critical after the
PIT analyses. Adjustable speed drives for example cannot ride through voltage interruptions but in
most cases are very well capable to withstand voltage dips with remaining voltage up to 70%. For
these types of equipment, different PIT’s for different voltage dip levels can be useful. In most cases,
information from manufacturers (voltage tolerance curves) is required to accomplish this refinement.
Flowchart for evaluating equipment behavior
Within the PIT procedure defined before, a piece of equipment always contains the contactor, breaker
or manual switch ,indicating how the equipment is connected to the supply. A breaker is immune to
dips as it is a pure mechanical device. Contactors however may trip due to a voltage dip or
interruption. When examining the equipment behaviour, this switchgear must be taken into account.
Figure 3-11 shows a flowchart to tackle this problem. Only sensitive electrical equipment is
considered. Protective devices in the process power distribution system are not considered. A
distinction is made between manual switches, breakers and contactors. Manual switches are typically
used with sensors and control systems. If the PIT for this equipment is small 3, UPS systems are
required to keep the equipment up and running. If no back up supply is available, the process will not
ride through.
Contactors are available with and without dip protection. A contactor without dip protection will trip and
disconnect the equipment from the supply. The equipment is no longer active. If there is no auto reset
(reclosing) for the contactor when the voltage has recovered and no restart mechanism of the
equipment, the process will not ride through. If restart is possible and the PIT is not to small, the
process can continue its normal operation.
3
In the flowchart (Figure 3-11), a 4 second limit is suggested. This value however depends
on the type of process and requirements to be achieved.
31
NRS 048-7:2008 (WG Draft 0)
Figure 0-11: Flowchart to analyse the behaviour of a piece of equipment based on the type of connector (breaker, contactor)
between the grid and the equipment.
For breakers and contactors with dip protection, the equipment stays connected to the supply system.
Whether this equipment will continue to operate depends on its protection settings. If the equipment
trips and an auto reset and restart can be realized within the PIT time, the process can ride through.
NRS 048-7:2008 (WG Draft 0)
32
Annex B
(informative)
Examples of generators and setup
Figures C.1a) and C.1b) show two possible test configurations for mains supply simulation.
To show the behaviour of the EUT under certain conditions, interruptions and voltagevariations are
simulated by means of two transformers with variable output voltages. Voltage drops, rises and
interruptions are simulated by alternately closing switch 1 and switch 2. These two switches are never
closed at the same time and an interval up to 100μs with the two switches opened is acceptable. It
shall be possible to open and close the switches independently of the phase angle. Semiconductors
switches constructed with power MOSFETs and IGBTs can fulfil this requirement. Thyristors and
triacs open during current zero crossing, and therefore do not meet this requirement.
The output voltage of the variable transformers can either be adjusted manually or automatically by
means of a motor. Alternatively, an autotransformer with multiple switchselected taps may be used.
Wave-form generators and power amplifiers can be used instead of variable transformers and
switches (see Figure C.1b)). This configuration also allows testing of the EUT in the context of
frequency variations and harmonics.
The generators described for single-phase testing (see Figures C.1a), C.1b) and C.1c) can
be also used for three-phase testing (see Figure C.2).
33
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NRS 048-7:2008 (WG Draft 0)
34