Download Comparison of harmonic limit allocations in IEC TR 61000-3

Comparison of harmonic limit allocations in IEC TR 61000-3-6 and ER G5/4-1 for MV Networks
IEC TR 61000-3-6
Stage 1 section 8.1.1 provides limits based on
ratio of agreed power, Si to short-circuit power,
Ssc. Stage 1 section 8.1.2 provides limits based on
ratio of sum of weighted distorting power, SDwi, to
short-circuit power, Ssc.
If the ratio Si/Ssc or SDwi/Ssc ≤0.2% then
connection is permitted without detailed
assessment.
ER G5/4-1
The nearest comparable part of G5/4-1 is Stage 2 Table 10
which permits 130kVA 6-pulse convertor, 250kVA 12pulse convertor and 150kVA 6-pulse AC regulator to be
connected without detailed assessment to 11kV; this
assumes Ssc = 100MVA. This equates to SDwi/Ssc of 0.26%,
0.13% & 0.104% for the three types of 6-pulse convertor in
the IEC, 0.125% for 12-pulse convertor & 0.105% for 6pulse AC regulator.
Stage 2 section 8.2.1 gives a simplified stage 2
assessment. This gives limits on harmonic
currents as a percentage of the current
corresponding to the customer’s agreed power.
Indicative limits are given in Table 5 applicable
when agreed power, Si ≤1MVA, Si/Ssc <1%,
background levels and not restrictive and there is
no power factor correction.
Stage 2 Table 12 gives current limits in Amperes. These
are based on a single customer raising the harmonic voltage
distortion levels by 25% of the planning levels. The Table
only applies where background levels ≤75% of the planning
level. This is a first come, first served approach and is not
based on the agreed power. For comparison, at Si = 1MVA,
G5/4-1 gives higher limits for h = 5, 7, 11 & 13 but not for
some higher orders.
Stage 2 section 8.2.2 gives a more complex
assessment method:
a) Establish the global harmonic voltage
contribution, GhMV+LV, that can be
allocated to the total of MV and LV
installations supplied from the MV node.
Note this requires knowledge of the
transfer coefficient, T hUM, from the
upstream system to the MV node by
simulation. Example using existing
The Table 12 limits have to be scaled according to shortcircuit power.
The detailed Stage 2 section 7.3 assessment method
involves prediction of voltage distortion taking account of
background voltage distortion, assumed network impedance
(based on 50Hz short-circuit power, harmonic order h and
standard values of ‘k’ allowing for resonance) and customer
requested emission currents up to the 50th harmonic. The
predicted values are compared with the planning levels for
5th harmonic and THD. The assessment can done using a
simple spreadsheet approach. It considers only the point of
common coupling so like Stage 2 section 8.2.2 of the IEC is
Comments
The levels permitted are broadly of the same order but with
IEC less conservative for all but one of the 6-pulse cases.
The assumption of Ssc = 100MVA underlying Table 10 of
G5/4-1 assumes a fairly robust network. It makes sense to
scale the Table 10 limits according to short-circuit power
but this is not mentioned in this part of G5/4-1. The IEC
approach is more general, allowing for scaling for shortcircuit power. It also covers 33kV so would allow 1MVA
sum of weighted distorting power for a typical 500MVA
short-circuit power at 33kV.
Both methods require knowledge of the background levels
and both take account of short-circuit power but the IEC
approach gives no guidance on what level of background
distortion is acceptable for use of this method.
The IEC approach is based on agreed power so applies the
equal rights approach whereas G5/4-1 uses first come-first
served.
The G5/4-1 approach is not limited to Si ≤1MVA and so
can be applied to a wider range of cases.
With the IEC approach there are some concerns:
a)
How can it be applied retrospectively as existing
customers may/will have taken part of the margin?
b) It is unclear how the future load growth would be
taken into account in deriving St.
c) It would appear to treat those with small import
capacity (e.g. generators) unfavourably if St relates
to import capacity and not export capacity.
d) Derivation of the transfer coefficient implies
G5/4-1 planning levels: At 33kV fed
from 132kV for h=5 with L5MV =2%,
L5Us =2%, α=1.4 and T5UM = 1 then
GhMV+LV = 0%! At 11kV fed from 33kV
for h =5 with L5MV =3%, L5Us =2%,
α=1.4 and T5UM = 1 then GhMV+LV =
1.65%.
b) Establish the total available power, St, at
the location under consideration. The
analysis does not include neighbouring
substations.
c) Allocate a portion of the global emission
limit based on the ratio of agreed power,
Si, total available power, St, to a root α.
limited to one node. No detailed modelling is required and
consequently the approach can be applied without resorting
to consultants. This Stage 2 approach is not applied to
connection at 33kV and above. Many more connections are
made at 11kV than 33kV and so this type of assessment
represents a significant assessment activity.
detailed modelling and so is more complex than
the G5/4-1 equivalent. This has cost and resource
implications.
e) The allocation policy is not foolproof –
background levels at LV could grow say due to
increasing penetration of electronic loads.
f) When LhMV = LhUS and ThUM = 1 then this
gives GhMV+LV = 0% and it is not clear how to
apply the technical report.
g) 8.2.2.2 and 8.2.2.3 highlight limitations of the
basic rules and offers alternatives adding to
complexity e.g. “For distribution systems with
long cables and overhead lines, where customer
installations are distributed along the length of the
feeders, the above approach may result in
specifying too strict harmonic currents, thus
penalising customers connected at some distance
down the line…’ The report gives Annex B where
it becomes clear that complexity is dramatically
increased over G5/4-1
h) The allocation of part of the margin could increase
cost for first ‘connectees’ and the margin reserved
for others may be never be utilised. The move
away from minimum cost may not be acceptable to
OFGEM.
Stage 3 section 8.3 involves conditional
connection where higher levels than allowed
under stage 2 may be conditionally allowed. The
text does not appear to explicitly state that the
effect on lower voltage networks must be
considered. Conditional connection may be
justified based on a number of reasons including
the Stage 2 approach is, after consideration,
judged to be too conservative.
Stage 3 involves the determination of harmonic voltages at
the PCC based on a harmonic impedance model of the
network to take account of any resonance. For connections
at 33kV and above the assessment takes account of the
effect on lower voltage networks. Use of a computer
analysis program is recommended.
Increased levels of detail of assessment using computer
analysis are implied. Stage 3 assessments are recognised as
being onerous in time, skill level and resources.