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Electricity Association Document
G5/4-1...
Background
As with all other harmonic
regulations world-wide, G5/4 was written to ensure that
levels of harmonic current in the Public Electricity
supply system do not give rise to problems for connected
users. Previously in the UK the harmonic distortion
was regulated by Electricity Council Recommendation
G5/3 (1976) and this still remains in place today for
existing supplies. Parts of G5/3, however, were made
obsolete by the introduction of BSEN61000-3-2 which
covers supplies and equipment up to 16A, so a replacement
was required.
Measurements
To ensure G5/4 compliance,
measurements must be taken at the Point of Common Coupling
with other users (usually the energy metering point
- for a customer with their own transformer this will
be 11kV). Measurements are generally needed for Stage
2 and Stage 3 assessment to give background distortion.
Responsibility for the background tests lies with the
DNO (Distribution Network Operator) company, although
the customer may be asked to assist.
Measurements must
be taken over a seven (7) day period to cover peak distortion
periods (Saturday & Sunday evening), unless the
process will not operate over these periods.
The Prospective
Fault level of the supply is a crucial element as it
determines the current distortion limits; the levels
stated in the document's tables should be scaled according
to the fault level. All the stated figures are based
on 10MVA for 400V supplies and 100MVA for 11kV supplies.
Where a system has a lower fault level (e.g. 50MVA for
an MV user) the allowable 5th harmonic current is halved
from 3.9 to 1.95A.
Voltage distortion
measurements are extremely important as the majority
of cases will be decided on the THD(V) figure for the
supply.
For the purpose
of the study (G5/4 compliance) short duration harmonic
generating loads such as Soft Starter Drives are exempt
(assuming they comply with P28 flicker regulation).
Stage 1
This stage applies to all supplies between 16 and 75A,
and connections at 400A. A 10MVA fault level is assumed
and allows for up to a maximum of four harmonic generators
to be connected to the supply. The levels of current
distortion must not exceed the figures given in Table
7.
Where these are
exceeded then a THD(V) of no more than 5% is allowed.
Typical sizes of drives that can be connected based
on typical drive spectra (Gambica G5/4 Guide 2001) are:
6 pulse - 30kW (limited
by 5th harmonic current)
12 pulse - 300kW (limited by 23rd harmonic current)
Where the current
and voltage distortion figures have been exceeded the
assessment must move to Stage 2 .
Stage 2
This stage applies to those
exceeding Stage 1 levels and those having a point of
common coupling below 33kV. At this stage the NOC is
obliged to measure background distortion and inform
the user of the fault level.
Total drive loads
of the following sizes can be connected without further
assessment:
6 pulse - 130kVA
12 pulse - 250kVA
If the level of
background distortion is less than 75% of the maximum
THD(V) allowable, then Table 12 should be used.
Excluding 3rd and
5th, any two orders (up to 19th) may exceed the values
by 10% or 0.5A, whichever is greater. Above 19th they
may exceed by 10% or 0.1A whichever is greater.
Where these figures
have been exceeded then the voltage distortion levels
given by Table 11 should be used for THD(V) and levels
of 5th harmonic voltage distortion (other harmonic order
limits are as per draft IEC61000-2-12).
Stage 3
This assessment applies
to circumstances which fall outside the scope of stages
1 and 2. It is applicable to any non-linear equipment
that has a PCC of 33kV and above.
Harmonic voltages
at the PCC are assessed on an impedance model of the
network, and possible resonance points. The modelling
and metering is the sole responsibility of the DNO.
For 33kV and above,
the assessment should take into account the effects
of harmonic distortion on connected lower voltage networks.
Circumstances
for Exceeding G5/4
Where the voltage distortion
levels are exceeded, the DNO will give a conditional
connection agreement which may need mitigation measures.
However the final decision on whether connection is
possible is left with the DNO (judge and jury).
Where circumstances
are uncertain (possible resonance) then the DNO may
give a conditional connection where remedial measures
can be implemented within a reasonable time-scale.
In exceptional circumstances,
where the site to be connected is remote from other
customers, it may be considered at Stage 2 with computability
levels appropriate to the system voltage.
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