Les Hunt takes a look at the essentials of the G5/4 recommendations and
discovers how drives suppliers are helping their customers to meet the
new harmonics targets
Earlier this year, the Electricity Association (www.electricity.org.uk)
published its G5/4 Engineering Recommendation, which sets new target
levels for harmonic currents imposed on the public electricity supply
network at the so-called 'point of common coupling' (PCC). They supersede
the G5/3 set of recommendations, which have been in existence since 1976,
and reflect increasing use of rectifiers in industrial, commercial and
domestic electrical equipment.
The PCC is an important concept, as it defines precisely where G5/4 is
applied. For example, an industrial consumer will probably be connected
to the medium voltage (11kV) network via a transformer, which reduces the
voltage to 420V for use within his factory or plant. If no other
consumers are connected to this 400V network, then the PCC for this
installation will be at 11kV and G5/4 will be applied at this point by
the network operating company (NOC). The levels of harmonic current and
voltage at intermediate points on the consumer's own 400V network,
however, are entirely a matter for him, as it will be his (not other
consumers') equipment that may be at risk of damage, if they happen to be
above recommended levels.
G5/4 sets out three stages of assessment (similar to G5/3, though not
wholly corresponding), which get progressively more complex. Detailed
descriptions of these stages are completely beyond the scope of this
article, but a lot of help and advice is available from commercial
sources, particularly the major drives manufacturers, and representative
bodies such as Gambica (www.gambica.org.uk), which has produced an
excellent guide on the subject, entitled 'Managing harmonics - a guide to
EA Engineering Recommendation G5/4'.
Eurotherm Drives' customer assurance manager, Martin Payn has very wide
practical experience gained from visiting existing and potential
customers' sites, where he regularly presents onsite seminars on
harmonics management in general and G5/4 in particular. Meeting the G5/4
recommendations are solely the responsibility of the end-user, who should
always investigate the harmonics issues raised by his operations without
having to be prompted into action!.
According to Dr Payn, the process may not be as onerous as the plant
operator at first envisages. A key thing to remember when deciding
whether a reassessment is necessary under G5/4 is that it is not imposed
retrospectively. Even when a refurbishment is undertaken, if the harmonic
signature that existed under G5/3 is achieved with the new equipment, and
documentation is available to verify this, then there is no need to meet
G5/4. However, if an installation is extended by the addition of new
equipment, it will have meet G5/4 via one or more of the three compliance
routes or assessment stages, generally dictated by the voltage at the
PCC. Harmonic distortion limits are not covered by statute; the only
'enforcing' document that exists is the supply connection agreement drawn
up between the site operator and his NOC. Normally, harmonic problems are
only reported by other users sharing a PCC when related faults are
noticed (described later in this article). At the very worst, the NOC can
exercise its power to turn a facility off, particularly at a time when
the interference is most active with regard to other users, but this
would only happen if all other avenues had been exhausted. While G5/4
applies to a very wide range of electrical equipment, for the purposes of
this article, only variable speed drives - a potential major source of
harmonic distortion - are considered. But first, how is the distortion
created, how do we measure it, what are its effects and how can it be
limited to ensure compliance under the new G5/4 recommendations? In a
normal inverter drive, diodes are used as a simple method of rectifying
the ac supply into a dc voltage. The latter is stored in the drive's dc
bus capacitors, prior to being converted into a variable (voltage and
frequency) supply for the motor.
Diodes will only allow current to flow in one direction and only when the
voltage level is higher on the anode (positive) side compared to the
cathode (negative) side. In the case of diodes charging up the dc bus
capacitors of an ac drive, as the capacitors become fully charged, the
diode stops conducting. When this happens, it does not behave like a
perfect switch, and a small current flows in the opposite direction,
adding harmonic currents to the mains supply. These harmonic currents
cannot be reclaimed and serve no useful purpose, but still contribute to
the volt-ampere loading in generating losses in the distribution system.
Measurement is the responsibility of the user, but many suppliers will
offer to carry this out on their customers' behalf. It is probably better
to go this latter route, as suppliers are likely to have better equipment
and the software necessary to complete the analysis. The main difference
between G5/3 and G5/4 is that the latest recommendations require
measurements to be taken over a seven day period in order to establish
the worst case operating conditions. Measurements are covered by a
European standard (EN 61000-4-7), which defines the class of accuracy for
the measuring equipment.
Symptoms of a harmonics problem on site include burnt-out neutral
conductors due to overload (single phase loads); interference with
communication equipment; overheating of transformers and induction
motors; higher than normal noise emissions from transformers and
switchgear, and in severe cases, failure of equipment such as power
factor correction capacitors. Extreme care should be exercised when
powering an inverter from a stand-alone generator, since the alternator
windings can easily be damaged by the effects of harmonics. Harmonic
currents add to the normal line currents, which is the main reason for
the input current to an ac drive being higher than the output current.
This can have the effect of making the apparent power factor of the drive
appear much lower than it actually is (in fact at the frequency of 50Hz,
it will be near unity in most cases). The power supply must be
approximately 1.4 times the size of the drive's kVA rating to accommodate
this. Harmonics from different sources tend to be cumulative, so the site
operator must account for the total harmonic distortion present at the
PCC.
These harmonic levels can be tough to meet. If the PCC is only 415Volts,
the largest unmodified six-pulse inverter that could be connected would
probably be around 45kW in size - and this would assume no other harmonic
distortion coming from anywhere else on the site! Things are easier if,
as is more usual, the PCC is 3.3 kV or above. The number of diodes used
determines the frequency of these harmonics, which are multiples of the
fundamental (50 Hz in Europe).
Eliminating unwanted harmonics
It is worth noting that a dc installation produces considerably less
harmonic distortion than an ac installation, thanks to the smoothing
effect of the armature reactance. The difference between ac and dc in the
crucial fifth and seventh harmonics, for example is about 40% and 60%
respectively. However, in ac drives installations, the inclusion of input
line reactors of 3-5% impedance can dramatically reduce harmonics and
peak currents. They work by forming an L-C circuit with the capacitors,
widening the conduction angle of each diode. Installing a sine filter of
matched kVA will have similar results, but this is a costly solution.
The other tried and tested method is to use a higher pulse number.
Standard three-phase drives rated to around 200kW use six-pulse
rectifiers, though the new recommendations would suggest that a six-pulse
inverter greater than 40kW (75kW for G5/3) will now be unsuitable for
direct connection to the supply. An active harmonic filter can be used in
conjunction with a six-pulse drive to reduce its harmonic output, but
this tends to be an expensive solution. A 12-pulse rectifier, on the
other hand, is designed to eliminate the all-important fifth and seventh
harmonics, which cause most of the problems as far as the NOC is
concerned. Rockwell Automation is currently building inverters using
isolating phase-shifting transformers and series rectifier bridges into
standard six-pulse units, converting them to true 12-pulse drives. The
company says it has observed increasing demand for 12-pulse systems since
the introduction of G5/4, and reckons it can have 12-pulse drives rated
up to 600kW ready within six to eight weeks of receiving an order.
In a direct response to the G5/4 initiative, Mitsubishi Electric has
introduced a high current (HC) active bridge unit, which effectively
replaces the usual six input diodes with six power transistors. These
switch in phase with the supply voltage sine wave, effectively
eliminating harmonic noise. The typical total harmonic distortion from a
standard variable speed drive across all the harmonic orders is about
30%. Mitsubishi claims its HC active bridge reduces this to 1% in driving
mode and down to 0.7% in regenerative mode, dramatically reducing
unwanted harmonics and increasing energy efficiency. Another bonus with
this arrangement is that energy can flow equally well from the drive back
to the incoming supply, thus conserving even more energy, especially with
overhauling loads. Mitsubishi adds that this technology is also a viable
replacement for four-quadrant control, which removes the need for dc
drives in most motor control applications.
Eurotherm Drives (Martin Payn):
Rockwell Automation (Paul Claxton):
Mitsubishi (Chris Cusick):