Proposal for a Change of 11 kV
Operating Voltage Range
11th January 2013
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Proposal Summary
The Network Code specifies a nominal voltage of 11 kV at the medium voltage level,
with a maximum variation of +2%, -5% from nominal during normal operating
conditions. MSA 50160:2010 allows a +/- 10% voltage variation on the medium
voltage network for the same conditions.
Given that the reference voltage of 11.2 kV at the busbars of the distribution centres
around Malta and Gozo is already at the upper limit in the Network Code, there is
currently no provision for the voltage rise across the 11 kV network caused by power
that is generated at sites that are remote to the distribution centres. Enemalta
would therefore only be in a position to accept the connection of renewable energy
sources to its medium voltage network if the injected power were not to exceed the
load on the 11 kV feeder at any given point in time.
The extension of the upper limit to a permissible voltage of 11 kV + 5% would make
it possible to connect medium sized renewable generation sources to Enemalta’s
medium voltage network, without having a negative effect on the voltage levels of
low voltage consumers and without causing excessive stress on component
insulation levels.
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Proposal Background
Distribution Centre 11 kV Busbars
The steady state voltage on the 11 kV busbars of distribution centres is regulated by
means of on load tap changers that ensure that the voltage is maintained within the
bandwidth of the pre-determined setpoint that is currently adjusted to 11.2 kV.
Normal variations are within +/- 1.5 % for older transformers and +/- 1% for the
newer transformers, as the connected load increases or decreases. The total
variation in voltage at the distribution busbars is therefore between 11.365 kV and
11.035 kV for the 11.2 kV setpoint.
The 11 kV Network
The traditional one way direction of power flow from the distribution centre towards
the end of a radially operated 11 kV feeder, results in a voltage drop along the 11 kV
network.
While the connection of realistic levels of distributed generation on the low voltage
network is unlikely to change the direction of power flow on the 11 kV network,
larger renewable energy sources connected at the 11 kV level may cause the voltage
at the point of connection to rise above that of the busbars at the distribution
centre.
Apart from being constrained by the upper voltage limit specified in the Network
Code, an excessive rise in voltage over the system rated voltage will stress the
insulation of network components, and lead to accelerated ageing of this equipment.
The maximum acceptable voltage on the medium voltage network is considered to
be 11 kV + 5%. This value should permit the connection of sizeable distributed
generation facilities to points on the network where their generated power exceeds
the feeder load at the corresponding time.
Reduction of Distribution Centre Reference Voltage
A reference voltage of 11.2 kV at the distribution centre (maximum voltage of
11.365) would then permit a 0.185 kV (1.68 %) rise along the 11 kV network as
power flows back along the 11 kV feeder to the distribution centre from renewable
sources.
If the reference voltage at the distribution centre was reduced to 11.1 kV (maximum
voltage of 11.265 kV), a voltage rise of 0.285 kV (2.59 %) would be possible along the
11 kV network. This permits the connection of larger renewable energy sources at
more distant points on the network.
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Further reductions in the reference voltage at the 11 kV busbars of Enemalta’s
distribution centres is not recommended, as this will influence the Corporation’s
ability to meet the lower voltage limit in areas that are remote from both a
distribution centre and a local substation.
Substation 11,000/433 V Transformers
A load-dependent voltage drop (or rise) will occur across a substation transformer,
and will affect the 400 V voltage level inside the substation. The voltage drop in %
across a transformer at full load is quoted as the ‘Impedance Voltage’ of that
transformer.
To counteract the voltage variations across the transformer impedance, each
transformer is equipped with an off-load tap changer. The secondary voltage may be
varied by up to +/- 5 % from nominal, in steps of 2.5 %.
Rated primary/secondary voltage of transformers installed in substations is
11,000/433 V, which is slightly different from the customary 11.2 kV reference at the
distribution centres and the nominal 400 V on the LV network.
A voltage of 11.55 kV at the primary winding of the substation transformer will in
fact result in a voltage of 250 V at the secondary windings when the transformer is
on no load and adjusted to minimum tap. Higher 11 kV voltages will cause the
voltage at the point of LV customers to exceed the +10% limit that is specified in the
Network Code for this voltage level, and are hence not recommended.
If the power generated by distributed generated sources on the low voltage network
exceeds the total connected load, the net power flow from the low voltage busbars
across the substation transformer causes a rise in the voltage level on the low
voltage side. One solution for generators with power that exceeds the substation
load would be to provide them with a connection to the 11 kV network to avoid this
problem.
The Low Voltage Network
Perhaps the greatest likelihood of observing a flow of power towards the traditional
source exists on the low voltage network during periods of high solar radiation.
Sizable PV installations at the end of low voltage feeders may easily create a voltage
rise along the feeder and a voltage at the point of connection that is higher than that
in the substation.
On the other hand, the high evening loads that are experienced in residential areas
at the time when the output from PV installations is zero give rise to a voltage drop
from the substation along the low voltage feeder to the customer.
In areas with a high penetration of PV generation, where the 11 kV voltage at the
substation is allowed to rise to the 5% upper limit, Enemalta will consider other
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measures to ensure that the voltage at the customer’s point of connection on the
low voltage network remains within the +10/-10 % limits in the Network Code.
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Conclusions
The current identical reference voltage setpoint and maximum voltage limit for the
11 kV network do not permit any voltage rise on the 11 kV network for power
flowing towards Enemalta’s distribution centres.
A slight reduction in 11 kV reference voltage may be implemented internally at
Enemalta without creating problems of exceeding the lower limit of the voltages
specified in the Network Code.
A modification to the upper limit of the 11 kV voltage in the Network Code to a value
of 11 kV + 5% is recommended to permit the connection of renewable energy
sources to the 11 kV network.
Enemalta will take measures to ensure that the voltage at the 400 V level remain
within the +/- 10% tolerances that are specified in the Network Code.
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