Tap changer for distributed power

Abstract

United Utilities have identified that the introduction of combined heat and power plant (CHP) into a new estate within their LV distribution network will give rise to the possibility of reversal of power flow through the distribution transformer at periods of low loading with consequent rise in line voltage. Since such a voltage might be outside the statutory limits, a cost effective method of limiting this voltage is essential. The solution identified is to switch between two predefined taps on the distribution transformer at preset times of the day based on historical loading data. Areva T&D have been studying advanced techniques for on-load tap changers (OLTC) resulting in several novel designs being demonstrated in the laboratory. The techniques are principally aimed at OLTC diverters for power transmission and have considered a range of technologies. The present tap changer uses a vacuum switch arrangement combined with a commutation circuit powered from the tap voltage. Modern control principles are required to obtain accurate point on wave switching, however the tap changer is operated from a single armature to give a simple and reliable scheme. The power circuit fits within a casing that integrates with the transformer tank, raising its height by 300 mm. The control is separate from the power circuit and fits into wall-mounted cabinet. Either a preset time setting or the secondary RMS voltage level can initiate a tap change according to a switch setting on the control unit. In both cases care has been taken to prevent the possibility of the tap change operation "hunting". The development is well advanced and should be ready to go into trial service in late 2007.

Full text

C I R E D 19th International Conference on Electricity Distribution Vienna, 21-24 May 2007
Paper 0420
CIRED2007 Session 3 Paper No 0420 Page 1 / 4
TAP CHANGER FOR DISTRIBUTED POWER
Colin OATES Alan BARLOW Victor LEVI
Areva T&D – UK Areva T&D – UK United Utilities - UK
colin.oates@areva-td.com alan.barlow@areva-td.com victor.levi@uuplc.co.uk
ABSTRACT
United Utilities has identified that the introduction of a
combined heat and power plant into a new estate in their
LV distribution network will give rise to the possibility of
reversal of power flow through the Distribution
Transformer at periods of low loading with consequent rise
in line voltage. Since such a voltage rise might violate the
statutory limits a cost effective method of limiting this
voltage is essential. The solution identified is to switch
between two predefined taps on the distribution transformer
at preset times of the day based on historical loading data.
The tap changer being developed uses a set of vacuum
switches operated from a single armature to give a simple
and hence reliable scheme. The power circuit is designed
to integrate within the transformer tank. The control is
from a single electronics board that mounts on the
substation wall for convenient adjustment and monitoring of
the tap changer settings. The primary operation is time
based although a means of switching the tap according to
the voltage level of the LV network has been included. In
both cases care has been taken to prevent the possibility of
the tap change operation “hunting”. The development is
well advanced and should be ready to go into service trials
in mid 2007.
INTRODUCTION
Combined Heat and Power (CHP) is a recognised method of
significantly improving the efficiency of power generation.
With increasing government incentives its use is growing
throughout the UK. United Utilities (UU) has been
studying the problems associated with the introduction of
CHP systems within the Manchester Area, [1]. They have
identified that during periods when the local load
requirement is reduced, for example in the early morning,
the generated power may reverse the power flow through
the Distribution Transformer. The level of AC regulation
present in the power system means that such a reversal will
cause a significant rise in the LV network voltage to a level
beyond the limits defined by security standards. A simple
solution to alleviate this is to set the distribution transformer
to a lower voltage tap than normal.
Conventional On-Load Tap Changers (OLTCs) are fitted to
distribution transformers but have a cost considerably in
excess of the transformer, have size comparable to the
transformer and have a reputation for “hunting”, i.e.
repeatedly tapping up and down. Furthermore the control is
not designed to manage reverse power. For this application
the requirement is to switch only between two preset tap
settings, equivalent to the diverter function in a
conventional OLTC.
Based on research into advanced techniques for OLTCs for
transmission transformers, Areva T&D proposed a design
based on a patented design approach using vacuum
switches, [2]. A development project was agreed with UU
to be funded by the UK regulator (Ofgem) Innovation
Funding Incentive (IFI) initiative under which the
development costs can be recovered on a limited basis.
VACUUM-SWITCH-BASED TAP CHANGING
Vacuum switches are commonly used for medium voltage
power breakers and are known to be high reliability
components. They are also commonly used in synthetic
laboratory testing of power equipment because of their
ability to switch relatively high levels of voltage and current
to a timing precision of better than 1ms. The proposed tap-
changing scheme takes advantage of the unique properties
that vacuum switches possess, specifically:
1. The armature movement is small for large voltage
withstand, typically 10kV/mm, enabling rapid
operation.
2. The arc voltage is very small and stable, being typically
20V to 60V.
3. The arc recovery is very rapid, being typically better
than 0.5μs.
L
B1
L
A1
L
A
2
C
Com
L
B2
V
S1
L
2
L
1
Referred
Load
Impedance
Primary
Winding
voltage
Ta p
Winding
voltage
VS /A
1
VS /B
1
MOV
1
Figure 1: Basic On-Load Tap Changer Circuit
Figure 1 presents the circuit on which the tap changer is
based. The values of the inductors and capacitor are related
to a commutation resonance associated with the fast
recovery time of the vacuum switch and so have negligible

C I R E D 19th International Conference on Electricity Distribution Vienna, 21-24 May 2007
Paper 0420
CIRED2007 Session 3 Paper No 0420 Page 2 / 4
voltage drop at 50Hz. This also means that the capacitor
voltage follows the tap voltage closely. The circuit
operation assumes that at a chosen instant there is a finite
tap voltage so the capacitor has charge. It is also assumed
that the load current will cause arcing as the conducting
vacuum switch is opened, maintaining the flow of current to
the load.
To cause the tap change a drive amplifier is switched into a
drive coil in the actuator that operates the vacuum switches.
A finite time is required for the current to build in the drive
coil to a sufficient level to overcome the actuator latching
force, typically 7ms. Additional time of about 3ms is then
required as the contacts open until a gap develops of about
2mm that is sufficient to hold off the tap voltage. The
second vacuum switch then closes and the capacitor is
discharged through both vacuum switches, causing a
resonant commutation process. As the resonant circulating
current swings it cancels the load current in the first vacuum
switch and the arcing ceases.
Timing Reference
Preset delay
Required Arcing period
Closing Actuator Traverse
Opening Actuator Traverse
Contact gap required at Commutation
Snatch Mechanism compression
Actuator movement
Vacuum Switch
Armature movement Phase Voltage
Vacuum Switch armature movement
A
ctuator movement
Figure 2: Tap Changer Operation
The timing of the commutation is critical since it must be
ensured that the commutation capacitors for all three phases
will have sufficient charge. This is assisted slightly by the
arcing voltage that offsets the ideal commutation point so
that one phase passes through zero current causing natural
commutation. Therefore only two commutation circuits are
required in practice. Figure 2 illustrates the tap change
process and highlights that, by adding a third preset delay
and by referencing the start of the process against the zero
crossing of the tap voltage, the tap change process can be
set to a specific point on wave.
Distribution transformers include an “Off-Load” tap
changer as standard, giving a range of ±5% in steps of
2.5%. To make the tap changer compatible with the most
common tap arrangement requires the basic tap changer
design to be extended to the form illustrated in Figure 3.
LT Win ding
L
N
L
B1
L
D1
L
A1
L
C1
L
A2
L
C2
C
1
C
2
L
B2
L
D2
VS1
VS /A
1
VS /B
1
MOV
1
MOV
2
W1T1CB1
CB2
CB3
CA3
CA1
CA2
W1T2
W1T3
W2T3
W2T2
W2T1
CT1
CT2
CT3
Figure 3: Full Tap Changer Power Circuit Arrangement
The tap changer must operate between any two of the taps
and the site engineer can set that this in-situ.
RELIABILITY & CONTROL
A study into the reliability of the tap changer design
highlighted that very high reliability could be achieved
provided the power circuit and the control circuit are
addressed separately. The estimate of reliability for an
equivalent transmission transformer scheme is of 30 years
guaranteed life, based on a total of 500,000 operations (BS
EN 60214: 1998). The vacuum switches and actuator have
a fatigue life pedigree of millions of cycles and the power
inductors also have a very high reliability. The main failure
modes are due to failure in the commutation capacitor and
in the insulation of the current transformers (CTs). The
capacitors are of a segmented type used for snubbing power
electronics and so internal failures are isolated. Thus they
will not fail short circuit but a gradual reduction in their
capacitance will be observed over time to a point where
they may need to be replaced. All the CTs for a single
phase have been encapsulated into a single box with
insulating tubes running through to carry the primary
conductors. A full fault current safety earth is also fitted
over the tube in case the insulation is compromised. Thus
the power circuit to be contained within the transformer
tank has been designed to have a high reliability.
In contrast to the power circuit, the failure rate of
electronics used for control is inherently poor. To maintain
operation in the event of a control fault occurring two-way
redundancy and three-way voting has been included. The
latching property of the actuator means that with careful
design the electronics board can be removed without the
need to de-energise the transformer, i.e. it can be “hot
swapped”. Thus a detected fault will raise an alarm and the
second drive channel will be used until the board is
replaced. It should thus be sufficient to check the tap
changer operation on a routine basis to ensure 30-year life
of the tap changer without interruption.

C I R E D 19th International Conference on Electricity Distribution Vienna, 21-24 May 2007
Paper 0420
CIRED2007 Session 3 Paper No 0420 Page 3 / 4
Figure 4 illustrates the realisation of the timing process
illustrated in Figure 2. A primary timing reference is taken
from integrating the output from a CT monitoring the
commutation capacitor current (CT1). The commutation
process is limited in terms of the load current that can be
cancelled and the minimum tap voltage that must be
present, the output from both the timing reference CT, CT1,
and additional CTs monitoring load current, CT2 and CT3,
must be checked for magnitude to ensure both load current
and tap voltage are within the required limits.
L
B1
L
D1
L
A
1
L
C1
L
A
2
L
C2
C
1
C
2
L
B2
L
D2
VS1
VS /A
1
VS /B
1
MOV
1
MOV
2
CB1
CB2
CB3
CA3
CA1
CA2
Buffer
Amplifier
Referenc e
Reference
CT1
CT2
CT3
Inhibit
Window
Comparator/
Slope selector
Rectify & filter
Rectify, filter &
Greatest wins
Varia ble
Delay
Time/Voltage control
Tap U p
Tap D own
Changeover
drive
Gating
Logic
Figure 4: The control functionality
MOV
1
L
B1
L
D1
L
A1
L
C1
L
A2
L
C2
C
2
L
B2
L
D2
VS1
VS /A
1
VS /B
1
MOV
2
CB1
CB2
CB3
CA3
CA1
CA2
C
1
MOV
1
L
B1
L
D1
L
A1
L
C1
L
A2
L
C2
C
2
L
B2
L
D2
VS1
VS /A
1
VS /B
1
MOV
2
CB1
CB2
CB3
CA3
CA1
CA2
CT voltage sense in
CT1 current sense in
CT2 current sense in
Control Board 2
CT voltage sense in
CT1 current sense in
CT2 current sense in
Control B oard 1
Supervisory B oard 1
CB1 operation sense
Control Board Changeover
CB2 operation sense
Figure 5: Full Redundant Control
The full redundant control is illustrated in Figure 5 in which
the four functional blocks are shown, the display panel, the
supervisory block and control blocks 1 and 2. The
supervisory block and the control blocks contain the logic
shown in Figure 4 with either of the two control blocks
being able to drive the actuator as selected by the voting
logic within supervisory block. The supervisory block
manages the control panel interface and relays the time
information to the control blocks so that each block contains
an independent record of the timing information.
Voltage regulation has been included to the control that will
cause a tap change if the RMS secondary voltage rises
above or falls below its terminal requirements. A
calculation based on ideal supply settings for a range of
load power factors shows that the two switch thresholds are
sufficiently separated to prevent “hunting” from taking
place.
Figure 6: Electronics control board
The electronics is mounted onto two boards, one for the
display and the other containing all the control electronics,
Figure 6. Analogue electronics is used for buffering and
processing the CT signals, while the remaining functionality
is contained within three Field Programmable Gate Arrays
(FPGA), one for each of blocks shown in Figure 5. The
drive for the vacuum switch actuator coil is from a two-
quadrant H bridge, the supply being from a pre-charged
50V, 100mF capacitance, divided into a array of capacitors
that are also mounted on the board. The auxiliary supply
for the logic is taken from a charged super-capacitor to
protect against loss of mains supply and give the electronics
a hold up of up to half an hour. This can be extended if
required by increasing the number of super-capacitors.
The card carrying the display electronics provides a
mounting for the display and controls and mounts straight to
the front panel. The selection of the electronics to be
incorporated onto this card has been chosen to permit future
upgrading. This will permit custom boards to be developed
so that the equipment can be easily integrated into remote
automation schemes specific to the client to allow remote
control and monitoring. This has other potential benefits in
that since the equipment contains means of monitoring the
line current, the equipment offers an inherent remote “Fault
Passage Indicator”.
NON INVASIVE MECHANICAL DESIGN
Several arrangements of the mechanical design for the
OLTC have been considered, including a version in which it
was fitted in a separate housing from the transformer.
Standard sub-stations within the UK do not have sufficient
clearance around the transformer inside the enclosure to
position a separate, free-standing tap changer but do have
adequate clearance above the tank cover making this the
best practical arrangement. Thus the present OLTC has
been situated on top of the transformer and is fixed in
position using a bolted flange that would normally attach

C I R E D 19th International Conference on Electricity Distribution Vienna, 21-24 May 2007
Paper 0420
CIRED2007 Session 3 Paper No 0420 Page 4 / 4
the transformer cover to the main tank, Figure 7. The tap
changer is not submerged in insulating oil therefore normal
oil levels above transformer windings are maintained and
standard oil level gauges can be used. This arrangement
also enables the tap changer to be retrofitted to any existing
distribution transformer with no modification to the tank.
The mechanism is made up of vacuum switches configured
on a single shaft arrangement to give simultaneous 3-phase
operation. A magnetic actuator has been adapted to deliver
the required speed and precision to achieve the correct
switch commutation to operate the mechanism. AREVA
T&D VS3-C vacuum switches have been selected as having
the characteristics required for this application:
Figure 7: Tap Changer fitted to a 1MVA Distribution
Transformer.
Figure 8: The Tap Changer Layout
A major consideration for the system is the method of
insulation. Mounting the system outside the oil means that
to achieve the creepage between phases the three pairs of
vacuum switches and the two commutation circuits must be
compartmentalised and sealed, Figure 8.
The control is housed in a separate box, Figure 9, which is
sealed against vandalism so the display panel is only visible
when it has been unlocked and the front cover opened. The
box is wall mounted with armoured cable coupling it to the
power system. The control panel is arranged to be simple to
operate and set.
Figure 9: Control box layout
CONCLUSIONS
A simple changeover type on-load tap changer has been
presented that has been custom designed to address a
potential problem created by the introduction of distributed
CHP. The rise of domestic power generation means that
there is likely to be an increasing need for such equipment
in future. The principle of operation is based on a novel,
patented scheme that utilises the unique properties of
vacuum switches to give a product that will integrate easily
with the transformer and that can link into remote substation
operating systems. The design is flexible and can be
adapted fit a wide range of existing transformer designs.
The simple form of the tap changer means that the projected
cost of the equipment should be much less than an
equivalent conventional tap changer. Maintenance should
only be required to service the electronic control; the power
circuit has a projected life in excess of 30 years. A
prototype tap changer is at present under construction and
should be ready for testing early in 2007. It is presently
planned to start field trials mid 2007.
REFERENCES
[1] V.Levi, M.Attree, M.Kay, I.Povey, 2005, “Design of
low voltage networks for premises with small scale
embedded generators”, Session No 4, 18th
International Conference of Electrical Distribution,
CIRED2005
[2] C.Oates, 2006, “An On-Load Tap Changer”,
Worldwide patent WO2006103268 (A2)