VARIABLE SPEED DRIVE “REGENERATIVE” TYPE
- LESSONS LEARNT -
Guy DESCORPS
TOTAL
Avenue Larribau
Pau
France
Philippe ESPAGNE
TOTAL
Avenue Larribau
Pau
France
Claudiu NEACSU
LEROY SOMER
Usine de CEB
Beaucourt
France
Philippe WESOLOWSKI
LEROY SOMER
Engineering Dpt.
Angoulême
France
During phase 1, the operating experience allowed us to notice
that the bound constraints engendered important problems.
Works over operations were necessary to replace pumps
because the sizing was not adapted to the productivity of wells.
Considering the acquired experience, for the phases 2 and 3
development, the VSD option was considered.
Abstract - Variable speed A.C. drives are used in many new
and already existing oil and gas applications because of their
well known benefits for energy efficiency and flexible control of
process.
During the past years, numerous publications [Réf] have been
published relating to the various technologies of electronic
Variable Speed Drives (VSD) for Electric Submersible Pumps
(ESP) application. This paper explains why a “Regenerative”
VSD new technology had been chosen in a specific project to
supply ESP (PART I). Moreover we will present also the main
benefits of using this technology when both motor and VSD
have to comply with ATEX category 2 or 3 (PART 2).
This paper also presents the precautions to be taken during
the various stages (design, construction & operation) of a
project and the feed back after a few years of operation and
lessons to be learnt
Index Terms – VSDS: Variable Speed Drive Systems, ESP:
Electric Submersible Pumps, THD: total harmonics distortion,
Regenerative
PART I: HOW TO MINIMISE HARMONIC NETWORK
POLLUTION?
I.
Figure 1: OFFSHORE PLATFORMS
II.
INTRODUCTION
VARIABLE SPEED ADVANTAGES
In general, the use of electronic VSD technology has main
advantages, more or less linked:
Offshore platforms located in the Arabian Gulf have been
developed in association with a well known Middle East Oil
and Gas Company in 3 phases:
Flexibility of regulation and functional optimization:
Facility for starting up with a programmable motor torque
Flexibility of functionality allowing the adaptation to the
driven machine with variable conditions of use and even
in some cases, to increase its useful duty range
Possibility to use motors with a speed higher than
frequency imposed by the network
Shaft line simplification
Phase 1 Offshore platforms Central development 1995/1997:
- 3 Well head platforms equipped with ESP
- Onshore Treatment train 1
Phase 2 Offshore platforms Eastern development 1999/2001:
- 2 Well head platforms equipped with ESP and VSD
- Onshore Treatment train 2
Energetic economy:
Capacity to realise significant energy savings because
electro mechanical efficiency is intrinsically higher
Possibility for an equipment to work permanently with the
best efficiency in all the practicable speed ranges and not
only in the dimensional maximum duty point
Phase 3 Offshore platforms North development 2002/2004:
- a Well head platform equipped with ESP and VSD
- a Water separation and injection platform
Taking into account that wells are not eruptive it is necessary
to activate them with ESP. For phase 1, variable speed drives
were not installed for various reasons.
Availability and maintainability:
High availability of equipments due to an improved
reliability and reduced repairs time
1
12 or 24 pulses VSD
This solution needs an additional and special transformer with
complex sets of phase shifted AC output windings, the rectifier
bridge of the VSD is design accordingly. So 12 or 24 pulse
VSD is not the appropriate solution for ESP application.
Repair facilitated by modular electronic sub assemblies
and possibility of implementing automatic fault detection
procedures with rapid replacement
Limitation of nuisances and constraints on the
equipments:
Reduction of the mechanical constraints (starting up
torque, disconnection, blow of ram, etc.)
Limited inrush current on the network during motor start
up
Reduction of Starting Power Requirements
Regenerative VSD
This technology was specially adapted for this project and is
developed in the chapter VI.
IV. PHASE 2 : PROJECT DEVELOPMENT STUDIES
For ESP application, the variable speed drive is perfectly
adapted to resolve the following problems:
Unknown Well Productivity
Maintaining Constant Pump Intake Pressure
Changing conditions of the well (evolution of the BSW, PI
decreasing, well head pressure)
Adaptation of the power according to needs
Reduction of the constraints on the ESP during the
starting up
Reducing starting power requirements
Changing well production conditions over time
The challenge for this fast track project was to evaluate and
determine the most appropriate concept without affecting the
schedule and with a reliability guarantee.
The main technical objectives expected for this project were
determined as following:
- Voltage variation at the VSD input 10% without any effect
on the motor voltage
- THD Harmonics limitation 3%
- No stresses on the motor (electrical and thermal)
- No disturbance in particular for the down hole monitoring
system
III. HARMONICS
During the engineering phase, a feasibility study was
performed:
Harmonics effects:
Some precautions are required when using VSD’s, because
electronic devices engender harmonic currents which circulate
due to the impedance of the network, creating harmonic
voltages for other consumers connected to the same network
with the following effects:
Motor: additional losses both in the copper and in the
iron, these losses create over heating notably in the rotor
cage
Oscillating torque produced by the harmonic current, this
torque can have harmful effects on the stability and even
on the mechanical resistance if their frequencies are the
same as the rotating frequencies of the shaft line
Transformers: impure sinusoidal current increase the
losses causing significant overheating and in some
cases, a resonant circuit is produced
Cables: increased losses and risk of overheating,
damage to cable insulation
Capacitors: production of resonant circuits
Disruption of the regulation devices, remote control,
measurements, counters, etc.
Network calculation
A complete study was performed including the choice of
voltage level, short circuit calculation, stability calculation on
the largest equipment starting up, harmonics calculations.
Due to the fact that the power generation is located 44km far
away from the platforms, the short circuit power is around
30MVA.
Solutions:
Passive filter
Economic solution, however the filter must be calculated for a
fixed installed power and a constant harmonic level which is
not the case in an ESP application because the number and
the power of equipments in service are always changing; the
risk is to destroy the filter. In addition, when the passive filter is
stopped, significant pulses affect the network.
Active filter
Interesting solution but the filtering is not completely assured
for Harmonic current exceeding the capacity of the filter.
Figure 2: Single line diagram
2
ESP sizing
The ESP has been defined for an operating frequency range of
between 40 and 60 Hz in order to cover a complete flow range.
Total VSD Power (kVA)
No filter
Passive filter
VSD type regenerative
DP2
2150
12.5%
4.9%
2.2%
DP3
2150
11.5%
4.9%
2%
With the passive filter option, the size of the filter was more or
less 500 kVAr by platform for a guaranteed THD less than 5%.
V.
6 PULSES VSD VERSUS REGENERATIVE VSD
The input stage of a non-regenerative AC drive is usually an
uncontrolled diode rectifier; therefore power cannot be fed
back onto the AC mains supply. By replacing the bridge diode
input rectifier with a voltage source PWM input converter, AC
power supply power flow can be bi-directional with full control
over the input current waveform and power factor. Current can
now be controlled to give near unity power factor and a low
level of line frequency harmonics. An active IGBT converter is
used as a sinusoidal rectifier and synchronized with the main
supply network.
Furthermore, by maintaining the DC bus voltage above the
peak supply voltage the load motor can be operated at a
higher speed without field weakening. Alternatively, the higher
output voltage available can be exploited by using a motor with
a rated voltage higher than the AC mains supply, this reducing
the current for a given power. REGEN inductors must be used
to ensure a minimum source impedance. The difference
between the PWM line voltage and the supply voltage occurs
across the regen inductors at the REGEN drive. This voltage
has a high frequency component, which is blocked by the
regen inductor, and a sinusoidal component at line frequency.
As a result currents flowing in these inductors are sinusoidal
with a small high frequency ripple.
Figure 3: Pump performance curve in variable frequency
If the consumed power of the pump is proportional to the cube
of the speed, we have to remember that the motor horsepower
output rating will increase directly with the ratio of the
frequency.
Regenerative main advantages for this application
- Low level of harmonics distortion on the main supply: ≤ 4%
- Power factor close to 1
- Output voltage can be higher than input voltage (+100 V
max for 400V of input voltage supply)
Figure 4: ESP power curve
In fact, the input stage of the REGEN is regulated at a higher
voltage value than the normal voltage value of the 6 PULSE,
so this kind of converter is not affected by mains voltage
fluctuation.
As shown on figure 4, the pump requires less HP than the
motor is capable of delivering up to a certain frequency and
then exceeds it.
Surface package
Step up transformer and ESP cable losses
The losses have been calculated for the maximum frequency
including a temperature derating factor
VSD sizing
VSD power was calculated for the maximum surface absorbed
power including the cables and transformer losses
Harmonics calculation
A complete study was performed with the most stringent
conditions (minimum short circuit power minimum, one
33kV/415V transformer etc…)
Figure 5: Regenerative structure of the VSD
3
Additional necessary equipment
Sin wave filter (LC type):
Main advantages:
- Excellent output wave form, reducing the voltage stress on
the transformer and/or motor.
- Low THD Current ≤ 3%
- Elimination of the switching frequency from the VSD,
typical range from 2 to 4 kHz.
- Elimination of the risk of voltage reflections phenomena
due to ESP cable length (2000 to 4000 meters).
Step up transformer:
The output transformer is required to elevate the surface
voltage. The typical voltage required to ESP driving is 2000 up
to 4000 V.
DP3 HARMONIC
THD - %
VI. FEEDBACK
U1
3
kW
load
1200
1000
2,5
Field tests
During the start up of each well, several tests were performed
and recorded as shown in the figure below: the results agree
with the harmonics calculations made during the engineering
phase.
2
800
1,5
600
1
400
0,5
200
0
+ ALK 36
+ ALK 34
initial
Input voltage
+ ALK 33
0
+ ALK 32
When the load is increasing during starting of all the wells, the
total voltage harmonic distortion tends to decrease and these
tests demonstrated that the THD values are within the IEC and
IEEE standards limit.
+ ALK 31
Output current
Figure 6: Comparison harmonic and load.
In terms of absorbed power, we can see that the power factor
at the VSD input is close to 1 in all cases.
WELL
21
Input current
Output voltage
4
VSD INPUT
P(kW)
PF
167
0,97
VSD OUTPUT
P(kW)
PF
Hz
151
0,66
46
Efficiency
90,4%
22
218
0,98
208
0,74
47,6
95,4%
23
156
0,98
143
0,61
49,6
91,7%
24
175
0,98
160
0,68
443
91,4%
28
128
0,98
122
0,67
46
95,3%
31
204
0,99
192
0,76
53,4
94,1%
32
236
0,97
221
0,79
55,5
93,6%
33
134
0,96
119
0,69
46
88,8%
34
172
0,97
160
0,70
50,2
93,0%
36
235
0,98
229
0,75
50,8
97,4%
39
139
0,98
132
0,73
46,9
95,0%
20
90
0,95
83
0,55
40,3
92,2%
40
136
0,96
125
0,64
48,5
91,9%
41
168
0,94
157
0,55
40,4
93,5%
42
204
0,97
189
0,57
44,3
92,6%
These modifications were implemented 5 years ago and there
has been no further problem since.
Failures
As expected with all new technology equipment, some
problems occurred at the beginning:
VII. LESSONS LEARNT
- Damage to electronic cards and control transformer when
a sudden shutdown occurs on the 33kV network.
As a result of this technology there has been many application
benefits:
The voltage was maintained by the rectifier for a small period
and due to the commutations on the synchronous rectifier the
input filter was excited and a high amplitude current and
voltage wave occurred at a level higher than the limits of some
diodes on electronic cards.
- the VSD input consumes only active energy, which
reduces the current and the losses in the sub sea cables
and in all surface equipments.
- Very low THD Level of harmonics on the network
independent of the short circuit power and the VSD power
- There is no limitation on the implementation of additional
VSD equipment
- It is not necessary to perform harmonics calculations such
are necessary with active or passive filters solutions
- VSD output voltage stresses reduced which result in
increased reliability of cables, motors, transformers
- Reduced magnetic noise on transformer
This problem has been analyzed and solved by:
- Voltage clipper and resistance installation upstream of the
control transformers
- Modification on interfaces cards
- Implementation of a new software on the rectifier and
inverter command cards
Following these modifications, there was no reoccurrence of
problems linked to the VSD.
PART II: ADVANTAGES OF THIS SOLUTION
ASSOCIATED WITH ATEX MOTORS
Voltage before modification
I.
INTRODUCTION
The aim of PART II is mainly to demonstrate the interest of
new REGEN VSD compared to a typical VSD (6 pulses PWM
inverter) when supplying an ATEX motor.
II.
IDENTIFICATION OF CONTRAINTS IN STANDARD
INVERTER FED MOTOR
It is now commonly admitted that the use of standard inverters
to drive asynchronous motors may be detrimental to the
insulation system. From a motor point of view, there are two
kinds of supplementary stress, compared to a standard 50Hz
or 60Hz supply: Thermal and Electrical.
Firstly, it is well known that an additional temperature rise
occurs when the motor is fed by an inverter compared to a
normal sine wave supply. This phenomenon can be explained
by the consequence of field weakening and the rich spectra of
voltage harmonics. Indeed, a voltage drop can occur inside the
inverter and the cable length. This can produce generally a
field weakening and for the same torque demanded by the
shaft, the losses will be increased (Joules and iron losses).
This is not acceptable for ATEX standards.
Voltage after modification
Particular attention must be paid when designing this kind of
motor. To avoid this problem the motor designers take into
account the field weakening by decreasing the number of
winding turns in the slot. In this case, the flux increases and
compensates the voltage drop stated above. This practice is
currently used for the lower frame sizes, and the flux is
increased by at least 7% compared to a standard motor.
Unfortunately this solution cannot be used in larger motors
because of the lower number of turns used. For example a
400mm frame size motor that has 4 turns/ slot cannot be
reduced to 3 turns/ slot because the flux will be 33% higher. In
5
this case, a power de-rating is generally used in order to keep
the temperature rise at the normal value to comply with the
thermal class of the insulation system.
1
2
The second factor that needs to be taken into account is
electrical stress. Indeed, the use of inverters to drive rotating
machines may be detrimental to the electrical insulation [1] [2].
Among the different explanations proposed, the most relevant
are linked to the existence of over voltages due to external
conditions such as large dV/dt, cable length, impedance
mismatches between the cable and the motor or to the
unbalanced voltage distribution in the winding [3]. Their impact
is well known. They may trigger Partial Discharges (PD) [4]
and lead to a reduction in the life time of the insulation.
To avoid this second problem an upgraded insulation system
is generally used, particularly in the enameled wire by using a
special corona resistant wire (Fibber glass wire, etc). In this
case the motor is more expensive.
One question arises: Is there another way to assure the
security and reliability of ATEX motor VSD supplied?
3
In the following we will try to answer this question
demonstrating the benefits of the new approach using a
REGENERATIVE inverter.
III. EXPERIMENTAL
The methodology utilized compares both VSD technologies (6
PULSES and REGEN) supplying a standard Direct on Line
(DOL) ATEX motor (*) with a normal supply from the main.
Focus was on the electrical and thermal points.
To achieve this goal a specific test bench was developed and
is shown in Figure 7.
(* 90kW 4 poles and 400V/50Hz)
IV. RESULTS AND COMMENTS
FLS D 280 – 90 kW – 4 poles – 400V/50Hz EEx d II2G IIB T4
Results at Nominal voltage:
DOL
Figure 7: Specific test bench
Nominal Torque (Nm)
VSD Voltage input (Vrms)
First harmonic voltage H1(V)
THD input voltage (%)
VSD input current (Arms)
First harmonic current H1(A)
THD Current input (%)
VSD output (Vrms)
VSD System voltage drop* (Vrms)
Firter output (Vrms)
Motor input voltage (Vrms)
Motor input current (Arms)
dv/dt motor input (kV/µsec)
Voltage peak motor (Vpk)
Winding temperature rise (K)
DE bearing temperature rise (K)
578
400
400,7
/
/
/
/
/
/
/
400
164,8
1,7 E-4
577,7
70
64,9
6 Pulses
REGEN
577
400,5
400,2
2,45
162
147
43
380
/
/
380
171
1,75
1115
87,3
79,2
578
401
400,4
1,4
142
141
3,7
415
14
401
401
164,6
1,7 E-4
588
71
64
* VSD system voltage drop: cable + input filter + inverter + output filter
6
Results at Nominal voltage minus - 10%:
Nominal Torque (Nm)
VSD input voltage (Vrms)
First harmonic voltage H1(V)
THD input voltage (%)
VSD input current (Arms)
First harmonic current H1(A)
THD input current (%)
VSD output (Vrms)
VSD System voltage drop* (Vrms)
Filter output (Vrms)
Motor input voltage (Vrms)
Motor input current (Arms)
dv/dt motor input (kV/µsec)
Voltage peak motor (Vpk)
Winding temperature rise (K)
DE bearing temperature rise (K)
PART III
DOL
6 Pulses
REGEN
577
/
360
/
/
157,8
/
/
/
/
360,1
177,4
1,7 E-4
520,6
78
77,6
577
360
359,6
3,13
174,8
162
36
340
/
/
340
186,6
1,75
1001
97,5
81,6
577
365
363
1,4
158
157
2,45
414
13
400,7
400,7
164,8
1,7 E-4
578
74
67,8
I.
This new technology presented an important challenge from a
technical and economic point of view. The production of this
oilfield was strongly linked to the reliability of the equipments
and the environing impact.
The results confirm that this REGEN technology is the most
adapted for many applications in the Oil and Gas industry.
II.
ACKNOWLEDGEMENTS
The authors would like to acknowledge and express their
thanks to their colleagues. There are too many to acknowledge
individually in this paper but there are a few we would like to
specifically mention for their key roles.
From LEROY SOMER:
Christian PETIT – Innovation & development manager
Michel GALAIS – ATEX standard specialist
Daniel EHANNO – Electrical environment specialist
François BOISAUBERT – ATEX standard specialist
Nicolas DOS SANTOS – Project Manager / Engineering Dpt
* VSD system voltage drop: cable + input filter + inverter + output filter
The results show two main things:
Firstly, we observe that the temperature rise in the motor is
70K when supplied with 400V sine wave voltage. The motor is
designed with a good thermal reserve. However, when
supplying the same motor with a standard PMW 6 pulses drive
and at the same output torque, the temperature rise increases
to 87K (97,5K when 360V at the drive input). In the case of the
REGEN drive, the temperature rise is almost the same as DOL
at 400V, but decreases at 360V. That means that REGEN
drive compensates up to 10% input voltage drop and keeps
the flux constant in the motor. In conclusion, from the thermal
point of view, the REGEN solution drive keeps a constant
temperature rise in the motor even if the input voltage
decreases.
III. REFERENCES
[1] Y.Shibuya, K.Kimura, H.Mitsui, "Winding insulating
materials degradation under repetitive impulse voltages", only
available in French, Cigré, session 15-104, 1994.
[2] E. Personn, “Transients Effects in Application of PWM
Inverters to Induction Motors“, IEEE Transactions on Industry
Applications, Vol 28 n° 5, 1095-1101, Sept/Oct 1992
[3] A. Bonnett,”Analysis of the Impact of Pulse Width
Modulated Inverter Voltage Waveforms on AC Induction
Motors” Proc. of the Intern. Conf. on Pulp and Paper, 68-75,
1994
Secondly, the REGEN drive will keep electrical stresses very
low due to the output LC filter. The dV/dt is completely
flattened and we don't have voltage peaks as in the case of a
standard 6 Pulses PWM inverter. The voltage shape is a
complete sine wave when a REGEN Drive fed motor. In this
case partial discharges could be not triggered.
[4] A.Mbaye, T.Lebey, Bui Ai "Existence of partial discharges
in low voltage induction machines supplied by PWM drives",
IEEE Trans.Diel. And El.Ins, vol 3, 4, 1996
Finally, we have demonstrated that the REGEN solution has
two advantages for motors: thermal and electrical. This will
increases the life time of the insulation system and also the
reliability. Moreover, when using a REGEN inverter we can use
a standard design ATEX motor without any problems.
V.
OVERALL CONCLUSION
IV. VITA
M. Guy DESCORPS: graduated in France with an electro
technic associated Degree in 1970. He worked for eleven
years for APAVE a worldwide independent Third Party
Inspection Agency.
He joined the electrical Department of ELF in 1983 and TOTAL
in 2000. He has worked as an electrical engineer in
engineering, commissioning and maintenance on several Oil &
Gas Projects throughout the World.
CONCLUSION PART II
The above results demonstrate the benefits of the REGEN
drive when supplying a standard ATEX motor. This solution
complies with the ATEX directives.
The subject of this approach is the influence of the type of
VSD used on the motor temperature rise, the incidence on the
constraints imposed when the motor works into an explosive
atmosphere and to help in the evolution of INERIS officials
documents.
M. Philippe ESPAGNE: received his degree in mechanic
technology from Paul Sabatier University, Toulouse France in
1980. Since graduation, he has been employed with
FORASOL Oil&Gas Drilling Company.
In 1986, he joined ELF as an electrical engineer involved in
maintenance,
commissioning
and
offshore/onshore
7
development project. He is presently employed by TOTAL as
Senior Electrical Engineer in Total Head Quarter.
M. Claudiu NEACSU: received the degree of Electrical
Engineering in 1997 from the University Politehnica of
Bucharest. He has also working for a Ph.D. at the “Laboratoire
de Génie Electique” of Paul Sabatier University in Toulouse &
Leroy Somer. The subject of his doctoral thesis was
"diagnostics of the failure in asynchronous motors fed by an
inverter". In 2003, he joined the R&D department of Leroy
Somer at CEB factory.
M. Philippe WESOLOWSKI: graduated in France with a
“Genie Electrique” associated Degree in 1985 CNAM.
He joined Leroy Somer Company in 1988 as a specialist of
drive applications in Oil and Gas market.
________________________________
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