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. ________________________________
8
