Download Application of VFD in Condenser Pumps in Thermal Power Plant 1

Application of VFD in Condenser Pumps in Thermal Power Plant
1. Preface:
Established in 1988, Zhangjiakou Thermal Power Plant of Beijing Datang Power Generation Group
Co., Ltd. is located in Zhangjiakou, Hebei. At present, it has 8 generators, and each has
capacity of 300MW, altogether is 2400 MW. It is the largest thermal power plant in the north
of China. In 2002, Beijing Datang Power Generation Group Co., Ltd. reformed the frequency
conversion of a large number of equipments with VFD through international bidding. Beijing
Leader & Harvest Electric Technologies Co., Ltd., as the only winner for the VFD domestic bid
package, supplied two sets of HARSVERT-A06/130VFD for the innovation of the condenser pumps
of #6 and #8 generators. Up to now the VFD of condenser pump #1 of the #6 generator has operated
stably for 2 years with obvious energy saving effect.
2. Operating Mode of Condenser Pumps
The steam finished working in the turbine, will condense in the condenser, it is collected
in the hot water well. The function of the condenser pump is to pump the condensed water into
the deaerator in time. To keep the condenser pump operating continuously and stably is an
important aspect for keeping the power plant operating safely and economically.
Monitoring and adjusting water level in a condenser is a main work for the operation of condenser
pump. Under normal operation condition, the water level in the condenser should not be too
high or too low. When the load of the generator increases, the condensed water volume increases
and the water level in the condenser rises correspondingly. When the load of the generator
decreases, the water level in the condenser lowers correspondingly.
The motor for the condenser pump is 6KV/1000KW and it is designed certain margin. Each generator
is equipped with two condenser pumps. One is operating and another one is reserved. Before
using VFD, the water level adjustment is performed through varying the opening of outlet valve
of the condenser pump. The linearity of adjustment is low and a large amount of energy is lost
in the valve. Meanwhile, frequent operation of the valve leads the reliability of the valve
decreasing and the stability of the operation of the generator is affected.
After using VFD, the outlet valve of the condenser pump basically needs not to be adjusted
frequently and the valve opening is kept in a rather wide range. Motor speed is changed by
varying the output frequency of the VFD to adjust the outlet flow and satisfy the requirements
for the operating mode.
3. Principle and features of VFD Model HARSVERT-A06/130
HARSVERT-A series VFD are produced by Beijing Leader & Harvest Electric Technologies Co., Ltd.,
enjoying the self-owned intellectual property rights. The system is of voltage resource type
and has features of stable operation, wide speed regulating range, good output waveform, low
input current harmonic, high power factor and high effectiveness, etc. It has low harmonic
contamination to the electric network. The total harmonic distortion THD is less than 4% and
complies with the standard of harmonic suppression of IEEE519-1992 directly and input harmonic
filters are not required to be used. It has high power factor and power factor correction
capacitors are not required. It has good output waveform, without additional heating of motor,
torque pulsing, noises, output dv/dt and common mode voltage, etc. caused by harmonic. Output
filters are not necessary and common asynchronous motors can be used.
HARSVERT-A series VFD use topology structure of cells in series, multi-level PWM (in brief
CSML). Direct high voltage output is implemented in the form of several low-voltage PWM variable
frequency power cells in series. Optic fibers connections are used between the high-voltage
main loop and the controller, safe and reliable. It has accurate failure alarming protection,
with functions of electronic protection of electric power and industrial electrical protection
to ensure the safety and reliability under normal operation and failure of VFD and motors.
Power cells are supplied by power network of 6kV decreased by isolation transformers with
multi-plied secondaries. Power cells are AC-DC-AC PWM voltage resource inverter structure with
three-phase input and single-phase output. Output ends of power cells connect in series each
other formed ‘Y’ structure to fulfill VFD’s direct medium output feeded the medium motors.
Every phase of 6kV output voltage is consisted of 7 power cells with 480V rated voltage connected
in series. Its output phase voltage is 3450V and line voltage is around 6kV. Every cell is
loaded with all the motor current but only put out 1/7 phase voltage and 1/21 power.
Each cell is powered by one group of the input transformer’s secondaries. Power cells and
transformer’s secondaries insulate each other. The secondaries are connected in the way of
extended sides triangle to fulfill multi-ply so as to decrease input harmonic current. Even
the unbalanced current happened, the generally same current can be ensured to reach the ideal
effect of harmony elimination. For the output voltage of 6kV, the 21 secondaries feeded 21
power cells are divided into 3 groups and each group is consisted of 7 different phase with
60/7 degree electric angle difference to form 42 pulse rectifier circuit structure. The input
current wave form is near sinusoidal and total harmony current distortion lower 4% and input
power factor can reach above 0.95.
The inverter output adopts the multi-level shifting-phase PWM technology. The same phase power
cells out put the basic wave voltage with the same magnitude and phase. But the wave load of
each cell in series has some electric degree to fulfill multi-level PWM and the output voltage
is almost sinusoidal waveform. Every level step of the output voltage is the same as cell DC
mother line voltage so the value of dv/dt is small. The lower switch on-off frequency is applied
to the power cell in order to decrease the wear of switches. Moreover the amount of output
level is increased. 6kV output voltage out put 15 levels phase voltage and 29 levels line voltage
and the equal output frequency is 9KHz, which is benefit to improve the output waveform, decrease
output harmonics. So the motor overheating, noise and torque pulse etc. caused by harmonics
are decreased deeply. And it can be used in driving the common asynchronous motor directly.
The failure of the motor insulation caused by surge voltage increase because of walk wave
reflection with long output cable will not happen.
It is the power cell connected in series not power components. It is unnecessary to connect
components in series because the highest voltage the components beared comes from cell DC mother
line. In this way, the even voltage caused by in series components is avoided. Using low voltage
IGBT power modules, the components failure occurs not easily working under low voltage. 6kv
VFD uses 42 couples 1200v low voltage IGBT. The drive power of low voltage IGBT is lower and
the circuit can be easily driven with low frequency of switch on or off. It is unnecessary
to adopt even voltage circuit and surge absorption circuit and the system efficiency is high.
At the same time the power cell is designed capacitance filter structure. In general, the VFD
technology is matured and reliable.
VFD can bear 30% drop of voltage and continues to operate. When the main power supply of 6KV
of VFD is out of power completely, it may continue to operate for 3 seconds, thus to satisfy
the requirements of continuous operation of VFD, when the power lines of the VFD switches.
In addition, when the main power supply of 6KV is low voltage, the VFD will continue to operate
with derated voltage and recover to the original speed when the voltage becomes normal.
Uncontolled diode rectifying circuits are used to the VFD. VFD can bear the surge voltage better.
The surge voltage caused by thundering or switching will pass through the transformers (the
impedance of a transformer is 8% usually) to produce surge current. And then the surge current
will charge the filtering capacitors through the rectifying diodes of the power cells, and
the filtering capacitors can fully absorb the surge energy that has entered into the cells.
In addition, surge absorbing devices of pressure sensitive resistors, which are set at the
primary side of the transformer have a protective function.
Power cells adapt module structure. All the power cells in the same VFD are completely identical
in structure and are exchangeable. It is very easy for maintenance. And only remove 3 AC input
terminals, 2 AC output terminals as well as 1 optic fiber plug, will the power cells be drawn
out. When one power module fails, the user may replace with a reserved power module through
simple operation in 5 minutes to ensure the reliable system operation. All power modules have
two indicator lights, one for indication of live status and the other for indication of operation.
The operation condition of the module can be known clearly.
Power cells are multi-level modules in series. When one module fails, it will be bypassed
automatically to enable to take corresponding measures at site. A parallel bypass circuit is
connected between the output terminals of each power cell. When the power cell fails, the
triggering signal for the corresponding power cell IGBT will be locked and make the bypass
SCR short, forming a channel for the motor current to pass. When bypassing the failed power
cell, two other corresponding power cells for the other two phases are also bypassed at the
same time to ensure the symmetry of the three-phase voltage. For the VFD of 6KV, each phase
has 7 power cells in series, when one cell has been bypassed for each phase, the other 6 power
cells will be left to operate for each phase, and their highest output voltage will be 86%
of the rated voltage, the output current up to 100% as well as the output power up to about
86%. The loaded pump speed may still be more than 92%. The production requirement can be
basically satisfied and the system operation reliability is increased greatly.
The input dry transformer is free of maintenance and has high reliability. The common protection
of the transformer and the VFD is a IP21 protective case, which may prevent the entering of
external solid matter with a diameter of more than 12mm as well as mice, snakes, cats and sparrows,
etc. from causing power failure for short circuit and provide safety guarantee for the live
parts.
High speed adjusting accuracy, precise output frequency resolution (0.01Hz) and continuously
adjustable output frequency 0.5Hz-120Hz can satisfy the operation mode requirements of
production technology for generators. Soft starting, soft braking and automatic control of
speed for motors can be performed. The starting current will be less than the rated current.
The starting time of motors can be adjusted continuously, and the affects to the electric network
will then be reduced.
Operating and monitoring software with self-owned, independent copyrights are preinstalled
in VFD and WINDOWS operating interfaces are used for local and remote starting and stopping,
function setting, parameters setting and operation log inquiring etc.; With color screen of
12.1”, the setting function of complete and common VFD parameters can be realized and the
operation charts can be printed out; Adjusting the touching screen, voltage & current waveform,
frequency and motor speed can be displayed at any time and the real-time status of motors at
any time can be displayed visually; It has high diagnostic and indicating capability and can
detect the operating conditions of various parts of the VFD. It has complete failure monitoring
circuits and precise failure locating capability. All the modules are of intellectual design.
When failures occur, the failure information will be sent to the main control unit and the
main control unit will switch off the main power cell IGBT in time, protecting the main circuit
and displaying failure location and classification precisely on the Chinese human machine
interface to see the failures at a glance. It is applicable to the skill level of ordinary
operating and service personnel.
PLC is built in VFD directly, thus the control logic is easy to modify, satisfying the
requirements of the versatile site, and real seamless interface with the DCS system of
generators can be implemented.
When external analog signals are used to control the output frequency of VFD (the VFD acts
as the executive mechanism of DCS), VFD can provide alarming signals and keep the original
output frequency unchanged simultaneously, if break-off or short of analog signals occurs.
The control power supply of VFD can accept AC 220V or DC 220V input and is equipped with UPS,
which enables the system continue to operate when the control power supply fails and provide
alarming simultaneously.