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APPLICATION otes
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Published for the Basler Electric Power Systems Group #EX-D200N1 • January, 2011
DECS-200N Negative Field Forcing Capability Offers
“New” Tuning Advantages for Rotating Exciter Systems
Voltage regulators of various types have been designed
over the years. On many machines, one quadrant voltage
regulators provide positive field control but no negative field
forcing into the exciter field in response to system disturbances
For generators having small machine time constants, the
lack of negative field forcing generally is not an issue. However,
as the size of the generator becomes larger, performance
expectations increase. A non-negative field forcing voltage
regulator will have limitations when asked to provide corrective
generator voltage change in response to system disturbances.
The lack of negative field forcing creates nonlinearity in the
system when fast response is
desired, such as during system
transients, because it limits the
exciter field decay. For hydros, Figure 3: Facilities with gas turbines requiring power system stabilizers
which are slow speed spinning benefit from Negative Field Forcing Voltage Regulators.
machines with large time
constants, tuning the digital DECS-200N is similar to the features and functionality of the
excitation system can be a Basler DECS-200 except instead of having a pulse width
challenge when negative field modulated bridge for field control, it utilizes a SCR bridge
forcing is not available. with the ability to provide both positive negative field
Yet, for machines that are forcing. Table 1 offers a summary of DECS-200N features
brush or brushless, up to 30-40 and functions.
MVA, cost may be a concern to Voltage regulators with negative forcing can improve the
purchase extra performance generator voltage response time during load disturbances.
capabilities. Figure 4 represents a typical response characteristic for a
Today, Basler Electric offers voltage step change on a generator that utilizes a negative
a solution to address the need for
Figure 1: Standalone improved performance - the
DECS-200N DECS-200N. See Figure 1. The
(a) KG=5 with negative field forcing
(b) KG=5 with no negative field forcing
Figure 2: At facilities with slow speed hydros, like the one above, there can
be tuning challenges without Negative Field Forcing Voltage Regulators. Figure 4: Performance differences with negative forcing
1
DECS-200N
forcing voltage regulator, compare the lower Bi-Directional No Negative
diagram with the same voltage step change. Note the Field Forcing Field Forcing A
generator being controlled by a Non-Negative
Forcing Voltage Regulator has much more sluggish
response, both with voltage steps up and even
slower for a voltage stepdown. The voltage over-
shoot can be minimized by lowering the gain to
make the system produce less voltage overshoot, but
this affects the overall system response by making it B
even more sluggish.
Yet another concern exists where higher
performance is needed, with increasing demand for
reliable power. Today, more and more excitation
systems are being equipped with power system
stabilizers to improve the collected response of C
machines connected to the transmission system.
In Western North America, machines 30 MVA
and higher, and plants having 75MVA of generation
or greater, must have a power system stabilizer. In
the past, power system stabilizers have been added to
various types of voltage regulator systems working
direct into the shunt field of the rotating exciter. The D
stabilizer often was added to a system to meet a
regulatory requirement without review of the
excitation system to determine if it had the perfor-
mance capabilities to respond favorably to system
disturbances.
For a power system stabilizer to be fully
beneficial for generators having rotating exciters, the E
voltage regulator should have fast response and a
wide frequency bandwidth, ideally in the range of
135 degrees phase lag at 1 Hertz. Where many
excitation systems used on turbine generators in the
30MVA range have only positive field forcing to
drive the exciter shunt, field performance is limited.
Adding negative field forcing into the excitation A - Exciter Field Voltage, VR B - Generator Field Voltage
systems with exciter shunt field restores the linearity C - Generator Terminal Voltage D - Power (MW)
and optimizes the unit response. E - Power System Stabilizer
Figure 5 is a simulation of an external fault. The Time equals 1 second for Bi-Directional performance. Time equals 3 seconds for No
first half of the record is for a full-controlled bidirec- Negative Field performance
tional bridge; at time equals 3 seconds, a single Figure 5: Simulation of an external disturbance
Basler DECS-200N Voltage Regulator Features and Functions
DECS-200N • Protection:
• 0.25% Voltage Regulation - Field Overvoltage
• Field Current Regulation (manual) - Generator Over/Under Voltage
• Var or Power Factor Control - Field Overcurrent
• Overexcitation limiter (off-line and on-line) - Loss of Voltage Sensing Detection/Transfer to
• Underexcitation limiter (custom 5 point or standard curve) Manual Control
• Dual PID settings groups optimize system response when - Brushless Exciter Diode Monitoring
a separate Power System Stabilizer (PSS) is active or - Loss of Field
inactive • HMI Metering, Operating Screen Communications
• Automatic Tracking for Bumpless Transfer • RS485 Serial Communications (ModBus protocol)
• Underfrequency Voltage Limited Compensation or • Operations Setup Software (BESTCOMS)
Selectable Volts/Hertz Ratio Limiter • Preposition Set point
• Automatic Voltage Matching • Reactive Droop
• Generator Voltage Softstart • Line Drop Compensation
• Oscillography (8 records) • Meets cURus & CE Markings
• Sequence of Events Recording (127 events) • IEEE Math Model
Table 1: DECS-200N Features and Functions
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directional bridge is implemented. The single directional 200N's can be provided on
bridge produces a generator terminal voltage increase of over a panel or in a cubicle with
10% . See Figure 5C. The settings for the PSS on these sequencing control relays
systems must consider this effect. Notice power swings and automatic transfer
dampen much faster when the bi-directional field control is capability to the backup
utilized. See Figure 5D. controller, with wire out
Where a high end excitation system like the DECS-400 terminal blocks for easy
may not be the most economical solution for these smaller customer interconnection.
machines, the DECS-200N may be the more favorable See Figure 6 and 7.
selection. For more information
Power for the bridge can be single phase and for high on the DECS-200N,
field forcing, three phase voltage input is suggested. For request Bulletin SZO.
generators having permanent magnet generators, the DECS- For more information
200N is compatible with PMG frequencies ranging from 50 on negative field forcing
to 500 Hertz. The DECS-200N has a 20 amp continuous voltage regulators and
current capacity (15 amps for power input frequencies greater power system stabilizers,
than 420Hz) and compatible with field voltages of 63 and 125 see the Technical Paper
Vdc with a minimum of 150% Field Forcing. “Suitability of Excitation
Basler Electric offers the Redundant Controller package Systems for PSS Applica-
in a fixed mounted configuration or a disconnectable DECS- tions” from the Power
200N version allowing on-line maintenance of the DECS- Engineering Society General Figure 6: Typical Dual
200N while the machine is running. Meeting, 2003 in the DECS-200N on a chassis
While Table 2 offers various solutions pre-engineered Download Section of the with automatic transfer
for end users to address performance expectations, Basler Basler web site - http://www.basler.com.
Electric can custom engineer virtually any chassis or cabinet
design requested to include all the tools you needed for a
successful excitation upgrade.
For redundant controller applications, the two DECS-
DECS-200N Kit Options Part Number Control Power
A) Single DECS-200N Kit shipped loose P/N 9397500100 125 Vdc Control Power, 50-160Hz., Operating Power
P/N 9397500101 24/48 Vdc Control Power, 50-160Hz., Operating Power
P/N 9397500102 125 Vdc Control Power, 161-420Hz., Operating Power
P/N 9397500103 24/48 Vdc Control Power, 161-420Hz., Operating Power
B) Single DECS-200N Kit shipped loose P/N 9397600100 125 Vdc Control Power, 50-160Hz., Operating Power
with Adapter Plate for Westinghouse P/N 9397600101 24/48 Vdc Control Power, 50-160Hz., Operating Power
SilverStat
C) Redundant DECS-200N, fixed P/N 9395500100 125 Vdc Control Power, 50-160Hz., Operating Power
mounted on Panel with Automatic Transfer P/N 9395500101 125 Vdc Control Power, 161-420Hz., Operating Power
Approx. Dimensions: 27W x 55L x 9H
D) Redundant DECS-200N, P/N 9395600100 125 Vdc Control Power, 50-160Hz., Operating Power
Unpluggable Disconnect, mounted on P/N 9395600101 125 Vdc Control Power, 161-420Hz., Operating Power
Panel with Automatic Transfer
Approx. Dimensions: 27W x 55L x 9H
Including:
In the Option A and B Kit DECS-200N setting files are included, along with pushbutton switches for Auto/Manual Transfer,
Start/Stop/ Raise/Lower, Mounting bracket and Schematic interconnect are all included. For Westinghouse SilverStats an
adapter plate also is provided.
Table 2: DECS-200N Kit Options
3
Figure 7: Simplified Interconnection of a Dual DECS-200N System with a Seperate Power System Stabilizer
P.A.E. Les Pins, 67319 Wasselonne Cedex FRANCE
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EX-D200N1 (1/11) 4
