Basler Electric BE1-40Q - Manual (Page 9)

SECTION 1
• GENERAL INFORMATION
PURPOSE
Loss of excitation protection is applied on nearly all synchronous generators. Reduced or complete loss of
excitation can cause loss of synchronism, instability and, possibly, damage to the generator from
overheating. Many modern excitation systems include minimum-excitation limiters to prevent
underexcitation; however, loss of excitation protective relays are still applied as backup to these
automatic controls. BE1-40Q Loss of Excitation Relays provide this protection by monitoring the field
excitation (measuring the magnitude and direction of var flow) and tripping the generator before serious
damage to the generator can occur.
Synchronous generators in parallel are normally operated in the overexcited (lagging) region, which
allows generation of reactive power (vars). Although the field excitation may be safely adjusted to cause
the generator to absorb vars (leading), this is usually avoided because stability is unreliable under this
condition.
When field excitation is not sufficient to maintain the terminal voltage of an interconnected generator, the
system will attempt to supply reactive power to excite the generator. If the system cannot supply the
required vars, the weakened field may allow the rotor to slip poles during disturbances such as load
changes or faults, causing loss of synchronism.
When the system can supply the necessary vars, the generator will act as an induction generator,
drawing excitation from the system. The machine voltage will remain above the setting of undervoltage
relays, but the current induced by the rotor slip will flow in the damper (amortisseur) windings. The
excessive heating caused by the current flow reduces machine life exponentially.
Under either condition, BE1-40Q Relays will detect the increased vars at the generator terminals as a loss
of excitation and trips the generator to prevent loss of synchronism or excessive heating within the
generator.
APPLICATION
Capability Curves
Generator manufacturers supply capability curves that specify the operating limits of a particular machine
(similar to those shown in Figure 1-1). The curves are derived from the heating characteristics that occur
on the stator end iron, the stator winding, and the rotor winding. Plotted on the complex power plane, real
power P (kW) is on the horizontal axis and reactive power Q (var) is on the vertical axis.
An additional limit is often included on these curves, as shown in Figure 1-2. Here, the steady state
stability limit further defines the safe operating limit of the generator. If the stability limit is exceeded, an
out-of-step condition can occur due to loss of synchronism.
BE1-40Q Operating Characteristics
BE1-40Q relay characteristics closely follow the generator capability curves. The response characteristic
is represented by a line eight degrees from horizontal, placed above the most restrictive limit of normal
operation. As shown in Figure 1-3, the attendant intercept of the line on the Q axis (at
–0.4 per unit vars in
this example) is used to establish the pickup of the relay. A front panel rotary switch is used to set the
TAP setting. Refer to Section 5, Setting and Testing, for specific information on determining the pickup
setting.
Time Delay
A time delay is included in BE1-40Q Relays to prevent misoperation for transient conditions such as
power swings due to synchronizing or external fault clearing. A definite time delay of 0.1 to 9.9 seconds
can be set on the front panel thumbwheels in increments of 0.1 second. Setting both thumbwheels to 0
causes an instantaneous trip signal to be sent when the TAP setting is exceeded. Refer to Section 5 for
specific setting information.
9171500990 Rev N BE1-40Q General Information 1-1