FIELD OF INVENTION
The invention relates a method for generating triggering pulses for thyristors to
control current in general and to a method for frequency compensation for
thyristor firing pulse generation circuit based on Ramp Comparator in particular.
BACKGROUND AND PRIOR ART
Thyristor controlled Reactors (TCR) are used extensively in FACTS (Flexible AC
transmission system) devices like SVC (Static VAR Compensators), CSR
(Controlled Shunt Reactors), TCSC (Thyristor Controlled Series Compensator)
etc, for controlling the reactive power flow. Typical circuit of a CSR is shown in
fig 1.
The current flow in a thyristor controlled reactor can be smoothly varied from
0% to 100% by triggering the thyristors at an angle from 180 degs to 90degs
with respect to the voltage across the TCR. The current in a reactor lags the"
voltage by 90 degs due to reactor being an inductor. Triggering the thyristors at
90 degs results in full current whereas there will be zero current at 180 degs.
The triggering pulses are generated by a firing pulse generator module. This
module requires the synchronizing signals (signals corresponding to the Voltage
across the TCR) and a control voltage corresponding to the angle to which
triggering is required for generating the firing pulses. A 3phase TCR system
requires 6 pulses (R+, R-, Y+, Y-, B+, and B-).

The firing pulse generator module used follows the relation between control
voltage and triggering angle correctly when the synchronizing signal is 50Hz. A
ramp of fixed rate is started at every zero crossing (each phase) and at a trigger
pulse is generated at point of coincidence of Control voltage and the Ramp. Any
change in frequency (generally +/- 3% from 50Hz) will result in a incorrect
angle. An angle intended to be 90 degs will be less than/more than 90 degs
depending if the frequency has reduced or increased from 50Hz. An angle less
than 90 degs is not desirable in TCR circuit since it could result in a offset cosine
current. To ensure the triggering angle is never less than 90 degs for the
frequency range, the control voltage is presently set to limit of not less than 96
degs.
The limit of 96 degs has a drawback since fully rated current (hence full reactive
power) is not allowed to flow for the frequency range. For a 50MVAr reactor
about 3-4MVAr reactive power will not be available at 50Hz frequency.
A search was carried in the following websites:
1) http://pfc.ora.in/db/db.htm
2) www.wipo.int
3) www.uspto.QOv/patft
A related patent US Patent no 4,546,424 dated Oct 8th 1985 wherein "a
compensating circuit maintains the peak value of the ramp signal close to its
calculated value despite variations in the actual value of the capacitor from its
normal or rated value and despite changes in power line frequency between 50
hertz and 60 hertz. This compensating circuit increases or decreases charging
current to the capacitor according to the type of compensation that is
necessary." has been used.

OBJECTS OF INVENTION
The primary object of the present invention is to generate firing pulses from of
90 degs to 180 degs over a frequency range of 45Hz to 55Hz ensuring full rating
of the reactor where a Ramp comparator based Firing pulse generator is used.
SUMMARY OF INVENTION
The method of frequency compensation implemented in a microprocessor based
controller calculates the frequency and in turn generates a calibrating value for
the required firing angle. The control voltage for the firing pulse generator is
given by the controller.
The operation of the controller with the feature of frequency compensation has
been tested and checked in field.
BRIEF DESCRIPTION OF ACCORDING DRAWINGS
Fig 1 (Prior art): Shows typical Circuit of a Controlled Shunt Reactor (CSR). The
Electrical firing pulses from Controller is converted to Optical form in BETM (Base
Electronics and Thyristor Monitoring) equipment before applying to Thyristor to
ensure isolation.
Fig 2 (Prior art): Shows illustrates the method used in the invention with the help
of a flow diagram.
Fig 3 (Prior art): Shows illustrates through graph the change of control voltage
with deviation of frequency.

DESCRIPTION OF DRAWING
The method used has been illustrated in fig 2. Three main blocks are used in this
scheme viz (1) +ve sequence component extraction block (2) Frequency
deviation calculation block and (3) Calibration of control voltage.
To calculate the frequency deviation, samples 20ms apart are compared. For a
50Hz signal samples 20ms apart are equal. For other frequencies these samples
are not equal in magnitude. The difference in magnitude of these samples is
related to the frequency deviation from 50Hz.
1) +ve Sequence component extraction block: The positive sequence
component of the three phase synchronizing signals is computed in this block.
V1 = l/3(Vr + a.Vy +a2.Vb)
Where a = -0.5 +j0.0866 and a2 = -0.5 -j0.0866
The symmetrical component is extracted to take care of frequency calculation in
case of unbalanced and line faults conditions.
2) Frequency deviation (Af) calculation block:
The deviation is calculated as follows:

S= arctangent ((abs(A) +abs (B))/(abs(C)+abs(D)))

Af = (F+S)/π, if ((A X D) - (B X C) is +ve, Af is -ve.
Where F= 50Hz
3) Calibration of control voltage block:
Depending on the frequency deviation, the amount of calibration required on the
control voltage is calculated using Fig. 3 as follows:

Calibrating voltage = Yx(V180 + VCOn)/VCon
Where Vcon is voltage corresponding to required firing angle and V180 is voltage
corresponding to 180 firing angle.
The controller with the feature of frequency compensation ensures availability of
full rated current in a TCR over a frequency range of 45 Hz to 55 Hz where a
Ramp comparator based firing generator is used.
Reference
1. A digital Multifunctional protective Relay. Murty V.V.S Yalla. Transactiona
on Power Delivery, Vol 7 Nol Jan 1992.

We Claim
1. Method for frequency compensation of thyristor firing pulse generation circuit
based on ramp comparator comprising steps;
- extraction of +ve sequence component of the three phase synchronizing
signals;
- determination of frequency deviation (Af) of the extracted symmetrical
component;
- calibration of control voltage depending on the frequency deviation;
Characterized in that the frequency compensation in the controller ensures
availability of full rated current in a Thyristor Controlled Rectors (TCR) over a
frequency range of 45HZ to 55HZ wherein a Ramp Comparator based firing pulse
generator is provided.
2. The method as claimed in claiml, wherein said extraction is carried out with
the symmetrical component for frequency determination in unbalance and line
fault condition also.
3. The method as claimed in claiml, wherein the frequency deviation (Af) is
determined on the basis of samples 20ms apart are compared.
4. The method as claimed in claiml, wherein said frequency deviation from 50Hz
depends on the difference of these samples.
5. The method as claimed in claiml, wherein said calibration control voltage is
determined depending on the deviation of frequency.

Method for frequency compensation of thyristor firing pulse generation circuit based on ramp comparator comprising steps;
- extraction of +ve sequence component of the three phase synchronizing
signals;
- determination of frequency deviation (∆f) of the extracted symmetrical component;
- calibration of control voltage depending on the frequency deviation;
Characterized in that the frequency compensation in the controller ensures availability of full rated current in a Thyristor Controlled Rectors (TCR) over a frequency range of 45HZ to 55HZ wherein a Ramp Comparator based firing pulse
generator is provided.