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The PureWave AVC consists of power elements including capacitor
stages and power-electronic switches, a microprocessor-based control-
ler, and auxiliary equipment such as potential and current transform-
ers.
Capacitor Stages
The per-phase reactive power supplied by the PureWave AVC is divided
into binary increments. If Q1 is the reactive power (in kVARs) gener-
ated by the first capacitor stage, then—for a four-stage PureWave
AVC—the second stage has a reactive power of 2
Q1 (twice the
kVARs of the first stage). The third stage has a value of 4
Q1 (four
times the kVARs of the first stage). And the fourth stage has a value of 8
Q1 (eight times the kVARs of the first stage). The total reactive
power of the PureWave AVC is calculated as 3
Q1
(2
0
+ 2
1
+ 2
2
+
2
3
). The number of stages and the value of Q1 are determined by the
requirements of the application.
Each capacitor stage has a discharge resistor that dissipates the
stored energy, reducing the voltage across the capacitor terminals to 50
volts within 5 minutes after disconnection.
Power-Electronic Switches
Each capacitor stage is controlled by a set of power-electronic
switches composed of an SCR and a diode connected in anti-parallel.
The diode performs two basic functions: It charges the capacitors in
each stage to the negative peak value of the supply voltage and it pro-
vides a path for the flow of the negative half cycle of capacitor current.
When the supply voltage crosses zero becoming negative, the diodes
conduct, charging the capacitors to the negative peak value of the
supply voltage. At this instant, the PureWave AVC current reverses
direction and the diodes stop conducting. All capacitors are thus pre-
charged and remain in that condition until a control signal is sent to the
SCRs to set them in the conducting mode. While the thyristors are not
conducting, the discharge resistors tend to dissipate the energy stored
in the capacitors. However, during this short time (one cycle), the
change in the stored energy and the voltage across the capacitors are
negligible.
Microprocessor Controller
The microprocessor controller continuously monitors the phase volt-
ages and the load current. Based on the magnitude of the imaginary
component of the load current, the controller calculates the appropri-
ate triggering sequence of the SCRs needed to compensate for the reac-
tive power requirement at that moment. Maximum compensation
capability is determined by the total rated kVAR of the PureWave AVC.
System voltage and current waveforms are used as references by the
controller. They are filtered of all harmonics and transient distortions
so that compensation is based on the 60-Hz or fundamental compo-
nents. The required reactive power is supplied by the PureWave AVC
and injected into the system during the following cycle, giving a maxi-
mum response time of one cycle, or 16.6 milliseconds for a 60-Hz
power system. Complete compensation is achieved when the line cur-
rent is in phase with its respective phase voltage.
MAIN COMPONENTS
Summary of Contents for PureWave AVC
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