Random switching of power system equipment may lead to high-frequency overvoltage transient and inrush current. This, in turn, may stress the equipment, leading to rapid aging or dielectric failure. What is ? Point on Wave Controller/ Switch? Point on Wave Switch (PoW), often called Point on Wave Controller is a high speed microprocessor based relay used for the controlled switching of circuit breaker of HVAC system. Controlled switching refers to open or close a breaker at a pre-determined point on the voltage waveform.

1. POINT ON WAVE
SWITCHING

2. Random switching of power system
equipment may lead to high-frequency
overvoltage transient and inrush
current.
This, in turn, may stress the equipment,
leading to rapid aging or dielectric
failure.

3. • What is ?
Point on Wave Controller/ Switch?
• Point on Wave Switch (PoW), often
called Point on Wave Controller is a
high speed microprocessor based relay
used for the controlled switching of
circuit breaker of HVAC system.
Controlled switching refers to open or
close a breaker at a pre-determined
point on the voltage waveform.

4. This controller is used for switching of
Shunt Reactors, Transmission Lines,
Power Transformers & Capacitor Bank.
• Point On Wave (POW) controllers, also
known as Synchronous Switching Controllers
(SSC), are high-speed microprocessor-based
relays used to open and close the contacts of
independent pole operation (IPO) circuit
breakers (CB) at the pre-determined point on
wave for minimizing the switching transients.

5. • This approach minimizes the disturbance to the
network, as well as the stresses to the
associated equipment.
• Very sophisticated algorithm implementations
are available in the market, allowing POW
controllers to perform optimum switching of
capacitor banks, shunt reactors, power
transformers, and compensated or
uncompensated transmission lines.

6. Why Controlled Switching is required?
Random switching of equipment may lead to
high frequency over-voltage transient and
inrush current.
This in turn may stress the equipment and
leads to rapid aging.
It is suggested to read Transformer Inrush
Current for better understanding of inrush
phenomenon.

7. How Controlled Switching by Point
on Wave Switch / PoW Controller
achieved?
• Point on Wave Switch basically synchronizes
the random closing / opening input order so
that individual poles of the breaker get close
/ open at pre-determined point on reference
voltage waveform.

8. Figure below shows the simplified
diagram of PoW controller operation
The above figure shows the use of PoW controller for switching Shunt Reactor. It can be
easily seen that, a Point on Wave controller takes the following inputs:

9. Source side PT supply
• Only one phase i.e. R-N of source side PT supply
is taken for reference. This PT supply acts as a
reference for system voltage and PoW controller
decides the target point on this system voltage.
In case of Shunt Reactor switching to minimize
the inrush current, it is desirable to close the
breaker at voltage peak. Since the load is reactive
in nature, therefore closing the breaker at
voltage peak will cause the shunt reactor to take
zero current (current through an inductor lags by
90°) and therefore the transients in current will
be minimized.

10. CB Parameter
• For PoW controller to decide the point of
switching on the system voltage
waveform, it is necessary for it to know
the breaker opening and closing time.
Therefore, breaker closing and tripping
parameter is fed.
• Breaker timing is obtained through the
timing test which is carried out along
with DCRM test.

11. CT Input
• CT input to PoW controller serves the purpose for
analysis of load current upon closing / opening of
breaker. It is also used for the estimation of breaker
opening / closing time. It shall be noted at this point
that breaker closing is the instant at which pre-
arcing at take place.
• Pre-arcing take place before the actual closing of
mechanical contacts. Normally pre-arcing time is
2ms. Similarly for opening operating, breaker
opening time is the instant between mechanical
contacts open and arc extinguishes. Therefore arcing
time must be considered for determination of target
point. Normally, arcing time is 6 ms.

12. Working Principle of Point on Wave Switch
• When a random closing / opening command is
given to the breaker through control switch, PoW
receives the command and process it before issuing
the closing / opening command to the breaker. It
introduces a time delay between the issue of
random command from control switch to the issue
of command by PoW Switch as shown in figure
below.
• In this time period, the controller does the
necessary calculation such that breaker may close at
defined point on reference voltage waveform. For
this calculation, breaker operating time is
considered and then PoW issues command in such a
time that breaker may close at the target point. In
the figure below, target point is at voltage zero.

14. Let us consider an example for better
understanding. Let us suppose that breaker
closing time is 104.78 ms and it is used for
switching shunt reactor. Therefore our target
point shall be voltage peak. Point on Wave
Switch has its own time delay and it adjusts its
delay in such a way that the sum of PoW delay
and Breaker closing time corresponds to
voltage peak. Suppose the controller delay be
37.80 ms, therefore expected target point
shall be (Breaker closure time + Controller
Delay = 104.78+37.80) 142.58 ms.

15. • But actual closure of breaker means the
instant of time where current start flowing
through the circuit.
• Thus pre-arcing time shall be taken into
consideration. Assuming pre-arcing time to
be 2 ms, the target point shall be 140.58 ms.
Figure below shows the graph of switching
instant for Shunt Reactor.

16. Following
points shall
be noted
from the
above
graph:
The target point for shunt reactor is at voltage peak to limit inrush current.
PoW Switch determines the target point for Y and B phase assuming balanced three phase
voltage as only one phase voltage input is given to it. Thus if R phase voltage is assumed
reference then VY = VmSin(ωt-120°), VB = VmSin(ωt+120°)
As voltage peak either positive or negative for Y phase is occurring first, therefore closing
command to Y pole of breaker will be issued first. Thereafter R and at last B pole will close.
Thus the difference in target point for two consecutive phases will be (30° = 10 ms / 6)
1.67 ms as evident from the graph above.
Carefully observe the current curve and see how smooth current transition is taking place.
It can be realized from the above graph that how inrush current is limited.

17. Adaptive Nature of Point on Wave Switch
• As we know that, Breaker opening and closing time
changes with time and aging. Further it is also
affected by the environmental condition. Therefore
it is necessary to compensate for these.
Compensation for environmental condition is
accounted by giving ambient temperature input to
Point on Wave Switch.
• For accounting change in breaker operating time
due aging, PoW is made adaptive. Adaptive means,
it can adapt its timing as per the previous switching
data so that required target point can be achieved.

18. Point on Wave Switch Bypass Feature
• A provision for bypassing PoW switch is generally
provided by the manufacturer. The controller gets
automatically bypassed in case of internal fault in
it.
• For manual bypass, two hand switches CLOSE and
TRIP with IN & OUT positions are provided to
bypass closing and tripping operation of breaker
manually when required.
• It shall be noted that, in case of protection
breaker always trips on PoW bypass mode. This is
done to avoid for any intentional introduction of
time delay for clearing fault.

19. Review of Switching Strategies
• One advantage of POW controllers is their capability to
perform more than one switching strategy to better adapt
when the network changes (e.g., changes in line
compensation levels or replacement of CBs with a different
operating mechanism). Our team can recommend new
operating strategies, improved settings, or potential
controller upgrades to fit the application.
Conlcusion: Point-on-Wave Switching
• It offers a cost-effective and compact electronic
solution to switching transients and can be installed at
initial build stage or as a retrofit to existing circuit
breakers.

20. Powertech Labs Point On Wave controller snapshot