Introduction
Distribution substations are responsible for a continuous supply of electricity to consumers. However, most of the time, the losses in this process often raise economic concerns for the transmission and distribution-providing company. According to International Energy Agency (IEA), the amount of electrical power lost in transmission and distribution is 19.245% of the total produced in India (2014). Thus, there is a need to decrease this loss by using the appropriate method.
Transmission and Distribution losses (T&D) losses can be divided into Fixed technical losses and Variable technical losses. Fixed losses do not vary according to current. and take the form of heat and noise and occur as long as a transformer is energized. Fixed losses include corona losses, leakage current losses, dielectric losses, open-circuit losses, losses caused by the continuous load of measuring elements, and losses caused by the constant load of control elements. Variable losses vary with the amount of electricity distributed and are, more precisely, proportional to the square of the current. The leading causes of Variable T&D losses are as follows:
· Lengthy Distribution lines
· Inadequate size of conductors
· Installation of Distribution transformers
away from load centers
· Low power factor of primary and
distribution system.
· Load factor effect on losses
· Transformer sizing and selection
· Balancing Three Phase load
Mostly, the average power factor in secondary distribution varies from 0.65-0.8. The low power factor forces the load to draw more current from the source leading to high power loss which leads to high stress on the power generation utility as it has to work more often to meet the demand and the losses generated.
This problem of low power factor can be solved locally by using shunt capacitors at distribution substations. Capacitor banks are locally commissioned in Primary and secondary distribution centers to counterbalance the reactive power demand which helps improve the quality of electrical power supplied to consumers and reduce the expenditure on providing economical electrical supply. Capacitor banks help electrical engineers provide a better electrical supply which is helpful to consumers and producers technically and commercially. In this paper, the author has described the practicality and technicality of shunt capacitors in the Distribution system.
Need for Shunt Capacitors
Apparent electric power (S/VA) comprises two types of power, Active power (P/W) and Reactive power (Q/VAR). Reactive power demand depends on voltage. As the voltage of the receiving end decreases due to high load, the reactive power demand increases simultaneously on that receiving end. Almost all the load in practical power systems is inductive in nature, thus this reactive power demand is lagging in nature (lagging VARS). To fulfill this reactive power demand, a local source of reactive power is used which is the Shunt capacitor because it provides lagging reactive power (lagging VARS) when acting as a source. Using a compensator in a power system helps maintain a better voltage profile, lower voltage regulation, and higher power factor. The calculation for the rating of the capacitor bank needed depends on the present power factor of the system, the desired power factor, and the load on the system. The load of the substation was calculated using the rating of the power transformers in the 33/11 KV substation. The ratings were as follows: 3960 KVAR, 12.65 kV for 10 MVA transformer, and 1980 KVAR,12.65 kV for 5MVA transformer. Several arrangements of capacitor banks are available in the market like pole-mounted RPC systems, Pad-mounted RPC systems, Station mounted RPC systems, etc. Every configuration has its own applications. However, in this paper, Station mounted RPC system is explained.
Station-mounted RPC System
· Structure
It can be divided
into two parts:
1. Off-load Isolator
2. RPC Body
1. Off-load Isolator
As the name
suggests it works as a mechanical switch to connect or disconnect the capacitor
bank to the supply. It is, however an isolator with an earth switch with an
interlocking mechanism. The interlocking mechanism restricts connecting the earth switch to the circuit
while the main isolator is still connected to the circuit in order to prevent
ground fault.
2. RPC Body
This is the main body
which contains capacitor units, series reactors, expulsion fuses, RVT,
Lightning arrestors, capacitor switches, and porcelain post insulators. All
these elements are mounted on the outdoor assembly. Apart from all this, an
APFC panel comprising of the different relays is installed inside the substation.
A general
arrangement drawing of capacitor banks is given in Figure 1.
The capacitors are installed into step banks, the total KVAR is divided into small segments of KVAR and each segment works as a single working unit under the control of the capacitor switch. e.g.: The 3960 KVAR Capacitor Bank is divided into four-step banks of capacity 1188 KVAR, 1188 KVAR, 792 KVAR, and 792 KVAR. Each 1188 KVAR step bank contains 3 nos. of single phase capacitor units of 396 KVAR and a step bank of 792 KVAR comprises 3 units of 264 KVAR. Each Capacitor has two terminals, one is connected to the reactor and the other is connected to the capacitor switch via the expulsion switch. The rating of reactors and expulsion switches depends on the rating of capacitor units. RVT is connected to the reactor end which completes the circuit. The internal connections in ROC are done by 50x3 mm and 25x3 mm Aluminium Flats. The connection between RPC Body and off load isolator is done using a dong conductor which connects each phase of the capacitor switch separately.
· Working
The bank gets its
supply from the 11 kV main bus. It is connected in parallel to other 11 KV
feeders. When the bank isolator is switched on, the supply reaches the
capacitor switch. RVT and switch are connected using 2 core cables. RVT
receives signals from the APFC panel to on/off the desired switch.
During the peak time,
capacitor banks are connected to the circuit and disconnected in off demand
period. When a bank is disconnected using the isolator, it is important for the
capacitor to discharge properly. Hence the grounding system needs to be very
efficient. The earthing switch in the isolator discharges the residual charge in the circuit and the reactor connected in series with a capacitor helps in proper discharging.
The power circuit
diagram of the capacitor bank is given in Figure 2.
Precautions while Installing and Commissioning
1. The earthing of capacitors, reactors, RVT, and APFC should be checked and made very efficient as it ensures the long life of the RPC system.
2. The connections within APFC and among APFC,
RVT, and 11KV Breaker need to be very precise and tight otherwise loose
connections will damage the panel.
3. In case of maintenance, the bank should be
made to discharge for at least 5-7 minutes and proper discharge should be
cross-checked using a discharge rod. Only then, any technician should be allowed
to work in the bank.
4. RPC system should be used mostly in peak demand time.
Conclusion
Reactive power compensators need to be widely used in the power distribution sector for more efficient power transfer. In this paper, Station mounted RPC is discussed which is commercially used nowadays in secondary distribution systems. RPC can also be used in specific big feeders extending to a long distance. However, proper maintenance and scheduling of such banks are equally important.
References
· https://data.worldbank.org/indicator/EG.ELC.LOSS.ZS?locations=IN&name_desc=false
·
https://www.powerquality.co.in/epcos.pdf
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