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Thermal Limits & Other Embedded Generation Issues

Renewable Energy Engineering presentation
by

Khaliun Boldbaatar

on 3 June 2014

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Transcript of Thermal Limits & Other Embedded Generation Issues

Thermal Limits
Overhead Lines & Cables
Transformers

Other Embedded Generation Issues
Flicker
Step & Dips

Renewable Energy Engineering
Presenter : KHALIUN
Boldbaatar

What will we learn by Understanding Thermal Limits?
Power flows in individual components may be increased, decreased or even reversed. Power flow requires current flows and this causes
Heating in Lines & Transformers.
Each of these components has a Thermal Limit, which effectively determines the maximum current it in carry, & these thermal limits can set an upper limit on the size of the RE generator.
Load flow(Power flow) analysis allows all the component currents to be calculated.
Overhead Lines & Cables
In lower Voltage Network,
Lines & Cables are normally operated at well
below their Thermal Limits
At higher Voltage Levels
, Voltage control is less demanding & Thermal Limits of Lines & Cables -->
WELL be the Limiting Factor
In lower Voltage Netwotk
(up to say 20kV)
At higher Voltage Levels
(33 & 132kV)
1. To keep Customer Voltages within range,
Voltage drops in conductors must be limited
--> Requires use of larger conductors than those dictated by Thermal Limits
2. Energy losses are less in larger conductors & a life-cycle cost analysis usually favours oversizing
Voltage & increased Fault Level considerations arising from the connection of additional generation
1. Care is needed regarding DEMAND that is assumed to be concurrent with the Generation in calculations
RE source is concurrent with the MAX Demand

2. Overhead Lines are naturally cooled be WIND, which increases their capacity during periods of High Wind
Lines --> WP (undersized relative) --> Conventional Design guides
Transformers
COMPLICATED to
determining TL
for Transformers
Natural Cooling
Fan Cooling
Cooling by Pumped Oil
Natural Cooling
(Convective)
Fan Cooling
Cooling by Pumped Oil
Transformers have long Thermal time constants --> overloaded for short periods of Time without causing overheating or significant DAMAGE
Flicker
Flicker in WT
CONCLUSION
Transformers
-->
maximum Demand
normally operating
Where
: Significant proportion of their TL
Why
: Standing losses are significant & efficiency of lightly --> Transformer is poor
Transformers -->
reverse power flow
up to the normal forward rating

PROBLEM
BUT -- On-load tap-chargers that have very limited reverse-power capability
The TL of existing transformers are NOT normally a limiting factor in the Installation of Distributed Generator, unless the Installed Capacity of Generators exceeds the maximum Demand in the area served
Voltage FLICKER occurs when heavy loads are periodically turned on and off in a weak distribution system.
If the distribution system's short circuit capacity is not large enough, voltage fluctuations will occur. Starting large motors require an rush of current, which causes a decrease in voltage.
WT --> to flicker
Why?
: Tower shadow & turbulent winds can cause rapid variations in active & reactive Power Output
Fixed speed pitch-regulated turbines are generally worse than stall-regulated ones at High Wind Speeds
Why?
: The pitch regulation action in response, a gust is slow to take effect due to limited pitch actuator capability & the inertial mass of the hole, while the aerodynamic action of stalling a
BLADE --> very fast
.

BUT :
WT--> Variable speeds UP to the Rated Wind speed
Rated ==> Marginal speed Variations <== Cut Out Wind Speed
CONCLUSION
Flicker is more of an issue for single turbines or small clusters on low Voltage distribution Networks. As the Power fluctuations from individual WT are NOT correlated, the effect of flicker is less pronounced for large WF.
Step and Dips
Switching generator

-> step change, due to
an associated abrupt change in
active or reactive Power flow
Where?
: in the Voltage
at the PCC
IF
The power changes -> more gentle

Voltage steps -> minimized
Where?
: at least during planned switching operations
Voltage DIPS
Voltage STEPS
repetitive or cyclic, may contribute to flicker
Steps & Dips,
IF
RE generator==>With the increasing use of power electronic interfaces<==The GRID
Steps & Dips during switching-- virtually eliminated!
THANK U 4 your ATTENTION

Any QUESTIONs
In the graphs, when the load is turned ON, it is seen that one phase is totally crushed to zero level, resulting in the increase of the other 2phases. This is called voltage FLICKER in industrial terms.
The aim of this test is check the equipment performance under short-term reduction of mains power or brief interruption of the supply. This can occur due to faults in the mains power distribution network.
It may surprising that a great many lines & cables are normally operated at well below their thermal limits.
If the power changes associated with switching can be made more gentle, then such voltage steps can be minimized, at least during planned switching operations.
Voltage STEP waveform
Thermal Limits in Wind Turbine
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