Chapter V – Coordinated Voltage Control for Active Network
6.2. New Generator Output Estimation Techniques
The load ratioEST, which is currently used in the SuperTAPP n+ scheme, is determined at the network analysis stage and applied into the relay when the AVC
of the scheme is fluctuation in this parameter. As has been discussed above, the load ratio in distribution networks is not constant and can significantly diverge from the set value.
With the aim of improving the efficiency and accuracy of the estimation technique, two new methods to calculate generator current injection at the remote point on the feeder are presented in this section. Both methods are still based on local measurements. However, relationships between the active and reactive components of the measured currents are used.
6.2.1. Dynamic Load Ratio
This method is based on the dynamic load ratio, EDY. Unlike the estimation method based on load share ratio ESTbetween the feeders that needs to be evaluated when generator output is zero, the EDY parameter is updated continuously. Calculation of the dynamic load ratio EDY is based on the premise that load power factor on the substation and on individual feeders is similar and the fact that distributed generation operates at unity power factor.
The load share between the feeders with generation and those without can be calculated using reactive components of the summed transformer current I and TL current on the feeder with DG IFG. This calculation is as follows:
( ) ( )
FG replacing the static load ratio EST with the dynamic ratio that gives:( )
(
DY TL FG)
FGG E I I I
I = ⋅ − − (47)
The advantage of this method is that the load ratio does not have to be changed in the relay if the load on the network changes, as it is a case for static load ratio algorithms.
These changes are reflected in the local measurements and automatically used by the relay. However if additional ‘new’ load is added, the reactive current will also increase in proportion resulting in the change of theEDY. As this is a predictable change suitable adjustment would be made to avoid any errors in the measurements.
Additionally, the method using EDY is susceptible to power factor deviations that affect the estimation accuracy.
6.2.2. Constant Power Factors
It has been described in previous chapters that the power factor on the distribution networks without generation is fairly constant [29], [36]. It is also common practice that DG is requested to operate at fixed power factor. The second method, called
( ) ( )
With the assumption that load power factor is constant the term
( ) ( )
TLvalue and with the assumption that generator operates at fixed power factor, the term
( ) ( )
FG FGI I Im
Re becomes a constant. Thus, the generator output can be calculated using the
current measurement on the feeder with DG and two constants CLOAD and CDGas follows:
( ) ( ) ( )
[
FG LOAD FG]
DGG I C I C
I = Im ⋅ −Re ⋅ (49)
As the generator current output calculation relies on a constant power factor for both load and generation, any deviation from the set value of either will cause an error in the estimation.
6.2.3. Simulation Results of Dynamic and Constant Power Factor Ratio
The simulation results based on the SuperTAPP n+ and the real network data have been used to investigate consistency and accuracy of the proposed generation estimation methods and compare the results with the estimation technique based on static load ratio. A three day sample of the simulation results is shown in figure 6.2.1.
Generation Estimation Techniques
-100 -50 0 50 100 150 200 250
14/12/09 00:00 15/12/09 00:00 16/12/09 00:00 17/12/09 00:00
Time
Generator output [A]
Est Edy
Const PF Measured
Figure 6.2.1 Generation estimation technique comparison.
Generation estimation techniques based on static, dynamic and constant power factors are represented by the green, red and blue lines respectively. The black line corresponds to the measured generator output at the point of connection. It can be observed that all three estimation methods have some errors. The least consistent is the technique based onEDY. Estimation based on EST tends to overestimate generator current injection to the network between 15:00 and 01:00. The constant power factors method, in this case is the most accurate, the generator output closely following the black line in the figure 6.2.1.
It has been observed that, depending on the network conditions, load profile on the feeders, generator operation regime and load power factor, one of the three proposed estimation techniques can provide better accuracy than the others. For each network
detailed analysis is necessary to select the most suitable technique in order to ensure the most accurate generation estimation based on local measurements.
The new methods offer alternative estimation techniques that can be used in the SuperTAPP n+ scheme to calculate generator current injection at the remote point on the feeder based on local measurements. The SuperTAPP n+ relay system has been design to facilitate multiple current measurements and each measurement can be programmed for a particular use. In this way it is a simple task to modify the program code for alternative estimation techniques. Both methods have been implemented in the SuperTAPP n+ relay and will be tested at SuperTAPP n+ trials to evaluate their functionality and accuracy in real networks.