1. The Influence of Load Modeling on Distribution Protective Relay Current
Luka Strezoski Marija Prica
Case School of Engineering, Case Western Reserve University, Cleveland, OH, USA
Introduction
Network Model
Results
It is common practice in recently developed short- circuit calculations to neglect loads in order to simplify the calculation procedure. This can lead to incorrect results when calculating currents at relay locations (relay currents) in distribution networks.
It is highly important to have accurate results for relay currents. Relay currents are used to ensure that relays will be able to protect their entire protective zones and clear the fault within predetermined time. Additionally, they are used to insure that protective devices along the circuit will coordinate with each other.
Test results show that neglecting loads could lead to error as high as 16% for small test network.
As the sequence domain is used, three-phase network is reduced to three single-phase networks, for positive, negative and zero sequence. These networks are modeled with  segment.
DGs are modeled either by ideal voltage sources behind corresponding impedances or by constant current sources, depending on their type [3].
Loads are modeled by three different linear models:
Constant currents,
Constant impedances,
Open circuits (neglecting loads).
Difference in modeling loads as constant currents and constant impedances is minor (Fig.4).
When short-circuit occurs at location considered far from the supply point, the error made by neglecting loads becomes significant. Error is as big as 15.6% for tested network (Table I).
There is no difference between currents at different relay location when loads are neglected (Table I).
For long rural networks (45 km) fault current becomes very close to normal operating current if loads are neglected. Thus, neglecting loads could lead to misuse of protective equipment by mixing normal operating current with miscalculated fault current (Fig. 5).
The percentage error made by neglecting loads increases rapidly with the increasing of distance between short-circuit location and the supply point (Fig. 5).
Figure 2 –  segment
Analysis
Calculation Procedure
The proposed approach is tested on 20 kV balanced distribution network (Fig. 3). Length of the base test network (distance between node 1 and node 9) is 15km. To simulate short urban and long rural networks, length L was increased from 9 km to 45 km.
The loads are selected such that current at the beginning of the feeder is equal to the maximal current of the section (Imax=400A).
A fault is located at node 9. Four different types of fault are analyzed.
Results are shown for relays located at:
Beggining of the feeder (I2, Fig. 3),
Beggining of the lateral (I8, Fig. 3).
The relative differences between relay currents when the load is modeled as constant current or open circuit is not minor. It could lead to error as high as 16%.
Short-circuit calculation is performed using four decompositions of the linear faulted network model:
TABLE I. RELAY CURRENTS – I2 AND I8 FOR SHORT-CIRCUIT AT NODE 9
Conclusions
Constant Current
Open
Circuit
Error
[%]
I2[A]
I8 [A]
I2 [A]
DI2[%]
DI8[%]
3LG
2401
2104
2252
6.6
7.0 2L
2135
1835
1951
9.4
6.3
2LG
2198
1891
2009
9.3
6.2
SLG
1601
1302
1351
15.6
3.76
A detail mutual comparison of relay currents results, provided through three types of load models: open circuits, constant currents and constant impedances is presented.
Results show that difference between modeling loads as constant currents and constant impedances is minor. However, neglecting loads lead to error as high as 16% for the tested network. Also, with the increasing of distance between short-circuit and supply point, percentage error increases rapidly.
These effects should be taken into consideration when performing short-circuit analysis for setting and coordination of protective equipment in distribution network.
The difference between Relay 1 currents when loads are modeled as constant current and open circuit, increases with the network lengths.
Figure 3 – 20 KV Test Network
References
The relative differences between relay currents when the load is modeled as constant currents or constant impedances is minor. It is no more than 4.5% even for long rural networks (45km).
Figure 1 – Four decompositions of the faulted network [1]
V. Strezoski, D. Bekut, “A Canonical Model for the Study of Faults in Power Systems”, IEEE Trans. on PS, Vol. 6, No. 4, Nov. 1991, pp. 1493–1499.
V. Strezoski, P. Vidović, “Power Flow for General Mixed Distribution Networks”, International Transactions on Electrical Energy Systems, Aug 2014, DOI: /etep.1974
T. N. Boutsika, S. A. Papathanassiou "Short Circuit Calculations in Network with Distributed Generation", Electric Power Systems Research, 78, 2008, pp
The calculation procedure is based on back/forward sweep (BFS) [2].
The procedure is originally developed for power flow calculation. It is adapted for short-circuit calculation here, through the linearization of non-linear models of the network pre-fault state [2]. The non-linearity of the pre-fault state models is introduced by distribution generators (DGs) and loads modeled as constant powers.
Figure 5 –Relay 1 current, for loads modeled as open circuits and constant currents, for different network lengths
Figure 4 – Differences in relay current, depending on load modeled as constant impedances and constant currents