1
Sizing Current Transformers for Line
Protection Applications
Héctor J. Altuve, Normann Fischer, Gabriel Benmouyal, and Dale Finney,
Schweitzer Engineering Laboratories, Inc.
Abstract—This paper discusses the factors to consider for II. CT BASICS
sizing current transformers (CTs) for line protection
applications. We first cover CT basics, with emphasis on errors A. CT Steady-State Operation
and ac and dc saturation. We also discuss the criteria to avoid
1) Ideal CT Behavior
CT saturation. Then we analyze the effect of CT saturation on
overcurrent, distance, directional, and differential elements.
Ideally, the secondary current of a CT is perfectly
Further, we present the advances in protection element design to proportional to the primary current. The ideal CT has no
improve security and speed under CT saturation conditions. losses or leakage flux and requires no magnetizing current.
Finally, we discuss the tools available to the protection engineer For a CT having nP primary turns and nS secondary turns, the
for CT sizing and provide some guidelines. ideal relationship between primary (IP) and secondary (IS)
currents is the following:
I. INTRODUCTION
I P n P = IS n S (1)
The transient response of current transformers (CTs) has a
significant impact on the performance of line protection. CT nP IP I
IS = IP = = P (2)
saturation during external faults can seriously affect the nS nS / n P n
security of the protection scheme, especially for dual-breaker
where:
line terminals where a large portion of the fault current can
enter and leave the line protection zone without flowing n is the CT turns ratio, n = nS/nP.
through the protected line. Equation (2) can be expressed in per-unit (pu) values as:
Selecting higher-ratio CTs to prevent saturation and match IS ( pu ) = IP ( pu ) (3)
the breaker load ratings may result in CTs that have
considerably higher nominal current than the line loading. 2) Real CT Behavior
Sensitivity may have to be sacrificed as a result. The degree to Real CTs have copper losses, core losses, and leakage flux
which the various line protection elements are impacted can and require a certain current to magnetize the core. As a result,
also vary. In the past, general rules were developed to allow the secondary current of a CT is not perfectly proportional to
the protection engineer to size the CT for a particular the primary current. For most operating conditions, CTs
application. These rules were used to determine the fault reproduce the primary currents well. However, under certain
current magnitude (including ac and dc components) beyond conditions, the CT core saturates and the CT fails to correctly
which saturation occurs and to determine the CT time to reproduce the primary current.
saturation for a given fault. These rules take into account the Fig. 1 depicts the equivalent circuit of a CT, referred to the
CT knee point, connected burden, and system X/R ratios. transformer secondary side. The CT primary current IP is
Other potentially important aspects, such as remanent flux, dictated by the power system because the CT primary winding
were typically not considered or left to the discretion of the is connected in series with the protected element. The current
protection engineer. These rules also did not consider CT source IP/n represents the power system in Fig. 1. CT leakage
saturation countermeasures available by design in modern impedances are R´P + jX´P for the primary winding (referred
relays, which reduce the impact of saturation. For a to secondary) and RS + jXS for the secondary winding. As a
transmission line, a typical CT sizing rule called for ratings result of the current source, the primary leakage impedance
that would ensure no saturation for the end-of-line fault. This has no practical effect on the CT behavior and can be
rule works well in single-breaker applications or with line CTs disregarded. The nonlinear excitation impedance ZE represents
but clearly has limitations in dual-breaker applications with CT magnetization. The excitation current IE flowing through
breaker CTs. the excitation impedance has two components. One
The advent of microprocessor-based relaying has allowed component is the magnetizing current (flowing through the
relay designers to incorporate novel methods for dealing with inductive component of ZE), which is needed to generate the
CT saturation. These methods improve relay performance in flux in the CT core. The other component of IE is the loss
the face of saturation and can allow CT sizing requirements to current (flowing through the resistive component of ZE),
be relaxed as a result. Hence, the CT can no longer be which mainly results from the core hysteresis and eddy losses.
evaluated without consideration for the particular relay to The secondary excitation voltage ES is the voltage induced in
which it will be connected. the secondary winding. Impedance ZB represents the total load
