RFL Electronics RFL 9300 - Section 3: Applications; Introduction; Series Compensated Lines

RFL 9300 RFL Electronics Inc.
August 25, 2000 3 - 1 (973) 334-3100
SECTION 3: APPLICATIONS
3.1 INTRODUCTION
The RFL 9300 is suitable for high-speed protection of both overhead and underground ac transmission and sub-
transmission lines. All line lengths and voltage levels can be accommodated.
Since the RFL 9300 operates from current-only, it shares properties with all current-based schemes. These in-
clude the following:
1. Mutual induction has no effect on performance.
2. Current reversals are easily handled.
3. There is inherent blocking during swing conditions.
In addition, the RFL 9300 is particularly well-suited to many difficult or unusual applications. Many of these ap-
plications will be discussed in this section.
3.2 SERIES COMPENSATED LINES
The RFL 9300 can be used to protect series compensated lines, or lines adjacent to series capacitors. Series
capacitors create three conditions that can affect the protection:
1. Voltage inversion.
2. Off-normal frequencies (particularly subnormal frequencies).
3. Severe phase unbalance.
These conditions do not affect the RFL 9300, because it is current-only, not frequency-dependent, and uses
per-phase operation.
3.2.1 VOLTAGE INVERSION
The relay voltage will be inverted if the impedance between the fault location and the relay potential location is
net capacitive. (See Figure 3-1.) A similar inversion may take place on external faults where the adjacent line
has line-end series capacitors. These voltage inversions affect distance relays, but they will have no effect on
current-only relays like the RFL 9300.
3.2.2 SUB-NORMAL FREQUENCIES
There is some resonant frequency below the fundamental where | Z
C
| = | Z
S
+ nZ
L
|. When the system is shock-
excited by fault F, the system may oscillate at this low frequency. Off-normal frequencies may affect distance
relays, and current-only relays that are based on operation at the fundamental frequency.
The RFL 9300 is based on the principle of Conservation Of Charge, which is independent of frequency. Its op-
eration is not affected by the off-normal frequencies that may appear during the post-fault period. These off-
normal frequencies may be caused by series capacitors or other power system parameters.