RFL 9300 RFL Electronics Inc.
October 29, 2001 2 - 1 (973) 334-3100
SECTION 2: SYSTEM OPERATION
2.1 INTRODUCTION
This section provides a basic description of how the RFL 9300 Charge Comparison System operates. A block
diagram of the RFL 9300 appears in Figure 2-1. After reading this section, refer to Section 24 if your system has
the Single Pole Option.
In the description that follows it is understood that if the relay is applied in a 3-terminal system there are two re-
mote stations.
The RFL 9300 Charge Comparison System (CCS) is a protective relaying system for ac power line protection.
Its operating principles are similar to those of the more common current differential type of protective relay. Cur-
rent differential relays compare the total currents entering and leaving the primary protection zone. They will trip
the auxiliary relays protecting this zone if the difference between these currents exceeds some pre-defined re-
straint limit.
For this comparison to be made, the current differential relay at the local station has to know the identical phase
current recorded at the remote station(s) for the same interval being considered at the local station. If a single
quantity is being compared, its composite phasor must be known. These requirements have always presented
problems for the current differential scheme. The exact communications delay between stations must be known
in order to make a meaningful comparison. In addition, each recorded current sample must be transmitted to the
remote station(s). This requirement places a heavy burden on the communications channel.
The RFL 9300 addresses this problem by comparing local and remote station readings on a half-cycle basis.
This drastically reduces the communications channel's throughput requirements. In order to accomplish this, the
RFL 9300 is referenced to the zero crossings of the power line current waveform supplied by current transform-
ers at the local station. The RFL 9300 samples the current waveform at a 2-kHz rate (60-Hz systems), inte-
grates all the samples for a half-cycle, and then transmits the result to the remote station(s).
The RFL 9300 has four phase controller modules: one for each phase, and one for ground. Each phase control-
ler has a microcontroller, and there are additional microcontrollers on the display, supervisor, and communica-
tions controller modules. A two-terminal RFL 9300 will have seven microcontrollers communicating with one
another while executing various control algorithms; a three-terminal RFL 9300 will have eight microcontrollers.
2.2 SYSTEM INITIALIZATION
At turn-on, the local RFL 93B CC Communications Controller Module will attempt to establish contact with the
RFL 9300 at the remote station. In order for this to occur, both the local and remote 9300’s must be pro-
grammed to have compatible transmit addresses. Communications channel delay will be measured once two-
way communications are established. To measure the delay, the RFL 9300 measures the round-trip delay of a
test (ping-pong) message sent to and returned by the remote RFL 9300. The Communications Controller will
not accept the measured channel delay value as being correct until it confirms the measurement with the re-
mote RFL 9300.
Four jumpers on the RFL 93B SV Supervisor Controller Module configure the system. The RFL 9300 uses these
jumper settings to configure itself to operate at 50 Hz or 60 Hz, with 1-ampere or 5-ampere auxiliary current
transformers (ACT's), as a two- or three-terminal system, or as a three-pole or single-pole system.
Finally, each RFL 9300 confirms that it is configured the same as the other RFL 9300. If the configurations do
not match, neither RFL 9300 will exit the initialization mode.
The RFL 93B SV cannot initialize the RFL 93B PC Phase Controller Modules until a valid channel delay com-
pensation (CDC) value is available. During the initialization process, the RFL 93B SV will test all programmed
variables stored in its non-volatile memory. These variables control phase/ground bias, phase/ground overcur-
rent, the fault detector setting, and other variables. If their values are valid, they are passed to the phase con-
trollers. None of the phase controller modules will exit the initialization mode until they have received and ac-
knowledged the CDC values and all programmed variables sent by the RFL 93B SV.
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