DC Control Circuits

A protection system consists of circuit breaker(s), instrument transformers, protective relay(s),and a dc system. Every component of this system must perform properly for the system to work reliably. This paper concentrates on measuring and improving the health of the dc portion of the protection system.The dc system consists of several parts:
 a power source including the battery and charger;
 wiring and connections;
 dc system protection;
 switches, including protective relay contact outputs, auxiliary relay contacts, breaker auxiliary contacts, and isolation switches; and
 loads, including protective relay contact inputs, auxiliary relay control coils, and circuit breaker trip and close coils.
We discuss a battery voltage monitor circuit suitable for inclusion in a protective relay. This circuit helps detect and locate inadvertent dc grounds. In addition, this circuit helps improve the security and reliability of the relay contact inputs.
Inadvertent dc grounds can falsely assert contact inputs and operate auxiliary coils. In extreme cases, these grounds can even operate circuit breaker trip and close coils. We review a method to decrease the impact of dc grounds on these circuits.
We discuss the benefits of a contact input circuit that goes beyond the traditional role of detecting contact or switch closure; it actually measures the applied dc voltage. We review how small metallic contacts interrupt dc current. Further, we discuss the benefits of a protective relay contact output with near instantaneous make-times and the capability to interrupt circuit breaker trip and close current.
Figure 1 shows a small portion of a typical dc system. The batteries usually are series strings of lead-acid cells. While we discuss 125 Vdc nominal battery systems, the following discussion applies equally well to other voltages.
Resistors R1 and R2 are replaced sometimes by lamps. The common connection of R1 and R2 is grounded. This references the battery to ground while still providing some isolation from ground. Isolation from ground is important, given that inadvertent shorts from the dc system wiring to ground do occur and the system must remain operative.
C1 and C2  Capacitance Due to Wiring, Surge Protection, Power Supplies, etc.
R1 and R2  Battery Ground Centering Resistors
43/CS  Panel Mounted Feature Control Switch
TRIP1  Protective Relay Trip Output
52A1  Breaker Auxiliary Contact
IN1 and IN2  Protective Relay Contact Input
DCM1  DC Monitor
FUSE  Relay Panel Fuse
TC 1  Trip Coil 1
52a  Breaker Auxiliary
For example, consider an inadvertent short-circuit from the +DC bus to ground. Before the inadvertent ground occurs, the current flowing through R1 and R2 is equal. Thus, under normal operation, R1 and R2 each support a voltage equal to half the total battery voltage. The inadvertent ground shorts out resistor R1. Therefore, the +DC bus drops to ground potential, and the -DC bus drops to the full battery voltage below ground potential. If Lamps 1 and 2 replace R1 and R2, respectively, Lamp 1 extinguishes during the positive dc ground, and Lamp 2 glows
brighter. Thus, the lamps detect and help locate the dc ground. However, lamps cannot notify remote personnel of a dc ground. A single dc ground is not destructive. If a second short circuit is applied from the -DC bus to ground, the dc protection system (panel fuses in Figure 1) operates to isolate the fault. It is
important to isolate and remove dc grounds before this happens. Unfortunately, the method most often used to isolate dc grounds involves removing panel fuses or opening panel circuit breakers.