While AC system protection is well known and applied throughout the power system industry, there is also protection needed for complex DC equipment.
This includes DC traction power systems associated with railway electrification. Below are some of the methods of protection that a Power System Engineer may often encounter in DC traction power.
Track Time Constant
To properly apply system protection for DC traction power systems, a good understanding of the track time constant (L/R ratio) is required. The L/R ratio is used to evaluate interrupting limits of a DC power circuit breaker.
L/R ratio
The L/R ratio of a DC system is like the X/R ratio of an AC system, and it is used by DC protective device
The L/R ratio of a DC system is like the X/R ratio of an AC system, and it is used by a DC protective device to determine an appropriate time to interrupt the circuit.
A track fault has an associated L/R time constant that is calculated based on the entire loop inductance and the resistance between the rectifier output and the fault point. The L/R time constant is then used to determine the initial rate of rise or increment of the fault current.
ANSI 7 – DC Rate of Rise Protection
ANSI 7 relays are often applied on the feeder breakers of DC switchgear lineups to protect against a track fault event. The rate of rise protection distinguishes between a fault event and a train starting event (including multiple simultaneous train starts), as each will have a distinguishable di/dt characteristic. This is accomplished by determining and comparing the rate of rise of the peak train load and the calculated fault current.
ANSI-7 protection (previously known as ANSI-150RR or di/dt relay) detects the “Rate of Rise” of the time/current profile and operates on this comparative philosophy.
ANSI 76 – DC Overcurrent Protection
ANSI 76 relays detect the Instantaneous Overcurrent on DC circuit breakers and function when the current in the DC circuit exceeds a given value. It is often used as backup protection for ANSI-7, Rate of Rise relays.
This protection element is akin to AC Instantaneous Overcurrent (ANSI 50) protection. However, the DC overcurrent element does more than its AC System counterpart. Correctly setting the ANSI-76 parameters on the DC breaker will also ensure that the breaker will trip under favorable conditions by generating its interrupting voltage to drive the current to zero.
ANSI 64C/64V – DC Hot/Grounded Structure Relay
An alarm operates upon failure of apparatus insulation to the ground or a hot structure
ANSI 64C/64V relays detect Stray Overcurrents or unacceptable voltage levels to Equipment or Station Ground. An alarm operates upon failure of apparatus insulation to the ground or a hot structure. To detect a hot structure or grounded equipment, the system voltage on selected parts of the DC System is monitored concerning station ground.
If this voltage goes too low, it can be assumed that there is a short to the ground because of a breakdown in the insulation. If the system voltage goes too high, it can be inferred that there is a fault within the structure.
Due to the destructive nature of DC Arc Flash events, a DC system should never be solidly grounded. But that is not the only reason since extended ground current will cause significant corrosion of all metallic structures near the track.
ANSI 32 – Reverse Current Protection
ANSI 32 relays detect the Reverse Power/Current from track feeders back to the DC Switchgear or Rectifier. This relay operates on a predetermined value of power flow in a given direction or upon reverse power flow.
Unless the rectifier is designed as a four-quadrant device that can provide energy recovery during train braking, the reverse power flow to the rectifier can be damaging. This protection element is akin to reverse power protection on AC systems, such as the protection against motoring of a generator upon loss of its prime mover.
ANSI 27 – Undervoltage Protection
If there is a loss of output from the rectifier, the main DC breaker will trip
ANSI 27 relays are based on the system voltage and its protection serves as a backup to internal rectifier failure protection. This relay operates when the DC input voltage is less than a predetermined value.
If there is a loss of output from the rectifier, the main DC breaker will trip. This protection element is akin to ANSI 27 Undervoltage settings in an AC system.
ANSI 26 – Diode and Heat Sink Overtemperature
ANSI 26 relays provide temperature protection to electrical equipment. This protection is a thermal element that functions when the temperature of the protected apparatus (in this case, the rectifier diode elements) exceeds a predetermined value.
This is similar to the thermal/temperature protection of a transformer or motor on an AC system.
