Substation DC Distribution Panel

What it is and where it is

The DC distribution panel sits between the source (charger and battery) and the substation loads. It receives the charger incomer and the battery incomer on a bus, and from it come the circuits to the protection and control panels, the power breaker cabinets, communications, emergency lighting and the converters and inverters. It carries voltage and current metering, the insulation monitor and the alarms.

It is the third part of the substation DC system, together with the charger-rectifier and the battery. Its AC counterpart is the low-voltage panel.

Parts and how each one works

• Incomers from the charger and the battery, with a breaker or fuses and, at times, a low-voltage disconnect contactor that protects the battery from deep discharge.
• Positive and negative buses, isolated from ground in most systems.
• Outgoing circuits with circuit breakers or double-pole fuses, each with a trip auxiliary contact for alarm.
• Metering: bus voltmeter and battery and charger ammeters, or a meter with communication.
• High and low voltage relays, and an insulation monitor according to IEC 61557-8.
• Bus tie between buses on panels with two sections, when the design allows it.

DC current does not cross zero. In AC the arc goes out by itself at the next zero crossing; in DC the device has to force it. That is why every breaker or fuse must have a DC voltage and breaking capacity assigned by its manufacturer. The IEEE Power System Relaying Committee (PSRC) publication on trip circuit design also recommends that its rated voltage exceed the charger’s equalizing voltage and that its breaking capacity exceed the available fault current, with margin.

Circuit organization

The same publication describes two schemes:

• One circuit per breaker. Each power breaker gets its own circuit, which feeds its trip, its close and its auxiliaries. If it has two trip coils, it gets two circuits. Protections that trip several breakers, such as bus differential or breaker failure, take their power from separate “relay” circuits, so the circuit of an open breaker can be de-energized without losing the protection.
• Branch circuit (or branch bus). One circuit runs through several panels and branches at each one, often with a fuse at each branch. It is suitable as a single bus only in very small substations.

Some PSRC criteria worth applying: arrange circuits logically so the effect of opening one is obvious, and provide enough circuits to take one out for maintenance without compromising protection. Also, never connect a load between the positive of one circuit and the negative of another. Closing circuits are protected on the load side of the trip protection, so that when closing there is always at least one trip coil with power.

How it changes by voltage level

• Medium and low voltage: a small panel or a section inside the charger cabinet, with few outgoing circuits.
• High voltage: a panel with circuits per bay, sometimes with a split bus and tie.
• Extra-high voltage: two independent panels, A and B, one per system. Main protection 1 and trip coil 1 of each breaker come from panel A; main 2 and coil 2, from B, following the logic of main and backup protections.
• Solar plants and industry: outgoing circuits are added for plant controllers, communications and 24 V or 48 V converters.

Sizing and selectivity criteria

• Short-circuit current: almost all of it comes from the battery; the charger, because of its current limitation, contributes little. The battery manufacturer publishes the value: a nickel-cadmium one gives it as a multiple of the ampere-hour capacity depending on the cell type.
• Selectivity: fuse and DC breaker curves vary between manufacturers. The PSRC recommends plotting them when brands are mixed.
• Voltage drop: to the farthest coil, with the battery at its end-of-discharge voltage.
• Poles and polarity: the DC rated voltage of a miniature circuit breaker changes with the number of poles in series, and some models must be connected with a defined polarity. Follow the manufacturer’s diagram.

Connection with protection, control and alarms

The panel receives signals for outgoing breaker open or tripped, high or low bus voltage, earth fault and battery and charger status. They are wired to the bay controller or the RTU. In addition, many relays supervise their own supply and the continuity of the trip circuit, so an open breaker in the panel shows up as an alarm in the affected relay.

Failures and what to check first

• One outgoing circuit trips and several protections are lost: a circuit shared by more loads than planned.
• The main outgoing circuit trips for a fault on a branch: lack of selectivity.
• A breaker that does not clear or is damaged in a fault: a device without enough DC capacity or connected incorrectly.
• Intermittent earth fault alarm: moisture in yard cabinets or damaged cables; see DC earth fault detection.

Testing and maintenance

At commissioning, the wiring is checked against drawings and the polarity of each outgoing circuit. The A/B separation is checked by opening each circuit and verifying what is lost. Alarms are checked through to SCADA, the insulation monitor with a simulated fault and the voltage drop under load. In maintenance: thermography, connection torque and periodic alarm verification.

Commissioning with Tiza Group

Our team verifies DC panels in substations, industry and solar plants, including circuit-by-circuit testing of the A/B separation (commissioning), in Colombia or elsewhere on request.

References

• IEEE Power System Relaying Committee, “Relay Trip Circuit Design”, special publication.
• IEEE 946, design of DC auxiliary power systems for generating stations.
• IEEE 1818, guide for design of low-voltage auxiliary systems for electric power substations.
• IEC 61557-8, insulation monitoring devices for IT systems.
• Manufacturer catalogs of DC breakers and fuses.

Frequently asked questions