In high-voltage substations, maintaining proper isolation between DC control circuits is crucial for reliable operation. This case study examines how we resolved persistent 110V DC mixing issues in 132 kV circuits, highlighting the importance of systematic troubleshooting and attention to detail.
The Initial Problem of DC Mixing Issues
Our maintenance team encountered abnormal behavior in the substation’s DC system due to DC mixing issues:
- Uneven current distribution between DC rectifiers
- DC-1 and DC-2 systems showing unexpected mixing when tested
- Potential safety and reliability concerns due to improper circuit isolation
Step-by-Step Investigation of DC mixing issues
Phase 1: Circuit Breaker Cubicle Inspection
During our initial investigation, we discovered incorrect shorting links in the 132kV circuit breaker cubicle. These links, connecting terminals X1:600-602 and X1:605-607, should have been removed during commissioning but were overlooked. After removing these links and rewiring the closing and opening circuits, we verified proper circuit breaker operation both locally and remotely.
Phase 2: Wiring Verification of DC mixing problems
Further investigation revealed swapped wiring in the E7Q1 MK panel. Despite correcting this issue, the DC mixing problem persisted, indicating a deeper underlying cause.
Phase 3: Systematic Isolation Testing
We conducted comprehensive isolation tests to pinpoint the source:
- With DC-1 OFF and DC-2 ON: No return voltage detected in DC-1 circuits
- With DC-2 OFF and DC-1 ON: Discovered 60V return on both positive and negative wires in 2Q21 MCB
Phase 4: Component-Level Diagnosis
The breakthrough came when tracing voltages through various components:
- Bus-2 terminal showed no voltage after wire removal
- MCB 210 exhibited unusual behavior with output voltage present despite no input
- Traced the issue to the KC Contactor, where DC-1 and DC-2 circuits were cross-feeding through NC and NO contacts
Root Cause and Solution of DC mixing problems
The investigation revealed that a faulty KC contactor was the primary culprit. The contactor’s deteriorated condition allowed unintended current paths between the DC-1 and DC-2 circuits through its normally closed and normally open contacts. Replacing the contactor completely resolved the long-standing issue.
Key Takeaways
- Always verify commissioning procedures are fully completed
- Use systematic isolation testing when troubleshooting complex electrical issues
- Don’t assume the first problem found is the only issue
- Component-level testing is crucial for resolving persistent problems
Conclusion
This case demonstrates the importance of methodical troubleshooting in high-voltage environments. While initial findings may point to obvious issues like missing commissioning steps, persistent problems often require deeper investigation at the component level.
This technical case study is part of our ongoing series on substation maintenance and troubleshooting. For more information about DC system maintenance in high-voltage substations, visit our technical resources section
DC mixing happens when two separate DC power supplies (for example, Battery Bank-1 and Battery Bank-2) that are supposed to be completely independent accidentally get connected. This means current from one system flows into the other, which should never happen. Think of it like two separate water tanks accidentally getting a pipe between them—the pressure in both tanks becomes unreliable.
Substations use two independent DC systems for redundancy and safety. If one DC supply fails, the other keeps critical equipment like protection relays, circuit breakers, and alarms running without interruption. The moment these two systems mix, you lose that backup protection—both systems become dependent on each other and can fail together.
Forgotten shorting links left behind during installation or commissioning that were never removed
Faulty contactors or relays whose deteriorated contacts create unintended bridges between two DC circuits
Incorrect wiring during panel modifications, extensions, or maintenance work
Damaged cable insulation where positive of DC-1 touches positive or negative of DC-2 through a fault path
Poorly maintained terminal blocks with loose or corroded connections causing leakage paths
Common earth faults on both DC systems simultaneously creating an indirect mixing path
Common real-world indicators include:
Both DC rectifiers or chargers showing unequal or unusual current readings
Battery charger trips or alarms activating without a clear reason
Earth fault relays operating on both DC systems at the same time
Unexpected operation or non-operation of protection relays and trip coils
Voltage appearing on a bus bar or terminal that should have zero voltage when isolated
MCBs (Miniature Circuit Breakers) tripping randomly across both DC panels
In a healthy DC system, neither the positive nor the negative wire should be connected to earth (ground). An earth fault means one of these wires has accidentally touched the earthed metalwork or structure. A single earth fault is a warning. But when both DC-1 and DC-2 each have an earth fault on opposite polarities (one on positive and one on negative), current can actually flow between the two systems through the earth—this is one of the sneakiest forms of DC mixing and is very hard to find without proper testing.
Practical tools used by substation engineers include:
Digital multimeter — to check for unexpected voltages on isolated circuits
DC earth fault locator / insulation monitoring device (IMD) — permanently installed to monitor earth fault resistance in real time
Megger (insulation resistance tester) — used during outages to check cable and panel insulation
Clamp meter — to check for unexpected current flow in wires that should carry no current
Loop impedance tester — used to trace unwanted current paths in complex panel wiring
A practical field approach:
Check rectifier/charger current readings—unequal loading is the first clue
Switch off DC-1, keep DC-2 live—check if any voltage appears on DC-1 circuits. If yes, DC-2 is feeding into DC-1
Switch off DC-2, keep DC-1 live—repeat the check in the opposite direction
Isolate sections one by one—disconnect feeders, panels, and cubicles systematically until the mixing disappears
Trace the problem panel—once identified, go component by component: check terminal blocks, contactors, relays, MCBs, and wiring
Test components individually — use a multimeter to check contactor contacts, relay contacts, and wiring continuity while the circuit is de-energized.
Absolutely yes — and this is one of the most important lessons from real-world experience. A single contactor or relay with worn-out or welded contacts can silently bridge DC-1 and DC-2. Because the path carries very little current in normal operation, it may go unnoticed for months or even years until a serious fault occurs or an engineer investigates unusual meter readings.
The consequences can be serious:
Protection relays may operate incorrectly — either tripping healthy equipment or failing to trip during a real fault
Battery backup becomes unreliable since both systems are now effectively sharing the load.
Persistent small fault currents can damage wiring insulation, contactors, and relay coils over time
In worst cases, it can lead to complete loss of DC supply during a critical fault, causing a blackout or equipment damage
It also becomes a fire risk if fault currents flow through unprotected or undersized wiring paths.
Key preventive practices:
Always use a DC isolation checklist before and after any panel modification
Never leave temporary shorting links in panels without tagging and scheduling their removal
Follow a strict permit-to-work (PTW) system that includes post-work verification of DC isolation
Conduct periodic insulation resistance (IR) testing on DC cables and panel wiring at least once a year
Install permanent insulation monitoring devices (IMD) on both DC systems with visible alarms in the control room
Maintain a clear DC circuit diagram that is updated every time a modification is made—many mixing faults happen because outdated drawings are followed.
Based on standard industry practice:
Daily: Check rectifier/charger current and voltage readings for any unusual variation
Monthly: Check earth fault relay status and insulation monitoring device alarms
Annually: Full insulation resistance (IR) testing of DC cables and panels during planned outages
After every major maintenance work: Verify DC-1 and DC-2 isolation before restoring the system to service.
No. DC mixing can and does occur in brand new substations too, usually due to commissioning errors like unremoved shoring links, incorrect wiring, or wrong panel configurations. In older substations, the causes shift more toward deteriorated contacts, aged insulation, and cable degradation. The fault-finding approach remains the same regardless of the age of the installation.
Always verify before assuming. Never assume a circuit is isolated just because an MCB is off or a switch is open. Always measure with a meter. Never assume the first fault you find is the only fault. And never restore a DC system to service after maintenance without performing a proper isolation verification test. In high-voltage DC control systems, a small oversight can lead to a very large and dangerous consequence.