# Application of a DC Distribution System in Korea: A Case Study of the LVDC Project
## 2 Design of DC Distribution System
### Grounding and Protection
#### 2.1.3 Protection  
The purpose of the power system protection is to secure people and equipment by eliminating faults or limiting voltage and current below a certain level. If a failure occurs in the DC distribution  
Figure 4: Proposed insulation monitoring device (IMD) operating procedures.  
system, the protection operation should be performed for the faulty part, and the fault should be detected, even when it does not require any protective operation [12; 13].  
In the DC distribution system, the protection system can be divided into three parts: power supply rectifier, DC distribution line, and consumer. The protection system should be determined by the system grounding method, voltage level, network configuration, and power converter structure. Protection of the system shall meet the following requirements.  
* The risk level should not be higher than the existing AC system.
* Faults should be detected and removed.
* Fault section should be separated.
* There should be no breakdown of equipment due to line failure.
* Safety should not be threatened by equipment failure.  
The protection requirements of the above system can be satisfied by protective devices such as converter protection system, circuit breaker (CB), overcurrent/overvoltage relay, surge protection device (SPD), IMD, grounding system, electrical isolation, and insulation cooperation.  
As shown in the Figure 5, the protection system of the DC distribution system was designed. First of all, a surge protection device needs to be installed on the primary side of the transformer to protect against overvoltage which can flow through the high-voltage line. The transformer does not need to be grounded on the secondary side because the flow of current through the earth can cause permanent failure. A DC circuit breaker (DCCB) was installed on the rectifier output side to prepare for overcurrent and short circuit faults that may occur in each pole. In addition, the DCCB can be used when the rectifier between the high voltage line and the DC line needs to be independent, allowing for the independent operation of the lines on each end. At this moment, the converter system can supply limited fault current when the fault occurs and control the DCCB by the measured fault results. Since the use of the IT grounding system, the IMD is installed to monitor grounding faults. If a grounding fault occurs in the DC line, the insulation monitoring device controls the DCCB, and if the grounding situation is not resolved, the DCCB between the transformer and the converter system is controlled to cut off the power supply. In addition, an SPD is installed to protect the converter system and the line against the DC line overvoltage input. Likewise, an SPD is installed in front of the customer's side to provide protection against overvoltage inflow to the customer's network and the inverter. In addition, a DCCB is installed in front of the customer's inverter to protect the DC line from the fault of the customer's network and the inverter. The inverter limits the amount of current passing through at the time of the fault to ensure the normal operation of the CB, while the CB installed on the customer's power supply line performs the protection operation if the operating characteristics curve in the event of failure.