Proceedings of 2013 IEEE International Conference on ID3031 Applied Superconductivity and Electromagnetic Devices Beijing, China, October 25-27, 2013

978-1-4799-0070-1/13/$31.00 ©2013 IEEE 89

System Protection for Vessel DC Zonal Electrical System Supplied by Medium Voltage DC

Rui Tian Wang, Li Jun Fu, Fei Xiao, Xue Xin Fan National Key Laboratory for Vessel Integrated Power System Technology,

Naval University of Engineering, Wuhan 430033, China wangrt4321@163.com

Abstract—The traditional radial AC distribution network in the vessel is provided with selective protection through the rational configuration of the circuit breakers based on the time-current coordination principle. However, the principle cannot be applied to the DC Zonal Electrical Distribution System (DC-ZEDS) which is a typical ring network. Furthermore, DC circuit breakers cannot take advantage of the current zero crossing, so the current-interrupted capacity of the electromechanical breakers is limited. And the wide use of DC breakers will lead to a great increase in cost, weight and size of the ship power system. Aiming at the DC-ZEDS supplied by Medium Voltage DC (MVDC), this paper has analyzed the problem of selective protection for 700V DC bus and proposed a method of protection configuration. The results obtained from the simulation model established in PSCAD/EMTDC verify the effectiveness of the method. Based on the researches, the paper makes a summary of the system protection for DC-ZEDS supplied by MVDC.

Keywords-DC zonal electrical distribution system; coordination protection; medium voltage DC; fault current limiting

I. INTRODUCTION According to the concept of power integration, the vessel

integrated power system (IPS) is established to implement a unified control of the electric power. The typical IPS consists of six modules: power generation module (PGM), power distribution module (PDM), power conversion module (PCM), electric propulsion module (EPM), energy storage module (ESM) and power management module (PMM), considering as the trend of future naval vessel power [1,2]. Due to the advantages of power density, size, weight, electromagnetic compatibility (EMC), vibration and acoustic signature, the Medium Voltage DC (MVDC) is becoming the standard form of PGM instead of Medium Voltage AC (MVAC) [3-5]. As for PCM, DC Zonal Electrical Distribution System (DC-ZEDS), which is characterized by good continuity, load flow flexibility and easy productability due to modular design, will become an indispensable part of the IPS [6].

Rational protection is necessary for the safety and stability of the DC-ZEDS. The traditional radial AC distribution network is provided with selective protection through the rational configuration of the circuit breakers based on the time-current coordination principle. However, the DC-ZEDS is a ring network, to which the time-current coordination principle cannot be applied. DC circuit breakers cannot take advantage

of the current zero crossing, so the current breaking capacity of the electromechanical breakers is limited. Because of the harsh operating conditions, namely, limited space, high temperature, high humidity, and salt mist, the extensive application of high-capacity and high-reliability militarized DC breakers in the ship power system will lead to a great increase in its cost, weight and size.

Reference [7] has made a study of the coordination protection of the low-voltage DC distribution network in DC-ZEDS. Reference [8] has made an investigation into the system protection for DC-ZEDS supplied by MVAC. Focused on the DC-ZEDS supplied by MVDC, this paper analyzes the problem of selective protection for the 700 V DC bus and then proposes a method of protection. The results obtained from the simulation model established in PSCAD/EMTDC verify the effectiveness of the method. Based on the researches, the paper makes a summary of the system protection for DC-ZEDS supplied by MVDC.

II. PROBLEM OF PROTECTION IN DC 700V BUS A simplified diagram of DC-ZEDS with 700 V DC Bus

supplied by MVDC is shown in Fig. 1 [1]. DC/AC/DC PCM (PCM-1) converts the 4 kV DC prime power to 700 V DC output power. Either of the two PCM-1s, placed in the forward and backward ship power plant respectively, feeds power to the port or starboard DC bus. The DC-ZEDS in Fig. 1 is divided into two zones. The transfer switch cabinets (K1~K4) conduct power transmission from DC 700 V bus to the distribution zone. There are two kinds of protection devices in the transfer switch cabinets. Switches, Q11~Q22, which cannot be safely closed or opened with high current, are available for remote control; DC circuit breakers, Q23~Q30, can break a fault current and isolate the fault in its zone.

As the 700 V DC bus plays an important role in the power system, positive measures should be taken to reduce the possibility of faults. Once a fault occurs, it should be isolated as quickly as possible. Compared to the DC circuit breakers, the introduction of the power electronics offers a good opportunity to implement fault response without increase in size, weight and cost. When a fault current is limited by the PCM, the switches will isolate the fault to protect the power system. The protection characteristics of the 700 V DC bus are as following:

This work was supported by the National Science Foundation of China (Grant No. 51077130, 51277178). Program for New Century Excellent Talents in University of Ministry of Education of China (NECT-11-0871).

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Figure 1. Simplified diagram of DC zonal distribution system.

dcC C+

−

L +

−

+−

Figure 2. Structure of the PCM-1.

(1) Faults in the DC 700 V bus. Taking a fault on L14 in Fig. 1 as an example, 1PCM-1 detects an over-current and shuts down immediately, and then opens the transfer switch Q14 to remove the fault. If the fault occurs in L24, 1PCM-1 should provide enough fault current in a certain interval for the breaker Q24 to isolate the fault. Therefore, PCM-1 is required to limit the fault current.

(2) Self healing of DC-ZEDS. If there is a fault in L14, 1PCM-1 shuts down to de-energize the fault current. After the fault is removed by Q14, 1PCM-1 will need to restore the power as soon as possible.

(3) Fault inside PCM-1. When an internal fault occurs, PCM-1 will shut down automatically, so that all the loads will be transferred to the other PCM-1.

From the above, the prime functions of the PCM-1 can be listed as following:

(1) Meeting the demand of the DC 700 V power.

(2) Limiting the fault current to a certain level for a period of time.

(3) Providing an overload capacity.

(4) Making autonomous internal fault detection and self-protection.

−∑oU

−

oI

orefU+ m

1g2g3gLrefI

4g∑

LI

Figure 3. Control block diagram of PCM-1.

TABLE I. CONFIGURATION OF DC CIRCUIT BREAKERS

DC Breakers

Protection Configuration

Long Delay Short Delay Instantaneous

Tripping current(A)

Time delay(s)

Tripping current(A)

Time delay(s)

Tripping current(A)

Q23, Q26 Q27, Q30 ─ ─ ─ ─

5Ie: 3215

Q24, Q25 Q28, Q29 ─ ─ ─ ─

5Ie: 357

III. PROTECTION METHODS FOR DC 700 V BUS The protection method refers to three aspects: structure

design, fault current limiting algorithm of PCM-1, and configuration of downstream DC circuit breakers.

A. Structure of PCM-1 In accordance with the requirements of the capacity,

reliability, and electrical isolation, the cascaded structure with medium frequency link is chosen for PCM-1, as shown in Fig. 2. Input and output series fuses (Fuse1) and contactors (K1) are protection devices. Fuse1 provides backup protection for K1. PCM-1 feeds power to port or starboard DC bus through the single-pole double-throw switch K2.

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B. Fault current limiting of PCM-1 The block diagram of the dual loop controller for PCM-1 is

shown in Fig. 3 [9,10]. Droop control algorithm is used for load sharing between the two PCM-1s. The deviation between the feedback and the set point is sent to the PI regulator, used to generate PWM pulse signals with a full-bridge phase-shift method. The upper limit of ILref is the fault current capacity of PCM-1.

C. Protection Configuration for DC 700 V Bus Faults in the DC 700 V bus, F2~F7, are shown in Fig. 7.

The fault response to F1 is fulfilled by the DC 4 kV protection devices, whose behavior leads to the shutdown of PCM-1. This case will be discussed here, too, which includes the configuration of the downstream DC circuit breakers, the determination of the fault current limiting parameters of PCM-2, and the influence of the short-circuit fault on the upstream DC 4 kV bus.

In the DC-ZEDS supplied by MVAC mentioned in [8], all the power transmission is carried out by the transfer switches, while none of the circuit breakers is used. Once the upstream PCM detects the fault in the DC bus, it shuts down. Then it reconfigures the transfer switches to isolate the fault, and re-energizes the DC bus as soon as possible. If the method of [8] is used for the network in Fig. 7, by choosing no-load switches for Q13~Q30, as a result of the fault F8 or F9, both of the PCM-1s will detect the fault and shut down autonomously, leading to the power loss of all the ship service loads. Such a case is never allowed to happen. However, if Q23~Q30 are DC circuit breakers, the fault in F8 or F9 can be removed quickly, with the help of the PCM-1 fault current limiting.

Instantaneous tripping currents for short-circuit of Q23~Q30 are set to be three times of the branch rated current, as shown in Table I. Ie is the nominal current of the corresponding branch.

The fault current of PCM-1, Isp1, should be greater than the maximum tripping current for short-circuit of the DC bus breakers, and within the SOA of power electronic devices. Here, Isp1 is set to be twice the rated current of the PCM-1.The fault current duration time Ts1 of PCM-1 should be longer than the trip time of the breakers Q23~Q30. Hence, Ts1 is configured to be 50 ms.

PCM-1 can prevent the fault from spreading out of the DC 700 V bus. For the fault current limiting and the low-impedance of the fault, the input active power of PCM-1 is negligible during the fault process. So the short-circuit fault in DC 700 V bus has little effect on the upstream 4 kV DC bus.

IV. SIMULATIONS To verify the above protection configuration method, a

simplified model of DC-ZEDS is established in PSCAD. In Zone1, a DC/AC PCM (PCM-2) supplies power to a 150 kW resistive-inductive load ZL1 and a 20 kVA motor M1, and a DC/DC PCM (PCM-3) powers two 15 kW resistive loads. There is a 200 kW resistive load in Zone2.

PCM

-1 O

utpu

t V

olta

ge (V

)1P

CM

-1 O

utpu

t C

urre

nt (A

)2P

CM

-1 O

utpu

t C

urre

nt (A

)

Zone

1 O

utpu

t V

olta

ge (V

)Zo

ne2

Inpu

t V

olta

ge (V

)

Figure 4. Simulation results of fault F5.

Figure 5. Simulation results of fault F6.

Figure 6. Simulation results of fault F8.

The faults, F1~F9, can be divided into three categories: F1~F5, after detecting the fault, 1PCM-1 shuts down, and all the loads are transferred to the starboard DC 700 V bus powered by 2PCM-1; F6~F7, 1PCM-1 limits the fault current, and the corresponding breakers open to clear the faults; F8~F9, two of the PCM-1 identify the fault and limit the fault current, then both the port and starboard breakers trip to remove the faults. Therefore, the simulation results of F5, F6, F8, only three of the nine faults, are given typically. The solution time step is 30 μs.

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TABLE II. SYSTEM PROTECTION FOR DC ZONAL ELECTRICAL DISTRIBUTION SYSTEM No. Fault Location Fault Detection Fault Response

F1 Input fault of 1PCM-1 Input overvoltage or under-voltage of 1PCM-1

1PCM-1 shuts down; protection devices of DC 4 kV respond to clear the fault; loads in Zone1 are transferred to starboard.

F2 Internal fault of 1PCM-1 Internal over-current, abnormal output voltege1, over-temperature, et al. 1PCM-1 shuts down; loads in Zone1 are transferred to starboard.

F3 Fault in the line between 1PCM-1and K1 Output over-current of 1PCM-1 1PCM-1 limits the fault current for 50 ms, and then shuts down; loads

in Zone1 are transferred to starboard.

F4 Internal fault of K1 Over-current of Q11 and Q13, zero current of Q14

1PCM-1 limits the fault current for 50 ms, and then shuts down; Q13, Q14, Q23, Q24 are opened; loads in Zone1 are transferred to starboard.

F5 Fault between K1 and K2 Over-current of Q11, Q13, Q14, zero current of Q15

1PCM-1 limits the fault current for 50 ms, and then shuts down temporarily; after Q14, Q15 are opened, 1PCM-1 restarts; loads in

Zone2 are transferred to starboard.

F6 Fault in the line between K1 and PCM-2 Over-current of Q11, Q13, Q23 Q23 is opened instantaneously; PCM-2 in Zone1 is transferred to

starboard.

F7 Fault in the line between K3 and PCM-2 Over-current of Q22, Q20, Q19, Q18, Q26 Q26 is opened instantaneously; PCM-2 in Zone1 is transferred to port.

F8 Input fault of PCM-2 Over-current of Q11,Q13,Q23 in port,

Q22, Q20, Q19, Q18, Q26 in starboard, input over-current of PCM-2

Q23 and Q26 are opened instantaneously; PCM-2 shuts down; ZL1, ZL2, M1, M2 lose power.

F9 Input fault of PCM-3 Over-current of Q11,Q13,Q24 in port,

Q22, Q20, Q19, Q18, Q25 in starboard, input over-current of PCM-3

Q24 and Q25 are opened instantaneously; PCM-3 shuts down; R1~R4 lose power.

F2a Internal fault of 1SSIM Internal over-current, abnormal output voltege1, over-temperature, et al. 1SSIM shuts down; loads of PCM-2 is transferred to 2SSIM.

F2b Output fault of PCM-2 Output over-current of SSIMs PCM-2 limits the fault current for 500 ms, and then shuts down.

F2c Fault in the line between PCM-2 and AC loads Over-current of Q6 PCM-2 limits the fault current; Q6 is opened instantaneously.

F2d Fault in the line between PCM-2 and AC load centre Over-current of Q8 PCM-2 limits the fault current; Q8 is opened after a short delay.

F2e Fault in the line between AC load centre and AC loads Over-current of Q8 and Q9 PCM-2 limits the fault current; Q9 is opened instantaneously, then the

voltage recovers, and ZL2 loses power.

F3a Internal fault of 1SSCM Internal over-current, abnormal output voltege1, over-temperature, et al. 1SSCM shuts down; loads of PCM-3 is transferred to 2SSCM.

F3b Output fault of PCM-3 Output over-current of SSCMs PCM-3 limits the fault current for 500 ms, and then shuts down.

F3c Fault in the line between PCM-3 and DC loads Over-current of Q1 PCM-3 limits the fault current; Q1 is opened instantaneously.

F3d Fault in the line between PCM-3 and DC load centre Over-current of Q3 PCM-3 limits the fault current; Q3 is opened after a short delay.

F3e Fault in the line between DC load centre and DC loads Over-current of Q3 and Q4 PCM-3 limits the fault current; Q4 is opened instantaneously, then the

voltage recovers, and R3 loses power.

A. Busbar Fault (F5) The simulation results are shown in Fig. 4. Uo1 and Uo2 are

the output voltage of the two PCM-1, while Io1 and Io2 are the output current of them. UPCM2_rms is the output voltage RMS of PCM-2 in Zone1. UPCM3 is the output voltage of PCM-3 in Zone1. Uzone2 is the input voltage of Zone2. Before a fault occurs, the two PCM-1s share the loads. After that, 1PCM-1 limits its output current to 5750 A for 50 ms, and then shuts down. Then all loads in the two zones, which are not influenced, are transferred to 2PCM-1 seamlessly.

B. Fault in the Line between PCM-2 and Transfer Switches (F6) The simulation results are shown in Fig. 5. IQ23 is the

current flowing through Q23, and IQ23op is the instantaneous tripping current of Q23. After a fault occurs, 1PCM-1 limits the fault current. IQ23 reaches the instantaneous trip current iQ23op (3215 A) and Q23 opens immediately to clear the fault. Then the voltage of 1PCM-1 recoveries and the PCM-2 in Zone1 is powered by 2PCM-2. The entire loads keep on working.

C. Input Fault of PCM-2 (F8) The simulation results are shown in Fig. 6. IQ26 is the

current flowing through Q26, and IQ26op is the instantaneous tripping current of Q26, which has the same value as IQ23op. After a fault occurs, the output voltage of the two PCM-1s collapses seriously. With the limited fault current, both of IQ23 and IQ26 reach the instantaneous trip current (3215 A), and then Q23 and Q26 response to the short-circuit fault and remove it, leading the bus voltage to restore. Because of the input fault, PCM-2 in Zone1 loses power, while other loads are not affected.

The simulation results show that the fault response function is as quick and accurate as to achieve self healing of the power system. Therefore, the proposed protection method for selective protection in the DC 700 V bus is effective enough to ensure the coordination of the PCM-1 and the downstream DC circuit breakers.

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Zone 1 Zone 2

DC4000V

Starboard DC 700V Bus

DC4000V

Q13 Q14

Q23

2PCM-1

Q21

Q22

L21

K1 K2

K3 K4

Q15Q24

Q16

Q25

Q17 Q18 Q20Q19

Q30

Port DC 700V Bus

1PCM-1

Q11

Q12

R4

Q4

Q5

15kW

10kW

10kW

10kW

M

M

150kW

Q9

Q10

150kW

Q1

Q2

Q3PCM-3 DC230V

1SSCM2SSCM

Q6

Q7

Q8PCM-2 AC400V

1SSIM2SSIM

Q27 Q28

Q29Q26

F1

F3

F4

F5

F6

F7

F8F2

F3b

F3c

F3d

F3e

F3a

F2b F2c

F2d

F2e

F2a

F9R1

ZL2

M1

M2

R2

R3

ZL1

Figure 7. Faults in DC zonal electrical distribution system.

V. PROTECTION METHOD FOR DC-ZEDS To illustrate the system protection of DC-ZEDS supplied

by MVDC, the protection method is summarized based on the above work and [7]. The identification and protection means of the 19 faults are Zone1 is listed in Table II, wherein, 1PCM-1 feeds power to the port DC 700 V bus, with Q11 closed and Q12 open; while 2PCM-2 feeds power to starboard, with Q22 closed and Q21 open.

The comparison of the DC-ZEDS supplied by MVDC in Fig. 1 and the US naval DC-ZEDS (Integrated Fight Through Power, IFTP) can get the following conclusions:

(1) The 4160 V, 60 Hz MVAC is used in IFTP, while the 4kV MVDC is used in the presented DC-ZEDS, for which there is difficulty selecting suitable medium voltage DC circuit breakers. Without the AC/DC PCM (PCM-4), the corresponding cost, size and weight will be reduced. However, PCM-1 in the presented DC-ZEDS needs to provide electrical isolation function.

(2) In IFTP, PCM-1 is the interface between the DC 1 kV bus and the distribution zones, and is responsible for preventing the fault spreading out of the zones. In the presented DC-ZEDS, the transfer switch cabinets perform power transmission, and DC circuit breakers are necessary for fault isolation.

(3) In the two schemes, the selective protection is implemented through the coordination between the fault current limiting of power electronics and the low-voltage protection devices like circuit breaker and switches.

VI. CONCLUTION In this paper, the DC-ZEDS supplied by MVDC with a ring

network structure, achieve the selective protection through the coordination between the power electronics fault current

limiting and the low-voltage protection devices, has quick and accurate fault response, and shows excellent reliability and continuity. Reconfiguration with advanced intelligent algorithms such as multi-agent system and expert system will be explored in the future.

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