THE RTDS SIMULATION AND ANALYSIS ON CSG OPERATION

CHARACTERS

Mengjun Liao, Weiqiang Han, Qi Guo, Yunwen Guo

Electrical Power Research Institute, China Southern Power Grid, Guangzhou, P.R.China 510000

Abstracts: Nuozhadu and Xiluodu DC transmission system in operation makes the AC/DC hybrid scale of CSG

enlarge to eight AC and seven DC transmission systems, which increases the complexity of AC/DC hybrid

transmission system operation. It’s indicates that the paralleled complex-circuit AC/DC hybrid transmission

system of CSG cannot be simulated accurately and cannot be analysed on some deeper problems by only using the

electro-mechanical transient simulation tools. This paper constructs the RTDS simulation platform of CSG above

220kV grid with 8 HVDC system, based on the closed loop connection and FDNE (Frequency Dependent

Network Equivalent), which recurs the electro-magnetic transient and response of (U)HVDC control and

protection system. At last the security risk assessment of multi-HVDCs commutation failures is present by this

simulation platform in some extreme AC fault.

Key words: Hybrid AC&DC power system, Real-time simulation, Interaction of AC&DC, Multi-HVDCs

commutation failure

1. Overview

China southern power grid (CSG) has institute

“eight HVDC and eight AC” pattern in the “West to

East” Power Transmission Project, while Nuozhadu

HVDC project and Xiluodu HVDC project are

operating in 2013. It operates one of the most

sophisticated and technologically advanced in the

world featuring long distance, extra high voltage and

hybrid AC&DC operation. Multi-HVDC projects

centre in the load area lead to complex interaction

between HVDC system and AC system, such as a

key issue of multi-HVDC commutation failure.

Considering a key factor of nonlinearity of HVDC

transmission system in the hybrid AC&DC operation,

traditional TSA (Transient Stability Analysis) may be

hard to simulate accurately in the electro-magnetic

transience of interaction between HVDC and AC

system and the response of control and protection

system. Therefore, this paper indicates to build a

real-time simulation platform of CSG to solve the

analysis issues of hybrid AC&DC operation. At last,

investigation about security risk of multi-HVDCs

commutation failure is present.

2. Feasibility and Challenge of Large Scale

Real-Time Simulation

2.1 Real-Time Simulation (RTS)

RTDS, RT-LAB, HYPERSIM and ADPSS are

typical RTS tools around the world. At the beginning

of being designed, they are used to verify actual

power system Control and Protection (C&P) devices

with hardware-in-loop (HIL) technology instead of

the traditional dynamic analog test. As an

Electro-Magnetic Transient (EMT) algorithm, RTS

simulates the real power system instant by building

the power model with high order differential

equation and partial differential equation. It is able to

respond nonlinearity of power electronic accurately.

Therefore, RTS presents significant advantages of

accurate simulation for HVDC and HIL technology

in hybrid AC&DC operation comparing with

TSA.[1]~[4]

2.2 Challenge of Hybrid AC&DC Operation

Real-Time Simulation

In hybrid AC&DC operation simulation, two

key issues below must be considered seriously: one is

the accurate modelling of HVDC transmission

system, including power electronic nonlinearity as

well as C&P response of HVDC. The other is

characteristic of large scale AC system. As

mentioned before, RTS presents significant

advantages of accurate simulation for HVDC and

HIL technology in hybrid AC&DC operation

comparing with TSA. However, the simulation scale

of RTS is limited to maintain the solution in real-time

which ensures HIL requirement strictly. Taking

Real-Time Digital Simulator (RTDS) as example,

there are specific limitations of node, component and

variable as well as interface channel number in each

unit of RTDS.

In the earlier investigation of power system, the

key issues of system stability concentrated upon the

generators electromechanical transient process to

judge whether two groups of coherent generators

would lose the synchronization during disturbance.

Power system researches usually reduce the AC

system scales in real-time simulation by equivalent

models. Dynamic equivalence of coherent generator

groups and network equivalence are two of the most

common methods. However, as an increasing of

system capacity and power electronic application,

new challenge arises in real-time simulation of

hybrid AC&DC system. One is the major factor of

power system stability turns to HVDC

electro-magnetic transience gradually from

generators electromechanical transient. Traditional

dynamic equivalence based on fundamental

frequency may lose the interaction characteristic of

high frequency. On the other hands, fast increasing of

AC system scales make dynamic equivalence more

difficulty than before.[5]~[6]

Some references present a new technology of

Frequency Dependent Network Equivalence (FDNE)

as an accurate solution of real-time simulation for

large scale power system. A wide-band multi-port

equivalence will be used to instead of less critical

power grid so that the simulation scales would be

decreased but a wide-band electro-magnetic

transience would be reserved in the simulation. In

this way, hybrid AC&DC power system is able to be

modelled in RTS [7~[9].

Considering advantages of RTS and FDNE

technology, this paper has build a real-time

simulation platform with whole transmission network

above 220kV based on RTDS. Some important C&P

devices of HVDC transmission projects and Special

System Protection (SSP) are HIL in the RTS

platform.

3. The RTS Platform with Whole Transmission

Network of CSG above 220kV

The RTS platform with whole transmission

network above 220kV is shown as Figure.1. It

includes eight HVDC transmission systems and their

C&P systems. Two sets of C&P systems are HIL and

the rest are used digital models in RTDS. Certainly,

other C&P systems can be switched as HIL

depending on different investigating objective. The

AC system models are separated into two parts. One

is electro-magnetic transient simulation model of full

power system surrounding the converter stations and

the “West to East” Power Transmission Project. The

other part is an equivalence of less important power

system using FDNE technology. By this way, it

decreases the simulation scales but maintains a high

simulation accuracy of hybrid AC&DC system.

HVDC Digital model

HVDC C&P model

HVDC C&P DeviceClosed-loop

BUS

Equivalence simulation Grid

using FDNE

Full EMT simulation Grid

HVDC SYSTEM AC SYSTEM

Fig. 1 Schematic diagram of the CSG RTS platform with whole

transmission network above 220kV

3.1 HIL of HVDC C&P System

HVDC transmission system modelling is the

most important section in the RTS platform. Power

electronics are full electro-magnetic transient

simulation models in RTDS, such as valve, converter

transformer, filters and so on. The parameters refer to

actual project. Set of C&P devices, including Station

Controller, Pole Controller, Pole Protection and so on,

are HIL with the specific GT interface cards. The

analogs of voltage and current as well as status of

breakers are sending to C&P devices by GT output

interface cards. After C&P system calculation,

references of alpha angle and orders of breakers

would be returned to the RTDS model for HVDC

operation.

Fig. 2 schematic diagram of HIL of HVDC C&P system

3.2 HIL of Special System Protection

In order to verify the C&P response of actual

power grid, some SSPs are necessary in the RTS

platform. Out-off-step separator, a system special

protection, is considered to be closed-in-the-loop,

which is used to separate power system into two

disconnected power grid to prevent instability of two

groups of coherent generators during disturbance.

Considering where oscillation between coherent

generators centres in the AC transmission lines due to

the multi-HVDCs commutation failure, out-off-step

separator of stations between Guangxi power grid

and Guangdong power grid are HIL in the RTS

platform.

3.3 Comparison of Simulation Result and PMU

Record

These years, there are some post-disturbances

in the multi-HVDCs infeed power grid leading to

multi-HVDCs commutation failures. With

comparison of simulation result and PMU record in

actual case, the RTDS simulation platform is proved

to be effective and high accuracy.

Figure 3.1 to 3.3 show a post-disturbance

simulation of “2012.8.11” failure. In this case, single

phase short-circuit was located on the T-line from

Zengcheng to Suidong Ⅱ which leaded to five

HVDCs commutation failure. Firstly, it is necessary

to adjust power flow and HVDC to meet the

operation before post-disturbance in the CSG RTS

platform with whole transmission network above

220kV. Then set a single phase short-circuit where it

was in “2012.8.11” failure. In this case, C&P systems

of YG UHVDC, TG HVDC and GG2 HVDC are

HIL in the RTS platform.

Fig. 3.1 The comparison of simulation result and PMU record –

YG HVDC power, voltage and current

Fig. 3.2 The comparison of simulation result and PMU record –

TG HVDC power

Fig. 3.3 The comparison of simulation result and PMU record –

power of AC transmission line

Table 1 shows the detailed difference of HVDC

commutation failure between simulation and actual

record. According to the simulation result and PMU

record, commutation failure of three HVDC projects

during post-disturbance present approximately same

response in power loss and recovery. Then, prony

method is taken to analyse the system oscillation

frequency and damping. The difference of oscillation

between simulation and actual record are shown as

Table 2.

Table 1 The detailed difference of HVDC

commutation failure

HVDC

PROJECT

Minimun power Recovery time

Simulation Record Simulation Record

YG UHVDC 0MW 0MW 177ms 158ms

TG HVDC 0MW 280MW 132ms 142ms

GG2 HVDC 1600MW 1800MW 58ms 22ms

Table 2 The detailed difference of oscillation

Transmission Line Oscillation Frequency Damping

Simulation Record Simulation Record

Wuzhou – Luodong I 0.37 Hz 0.36 Hz 6.6% 11.5%

Hezhou – Luodong I 0.36 Hz 0.37 Hz 6.5% 12.5%

Guilin - Xianlingshan I 0.37 Hz 0.36 Hz 7.3% 11.6%

4. Power system security risk of multi-HVDCs

commutation failure

As it shows above, short-circuit fault in

multi-infeed power system may lead to

multi-HVDCs commutation failure. As if this fault is

not able to be clear in time, multi-HVDCs

commutation failure will last longer and system

disturbance becomes more critical. Even, it will

cause system instability and damage.

In order to investigate the power system security

risk of multi-HVDCs commutation failure, this

section simulates largest load and power generation

of strict operation in RTS platform and sets a series

of faults about single phase short-circuit and

mid-breaker failure located in important 500kV

substations of Guangdong power grid.

The main electrical connection of 500kV

substation is shown as figure 4. Two transmission

lines or transformers are connecting to the electrical

buses of substation by three breakers. If a

short-circuit fault locates in LINE 2, breaker 5021

and 5022 will be open to clear the fault. However, if

the mid-breaker 5022 doesn’t open as well as

expected, the Breaker Failure Protection will be

active after a delay to open breaker 5021 and 5023.

At the same time, the relevant breakers of the other

side of LINE 2 and LINE 5 will be open, too. In this

situation, two transmission lines would be power off.

LINE 1 LINE 2 LINE 3

LINE 4 LINE 5 LINE 6

Short-circuit fault

#1 Bus

#2 Bus

5021

5022

5023

A

5011 5031

5012

5013

5032

5033

Fig. 4 The main electrical connection of 500kV substation

Take Beijiao 500kV substation as example.

According to the main electrical connection of Huad

substation, if a short-circuit fault and mid-breaker

failure locate in Beijiao-Zengcheng I transmission

line, Breaker Failure Protection acts after 300ms and

cut-off Beijiao-Zengcheng I transmission line and #1

transformer. As Figure 5 and Table 3 shown below,

eight HVDC projects present commutation failure at

the same time during this disturbance. The minimum

power of four HVDC projects reach 0 MW and all

HVDC projects take more than 344 micro-second to

be recovery. Because of the long-time multi-HVDCs

commutation failure, system becomes instable and

out-off-step oscillation is located in the AC

transmission lines of Guangdong section. As Figure 6

shown, about 3 seconds after the short-circuit fault,

Special System Protection acts and cut off the AC

transmission lines. Power grid is separated into two

disconnected parts.

Besides, system instability will be caused by a

short-circuit fault and mid-breaker failure in another

nine substations of Huadu, Suidong and so on.

Fig. 5 Simulation result of HVDC power

HVDC Project

Minimum Power

of HVDC

(MW)

Recovery

XLD HVDC 0 400ms

NZD UHVDC 820 462ms

YG UHVDC 0 618ms

GG2 HVDC 422 517ms

GG1 HVDC 0 344ms

TG HVDC 234 548ms

SG HVDC 0 370ms

Fig. 6 Simulation result of AC transmission lines of

Guangdong section

5. Conclusion

CSG operates one of the most sophisticated and

technologically advanced in the world featuring long

distance, extra high voltage and hybrid AC&DC

operation. Multi-HVDC projects centre in the load

area lead to complex interaction between HVDC

system and AC system. Facing the difficulty and

challenge of simulation of hybrid AC&DC power

system, this paper investigates to build a real-time

simulation platform for the whole transmission

network of CSG above 220kV. With comparison of

simulation result and PMU record in actual case, the

RTDS simulation platform is proved to be effective

and high accuracy.

Another investigation of this paper is power

system security risk of multi-HVDCs commutation

failure. Multi-HVDCs commutation failure will last

longer, or even lead to system instability if

post-disturbances in the multi-HVDCs infeed power

grid does not be clear in time. A series of single

phase short-circuits and mid-breaker failures in

important 500kV substations of Guangdong power

grid have been simulated in the RTS platform.

According to the simulation result, single phase

short-circuits and mid-breaker failures in Beijiao and

other nine substations may cause system instability in

strict operation of largest load and power generation.

References

[1] KUFFEL R, GIESBRECHT J, MAGUIRE T, et al. RTDS-A

Fully Digital Power System Simulator Operation in Real

Time[C]// Proceedings of First International Conference on

Digital Power System Simulators (ICDS’95), 5−7 April 1995,

College Station, TX, USA: IEEE, 1995: 19−24.

[2] DUCHEN H, AGERKVIST M, KUFFEL R, et al. HVDC

Simulation and Control System Testing Using a Real-Time

Digital Simulator (RTDS) [C]// Proceedings of First

International Conference on Digital Power System

Simulators (ICDS’95), 5−7 April 1995, College Station, TX,

USA: IEEE, 1995: 213.

[3] KUFFEL R, WIERCKX R P, DUCHEN H, et al. Expanding

an Analogue HVDC Simulator's Modelling Capability Using

a Real-Time Digital simulator (RTDS) [C] // Proceedings of

First International Conference on Digital Power System

Simulators(ICDS’95), 5−7 April 1995, College Station, TX,

USA: IEEE, 1995: 199.

[4] ANAYA-LARA O, ACHA E. Modeling and Analysis of

Custom Power Systems by PSCAD/EMTDC [J]. IEEE Trans

on Power Delivery， 2002，17（1）：266−272.

[5] HEFFERNAN M D，TURNER K S，ARRILLAGA J，et al.

Computation of AC-DC System Disturbances, Part I：

Interactive Coordination of Generator and Converter

Transient Models [J]．IEEE Trans on Power Apparatus and

Systems，1981，100（11）：4341−4348.

[6] ANDERSON G W J. Hybrid Simulation of AC-DC

Power Systems [D]．New Zealand：University of Canterbury，

1995

[7] Y. Liang, X. Lin, A. M. Gole and M. Yu, “Improved

Coherency-Based Wide-Band Equivalents for Real-Time

Digital Simulators”, IEEE Trans. Power syst., vol.26, no.3,

pp.1410-1417, 2011.

[8] X. Lin, A. M. Gole, and M. Yu, “A wide-band

multi-port system equivalent for real-time digital power

system simulators,” IEEE Trans. Power Syst., vol.24, no.1,

pp.237-249, 2009.

[9] B. Gustavsen, A. Semlyen. Enforcing Passivity for

Admittance Matrices Approximated by Rational Functions

[J]. IEEE Trans. on Power System, 2001,16(1): 97-104.

[10] HAN, Weiqiang, LI Zhangying, GUO Qi, et al.

Research on the Feasibility and Mode of RTDS Analysis of

Power System Operation. [J]. Southern Power System

Technology, 2010, Vol.4, No.3, pp: 51-56.

[11] LIAO Mengjun, GUO Qi, GUO Yunwen, et al. Study on

risk of system transient instability by breaker refused [J].

Southern Power System Technology, 2014, Vol. 8(S2):

199-204.