Operational Problems and Chalenges in Power System of Vietnam (Part II of III)

Ngày cập nhật: 30/11/2011
Ngo Son Hai, Nguyen The Huu, National Load Dispatch Centre of Vietnam

II. OPERATING PROBLEMS OF VIETNAMESE POWER SYSTEM


A. EHV transmission stability at 500kV


The 500kV line, as the backbone of the national power network, plays an important role in balancing the power supply and demand. The geographical characteristics of the country, long and narrow, have affected the distribution of generation sources in Vietnam. Main generation sources include hydro plants, coal plants, and gas turbine plants (single cycle and combined cycle). Large hydro plants mainly centered in the North and West areas of the Northern Region. Coal plants are constructed near primary energy sources in the East area of the Northern Region. Gas turbines are built in the Southern Region. Because of this uneven distribution and the diverse nature of primary sources as well as different operating modes, the 500kV North-South line frequently has to transmit a large amount of power between the North – South regions. Experience has proved that the power flow is often limited by the boundary values of: (1) Voltage stability, (2) Transient stability. 


The voltage collapse limit of the 500kV line needs to be verified when the voltages of the busbars at receiving ends decreases. This can happen when the line is heavily loaded (around 1000 MW for a single line and 1500 MW for two parallel lines) provided that this happens during peak hours. Calculations show that the system becomes susceptible to instability when the voltage at the receiving end is lower than 0.9 pu.

The transient stability limit of the 500kV line is violated when the system becomes unstable after large disturbance such as the tripping of a 500kV circuit breaker. Internal simulations carried out in NLDC have shown that instability can happen when the line is heavily loaded (1200MW for a single line and 1700MW for two parallel lines). The risk of instability is especially higher when 500kV substations Da Nang, Pleiku operate with incomplete configurations (lacking one or more circuit breakers).

The Danang – Pleiku connection is the middle link of the 500kV system and the transmitted power is usually high. Real operation has experienced incidents where power swing happens, activating the distance relay (F21) on this line.

Many instability incidents have been recorded since the operation of the 500kV lines. Typical contingencies in the past that resulted in instability are: (1) One incident at 14h:43 on December 27, 2006 at 500kV Pleiku substation. A 500kV circuit breaker did not trip as commanded due to the loss of its DC source. A Breaker Failure relay acted upon the failure, tripping a busbar of Pleiku substation. Unfortunately, during that time, another circuit breaker connected to the other busbar of Pleiku was out of operation, causing the power to flow from Ialy to Phu Lam and back to Pleiku busbar and then to Da nang. The power swing happened in this case because of the weak interconnection. Network configuration at the time of the incident occurs is presented in the Figure 2. Simulation of power swing is recorded in Figure 3.
 

Fig. 2. Network configuration at the time of power swing, Dec 27th , 2006
(Source: National Load Dispatch Centre of Vietnam)

 

Fig. 3. Simulation of power swing in DaNang-Pleiku line, Dec 27th , 2006
(Source: National Load Dispatch Centre of Vietnam)


(2) Power swing at 500kV Da Nang substation: at 11h30 on April 24, 2008, there was power swing on the 500kV Pleiku – Da nang line. Distance relay F21 at Da nang station recognized the swing on the line. At this time, the 500kV Da Nang – Ha Tinh link was operating with a single circuit. The Pleiku – Da Nang was transmitting 1040MW, Da Nang – Ha Tinh was transmitting 690MW.


The swing caused the circuit breakers to trip the 500kV Da Nang – Ha Tinh, Da Nang – Pleuku lines. The system was separated at Ha Tinh – Da Nang. The over-voltage relay followed and tripped the Ha Tinh – Nho Quan line. Record of the swing is shown in Figure 4.
 

Fig. 4. Power swing recorded at Da nang 500kV Substation, Apr 24th , 2008
(Source: National Load Dispatch Centre of Vietnam)

The system’s resistance to instability has increased significantly when most of the substations operate with transformers, especially when the link is operating with two complete parallel lines. With two parallel lines, the voltage and transient stability limits have both improved.


B.    Voltage regulation of EHV 500kV network


Voltage issues: voltage regulation of power system that includes 500kV faces many obstacles because of long lines and the load curve. Voltages on 500kV busbars normally range between 0.95 – 1.02 pu. However, in some cases, the voltage can fluctuate in a wider range of 0.9 – 1.04 pu. Operation in such conditions poses many risks such as: insulators ageing faster, reactive power pushed to 220kV causing transformer overload, etc.

Low voltage usually occurs at 500kV busbars at the receiving ends during peak hours plus the line is heavily loaded. Low voltage increases the loss and threatens the system stability.

High voltage usually occurs during holidays or at late night, early morning, when the system load is low. In some circumstances, operators have to trip one of the two parallel lines to reduce the voltage, lowering the system’s supplying reliability.

Another difficulty for voltage regulation is that most of the shunt reactors are directly connected to the lines without circuit breakers, which causes inflexibility in operation of the system. Therefore, in the near future, Vietnam is promoting research and development of the application of switchable inductors and/or controllable compensators. At the present, Vietnam has already installed 500kV shunt reactors with circuit breakers at Nho Quan, Dak Nong, Tan Dinh, O Mon substations.

C.    High fault current  in 220kV network

With a total of 10,015 km of line length and 102 substations where 220kV level is present (including 500/220kV and 220/110kV transformers), the 220kV lines are the main regional transmission link. The installed capacity of generators connected to 220kV network is 16,478 MW by the end of 2010 (63.7% of the national system’s installed capacity). The short-circuit current at some busbars in the 220kV grid has exceeded the rated current of circuit breakers. Computation from NLDC proved that the fault current at large 220kV substations located near load centres and connected to a number of generators is higher than 40kA, which is the common rated current of most of 220kV circuit breakers. This causes the threat of fault spreading and in the worst case, might corrupt a part of the system.

Currently, in order to secure the operation, a temporary solution is applied, which separates the busbars to reduce the concentration of the fault current at busbars of Phu My, Phu Lam in the South. Most recent calculations (2011) have shown that others nodes also require the separation of busbars including Hoa Binh, Pha Lai (in the North) and Nam Sai Gon (in the South). Nonetheless, this temporary method also results in some negative consequences: (1) increased loss because the power flow is forced, (2) reduced flexibility in operations, (3) lower reliability, (4) less efficiency in economic operation.

Vietnam is looking into other solutions such as installing series reactors to reduce the fault current, requesting new generators not to worsening the fault current by installation of fault current limitation equipment at generator’s sites and taking into account the problem of short-circuit current into power system planning. Among them, the idea of series reactors is an actual method which yields positive results in the short-term. Nevertheless, Vietnam is not experienced in this matter. Thus, the problem of short-circuit currents is not expected to be satisfactorily solved in the next few years. A complete solution to this problem is still being researched.

(to be continued...)

REFERENCES

[1]    National Load Dispatch Centre, Annual Operating Summary 2010 and appendices.
[2]    Decision No. 26/2006/QD-TTg dated January 26, 2006 issued by the Prime Minister of the Socialist Republic of Vietnam, approving the development phases and establishing requirements of a power market in Vietnam.
[3]    Decision No. 12/2010/TT-BCT dated April 15, 2010 issued by the Minister of Ministry of Industry and Trade, approving the Grid Code.
[4]    Decision No. 1208/2011/QD-TTg dated July 21, 2011 issued by the Prime Minister of Vietnam approving the Master plan of Vietnam power developments for the period of 2011 – 2020, taking into accounts year 2030.
[5]    Dantri Online, http://dantri.com/vn/c76/x76-494482/khoi-dong-thi-truong-phat-dien-canh-tranh.htm
[6]    Grid Investments from a Nordic Perspective – Nordic Energy Regulators recommendations, https://www.nordicenergyregulators.org/.../Nordic_grid_conclusions.pdf
[7]    ACCC (Australian Competition & Consumer Commission) - State of the Energy market 2009 – Chapter 5, Figure 5.2, Page 128.


BIOGRAPHIES



Ngo Son Hai was born in Quang Ninh, Vietnam, in 1968. He received the B.E. degree in power electrical engineering from Hanoi University of Technology, Hanoi, Vietnam, in 1991, and the MEngSc degree in power electrical engineering from University of New South Wales, Sydney, Australia, in 1999, and the EMBA degree from Asian Institute of Technology, Bangkok, Thailand, in 2008.
In 1991, he joined the Dispatching Department of Hanoi Power Company, as a dispatcher. Since 1994, he has been with the National Load Dispatch Centre (NLDC), Hanoi, Vietnam where he was an Power System Operator in 1994, became a Deputy Manager of Dispatching Department in 1997, and a Manager of Dispatching Department in 2001, a Manager of Economic Operation Department in 2005, became a Deputy Director of NLDC in 2006, and a Director of NLDC in 2011. His experiences include power transmission network operation, power system operation planning, power generation scheduling and dispatching in the large system, power market operation.



Nguyen The Huu was born in Vietnam, in 1979. He studied electrical engineering at Hanoi University of Technology in Vietnam, graduating with with a B.E. degree in 2001. He joined the National Load Dispatch Centre (NLDC), Hanoi, Vietnam in 2001 and working as power system analysis engineer. From May 2011, Huu became deputy manager of Power System Analysis and Planing Department, NLDC. His special field of interest include electric power systems operation and planing, power market operation.

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