Legislative Council Panel On Economic Services



The systems of The Hongkong Electric Co. Ltd (HEC) and China Light & Power Co. Ltd. (CLP) have been interconnected since April 1981. The interconnector installed capacity was increased to its current level of 720MVA (3 x 240MVA) in 1991. The interconnection between HEC and CLP is designed to provide mutual benefits to both systems including generating capacity support, economic interchange of electricity and mutual emergency backup.

2. The CLP system has also been interconnected with the Guangdong Electric Power Holding Company (GPHC) since 1979. The interconnection with GPHC is used to deliver CLP’s share of the nuclear and pumped-storage energy to the CLP system and for bulk energy sales from CLP to GPHC. The interconnection also serves for emergency backup between the two systems.

Functions of existing interconnectors

3. The functions of the existing interconnectors between CLP and HEC are as follows:

  1. Reliability of Service

    With the existing interconnection in place, HEC has assumed a firm power support of 480MVA from CLP and has relaxed its generating capacity planning criterion from a Loss of Load Probability (LOLP) index of 2.5 hours per year to 20 hours per year, having regard to its system alone. In other words, the interconection allows HEC to achieve its target reliability level with less generating capacity. CLP obtains similar benefits from the interconnection.

  2. Economic Interchange of Energy

    During daily operation, a power company (CLP or HEC) might find it more economical in meeting peak demand to purchase power from the other company, if available, than to start up another coal-fired unit or run diesel-fired gas turbines. In such circumstances, economic interchange of electricity can provide savings in operating costs.

The amount of savings from economic interchange between CLP and HEC for the past six years are shown below :

Savings from economy interchange (HK$M)





















  1. Sharing of Spinning Reserve

    A certain amount of generating capacity known as "spinning reserve" is required to be kept in the system to cater for load fluctuations or sudden outage of a generator. This consists of running generators ready to supply additional power instantaneously.

With the interconnection, the amount of spinning reserve required in the system can be shared among the interconnected parties, i.e. CLP, HEC and Guangdong. The shares of spinning reserve of CLP and HEC are 240MW and 135MW respectively.

Examination of need for new interconnector

4. In January 1996, the Administration commissioned Burns and Roe Company to study the future demand for electricity in Hong Kong to 2005 and ways of meeting the demand. The consultants indicated that a new interconnector between CLP and HEC at an estimated cost of about HK$468M would be required if CLP were to sell excess power to HEC during the period 2003 to 2005.

5. However, CLP indicated during the LegCo Economic Services Panel meeting on 9 December 1996 that no additional cost for equipment, including a new interconnector, would be required for selling power to HEC.

  1. CLP’s interconnection analysis

6. CLP submitted a reliability analysis report to the Administration on 10.1.97 explaining why no additional interconnector is required. Their assumptions are highlighted as follows:

  1. Reliability of Service

    CLP assumed a cyclic rating of 265MVA for the existing interconnectors - that is, 110% of the continuous rating of 240MVA for the existing interconnectors. This is different from the planning criterion of using continuous rating (i.e. 100%) for transmission planning used by CLP in the 1992 Financing Review. It should be noted that one out of the three connectors is regarded as a spare.

    CLP assumed a power factor of one, that is current and voltage completely in phase for the load transferred through the interconnectors. However, for large power transfer, a power factor of 0.9 should be used.

    In calculating HEC’s LOLP for extended power transfer of 350MW from CLP to HEC, CLP used HEC’s relaxed LOLP criterion of 20 hours/year, though firm power support will no longer be 480MVA (para. 3(a) above) if the interconnectors are used for extended power transfer.

  2. Sharing of Spinning Reserve

    In calculating the emergency transfer required by HEC in case of a generator failure, CLP assumed that HEC will have 180MW reserve of its own, including capacity available within 3 hours, instead of 135MW spinning reserve share of HEC available instantaneously (para. 3(c) above).

7. CLP submitted a report on interconnection provided by its consultant, Hong Kong Polytechnic (HKP) to Government on 17 February 1997. The findings of the report are summarised as follows:

  1. The feasibility of using existing interconnectors for firm power transfer is assessed based on the physical capability, such as maximum temperature withstand capability, of the equipment under different scenarios.

  2. If one HEC 350MW generator fails while one interconnector is out, some switch-gear will be overloaded while the temperature rise of the cables will still be within limit.

  3. When one interconnector circuit is out, the power transfer should be limited to 240MW for reasons of operational security.

    The reliability of HEC generation system, i.e. LOLP, was not evaluated.

    1. HEC’s views

8. HEC have stated to the Administration that they disagreed with CLP’s calculations using cyclic rating, power factor of one, and HEC’s own reserve of 180MW. HEC also raised the concern of the possibility of disconnection of interconnectors due to dynamic stability problem in the form of oscillation. Low frequency oscillations which have been disturbing the HEC-CLP-Guangdong interconnected system are currently suppressed by the stabiliser installed in HEC’s and CLP’s generators. However, system damping decreased as interconnector flow is increased due to the firm power transfer, the problem of dynamic instability or power oscillations may once again occur. HEC’s consultant concludes that a fourth interconnector is needed in order to carry a firm power transfer of 350MW.

The Administration’s views

9. The Administration considers that the assumptions used by CLP in assessing the capability of the existing interconnectors are not suitable. CLP’s calculation of the capacity of the existing interconnectors using cyclic loading is not consistent with the criteria reported and agreed in its 1992 Financing Review. Although the Administration has no dispute with CLP’s consultant in the method of assessing the feasibility of using existing interconnectors for firm power transfer based on temperature rise limit of cables, the Administration considers that using temperature rise simulation for particular situations is not a prudent approach for long term planning. CLP’s consultant also recommended that the firm power transfer should be limited to 240MW when one interconnector circuit is out and highlighted that some switch-gear will be overloaded under a possible scenario. The Administration therefore does not support using the existing interconnectors for 350MW firm power transfer.

10. While CLP’s consultant has not assessed the LOLP of the HEC system, the Administration has calculated HEC’s LOLP using computer modelling and found that HEC’s LOLP planning criterion will not be fulfilled by 2004 if the interconnectors are used for firm power transfer of 350MW and is therefore not acceptable.

11. The Administration’s consultants concluded in their original assessment in July 1996 that, from the economic point of view, among all the scenarios studied, including sales of electricity from CLP to HEC during 2003 to 2005, five year deferral of CLP’s Black Point Units 5 to 8 would be the most economical option. Based on the latest load forecasts of the power companies, HEC would not need the purchase of firm power from CLP before 2003 and CLP would not have excess power for sale by 2006. Moreover, CLP has indicated in its report presented to the Panel on 17 February 1997 that deferment of Black Point Units 7 and 8 for five years is possible and has proposed decommissioning of 442MW of existing gas turbines at Castle Peak and Tsing Yi power stations. If these two actions are implemented, by 2003 CLP would not have surplus capacity with which to generate power for sale to HEC.


12. Based on the information provided by CLP and HEC, and the assessment of the Administration with the assistance of its independent consultants, it is concluded that CLP’s proposal for firm power transfer of 350MW to HEC using the existing interconnectors is not feasible under certain scenarios and will degrade the security of the HEC system. CLP’s proposal is therefore unacceptable to the Administration.

Electrical and Mechanical Services Department
March 1997

Last Updated on 14 August 1998