___________________________________________________________________* Tour 800, 12th  floor, boul. de Maisonneuve-Est, Montreal, Quebec, Canada, H2L 4M8
LEYTE-LUZON HVDC POWER TRANSMISSION:
COMMISSIONING HIGHLIGHTS, PERFORMANCE MEASUREMENTS
AND OPERATING EXPERIENCE
by
J. F. ALLAIRE*  C. D. CLARKE L. R.WILHELMSSON M. P. GARCIA G. L. DÉSILETS G. MOREAU
B. S. S. EKEHOV NAPOCOR Hydro-Québec ABB Power Systems
SNC Lavalin (Canada)
(Sweden)
(Philippines)
Abstract - The Leyte-Luzon HVDC power transmission system has achieved a converter availability of 98%during its first 15 months of commercial operation.Despite AC network constraints the commissioning tests were able to optimise the controls and demonstrate the effectiveness of the special frequency and emergency power controls in stabilising both the Leyte and Luzon networks.  The performance measurements confirmed that the specification requirements have been met with no appreciable level of interference.
Keywords - HVDC - Control - Interconnection -Commissioning - System test - Geothermal generation -Availability - Utilisation – Disturbance 1.
INTRODUCTION
The 440 MW Leyte-Luzon HVDC Transmission Project connects the power system of Luzon, the major island of the Philippines and includes the capital, Manila, to that of Leyte which is part of the Visayan island group. This project is part of National Power Corporation's (Napocor) overall plan to connect the existing Luzon,Visayas, and Mindanao grids into a single national grid (Figure 1) [1].
The demand for electrical energy is increasing rapidly in the Philippines and the Leyte-Luzon interconnection makes it possible to transmit environmentally friendly geothermal energy to the Manil
a area. It is also a vital part of Napocor´s development program as it permits a more efficient balance of energy supply and demand.This is the first time a HVDC system has been connected to a network almost completely supplied by geothermal power and is by far the largest load on the Leyte system - about 2/3 of the installed capacity. It also represents the largest power source on the island of
Luzon and, as this power has to be transmitted from Naga in the south-east some 250 km to Manila, the effects on the Luzon grid are considerable.
This paper describes some of the special control features which are necessary to ensure the stability of the Visayas and Luzon power systems and to allow the geothermal power plants to be operated at maximum efficiency. The results of several tests of these special control functions carried out during commissioning are presented as well as a brief discussion on equipment
related problems and initial operating experience.
It is considered that the thorough testing contributed to the excellent performance of the HVDC system during its first full year of operation - the converter availability has exceeded the guaranteed value of 98%, and high availability is expected in the future.
2.GENERAL SYSTEM DESCRIPTION
The Ormoc converter station is located on Leyte, one of the islands of the Visayas group, and acts normally as a rectifier. The five major islands of this group are connected by AC lines and undersea cables at 230 or 138 kV. Cebu has the largest load and mainly fossil fuel fired conventional thermal g
eneration, it imports power from geothermal and thermal resources on Negros and Panay and up to 200 MW from Leyte. Leyte is also connected to Samar, and these two islands (Leysam) have a load between about 70 MW and 120 MW.
As shown on Figure 2, the geothermal generation on Leyte has a rated capacity of just over 700 MW. With all units available the firm capability is about 665 MW.
A 30 MW stand-by gas turbine was available during commissioning of the HVDC system.
The Naga converter station is located in the south-east corner of the island of Luzon and acts principally as an inverter.  As shown on Figure 3, it is connected to the 500 kV double circuit Naga-Tayabas (operating at 230 kV) and Naga-Labo transmission lines (and thence to the Manila area). These lines also transmit power from the Tiwi and Bacman geothermal plants.
The Leyte-Luzon HVDC interconnection is monopolar as shown in Figure 4. The converter valves, smoothing reactors (240 mH) and converter transformers are the same design at both stations. The three 35 Mvar filter banks at Ormoc are identical while, at Naga, there are two identical 70 Mvar banks and a 78 Mvar high pass filter. The AC filter circuit breakers are equipped with synchronous closing devices. A passive DC filter (12th and 24HP) is also installed at each converter station. Shore
electrodes made of 40 vertical sub-electrodes are connected by overhead electrode lines. As the HVDC line is bipolar, three operating modes are possible, the first one being preferred for its lowest losses:
•two HV conductors in parallel with sea return •one HV conductor with sea return
•metallic return.
3.MAIN CONTROL FUNCTIONS
The Leyte–Luzon HVDC transmission system normally operates in power control mode (Figure 5). Current control can be used as a back-up mode. A back-up synchronous control is automatically activated in the event of loss of telecommunication. In this mode the current response is used as the inverter current order, so assuring that the current margin is always maintained. In addition, the reduced voltage (80%) operating mode can be ordered by the operator as well as being applied automatically after repeated line faults.
To avoid Leyte system collapse should generation be reduced, the HVDC link is normally operated in Leyte frequency control mode. This is the principal control mode of the Leyte-Luzon HVDC system. The frequency controller (FC) is a proportional regulator with a pre-set dead band.
Both converter stations have reactive power controls (RPC) which maintain the reactive power balance with the networks and determine when to switch filters. The
MALITBOG (250 MW)
UPPER
MAHIAO
(130 MW)
MAHANAGDONG
(200 MW)
Figure 2 - AC system around Ormoc
LABO TAYABAS
TIWI "A"
(110 MW)
TIWI "B"
(110 MW)
TIWI "C"
(110 MW)
TOWARDS METRO MANILA Figure 3 - AC system around Naga
RPC is provided with a step voltage (flicker) controller to minimise voltage steps on filter switching, described in section 5.6. The net reactive power balance can be set by the operator and the RPC also ensures that sufficient harmonic filtering is always connected.
To avoid overloading transmission lines on Luzon, five emergency power control modules (EPC) mo
dulate or limit the transmitted power based on the status of the transmission lines or system frequency. These are described in Section 5.5.
The Line Fault Locator (LFL) covers the total distance between the two converter stations and quickly informs the operator on which of the two DC line conductors the fault occurred as well as the distance to the fault.4.
GEOTHERMAL GENERATION
Geothermal power plants use the heat in high quantities of relatively low quality naturally occurring steam to produce high pressure and temperature steam to drive a conventional steam turbine. The raw steam is collected from wells located over a fairly wide area, referred to as a steam field. The geothermal power producers on Leyte request up to 24 hours notice of the need to open or shut a given steam field. Once open, a steam field takes up to 4 hours to achieve full capacity.
The rate of change of power from a set of generators is also limited. During the commissioning of the HVDC system it was generally attempted to limit this to about 200 MW/h. Power order ramp rates of  20 MW/min could be followed by the geothermal plant operators if the total change was limited to about 100 MW. Rapid power reduction in an emergency can be achieved by releasing high pressure
steam to the atmosphere and,
after such an action, power can be increased at a fairly fast rate since the steam is available.
Due to the relatively long steam side time constants the HVDC system is usually operated in Leyte frequency control mode so that sudden loss of generation capacity or reduction in local load (eg loss of the cable to Cebu)is immediately compensated for by a change in HVDC power transmission. This allows the Visayas system to be run with virtually zero spinning reserve and limits the probability of having to vent steam.
Frequency control cannot prevent sudden reduction in HVDC transmitted power (due to blocking, EPC action,etc.) which results in significant acceleration of the Leyte generators. A sequence of generator tripping based on frequency was developed to ensure that only the minimum amount of generation needed to preserve stability is tripped. Planned and inadvertent blocking from 440 MW did occur during commissioning and did result in tripping of some Leyte generators, however the units which were tripped were sufficient to stabilise the frequency without causing loss of any of the major generators or other system load.5.
TRANSMISSION TESTS
5.1General Aspects
The transmission tests were performed in two stages between March and August, 1998. All basic control and protection functions were verified at lower power to reduce the impact on the AC systems. Clearly it was also necessary to verify the performance of the HVDC system at rated power which involved both deliberate and unintended blocking from 440 MW.
23
176256Figure 4 - Schematic of the HVDC interconnection
Network constraints, official holidays and an election period extended the total commissioning duration since testing could not be performed continuously. Problems with the Leyte-Cebu AC interconnection forced some of the testing to be done with the HVDC system islanded on the Leyte system.  In this configuration the number of connected geothermal machines and the Leyte network voltage were critical, particularly during de-blocking and until a sufficient power level was reached. The non-availability of the Naga reactor and the Naga-Tayabas transmission lines imposed operating restrictions and contributed to delays in the commissioning; however the fact that the HVDC system was able to function well with the reduced AC system capabilities is significant.
5.2Control Instabilities
At the beginning of high power testing an oscillation in the DC quantities at about 97 Hz was noted which had not been seen during low power testing. It was found that a software error had caused a normally temporary high value of the gain of the current control amplifier to be present continuously. This was corrected easily by a change in the software logic.
5.3Operation with 5% Minimum Current
An important feature of the Leyte-Luzon HVDC system is that it is designed to operate continuously at 5% of rated current. This minimises the disturbance to the AC networks during start/stop and is particularly needed when the Leyte system is islanded.The valve design was shown to be able to maintain this level without discontinuous current. The converters can, however, be exposed to more frequent operation with discontinuous current during AC voltage reductions at the rectifier due to remote faults with long clearing times or transformer energisation. Protective functions (such as valve misfire protection) which could mis-operate during such events had to be tested to ensure correct performance.
5.4Frequency Control in Leyte
To make full use of the Leyte geothermal resources the Visayas system is operated with virtually zero spinning reserve. The HVDC power transmitted to Luzon must be reduced if significant Leyte generation is lost.
The Frequency Controller (FC) was deactivated during the first set of commissioning tests in order to test all the basic control functions. However, the FC was active during the major part of the testing period and so was exposed to many different disturbances and operator actions, e.g. power ramping,
start/stop, control mode changes, DC line faults (figure 9), and AC system disturbances on both Leyte and Luzon sides.
Several tests were performed to verify the performance of the Frequency Control, including trips of complete geothermal plants. Tests were performed both with and without telecommunication in service. In all tests the FC acted correctly to change the transmitted power and bring the Leyte network frequency back close to 60 Hz.
Other
f
modulate
Converter
Interface Figure 5 - Simplified  block diagram of the converter controls
For reference, a plot of the HVDC quantities and system frequency against time is shown as Figure 6 for the case of a 158MW load rejection at Mahanagdong.5.5Emergency Power Control
The Emergency Power Controller (EPC) reduces the transmitted power to help the Luzon AC networ
k. The EPC over-rides Leyte frequency control and consists of five power reductions which are imposed automatically if specific system conditions are encountered (Table I).The first one is always active while the other four can be individually activated by the operator.
Table I – Description of the EPC functions EPC Initiated by Action
1Loss of both Naga-Tayabas lines Block DC link
2
Loss of one Naga-Tayabas line
Reduce power to
440 MW at 15 MW/s 3Loss of both Naga-Labo lines
Reduce power to 80%
of prefault at
1000 MW/s
4
Loss of telecommunication Reduce power to
440 MW at 15 MW/s 5
Luzon frequency >60,6 Hz Reduce power until f < 60,3 Hz To detect if an AC line is not available the EPC system
uses either breaker status or line power measurement.Special care has to be taken when using measurement of line power to prevent inadvertent initiation of the EPC during power swings.
The tests of the EPC activation modules were initially performed by simulating the status of the breakers of the critical AC lines or the power measurement on the AC line. Later "natural" events occured which led to EPC actions. Figure 7 shows the initiation of EPC 3 after an AC fault resulted in the loss of both Labo lines.5.6Step Voltage Control
Reduction of the voltage step when switching a filter at an inverter has been implemented on previous projects (appendix 8.2 of [2]). As control actions are only taken at the inverter this is relatively simple and is effected by temporarily increasing the extinction angle at the instant
of filter breaker closing to increase the reactive power consumption. The extinction angle is later ramped back to normal over a few seconds. Prior to switching a filter off, the extinction angle is slowly increased and is then stepped back to normal as the breaker opens. During the sequence the active power is held relatively constant by a fast controller. This function is installed at Naga and tests showed that the voltage step is reduced from 4% to 3% without much of a transient at the rectifier. If needed, the voltage step could probably be reduced further.
For Leyte-Luzon, a similar function was also added at the rectifier. This requires  co-ordination of actions with the inverter since, at filter switch in, the angle has to be increased simultaneously at both ends at the instant the breaker is closed and then ramped back slowly. At filter switch off, both angles have to be increased slowly and stepped back at the instant of the breaker opening. This control reduces the step voltage from 1,2 to 0,6% at the rectifier; but causes a 1% voltage step at the inverter, as shown in Figure 8. This must be considered when
evaluating the efficacy of such a function at a rectifier.
α
f Ud Io Id
00,8
59,659,4
325
3251,059,865065060,0Io (A)
Id (A)Ud (pu)f (Hz)  4 seconds
Figure 6 - Load rejection test at Mahanagdong
α102030( )
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