THE AMERICAN SOCIETY OF MECHANICAL. ENGINEERS 345 E. 47th St., New York, N.Y

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1 THE AMERICAN SOCIETY OF MECHANICAL. ENGINEERS 345 E. 47th St., New York, N.Y S The Society shall not be responsible for statements or opinions advanced in papers or discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. Discussion is printed only if the paper is published m in an ASME Journal. Authorization to photocopy material for internal or personal use under circumstance not falling within the fair use provisions of the Copyright Act is granted by ASME to libraries and other users registered with the Copyright Clearance Center (CCC) Transactional Reporting Service provided that the base fee of $0.30 per page is paid directly to the CCC, 27 Congress Street, Salem MA Requests for special permission or bulk reproduction should be addressed to the ASME Technical Publishing Department 95 _61_273 Copyright O 1995 by ASME All Rights Reserved Printed in U.S.A. SYNCHRONOUS CONDENSING USING THE GENERATOR OF PEAK LOAD PLANT Morgan L. Hendry SSS Clutch Company, Inc. New Castle, Delaware John C. McGough B.C. Hydro Vancouver, B.C., Canada ABSTRACT This paper will describe how a generator of a peak load turbine, when operated as a synchronous condenser, can help improve electrical system power factor and therefore help maintain reactive power balances between sources of generation and demand points. It is particularly appropriate to use peak load plant with either industrial or aero-derivative gas turbines for synchronous condensing. By using a clutch to disconnect the turbine from the generator, losses from the turbine are avoided enabling more efficient synchronous condensing operation. Details of specific clutch designs for turbine generator synchronous condenser applications and experience at various B.C. Hydro installations are provided. INTRODUCTION The negative effects of low power factor on voltage control and system operating costs are well documented. For example, low power factor can reduce generator, cable, transformer, and transmission line capacity and cause low voltage as well as voltage swings at remote load sites. There are a variety of methods available today for correcting these conditions. These methods include capacitor banks, reactors, static Volt Ampere Reactive (VAR) compensators and one of the oldest methods, synchronous condensers. These solutions are generally described as power factor correction or VAR compensation. The chosen method of dealing with power factor is largely dependent on the particular system circumstances. This paper describes how synchronous condensers are used and under what circumstances they can become the preferred method of choice. POWER FACTOR First it is necessary to define power factor. Very simply, power factor is a measure of how much current supplied to a system or load is used effectively to produce useful work. Although this is an electric phenomena, a simple mechanical analogy can also be made. Mr. G.L. Oscarson, Electric Machinery Company, Inc., made the following analogy in his excellent paper entitled, "The ABC of Power Factor" in In a mechanical system as shown in Figure la, the force and motion are both in the same direction and there is no difference between apparent and actual power required to move the train. However, as shown in Figure lb, when the force and the motion are not in the same direction, a correction factor must be applied to ^r ^' ^'^III11N11`I Fig. la Force applied in the direction of motion :I:::I::: ; \ Fig.1c Force applied at an angle to the direction of motion ^000 im. A n n nq 45 i ^I i Fg.1 b y Vector 9 relationship 0y offorces a c Figure 1 - Apparent Versus Actual Power Presented at the International Gas Turbine and Aeroengine Congress and Exposition Houston, Texas - June 5-8,1995

2 the apparent power to determine the actual power being exerted. This correction factor is the cosine of the angle between the apparent and actual power as shown in Figure lc. A similar angle, known as a phase angle, exists in an alternating current circuit between the force (Volts) and the rate of motion (current stated in Amperes) and a correction factor must be correspondingly applied. Electrically this correction factor is known as power factor and is used to multiply apparent power to obtain actual power. Figure 2 shows that power factor is the ratio of active power in Watts to the supplied Volt Ampere Reactive (VAR's). Inductive loads cause a lagging power factor and capacitive loads cause a leading power factor. For power (Watts) and Volt Amps to be equal, COS 0 must be unity or 0 = 0. Power Factor = Power (Watts) = COS 0 Volt Amps Reactive WATTLESS OR REACTIVE COMPONENT (VAR5) CAPACITIVE WATTLESS OR REACTIVE COMPONENT (VARS) INDUCTIVE IlAMP51 LEADING (CAPACITIVE) V(VOLTS) ENERGY OR ACTIVE COMPONENT (REAL POWER-WATTSI 0I V(VOLTS) [(AMPS) LAGGING (INDUCTIVE) Figure 2 - Active & Reactive Components of Current To achieve this ideal condition of unity power factor in practice, capacitive reactance must be added to an inductive system and vise versa. Few devices have the capability of doing both. However, a synchronous condenser can provide either capacitive reactance or inductive reactance and can provide these VAR's in infinitely variable increments. It is this particular ability which makes a synchronous condenser an attractive power factor correction device. SYNCHRONOUS CONDENSING What is a synchronous condenser? A synchronous condenser is a synchronous machine connected to an electrical system and being driven from it and supplying or absorbing reactive power. Considered as a motor, the synchronous condenser absorbs leading VAR from the supply system when over excited and lagging VAR when under excited. Alternatively, when considered as a generator, it can supply lagging VAR when over excited and leading VAR when under excited. This is illustrated in Figure 3. Operating Regions of a Synchronous Machine Generating Generating Supplying leading e Supplying lagging reactive current reactive current Under excited I I Over excited Under excited Over excited Compensating j] Compensating Absorbing lagging Absorbing leading reactive current reactive current Motoring ' Motoring Figure 3 - Operating Regions of a Synchronous Machine Almost any synchronous generator has the potential to become a synchronous condenser. All that is required is a means of accelerating the generator to synchronous speed and a means of detaching the prime mover from the generator after electrical synchronization has been achieved. This leaves the generator being driven (motored) by the electrical system and running at synchronous speed. In a particular electrical supply system, synchronous generators, when driven by a prime mover, provide real power and Wattless power (VAR's) according to their relative capacities. However, when the synchronous machine's limit of either Watts or Wattless power is reached, additional capacity must be added to the system with peak load generators. However, if the VAR capacity is exceeded, more VAR's from the same machine means this same machine will produce less Watts. This is the first case where synchronous condensers can be valuable. If spare synchronous generators are available to provide Watts occasionally when needed (peaking units for example), why not make them available for peak VAR's when needed as well? Low power factor can also reduce the capacity of a transmission line to deliver the power to the load. The line capacity can be used up delivering Wattless VAR's, resulting in low voltage at the end of the line. If there is a synchronous condenser at the load end of the line, the drop in voltage can be recognized by its Automatic Voltage Regular (AVR). The AVR can respond by adjusting the excitation of the synchronous condenser to compensate for the low power factor by producing the precise amount of VAR's needed, thereby restoring the voltage and the transmission line capacity. Conversely, if the voltage rises at the load end of the line due to a load change which reflects a surplus of VAR's, the AVR can adjust the excitation to reduce the VAR output of the unit and restore normal voltage. The synchronous condenser therefore has a stabilizing effect on system voltage and the additional mechanical inertia can help when system loads vary quickly.

3 PEAK LOAD GAS TURBINE GENERATORS Gas-turbine-driven synchronous generators have historically been used for peak load power generation requirements and are often sited near centers of high inductive load and/or at the end of long transmission lines. This makes them attractive for the dual purpose of producing Watts and VAR's. In most instances, these peak load generators only operate for a small percentage of the year and for short durations at a time, and there often is a need for reactive power and voltage control at times other than when generating peak power. As the load falls, the units can be converted to synchronous condenser mode by shutting down the gas turbine and allowing the generator to be motored by the system. This only requires a disconnect device (clutch) between the gas turbine and the synchronous generator/condenser. If this device is inherently self-synchronizing, the next peak load requiring Watts can be met by a re-start of the gas turbine and an automatic re-engagement of the clutch at the instant the turbine tries to go faster than the generator. The synchronous machine will then smoothly convert from motoring (synchronous condensing) to generating. shift into mesh at rest or at speed, phasing and engagement of the clutch teeth at synchronous speed is accomplished automatically without any external controls and without possibility of error. Also, unlike a tooth coupling, disengagement of the clutch will occur whenever the input slows down relative to the output without the need to maintain an unloaded turbine condition for disengagement. The principle of operation of the synchronous self-shifting clutch can be seen from Figure 4. When the speeds of the clutch input and output pass through synchronism, the pawls on one clutch element engage with ratchet teeth on the other to phase the teeth precisely for interengagement. Additional shaft relative rotation causes the pawls to provide the small force to move the sliding component along helical splines, thereby engaging the driving and driven teeth smoothly and positively. History Thousands of simple-cycle, gas turbine generators have been installed worldwide to meet peak load generating requirements. These machines were often ordered and delivered quickly to meet immediate power requirement short falls. It is fundamental, however, that utility system needs change considerably over the life of the installed generation unit and often these machines are operated less for power generation than initially intended. Requirements for reactive power change from year to year and existing equipment needs to be flexible. Most of the peak load gas turbine generators which have been installed were specified and purchased on the basis of lowest cost per kilowatt hour. For a comparatively small additional investment, a compact synchronous, self-shifting clutch could have been installed and these machines used for synchronous condensing. There are more than 600 machines worldwide which have clutches installed between the gas turbine and generator to enable synchronous condensing during off-peak periods. Clutches for both aero-derivative as well as industrial frame-type gas turbine driven generators have been used and clutches from 10 MW at either 3,000 rpm or 3,600 rpm up to 140 MW at 3,000 rpm are in service. The highest power clutch in service is 300 MW at 3,000 rpm on a compressed air energy storage (CAES) plant in Germany. Most medium to high power gas turbine/generator manufacturers have experience with such clutches in peak load gas turbine synchronous condensing applications. SYNCHRONOUS SELF -SHIFTING CLUTCHES A synchronous self-shifting clutch is an automatic, engaging/disengaging, freewheel-type overrunning clutch which transmits torque through the full surface contact of gear coupling teeth. Unlike a servo-actuated tooth coupling which is difficult to sliding component axially to engage teeth at synchronous speed. Figure 4 - Synchronous Self-Shifting Clutch Operating Principle The pawls do not transmit any driving torque because they move out of contact with the ratchet teeth by either axial or slight rotational movement before the clutch teeth have shifted into full driving engagement. The clutch completes its travel when the sliding component moves against a shaft abutment and then full torque passes through the helical splines and the fully engaged clutch teeth. As soon as the clutch input speed reduces relative to the clutch output speed, the clutch automatically disengages due to the reversal of torque on the helical splines. Clutch Types and Sizes The basic types of synchronous self-shifting clutches which are used for peak load gas turbine generators are either: 1) Semi- Rigid Type; or 2) Spacer Type, as shown in Figure 5. The semirigid type is simpler and is mounted between shafts in good alignment. The semi-rigid clutch when engaged acts as a solid

4 coupling; and as torque is applied, the driving and driven teeth force the shafts toward a concentric condition. Any slight amount of machinery misalignment with a semi-rigid clutch installation is accommodated by the flexibility of the driving and driven shafts plus the ability of either the driving or driven shafts to be displaced very slightly within the clearances of their respective bearings. The spacer type clutch acts as a flexible coupling to accept slight angular or offset misalignment between the clutch driving and driven shafts when it is engaged and transmitting torque. Typically, a non-geared gas turbine generator which is designed to drive the generator from the exhaust, or hot end, uses a spacer type clutch whereas either a spacer type or semi-rigid clutch is used with a machine designed to drive from the cold end, or for a geared machine. Clutch Installation and Optional Features When considering the installation of a clutch in a peak load, gas turbine generator, the first aspect which needs to be considered is whether the turbine and generator can support the clutch weight. Figure 6 shows various possible methods of installing a clutch and illustrates how additional bearings can be added to the clutch package if overhung weight proves to be excessive for either the Turbine (T) or Generator (G). If the clutch is for a geared gas turbine generator, the clutch can be installed on the turbine side of the low-speed gear and a quill shaft used to connect the clutch output to the generator through the center of the low-speed gear, as shown in Figure 7. SEMI-RIGID CLUTCH Disengaged m --i' o) m ^^ c^) (a) Clutch weight supported entirely (b) Clutch weight shared between the driving be the driving and driven machines. machine and a bearing within the clutch casing. Input Output Th (c) Clutch weight shared between the driven (d) Clutch weight supported entirely bt machine and a bearing within the clutch caring. bearings within the SSS clutch casing. Figure 6 - Typical Clutch Installations Engaged SPACER CLUTCH FOR MISALIGNMENT Tu rlk nnnnorl generator Input - Output Figure 7 - Typical Installation in a Gearbox Figure 5 Because torque is transmitted through gear coupling teeth, synchronous self-shifting clutches are compact devices. As an example, the main overall dimensions of a typical 60 MW, at either 3,000 rpm or 3,600 rpm, semi-rigid clutch is about 15 inches long and 24 inches in diameter; and a spacer type clutch would be about 32 inches long and about 29 inches in diameter. The highest power synchronous self-shifting clutch in service is a 300 MW at 3,000 rpm semi-rigid clutch and it is about 28 inches long and about 40 inches in diameter. The basic synchronous, self-shifting clutch as described above and in Figures 4 and 5 can be used for any gas turbine generator which has the starting system at the gas turbine end of the machine. With the entire machine at rest, the clutch will automatically engage as soon as the gas turbine starts to rotate and it will automatically disengage when the turbine speed is reduced relative to the generator so the generator can be left connected to the electric grid. Re-engagement at speed is simply achieved automatically when the gas turbine is accelerated to the same speed as the generator. For a gas turbine driven generator with the starting system at the generator end of the machine, the clutch must be initially automatically engaged at rest with the gas turbine turning gear and then locked into engagement. This clutch lock will then transmit the starting torque from the generator end to the gas turbine until the 4

5 gas turbine reaches its self-sustaining condition. At that point, torque is transmitted from the gas turbine to the generator through the clutch helical splines and teeth, and the lock will become unloaded. To change to synchronous condensing after power generation, the lock must first be disengaged and then the clutch will disengage automatically when the gas turbine is shut down. Should re-engagement of the gas turbine to the generator at full speed be desired, an optional fluid coupling can be incorporated inside the clutch to enable the rotating generator (acting as a motor) to accelerate the gas turbine to its self-sustaining speed. At that point, the gas turbine will then accelerate itself to full speed to engage the clutch teeth and produce power. A number of clutches with hydraulic couplings are in service; but for most of these machines, it is accepted that the clutch is only engaged at rest most of the time. Province's mainland. B.C. Hydro has approximately 10,800 MW of generating plant of which most is for base load power and these plants are located in the North and East within the Province. Figure 8 is a diagram showing the major transmission system for B.C. Hydro and Figure 9 shows more detail of the lower mainland network in the Vancouver vicinity. Generating stations are shown with square symbols and those specific stations referred to in this paper are designated by a solid square. B.C. Hydro's peak load generating units are either at the end of long transmission lines and/or near to centers of high inductive load; both where synchronous condensing is a valuable asset. 1989/90 B.C. Hydro MajorTransmission System 500 kv Lines Only (except where noted) Clutch Reliability. Retrofitability and Cost xexxcr. Pig 5.ISr..E S Experience has shown that synchronous self-shifting clutches require little or no maintenance throughout the life of the plant. Mean time between failures (MTBF) in excess of 100,000 hours are typical for clutches in synchronous condensing applications regardless of size. For example, the 300 MW at 3,000 rpm CAES clutch has been in service for more than 15 years with thousands of engagements/disengagements and has required no maintenance. Retrofitting clutches into existing turbine generator plant is most often very difficult as there usually is insufficient space between the turbine and generator to accommodate the clutch. If a flexible coupling has been used in the original plant, the clutch can sometimes be designed to fit in the same space; or if the generator is driven by a geared turbine such as a General Electric Frame 5 or 6, the clutch can often be installed within the gearbox with some gearbox modifications. In most instances, however, movement of either the turbine or generator would be required to create the axial space between the two machines to accommodate a clutch. G,eo If a new plant is being installed, a spacer shaft could be installed in the shaft system between the turbine and generator to enable substitution by a clutch at a later date when synchronous condensing is desired. However, care must be given in designing the initial shafting and bearing support to ensure that when a clutch is installed, the clutch is supported correctly. \ The cost of a synchronous self-shifting clutch would depend on the type, design, and installation method required for a specific turbine generator. However, for example, a spacer type clutch designed for 60 MW at 3,600 rpm rating, together with a casing as shown in Figure 6(b), would be approximately $2.5/kW. A semirigid clutch of the same power and installation method would be slightly less. These figures could possibly be doubled when the installations costs, etc. are taken into consideration. EMUw.x, l^[sig ue ^^.G^Ea.:Ix4 5*.r:tx ic^ CnC,i <,J>, IiJL -E E Keogh.:ca.^x^ Generating + `y µum.. Station ^,,l `"hl,^ n,c.rz `.....,m _ r N,Ia ST re^or Ewe., a x.. 0. For detail of lower mainland network sec Fig. 8 e esc , fo.c^ art Figure 8 ^B I. S i Lower Mainlantl Network -""'1 (tor SC Hydro 500 W System) oc 2^sc -i BurrardThermal cok 2633 Of` Port Mann csc B.C. HYDRO'S EXPERIENCE 2Ea I NC fl'". 'cp B.C. Hydro is the provincial utility for British Columbia, Canada, with headquarters in Vancouver, in the Southwest comer of the Figure 9

6 Like many other utilities, B.C. Hydro recognized many years ago the operational and economic advantages of providing peak electrical generating machines with both generating and synchronous condensing capability. In many cases, quality of product and customer service were the primary motives for adoption of synchronous condensing, even when there was no specific net economic benefit. Two such examples are the B.C. Hydro peak load generating sites given in Tables 1 and 2. Table I gives operating hours logged on industrial type peak load gas turbine generators at B.C. Hydro's Port Mann Power Station. Table 2 gives generating and synchronous condensing hours at B.C. Hydro's Keogh site where aero-derivative gas turbines are installed. Synchronous condensing at Keogh has proven to be very important because if these machines were not available for VAR's, transmission line voltage could drop from 138 kv to as low as 108 kv and load shedding would be necessary under Winter load conditions. PORT MANN - Table 1 4 X 25 MW Units Operating Hours (Lifetime Figures) Unit 1 Unit 2 Unit 3 Unit 4 Gas Turbine * Generator * * Note: Synchronous condensing hours are not specifically recorded. Most of the difference between generator and gas turbine operating hours are synchronous condensing hours. KEOGH - Table 2 2 X 25 MW Units Operating Hours (From Jan 1/90 to Oct 6/94) Unit 1 Unit 2 Generating Synchronous Condensing The value of synchronous condenser capability to B. C. Hydro is further illustrated by the experience at their Burrard Thermal Generating Station. This installation of six 150 MW steam turbine generator sets was the subject of a paper in 1990 to the American Power Conference. The Burrard units fall into a different peaking unit category as they have been used primarily in Summer when hydro-electric is less available as reservoirs are being replenished. Considered for retirement in the early 1980s, it was decided to disconnect two of the generators from their turbines and install a pony motor run-up facility at the exciter end of the generator. A synchronous, self-shifting clutch automatically disconnected the pony motor drive from the generator once it was connected to the electrical grid for synchronous condensing. To date, four units at Burrard have been converted and a fifth is being considered. Table 3 gives the number of hours of operation of each Burrard unit since conversion to dual-mode operation. BURRARD THERMAL - Table 3 4 of 6 X 150 MW Units Operating Hours (Periods as Indicated) Unit I Unit 2 Unit 3 Unit 4 Nov/88 to Dec/86 to Oct/86 to Nov/89 to Dec/93 Dec/93 Dec/93 Dec/93 Generating ,606 Synchronous 30,176 47,467 49,468 21,394 Condensing Recently, the Burrard plant was re-assessed for rehabilitation to provide generation/synchronous condensing capability until the year The new operational scenario is to have 6 x 150 MW units available from April to October each year for generation. This allows water storage in the hydro-electric system during the Summer. In the Winter when the residential demand for natural gas restricts the availability of fuel for the six steam turbine generator units at Burrard, four units will be shifted back to synchronous condensing to maximize the electrical system capacity to transmit energy to the load centers and help maximize electricity export to the United States. Synchronous condensing at Burrard has helped B.C. Hydro improve the voltage on its 230 kv and 500 kv transmission lines which interconnect with Bonneville Power Authority, enabling 300 MW of additional export power sales to the United States. With about $2 Million Canadian (1986 dollars) total conversion cost for these four generators and a total synchronous condensing capacity of +560 mvar (overexcited) and -360 mvar (underexcited), this investment and the cost to operate the plant is an attractive alternative to an equivalent static VAR compensation system which B.C. Hydro installed elsewhere in their system in 1992 for $22 Million Canadian. WORLDWIDE EXPERIENCE It is interesting to note that many peak load gas turbine generating sets installed by other utilities in Canada as well as by utilities in various countries in each of the other continents worldwide have incorporated clutches so that the generator can be used as a synchronous condenser. As a brief example of this worldwide experience, typical operating hours are given in Table 4 for a number of high-power, industrial-type, peak-load gas turbine generator sets (Fiat/Westinghouse type) in Italy; and in Table 5, operating hours for two recently installed GE peak-load gas turbine sets in Montevideo, Uruguay. 6

7 In spite of this extensive usage worldwide, the application does not seem to have been adopted generally in the USA. Many utilities in the USA have realized the potential benefit of synchronous condensing and have investigated the practicality of retrofitting clutches within their existing machines. However, in most instances, there has been insufficient space to install a clutch and the cost to move either the turbine or generator to create the space necessary to install a clutch was prohibitive. ENEL - ITALY - Table 4 7 X 110 MW Gas Turbine Generating Sets Operating Hours 1st. Yr. of Generating Synchronous Number of Operation Condensing Starts ,844 33, ,202 30, , , ,965 26,602 1, ,189 22, ,141 20, UTE - URUGUAY - Table 5 2 X 140 MW Gas Turbine Generating Sets Operating Hours 1st. Yr. of Generating Synchronous Number of Operation Condensing Starts ,979 4, ,973 7, As utilities in the USA and in other countries become further deregulated, future sources of generation and future loads will be less predictable in advance. It seems desirable, therefore, that if a clutch is not initially installed in peak load plant, that consideration be given to at least allowing space for a clutch to be installed to permit easy conversion for synchronous condensing operation in future years. generation/transmission changes which will occur over the lifetime of these peak generation units. References 1. "Ancillary Services"; Ancillary Services Business of the National Grid Company, Inc.; March, "Application and Design of A.C. Generators for Turbine Drive"; Fogwill, T.J.; Brush Electrical Machines Ltd. 3. "Clutches for Synchronous Condensing"; "Diesel & Gas Turbine Progress" magazine, March, "Clutches Give New Lease of Life to Gas Turbine Generation"; Simmons, C.R.; SSS Gears Limited. 5. "Correcting Mill Power Factor With a Synchronous Condenser"; Becker, Kevin; 1991 Engineering Conference. 6. "Highlights of the B.C. Hydro Burrard Generating Units Operating As Synchronous Condensers"; LeFrancois, M.J. and McGough, John C.; "Proceedings" of the American Power Conference, "Rebuild FT-4 Powerplant to Save Austin a Ton of Money"; "Gas Turbine World" magazine, May-June, "Synchronizer - The ABC of Power Factor"; Oscarson, G.L.; Publication No. 200-SYN-50, "The Economics of Electric Utility Power Factor Pricing"; Petniunas, Raymond V.; American Society of Mechanical Engineers, Publication No. 84-Mgt-9; Presented at Energy Sources and Technology Conference and Exhibition, February, "The Return of the Synchronous Condenser"; Clark, Donald L.; Scholl, Paula L.; Sargent & Lundy; Presented at the American Power Conference, April, "Incorporation of Synchronous Condensing Capabilities in Large Turbo-Generators"; Mukherji; New Brunswick Electric Power Commission; Presented at the Canadian Electrical Association, May, CONCLUSION In summary, peak load gas turbine generators are ideally suited for synchronous condensing duty when sited near centers of high inductive load and/or at the end of long transmission lines. The proportionate small cost for the addition of a synchronous-selfshifting clutch provides a way to obtain a greater utilization of the high capital investment of these machines and offers greater operational flexibility to meet the inevitable