Abstract— This paper is written to study three projects

using synchronous condensers to control system voltage,

which are Granite Substation, MTA (Tennesse Valley

Authority) grid and Basslink HVDC. These projects are

using only synchronous condensers not any other FACTS

devices because synchronous condenser provides

additional features with dynamic reactive power. This

paper has also discussed structure, working and

performance of two new synchronous condensers. One is

developed by GE, which is also called next generation

synchronous condenser and other is developed by

American Superconductor that is called SuperVAR. Both

these synchronous condensers are good to provide

dynamic reactive power and have their own special

features.

Index Terms—condenser, Reactive Power Control, capacitor,

dynamic synchronous condensers, FACTS (Flexible AC

Transmission System), Inertia, reactive power, Super VAR,

SVC ( Static Var Compensator ) , Low voltage ride through

(LVRT).

CASE STUDY 1

I. INTRODUCTION

ERMONT Electric Power Company (VELCO) upgraded

their transmission project to meet the increasing demand

of electricity. These upgrade projects will do the following: 1)

improve the reliability of the electric system, 2) preserve

Vermont’s future energy options, and 3) contribute to

economics stability of the state.

II. ONE OF THE UPGRADED PROJECTS AND PROBLEM

Northern Vermont Reliability Project (NRP), includes

construction of 36 miles of 345 KV transmission lines

between existing line with new 115 KV transmission line,

and the upgrade of 13 stations between Williamstown and

Barre. The Granite 230 KV /115 KV station is located in

Williamstown, VT.

Second problem is that 50% of the summer peak load is

met by in-state generation and in northern Vermont, about

90% of the peak load is supplied with imported power

through the Highgate HVDC terminal on the north, the Sandy

Bar phase angle regulator to the west of Burlington, and the

230 KV line from New Hampshire via the Granite station on

the east side of the state.With a forecasted summer peak load

of 1200 MW, the Highgate HVDC back-to-back terminal out

of service, and with the subsequent loss of the 345 KV line

between Coolidge and Vermont Yankee, the flow on the 230

KV line connecting Granite 230 KV to Comerford 230KV

could reach 300 MW. To maintain an acceptable voltage at

Granite, approximately 180 MVArs is required with a portion

being dynamic compensation. Otherwise, voltage will

collapse.

III. DYNAMIC COMPENSATION EQUIPMENT

Several dynamic compensators are static var compensator

(SVC), a STATCOM and synchronous condensers.

IV. SELECTION CRITERIA

A wide range of factors were considered: equipment cost,

cost of losses, system harmonics, steady state overload, short

time overcurrent, low voltage ride-through capability and

interaction with other static devices with fast acting controls

on the VELCO system. From SVC static var compensator

(SVC), STATCOM, shunt capacitor bank and synchronous

condenser, Synchronous condenser is selected because of the

following advantages:

a) Synchronous condensers are harmonic free equipment.

b) Condensers are readily stimulated in load flow and stability

studies.

c) Condensers are active devices and can stabilize the local

power system by contributing short circuit current.

d) Synchronous condensers have internal voltage and

mechanical inertia that allows the synchronous condenser

to increase MVAr output during voltage dip because of

local fault.

e) Output of the condenser will never drop abruptly like other

static equipment during local fault.

f) Have inherent overload capacity.

Upgrading of the Granite station includes four (4) +25/-

12.5 MVAr synchronous condensers and four (4) 25 MVAr

115 KV shunt capacitors. Two condensers are connected to

each tertiary winding of two autotransformers. Tertiary

winding of autotransformer is rated 100 MVA to

accommodate 4 condensers, if needed.

Voltage Control through Synchronous

Condensers

R. Sharma and R.K. Varma, Senior Member, IEEE

V

Fig. 1 One line diagram of Granite Substation pixel (grayscale)[1].

V. GRANITE RPD SYSTEM PERFORMANCE

A. Steady State Rating

Each Granite synchronous condenser ranting is +25/-

12.5 MVAr continuous at 13.8 KV. Total supply is 100

MVAr (-Q) and absorb up to 50 (+Q), when four condensers

are used. Nominal Operating range of the granite condenser

unit is 150 MVAr. Condensers are connected to tertiary

winding of the transformer because of this MVAR deviates

slightly from the nameplate.

Fig. 2 Granite Condenser Q-V profile (1 p.u. service factor, graph not to

scale)[1].

Above graph (2) shows the working range of

synchronous condenser by Q-V graph from .90 to 1.1 p.u.

Upper graph (2) shows the overexcited operation and range of

supplying MVAR and lower graph (3) shows the absorption

of MVAr. Granite condensers have 1.15 service factor, which

means current is 1.15 p.u. of rated current at 1.0 p.u. terminal

voltage in the overexcited operation. At 1 p.u. voltage at

granite 115 KV, the four condensers can deliver up to +106/-

48 MVAr, which is slightly above the nameplate of

synchronous condensers

Fig. 3. Approximate Q-V profile at Granite 115 kV with 1.15 service factor and

the PSTs at maximum phase shift [1].

B. Dynamic Performance

Dynamic performance shows how synchronous condenser

reacts when there is a fault in the transmission line.

Fig 4. PSLF simulation of fault clearing on critical 345 kV line[1].

Above figure (4) shows that initially voltage of condenser

(blue) and 115 KV transmission line (red) were at 1.02 p.u. ,

but after the fault at 1 sec voltages drop up to .8 for .25 sec

and after that voltages recover back to 1.01 p.u. within 1 sec.

This is the performance of synchronous condenser which

helps the system to attain its normal voltage after few cycles

of fault.

VI. SYNCHRONOUS CONDENSERS

One of the unique characteristics of Granite synchronous

condenser is overloading capability on both steady state and

transient state. Synchronous condenser which is used in

granite substation was made by GE.

A. Features of synchronous condenser

In low-ambient temperature, the cooling capability of the

TEWAC (totally enclosed water-air cooling system with

50%/50% ethylene glycol) increases with increase in +MVAr

above the rating as shown in table below.

Fig. 5 Low-Ambient temperature condenser rating [1].

In elevated steady state, overloading capability increases for

short duration as shown in below table and in which under

excited capability will never change.

Fig. 6 Condenser short-time overload capability [1].

B. Excitation System

Next- generation condenser excitation system has:-

1) An alternator and rotating Diode bridge protected by

metal-oxide varistor- gated crowbar SCRs.

2) GE EX-2100 digital voltage regulator.

3) Digital relays for protection.

Excitation system for the Granite synchronous condensers are

designed for nominal 300 % excitation for 3 seconds.

C. Cooling

GE synchronous condensers utilize TEWAC (totally

enclosed water-air cooling) system. A radiator located above

the stator winding within the condensers enclosure cools

warm air from stator. A solution of water/glycol circulates in

the radiator that is cooled outside the condenser building in

three air cooled heat exchanger per condenser.

D. Switchgear

Per Fig. 1, a tiebreaker between the two autotransformer

will maximize the operating flexibility of the Granite RPD

together with twelve 15 kV class metal enclosed vacuum

circuit breaker. Any number of synchronous can be connected

to auto transformer.

Installation of circuit breaker near salient-pole synchronous

condensers contributes to higher short circuit currents and

transient recovery voltages (TRV) especially when the

condensers are overexcited [1]. Both short circuit current and

TVA requirements led to the selection of granite circuit

breaker.

E. Surge Protection

Surge protection is needed to protect from high TRV

frequencies. Granite substation MVA rating is several

multiple of condenser MVA rating thus minimizing the

transformer leakage and contributing to high TRV

frequencies. Every synchronous condenser in granite

substation is protected with surge arrestors and surge

capacitor.

F. Lubricant

In normal operation, oil is circulated in bearings via a pump

with a back-up unit.

VII. DISADVANTAGE

A. Static Var Compensator (SVC)

1) Produce harmonics.

2) Control interaction in between multiple electronic

device creates problem.

3) Does not posses any short term over current

capability.

4) Output MVAr is dependent on nominal voltage.

5) Output power is equal to square of the voltage

terminal.

B. STATCOM

1) High cost.

2) High audible noises.

3) Are designed to shut off in case to large voltage sag.

C. Disadvantages of Synchronous condensers

1) Maintenance of synchronous condenser because of

moving parts.

2) Slower response time than SVC and STATCOM.

3) Higher level of losses.

CASE STUDY 2

I. INTRODUCTION OF SUPERVAR

High Temperature Superconductor (HTS) SuperVAR

dynamic synchronous condenser (DSC) was developed by

American Superconductor. This synchronous condenser has

small foot print, easily transportable and is economic option

for providing peak and dynamic reactive compensation to a

power system. Transmission line voltage collapse when local

sources are not able to meet the reactive power demands.

Voltage of the transmission line drops and line current

increase, which further drops the voltage of transmission line

more. For these situations, static VAR devices are used

because their output varies in proportion to V or V2 – thus

making less efficient. DSC output is independent of system

voltage and can be increased up to 4 times in a second by

changing its excitation on the field winding.

Fig. 7 +/- 8 MVAR SuperVAR machine with key characteristics highlighted[3].

II. SUPER VAR DYNAMIC CONDENSER FEATURES

a) Field current of a conventional machine must be

increased by 3X between no- load and full- load, where

as High temperature superconductor (HTS) wire in the

rotor field winding requires only a small change in field

current between no-load and full load and it always

operate at a constant temperature.

b) HTS DSC is estimated to be 98.8% efficient, typically 1

% more efficient than copper based units and it

maintains this efficiency down to partial load of 25 %.

c) New DSC is expected to be very economic option for

providing peak and dynamic reactive compensation to a

power system.

d) DSC is highly reliable because in case of fault, it can

provide twice as nominal rating for about one minute

(peak rating) without change in temperature of winding.

e) DSC can also handle transients.

DSC support system consisting of the following subsystem:-

a) HTS Rotor

b) Stator lubrication and cooling systems

c) Refrigeration

d) Exciter, Control and Communication system

e) Start-up motor and controller.

A. Cooling

Cryocooler is a device used to reach cryogrnic temperatures

by cycling certain gases. Liquefied gases are liquid nitrogen

and liquid helium. In DSC helium gas is used.

Fig. 8 DSC support system [3].

B. Characteristic of HTS DSC VS Synchronous Condensers.

Figure (9) is comparing performance of SuperVAR with

conventional synchronous condensers. SupeVAR is taking

less excitation current form zero load to full load performance

where as conventional synchronous condenser is taking three

times excitation current to perform the same.

Fig. 9 V-curves for conventional and SuperVAR machines

Above fig represents that 3 times more excitation current is needed by

conventional synchronous condenser for the same performance as by HTS

DSC[3].

III. FACTORY TEST RESULTS

DSC machine was factory tested according to IEEE 115

standard and results are listed below:

a) Measure 98.8% efficiency.

b) Temperature rise curve shows that at rated load,

temperature of DSC is always less than allowed limit

(105 oC at a 25 oC ambient).

Fig. 10 DSC heat run –temperature rise of various stator components[3].

c) Open short circuit and short circuit results show that

machine exhibit any saturation effect over the operating

range (1.3 p.u. ).

Fig. 11 Open-circuit and short- circuit measurements on the

DSC[3].

IV. SIMULATION POWER RESULTS

Fig. 12 A simulation of a utility system power quality problem concerned with a

DSC[3].

Whenever a large motor starts it causes optional voltage drops

which is cover up by installing 8 MVAR Super VAR DSC

which increases local MVAR capability to mitigate large

motor starting voltage sags and other transient voltage

problems.

V. RESULTS

Fig . 13 A Simulation of power quality problem fixed by DSC – shows voltage

variation with and without DSC and MVARS contributed by DSC[3].

Above figure (13) show the working of HTS DSC working in

grid.

VI. ARC FURNACE TESTED

HTS SDC helped in reducing flicker caused by arc furnace.

MVARs supplied by the machine during a typical melt cycle

is shown in below figure

Fig. 14 MVARS supplied by HTS synchronous condenser during an Arc

Furnace Burn cycle[4].

The machine was absorbing high negative and zero

sequence current and producing heat, which was sunk by

damper winding in the form of copper shell.

VII. FIND FARM APPLICATION (SIMULATION FOR 78 MW

MIDWESTERN U.S.)

Simulation was performed to study the performance of

synchronous condenser on wind farm. Wind farm had

inadequate voltage regulation and low voltage ride though

capabilities (LVRT), which resulted in tripping of circuit

breaker before implementing HTS synchronous condenser.

Approximately, 40 MVAR of additional capacitive equipment

was needed to meet the voltage regulation and power factor

requirement.

To improve low voltage ride though (LVRT) capability of

virtual farm, two 12 MVA HTS dynamic synchronous

condensers were used. These condensers were connected to

34.5 kV bus via step up transformer of 13.8 kV.

Fig. 15 Wind farm with two HTS Dynamic synchronous condensers units

installed to improve LVRT [4].

With the installation of HTS synchronous condenser, LVRT

capability of solution was significantly improved.

Fig. 16 Wind Farm Bus Voltage and MW Output with and without superVAR

[4].

Above figure (16), shows the working and performance

characteristics of HST condensers. First figure shows the bus

voltage with and without condensers. During LVRT (without

HST synchronous condensers) voltages dropped to tripping

condition, where as with synchronous condenser voltage did

not drop to tripping condition. Second figure (16) shows

output of wind farm, at the condition of LVRT, voltage

dropped and tripped the farm and output dropped to zero, but

with synchronous condenser this output power not dropped to

zero.

CASE STUDY 3

I. INTRODUCTION

The Basslink HVDC interconnection is a connection

between the island of Tasmania and the mainland of

Australia for electricity exchange. Basslink is a 290 km

undersea HVDC link, which export 630 MW and import 480

MW. This HVDC link is monopolar with rated DC voltage of

400 KV, rated DC current of 1250 A and rated continuous

power of 500 MW. Dynamic power transfer capacity is 626

MW from Tasmania to Australia. Tasmania is expected to

export 630 MW energy to Victoria, generated through

Tasmania hydro generation and 140 MW of wind generation.

It also imports 480 MW of energy from Victoria to Tasmania.

To provide reactive power demand of the DC converter for

630 MW, a total of 313 MVAr shunt capacitance is required.

The available compensation was subdivided into five filter

sub-banks of 43 MVAr and one additional 98 MVAr to

comply both with the limitation on AC voltage change due to

filter switching and specified flicker limits. All five filters are

tuned to remove harmonics ( 3,5,7,11,13,23,etc), but 98

MVAr is specially tuned to attenuate the effect of 5th

harmonics. Triple tuned harmonic filters were foreseen at

George Town converter station to meet the specified

harmonic performance requirements.

Fig. 17 Representation of HVDC connection of Tasmania to mainland

Australian and interconnection into eastern seaboard national electricity market

[6].

Although 500 kV AC system at Loy Yang is very strong in

comparison with George town, then still reactive power is

needed to be provided. HVDC link is capable of operating at

.95 lagging power factor. Two types of triple-tuned filters,

each of 105 MVAr rating are used to provide filtering of

harmonics.

Fig. 18 Key physical components of Bsslink[7].

A. Power Transmission Capacity

Basslink HVDC has continuous rating of 500 MW (+/-

400kV , 1250 A) defined at DC rectifier side with nominal

range and maximum ambient temperature. The rated

transmission power will be met without any redundant

cooling system until maximum ambient temperature 30oC at

George Town and 40 oC Loy Yang.

B. Performance Requirement of HVDC link

Design of HVDC link was designed to have low loss for

technical and economical optimization, to achieve this

converter at both stations should have total loss less than 1.4

percent at 500 MW power transfer. At rated transmission

capacity the main source of losses is DC cables and DC

overhead lines with approximately 2.7 percent and 1.0

percent respectively.

II. PROBLEM AND SOLUTION

Inter link was suffering from two problems.

a) Rotating Inertia

b) Short circuit strength

A. Rotating Inertia

Thermal power sources provide inertia to the power system,

whereas renewable sources are not. Wind turbines can

provide inertia to the system, if they are used properly.

System inertia in Tasmania can be low enough, especially at

the time of import during low demand. System also faces

shortage of fast acting frequency control ancillary services

(FCAS) because of slow response of hydro generators to

frequency.

―Transend‖ studies indicate that rate of change of frequency

approaches 3 Hz that can lead to HVDC link trip because of

import of energy during low system inertia[6]. As more

renewable sources like wind farm attached to the system,

FCAS demands will increase. As situation is tight, Tasmania

is used Basslink to transfer FCAS from mainland, which

reduces power transfer capability of HVDC link. ―Transend‖

also identified potential future voltage stability issues because

of future load increase. In this case SVC’ and STATCOM are

not good because they only provide only voltage stability not

any support to inertia. Solution to this problem is Modern

synchronous condenser, which can provide dynamic MVAr as

well as inertia.

B. Short Circuit Strength

For a three phase fault at local transmission bus, the first

approximation of the symmetrical short circuit current,

ignoring resistance, is determined by dividing the voltage

behind sub transient reactance by the sum of sub transient

reactance and transformer leakage impedance.

Fig. 19 One line diagram of synchronous condenser short circuit[6].

In case, when synchronous is floating i.e. not changing

MVAr with system at that time E is 1 p.u. During

overexcited operation E is greater than 1 and MVAr is

supplied by synchronous condensers. A higher short circuit is

also expected. One of the ways to improve synchronous

condenser performance is by decreasing the impendence of

both the synchronous condenser and leakage transformer.

Initially, Minimum fault level was 1200 MVA, which was

not met after the installation of wind farm. After installing

four 25 MVAr GE synchronous condensers, minimum fault

level was 1440 MVA.

III. FACTS DEVICES

a) SVC

b) STATCOM

c) Synchronous condenser

SVC and STATCOM cannot be used in Basslink HVDC

because utility is facing problem with inertia and short circuit

level, which SVC and STATCOM does not control. To solve

this problem with dynamic reactive power control,

synchronous condensers are the best options.

IV. COCLUSION

A. Case Study 1

In granite substation many factors are considered before

placing synchronous condensers instead of other FACTS

Compensators. Some of the factors are harmonics, costs,

steady state overload, short time overcurrent, low voltage ride

through, interaction with other static devices, etc.

Synchronous condenser is selected because it is harmonic free

device, readily simulated in load flow studies, internal inertia

and internal voltage allows the condenser to readily increase

MVARs and during fault, output of synchronous condenser

never drops abruptly. After considering all factors and

dynamic performance, then next generation GE synchronous

condensers is installed at Granite Substation. Total four

condensers are installed in Granite Substation to support 180

MVAr requirements for normal transmission operation.

Results shows that in case of fault, synchronous condensers

are working and uplifting the dropped voltage within few

cycles.

B. Case Study 2

American Superconductor developed a new synchronous

condenser, which is also called SuperVAR or High

Temperature Superconductor DSC (Dynamic Synchronous

Condenser). This condenser is unique because of its some

features. DSC output voltages can be increased up to 4 times

in a second by changing field excitation with a faster exciter.

In case of other FACTS devices except synchronous

condensers, output voltage controlled by that device is

dependent on system voltages. When fault occurs, FACTS

devices performance also gets affected. This synchronous

condenser is installed at TVA ( Tennesse Valley Authority)

grid, which was suffering for harmonics generated by arc

furnace situated near grid. After installation of SuperVAR

there is no disturbance in grid because of harmonics and low

voltage. All the generated harmonics are suppressed by

SuperVAR. HTS Dynamic synchronous condenser is also

installed in 78 MW Midwestern (U.S.) because system was

suffering from LVRT (low voltage ride through) and in this

case system voltage was dripping so low that it was causing

whole system to be shutdown. After installing SuperVAR,

system voltage is recovering before it will lead to shutdown.

Synchronous condenser has small foot print, are readily

transportable and economic option for providing peak and

dynamic reactive power.

C. Case Study 3

In basslink HVDC, synchronous condenser is installed for

voltage control as well as for two special purposes. First, for

providing inertia and second, to control short circuit level.

Basslink HVDC is constructed between island of Tasmania

and the mainland of Australia for electricity exchange. This is

a second largest underwater cable i.e. 290 KM. During

transfer of electricity from Mainland to Tasmania at low load,

system inertia is low. Renewable resources have less inertia.

Synchronous condenser is only device that can provide

control over inertia, short circuit level, overloading capability

and dynamic voltage control together. Four synchronous

condensers are installed for proper working of the HVDC

system and improving power transfer capability.

D. Overall from all three case studies

It is concluded that after having all other fast dynamic

FACTS devices, still synchronous condensers are used in

utilities. Synchronous condensers have some special feature

for which it is used. Installation of synchronous condenser is

economical and can provide dynamic reactive power. It can

provide control over inertia, short circuit level and provide

overloading capability without harmonics, which any other

FACTS device cannot provide.

REFERENCES

[1] J. Skliutas , LaForest, R.D’Aquila, D.Derr and E.

Kronbeck,‖Next-Generation Synchronous Condenser

Installation at the VELCO Granite Substation‖.

[2] Paul E. Marken, M. Henderson, LaForest, J. Skliutas, J.

Roedel and Todd Campbell, ― Selection of synchronous

condenser technology for the Granite Substation‖.

[3] S.S. Kalsi, D. Madura and Mike Ingram,‖

Superconductor Synchronous Condenser for Reactive

Power support in an Electric Grid‖, IEEE Trans. On

Applied Superconductivity, Vol. 15, No. 2, June 2005.

[4] S. Kalsi, D. Madura and Mike Ross,‖ Performance of

Superconductor Dynamic Synchronous Condenser on an

Electric Grid‖, IEEE/PES Transmission and Distribution

Conference and Exhibition: Asia and Pacific Dalian,

China.

[5] S.S. Kalsi, D. Madura, G. Snitchler, M.Ross, J. Voccio

and M. Ingram,‖ Discussion of Test Results of a

Superconductor Synchronous Condenser on a Utility

Grid‖, IEE Trans. On Applied Superconductivity, VOL.

17, NO. 2., June 2007.

[6] Paul E.Marken, Arthur C. Depoian, John Skliutas and

Michael Verrier, ― Modern Synchronous Condenser

Performance Considerations‖, 978-1-4577-1002-

5/11/IEEE.

[7] Th. Westerweller and J.J. Price,‖ Basslink HVDC

Interconnector- System Design Considerations‖, IEEE.