Synchronous Condenser

SCs have been used to support electric power systems almost as long as power systems have existed. SC as a synchronous machine is not a part of FACTS group. The SC is con- nected to the high voltage grid through a step-up transformer as well as synchronous generators (see Fig. 3.22).

The superior response times can be achieved by making use of a modern generator excitation system, namely rotating exciters. The rotating exciter, located at one end of the rotor shaft, is directly connected to the machine field without the use of brushes and slip rings. Since the brushes require periodic maintenance, which is influenced by the local service conditions, it can be counted as another advantage of SCs. IGBT bridge

is provided with a back-up source of power. This source is able to supply power for the bridge when AC power is not available, as in the case of a local three-phase fault. This allows the synchronous condenser the opportunity to provide reactive power support under more extreme conditions [67].

𝑈 _AC 𝐼 𝑈_T(𝐸_fd) Coupling Transformer Synchronous Reactance Exciter 𝑋_d

Figure 3.22.: The Basic Scheme of a SC.

Fig. 3.23 shows u_{− I and u − Q curves of the SC. The reactive power output in the right} half of in u_{− Q curve could be held constant as voltage decreases, up to a certain point.} The current needs to be increased in order to achieve the rated power at low voltages. This increase in current can be observed in the associated to the left half of u_{− I curve.} The limit of the low voltage depends on a number of system design factors including the SC itself, the control system, and even auxiliary components such as lubrication and cooling. Considering these facts, it can be seen that the SC performs similarly to the STATCOM. However, SC has the significant overload capability compared to STATCOM. For periods of a few seconds, it is possible for a SC to remain reactive power output constant as the voltage decreases [68]. The very compact STATCOM IGBT switches have very limited thermal buffers and therefore limited or no overload capacity [48].

Q = U

2

T− UTUAC

X_tot (3.15)

Assuming lossless operation and a solid pole machine, the reactive power can be calculated by Eq. 3.15 and is proportional to the terminal voltage U_T. The internal voltage U is controlled via the rotor excitation. Xtot represents the total connection

reactance consisting of machine reactance and transformer leakage reactance. During disturbances, a synchronous machine passes the sub-transient and transient stage. The effective inductivity of the machine decreases in order to maintain the internal fluxes. Therefore, following relation for the sub-transient reactance X00

d and transient reactance

X0 dis applicable: X00 d < X 0 d< Xd (3.16)

I_C,max I_L,max 1.0 0.8 0.6 0.4 0.2 u_T transient rating (a) u_{− I Characteristics} transient rating Q_C,max 1.0 0.8 0.6 0.4 0.2 uT (b) u_{− Q Characteristics}

Figure 3.23.: SC u_{− I and u − Q Characteristics [49, 57].}

As it is mentioned above SCs have greater overloading capability than STATCOMs. This is also indicated in Fig. 3.23. The amount of overload depends on the design and utilized technologies [68]. When a larger system disturbance occurs, the condensers can be- come over-loaded for few seconds, if necessary, until the shunt capacitors are switched. The capacitors then bring the condensers back within rated steady-state capability. For example, a newly built condenser is able to produce more than 200% reactive power beyond five seconds from the disturbance showing that SC can have very high transient ratings [67].

The SC rotates synchronous to the system frequency. It is therefore subject to the frequency deviations and can react to them like typical synchronous generator [49]. SCs can provide rotating inertia to a power system and can also increase system short circuit strength. These traits can be helpful as systems adapt to higher penetrations of renewable power sources such as wind or solar [68].

The voltage support capability and short circuit strength of SCs are together a very favorable combination for utilities in case of commutation failures especially in weak AC systems (e.g. HVDC applications) [48, 54].

SCs are characterized by relatively slow control responses because of their large field- time constants. Since SCs are the rotating devices, they require regular maintenance and become more expensive than equivalent-rating static compensators (due to slip rings or brushes in the converted SCs). Additionally, they need significant space especially for high var ratings so that it can be very costly [54].

The reactive power of the SC can be continuously controlled by varying its excitation cur- rent. When it is overexcited, it behaves like a capacitor, injecting reactive power into the system. The machine is normally excited at the base current when its generated voltage equals the system voltage; it thus floats without exchanging reactive power with the sys- tem. When it is underexcited, it absorbs reactive power from the system like an inductor.

The basic operating requirement of the excitation control system is adjusting the field current to maintain the desired voltage at the terminal of the machine. There are several excitation systems available as listed below [20]:

1. DC excitation 2. AC excitation 3. Static excitation

The control scheme of a general excitation system schematically shown in Fig. 3.24. DC excitation systems utilize DC generators as sources of excitation power. They provide DC power to the machine field windings and current to the rotor of the synchronous machine through the slip rings. DC excitation systems are gradually disappearing, as many older systems are being replaced by AC or static type systems [20].

u_AC u_F

u_err u_R e_fd u_T

Figure 3.24.: General Excitation Control System [20, 49].

The block diagram of IEEE DC2A excitation system is displayed in Fig. 3.25 and the related parameters are given in Table 3.3 (T_A and T_B are "0"). The resulting signal u_err is amplified in the controller. The major time constant T_A and gain K_Aassociated with the voltage controller. The controller is non-windup type having the limits of u_R,maxand

u_R,min that represent saturation or amplified power supply limitations. Additionally, u_C represents the compensated terminal voltage. The time constants, T_Band T_Cinherent in the voltage controller are small enough to be neglected [69].

u_AC u_F u_R e_fd u_x u_Rmax u_Rmin u_C

Regulator Excitation System

u_err

Table 3.3.: Selected Parameters of a SC Excitation System.

Parameter Definition Unit

K_A Controller Gain p.u.

T_A Controller Time Constant s

K_E Exciter Gain p.u.

T_E Exciter Time Constant s

KF Stabilization Path Gain p.u.

T_F Stabilization Path Time Constant s

E₁ Saturation Factor 1 p.u.

S_E(E₁) Saturation Factor 2 p.u.

E₂ Saturation Factor 3 p.u.

SE(E2) Saturation Factor 4 p.u.

u_Rmin Minimum Controller Output p.u.

u_Rmax Maximum Controller Output p.u.

The excitation itself usually is represented by an integrator block with feedback elements for saturation SE (Efd) and self-excitation KE. A signal derived from generator field voltage

is normally used to provide excitation system stabilization via the rate feedback block with gain K_Fand time constant T_F[69].

The feedback element block in the general excitation control system in Fig. 3.25 can include over- and under-excitation limiters and power system stabilizers [49].