{FREQUENCY RELAY}

I. INTRODUCTION
Nowadays, distributed generation has recently gained a lot of attention related to its connection
to distribution network (Xu et al., 2004). Although distributed generation units have many
benefits such as stability and economy, it suffers from some critical problems that may affect
these benefits. One of these problems is “islanding phenomena”. So far, frequency relays have
been recognized as one of the most sensitive and economic devices used for islanding detection
technique. However, the settings of these relays should be carefully selected to avoid false
tripping .To avoid this problem it is required to balance between sensitivity and dependability.
Variation in performance of commercially available frequency relays lead to difficulty in
selecting the right setting of these relays, despite being subjected to the same network
disturbance. In this paper different operating responses of frequency relays are being
investigated as a result of change in the used internal algorithm, i.e. frequency measuring
technique, measuring windows, time delays and under-voltage interlock function.

II. Overview of frequency relay

An equivalent circuit of a distributed generator with frequency relay operating in parallel with a
distribution utility is showed in Fig. 1. The distributed generator (DG) is inserted to feed a load.
The distribution utility may provide or consume power, this depend on power supplied by
distributed generator. Therefore, the system frequency remains constant. If the circuit breaker
(CB) opens, due to a fault for example, the system composed by the distributed generator and
the load becomes islanded. In this case there is active power mismatch between generation and
consumption due to, the loss of main power from utility.

Fig. 1. Equivalent circuit of distributed generator equipped with frequency relay operating in parallel with
utility.

This mismatch in power causes transients in the islanded system and the system frequency
starts to vary dynamically. The islanding condition can be detected easily depending on this
mismatch or imbalance in active power between source and load. Thus frequency relay can be
an effective method in islanding detection in this situation. The generic Frequency relay
computational model is illustrated in Fig. 2

Usually, frequency relays calculate system frequency considering a measuring window over a
few cycles and the resulting signal is processed by filters. This process is being implemented by
using first order transfer function (1/(1 + sTf)) designed to eliminate high frequency transient
where the time constant Tf represents both the time constant of the filter and the adopted
measuring window. The resulting signal fr is then compared with the relay settings (i.e.), if it is
larger (smaller) than the over frequency (under frequency) settings β1 (β2), the frequency relay
send a trip signal to the generator circuit breaker. However, network transient events may cause
change in system frequency resulting in mal operation of frequency relay which should be more
investigated. In systems with multiple distributed generations, false operation of distributed
generators may directly disturb reliability of the system . In addition, commercially available
frequency relays from different manufactures respond rather differently to the event, even
when they are configured with the same setting . This happens because of different factors
affecting the relay operation, which can be one of the following:

· Frequency measuring techniques

· Time delay

· Measuring windows

· Under voltage interlock

Fig. 2. Generic frequency relay computational model

III.Frequency relay performance with multiple distributed generators

Fig. 3. shows utilized distribution network to analyze cases with multiple distributed generators.
This system comprises 132 kV, 50 Hz grid with short circuit level 1500 MVA which feeds 33 kV
bus bar through two parallel 132/33 kV transformers. In this system there are two identical
synchronous generators, both with 10 MW connected at buses 3&5. Each generator is equipped
with frequency relay and circuit breaker. This simulation aims to investigate the possibility of
interference between various frequency relays in terms of both dependability and security.

Fig. 3. Single line diagrams of multiple distributed generators.

(Frequency relay SPAF 340 C)

Application:
The frequency relay SPAF 340 C is used for load shedding in situations where power
consumption exceeds the available power of the network. In such a situation of unbalance, the
network frequency tends to fall. The frequency relay can control four circuit breakers, allowing
four feeders to be disconnected from the network, one by one. Should the power deficiency still
persist, the relay disconnects the plant for island operation. The operation of the relay can be
based on set frequency values (f<), on the rate of decrease of frequency (negative df/dt), or on
both criteria (f< and df/dt). In addition, output of each stage can be obtained via two separate
time circuits and so eight different frequency/time combinations are available. Should the
network frequency drop rapidly, this feature allows fast disconnection of various loads.

Design:
The frequency relay SPAF 340 C is a secondary relay, which is connected to the voltage
transformers of the network section to be protected. The relay incorporates one frequency relay
module type SPCF 1D15, which includes a definite time overfrequency and/or underfrequency
unit and a rate of change of frequency unit. The relay module includes four frequency stages.
Each frequency stage can be set to operate as an overfrequency (f>) stage or an underfrequency
(f<) stage with definite time characteristic. Further, each stage can be set to function as a rate of
change of frequency (df/dt) stage. When the start frequency of a stage is set below the rated
frequency, the stage operates as an underfrequency stage. Correspondingly, the stage has the
function of an overfrequency stage, when the start frequency is set above the rated frequency.
The frequency setting cannot be the same as the rated frequency. The operation of the df/dt
function of a protection stage is based on the same principle as the frequency function, which
means that if a stage operates as an underfrequency stage, the sign of the df/dt function is
negative. Then the df/dt function starts once the absolute value of the rate of frequency drop
exceeds the set df/dt value. When required by the application, the definite time principle and
the rate of change principle can be combined so that the criteria for operation of both functions
have to be fulfilled at the same time to enable operation of the stage. Once a preset condition is
fulfilled, the stage starts and, at the same time, it activates a timing circuit. When the stage
times out, the relay produces a trip signal. The trip signal can be assigned to the desired output
relay.

Data communication:
The feeder protection relay is provided with a serial interface on the rear panel. By means of a
bus connection module type SPA-ZC21 or SPA-ZC 17 the feeder protection relay can be
connected to the fibre-optic SPA bus. The bus connection module SPA-ZC 21 is powered from
the host relay, whereas the bus connection module type SPA-ZC 17 is provided with a built-in
power unit, which can be fed from an external secured power source. The relay communicates
with higher-level data acquisition and control systems via the SPA bus.

{REVERSE POWER RELAY}

1. INTRODUCTION
Protection relays play a very important role in the safe and reliable operation of power system.
Insecure or failed protection systems may make the situation worse and lead to the system
blackouts. All faulted conditions do not lead to such situations. A fault that causes such
situations include N-1 contingency of line, overloads, reverse power flow (loss of mechanical
input) and others. A typical protection scheme is an arrangement of various types of relays such
as over current, short circuit relay, over-under voltage, over- under frequency relays and others.
In 90’s, most of the relays in power system were electromechanical, later on replaced with solid-
state. Now both types of relays are being replaced with digital relays. Digitals relays offer
advantage of fast in operation, small in size and reliable in operation in case of power system
fault. The relay also offers advantage in terms of their sensitivity andwide range controlling .

2. REVERSE POWER RELAY

Reverse Power Relays (RPR) are commonly used in power system for detecting motoring action
of synchronous generator. This condition normally occurs when the prime mover (engine or
turbine) fails, however the field winding is still connected with the excitation system. This
resulted in motoring action and the machine behaves like a synchronous motor connected with
large power system. In such condition, the turbines become the active load on that machine.
Motoring action draws power from the system to drive the prime mover and can cause severe
damage to the prime mover. This condition is not desirable and there is an objectionable
temperature rise in case of steam turbine. Therefore such conditions need to detect quickly and
the GCB should be tripped. Diesel engines and gas turbines are less susceptible to immediate
damage, but unburned fuel may present a fire or explosion hazard.

3. Reverse power relay Construction and operation

The relay is made of a lightweight non-magnetic aluminum disc between two soft laminated
iron core electromagnets, and fixed on a spindle running on low friction bearings. The upper
electromagnet is wound with a voltage coil which is then supplied from one phase and an
artificial neutral of the generator output. The other magnet has a current coil from supplied
from a current transformer connected to the same phase as the voltage in the upper
electromagnet.

The voltage coil has a high inductance, designed in a way that the voltage lags the current in the
coil by about 90 degrees. This lag ensures that the magnetic field generated from the current in
the upper coil lags the magnetic field produced by the current in the lower electromagnet.

The two magnetic fields which are out of phase, produces eddy current in the aluminium disc,
and this creates a torque that tries to rotate the disc.

Under normal condition when power is flowing as expected, the trip contacts of the relay are
open, and the disc is against a stop. If a reverse power starts to flow, the disc rotates in the
opposite direction, moves away from the stop and towards the trip contacts that activates the
trip circuit.
Figure 1: Construction of a reverse power relay

Most of the reverse power relays have adjustable settings to allow the customer do the settings
according to the installed equipment. The trip point is usually adjustable to between 2 and 20
percent of the input current while the time delay is adjustable from 0 to 20 seconds.

A 5 second time delay is often used to avoid tripping the circuit during synchronization. In most
practical applications, the reverse power settings are between 8 to 15 percent for diesel engines
and between 2 and 6 percent for turbine power movers.

4. PRINCIPLE OF REVERSE POWER RELAY

A reverse power relay is a directional relay that is used to monitor the power flow from
generator (running in parallel with another generator or the utility) and in case of abnormal
condition take appropriate action. Under abnormal condition, the direction of power changes
from the bus bar into the generator. This condition normally occurs when prime mover fails. The
real power drawn from the grid is quite small compared with the generator rating. However
stator current undergoes 180" phase shift normally referred as Maximum Torque Angle (MTA)
this suggests that if we use a directional relay with MTA of 180" (using generator phase angle
conventions) then it could detect the loss of prime mover as the current phasor would reverse
and enter the trip region. However the magnitude of this reversed current phasor is quite small
compared to the forward current as the generator draws just enough real power to meet the
losses and drive the turbine. Hence, the directional relay for detecting the loss of prime mover
needs to have a high degree of sensitivity compared to directional relays used for over-current
application. The installation of RPR on a power system is shown in Fig.2

Fig.2 . Reverse Power Relay in Power System

For applications where a protection sensitivity of better than 3% is required, a metering class CT
should be employed to avoid incorrect protection behavior due to CT phase angle errors when
the generator supplies a significant level of reactive power at close to zero power-factor. The
reverse power protection should be provided with a definite time delay on operation to prevent
spurious operation with transient power swings that may arise following synchronization or in
the event of a power transmission system disturbance.

5. Direction of Power and Current during

the Reverse Power Detection Fig. 3(a) shows the concept of three phase current direction and
sign of cos  and power factor (PF) and (b) shows the same concepts, but on a PQ-power
plane, respectively.

Figure 3. a) Quadrants of voltage/current phasor

plane
b) Quadrants of power plane

6. DATA COLLECTION SYSTEM

Reverse power data are collected by using the generator protection relays. The relays are also
able to detect disturbances. When these disturbances occur, all digital and analogical signals are
stored in its memory, including the pre-fault, fault and post-fault intervals. Disturbance recorder
sampling rate is selected as 100 S/s. Hence, total data number size is 796 S. In Fig. 4, the reverse
power data measurement system is given generally.

Figure 4. Reverse power data collection system

7. Advantages of reverse power relay

· Prevents power from flowing in the reverse direction and damaging the generator stator

· Prevents fire or explosions that may be caused by unburned fuel in the generator
· Prevents damage to the prime mover