Power Blackout Preventing By Power Swing Detection

Power Blackout Preventing By Power Swing Detection and Out-Of-Step Protection

 Power system disturbance, such as fault occurrence, automatic reclosing, and large loads disconnection. These conditions force the generating units to adapt with the new load condition but the generators can’t do this instantaneous due to the inertia. This cause a power swing. It may be stable power swing or un stable power swing. In the stable power swing the generator can return to the new equilibrium state. On the other hand, in the un stable power swing the generator can’t reach to the steady state operation and run out of step and may lead to the blackout. in the case of power swing the load impedance may enter into the operation characteristic of the distance relay and case unwanted tripping for the transmission lines. So, the distance relay isn’t supposed to tripe under the power swing if stable or unstable to give the system the availability to return to its steady state. A power swing block “PSB” it’s a function inside the modern distance relays that prevent unwanted tripping by block the tripping signal in case of stable or unstable power swing. However, when the fault happens due to a Power swing it must be cleared very rapidly with high level of dependability and selectivity. Out-of-step trip (OST) in is a function that included in the modern relays to achieve the separation of the power system under the unstable power swing. The major aim of this function to differentiate between the stable and unstable power swing and separate the power system into predetermined zones to achieve the stability and continuity of the service.

Key words: Power Swing Detection; Out of Step Protection; Distance Protection.

In the previous few years we have suffered from big disturbances in the power system which caused complete blackout and millions of users including the industry have suffered from big economical losses. These disturbances cause big fluctuations in active and reactive power, low voltage, voltage instability and angular instability between the generated power and consumed power which results in loss of generation and load which effected on both sides the power generation and the end customers.

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During the steady state operating condition, the power systems operates on the nominal frequency (50Hz or 60Hz) +/- 0.02 Hz and Voltage=Nominal voltage +/- 5% [1]. The complete synchronism of nominal frequency and voltage at the sending and receiving ends make complete balance of active and reactive power between generated and consumed active and reactive powers.

Power system faults, line switching, generator disconnection, and the loss of large blocks of load result in sudden changes to electrical power, which is due to the causes shown in Fig 1.

1.1. Blackout History

1.1.1 Some blackouts became in 2003 (during six weeks).

in the northeastern USA and central Canada and in Europe, which affected more than 100 million people.

On 14 August – the northeastern USA and central Canada:

• 62 GW power outage, affected 50 million people.

• power supply restoring took some days.

On 14 August – London

• 724 MW power outage, affected 410 thousand people.

• power supply restoring took 40 minutes.

On 2 September – the southern Malaysia

• affected 5 states (out of 13) in Malaysia, including the capital Kuala Lumpur.

• power supply restoring took 5 hours.

On 5 September – Birmingham

• 250 MW power outage, affected 220 thousand people.

• power supply restoring took 11 minutes.

On 19 September – nine US states and parts of Ontario, Canada

• affected 4.3 million people.

• cause – hurricane Isabel.

On 23 September – Denmark and Sweden

• affected 5 million people.

• power supply restoring took 4 hours.

On 28 September – Italy

• affected 57 million people.

• power supply restoring took 4 hours. [1], [2], [3]

1. Blackout in the Northeastern USA and Central Canada – 14 August 2003

Initially the event was the tripping of the power line that caused by the short circuit to ground due to tree contacts.

Blackout in the northeastern USA and central Canada was studied by The North American Electric Reliability Corporation (NERC) and some contravention of safety and reliability standards was identified:

• following the outage of the first 345-kV line, dispatcher of the power system did not make the necessary actions to return the system to a safe operating state within 30 minutes to fulfillment of “N-1″criterion which mean ”Most security rules therefore call for the system to be able to withstand the loss of any single component. When a power system satisfies this criterion, it is said to be “N-1 secure””

• dispatchers of neighboring transmission system were not informed about this situation.

• insufficient training of dispatchers.

• functionless monitor system in part of the power system.

Reliable and safety operation of the power system

The North American Electric Reliability Council (NERC) have developed the operating of the system and planning standards to confirm the reliability and the stability of a transmission network that are depend on seven key concepts:

1. Balance power generation and demand continuously.
2. Balance the reactive power supply and the demand to keep scheduled voltages.
3. Monitor the power flow over the transmission lines and other facilities to ensure that thermal (heating) limits are not exceeded.
4. Keep the system in a stable condition.
5. Operate the system so that it remains in a reliable condition even if a contingency occurs, such as the loss of a key generator or transmission facility (the “N-1″criterion).
6. Plan, design, and maintain the system to operate reliably.
7. Prepare for emergencies

2. Blackout in Italy on September 28, 2003

The situation in the Italian power system before blackout wasn’t exception.

• total load of Italy was 27 444 MW.

• 3 487 MW – pump load.

• 6 951 MW – physical import to the Italian power system.

• some transmission lines in neighboring power systems were out of service by reason of scheduled maintenance.

• the Italian power system was connected to neighboring power systems via 15 transmission lines.

Sequence of events of the blackout in Italy

The initial event was the 380 kV line tripping in Switzerland. Fewer than 25 minutes after this event the Italian power system ceased to operate synchronized to the UCTE system.” Union for the Coordination of the Transmission of Electricity”

03:01:42 – the 380-kV line tripping in Switzerland (Lavorno – Metlen) – line was heavy loaded at 86%.

• The cause of tripping – a wire contacted to a tree.

• The attempts of single-phase auto-reclosing and also the attempt of the operators to put this line back into operation were not successful and the line was disconnected by its protection device, due to high phase angle (42°).

• After the Lavorno – Metlen line tripping, the other 380 kV line in Switzerland (Sils – Soaza) became overloaded.

03:11 – the Swiss dispatch asked the Italian dispatch to reduce the import of Italy by 300 MW (on scheduled value).

03:25:21 – the second overloaded 380 kV line tripping in Switzerland (Sils – Soaza).

03:25:25 – the third overloaded 380 kV line tripping in Switzerland.

03:25:26 – the interconnection line Austria – Italy (Lienz – Soverenze) tripping.

03:25:33 – the Italian power system started to disconnect from the UCTE system After disconnection Italy from the UCTE system the frequency in the Italian power system dropped suddenly, caused by the negative imbalance between power injection to the system and system load. The blackout in the Italian power system became within fewer than 3 minutes.

21:40 – official announcement about emergency cancellation in whole power system of Italy.

Main causes of the blackout in Italy

The initial event was the 380-kV line tripping in Switzerland in consequence of the wire – the tree contact. The attempts of single-phase auto-reclosing and also attempt of the operators to put this line back into operation were not successful and the line was disconnected by its protection device, because phase angle was too high. We can see two causes:

• insufficient protective zone under the transmission lines.

• overloaded lines.

After the first 380 kV line tripping in the Swiss power system other power lines became overloaded. That means that the safety “N-1″criterion was non fulfillment in the Swiss power system.

We can see other causes:

• Violation of basic safety “N-1″criterion.

• high import to Italy.

• incomplete information about neighboring power system.

• pump load was stopped too late, by automatics.

1.1.2. System disturbance in the UCTE system on 4 November , 2006

The system disturbance in the UCTE system on 4 November, 2006 was the most serious incident in the UCTE system within an interconnected Europe history.

The system disturbance started in the German transmission system on 4 November 2006, around 10 p.m. This disturbance split the UCTE system into three separate parts (West, North-East and South-East). More than 15 million households were affected by an interruption of the electricity. The UCTE system resynchronization was completed 38 minutes after the splitting. [4]

What happened?

On 18 Sept. 2006, the shipyard (Meyerwerft) sent a request to E.ON Netz to disconnect two 380 kV line Conneforde-Diele for the transport of the ship through the Ems River to the North Sea on 5 November at 01:00. As a switching in the last several times. The E.ON Netz operator did a power flow calculation and verified fulfillment of safety “N-1″criterion using numerical computation. Analysis did not show any problem and so the operator provisionally approved the request of the shipyard and informed neighbouring transmission system operators (RWE – Germany and TenneT – Niederland). On 3 November (around 12.00) came a new request to E.ON Netz for a time change of two power lines Conneforde- Diele switching – on 4 November at 22:00. A provisional agreement was given by E.ON Netz after a new analysis. But RWE and TenneT operators were not informed about this change at the same time. Only at 19:00 on 4 November E.ON Netz informed TenneT and RWE TSO about the new time for switching off the Diele-Coneforde line.

At 21:29 according to the load flow calculation made by E.ON Netz did not indicate any violation of limit values. But “N-1″criterion was checked without numerical computation, was checked based on an empirical evaluation of the grid situation only.

Sequence of events

• At 21:38 a 21:39. power lines Conneforde-Diele were disconnected.

• At 21:39. after the switching operation, two 380 kV lines was overloaded.

• At 21:41. RWE dispatcher informed E.ON Netz about the safety limit value on the line Landesbergen-Wehrendorf (an interconnection line between E.ON Netz and RWE TSO). Later investigation uncovered different protections setting on this line.

• Between 22:05 and 22:07, the load on the 380 kV line Landesbergen-Wehrendorf increased and the RWE operator called E.ON Netz at 22:08 with the request for urgent intervention due to return to the stable grid condition.

• E.ON Netz made an empirical assessment of corrective switching measures without making load flow estimation for checking the “N-1″criterion. E.ON Netz expected that coupling of the bus-bars in the substation of Landesbergen would reduce of load on the 380 kV line Landesbergen- Wehrendorf. But this line was overload more than 100% and so was tripped and the other lines became overloaded.

• At 22:10:28 – the UCTE system splitted to three areas after the power lines switching in E.ON Netz, RWE, the Austrian power system and after disconnection of interconnection lines: Croatia – Hungary and Marocco -Spain. The initial event of the system disturbance in the UCTE system was scheduled switching off two power lines.

Main causes

The investigation identified two main causes of the disturbance as well as some critical factors which had significant influence on its course: non fulfillment of the “N-1″criterion or verified its fulfillment without numerical computation.

• insufficient co-ordination between the transmission system operators.

• dissatisfied power plant operation in emergency: tripping of generation units (particularly wind power plants) during disturbance and uncontrolled reconnection of generation units.

• limited range of action available to dispatchers.

• insufficient training of dispatchers.

1.2. Blackout Causes and Risks

A blackout is a power outage. This state means the loss of the electricity supply for a part of the power system or the whole power system.

A. Blackout Causes

Natural causes

• Lightening.
• Rain.
• Snow.
• wind storm.

Technical Failures

• Transformer faults.
• Short circuits.

Human error

• Error of judgment.
• Insufficient co-ordination between the transmission system operators.
• Insufficient training of dispatchers.

Terrorism

• Bombing.
• cyber-attacks.

B. Blackout Risks

• power system equipment damage.
• heavy economical losses.
• jeopardy of economy functioning.
• life paralysis in stricken parts of country.

Several blackouts cases became in the world of last years. Causes of these disturbances were various – technical, bad weather conditions, human failing.

C. Some impacts of Northeastern Blackout

Ford Motor Company

The stoppage of factory’s furnace cause to convert the molten metal to solid metal. The company reported that it need a week to clean the furnace.

Marathon Oil Corporation’s

The blackout was responsible for making suddenly shutdown procedures at Marathon refinery. in those procedures, a carbon monoxide boiler failed to shut down correctly, causing a small explosion. As a precautionary measure, the police vacate one-mile sector around the complex.

Daimler Chrysler

Daimler Chrysler lost production at 14 of its 31 factories. 6 of those were assembly factories with paint shops. The company reported that, in total, 10,000 vehicles were moving through the paint shops during the blackout had to be scrapped.

New York City

New York City’s mayor estimated that the city would pay almost USD 10 m in overtime related to the outage.

Airports

Airports were closed in Toronto, Newark, New York, Montreal, Islip, Cleveland, Erie and Hamilton. Together they cancelled over 1,000 flights

2.1. Introduction

Under steady state conditions the power generation and the load are balanced and the power systems operate in nominal frequency 50 Hz or 60Hz with some deviations 0.02Hz for the large system and 0.05 for the small system.

Taking into consideration the two-machines shown in Fig. 2, the power transmitting can be denoted by the following equation:

Where:

ES – is the voltage of machine S

ER – is the voltage of machine R

δ – is the angle by which ES leads ER = QS-QR

QS – is the rotor angle of machine S

QR – is the rotor angle of machine R

ZT – is the total impedance between the two machines consisting of ZS, ZR, and ZL

ZS + ZL + ZR

ZS – is the impedance of source S

ZR – is the impedance of source R

ZL – is the impedance of the transmission line

The Power angle curve in Figure 3 clears the relation between the power transmit and the angle δ (the angle between the two ends). The relation clear that the power transmit increase with nonlinear direct proportion with the angle δ when δ lie between 0° : 90° . After δ equal 90° the power decrease with nonlinear inverse proportion with the angle δ. The power system are worked well at the point of maximum power at δ = 90° so The maximum power is presented as the following equation:

Power Swing definition

It is a changing in the Electrical power due to the changing of the rotor angle (δ) either Increasing or decreasing response to : line switching, Load disturbances , loss of generation, short circuit faults and other power system disturbances.

During the large disturbance in the power system the transmit electric output power suddenly decreased from P0 to Pf as shown in Fig.4 but the mechanical torque (that connected to the generator and equivalent to the output electrical power P0 at the moment before reducing the electrical power) can’t reduce suddenly to equivalent Pf so this unbalance cases accelerating in the rotor of the machine and increasing in the angle (δ). The assume of this analysis is neglects the operation of the voltage regulator that control on the excitation and change the machine voltage, and the governors that change the mechanical input power

If the fault is cleared at the point of δc the output electric power will increase and become greater than the input mechanical power so the machine will start to decelerate but because of the inertia the machine rotor can’t decrease suddenly and reached to δe.

Assume area 1 refer to the accelerating energy, and area 2 refer to decelerating energy, so when the fault cleared quickly the two areas can be equal before the angle reach to the limiting angle δL and the system will return after some oscillation to the last operating point at δ0 and that call Stable power swing.

On the other side if the fault didn’t clear quickly and spend more time, the angle δ will move far away enough to make the two areas (accelerating area & de accelerating area) not equal before the angle reaches to δL as shown in Fig. 5 and due to the inertia the the angle will reach to the limiting angle and after this point the electrical output power will decrease again to be less than the mechanical input power so the machine rotor will accelerate and the rotor angle will increase above 180 and the pole slipping will happen . this case called out of step condition or unstable power swing condition

Power Swing Effect on the Distance Relay

Under Steady State Operation the impedance of the load not enter into the operation characteristic of the distance relay but in the case of power swing the load impedance may enter into the operation characteristic of the distance relay and case unwanted tripping for the transmission lines, and may be cause cascading tripping and stoppage of major sections of the power system.

The distance relay isn’t supposed to tripe under the power swing if stable or unstable to give the system the availability to return to its steady state. A power swing block “PSB” it’s a function inside the modern distance relays that prevent unwanted tripping by block the tripping signal in case of stable or unstable power swing thus, The Power Swing Block function is used to differentiate between the power swing and the fault. However, when the fault happens due to a Power swing it must be cleared very rapidly with high level of dependability and selectivity. The zones must be separated hoping to avoid causing large deference of the rotor angle between groups of the generators and loss the synchronism between them, equipment damage and the blackouts. Perfectly, the power system should be separated into predetermined zones to achieve a balance of load-generation in each of the separated zones. In some cases, load shedding is essential to avoid a whole blackout of the area where the load of the separated area is more than the local generation

Controlled tripping of the power system relays in the case of an Out-of-Step condition is very essential to prevent extensive outages, equipment damage and shutdown of large areas in the power system. Out-of-step trip (OST) in is a function that included in the modern relays to achieve the separation of the power system under the unstable power swing. The major aim of this function to differentiate between the stable and unstable power swing and separate the power system into predetermined zones to achieve the stability and continuity of the service.

2. Power Swing Detection Methods.

Following is a brief discussion of the methods that have been used for detecting power swings and schemes used for OSB and OST functions. Traditional method based on rate-of change of impedance and the recently method is Power-Swing-Detection Algorithm Based on Continuous Impedance Calculation.

2.2.1 Traditional Rate – of – change of Impedance Schemes for Detecting Power Swings.

In the conventional method, the relay measure and calculate the positive sequence impedance and the change rate of the impedance. During the steady sat operation the measured impedance depends on the load impedance and the distance between the protection zone of the distance relay and the point of measurement. The impedance move in trajectory depends on the operation case. In the case of the fault if the impedance moved rapidly to the fault zone. On the other case during the power swing the impedance moved slowly at some trajectory with a rate depend on the slip frequency between the generators. Thus, the variation of the impedance movement speed used to distinguish between the power swing and the faults. The concept of this method that using of two measuring impedance and separated them by ΔZ and used a calculated timer. When the measured impedance moved through the first one the timer will start running. If the measured impedance cross the second impedance before the timer expire, that indicated to a fault case. If the timer expired and the impedance didn’t reach to the second impedance that indicated to a power swing case.

Two-Blinder Scheme for PSB and OST Functions

The two blinder scheme of the rate-of-change of impedance method is used in many relays, separated by ΔZ, and a calculated timer TOSB. The two-blinder scheme is shown in Fig. 7. This figure shows Two parallel blinders are on the right side of the X-line impedance and another two parallel blinders are on the left side. The timer TOSB is start running when the impedance move through the outer blinder

During the fault condition the impedance moved rapidly through the outer blinder to the fault zone and cross the inner blinder before the TOSB expire. On the other case during the power swing the impedance moved slowly through the outer blinder and stay between the two blinders even TOSB will expire. This scheme can be used for a power swing blocking function to block the distance relay to prevent unwanted tripping.

During Power Swing Blocking declared a reset timer should be used in order to force the relay to delay the assert when TOSB expired.

The Out-Of-Step tripping (OST) can use the previous scheme expect that the calculated timer for out of step (TOST) is shorter than TOSB. In the case of power swing the impedance locus cross the outer blinder and at this point the timer will start running. If the timer TOST expires before the impedance locus cross the inner blinder, the power swing condition will declare, and if the impedance locus cross the inner blinder before timer TOST expires, the out of step condition will declare. The Out-Of-Step tripping function can be set to tripe instantly or wait for a time. The Instantly tripping referred to as “predictive tripping” or “early tripping”. The other case is referred to as “delayed tripping”. In some applications, the tripping is required only when the out of step condition occurs a determined number of times.

It is essential for some application to use both OSB and OST functions, and it available to use them in the Same relays using the two blinder scheme technique. According to difference in speed of impedance trajectory in the case of stable power swing and unstable power swing, two timers are required TOSB and TOST whereas TOST is shorter than TOSB to be fit with the rapidly speed of the impedance trajectory in case of unstable power swing

2.2.2. Power Swing Detection Algorithm Based on Continuous Impedance Calculation

The traditional methods of power swing detection are depend on a complex study for the power system to reach to the correct settings, but the setting are fixed and not adapt to any change in the system condition because of it is not possible to study the system under unstable power swing. therefore, the power system study doesn’t deem the worst cases and the bad conditions, so in the case of power swing or out of step condition it may lead to unwanted tripping.

The following method described below solves these problems. It is based on a continuous impedance calculation [5].

This method has the following features:

1. It is not required for settings or complex calculation
2. detect the power swing with frequencies range from 0.1: 10 Hz.
3. Detect the power swing in the case of single-pole open condition or fault condition.
4. Unblocking of distance protection relay if there both faults and power swing.
5. Out-of-step tripping during unstable power swing.

The Continuous Impedance Calculation method depend on taking four sample of the impedance per one cycle of the power system frequency the three phase each phase separately. During the power swing the impedance trajectory moved on an oval path, and when it enters the power swing area, as shown in Figure 8, the algorithm of power swing start its analysis for each phase and calculate the power swing area automatically.

In the case of power swing, the power swing detection will detect it, and stay active even if the trajectory of the impedance leave the power swing zone. The algorithm computes the new values of R & X, and compares them with the previous values that placed in the memory. The main criterias of the power swing detection methods are monotony, continuity and smoothness. The thresholds are calculated dynamically. This automatic adaptability to the trajectory speed change, enables the algorithm to detect the low frequency and the high frequency power swing.

Monotony: The algorithm will check the direction of the derivat